Classifications

• • 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
  • C03C23/00—Other surface treatment of glass not in the form of fibres or filaments
  • C03C23/0005—Other surface treatment of glass not in the form of fibres or filaments by irradiation
  • C03C23/0025—Other surface treatment of glass not in the form of fibres or filaments by irradiation by a laser beam
• • 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
  • C03C3/00—Glass compositions
  • C03C3/12—Silica-free oxide glass compositions
  • C03C3/16—Silica-free oxide glass compositions containing phosphorus
• • 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
  • C03C3/00—Glass compositions
  • C03C3/12—Silica-free oxide glass compositions
  • C03C3/253—Silica-free oxide glass compositions containing germanium
• • 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
  • C03C4/00—Compositions for glass with special properties
  • C03C4/04—Compositions for glass with special properties for photosensitive glass

Definitions

• the present invention relates to new compositions of photosensitive and transparent oxide glasses in the visible and infrared. More particularly, the present invention relates to photosensitive and transparent glasses for wavelengths between 400 nm and 800 nm in the visible spectral range and between 800 nm and 8000 nm in the infrared range.
• the present invention also relates to a process for inscribing structures of variation of refractive index by volume in such a transparent photosensitive glass by irradiation of a femtosecond laser beam.
• the method is suitable for producing three-dimensional structures of refractive index variation forming a Bragg grating.
• a Bragg grating designates, in general, a periodic modulation of the optical index of refraction made of a transparent material with the aim of filtering the incident light.
• the Bragg grating reflects incident light at a particular wavelength, called the Bragg wavelength, and transmits the other wavelengths of the spectrum.
• a Bragg grating can be produced in a guided configuration, in the core of an optical fiber or in free space, in the volume of a substrate.
• it is a volume Bragg grating which is an essential optical component used in particular for the wavelength stabilization of lasers on the one hand, and also for spectral filtering in high resolution spectroscopy on the other hand .
• a conventional way of obtaining a Bragg grating in a transparent material consists in subjecting a photosensitive transparent material to illumination with a sinusoidal type spatial profile obtained by interference of two beams at the sensitivity wavelength of the material, to modulate the refractive index caused by a variation in the charge distribution within the glass.
• the network is then stabilized and made permanent by annealing techniques.
• Germanium-doped silicate material is known for producing optical fibers in telecommunications.
• the amplitude of the variation in optical index of refraction induced during UV exposure is most often limited to a few 10' 5 .
• PTR photothermoreactive
• Silica silica
• Zinc Zinc
• Aluminum doped with photosensitive silver, fluorine and cerium ions.
• the index variations are obtained by a photo-thermal process, based on the precipitation of dielectric microcrystals inside the glass, once it has been exposed to UV radiation and heat treated above the temperature glass transition.
• This material can be put in the form of a thin and easily polishable blade due to its composition and its vitreous nature.
• the glass obtained is transparent in the visible and offers a transmission range generally between 0.3 and 3 microns.
• Another object of the present invention is to propose a photosensitive transparent glass having a composition adapted to allow photo-structuring in volume by a laser beam of short and ultra-short pulses, in order to be able to produce three-dimensional structures for modulating high refractive optical index, generally greater than a few 10' 3 , with submicron spatial resolution, and with high repeatability.
• An object of the present invention therefore relates to transparent glasses based on silica, phosphate or germanium oxides containing photosensitive silver ions suitable for volume inscription of a structure by a femtosecond laser beam.
• the transparent glass according to the present invention comprises at least 99% to 100%, by mass, relative to the total mass of the material, of a composition of formula (I) below:
• Oxy1 is a glass-forming oxide selected from silicon oxide SiC, germanium oxide, or phosphate oxide, and
• Oxy2 represents an oxide chosen from Ga2Os, Al2O3, ZnO,
• Oxy3 represents an oxide chosen from MgO, CaO or BaO, and
• the glass according to the present invention comprises at least 99%, by mass, relative to the total mass of the material, of a composition of the following formula (II):
• Oxy2 represents oxides such as Al2O3, Ga2Os, ZnO, preferably Ga2O3
• Oxy3 represents an oxide chosen from CaO, MgO, or BaO, preferably MgO
• Germanates [0017]
• the oxide chosen to form the viral matrix is a germanium oxide.
• the compositions according to this embodiment will be called germanates.
• the glass according to the present invention comprises at least 99%, by mass, relative to the total mass of the material, of a composition of the following formula (HI):
• Oxy2 represents an oxide chosen from Ga2Os, Al2O3, ZnO,
• Oxy3 represents an oxide chosen from MgO, CaO or BaO, preferably BaO
• the glass also comprises halogenated compounds (fluoride, chloride, bromide) which have the function of modulating the photosensitivity or of facilitating the shaping and purification of the glass.
• halogenated compounds fluoride, chloride, bromide
• the glass further comprises dopants in addition to the composition of formula (I), (II) or (III) to reach 100% by weight.
• the dopants are chosen from the following metal ions: Ag + , Au 3+ , Cu + .
• the glass as defined above has a transmission greater than 90% in a range between 400 nm and 8000 nm.
• Another object of the present invention relates to a process for inscribing a three-dimensional structure of refractive index variation by a femtosecond laser beam in a transparent photosensitive glass of oxides comprising silver ions as defined below. above, the method comprising:
• the number of pulses, the repetition rate of the pulses and the irradiance at each irradiation point being controlled to induce an accumulation of silver aggregates localized in an annular peripheral zone around an irradiation point, said accumulation of silver aggregates generating a variation in optical index of refraction in the annular peripheral zone around the point of irradiation and to erase a refractive optical index variation in a portion of an annular peripheral zone generated around another irradiation point when said portion of the peripheral zone coincides with a zone of the laser beam.
