WO2014100702A2 - Dispositifs, systèmes et procédés pour applications optiques ultrarapides - Google Patents

Dispositifs, systèmes et procédés pour applications optiques ultrarapides Download PDF

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Publication number
WO2014100702A2
WO2014100702A2 PCT/US2013/077166 US2013077166W WO2014100702A2 WO 2014100702 A2 WO2014100702 A2 WO 2014100702A2 US 2013077166 W US2013077166 W US 2013077166W WO 2014100702 A2 WO2014100702 A2 WO 2014100702A2
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optical
ultrafast
pulse
nlo
nonlinear
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PCT/US2013/077166
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WO2014100702A3 (fr
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James Fan HSU
Carek Fuentes HERNANDEZ
Bernard Kippelen
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Georgia Tech Research Corporation
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Priority to US14/764,100 priority Critical patent/US9658510B2/en
Publication of WO2014100702A2 publication Critical patent/WO2014100702A2/fr
Publication of WO2014100702A3 publication Critical patent/WO2014100702A3/fr

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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/0126—Opto-optical modulation, i.e. control of one light beam by another light beam, not otherwise provided for in this subclass
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
    • C03C17/34—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions
    • C03C17/36—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal
    • C03C17/3602—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer
    • C03C17/3607—Coatings of the type glass/inorganic compound/metal
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
    • C03C17/34—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions
    • C03C17/36—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal
    • C03C17/3602—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer
    • C03C17/3618—Coatings of type glass/inorganic compound/other inorganic layers, at least one layer being metallic
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
    • C03C17/34—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions
    • C03C17/36—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal
    • C03C17/3602—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer
    • C03C17/3639—Multilayers containing at least two functional metal layers
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
    • C03C17/34—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions
    • C03C17/36—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal
    • C03C17/3602—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer
    • C03C17/3644—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer the metal being silver
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
    • C03C17/34—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions
    • C03C17/36—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal
    • C03C17/3602—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer
    • C03C17/3649—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer made of metals other than silver
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
    • C03C17/34—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions
    • C03C17/36—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal
    • C03C17/3602—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer
    • C03C17/3657—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions at least one coating being a metal the metal being present as a layer the multilayer coating having optical properties
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/011—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  in optical waveguides, not otherwise provided for in this subclass
    • G02F1/0115—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  in optical waveguides, not otherwise provided for in this subclass in optical fibres
    • G02F1/0118—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  in optical waveguides, not otherwise provided for in this subclass in optical fibres by controlling the evanescent coupling of light from a fibre into an active, e.g. electro-optic, overlay
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/0147—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on thermo-optic effects
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/3515—All-optical modulation, gating, switching, e.g. control of a light beam by another light beam
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/3515—All-optical modulation, gating, switching, e.g. control of a light beam by another light beam
    • G02F1/3517—All-optical modulation, gating, switching, e.g. control of a light beam by another light beam using an interferometer
    • G02F1/3519—All-optical modulation, gating, switching, e.g. control of a light beam by another light beam using an interferometer of Sagnac type, i.e. nonlinear optical loop mirror [NOLM]
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/3523—Non-linear absorption changing by light, e.g. bleaching
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/08—Mirrors
    • G02B5/0816—Multilayer mirrors, i.e. having two or more reflecting layers
    • G02B5/085—Multilayer mirrors, i.e. having two or more reflecting layers at least one of the reflecting layers comprising metal
    • G02B5/0875—Multilayer mirrors, i.e. having two or more reflecting layers at least one of the reflecting layers comprising metal the reflecting layers comprising two or more metallic layers
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference
    • G02F1/213—Fabry-Perot type
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2201/00—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00
    • G02F2201/34—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00 reflector
    • G02F2201/346—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00 reflector distributed (Bragg) reflector

Definitions

  • the present invention relates generally to devices, systems and methods for ultrafast optical applications, and more specifically to a nonlinear optical mirror, and uses of same in optical system, providing stronger modulations of the reflectance or transmittance in the visible and near-infrared spectral ranges at lower pump energies than conventional nonlinear optical materials or devices.
  • nonlinear optical devices are more easily implemented in the near-infrared and infrared spectral ranges.
  • nonlinear response of common materials is typically weak, when incorporated in device configurations they require large switching energies and/or long interaction lengths to produce significant effects.
  • other restrictions such as fulfillment of wave-vector matching conditions further complicate the incorporation of these materials into active optical devices.
  • Nonlinear mirrors have been realized by: the combination of a second harmonic generation (SHG) crystal and a dichroic mirror and used to demonstrate intra-cavity passive mode-locking operation for picosecond-pulse generation in the visible range; Bragg-periodic structures comprising semiconductor layers with Kerr-type optical nonlinearity in the infrared spectral range have been reported.
  • Ultrafast optical shutters or loop mirrors are known to be realized using a nonlinear Sagnac interferometer wherein an intra-loop nonlinear optical element is placed off-center to define the opening time of the shutter.
  • the nonlinear optical element in these cases is known to be a Kerr medium, a saturable absorber or a high gain laser media.
  • Noble metals are known to have an extremely large and ultrafast NLO response in the visible spectral range, much larger than most known organic or inorganic materials, but are seldom used as NLO materials due to their limited optical transparency and large reflectance in the visible (Vis) and near infrared (NIR) spectral ranges.
  • Ultra- thin layers of noble metals can be semi-transparent in the visible range if their thickness is around the skin depth of metals (typically between 10 to 20 nm). Thicker metal layers rapidly loose transparency and become highly reflective due to the inherent large admittance contrast between metals and the dielectric environment. It is well known in the art that the dielectric environment of a metal can be engineered, by nanostructured dielectric layers, to relief the admittance contrast between the metal and its environment.
  • metallic nanostructures offer a unique opportunity to engineer optical devices with tailored linear optical properties in the Vis and NIR spectral ranges.
  • the ability to engineer the linear absorption in a thin metal layer within a nanostructure is also important because the NLO response of noble metals arises from the electron and lattice heating caused by the absorption of energy from an ultrafast optical pulse.
  • the linear and NLO responses of noble metals are determined by the inherent electronic properties that define their dielectric permittivity.
  • interband and intraband transitions The electronic properties of noble metals in the visible spectrum are characterized by two separate mechanisms, namely interband and intraband transitions, that dominate in different spectral regions within the visible spectrum, and consequently lead to different properties.
  • Electronic interband transitions dominate in the visible or ultraviolet (UV) spectral region and arise from bound electrons excited from fully occupied electronic states within the d-band, below the Fermi energy level, to the half-filled s-p electronic bands in the conduction band. At lower energies, electronic intraband transitions occur from free electrons stimulated within the conduction band.
  • UV visible or ultraviolet
  • the absorbed optical intensity raises the temperature of the electron cloud and smears the electronic distribution around the Fermi energy (Fermi- smearing), causing a very strong change of the dielectric permittivity of the metal around the interband transition onset.
  • the latter is defined by the wavelength or photon energy where the electronic properties stop being dominated by interband transitions and start to be assigned to intraband transitions.
  • nonlinear optical devices exploiting the nonlinear optical response of thin noble-metal films are induced- transmission filters (ITFs) and metallo-dielectric bandgap structures (MDPBGs). These devices are known to amplify the response of a single thin metal film and typically display nonlinear reflectance or transmittance changes smaller than 10% at moderate fluences smaller than 10 J/cm .
  • ITFs induced- transmission filters
  • MDPBGs metallo-dielectric bandgap structures
  • the present invention comprises devices, systems and methods for ultrafast optical applications.
  • the present invention can comprise an optical device capable of an ultrafast and large change of its reflection or absorption coefficient upon being excited by an ultrafast optical pulse with wavelength in the visible (Vis), near-infrared (NIR) or infrared (IR) spectral regions.
  • Vis visible
  • NIR near-infrared
  • IR infrared
  • the present invention can further comprise systems and uses of such an optical device, for example, in an ultrafast all-optical shutter, and/or an ultrafast framing camera or to produce a pulse shaping device that could be used for laser manufacturing or other applications.
  • the present optical device comprises, preferably in sequential order, a first thick metallic layer, a first dielectric layer, a second thin metallic layer, and a second dielectric layer.
  • the present optical device can act as a nonlinear mirror that presents a large reflectance at low irradiance and a low reflectance at large irradiance.
  • the present optical device acts as a nonlinear mirror that presents a linear and nonlinear reflectance with a large angular bandwidth.
  • the present optical device acts as a nonlinear mirror that presents a high reflectance at a first angle of incidence and a high absorptance at a second angle of incidence, wherein an optical pulse impinging on the nonlinear mirror with a high irradiance at the second angle of incidence causes a strong reduction of the reflectance of the nonlinear mirror at the first angle of incidence.
  • the present optical device acts as a non-degenerate nonlinear mirror, wherein the reflectance at a first wavelength and at a first angle of incidence is controlled by the absorption of a light pulse at a second wavelength and at a second angle of incidence.
  • the present optical device acts as a reflective saturable absorber that presents a high absorptance at low irradiances and a lower absorptance at high irradiances.
  • the present optical device acts as a non-degenerate switch wherein the absorptance or reflectance of the device at a first wavelength is modified by the absorption of light at a second wavelength.
  • the present nonlinear optical device comprises in sequential order a first thick metallic layer, a first dielectric layer, a second thin metallic layer and a second dielectric layer.
  • the first thick metallic layer can be a substrate or a superstate and can be known metal with a large reflectivity in the Vis, NIR or/and IR spectral ranges.
  • the thickness of the first metallic layer is at least 50 nm, preferably more than 100 nm, and more preferably more than 200 nm.
  • the first dielectric layer can comprise a bulk or a nano structured material of known organic or inorganic materials with insulator or semiconductor properties having a low absorption in the spectral region of operation of the nonlinear mirror.
  • the refractive index of the first dielectric layer has a value of at least 1 in the spectral region of operation of the nonlinear mirror.
  • the second thin metallic layer constitutes the active nonlinear optical component on the structure and can be a noble metal such as Au, Ag, and Cu.
  • the thickness of the second metallic layer is at least such that the metallic layer is electrically continuous, above the percolation threshold, typically more than 5 nm, preferably more than 10 nm, but not larger than 40 nm or a couple of times the maximum metal skin depth in the spectral region of interest.
  • the second dielectric layer can be a substrate or a superstate comprising a bulk or a nano structured material, such as a Bragg stack, of known organic or inorganic materials with insulator or semiconductor properties having a low absorption in the spectral region of operation of the nonlinear mirror.
  • the refractive index of the second dielectric layer has a value of at least 1 in the spectral region of operation of the nonlinear mirror.
  • the thickness of the first and second dielectric layers are selected to produce the desired amount of absorption in the second thin metallic layer to produce the desired nonlinear optical change of the reflection or absorption coefficient of the entire nonlinear optical device.
  • the absorptance in the second thin metallic layer is a periodic function of the optical path length, defined as the thickness of a layer times its refractive index, of the first and second dielectric layers.
  • the periodicity of the absorptance in the second metallic layer is half-wave the optical path length of the first and second dielectric layers.
  • the linear absorption in the second metallic layer is significantly increased compared to the linear absorption of a single metallic layer of the same thickness at a wavelength in the Vis, NIR or IR.
  • the increased absorption in the second metallic layer produces an increased thermal nonlinearity of this layer, and consequently increases the nonlinear optical response of the present optical device, making it much stronger than the nonlinear response of a single metallic layer of the same thickness on glass.
  • the present nonlinear optical device displays a higher rate of change of reflectance (R) or absorptance (A) per change of refractive index of the second metallic layer, dR/dn or dA/dn, than that of a single metallic layer of the same thickness on glass.
  • the present nonlinear optical device can reduce the sensitivity of R or A to changes in the imaginary part of the refractive index of the second metallic layer, which are reflected by smaller values of dR/dk or dA/dk than those obtained for a single metallic layer of the same thickness.
  • the large nonlinear response of the present nonlinear optical device arises from: 1) a large absorptance in the second metallic layer and 2) high sensitivity of the present nonlinear optical device to subtle changes of the real part of the refractive index of the second metallic layer.
  • Some exemplary embodiments are or include nonlinear optical devices wherein the nonlinear reflectance change is bigger than 10% at fluences smaller than 10 J/m .
  • the present nonlinear optical device is designed to act as a nonlinear mirror having a large reflectance at low irradiances and low reflectance at large irradiances, wherein the reflectance changes is at least 10% for an optical fhience smaller than 10 J/m .
