CN111399262B - Adjustable terahertz lens and preparation method and application thereof - Google Patents
Adjustable terahertz lens and preparation method and application thereof Download PDFInfo
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- G—PHYSICS
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- 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
- G02F1/13—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 liquid crystals, e.g. single liquid crystal display cells
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- G—PHYSICS
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- 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
- G02F1/13—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 liquid crystals, e.g. single liquid crystal display cells
- G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
- G02F1/1333—Constructional arrangements; Manufacturing methods
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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
- G02F1/13—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 liquid crystals, e.g. single liquid crystal display cells
- G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
- G02F1/1333—Constructional arrangements; Manufacturing methods
- G02F1/1337—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers
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- 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
- G02F1/13—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 liquid crystals, e.g. single liquid crystal display cells
- G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
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- G02F1/1343—Electrodes
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- G02F2203/00—Function characteristic
- G02F2203/13—Function characteristic involving THZ radiation
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Abstract
The invention discloses an adjustable terahertz lens, a preparation method and application thereof, wherein the adjustable terahertz lens comprises a substrate, a medium layer and a liquid crystal layer, wherein the substrate and the medium layer are oppositely arranged; the outer side surface of the dielectric layer, which is far away from the substrate, is provided with a dielectric super-structure surface layer, the dielectric super-structure surface layer comprises an anisotropic dielectric column array, and the width of the dielectric column is gradually reduced from the center to the edge along the radial direction; the opposite inner sides of the substrate and the dielectric layer are respectively provided with an electrode layer and an orientation layer; the alignment layers are all aligned in a periodical 0-180-degree gradual change distribution along the radial direction to induce the liquid crystal molecules to be aligned; the directors of liquid crystal molecules in the liquid crystal layer are periodically distributed in a gradual change way of 0-180 degrees along the radial direction. The invention has the characteristics of wide-band application, miniaturization, easy integration, light weight and thinness, can dynamically switch chromatic aberration or achromatic lens according to different applicable scenes, and has great application potential in the aspects of terahertz communication, imaging, sensing and the like.
Description
Technical Field
The invention relates to an optical device and a preparation method and application thereof, in particular to an adjustable terahertz lens and a preparation method and application thereof.
Background
Terahertz waves are electromagnetic waves with frequencies between 0.1 and 10THz (corresponding wavelengths of 30 to 3000 μm) and have unique properties. Terahertz wave energy penetrates through nonpolar and nonmetallic materials, and the ionizing radiation is small; in addition, many organic or biological molecules have a collective vibration mode in this band, creating a unique "terahertz fingerprint". These features provide great opportunities for terahertz human security, industrial inspection, and medical diagnosis. In these applications involving terahertz imaging, lenses are important elements. Super-structured lenses composed of metal or dielectric resonator arrays of specific sub-wavelengths have been developed and used for terahertz wave front modulation, replacing conventional crystalline or polymeric refractive lenses, exhibiting a trend toward miniaturization and integration. However, their function is mostly static. Up to now, dynamic functions, in particular continuous zoom or adjustable chromatic aberration, remain a great challenge.
Because of the broad band of the terahertz spectrum, the chromatic aberration of the terahertz super-structured lens is more remarkable than that of the optical or near-infrared band. The chromatic aberration of the lens causes the focal length to change along with the frequency, and the spectral resolution of wide-spectrum terahertz imaging is greatly reduced. To solve this problem, mechanical scanning along the terahertz optical path is required to capture images of different frequencies, making detection and analysis complex and time-consuming. On the other hand, in applications such as spectrometers and tomography, large lens chromatic aberration is required to spatially separate focal spots of different frequencies without cross-talk, and dispersive lenses are therefore also widely used. Dynamic switching of achromatic focusing and dispersive focusing characteristics can be achieved with a single super-structured lens, and practical application development of terahertz spectrum and imaging systems is greatly promoted.
Disclosure of Invention
The invention aims to: one of the purposes of the invention is to provide an adjustable terahertz lens which can realize the dynamic switching of broadband achromatic focusing and chromatic dispersion focusing functions so as to solve the technical problems of single terahertz lens function and application in the prior art; the second purpose of the invention is to provide a preparation method of the adjustable terahertz lens; the invention further provides an application of the adjustable terahertz lens.
The technical scheme is as follows: the adjustable terahertz lens comprises a substrate, a medium layer and a liquid crystal layer, wherein the substrate and the medium layer are oppositely arranged; a dielectric super-structure surface layer is arranged on one side of the dielectric layer away from the substrate, namely, the dielectric super-structure surface layer is arranged on the outer side surface of the dielectric layer away from the substrate; the dielectric super-structure surface layer comprises an anisotropic dielectric column array structure, and the width of the dielectric column is gradually reduced from the center of the super-structure surface to the edge along the radial direction;
the length and width of the dielectric pillars are unequal, the array of dielectric pillars continuously varies along the radial structural parameter (wide dimension), while the height and period of the different caps of dielectric pillars are unchanged, and the length is basically unchanged.