• the refractive index variation An is a positive variation of at least greater than 10' 3 .
• the sample is moved in translation along a direction so as to form a beam passage line formed of a set of irradiation points, the distance between two irradiation points being substantially equal to half the diameter of the beam laser so that the laser beam passage forms two refractive index variation planes on either side of the beam passage line;
• the sample is moved in another direction between two passing lines of laser beam so as to form a succession of beam passage lines, the distance between two beam passage lines being less than the diameter of the laser beam so that the succession of laser beam passages form an array of variation planes d index of refraction parallel to the laser beam pass line;
• the repetition rate is greater than 10 kHz
• the pulse duration of the laser beam is between 100 femtoseconds and 0.5 picoseconds and, the duration being shorter than the characteristic thermalization time of the glass so as to achieve excitation at the point of irradiation by multi-photonic interaction ;
• the irradiance is between 7 TW. cm -2 and 8.4 TW. cm -2 ;
• the laser beam is emitted at a wavelength between 515 nm and 1200 nm, preferably at 1030 nm;
• the sample is moved relative to the laser beam at a speed, VD, of between 50 ⁇ m.s -1 and 1000 ⁇ m.s -1 .
• the structure produced is formed of at least one refractive index variation plane, the thickness of said plane being less than 200 nm, substantially equal to 80 nm.
• the structure produced is a periodic structure comprising a plurality of refractive index variation planes to form a volume Bragg grating, with a grating pitch A between 200 nm and 1.5 ⁇ m.
• a volume Bragg grating comprising a network of refractive index variation planes, the refractive index variation being greater than 10 -3 , the the thickness of each plane being less than 200 nm, preferably substantially equal to 80 nm, the pitch of the grating being between 200 nm and 1.5 ⁇ m.
• FIG. 1 shows a device implementing the process for inscribing structures of variation of refractive index by volume in a photosensitive glass according to the invention
• FIG. 2 schematically illustrates a spatial distribution of silver aggregates around an irradiation point during point irradiation according to the method of the invention
• FIG. 3A schematically illustrates the inscription in a sample during a translational movement of the sample in the direction X, to form during a laser passage, a distribution of variation in optical index of refraction corresponding to two zones of positive index variation on the edges of the focal point, these zones being separated by a distance D which reflects the distance between the modifications on either side of the focal point of the laser beam, the distance D being defined by the size of the focused laser beam, the dose of energy deposited which depends on the number of pulses accumulated at the point of focus and the irradiance used;
• FIG. 3B shows the inscription of Figure 3A followed by a second laser pass made in the opposite direction or in the same direction, with a center-to-center lateral displacement Ay to inscribe , which can then be generalized to N passes laser and Ay defining the periodicity of the Bragg grating; the center-to-center lateral displacements being less than the separation distance of the two index variation zones during the previous pass, such that Ay ⁇ D, and chosen so that one of the two index variation zones written during the first laser pass is covered by the second laser pass, resulting in the erasing of the optical refractive index variation of this zone, while the other zone of index variation written during the first laser pass remains.
• this capacity for rewriting within the photosensitive glass is a central point of the method of the present invention, which makes it possible to retain, laser pass after laser pass, only one of the two index variation zones, with the spatial period imposed by the center-to-center lateral displacement Ay of the laser;
• FIG. 4 schematically illustrates in more detail in a top view the principle of formation of two refractive index variation planes on either side of the line of passage of the laser beam of FIG. 3A from a succession quasi-continuous array of irradiation points, the distance between two irradiation points Ax being much smaller (up to 100 nm) than the diameter of the laser beam D which is of micron dimension, in connection with the pairs of parameters applied which are the high repetition rate of the laser and the moderate speed of movement of the sample;
• FIG. 5 schematically illustrates in a side view the formation of two refractive index variation planes on either side of each laser beam passage line, when the distance between two laser beam passage lines Ay is greater the diameter of the bundle; this does not correspond to the embodiment of the method of the invention because the overall periodicity of the pattern is not suitable;
• FIG.6 represents the formation of a network of refractive index variation planes after a succession of laser beam passage lines according to one embodiment of the method of the invention, the distance between two laser beam passage lines Ay being on the one hand smaller than the diameter of the beam and on the other hand adjustable, making it possible to control the spatial periodicity required for the production of the volume Bragg grating;
• FIG. 7 represents a refractive index evolution at 480, 589, 644 and 656 nm for a series of germanium-gallium-barium-potassium glasses doped with silver ions (GGBK) as a function of the rate BaO;
• GGBK silver ions
• FIG.8 represents a spectrum of the absorption coefficient in the average UV-Visible-IR region for potassium and barium germano-gallate glasses (BaO: 0%), GGB5K (BaO: 5%) , GGB10K (BaO: 10%) and GGB15K (BaO: 15%);
• FIG.9 represents an evolution of the absorption coefficient in the average UV-Visible-IR region for GGB15K (BaO: 15%) and BGGK (BaO: 37.5%) glasses with in the inset a zoom in the UV-blue range;
• FIG.10 represents excitation and emission spectra of GGB15K and BGGK glasses
• FIG.11 represents (a) a fluorescence confocal microscopy image under excitation at 405 nm showing a matrix of structures inscribed in the BGGK glass at different irradiances (7.3 TW.crrr 2 - 8.9 TW.cm -2 ) and at different speeds (50 pm.s' 1 - 1100 pm.s' 1 ), (b) a zoom of the image (a) showing one of the structures inscribed with an irradiance of 8.4 TW.