  • the present optical device acts as a nonlinear mirror that presents a linear and nonlinear reflectance with a large angular bandwidth.
  • the present optical device acts as a nonlinear mirror that presents a high reflectance at a first angle of incidence and a high absorptance at a second angle of incidence, and wherein an light impinging on the nonlinear mirror with a high irradiance at the second angle of incidence causes a strong reduction of the reflectance of the nonlinear mirror at the first angle of incidence wherein the reflectance change induced in the present nonlinear optical devices is at least 10% for an optical fhience smaller than 10 J/m .
  • the present optical device acts as a non-degenerate nonlinear mirror, wherein the reflectance at a first wavelength and at a first angle of incidence is controlled by the absorption of a light pulse at a second wavelength and at a second angle of incidence.
  • the present optical device acts as a reflective saturable absorber that presents a high absorptance at low irradiances and a lower absorptance at high irradiances.
  • the present optical device acts as a non-degenerate switch or modulator wherein the absorptance or reflectance of the device at a first wavelength is modified by the absorption of light at a second wavelength.
  • the present nonlinear optical device can be incorporated into an optical apparatus to control the amplitude or phase of an optical beam.
  • an optical apparatus can include but not be limited to a laser cavity, an interferometer, a detector and an imaging system.
  • the present invention comprises a nonlinear optical device having an ultrafast nonlinear optical response in visible and near infrared spectral range.
  • the nonlinear optical device comprises a metallic mirror, a first dielectric layer deposited on top of the metallic mirror, a semi-transparent thin noble metal layer deposited on top of the first dielectric layer comprising a single or a combination of noble metals (such as Au, Ag and Cu) and a second dielectric layer, wherein a single layer or a nanostructured dielectric multilayer is deposited on top of the noble metal layer.
  • the thickness of the noble metal layer is around the skin depth of the metal used.
  • the thickness of the first and second dielectric layers is chosen to tailor the linear absorption of the noble metal layer and the overall linear and nonlinear spectral reflectance and absorptance of the nonlinear optical device.
  • the nonlinear optical reflectance or absorptance of the present nonlinear optical device in response to strong optical pulse is at least one order of magnitude larger than the same pulse produced by a single isolated layer of the noble metal used in the nonlinear optical device structure.
  • the response time of the nonlinear optical device is ultrafast faster than lOOps.
  • the nonlinear optical device is designed to provide the linear and nonlinear reflectance invariant with respect to the angle of incidence.
  • the nonlinear optical device provides a high linear reflectance and low linear absorptance at a first angle of incidence and a lower linear reflectance and high linear absorptance at a second angle of incidence.
  • the nonlinear optical device is designed to have a very high absorptance at a low irradiance and smaller absorptance at a high irradiance.
  • the nonlinear optical device is used in optical apparatus used to control the optical beam in interferometers, detectors, and imaging systems.
  • an objective of the present invention is to advance the science and engineering of metal-dielectric thin-film structures for ultrafast all-optical applications.
  • Another object of the present invention is to utilize the beneficial advantages of linear and nonlinear optical (NLO) properties of Au and Ag/Au bilayer metallic thin films, and how bilayer metallic films can be tuned by controlling the mass-thickness ratio between Au and Ag.
  • NLO linear and nonlinear optical
  • Yet another object of the present invention is the design of a nonlinear device useful in an ultrafast all-optical shutter.
  • a further object of the present invention is the development of bilayer films that are attractive for active plasmonic applications and nonlinear optical filters.
  • Figs. 1 (a) real and (b) imaginary effective refractive index values of bilayer Ag/Au metal thin films: Ml, M2 and M3; Au (23 nm) and Ag (20 nm) are shown as reference.
  • Figs. 2 (a) Quality factor spectra for localized surface plasmon and (b) quality factor spectra for surface plasmon polariton of bilayer Ag/Au metal thin films: Ml, M2 and M3; Au (23 nm) and Ag (20 nm) are shown as reference.
  • Fig. 3 Simulated maximum potential transmittance spectra of bilayer Ag/Au metal thin films: Ml, M2 and M3; Au (23 nm) and Ag (20 nm) are shown as reference.
  • Figs. 4 (a), (b) Spectral dependence of transmittance and reflectance changes ( ⁇ ( ⁇ , tpeak) and ⁇ ( ⁇ , tpeak)) measured from WLC pump-probe experiment (solid line) and simulation by two-temperature model (dashed line) of samples Rl, S I, S2 and S3 with a pump fluence of 25 J/m2. The excitation wavelength in all cases was 560 nm.
  • Figs. 4 (a), (b) Spectral dependence of transmittance and reflectance changes ( ⁇ ( ⁇ , tpeak) and ⁇ ( ⁇ , tpeak)) measured from WLC pump-probe experiment (solid line) and simulation by two-temperature model (dashed line) of samples Rl, S I, S2 and S3 with a pump fluence of 25 J/m2. The excitation wavelength in all cases was 560 nm.
  • Figs. 9 (a) Comparison of measured (symbols) and simulated (lines) transmittance (T), reflectance (R), and absorptance (A) spectra in the visible range of the fabricated Broadband NLO Device sample.
  • the inset shows the generalized thin film structure of the NLO device
  • Figs. 10 Simulated (a) absorptance A(j, k) and (b) reflectance R(j, k) at 550 nm of the general NLO device structure (shown in the inset of Fig. 9) with different thickness combinations of reflection modifier (]) and cavity (k) layers.
  • Figs. 11 Spectral comparison of simulated (a) real and (b) imaginary refractive index changes between the 23 nm-thick Au film inside the Broadband NLO Device (light lines) and a 23 nm-thick Au film on a glass substrate (Au Ref) (dark symbols). Simulations were shown for an incident peak irradiance of 13 GW/cm2 at 550 nm. (c) Spectral comparison of simulated dR/dn and dR/dk for the same Au containing structures.
  • Figs. 13 Reflectance changes (AR) of the Broadband NLO Device pumped at 550 nm of (a) spectral spectrum AR( , tpeak) with respect to different incidence angles of ⁇ 5° (dark line), 10° (lighter line), 15° (lighter line), and 20° (dark line); (b) AR( , tpeak) at the peak response time tpeak, and (c) temporal dependence AR(600 nm, t) at probe wavelength 600 nm for different peak pump irradiance 10 (dark line), 21 (dark line), 31 (dark line) and 51(dark line) GW/cm2.
  • Figs. 14 (a) Irradiance dependent reflectance (%) at 550 nm (dark line and symbol) and 600 nm (dark line and symbol) by adding linear reflectance R0(X) with measured AR( , tpeak) at the peak response time tpeak, and (b) temporal dependence AR(600 nm, t) at probe wavelength 600 nm for different peak pump irradiances of the Broadband NLO Device sample pumped at 550nm.
  • Fig. 15 Non-collinear beam geometry imposed on the NLO device with a signal pulse at the normal incidence and a control pulse at the angle of incidence ⁇ .
  • Narrowband NLO Device lines and Broadband NLO Device (symbols).
  • Fig. 12(a) non-collinear
  • Fig. 12(b) collinear beam geometry
  • Fig. 20 Structure geometry of the reflective saturable absorber with two pairs of quarter- wave stacks
  • Fig. 21 Absorptance of the reflective saturable absorber as a function of the thickness of the dielectric cavity and spacer layers.
  • Fig. 22 Simulated linear spectra of the reflective saturable absorber using the matrix transfer method
  • Fig. 23 Simulated nonlinear response of the reflective saturable absorber pumped by a
  • Fig. 24 Schematics of 4f Nonlinear Sagnac Interferometer.
  • L represent a lens
  • M represent a mirror
  • BS is a 50/50 beam splitter.
  • Fig. 25 Schematics of 4f Nonlinear Sagnac Interferometer with Intraloop Fourier Lenses.
  • L represent a lens
  • M represent a mirror
  • BS is a 50/50 beam splitter.
  • Fig. 26 Temporal evolution of the transmitted intensity captured with the image camera setup demonstrating temporally tunable shutter operation.
  • Fig. 27 Schematics of an ultra high-speed camera setup.
  • L represent a lens
  • M represent a mirror
  • BS is a 50/50 beam splitter.
  • Fig. 28 Experimental setup of an all-optical shutter, where BS and PBS are the nonpolarized and polarized beam splitters, respectively, WPl is the half- wave plate, WP2 is the quarter- wave plate, Ml and M2 are silver mirrors, the Angular NLO Mirror is the nonlinear optical device and the transparent slab is a slab of a transparent material;
  • the corresponding polarizations in the light path are labeled in the figure as P-polarization, ⁇ -polarization, and Circw/ar-polarization.
  • a signal pulse incident from the input port with a peak irradiance of 21 o and a control pulse injects on the Angular NLO Mirror with a peak irradiance of Io at the incidence angle of ⁇ .
  • the sampled signal pulse transmits to the output port with a peak irradiance of I out , and then is focused by a microscope objective (MO) or a lens.
  • MO microscope objective
  • Fig. 29 Experimental setup of an all-optical shutter, where BS is the non-polarized beam splitters, Ml and M2 are silver mirrors, the Angular NLO Mirror is the nonlinear optical device and the transparent slab is a slab of a transparent material; A signal pulse incident from the input port with a peak irradiance of 21 o and a control pulse injects on the Angular NLO Mirror with a peak irradiance of Io at the incidence angle of ⁇ . The sampled signal pulse transmits to the output port with a peak irradiance of l ou and then is focused by a microscope objective (MO) or lens.
  • MO microscope objective
  • Fig. 30 Experimental setup of an ultrafast all-optical shutter with a diffractive optical element.
  • Fig. 31 The general configuration of the terahertz optical asymmetric de-multiplexer
  • Fig. 32 Experimental setup of an ultrafast all-optical shutter, where BS is the nonpolarized beam splitters, Ml and M2 are silver mirrors, the NLO Device is a nonlinear optical device (Narrowband NLO Device described in Section 3.2), LI, L2, and L3 are lenses and variable delay stage is an optical delay line; A signal pulse incident from the input port and a control pulse injects on the Narrowband NLO Device at normal incidence. The temporally sampled signal pulse transmits to the output port, and then is focused by a microscope objective (MO) down to the focal plane and causes non-thermal ablations.
  • BS is the nonpolarized beam splitters
  • Ml and M2 are silver mirrors
  • the NLO Device is a nonlinear optical device (Narrowband NLO Device described in Section 3.2)
  • LI, L2, and L3 are lenses and variable delay stage is an optical delay line
  • a signal pulse incident from the input port and a control pulse injects on the Narrowband NLO Device at normal incidence.
  • Figs. 34 The temporal analysis of (a) irradiance profile (I(t)) of the control pulse, (b) the complex amplitude modulation (A(t)) for the transient reflectance coefficient of the Narrowband NLO Device, and (c) the irradiance profile (Iout(t)) of the signal pulse sampled at the output port.
  • the shaded area represents the sampled portion which has experienced A(t).
  • the local time (t) is with respect to the position.
  • Figs. 35 The propagation analysis of the signal pulse with a spatially dependent irradiance profile (I(Z)) of (a) the incidence signal at the input port, (b) two split signals (clockwise and counter-clockwise propagating pulses) at the BS, (c) two split signals at the nonlinear optical device, (d) two recombined signals back to the BS, (e) the reflected signal at the input port, and (f) the sampled signal pulse at the output port, where Z is the local position, AL defines the adjustable sampling window of At, and 210 and 10 are peak irradiances of the incidence signal before and after split, respectively.
  • I(Z) spatially dependent irradiance profile
  • Fig. 36 Temporal-scan experiment of an ultrafast all-optical shutter to temporally scan its impulse response and measure the temporal profile of its adjustable opening window, where BS and PBS are the non-polarized and polarized beam splitters, respectively, WP is the half- wave plate, PI and P2 are polarizers, Ml to M6 are silver mirrors, the NLO Device is a nonlinear optical device (Narrowband NLO Device described in Section 3.2), LI to L4 are lenses, and Variable delay stage 1 and Variable delay stage 2 are optical delay lines; A femtosecond laser pulse is split into a high peak-irradiance control pulse and a low peak-irradiance signal pulse, where the control pulse injects on the NLO Device at normal incidence, the signal pulse incident from the input port, transmits to the output port, and then is focused down to the Photo-detector.