Electrode layers are respectively arranged on the inner side surfaces of the substrate and the dielectric layer, and orientation layers are respectively arranged on the inner side surfaces of the substrate and the dielectric layer, and are marked as a first orientation layer and a second orientation layer; the first orientation layer and the second orientation layer have the same orientation direction, and are all oriented in a periodic 0-180-degree gradual change distribution along the radial direction to induce liquid crystal molecules to orient; the liquid crystal layer is arranged between the first orientation layer and the second orientation layer, the orientation direction is parallel orientation along the direction perpendicular to the orientation layer, and periodical 0-180-degree gradual change orientation is formed in the radial direction in the orientation layer.
The requirement that the dielectric post at each position in the dielectric super-structure surface of the present invention needs to meet is that the resonant phase difference of two frequency endpoints (such as endpoint values 0.9 and 1.4THz in the embodiment of the present invention) from center to edge gradually decreases, so that the parameters corresponding to the width on the super-surface structure also gradually decrease. The structural units on the super-structured surface of the medium are anisotropic columns, such as elliptic medium columns, rectangular medium columns and the like; the specific parameters of the dielectric column at each position can be used for determining the length, width and height parameters of the silicon column at different positions in the super-structure surface structural unit and the period parameters of the structural unit through the early simulation design. Wherein, the orientation layer can adopt a photo-oriented layer or a friction orientation layer.
Further, the dielectric pillars of the dielectric super-structured surface layer satisfy: when the incident terahertz wave passes through the dielectric pillars, the change of the resonance phase of each dielectric pillar with frequency satisfies the following formula:
where F is the frequency, c is the speed of light in vacuum, F is the focal length, r is the radius of the lens, k and d are constants,is the lens phase.
The principle of the invention: according to the adjustable terahertz lens and the preparation method thereof, the achromatic focusing effect in a broadband can be realized by superposition of the two phases through the structure orientation liquid crystal layer based on the geometric phase and the dielectric super-structured surface layer based on the resonance phase; when the saturated voltage is added to the graphene electrodes on the upper substrate and the lower substrate of the liquid crystal layer, the liquid crystal molecules are aligned completely perpendicular to the substrates, so that the geometric phase type modulation disappears, only the resonance phase of the super-structured surface of the medium is left to act, the dispersive focusing effect is displayed, and the focal length is reduced along with the increase of the frequency. The dynamic switching of the two functions ensures the realization of a multifunctional adjustable terahertz super-structured lens, and solves the technical problems of single functions and application of an adjustable terahertz function device in the prior art.
Terahertz waves are electromagnetic waves having a frequency of 0.1 to 10THz (corresponding to a wavelength of 30 to 3000 μm). Terahertz waves have the following unique properties: 1) The photon energy is low, and the method is suitable for living body examination of biological tissues; 2) The backbone vibration and rotation energy levels of many biomolecules and condensed substances, the intermolecular interaction energy levels (hydrogen bonds, etc.), are all in the terahertz frequency band; 3) Many nonmetallic and nonpolar materials have small absorption to terahertz waves and high transmittance; 4) Compared with visible light and infrared rays, the terahertz wave has extremely high directivity and strong cloud penetration capability, and can realize wireless transmission rate above Gbit/s. The terahertz technology has wide application prospect in the fields of safety inspection, biomedicine, high-speed wireless communication and the like. Terahertz lenses are widely used in these fields, are generally composed of crystals and polymers, are large in size, have no tunability, and limit integration of terahertz systems. The super-structure material is an artificial electromagnetic medium, and can realize some unique properties which natural materials do not have, such as artificial magnetism, negative index materials, electromagnetic stealth and the like, through artificial design of a unit structure. The super-structured surface is in a two-dimensional form of super-structured material, so that the design and the processing are more convenient. The super-structured surface is used in the terahertz modulation field, however, once the super-structured surface is prepared, the structure is fixed, the functions are also fixed, and dynamic regulation and control cannot be performed, so that the search for the tunability of the super-structured surface becomes a big hot spot in the research field. The orientation distribution of the liquid crystal can be arbitrarily controlled by the photo-alignment technology, so that the liquid crystal is very suitable for designing and manufacturing the geometric phase element.