• FIG.12 represents respectively confocal fluorescence and phase contrast microcopy images for the structures inscribed in the BGGK glass with an irradiance of 8.4 TW.crrr 2 and a speed of 50 pm.s' 1 (images a and c) and with an irradiance of 7.3 TW.crrr 2 and a speed of 350 pm.s' 1 (images b and d);
• FIG. 13 [0042] [Fig.13] represents a superposition of the fluorescence intensity and refractive index variation profiles in a direction indicated by dashed lines in Figure 12;
• FIG.14 represents confocal fluorescence microscopy images under 405 nm excitation (images a, c, and e) and phase contrast images (images b, d and f) of three structures inscribed in the BGGK glass with a laser pass density per micrometer of 1
• FIG.15 represents the numerical simulation typically representing a refractive index variation structure in the form of tubes inscribed in a gallium-phosphate-sodium glass doped with silver ions (GP) during a point irradiation in a perspective view (a), in a top view (b) and in a side view (c);
• GP silver ions
• FIG.16 represents the digital simulation typically representing a structure formed of two planes of refractive index variation inscribed in the GP lens when the lens is moved in translation relative to the beam in a direction to produce a line passage of the laser beam, the image being shown in a perspective view (a), in a top view (b) and in a side view (c);
• FIG.17 represents the digital simulation typically representing a network of refractive index variation planes inscribed in the GP lens when the lens is moved in translation relative to the beam in one direction to produce a succession of lines passage of the laser beam at regular intervals, the image being shown in a perspective view (a), in a top view (b) and in a profile view (c);
• FIG.18A represents a phase contrast image of a refractive index variation structure inscribed in a GP lens with a passing line of the femtosecond pulsed laser beam;
• FIG.18B represents a refractive index variation profile of a portion of the structure of FIG. 18A along a line shown in the picture;
• FIG.19 shows a high resolution fluorescence image of a periodic structure of refractive index variation planes inscribed by re-inscribing property in GP glass with the distance between two laser beam pass lines equal to 1.1 ⁇ m, this being less than the diameter of the laser beam.
• glass means an amorphous inorganic solid exhibiting the glass transition phenomenon. A glass is obtained by cooling from a liquid phase.
• the term "transparent” means a material that can be seen through.
• the transparency of a material is specified by transmission measurements of a light beam.
• a material is considered transparent for a given wavelength when its transmittance is greater than or equal to 90% excluding Fresnel reflection.
• the terms “material” or “materials” denote the transparent glasses of the present invention.
• the numbers x, a, b, c and d relating to the reference composition of formula 1 represent molar proportions. Further, in the present invention, when a number is indicated between two values, the indicated limits are included in the range of values. Thus, by “x is between 25 and 35”, x is meant between 25 and 35, 25 and 35 being included.
• femtosecond laser means a laser which delivers pulses of duration between a few femtoseconds and a few hundred femtoseconds.
• the term "repetition rate” means the number of laser pulses per second.
• the delay between two successive pulses is shorter than the thermal relaxation time of the glass, there is thermal accumulation and the temperature of the material at the point of impact of the beam increases progressively.
• This thermal load induces a zone of physico-chemical modification around the irradiation point, in order to inscribe a structure of refractive index variation. It should be noted that the thermal build-up remains low in the present process, with a temperature rise well below the glass transition temperature.
• focusing zone means an interaction zone resulting from the impact of the spot of the laser beam in a focal plane located at a depth in the glass.
• writing of a volume structure in a lens means writing a structure of variation or local modulation of optical index of refraction at a depth of the lens induced by impacts of the laser beam, in connection with the result of the photochemistry induced on the silver elements without however modifying the structure of the vitreous matrix.
• submicron resolution means a spatial resolution of between 5 nm and 1 ⁇ m, preferably between 5 and 500 nm.
• sub-diffraction means a resolution lower than the optical resolution limited by the diffraction of light at the wavelength considered.
• the glasses are produced according to a conventional glassmaking process associated with the choice of the compositions of formula (I) of the present invention.
• the manufacturing process comprises the following successive steps:
• the mixture is then melted at a temperature between 800°C and 1700°C.
• This melting time is adapted to guarantee homogeneous dispersion of the Ag+ ion at the atomic scale in order to obtain glasses optically adapted to receive femtosecond laser irradiation points.
• the heating can be carried out in a conventional oven;
• the mixture in the molten liquid state in the crucible, is then subjected to water quenching in order to freeze the mixture while ensuring the homogeneity of the mixture;
• the mixture is then subjected to thermal annealing, at a temperature below the glass transition temperature of the glass.
• the glass is cut to a given thickness, for example to 1 mm thick.
• This thickness can be adapted to greater thicknesses as required, in particular for the production of volumetric Bragg gratings whose height can be several mm, then optically polished on two parallel faces for the structuring phase by a femtosecond laser beam.
• the starting oxides and their possible precursors are in the form of conventional commercial powders.
• the oxide precursors can be in a carbonate form.
• a Na2O precursor can be Na2COs and K2O as K2CO3.
• the mixture then undergoes a decarbonation treatment in order to eliminate the CO2 in order to obtain the oxide of the composition.