  • BS and PBS are the non-polarized and polarized beam splitters, respectively
  • WP is the half- wave plate
  • Fig. 37 Time-averaged output intensity I(t) of the ultrafast all-optical shutter versus time delay of the signal pulses with respect to the control pulses.
  • This temporal-scan experiment measures a profile of the adjustable opening window opened by the control pulse between the input and output of the shutter.
  • the figure shows 12 different settings of adjustable opening window (At) decreasing from 13 ps to 3.2 ps with a constant interval of 0.8 ps.
  • Figs. 38 (a) and (b), a femtosecond pulsed-laser surgery study between (a) temporally dependent irradiance profiles (I(t)) of a single Gaussian pulse with different fluences per pulse (F) and (b) ablation diameter (Yl axis on left and labeled by triangular symbols) and depth (Y2 axis on right and labeled in rectangular symbols) per pulse with different F.
  • I(t) temporally dependent irradiance profiles
  • F fluences per pulse
  • F ablation diameter
  • Figs. 39 Two different surgical operations are illustrated in (a) and (b), where (a) is the conventional dose control by the counting number (N) of multiple pulses incidence and (b) is the new dose control by sampling a single nanosecond pulse by the ultrafast all-optical shutter with an adjustable sampling window ( ⁇ ).
  • Fig. 40 Cascaded beam geometry imposed on the Narrowband NLO Device by a glass slab waveguide, where a signal pulse enters the waveguide at the incidence angle of 45°, double passes through the device in a reflection mode, and overlaps temporally and spatially with a control pulse coming at the normal incidence; Prism 1 and 2 are two right-angle prisms for coupling in and out the signal pulse.
  • substantially free of something can include both being “at least substantially free” of something, or “at least substantially pure”, and being “completely free” of something, or “completely pure”.
  • the present invention comprises metal-dielectric thin-film structures for ultrafast all- optical applications.
  • the linear and nonlinear optical (NLO) properties of Au and Ag/Au bilayer metallic thin films are useful in numerous applications, including an ultrafast all-optical shutter and camera.
  • the linear and NLO properties of bilayer metallic films can be tuned by controlling the mass-thickness ratio between Au and Ag.
  • NLO devices with broad spectral and angular bandwidths in the visible spectral region can be fashioned.
  • the narrow band dependent NLO response of the NLO device leads to all-optical controls of high peak- power optical signal pulses.
  • the more general NLO device technology can be integrated into an ultrafast all-optical shutter, which allows temporal sampling windows as short as a few ps.
  • Ultrafast all-optical shutter potentially can temporally shape high peak-power nanosecond optical pulses, which could benefit biomedical and micromachining applications.
  • Other possible optical applications such as short electron, X-ray pulse generations, ultrafast photography, and biomedical imaging.
  • the linear and NLO properties of Au and Ag/Au bilayer metallic thin films have been studied with physical models and compared with experiments.
  • An exemplary noble-metal NLO device structure is disclosed and shown to lead to extremely large and ultrafast reflectance changes upon being excited by femtosecond optical pulses.
  • the angular and spectral bandwidth of NLO devices are studied using two different designs: a broadband NLO device and narrowband NLO device.
  • the present NLO device technology can be used to develop an ultrafast all-optical shutter capable of offering the ability to easily adjust the opening window (i.e. actively control the time during which the shutter remains open) in the ps to ns temporal regimes.
  • Noble metals such as Ag, Au, and Cu
  • Ag, Au, and Cu can have a negative permittivity that allows for the excitation of surface plasmon polaritons (SPPs), which have been actively exploited in recent years for sub- wavelength photonic circuits.
  • SPPs surface plasmon polaritons
  • This negative permittivity also enables artificial meta-material structures that allow remarkable control over the dispersion and sign of the refractive index of a material and consequently over the flow of electromagnetic energy throughout its structure.
  • highly transparent metallic structures have been reported and are attractive as optical filters with high out-of-band rejection and thin-film metal electrodes in organic light-emitting diodes.
  • Noble metals have also attracted great attention because of their extremely large and ultrafast non-linear optical (NLO) response can be exploited to achieve all-optical control of metallic nano structure.
  • NLO non-linear optical
  • This NLO response is described as the first-order nonlinear susceptibility ⁇ (1) process caused by electron and lattice heating.
  • ⁇ (3) optical nonlinearities of noble metals have been interpreted in the context of the third-order nonlinear susceptibility ⁇ (3) process measured by standard techniques such as Z-scan and degenerate four-wave mixing (DFWM) for the characterization of the NLO properties.
  • the third-order nonlinearity is an intensity-dependent nonlinear effect so-called optical Kerr effect, wherein the value of ⁇ (3) coefficient is an intensity-independent constant, and the nonlinear response is instantaneous on the sub-femtosecond time scale.
  • optical Kerr effect the instantaneous Kerr effect was found inadequate to describe the NLO properties of noble metals through experimental studies.
  • the ⁇ (3) coefficients of noble metal thin films measured from Z-scan techniques are not intensity-independent constants and their values could vary significantly by two order-of-magnitudes with different pulse intensities and pulse widths.
  • non-instantaneous ⁇ (3) coefficient is still derived from the two-temperature model, but offers an extended version to interpret ultrafast thermal nonlinearities of noble metals. It is important to note that non-instantaneous ⁇ (3) coefficients of noble metals are pulse width dependent and intensity dependent. For instance, the imaginary part of ⁇ (3) is sometimes discussed in form of nonlinear absorption coefficient ⁇ to describe the nonlinear absorption process, and the effective value of reported to change by nearly two orders of magnitude from 6.8 —7 to 6.7 —5
  • ⁇ (3) coefficient values are listed below as a general reference on magnitudes.
  • Most NLO materials display values of ⁇ (3) that are orders-of-magnitude smaller than those estimated for noble metals, for example, glasses such as fused silica or BK-7 glass at 1.91 ⁇ displays a value of 10 " esu
  • nanoparticles such as CS-doped glass (Commercial glass, Corning #3484: CS3-68) at 532 nm or Au-doped glass at 550 nm displays values of 10 "9 - 10 "7 esu (10 "17 -10 "15 m 2 /V 2 ); Polymers such as PTS (p-toluene sulfonate) at 1.06 ⁇ displays values of 10 "8 esu (10 "16 m 2 /V 2 ).
  • the linear and nonlinear optical responses of a noble metal and their potential applications are determined by the inherent electronic response which gives rise to their dielectric permittivity.
  • the electronic response of a noble metal in the visible spectral region can be divided into two separate mechanisms: interband and intraband electronic transitions.
  • Electronic interband transitions in the visible or ultraviolet (UV) spectral region arise from bound electrons excited from fully occupied electronic states within the d-band, below the Fermi energy level, to the half-filled s-p electronic bands in the conduction band.
  • UV visible or ultraviolet
  • metals are opaque because optical fields are strongly absorbed.
  • electronic intraband transitions occur from free electrons stimulated within the conduction band.
  • metals are opaque mainly because the optical fields are reflected off its surface, rather than absorbed in the bulk.
  • Noble metals are known to have an extremely large and ultrafast NLO response in the visible spectral range, much larger than most known organic or inorganic materials, but are seldom used as NLO materials due to their limited optical transparency and large reflectance in the visible and near-infrared spectral ranges.
  • Ultra- thin layers of noble metals can be semi- transparent in the visible range if their thickness is around the skin depth of metals (typically between 10 to 20 nm). Thicker metal layers rapidly loose transparency and become highly reflective due to the inherent large admittance contrast between metals and the dielectric environment. It is well known in the art that the dielectric environment of a metal can be engineered, by nanostructured dielectric layers, to relief the admittance contrast between the metal and its environment.
  • the nonlinear optical changes of an Au thin film has been extracted from pump-probe experiments and shown to be strong, spectrally broad, and ultrafast.
  • the present invention in an exemplary embodiment is an Au based NLO device structure that allows all-optical control of its reflectance will be presented.
  • the easily fabricated four-layer metal- dielectric thin-film structure will provide large and ultrafast reflectance changes in the visible spectral range. Its adjustable spectral and angular bandwidth will also be demonstrated in two structure designs: broadband NLO device and narrowband NLO device.
  • NLO nonlinear optical
  • SHG second harmonic generation
  • dichroic mirror With nonlinear reflectors, for example, the combination of a second harmonic generation (SHG) crystal and a dichroic mirror has been used to demonstrate intra-cavity passive mode-locking operation for picosecond-pulse generation in the visible range.
  • SHG second harmonic generation
  • dichroic mirror With nonlinear reflectors, for example, the combination of a second harmonic generation (SHG) crystal and a dichroic mirror has been used to demonstrate intra-cavity passive mode-locking operation for picosecond-pulse generation in the visible range.
  • SHG second harmonic generation
  • dichroic mirror With nonlinear reflectors, for example, the combination of a second harmonic generation (SHG) crystal and a dichroic mirror has been used to demonstrate intra-cavity passive mode-locking operation for picosecond-pulse generation in the visible range.
  • SHG second harmonic generation
  • dichroic mirror With nonlinear reflectors, for example, the combination
  • NLO devices have mostly been implemented in the near-infrared and infrared spectral ranges because NLO materials in the visible spectral region are scarce.
  • the most commonly used NLO materials are semiconductors, but their prominent nonlinearities mainly locate at infrared or near infrared wavelengths.
  • All-optical applications in visible region have been limited by the lack of materials and devices with a strong NLO response.
  • the rapid-growth development of biomedical applications and micromachining using ultrafast optical pulses in the visible spectral range creates a strong need for developing ultrafast optical devices that can operate at visible and near-infrared wavelengths.
  • These devices can perform all-optical control over the spatial and temporal irradiance profiles of high energy optical pulses and could potentially be attractive for a multiple applications related to the use of ultrafast optical pulses.
  • Ultrafast laser pulses with nanosecond, picosecond, and femtosecond temporal pulse widths are used to drill, cut, or machine products for the microelectronics, biotechnology, photonics, precision engineering and medical industries.
  • Pulsed lasers offer fast and cost- efficient prototyping of products made from any material, like ceramics, glasses, plastics, or metals, just to mention a few.
  • pulsed lasers are now also used in a wide variety of surgical procedures such as eye, dental, blood-vessel, and endoscopic surgeries (i.e. removal of deep-sealed tumors or treating scarred vocal folds). This is because high peak-power laser pulses are able to ablate materials with a high spatial precision in a fast and cost-effective way.
  • the commercialization potential of the ultrafast all-optical shutter was investigated through a National Science Foundation funded program (NSF-iCORPS).
  • NSF-iCORPS National Science Foundation funded program
  • An extensive study has been conducted by interviewing people in a wide variety of companies such as laser contracting service, laser integration, laser tattoo removal, laser engravers, pulsed laser deposition (PLD), and research institutes conducting ultrafast spectroscopic and imaging studies. It was found that if applied to slice ultrafast laser pulses, the ultrafast all-optical optical shutter technology will need further developments and demonstration of outstanding cutting resolutions and speeds to compete with existing advanced sub-nanosecond pulsed laser.
  • Ag, Au, and configurations of Ag/Au bilayer thin films have also been used to solve multiple engineering problems. For instance, high chemical stability and high sensor sensitivity have been achieved by using Ag/Au bilayer films in different surface plasmon resonance (SPR) sensor designs. Al/Ag bilayer films have also been used as transparent electrodes in top-emitting organic light-emitting diodes. Additionally, the interactions between multi-layer metal films and femtosecond laser pulses have been studied in terms of increasing the damage threshold of laser mirrors. However, little attention has been directed at understanding the linear and nonlinear optical properties of Ag/Au bilayer thin films by controlling the thickness ratio between the two metals.
  • SPR surface plasmon resonance
  • R2 Glass/ Ag (20 nm), where Au (23 nm) and Ag (20 nm) corresponds to a 23 nm and 20 nm thick Au and Ag film, respectively, and Ml corresponds to Ag (4 nm)/Au (14 nm), M2 to Ag (10 nm)/ Au (10 nm), and M3 to Ag (15 nm)/ Au (6 nm) bilayers.
  • the layer thicknesses (shown inside the parentheses) were individually estimated by matching transfer matrix method simulations with measured values of the transmittance (7), reflectance (R), and absorptance (A) spectra taken by a Shimadzu UV-Vis-NIR scanning spectrophotometer.
  • Refractive index values of deposited Au (23 nm) and Ag (20 nm) films were obtained by modeling spectroscopic ellipsometric (SE) data (J. A. Woollam M-2000UI), taken on individual films, as a perfectly flat continuous layer.
  • SE spectroscopic ellipsometric
  • NLO nonlinear optical properties
  • WLC white-light continuum
  • Helios ultrafast system
  • the pump pulse obtained from an optical parametric amplifier (TOPAS-C, Spectra-Physics) was tuned to a wavelength of 560 nm.
  • a laser beam from a Ti:Sapphire regenerative amplifier (Spitfire, Spectra-Physics) operating at 800 nm pumped the TOPAS-C, while a small portion of this beam generated the WLC (420 - 950 nm) probe pulse.
  • the WLC probe pulse measured 60 ⁇ half-width- l/e (HW 1/e) at the sample position using a knife-edge scan, and the pump pulse was 285 ⁇ (HW 1/e). Because the probe size is significantly smaller than the pump, it is assumed that the probe overlaps with a region of approximately constant peak fluence from the pump.
  • the pump has a pulse width of 60 fs (HW 1/e) and the total instrument response time is 150 fs full-width-half-maximum (FWHM).
  • the pump beam was chopped at 500 Hz with a 50% duty cycle to obtain pumped (signal) and non-pumped (reference) probe spectra sequentially.
  • the transmittance spectra change ( ⁇ ( ⁇ , t)) and reflectance spectra change (AR( , t)) of the WLC probe pulses were calculated from measured ⁇ ( ⁇ , t) as a function of delay time for a variety of pump fluences by
  • Figs. 1(a) and 1(b) show the N ej f values of bilayers Ml, M2 and M3, as well as the refractive index values of single layer Au (23 nm) and Ag (20 nm).
  • the index values are close to literature values, with the interband transition onset of bulk Au located at 520 nm and the interband transition onset of bulk Ag at 313 nm. At these wavelengths, inflection points are present in the real part of refractive index.
  • QLSP is defined by the ratio - ⁇ ' I ⁇ ", which can be directly related to, for instance, the resolving power of a single layer or multilayer superlens.
  • QSPP relates to the proficiency of a metal to sustain surface plasmon polartions (SPPs).
  • QSPP is defined by the ratio ( ⁇ / ⁇ ", which is directly related to the propagation length of SPPs in plasmonic waveguides.
  • Figs. 2(a) and 2(b) shows the spectral dispersion of QLSP and Qspp calculated for Ml, M2,
  • QLSP improves from a value of 0.47 for Au (23 nm), to values of 0.79 for Ml, 2.3 for M2, and 4.5 for M3.
  • QLSP 9.8 for Ag (20 nm).
  • Qspp improves from a value of 0.89 for Au (23 nm), to values of 1.9 for Ml, 11 for M2, and 23 for M3, while for Ag (20 nm) it has a value of 78, although for wavelengths above -600 nm, the quality factors for Au (23 nm) are higher than for any of the bilayers.
  • the concept of maximum potential transmittance is useful since it provides an upper limit to the transmittance of an absorbing film, after all reflectance losses are suppressed.
  • the maximum potential transmittance, ⁇ , for a single metallic film has been defined as a function of the layer thickness and the refractive index.
  • the values of the maximum potential transmittance for Ag (20 nm), Au (23 nm), and bilayer films are compared by using the N e ff values previously derived and assuming films of equal thickness, 20 nm, to provide a fair comparison.
  • Au (23 nm) always displays lower values of ⁇ than Ag (20 nm).
  • strong absorptive losses in Au (23 nm) due to interband transitions limits the values of ⁇ compared to Ag (20 nm), which has an interband transition region in the UV range.
  • decreasing the ratio of Au-to-Ag leads to larger values of ⁇ in all Ag/Au bilayer films compared to Au (23 nm), in the 350- 500 nm wavelength range.
  • ⁇ improves from 70% for Au (23 nm) to 77% for Ml, 79% for M2, and 84% for M3, as the thickness ratio of Au-to-Ag decreases as shown in Fig. 3.
  • Bilayer films do not display improved values of ⁇ but remain still higher than 90%.
  • Figs. 4(a) and 4(b) show the values of ⁇ ( ⁇ , t pea k) and AR ⁇ , t pea k) measured in WLC pump-probe experiments in samples Rl, S I, S2, and S3 for a pump fluence of 25 J/m 2 .
  • the subscript peak denotes the maximum value of the NLO response in the spectral and temporal ranges studied.
  • the maximum ⁇ ( ⁇ , t pea k) ⁇ are at least an order of magnitude smaller ( ⁇ 0.5% for a pump fluence of 50 J/m ) than those found in bilayer or single Au layer films, so their values are not included in Figs. 4-5.
  • the ⁇ ( ⁇ , t pea k) and AR( , t pea k) spectra display similar dispersion characteristics found in a single Au layer (Rl).
  • the peak- to -valley magnitude of ⁇ ( ⁇ , t pea k) and AR( , t pea k) gradually reduces as the mass thickness ratio of Au-to-Ag decreases from Rl, SI, S2 to S3 (Figs. 4(a) and 4(b)).
  • T e and ⁇ / The changes in electron and lattice temperatures, T e and ⁇ / , respectively, calculated through the two temperature model depend upon intrinsic material properties such as the electron and lattice specific heats, C e and Q, respectively, and the electron phonon coupling constant, G, and are driven by the density of absorbed power within each individual layer, P(t).
  • C e (T e ) (62 + 5) x T e [J/m 3 K]
  • Q (3.2 + 0.4) x T e [J/m 3 K]
  • G (1.8 ⁇
  • ⁇ ( ⁇ , ⁇ ⁇ ⁇ , ⁇ ( ⁇ ) e intra (co, T t ⁇ , ⁇ (t)) + e inter ( ⁇ , ⁇ ( ⁇ ) (2)
  • ⁇ is the free space optical frequency
  • t is the time
  • 7 ⁇ (t) and T e (t) are temporal dependent lattice and electron temperatures, respectively.
  • 3 ⁇ 4 is the background dielectric constant
  • ⁇ is the damping constant
  • ⁇ ⁇ is the bulk plasma frequency
  • ⁇ , yi, y 2 ,and y ⁇ are constant coefficients.
  • Table 2 Constant coefficients of an Au permittivity model of the first and second interband transition term (Eq. 4), respectively.
  • AR An , Ak ,
  • AV e ff is extracted from the experimental data.
  • the partial derivatives, dT/dn, dT/dk, dR/dn and dR/dk were approximated by their differentials ( ⁇ / ⁇ AT/Ak ej , and etc.) by introducing a small perturbation to N e ff in the entire bilayer.
  • N S i m is related only to the thickness of the Au layer ( ⁇ 3 ⁇ 4J in the bilayer structure, while AV e ff is ascribed to the total bilayer thickness (dr).
  • the optical properties of Ag/Au bilayer metallic thin films with a total thickness of approximately 20 nm and with different Ag/Au mass-thickness ratios were studied.
  • the effective refractive index values were found to be spectrally tunable by controlling the mass- thickness ratio between Au and Ag.
  • the optical loss introduced by interband transitions in Au layers can be reduced.
  • improvement of the quality factors (QLSP and Qspp) derived for plasmonic applications and the potential transmittance ( ⁇ ) for optical filter applications are calculated within the visible range.
  • the NLO response in the bilayer films is dominated by the ultrafast dynamics of the thermal exchange between the absorbed optical field and the electron cloud and the lattice in the Au layer.
  • the combined properties of these bilayers could therefore be attractive for a variety of linear and nonlinear photonic applications.
  • Fig. 9(a) shows a generalized thin film structure of the broadband NLO device. It was fabricated by deposition of the Ag layer onto a glass substrate followed by the deposition of the Au layer sandwiched and protected by Si0 2 thin films with the following geometry: broadband NLO device: Glass/Ag(100 nm) /Si0 2 (81 nm)/Au(23 nm)/Si0 2 (81 nm).
  • Fig. 9(a) shows the comparison between measured values of the transmittance (7), reflectance (R), and absorptance (A) spectra taken by a Shimadzu UV-Vis-NIR scanning spectrophotometer and the simulated spectra by transfer matrix method. The good match simultaneously verifies both the fabrication process and the simulation of the transfer-matrix method.
  • Spectroscopic ellipsometric (SE) data J.A. Woollam M-2000UI
  • the angular dependence of linear reflectance was measured using the SE apparatus.
  • the spectroscopic ellipsometry also measures the reflectance spectra at varied angles shown in Fig. 9(b) to determine the angular bandwidth of the sample.
  • the nonlinear optical properties (NLO) of the broadband NLO device were characterized by a commercially available white-light continuum (WLC) pump-probe spectroscopy system (Helios, ultrafast system).
  • the pump pulse was tuned to a wavelength of 550 nm with a pulse width of 60 fs half-width-l/e (HW 1/e) and a spot size of 347 ⁇ (HW 1/e) at the sample position, measured using a knife-edge scan.
  • the WLC (420-950 nm) probe pulse had a spot size of 80 ⁇ (HW 1/e) at the sample position and a low enough fluence to produce no observable NLO response in the sample.
  • the total instrument response time is 150 fs full- width-half - maximum (FWHM). Because the probe spot size is smaller, it is assumed that the probe overlaps with a region of approximately constant peak fluence from the pump.
  • FWHM full- width-half - maximum
  • the structure of the broadband NLO device shown in the inset of Fig. 9(a) can be seen as a Fabry-Perot filter comprising a central dielectric layer (cavity) sandwiched by two reflectors; the first one is the thick Ag layer and the second one is the thin Au layer covered by a dielectric layer (reflection modifier) on top.
  • This structure can provide a wide variation of absorptance values by varying the thickness distribution of its two dielectric layers.
  • Fig. 10(a) shows a contour plot of absorptance A(j, k) values as a function of the thickness j of the top dielectric layer which is called the reflection modifier, and the thickness k of the dielectric layer sandwiched between the two metal layers and which is referred to as the cavity thickness.
  • the absorptance A(j, k) was simulated using the transfer-matrix method at a wavelength of 550 nm.
  • the structure can provide a wide variation of absorptance values, ranging from 2% to 96%. These variations are periodic with respect to the thicknesses of the reflection modifier and cavity layers, as can be seen from regions [(1,1), (2,1), (3, 1), (1,2)] in Fig. 10(a).
  • these regions have a length of 188 nm corresponding to the half-wave optical thickness of Si0 2 at 550 nm.
  • the simulated reflectance is complementary to the absorptance at 550 nm (as expected from the negligible transmittance at this wavelength) and therefore shares the same periodicity.
  • the dominant contribution to the total absorptance of the broadband NLO device is primarily attributed to the Au thin film (53%), with negligible absorption in its component Ag layer (2%). This is in part because of the electric field distribution imposed by the NLO device geometry, and in part because Au is highly absorptive at wavelengths close to the interband transition onset, in the middle of the visible spectral range.
  • the interband transition onset of Ag located in the ultraviolet (UV) spectral range causes light with photon energy far below the onset to be mostly reflected.
  • the Si0 2 films are lossless based on SE data.
  • the broadband NLO device was designed to have a linear absorptance of 55% and a linear reflectance of 45% at 550 nm. This choice was arbitrary, but it is limited by a trade-off between the linear reflectance and the strength of the NLO response of a NLO device. Although both properties are preferred in general all-optical control applications, a compromise was made here to pick up the broadband NLO device. The trade-off is because that a higher linear absorptance (A) drives the stronger nonlinearity of the broadband NLO device, on the other hand, it also decreases the linear reflectance (R), which is expected because R and A spectra are complementary in the whole visible spectral range as shown in Fig. 9(a).
  • the measured R spectrum also shows that the fabricated broadband NLO device sample is a long wavelength pass optical filter with a broad spectral bandwidth.
  • the measured R spectrum as a function of angle of incidence in Fig. 9(b) is shown to be angularly insensitive. Within incidence angles up to 65 ° , the R spectrum barely changes compared to the normal incidence.
  • the linear optical response of the broadband NLO device shows both broad spectral and angular bandwidth.
  • the thermal nonlinearity of this layer is enhanced and consequently the NLO response of the NLO device.
  • simulations of the refractive index change have been carried out on Au using the two-temperature model and a physical model describing the dielectric permittivity outlined above.
  • the linear absorptance in the Au film is calculated using the transfer-matrix method.
  • the absorbed power is introduced as the source term in the two-temperature model.
  • This model describes the temporal evolution of electron and lattice temperatures as a function of the absorbed power and thermal properties of the electrons and lattice in the metal.