• the glass according to the present invention comprises a composition of formula (I) below:
• Oxy1 represents a forming oxide, chosen from P2O5, GeC or SiO2,
• Oxy2 represents an oxide chosen from Ga2O3, Al2O3, ZnO,
• Oxy3 represents an oxide chosen from MgO, CaO or BaO, and
• the oxides Oxy1 represent the glass-forming oxides.
• silicon, germanium or phosphate oxides are combined with gallium oxides.
• the two oxides represent the two essential constituents of the materials of the present invention.
• the materials according to the present invention comprise a significant content of Na2O and BaO.
• the addition of Oxy3 oxides makes it possible to contribute to the mobility of the silver ions and to confer particular properties of inscription and reinscription of structure of variation of index of refraction by femtosecond pulse duration laser beam.
• Oxy2 oxides reduce the melting temperature and minimize crystallization problems.
• the material of the present invention further comprises silver ions to impart the material's photosensitivity property.
• This characteristic is essential to the direct structuring induced by femtosecond laser of photoluminescent patterns resulting from a non-linear phenomenon caused by the multi-photon absorption of the material which makes it possible to form silver aggregates.
• the materials of the present invention are favorable to the formation of silver aggregates linked to the interaction of silver ions with the high repetition rate femtosecond laser and to a local spatial distribution of these aggregates, allowing the inscription of refractive index variation structures.
• the present invention by judiciously associating ions such as Na2O and BaO with silver ions, the applicants note that it is possible to reinscribe a refractive index variation structure in an area which has already undergone irradiation.
• the materials of the present invention are also transparent in the visible range and in the infrared range. This characteristic is necessary to allow the use of these materials to produce optical components such as effective volume Bragg gratings for the visible, between 400 nm and 800 nm and the infrared between 800 and 8000 nm.
• the glass is a silver-doped phosphate-gallium glass in which the composition is formulated according to the following relationship (II):
• composition (II) An example of glass prepared according to composition (II) will be presented below.
• the glass is a germanium-gallium glass doped with silver in which the composition is formulated according to the following relationship (III):
• Oxy2 represents an oxide chosen from Ga2Os, Al2O3, ZnO,
• Oxy3 represents an oxide chosen from MgO, CaO or BaO, preferably BaO
• Oxy4 represents an oxide chosen from Na2O or K2O
• Rb2O or Ü2O preferably K2O x is between 35 and 45, preferably 43.9
• a is between 0 and 40
• preferably 8.8 b is between 0 and 50
• preferably 42.1 c is between 0 and 50
• preferably 3 d is between 0.1 and 10
• a femtosecond laser writing device 100 comprising a femtosecond laser source 101 comprising two amplifying media (Yb: KGW) which generates a laser beam 105.
• the laser beam is consisting of a series of ultrashort light pulses.
• a femtosecond laser source of the Sapphire-Titanium type is also suitable, another wavelength remaining generally suitable due to the non-linear nature of the energy deposition and activation of the photochemistry of the silver.
• the femtosecond laser used is a t-Pulse 500 laser (marketed by Amplitude Systems).
• the maximum power is 2.6 W.
• the femtosecond laser emits a laser beam having a wavelength between 1000 nm and 1100 nm.
• the wavelength of the laser is chosen so as to be at least twice the cut-off wavelength of the glass of the present invention, the wavelength from which the glass absorbs light.
• the wavelength can be chosen close to 1030 nm.
• the emission wavelength of sapphire-titanium around 800 nm would also be suitable.
• the laser is a femtosecond laser. But the invention can be implemented when the duration of the pulse is less than 1 picosecond, preferably between 0.5 ps and 500 fs.
• the method for writing structures comprises a configuration in which the chosen repetition rate is between 10 kHz and 100 MHz. If a major part of the demonstrations of silver photochemistry activation have been carried out around 10 MHz, observations at 80 MHz, based on a laser/glass interaction from a Sapphire-Titanium oscillator have already been carried out. Indeed, this range of repetition rates makes it possible to promote the formation of aggregates and to stabilize them.
• the parameters of the laser beam such as the repetition rate, the number of pulses and the irradiance, are adapted and controlled to irradiate the glass of the present invention so as to be able to register and re-register three-dimensional structures of variation of optical index of refraction at a given depth of the glass without modifying the crystalline structure of the glass.
• the device further comprises an acousto-optic modulator 102 (AOM for acousto-optic modulator) placed at the output of the laser source, in the path of the laser beam.
• AOM acousto-optic modulator
• the device comprises a microscope objective 103 which makes it possible to focus the material at a determined depth in the volume of the glass.
• the numerical aperture of the microscope is between 0.4 and 1.57 in the case of oil immersion objectives with a very high numerical aperture.
• a compromise in the numerical aperture can be envisaged according to the thickness of the volume Bragg grating to be produced, according to the refractive index of the matrix, vitreous or even also the period targeted for the Bragg wavelength targeted for a first-order effective resonance: ideally, to obtain ideal periodicities and therefore optimal efficiencies, it should be remembered that the size D must preferably be greater than the targeted period, while taking care, however, to obtain the largest index modulations possible.
• the structures were created in volume, typically at a depth of 160 pm below the surface of the sample, the realizations having been made with air and oil objectives, with numerical apertures of 0.75 and 1.3, respectively. Thus structures can be formed at different depths below the surface of the glass.