  • the electron temperature derived through this model is used to calculate a temperature-dependent dielectric permittivity function that contains two terms: a first one describing interband transitions using the approximation that electronic transitions occur from a flat d-band to a parabolic conduction band; and a second one describing contributions from intraband transitions using a Drude function.
  • the refractive index of the metal, before and after optical excitation is taken as the root-square of the calculated dielectric permittivity.
  • t pea k denotes the delay time of the probe pulse that yields maximum NLO response in the temporal ranges studied.
  • the peak wavelength of ⁇ ( ⁇ , t pea k) and Ak( , t pea k) are always located around the interband transition onset of Au at 520 nm.
  • the peak-to-valley magnitude of An and Ak doubles. This is achieved by increasing the linear absorptance at 550 nm in the Au thin film from 19% for Au Ref to 53% in the broadband NLO device.
  • the correspondent reflectance changes as a function of wavelength (X) and delay time (t) can also be calculated by the following Eq. 6.
  • AR( t) R(N Au ( ⁇ ) + AN Au (A, t)) - R 0 (A) (6)
  • the reflectance changes AR( , t) can be calculated exactly and have been used in the optimization process while designing the broadband NLO device, developing a simplified analysis is attractive to illustrate the enhancement mechanism of AR.
  • the measured AR(550 nm, t peak ) increases from -1% for Aw Ref to -16% for the broadband NLO device excited by a peak pump irradiance 13 GW/cm at 550 nm.
  • the first order approximation to the Taylor expansion of AR is taken as functions of AN AU an d yields the following equation.
  • FIG. 11(b) shows that ⁇ &(550 nm, t pea ]d values for the broadband NLO device and Au Ref are both close to zero, and from 515 nm to 800 nm the calculated dR/dn for the broadband NLO device is always at least ten times larger than dR/dk, as shown in Fig. 11(c).
  • AR can be decomposed into two components: An and dR/dn.
  • An is an intrinsic property of the nonlinear material, although is dependent on the absorbed optical power as has been shown in Figs. 11(a) and 11(b).
  • the broadband NLO device generally displays higher dR/dn values than the Au Ref i the visible spectral range. For instance, at 550 nm, dR/dn improves from a value of -8 for the Au Ref to a value of -85 for the broadband NLO device. Note that the broadband NLO device not only doubles the An of the Au film due to an increased linear absorptance from 19% for Au Ref to 53% in the broadband NLO device, but also introduces a better structural design which improves the sensitivity of R to small changes on the real part of the refractive index of Au.
  • the device structure reduces the sensitivity of R to changes in the imaginary part of the refractive index of Au; as revealed by the smaller values of dR/dk found for the device structure than for Aw Ref.
  • AR is expected to improve by around 20 times compared to Au Ref. This evaluation was verified by the measured AR(550 nm, t pea i), which increases 16 times from Aw Ref to the broadband NLO device.
  • the maximum magnitude of dR/dn is not at 550 nm, where it reaches a magnitude of -85, but instead at 600 nm, where it reaches a magnitude of -101.
  • the broadband NLO device was designed for degenerate pump-probe operation, which means that AR(550 nm) was maximized for this geometry.
  • the broadband NLO device is excited and probed at 600 nm, a smaller magnitude of AR is expected than the value when excited at 550 nm. This is because the linear absorption at 600 nm is only 31%; much smaller than the 55% obtained at 550 nm.
  • differences in An between excitation at 550 nm and at 600 nm, ⁇ «(550 nm) » ⁇ «(600 nm) are expected to be more significant than the difference in dR/dn found at these wavelengths.
  • the optimization process for designing the thickness distribution of the broadband NLO device is explained in detail.
  • a higher linear absorptance (A) on the broadband NLO device leads to stronger NLO changes in the Au layer, the trade-off is that the linear reflectance (R) is also reduced.
  • the broadband NLO device was set to have a linear absorptance (A) of 55% and a reflectance (R) of 45% at 550 nm.
  • a nonlinear mirror was fabricated with the following geometry: Glass/Ag(100 nm) /Si02(Dl nm)/Au(23 nm)/Si02(D2 nm).
  • the nonlinear mirror has, as noted above, a linear absorptance of 55% and a reflectance of 45% at 550 nm. This choice is arbitrary.
  • the total absorptance of the mirror is a periodic function of the thicknesses of the layers, so there are an infinite number of thickness combinations which will yield a given targeted absorptance value.
  • dielectric layer thickness combinations (j, k), a(81, 81), b(116, 98), c(30, 65), and (156, 94) were picked as examples to illustrate the design process.
  • the units of the thicknesses denoted inside the parenthesis have been dropped for convenience. This process required calculations of AR( , tp ea k) for multiple structures along this line in order to estimate the structure that maximized AR(550 nm, t peak ).
  • Fig. 12(b) shows the AR( , t pea k), calculated by Eq. 6 using the values of
  • Figs. 11(a) and 11(b) at the peak response time, t pea k, for the broadband NLO device structures excited at 550 nm.
  • the results show that AR(550 nm, t pea k) improves gradually from points (156, 94) through a(81,81) and starts decreasing from a(81,81) to c(30, 65).
  • the maximum magnitude of AR(550 nm, t pea k) was found to be located at a(81, 81).
  • the value of AR(550 nm, t pea k) was found to be the same in other regions provided that the dielectric thicknesses were increased by half-wave optical thickness of Si0 2 at 550 nm.
  • Fig. 13(a) shows that angular dependence of AR( , t pea k) measured in pump-probe experiments conducted on the broadband NLO device at a pump irradiance of 51 GW/cm at 550 nm.
  • t pea k denotes the maximum magnitude of AR in the temporal ranges studied.
  • AR( , t pea k) displays a broad spectral and angular bandwidth. For instance, for angles ⁇ 20 ° it was found that AR( , t pea k) across the visible spectral range changes by no larger than 7% with respect to data acquired near normal incidence ( ⁇ 5 ° ).
  • Fig. 13(b) shows the values of AR( , t pea k) measured for peak pump irradiances of 10, 21, 31, and 51 GW/cm 2 at 550 nm.
  • ⁇ ( ⁇ , t pea k) spectra consistently displays broad spectral bandwidth characteristics. It was confirmed that the highest irradiance did not damage the sample by acquiring multiple data sets at high and low irradiance levels on the same spot.
  • Fig. 14(a) shows that the broadband NLO device offers significantly strong reflectance suppression from a reasonably high Ro . , such as from 49% to 23% at 550 nm and from 69% to 32 % at 600 nm by increasing pump irradiance to 51 GW/cm .
  • the power dependent reflectance of the broadband NLO device can meet multiple engineering requirements in the visible spectral range by having an extreme large magnitude of AR, ultrafast response, and fairly broad spectral and angular bandwidths.
  • Fig. 14(b) shows the temporal evolution of AR(600 nm, t) as a function of the peak pump irradiances.
  • the peak wavelength has been selected to illustrate the ultrafast temporal evolution of the NLO response of the broadband NLO device.
  • AR(600 nm, t) shows a larger magnitude and a delayed relaxation of the reflectance modulation.
  • a very similar behavior has been reported in single Au films, and has been attributed to a change of the electronic specific heat with increased temperature.
  • Fig. 15 shows a generalized thin-film structure, comprising of a 100 nm- thick Ag and a 23 nm- thick Au sandwiched between two dielectric layers such as Dl and D2.
  • This four-layer thin-film structure is used in broadband NLO device, which provides large reflectance changes in response to a high peak-power optical pulse.
  • the broadband NLO device represents a conventional all-optical control in the sense that it allows a low peak-power signal pulse to be modulated in amplitude and phase by a high peak-power control pulse.
  • the narrowband NLO device shown in Fig. 15 is designed to operate in a non- collinear beam geometry, wherein the high angular dependence is designed such that the control pulse experiences a high linear absorptance at a first angle of incidence ⁇ , and the signal pulse experiences a low linear absorptance at a second angle of incidence (i.e. the normal incidence).
  • the narrowband NLO device presents a highly angularly dependent linear absorptance (A).
  • This four layer structure with a high angular A dependence is designed to maximize the linear absorptance of the control pulse, which leads to accessing the maximum NLO response of the device, but simultaneously limits the loss experienced by the signal; thus offering a better compromise between NLO response and linear loss than the broadband NLO device.
  • the present narrowband NLO device aims to show a high angular sensitivity in its NLO response, which can open up a new type of all- optical control for high peak-power signal pulses, for example, laser material processing applications such as laser manufactures and pulsed-laser surgeries.
  • All-optical controls have been primarily applied in the context of optical signal processing applications because advantages offered by its ultrafast operation, as shown in a variety of devices such as all-optical switch, all-optical modulator, and all-optical logic gate. These devices use nonlinear optical (NLO) materials to modulate the amplitude and phase of one light beam (signal) through their NLO properties by the other light beam (control). Different types of NLO properties such as Kerr nonlinearity, and saturable absorption have been used for all-optical control applications. However, a common inherent limitation of all these approaches is that the peak power of the controlled signal beam needs to be weaker than a control beam to selectively excite the NLO response of the material.
  • NLO nonlinear optical
  • Andrew M. C. Dawes et al have overcome some aspects of this limitation by using a highly sensitive light-induced scattering in a warm laser-pumped rubidium (Rb) vapor.
  • a weak control light is sufficient to redirect other strong signal lights.
  • This approach opens up new possibilities for all-optical devices to be cascaded and single-photon excitable. It needs to be noted that both signal and control lights here were using continuous wave (CW) lasers rather than pulsed lasers as extensively discussed herein. This new all-optical control operates within the so-called low-light-level regime, and has been achieved by other methods as well.
  • CW continuous wave
  • Cu are relatively easier to excite using the absorption of a single ultrafast optical pulse.
  • the absorbed energy and power of an optical pulse arises electron and lattice temperatures of a metal thin film, then smears the electronic distribution of the metal around the Fermi energy level (Fermi-smearing), and causes a large and ultrafast change of transmittance and reflectance on the other pulse (signal pulse) at visible wavelengths.
  • a metal-dielectric thin-film structure (broadband NLO device) was developed. This structure amplifies the ultrafast NLO properties for an Au thin film and consequently enhances the NLO response of the whole Au based thin film structure.
  • a NLO device with a high angular dependence was designed using the theoretical formalism presented above and fabricated by deposition of the Ag layer onto a glass substrate followed by the deposition of the Au layer sandwiched and protected by Zr0 2 thin films with the following geometry: narrowband NLO device: Glass/Ag(100 nm)/Zr0 2 (389 nm)/Au(23 nm)/Zr0 2 (133 nm).
  • the NLO properties were characterized by a commercially available white-light (WLC) continuum pump-probe spectroscopy system (HELIOS, ultrafast system), described in detail in reference.
  • WLC white-light
  • HELIOS ultrafast system
  • the WLC (420 - 950 nm) probe pulse (representing as a signal pulse in Fig. 15) had a beam radius of 80 ⁇ (HW 1/e) at the sample position and a low enough peak irradiance to produce no observable NLO response in the sample.
  • the total instrument response time is 150 fs full-width-half-maximum (FWHM).
  • the probe beam radius is smaller than the pump beam radius, it is assumed that the probe overlaps with a region of approximately constant peak irradiance from the pump.
  • the change in optical density ( ⁇ ( ⁇ , t)) was recorded as a function of wavelength (X) and delay time (t).
  • the choice of wavelengths and incidence angles of either control or signal pulses is arbitrary and can be tuned by varying the thickness combination of two ZrO 2 layers simulated through the transfer-matrix method. The same design process simulated by the transfer-matrix method has been explained in details and shown in Fig. 9.
  • This very high linear absorptance of the narrowband NLO device is primarily attributed to its component 23 nm-thick Au thin film, with negligible absorption in its component 100 nm- thick Ag film, and no absorption in its two Zr0 2 films.
  • the NLO properties of the narrowband NLO device were characterized by a commercially available white-light (WLC) continuum pump-probe spectroscopy system (HELIOS, ultrafast system).
  • Measured values of AR( , t pea k) of the narrowband NLO device are shown in Fig.
  • t pea k denotes the delay time between a probe and a pump pulse that yields the maximum value of AR( , tp ea k) in the temporal ranges studied.
  • the two pump-probe experiments were individually operated at the same narrowband NLO device sample to characterize its angularly dependent NLO properties by different beam geometries.
  • the first beam geometry non-collinear beam geometry
  • Pump-probe experiments with both beam geometries have been illustrated in Fig. 18, and the AR( , t) values measured across the whole visible spectrum by a WLC probe pulse suggesting control (pump) and signal (probe) pulses in all-optical control applications can have a wide range of combinations between their wavelengths and incidence angles.
  • narrowband NLO device Glass/Ag(100 nm)/Zr0 2 (389 nm)/Au(23 nm)/Zr0 2 (133 nm)
  • broadband NLO device Glass/Ag(100 nm)/Si0 2 (81 nm)/Au(23 nm)/Si0 2 (81 nm)
  • dielectric layers and thicknesses have been chosen to allow the structures to present narrow or broad spectral and angular bandwidths and to ease their fabrication.
  • the narrowband NLO device were firstly designed and fabricated using the same dielectric layers Si0 2 , and it was found that the required Si0 2 thickness is too thick (up to 528 nm) to allow fabrications by the e-beam deposition system used here, causing a delamination of the whole thin-film structure.
  • the linear optical properties of the narrowband NLO device design are also preserved with a faster e-beam deposition process after reducing the total layer thickness.
  • the NLO response of the narrowband NLO device presents narrow spectral bandwidth characteristics on AR( , t peak ), as shown in Fig. 17, in contrast to the NLO response of the broadband NLO device.
  • the required peak pump pulse irradiance required to operate the narrowband NLO device is reduced by at least four times compared to those required to operate the broadband NLO device. This reduction in pump power is clearly attractive for all-optical applications.
  • the narrowband NLO device works best in non-degenerate pump-probe operation.
  • the narrowband NLO device presents a high angular dependence in its linear and NLO properties.
  • the customized angular dependence can simultaneously enhance the NLO response excited by the control pulse and reduce the self-induced NLO responses from high peak-power signal pulse.
  • the extremely large magnitude of AR, ultrafast response, and adjustable angular and spectral bandwidth are attractive for NLO devices to ail-optically control for high peak power signal pulses with different engineering requirements.
  • NLO nonlinear optical
  • the narrowband NLO device not only reduces the peak power consumption of a control pulse used in exciting ultrafast reflectance changes, but opens up a new design of all-optical controls for high peak-power signal pulses.
  • This attractive optical device will be further integrated and developed into a novel dose-control system (ultrafast all- optical shutter) for medical and laser manufacturing applications presented hereinafter.
  • the nonlinear reflective saturable absorber is designed to maximize linear absorptance at a specified wavelength.
  • the wavelength In order to simulate the structure and take advantage of metal nonlinearities, the wavelength must be spectrally above the interband transition onset for a given metal. Silver is used because its interband transition occurs at 313 nm, allowing 400 nm to be the chosen wavelength in order to demonstrate capabilities in the full visible spectral range.
  • the exemplary structure is shown in Fig. 20.
  • the second dielectric layer, D2 in the nonlinear optical device structure comprises a quarter-wave stack and an extra layer, hereafter referred to as cavity.
  • the quarter-wave stacks are added in order to maximize absorption at 400 nm within the structure.
  • the number of quarter-wave stacks is initially chosen depending on the thickness of the Ag thin film layer and affects the full-width half-maximum as well as the absolute maximum of the absorptance peak.
  • the quarter-wave stacks alternate between high and low index materials, which in this case are assumed to be Zr0 2 and Si0 2 , respectively.
  • the geometry is as follows: Glass/Ag(120 nm) /ZrO 2 (cavity)/Ag(20 nm)/ SiO 2 (spacer)/ZrO 2 (50.3 nm)/Si0 2 (67.8 nm)/ ZrO 2 (50.3 nm).
  • the thicknesses of Dl and cavity layers are chosen to maximize absorptance (Fig. 21).
  • the absorption is periodic with respect to the Dl and cavity thickness.
  • the smallest possible thickness values are chosen for each layer such that the absorptance reaches a local maximum.
  • the final simulated structure has the following geometry: Glass/Ag(120 nm) /ZrO 2 (40 nm)/Ag(20 nm)/Si0 2 (67.8 nm)/ZrO 2 (50.3 nm)/Si0 2 (67.8 nm)/ ZrO 2 (50.3 nm).
  • the simulated linear spectra are shown in Fig. 22.
  • the nonlinear response of the reflective saturable absorber are simulated using a two- temperature model and a physical model describing the dielectric permittivity for silver.
  • a pump pulse at with a temporal width of 60 fs and wavelength of 400 nm is used for simulating the nonlinearity of the reflective saturable absorber.
  • Pump intensities are varied from 3 GW/cm to 12 GW/cm 2 as shown in Fig. 23.
  • Fig. 24 provides an example of a preferred embodiment according to the present invention wherein the a 4-f nonlinear Sagnac interferometer comprises: a first imaging lens, LI; a 50/50 beam splitter BS at the entrance of the ring interferometer; a ring interferometer with a path defined by the BS at the entrance port, a first mirror, Ml, a NLO device as described in the present invention, a second mirror, M2, a variable delay line and the BS at the output port; a second lens L2; and a point detector or a electronic image detector; and a third mirror M3 used to direct the control optical pulse to overlap with the Fourier spectrum of the image plane at the NLO device.
  • d' the distance from the electronic image detector to the NLO device
  • f 2 the focal length of the second lens.
  • the image plane can be at infinity and that the size of the control beam is typically adjusted to overfill the Fourier spectra of the image at the NLO device.
  • This configuration can be used as an ultrafast framing camera or to produce a pulse shaping device that could be used for laser manufacturing or other applications.
  • Fig. 25 provides an example of a preferred embodiment according to the present invention wherein the ultra high-speed framing camera comprises: a 50/50 beam splitter BS at the entrance of the ring interferometer; a ring interferometer with a path defined by the BS at the entrance port, a first mirror, Ml, a first imaging lens, LI, a NLO device, a second imaging lens, L2, a second mirror, M2, a variable delay line and the BS at the output port; a second lens L3; and an electronic image detector; and a third mirror M3 used to direct the control optical pulse to overlap with the Fourier spectrum of the image plane at the NLO device.
  • the two imaging lenses have a focal distance/.
  • Fig. 26 displays the temporal response of the 4f Nonlinear Sagnac Interferometer with Intraloop Fourier Lenses for the special case where the input beam is collimated.
  • Fig. 27 provides an example of a preferred embodiment according to the present invention wherein a 12-f Nonlinear Sagnac Interferometer comprises: a first imaging lens, LI ; a 50/50 beam splitter BS at the entrance of the ring interferometer; a ring interferometer with a path defined by the BS at the entrance port, a second lens, L2, a first mirror, Ml, a third lens, L3, a NLO device, a fourth lens, L4, a second mirror, M2, a variable delay line, a fifth lens, L5, and the BS at the output port; a six lens L6; and an point detector or electronic image detector; and a third mirror M3 used to direct the control optical pulse to overlap with the Fourier spectrum of the image plane at the NLO device.
  • a 12-f Nonlinear Sagnac Interferometer comprises: a first imaging lens, LI ; a 50/50 beam splitter BS at the entrance of the ring interferometer; a ring interferometer with a path defined by
  • the image plane can be at infinity and that the size of the control beam is typically adjusted to overfill the Fourier spectra of the image at the NLO device.
  • This configuration can be used as an ultrafast framing camera or to produce a pulse shaping device that could be used for laser manufacturing or other applications.
  • Another embodiment of the present invention includes an ultrafast all-optical shutter, as described in Fig. 28, comprising a 50:50 non-polarized beam splitter (BS) and a polarized beam splitter (PBS); two wave plates: a half-wave plate (WP1) and a quarter-wave plate (WP2); two silver mirrors: Ml and M2; a nonlinear optical (NLO) device: Angular NLO mirror; and a linear optical element comprising a slab of a transparent material (transparent slab).
  • BS non-polarized beam splitter
  • PBS polarized beam splitter
  • WP1 half-wave plate
  • WP2 quarter-wave plate
  • Ml and M2 silver mirrors
  • NLO nonlinear optical
  • Angular NLO mirror Angular NLO mirror
  • a linear optical element comprising a slab of a transparent material (transparent slab).
  • a first and a second signal optical pulse impinge upon the Angular NLO Mirror at normal incidence, while the control optical pulse impinges on it at an angle of incidence ⁇
  • the Angular NLO Mirror is designed to present a high linear reflectance at normal incidence and a low linear reflectance (high linear absorptance) at the angle of incidence ⁇
  • the high linear absorptance of the control pulse drives the strong thermal nonlinearity of the Angular NLO Mirror and produces a strong change of the reflectance at normal incidence.
  • the choice of wavelengths and incidence angles for the signal and control pulses is arbitrary and can be engineered when designing the Angular NLO Mirror.
  • the optical path length introduced by the transparent slab is defined as the product of the effective thickness traveled by and optical pulse and its refractive index.
  • This optical path length defines the adjustable sampling window of the shutter.
  • the sampling window can be adjusted by rotating the transparent slab or by increasing its thickness and controlled down to a few nanometers or, equivalently, down to a few fs. Note that this setup can be used to produce a pulse shaping of an ultrafast optical pulse by controlling the shape and size or the opening window. Such device could find applications in the processing of materials.
  • the ultrafast all-optical shutter has an optical layout of Fig. 29 comprising a 50:50 non-polarized beam splitter (BS), two silver mirrors: Ml and M2; a nonlinear optical (NLO) device: Angular NLO mirror; and a linear optical element comprising a slab of a transparent material (transparent slab).
  • BS non-polarized beam splitter
  • NLO nonlinear optical
  • Angular NLO mirror Angular NLO mirror
  • a linear optical element comprising a slab of a transparent material (transparent slab).
  • the high linear absorptance of the control pulse drives the strong thermal nonlinearity of the Angular NLO Mirror and produces a strong change of the reflectance at ⁇ 45°.
  • the choice of wavelengths for the signal and control pulses can be engineered when designing the Angular NLO Mirror. Note that this setup can be used to produce a pulse shaping of an ultrafast optical pulse by controlling the shape and size or the opening window. Such device could find applications in the processing of materials.
  • Fig. 30 provides an example of a preferred embodiment according to the present invention wherein an ultrafast all-optical shutter comprises: a polarizing beam splitter PBS, a variable waveplate, a diffractive optical element splitting the input beam into three beam, a zero- order beam and two first order beams, a first lens LI; a delay plate; a compensating plate in another arm and a compensation plate in the other arm or a beam shaping plate in addition to a half-waveplate to rotate the polarization of this beam in order to avoid exiting at the output port; a second lens, L2, a NLO device.
  • a polarizing beam splitter PBS a variable waveplate, a diffractive optical element splitting the input beam into three beam, a zero- order beam and two first order beams, a first lens LI
  • a delay plate a compensating plate in another arm and a compensation plate in the other arm or a beam shaping plate in addition to a half-waveplate to rotate the polarization of this beam in order to avoid exiting
  • the diffractive optical element is placed at the front focal plane of the lens LI and the distance between LI and L2 is ⁇ 1+ ⁇ 2, where fi are the focal length of the lenses.
  • the compensation plates are used to assure equal arrival time of at least two pulses at the NLO device plane, the temporal delay of the third pulse is controlled by a variable delay line in the form a transparent slab that is thicker or thinner than the compensation plate or preferably two sliding wedges wherein the total thickness is varied to control the temporal delay.
  • a variable waveplate is used to rotate the polarization of the reflected beams to make it perpendicular to the polarization of the input beam.
  • the present ultrafast all-optical shutter adopts a timing mechanism derived from a known terahertz optical asymmetric demultiplexer (TOAD).
  • Fig. 31 shows the TOAD configuration comprising a fiber loop with a first 2 x 2 coupler.
  • the first coupler is used for introducing a series of signal pulses.
  • the second intra-loop 2 x 2 coupler is used for introducing a control pulse to optically pump a nonlinear optical element.
  • the position of the nonlinear optical element is offset from the midpoint of the loop by a distance (AL) defined as an optical path length.
  • the TOAD architecture uses the off-center position of the nonlinear optical element to determine an adjustable sampling window (At).
  • At adjustable sampling window
  • Fig. 31 illustrates that three signal pulses enter at the input port: A, B, and C.
  • the de-multiplexer lets the pulse B pass only based on the adjustable sampling window (At).
  • An object of the present invention is to use a similar timing mechanism as used in TOAD, adopts it for the desired function of the ultrafast all-optical shutter as shown in Fig. 39(b), and disclosed in the ultrafast all-optical shutter embodiments in detail.