• the microscope air objective focuses the laser beam with a numerical aperture of 0.75, which corresponds to a focal spot of the order of 1.5 ⁇ m in diameter leading to modifications indices distant from D ranging from 1.6 to 1.8 ⁇ m, typically.
• NA 1.3
• beam diameters and therefore distances D ranging from 600 nm to 800 nm were obtained, typically.
• Focuses with NA ⁇ 0.7 are often to be avoided because they can accompany additional nonlinear processes of self-focusing, leading to possible distortions of the focus and therefore to less well controlled and less well localized energy deposition spatially.
• the laser beam is focused 160 ⁇ m below the glass surface.
• the device can comprise fluorescence and phase contrast microscopy to respectively visualize the distribution of the silver aggregates which emit fluorescence and the modification of the refractive index in the structured areas of the sample after irradiation according to the method of the present invention.
• the sample 10 is placed on a high precision plate 104 motorized in translation in the three directions with a precision of the order of 30 nm, in order to ensure the correct positioning of the laser beam in the glass.
• the sample is placed so that the incident radiation of the beam is preferably in normal incidence on the sample.
• the sample extends in a plane (XY) and the axis of propagation of the laser beam extends along an axis Z which is perpendicular to the plane (XY).
• the glass is translated perpendicularly to the axis of propagation of the laser beam, at controlled speeds of 10 to 1050 pm.s -1 respectively.
• the displacement of the sample during the laser inscription process makes it possible to produce three-dimensional structures of complex optical refractive index variation (truly 3D type structures and not only of the 2D type corresponding to multiplane approaches).
• the applicants further show that by controlling the parameters of the laser beam, it is also possible to erase the refractive index generated during a previous irradiation in a portion of this refractive index variation zone, by making the portion of this zone coincide with a zone intensity of the laser beam (not necessarily the center of the beam) where the intensity is high enough on this portion to induce photodissociation of silver aggregates accumulated around the irradiation point, which leads to erasing the index variation generated by the distribution of silver aggregates which are then photodissociated.
• the applicants show that it is possible to reinscribe a refractive index variation zone in a zone that has already undergone an optical index variation erasure.
• the parameters of the laser beam are controlled so as to always maintain in a zone of the glass having undergone irradiation a reservoir of silver ions sufficient to ensure rewriting, that is to say to be able to generate new an accumulation of silver aggregates in a peripheral area around the point of irradiation.
• FIG. 2 is illustrated a top view of the different phases of the process of a point interaction of the femtosecond laser beam in a lens of the present invention.
• the laser irradiation point 11 can be materialized by a circle. This laser inscription or local structuring of the material therefore takes place in a laser interaction volume via multiphoton absorption processes leading to the formation of electron traps by Ag + ions which are transformed into Ag° then to the distribution and to the stabilization of Ag m x+ type silver aggregates with m: number of atoms, m ⁇ 20 and x: degree of ionization 1 ⁇ x ⁇ m.
• the glass is photoexcited by non-linear absorption. This results in the generation of a gas of quasi-free electrons which are rapidly trapped by the Ag + ions to form Ag° atoms.
• the non-linear nature of the interaction confines the distribution of Ag° atoms to an area slightly smaller than the diameter of the laser beam, represented by a dotted circle in Figure 2.
• the temperature of the glass increases locally during the successive deposition of the pulses and generates a diffusion of the metallic species Ag m x+ from the center (highly concentrated) towards the periphery (weakly concentrated). This migration is represented by the arrows in Fig. 2. The temperature of the glass does not exceed the Tg during the laser interaction process and the glass is maintained in the solid state.
• the temperature rise in the glasses of the present invention is less than 300° C., which is sufficient to cause the thermal activation of the silver ion diffusion processes on the one hand and of the chemical reactivity on the other hand.
• Ag m x+ 14 metallic aggregates are formed between the Ag° mobile species and the Ag + ions.
• the glass contains only silver ions.
• the metal aggregates are gold or copper aggregates.
• the material comprises ions of different natures such as gold, copper or silver in different or equal quantities.
• the following pulse has the effect of destroying the silver aggregates by a process of photodissociation in the central region of the interaction volume where the intensity is greater than an intensity sufficient to degrade the silver aggregates previously registered. . Simultaneously, this new pulse regenerates free electrons which are trapped again to form aggregates on the peripheral zone only.
• This sequence of physico-chemical phenomena and the succession of pulses lead to a progressive pulse-after-pulse accumulation of localized aggregates in the peripheral zone of the laser beam, that is to say at the place where the laser intensity and glass temperature are low enough to prevent photodissociation.
• the structured zone is in the form of a tube whose axis is carried by the direction Z of propagation of the laser beam.
• image (c) in FIG. 2 it is in the form of a ring 15 having a very submicron thickness, of which very high resolution electron microscopy imaging has leads to an estimate equal to 80 nm.
• the diameter of the tube is of the order of the diameter of the beam comprised between 0.5 ⁇ m and 3 ⁇ m.
• the femtosecond laser irradiation in the oxide glass of the present invention induces a variation of index of refraction in the annular zone around the point of irradiation of the beam.
• the laser beam acts like an optical brush which makes it possible to induce in 3D a variation of optical index of refraction on its peripheral zone and to erase it in its center.
• the displacement of the sample is represented by an arrow in the plane (X, Y) along the X axis and the Y axis and is perpendicular to the propagation axis of the laser beam.
• FIG. 3A schematically illustrates the inscription in a sample during a translational movement of the sample in the direction X, to form an index modulation distribution corresponding to two zones of positive index variation on the edges of the point of focus, these two zones being separated by a distance D.