  • Timing configurations could also be used to define an adjustable sampling window (At) in the ultrafast all-optical shutter, such as colliding-pulse Mach-Zehnder (CPMZ) and symmetric Mach-Zehnder (SMZ). All these architectures have been developed as all-optical interferometric switches, mainly, for high bandwidth de-multiplexing in telecommunication signals. Otherwise, these architectures have not been exploited for all-optical control at visible wavelengths due to the limited number of materials having a large nonlinear optical response in the visible spectral region.
  • Fig. 32 shows an experimental setup of the ultrafast all-optical shutter, where the nonlinear optical element is a narrowband NLO device, and the nonlinear optical device discussed previously.
  • An optical delay line (variable delay stage shown in Fig. 32) comprising a translation stage with a retro-reflector is used to define AL, and consequently At.
  • the resolution of the AL, and consequently of At can be can be precisely controlled down to a few tens of micrometers.
  • At is easily, and very accurately adjustable by varying the value of AL with a resolution up to hundreds of femtoseconds.
  • the limitation to the minimum and maximum value of At that can be used, as shown in Eq. 8, is the temporal response of the nonlinear optical element used.
  • the narrowband NLO device displays transient reflectance changes (AR) with rising times on the order of T rise ⁇ 500_ s and relaxation time in the rage of ⁇ fall > 80/?s , when optically pumped by a 60 fs half-width-l/e (HW 1/e) pulse with a peak irradiance 19 GW/cm 2 (see Fig. 18).
  • HW 1/e half-width-l/e
  • the ultrafast all-optical shutter will be broadly tunable with adjustable sampling windows from picosecond to femtosecond response times ( 500/5 ⁇ At ⁇ ⁇ fall where ⁇ fall > 80/?s ).
  • the ultrafast all-optical shutter has an optical layout comprised of: a 50:50 non-polarized beam splitter (BS); two silver mirrors: Ml and M2; three lenses: LI to L3; a nonlinear optical (NLO) device: narrowband NLO device; and a linear optical element comprising an optical delay line (variable delay stage) comprising a translation stage with a retro-reflector.
  • BS non-polarized beam splitter
  • Ml and M2 three lenses: LI to L3
  • NLO nonlinear optical
  • narrowband NLO device narrowband NLO device
  • linear optical element comprising an optical delay line (variable delay stage) comprising a translation stage with a retro-reflector.
  • An optical loop (ring interferometer) is formed for a signal pulse through the use of a 50:50 non-polarized beam splitter (BS), two silver mirrors: Ml and M2, one nonlinear optical device: the narrowband NLO device, and an optical delay line: variable delay stage to introduce the adjustable sampling window of At.
  • the variable delay stage offsets the narrowband NLO device from the midpoint of the loop by AL.
  • a signal pulse incident from the input port enters the loop and is firstly split into two pulses by the BS to create a clockwise propagating pulse and a counter-clockwise propagating pulse. For example, consider a clockwise propagating pulse: it reflects from BS into Ml, and then focuses through L2 on the narrowband NLO device.
  • the clockwise propagating pulse subsequently reflects off the narrowband NLO device, is then converted back to a collimated pulse by LI, reflects off M2, and transmits through a variable delay stage. Finally, the pulse transmits through BS, and returns to the input port.
  • Both clockwise and counter-clockwise propagating pulses experience exactly the same loop path only in a reversed direction. Hence, both signal pulses temporally overlap with each other at the output port.
  • Figs. 33(a)-33(f) subsequently shows the propagation analysis of the above process by a spatially dependent irradiance profile (/(Z)).
  • Fig. 33(a) initially shows the irradiance profile of the incidence signal pulse at a peak irradiance 21 o assuming a Gaussian profile.
  • the BS then splits the incidence signal pulse into a clockwise and a counter-clockwise propagating pulse of equal peak irradiance Io and their irradiance profiles are overlapped as shown in Fig. 33(b).
  • Fig. 33(a) initially shows the irradiance profile of the incidence signal pulse at a peak irradiance 21 o assuming a Gaussian profile.
  • the BS then splits the incidence signal pulse into a clockwise and a counter-clockwise propagating pulse of equal peak irradiance Io and their irradiance profiles are overlapped as shown in Fig. 33(b).
  • FIG. 33(c) shows that there is a temporal offset of At between two split signal pulses at the narrowband NLO device, because the clockwise propagating pulse travels an extra distance of AL introduced by the optical delay line compared to the counter-clockwise propagating pulse, as shown in Fig. 32.
  • This At results in the clockwise propagating pulse arriving at the narrowband NLO device after the counter-clockwise propagating pulse.
  • Fig. 33(d) shows that the irradiance profile of two split signal pulses temporally overlap after the two pulses travel the whole loop in a reversed direction and recombine at the BS as shown in Fig. 32. Both clockwise and counter-clockwise propagating pulses pass through the optical delay line once.
  • Fig. 33(e) shows completely constructive interference of two recombined signal pulses at the input port and Fig. 33(f) shows a completely destructive interference at the output port.
  • a control pulse When a control pulse is introduced in the setup, as shown in Fig. 32, it excites the narrowband NLO device at the time point to as shown in Fig. 34(a). This excitation induces a strong transient reflectance coefficient change (including amplitude and phase modulations) which are symbolized in a temporally dependent function of complex amplitude (A(t)), shown in Fig. 34(b) and shaded.
  • the nonlinear optical response of the narrowband NLO device presents a fast rising time and a slow relaxation time upon being illuminated by a control optical pulse. For instance, a 60 fs control pulse should present a fast rising time less than 500 fs and a slow relaxation time about several tens of ps.
  • the preliminary experiments of the ultrafast all-optical shutter using a 60 fs control pulse will be later introduced in details.
  • Fig. 34(c) shows the irradiance profile (I out (t)) at the output port.
  • the ultrafast all-optical shutter therefore opens during the adjustable sampling window (At) and closes outside the window. The detailed temporal analysis will be discussed later. During this period the ultrafast all-optical shutter remains open, leading to an output irradiance profile of I out (t), shown in Fig. 34(c) (filled area).
  • the peak output irradiance of I oM (t) can be calculated by following a basic interferometric equation:
  • Fig. 35(d) shows that at the output port, the irradiance profiles of these signal pulses overlap, as initially shown in Fig. 35(b); since the two pulses travel the same loop path just in a reversed direction and recombine at the BS as shown in Fig. 32.
  • Fig. 35(d) shows that both recombined signal pulses are decomposed into three subsequent parts: A, B, and C.
  • Fig. 35(d) shows both recombined signal pulses are symmetrically un-modulated for the part A and symmetrically modulated for the part C. This leads to a completely destructive interference both in the part A and C which results in zero irradiance at the output port.
  • Fig. 35(d) shows only the part B of the recombined signal pulses, which is anti-symmetrically modulated. It is modulated for the clockwise propagating pulse but un-modulated for the counter-clockwise propagating pulse.
  • the part B of recombined signal pulse at the output port undergoes a constructive interference, thus producing an output. Therefore, Fig. 35(f) shows that the ultrafast all-optical shutter opens only in the part B.
  • the duration of the part B matches with the adjustable sampling window At.
  • the ultrafast all-optical shutter is designed to provide an ultrafast timing mechanism, with opening and closing times on the order of several hundreds of fs to ps, the adjustable opening window (adjustable sampling window as shown in Fig. 34) is easily pre-set and adjusted by varying the optical delay within the ring interferometer.
  • the dynamic of the ultrafast all-optical shutter will be characterized, and be described as an impulse response of an optical system.
  • a temporal-scan experiment is set up as shown in Fig. 36.
  • the experiment is set to measure the impulse response of the ultrafast all-optical shutter.
  • the impulse response can be widely used not only in simulating the sampling process, but also calculating a temporal response of shutter output with response to any control pulse with an arbitrary temporal profile by a convolution between the control pulse profile and the impulse response.
  • the impulse response as a function of time will be scanned in a time-resolved experiment as a proof-of-principle of the ultrafast all-optical shutter to show the temporal profile of its adjustable opening window.
  • Ultrafast applications enable by this ultrafast all-optical shutter such as ultrafast photography, time-gated spectroscopy, and photo-electric effects with temporally shaped pulses, which will be further discussed hereinafter
  • the temporal-scan experiment uses a femtosecond laser system, where its femtosecond laser pulse is obtained from an optical parametric amplifier (OperASolo, Coherent).
  • An optical parametric amplifier (OperASolo, Coherent)
  • a seed laser beam from a Ti:Sapphire regenerative amplifier (Libra, Coherent) operating at the wavelength of 800 nm generates a femtosecond pulse through the OperASolo, and the wavelength of the femtosecond pulse can be tuned within the visible spectra range.
  • the femtosecond pulse has a temporal pulse width of 100 fs half-width-l/e (HW 1/e).
  • Fig. 36 shows that the temporal-scan experiment of the ultrafast all-optical shutter has an optical layout comprised of: polarized and 50:50 non-polarized beam splitter: PBS and BS; five silver mirrors: Ml to M6; four lenses: LI to L4; a nonlinear optical (NLO) device: narrowband NLO device; and two optical delay line: Variable delay stagel and Variable delay stage 2, comprising a translation stage with a retro-reflector, where Variable delay stagel and Variable delay stagel uses a computer-controlled motorized translation stage and non-motorized translation stage, respectively.
  • NLO nonlinear optical
  • a single femtosecond laser pulse at the wavelength of 594 nm is split into a strong control pulse and a weak signal pulse with a peak irradiance of 123 and 0.35 GW/cm measured at the position of the narrowband NLO device, respectively, after passing through WP and PBS.
  • the polarization angle of WP is adjusted to maximize the peak irradiance of a control pulse, leading to optimize the excitation of the narrowband NLO device at the normal incidence, and consequently optimize the operation of the ultrafast all-optical shutter.
  • the control pulse was tuned to a spot size of 190 ⁇ (HW 1/e) at the position of the narrowband NLO device, measured using a knife-edge scan.
  • the signal pulse had a spot size of 100 ⁇ (HW 1/e) at the same position.
  • the signal pulse passes through the Variable delay stage 1, which introduces a variable optical path difference between the signal and control pulse, resulting in a time delay between two pulses.
  • This Variable delay stagel is gradually scanned through a series of offset positions, and the time-averaged output intensity I(t) of the ultrafast all-optical shutter recorded as a function of delay time (t) after averaging over one thousand signal pulses injecting from the input port of the ultrafast all-optical shutter and passing through the output port to the Photo- detector.
  • time-averaged output intensity I(t) can be sensitive to the jitter of laser intensity during acquisition time, so I(t) is calculated from measured time average output I ou t(t) by Iout(t)/ Iref(t) to discriminate signal from noises originated from laser jitters itself, where I re f(t) is measured time-averaged reference intensity sampled out of the same optical setup.
  • Fig. 36 also shows that a signal pulse passing through the input and output port of a ring interferometer, where the propagation analysis has been explained in details in Fig. 32.
  • a first and a second split signal optical pulse after BS impinge upon the narrowband NLO device at +45 , while the control optical pulse impinges on it at normal incidence.
  • the narrowband NLO device is designed to present a high linear reflectance at +45 and a low linear reflectance (high linear absorptance) at normal incidence.
  • the high linear absorptance of the control pulse drives the strong thermal nonlinearity of the narrowband NLO device and produces a strong change of the reflectance at +45°.
  • variable delay stage 2 introduces the adjustable opening window of At for the ultrafast all-optical shutter.
  • variable delay stage 2 offsets the narrowband NLO device from the midpoint of the loop (ring interferometer) by AL, the adjustable opening window can consequently be preset by At.
  • the general timing mechanism of the ultrafast all-optical shutter has been shown in the last section, the temporal- scan experiment here is used to scan the temporal profile of this adjustable opening window.
  • two polarizers PI and P2 are added into the optical layout shown in Fig. 36, because it is found that varying the combination of polarization angles of PI and P2 can further increase the signal-to-noise ratio measured by time-averaged output intensity I(t) of the ultrafast all- optical shutter.
  • Fig. 37 shows the impulse response of the ultrafast all-optical shutter by measuring time- averaged output intensity I(t) with different time delays between a signal and control pulse.
  • the shutter is opened between its input and output port by a femtosecond control pulse, and has a larger value of I(t) within the preset adjustable opening window compared to outside the opening window.
  • the results show that all adjustable opening windows display a consistent temporal shape with a sharp rising edge and a sharp falling edge, both below 1 ps.