• this distance D translates the distance between the modifications on either side of the focus of the laser beam.
• the distance D depends on the size of the focused laser beam but also on the dose of energy deposited which depends on the number of pulses accumulated locally and the laser irradiation used.
• Figure 3A shows the case of a first laser pass along the X axis.
• Figure 3B shows the case of the second laser pass, which can then be generalized to N laser passes.
• the second laser pass is carried out, in the opposite direction or in the same direction, with a center-to-center lateral displacement Ay which determines the periodicity of the Bragg grating.
• the center-to-center lateral displacements are much less than the separation distance of the two index variation zones during the previous pass, such that Ay ⁇ D. index written during the first laser pass, which is then covered by the second laser pass, is then erased, while the other zone of index variation written during the first laser pass remains.
• two new areas of optical refractive index variation are re-inscribed. This capacity for rewriting within the photosensitive glass makes it possible to retain laser passage after laser passage - only one of the two index variation zones, with the spatial period imposed by the center-to-center lateral displacement Ay of the laser.
• Figure 4 illustrates in more detail in a top view the principle of formation of two refractive index variation planes on either side of the line of passage of the laser beam of Figure 3A from a quasi-continuous succession of irradiation points, the distance between two irradiation points Ax being much smaller (up to 100 nm) than the diameter of the laser beam D, in connection with the pairs of parameters applied which are the high repetition rate of the laser (greater than 10 kHz) and the speed of movement of the sample.
• the intensity of the laser beam having a Gaussian profile it follows that the most energetic zone allowing multiphoton absorption is located in a central zone of each irradiation point where the phenomenon of photodissociation occurs when species of silver already inscribed find themselves in a zone of strong irradiation.
• the central zone of the laser beam passes again substantially on the front edge of the previously inscribed ring.
• the aggregates formed on the front edge of the beam of the reference irradiation point j are exposed by the beam of the next reference irradiation point j+1 (diagram which is not to scale for reasons of clarity because the distance between points j and j+1 is very small compared to the size of the diameter).
• the method comprises the following steps:
• the method of laser inscription in the oxide glasses of the present invention makes it possible to produce on each passage of the laser beam the creation of two planes of variation of optical index in the volume of the glass, by controlling the parameters of beam irradiation.
• passing a laser beam through the glass makes it possible to form two planes having a variation in refractive index.
• This process based solely on the photochemistry of silver ions and comoving ions, makes it possible to achieve submicron dimensions which are not very limited by the focusing of the laser beam and therefore by the spatial extension of the point of irradiation and energy deposition. by multi-photon absorption.
• This process therefore combines both deposition by nonlinear optical process and photochemistry whose characteristic dimensions are much lower than the characteristic lengths of energy deposition on the one hand and thermal diffusion on the other, making it possible to obtain very contrasting internal dimensions (An of some 10' 3 ) while having transverse dimensions on the mesoscopic scale (less than 200 nm or even up to 80 nm in thickness).
• each passage of the laser beam is also conditioned by the distance y between two successive passages.
• the spacing y between two laser beam passages is greater than the distance between the two planes which substantially correspond to the diameter of the irradiation point y> D/2)
• the passages of the laser beam do not overlap and make it possible to register at each passage two planes of variation of optical refractive index on either side of the line of passage of the laser.
• Figure 5 illustrates an example of three beam passes. Each passage makes it possible to register two planes of variation of optical index of refraction, the spacing between the two planes being substantially equal to the diameter of the irradiation point D.
• FIG. 6 illustrates an example of three laser beam passes through the glass.
• a first beam pass makes it possible to inscribe two planes of variation of optical index of refraction.
• a second beam passage whose center of the Gaussian profile of the beam passes substantially at the level of one of the two planes previously inscribed in the first passage. By effect of photodissociation, the second pass makes it possible to register two planes on either side of the erased plane at a distance substantially equal to D/2.
• a third passage makes it possible to register three planes P1, P2, P3 spaced at regular intervals by Ay and a fourth plane P4 spaced by D with respect to the plane P3.
• a series of N beam passages makes it possible to inscribe N planes of refractive optical index variation with a pitch A between two planes substantially equal to Ay and an N+1 th plane spaced from N th plane by a distance of D.
• the laser irradiation carried out comprising both the intensity per pulse and the cumulative number of pulses at each point must be adapted so as to maintain a reservoir of silver ions sufficient to allow re-registration and/or to ensure photodissociation in silver species sufficiently remobilizable during the next pass.
• the method of the present invention thanks to a combination of suitable parameters, namely the lateral spacing between two laser beam passages, the irradiance and the number of pulses, makes it possible to produce a network of variation planes with an optical index of refraction of dimension less than 200 nm or even up to 80 nm, with a grating pitch of between 200 nm and 1.5 ⁇ m (which corresponds to the diameter of the focused beam here). Structures with a double line of index variation can also be produced for longer periods.
• Example 1 BGGK (germanium-gallium-barium-potassium glass doped with silver)
• Example 1 relates to a series of silver-doped germanium-gallium-barium-potassium glasses comprising a composition of formula (III). Glass is prepared from gallium oxide, germanium oxide, barium carbonate and silver nitrate.
• the glass is prepared according to a conventional melting-quenching method from high purity reagents.
• the reagent powders are weighed and are introduced into a platinum crucible to be melted between 1350 and 1400°C for about fifteen hours.
• This melting time is adapted to guarantee a homogeneous dispersion of the Ag-i- ion at the atomic scale in order to obtain glasses optically adapted to receive femtosecond laser irradiation points.
• the mixture in the molten liquid state in the crucible, is subjected to water quenching in order to freeze the mixture while ensuring the homogeneity of the mixture.
• the mixture is then subjected to thermal annealing, at a temperature 30° C. below the melting temperature Tg for 4 hours.
• the sample is cut to 1 mm thick and then optically polished on two parallel faces.
• Table 1 are reported the experimental compositions in molar mass of a series of germanium-gallium glasses doped with silver by varying the rate of BaO.
• Figure 8 is shown the optical transmission given in absorption coefficient for the 4 samples GGK, GGB5K, GGB10K and GGB15K.
• the measurements show an absorption front in the UV region of 310 nm invariant with the barium level with an extended transmission in the infrared up to 5.5 pm. At 6.3 pm, an increasing evolution is observed.
• the curve GGB15K represents the evolution of the linear absorption coefficient of the germanate-gallate glass with a BaO content of 15% and the curve BGGK the evolution of the linear coefficient of the barium germanate glass with a BaO level of 37.5%.
• barium germanate glass which has a BaO level of 37.5, has a shorter transmission in the UV and more extended in the infrared.
• BGGK glass is a very good candidate for optical applications requiring an enlarged transmission window in the infrared.
• FIG 10 In Figure 10 are illustrated emission spectra at 270 nm and 320 nm and excitation at 350 nm and 450 nm for the GGB15K and BGGK glasses.
• the curves C6 and C7 respectively represent the excitation spectra at 350 nm and 450 nm and the curves C8 and C9 respectively represent the emission spectra at 270 nm and 320 nm.
• curves C10 and C11 correspond respectively to the excitation spectra at 350 nm and 450 nm.
• Curves C12 and C13 represent emission spectra at 270 nm and 320 nm.
• a 50x50 ⁇ m 2 "velocity-irradiance" irradiation matrix was produced in BGGK glass at a depth of 160 ⁇ m under a femtosecond infrared laser with an irradiance ranging from 6.3 to 8.9 TW.cnr 2 and a displacement speed of the plates ranging from 50 to 1100 ⁇ m.s′ 1 .
• the image (a) of Figure 11 represents a fluorescence confocal microscopy image of such a "velocity - irradiance" irradiation matrix inscribed in the BGGK glass, acquired with a 10x microscope-objective and an aperture numeric of 0.3.
• Image (b) of Figure 11 represents a zoom of a structure inscribed at 8.4 TW. cm -2 and at a speed of 50 pm.s -1 . It is observed that the structure exhibits a double fluorescence line behavior.
• Image (c) of Figure 11 represents a zoom of a structure inscribed at 7.3 TW. cm' 2 and at a speed of 350 pm.s' 1 .
• the structure in image (c) shows very low luminescence with a single line of fluorescence. It is also observed that beyond 8.9 TW.crrr 1 , microexplosions are observed for all speeds greater than or equal to 550 pm.s' 1 .
• Figure 12 respectively shows a confocal microscopic high resolution fluorescence image of the inscribed structure with an irradiance of 8.4 TW. cm -2 and at a speed of 50 pm.s -1 (image a) and an irradiance of 7.3 TW.cnr 2 and at a speed of 350 pm.s -1 (image b). Images (c) and (d) of Figure 12 respectively show a phase contrast image of these same structures.
• FIG. 14 are represented respectively the confocal microscopy images of fluorescence under excitation at 405 nm and of phase contrast of the structures inscribed with a laser passage density per micrometer of 1, 2 and 5 ⁇ m.s′ 1 .
• the applicants observe a maintenance of the fluorescence and of the variation in refractive index at all the densities of laser passage per micrometer.
• a Bragg grating consists of a periodic modulation of the refractive index of the material.
• the Bragg gratings obtained according to known methods in conventional glasses are generally effective in the infrared range down to the red (650 nm) but cannot be used in the entire visible range without resorting to higher orders of diffraction making then drop their effectiveness.
• Bragg gratings effective in the visible at the first diffraction order have been produced using a UV laser but reducing the spatial selectivity conferred by a 3D laser inscription.
• Example 2 relates to a photosensitive glass comprising a composition according to relation (II) made from gallium oxide, sodium carbonate, phosphoric acid and silver nitrate. Once the precursors are weighed, they are placed in a beaker to become a solid which is then ground. The powders are introduced into a platinum crucible to be melted at 1400°C for 24 hours. This melting time is adapted to guarantee the stabilization and the homogeneous dispersion at the atomic scale of the Ag + ions in order to obtain optically adapted glasses to receive reproducible femtosecond laser irradiation points.
• II gallium oxide, sodium carbonate, phosphoric acid and silver nitrate.
• the mixture in the molten liquid state in the crucible, is subjected to quenching with water in order to freeze the mixture while ensuring the homogeneity of the mixture.
• the mixture is then subjected to thermal annealing, at a temperature of 30° C. below the melting temperature Tg for 4 hours.
• thermal annealing at a temperature of 30° C. below the melting temperature Tg for 4 hours.
• the sample is cut to 1 mm thick and 150 ⁇ m then optically polished on two parallel faces.
• Table 2 shows the composition by molar mass of the various constituents of this glass.
• the rate of silver is fixed at 2 mol%.