  • the preset adjustable opening windows of At matches faithfully with the measured values as shown in Fig. 37.
  • the adjustable window time is easily set, and is accurately controlled by the variable delay stage 2 as shown in Fig. 36, though the optical alignment of the optical delay line can still be further refined to keep values of I(t) be constant when the shutter closes. It is interesting to note that values of I(t) changes exponentially, which is expected from the exponential decay of thermal nonlinearities of narrowband NLO device excited by a femtosecond laser pulse as shown in Fig. 18.
  • the shape of opening time window should be tunable, for example, a flat-hat shape can be achieved through a slowly decayed relaxation of thermal nonlinearities either from saturated nonlinearities excited by a very high peak-irradiance control pulse as mentioned in previously or nonlinearities excited by a temporally shaped nanosecond control pulse with a fast rising time and a slow decay time.
  • a present challenge in laser manufactures and pulsed-laser surgeries is to achieve precisely all-optical control over high peak power signal pulses.
  • a low intensity signal pulse is modulated in amplitude and/or phase by a high intensity control pulse through the NLO effects excited by the control pulse alone.
  • the all-optical control of non-thermal ablation applications require a high peak-power signal pulse since it needs to have enough energy and power to ablate the targeted materials including tissues.
  • the linear and NLO properties need to be precisely engineered, so that the nonlinearity can be selectively driven by the control pulse even if the signal pulse also has a high peak power.
  • the narrowband NLO device with its extremely large and ultrafast NLO response, offers such unique all-optical control of intense pulses in the visible spectral range and is ideal to develop the present ultrafast all-optical shutter design.
  • the ultrafast all-optical shutter is present to sample a high peak-power nanosecond optical pulse with a wavelength in the visible spectral regions to produce a high peak-power picosecond or femtosecond optical pulse of the same wavelength.
  • the potential impact of the shutter can enable high-repetition rate and low-cost nanosecond pulse laser systems to be used to produce femtosecond or picosecond laser pulses with properties that are well suited for the nonthermal ablation of materials.
  • the whole optical system should be integrated under a single nanosecond laser system to reduce capital costs.
  • the fast rising edge of a ns control optical pulse needs to be generated by passing a ns laser pulse through a slow saturable absorber, for example, an organic dye DODCI (3,3'-diethyloxadicarbocyanine iodide).
  • DODCI 3,3'-diethyloxadicarbocyanine iodide
  • Each pulse profile of 1(f) was experimentally confirmed to be close to a Gaussian pulse with similar temporal pulse width ( ⁇ ). These pulses can be described as below.
  • I(t) I peak cxp(-( ⁇ - ⁇ ) 2 )
  • Equation 10 shows a relationship between F and I pea k for any Gaussian pulse, and one set of F t h and I t h is labeled in Fig. 38(a).
  • Fig. 38(b) shows a relationship between the ablation diameter (Yl axis on left) and depth (Y2 axis on right) per pulse for a set of pulses of varying fluences shown in Fig. 38(a) as reported by others.
  • the ablation diameter and depth per pulse represent averages from measurements of multiple pulses, as illustrated in Fig. 39(a), and will be explained later.
  • Fig. 38(b) shows there is a minimum fluence /3 ⁇ 4 for a single pulse to reach observable ablation.
  • F t h or I t h is therefore the damage threshold for ablation and can be determined experimentally.
  • the damage threshold of femtosecond pulsed ablation originates in the optical breakdown mechanism called multi-photon ionization.
  • Fig. 38(b) also shows that both an ablation diameter and depth per pulse achieves a maximum point at a fluence per pulse of F 2 ; resulting in a maximum volume removal rate for ablation by a single pulse.
  • Fig. 39(a) exemplifies a conventional surgical operation requiring multiple pulses wherein the fluence per pulse (F) is set by adjusting a variable optical attenuator.
  • F 2 is selected here to ablate the medium for the best volume removal rate.
  • a mechanical shutter lets through a variable number of pulses (N) to achieve the desired ablation volume after a total exposure fluence (dose)
  • the cut size can be accurately controlled by varying the numbers (N) of incident pulses. Note that to reduce a single cut time using this method one would have to increase the repetition rate of the used laser.
  • a mechanical shutter lets through 100 pulses with a repetition rate of 3 Hz, so the whole single-cut process takes 33 s.
  • a new dose control system is present here to reduce the single-cut time within the temporal pulse width of a single nanosecond pulse without losing the cut precision and controllability of femtosecond pulsed-laser ablation.
  • the pulsed- laser in this new approach is replaced by a nanosecond laser, potentially reducing the capital equipment costs by avoiding the need of a more expensive femtosecond laser.
  • Fig. 39(b) shows that this new dose-control method will be able to sample a slide out of a single nanosecond pulse and ablate tissues by this sampled portion of the laser pulse.
  • This new dose-control enables nonthermal ablations with nanosecond lasers, because the duration of sampled portion of the laser pulse will be reduced down to picosecond to femtosecond region by this method.
  • the total exposure fluence (dose) is now controlled by t 0 +At
  • the present sampling process requires a shutter with an ultrafast temporal response, which mechanical and electrical shutters do not allow.
  • this research aims to develop an ultrafast all-optical shutter which allows implementation of the newly invented dose-control scheme presented in Fig. 39(b), and wherein the adjustable sampling window, At, can be precisely and easily tuned to meet real-time feedback dose-control requirements for laser surgeries.
  • the ultrafast all-optical shutter enables a dramatic reduction of surgical times and costs by allowing the use of less expensive nanosecond lasers emitting at visible wavelengths.
  • the optical layout of the ultrafast all-optical shutter comprises a ring interferometer and a nonlinear optical device producing strong reflectance changes at visible wavelengths optically excited by a control pulse.
  • the impulse response of the shutter is measured using a temporal- scan experiment.
  • the ultrafast temporal response shows that the shutter opens and closes faster than 1 ps controlled by a femtosecond laser pulse, and the adjustable opening time of the shutter is easily and precisely preset between 3.2 ps to 12 ps with a constant interval of 0.8 ps.
  • Ultrafast optical applications using all-optical controls at visible wavelengths have been limited by the lack of materials and devices with a strong nonlinear optical (NLO) response.
  • the transmittance and reflectance changes provided by a single noble metal thin-film layer are still not strong enough compared to large amplitude and/or phase modulations of optical signals needed in all-optical control applications.
  • the research was divided into three parts to understand how the linear and NLO properties of these noble metal thin films can be engineered and optimized to overcome a wide variety of requirements in ultrafast optical applications.
  • the NLO properties of Au and Ag/Au bilayer metallic thin films are described as an ultrafast electron and lattice heating process using comprehensive physical models compared with experiments. This shows that the linear and NLO properties of bilayer metallic films can both be tuned in the visible spectral region by controlling the mass-thickness ratio between Au and Ag. The combined properties of these bilayers are therefore attractive for different photonic applications illustrated by plasmonic devices and optical filters.
  • the understanding of NLO response of metallic films leads to the development of
  • the NLO device structure comprises four thin-film layers and can be engineered to display strong reflectance changes with very broad spectral and angular bandwidths across the visible spectral region.
  • the reflectance change of the NLO device is ultrafast.
  • the linear and NLO properties of these NLO devices can also be engineered to present narrow spectral and angular bandwidths while preserving a strong NLO response.
  • the adjustable bandwidth of NLO device designs transforms all-optical controls not only for low peak-power but also for high peak-power signal pulses applications.
  • the temporal-scan experiments show that the ultrafast all-optical shutter opens and closes by an ultrafast temporal response faster than 1 ps excited by a femtosecond control pulse, and its adjustable opening window was demonstrated to be precisely and easily tunable from 3.2 to 12 ps.
  • the metal-dielectric thin-film structures (Broadband and narrowband NLO devices) contain four components layers by two dielectric layers and two noble metal (Au and Ag) layers, it is possible to incorporate dielectric layers with strong NLO properties. Therefore, the nonlinearities of the whole structure could be completely exploited and cause further enhancement by nonlinearities not only from the metal layer but also superimposing with nonlinear dielectric layers. Note that the initial structure is designed only to amplify ultrafast thermal nonlinearities of its component Au layer. When superimposing the other NLO properties from dielectric layers, the total enhancement effect should not be decreased.
  • the NLO dielectric layer candidate should have a relatively low linear absorptance to preserve the linear absorption properties of the metal layer in order to excite its NLO response.
  • NLO dielectrics such as Au or Ag nanoparticles doped dielectric composite films, where their dielectric host are transparent such as Si0 2 and polyvinylpyrrolidone (PVP), which shows large nonlinear absorptions and nonlinear refractive indices based on the third-order nonlinear susceptibility ⁇ (3) process.
  • PVP polyvinylpyrrolidone
  • Fig. 40 shows possible geometry for coupling a signal pulse into a glass slab waveguide and form the cascade connection at the narrowband NLO device.
  • a 1.5 mm-thick glass substrate (Ex: VWR Micro Slides glass) is used as a glass slab waveguide, and the four layer metal-dielectric thin-film structure can be deposited on top of this glass substrate with a patterned length of 6 mm by an e-beam deposition system with a shadow mask.
  • Prism 1 and 2 are two right-angle prisms for coupling the signal pulse in and out the glass slab waveguide.
  • the signal pulse can be coupled into the slab waveguide at the incidence angle of 45°, which matches the total reflection angle at the interface between air and glass.
  • the result of doubling NLO interactions between a signal pulse and the narrowband NLO device cascades two amplitude and/or phase modulations.
  • the total cascaded signal modulations is expected to be increased compared to a single non- cascaded modulation in a simulation study.
  • NSF-iCORPS National Science Foundation
  • the ultrafast all-optical shutter was first investigated as a way to extract a high peak- power sub-nanosecond laser pulse out of a nanosecond laser pulse.
  • the potential impact of such an approach is that it could enable a low-cost, high speed, and easily maintenance sub- nanosecond laser to provide high resolution non-thermal ablation.
  • Further development of a high peak-power sub-nanosecond laser pulse extraction system will require integrating the ultrafast all-optical shutter into a single nanosecond laser.
  • a major challenge includes sharpening the fast rising edge of a control optical pulse from a nanosecond seed pulse.
  • a possible solution is to generate a sharp rising edge optical pulse by passing a nanosecond pulse through a slow saturable absorber, for example, an organic dye DODCI (3, 3'- diethyloxadicarbocyanine iodide).
  • a slow saturable absorber for example, an organic dye DODCI (3, 3'- diethyloxadicarbocyanine iodide).
  • DODCI 3, 3'- diethyloxadicarbocyanine iodide
  • the temporally shaped optical pulse could be converted into a short electron pulse by passing through a photocathode based on the photon-electric effect.
  • the high photoelectrons generation efficiency and an adjustable temporal pulse width within tens of picoseconds could be enabled.
  • the short X-ray pulse is able to be emitted by passing electron pulses through the X-ray tube. Both short electron pulses and X-ray pulses are powerful laboratory tools in fundamental studies, and X-ray pulses also have potential impacts in medical imaging applications.
  • the second path is to use the ultrafast all-optical shutter as an ultrafast camera for ultrafast photography.
  • ultrafast events such as visualizing light propagation can be captured by a streak camera, each image frame needs to be scanned and reconstructed through a multiple of captured lines.
  • the present ultrafast camera with a single shot per imaging frame ability with the ultrafast all-optical shutter can improve the image quality and recording speed.
  • a third path is to use the ultrafast all-optical shutter as a biomedical imaging device for seeing through scattering medium, for example, imaging blood vessels beneath skins.
  • the ultrafast all-optical shutter is used to demonstrate ballistic-photon imaging using a time-gated optical image technique.

Abstract

La présente invention porte sur un dispositif optique apte à un changement ultrarapide et large de son coefficient de réflexion ou d'absorption lorsqu'il est excité par une impulsion optique ultrarapide ayant une longueur d'onde dans les régions spectrales du visible, du proche infrarouge ou de l'infrarouge. Le dispositif optique comprend, en ordre séquentiel, une première couche métallique épaisse, une première couche diélectrique, une seconde couche métallique mince et une seconde couche diélectrique. Le dispositif optique agit en tant que miroir non linéaire qui présente une réflectance grande à une irradiance faible et une réflectance faible à une irradiance grande. Le dispositif optique peut en outre agir en tant que miroir non linéaire qui présente une réflectance linéaire et non linéaire ayant une largeur de bande angulaire grande.
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