• This glass has a low glass transition temperature of 368°C and almost 50% NaO2 element. Such a composition allows a highly photosensitive and chemically durable.
• the GPN glass was subjected to nanosecond ultraviolet laser irradiation.
• the emission spectrum obtained for an excitation wavelength at 355 nm shows that the GPN glass has a wide band in the visible range centered around 550, highlighting the majority presence of silver aggregates.
• the refractive index n of glass is 1.541 at 589 nm.
• the volume density p is 3.08 g.cm-3.
• This glass has infrared transparency up to about 3.2 - 3.3 ⁇ m, the limitation of which is associated with the vibration energies of the groups. phosphate giving rise to various absorptions from 3 pm. In the ultraviolet, they present an absorption front between 250 nm and 350 nm linked to the presence of silver ions in this glass.
• the device of Figure 1 is used to produce refractive index variation structures in GPN glass.
• the GPN glass slide is irradiated by laser pulses focused at a depth of 160 ⁇ m below the surface of the glass thanks to the microscope objective with a numerical aperture of 0.75 and a magnification of 20x.
• the irradiation pulses have a wavelength of 1030 nm, a pulse duration of 390 fs, a repetition rate of 9.1 MHz and a maximum power of 2.6 W. of refractive index shown in FIGS. 16 to 18, it was chosen to irradiate the GPN glass with an irradiance of between 5 TW. cm -2 and 10 TW. cm' 2 at a speed of between 20 pm.s' 1 and 200 pm.s' 1 .
• Figure 15 represents a simulated graphical representation of the refractive index variation formed during a point inscription in the GPN glass.
• the stimulated structure is shown in Figure 15 in a perspective view (a), a top view (b) and a profile view (c).
• the multi-photon nonlinear process induces a radial distribution of silver clusters around the center of the point irradiation point. It forms a refractive index variation structure in the form of a tube 30 oriented along the axis of propagation of the laser beam.
• the wall 31 of the tube 30, corresponds to the zone having a variation in refractive index, is formed of molecular entities based on silver aggregates and has a thickness of less than 200 nm or even about 80 nm of minimum thickness.
• the diameter of the tube is similar to that of the irradiation light beam.
• Figure 16 shows an inscription during a translation of the glass in an X direction as illustrated in Figure 4.
• a phenomenon of photodissociation then occurs when the zones of sufficient intensity of the light pulses irradiate silver aggregates formed during a previous irradiation. The silver aggregates are then redissolved in the form of ions in the glass.
• a quasi-continuous succession of aligned irradiation forms an effective structure, whose distribution of silver aggregates separated by the order of ten nm, typically, thus corresponding to a continuous distribution on the length scale.
• This distribution of aggregates and therefore of variation of index, is in the form of a double plane 40 whose wall 41 has a positive variation of refractive index.
• the inscribed structure is shown in Figure 16 in a perspective view (a), a top view (b) and a side view (c).
• Figure 17 represents an inscription of a network of parallel planes of refractive index variation by repeating the beam passage of Figure 17 by moving the sample in an X direction.
• the zones of high intensity of the laser beam make it possible to dissolve part of the silver aggregates previously inscribed during the preceding laser pass.
• the silver elements are then in the form of ions in the glass while two new planes of refractive index variation are formed on either side of the line of passage of the beam via the photochemical phenomenon of creation of new aggregates with the silver ions dissolved in the matrix.
• the inscribed periodic structure is shown in Figure 17 in perspective view (a), top view (b) and profile view (c).
• Figure 18. B represents a refractive index variation profile along a direction perpendicular to a passage single laser (schematized by a line in Figure 18. a).
• a refractive index variation An of 2.1 ⁇ 10′ 3 is determined in the modified zone, with two index variation planes separated by 1.4 ⁇ m typically corresponding to the diameter of the laser beam.
• the applicants show that it is possible to inscribe and re-inscribe structures of positive refractive index variation in the GPN glass comprising sodium ions which are co-mobile with the silver.
• the applicants show that it is possible to gradually inscribe line by line to form a periodic structure of refractive index variation planes with a thickness of less than 200 nm or even of the order of 80 nm, with a periodicity sub - micron controlled by laser inscription with lateral displacements Ay ⁇ D. Thanks to the combination of the nanometric dimension of the structure and a small periodicity, it is possible to produce Bragg gratings acting in the visible to the first order of diffraction.
• Figure 19 is represented a microscopy image showing step by step the realization of a periodic structure.
• This image was obtained by confocal fluorescence microscopy of silver aggregates under excitation at 405 nm.
• the periodic structure was obtained by re-writing property with an irradiance in the range 5-10 TW.cm-2 and a speed of 200 pm.s-1.
• the beam diameter is about 2.2 ⁇ m and the distance between two laser beam passes is half of the beam diameter i.e. 1.1 ⁇ m.
• the oxide glass of the present invention is of interest and has many advantages in the photonics field for producing optical components such as volume Bragg gratings, Bragg gratings in a waveguide or in the heart. of an optical fiber. Thanks to the specific vitreous composition of the various oxides of the present invention, the glasses have on the one hand a high photosensitivity and on the other hand a rewriting property due to the presence of ions which are co-mobile with the silver ions. . In addition, the glass exhibits a broadened transmission spectral range by compared to standard glasses in the infrared range.
• the glass of the invention is particularly suitable for femtosecond laser beam-assisted inscription to fabricate a Bragg grating with nanometric dimension variation lines and submicron grating pitches which can be configured according to the requirement of the applications.

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