The adjustable terahertz lens can realize the functions of achromatic focusing and dispersive focusing by electrically-powered dynamic switching. In the unpowered state, the geometric phase of the liquid crystal and the dynamic phase of the dielectric superstructural surface together provide the phase of the achromatic lens, the principle being as follows:
for an achromatic lens, its phase can be written as:
where F represents the frequency, c is the speed of light in vacuum, F represents the focal length, r is the radius of the lens,is an additionally introduced phase factor, which is related to f only. At->And f are introduced into a linear relationship, i.e.)>(k and d are both constant), then achromatic lens phase +.>Can be deformed into:
in the course of this formula (ii) the formula,and f are linear. Such linear phase dispersion can be achieved by introducing waveguide resonance modes at the super-structured surface of the medium. The latter term d is a constant term, except for the former linear dispersion term in the formula, and is not changed with frequency, and can be achieved by introducing the geometric phase of the liquid crystal.
Preferably, the substrate is made of a material with high transmittance in a terahertz wave band, such as quartz, polyimide, high-resistance silicon and the like; the dielectric layer is made of a material with high refractive index and high transmittance in a terahertz wave band, and preferably high-resistance silicon. Wherein the resistivity of the high-resistance silicon is 5000-8000 Ω cm.
Preferably, the structure of the dielectric super-structure surface is a rectangular dielectric column array, the resonant phase of the dielectric super-structure surface is in a linear change relation with the frequency, structural parameters are matched with different wavelengths, and the length, the width and the height of the dielectric column are preferably 10-50 mu m, 10-50 mu m and 150-300 mu m respectively in the applicable wavelength range of 0.9-1.4 THz; according to the phase distribution of the lens, the dielectric pillars with larger resonance phases of the same frequency are distributed in the center of the super-structure surface, so that the widths of the dielectric pillars gradually decrease from the center to the edge.
The materials of the first electrode layer and the second electrode layer are materials with high transmittance and high conductivity in terahertz wave bands, such as few-layer graphene, PEDOT, ITO nanowhiskers and the like, and preferably few-layer graphene. The first orientation layer and the second orientation layer are photo-oriented layers, control patterns of the photo-oriented layers are erasable, and the photo-oriented layers are made of azo dyes.
Preferably, the liquid crystal material of the liquid crystal layer is a birefringent material, and has a first refractive index and a second refractive index; when the frequency range of the incident light entering the adjustable terahertz lens is between 0.5 and 2.5THz, the difference between the first refractive index and the second refractive index is the double refractive index delta n, and delta n (double refractive index) is more than or equal to 0.2 and less than or equal to 0.4; the orientation of the liquid crystal layer is periodically and gradually distributed at 0-180 degrees along the radial direction, so as to generate the geometric phase lens.
The tunable terahertz lens further includes spacer particles located between the substrate and the dielectric layer, the spacer particles being for supporting the substrate and the dielectric layer to form a filling space of the liquid crystal layer; preferably, the thickness of the liquid crystal layer is 200 to 300 μm, and the preferred range of the thickness of the liquid crystal layer is due to the phase retardation conditionWhere Δn represents the birefringence (difference between extraordinary refractive index and ordinary refractive index) of the liquid crystal, h represents the thickness of the liquid crystal layer, and at a specific wavelength λ, the operating efficiency of the geometric phase optical element can be maximized when a half-wave condition is satisfied. For the thickness of the liquid crystal layer of the present invention, the theoretical preferable value is 200 to 300 μm, and if h is smaller than this value, the geometric phase generation efficiency gradually decreases because the phase accumulation of the terahertz wave passing through the device is far short of the half-wave condition of the terahertz band. The half-wave condition of this band is more approached at a thickness of more than 300 μm, but the alignment effect of the liquid crystal layer becomes worse, so in this embodiment, the thickness of the liquid crystal layer can be designed to be 200 μm to 300 μm, and further, the preferable value of the thickness h of the liquid crystal layer is 200 μm, and the alignment effect is good at this thickness condition and has high efficiency.
The invention also provides a preparation method of the adjustable terahertz lens, which comprises the following steps:
providing a substrate and a dielectric layer with a super-structure surface layer on one side;
sequentially preparing a first electrode layer and a first light control orientation layer on the surface of one side of the dielectric layer, on which the super-structure surface layer is not arranged;
sequentially preparing a second electrode layer and a second photo-alignment layer on one side surface of the substrate;
arranging spacer particles on one side of a second photo-alignment layer of the substrate, packaging the dielectric layer and the substrate, enabling the substrate and the dielectric layer to be arranged oppositely, positioning the super-structure surface layer on one side far away from the substrate, enabling the first photo-alignment layer to face the substrate, and enabling the second photo-alignment layer to face the dielectric layer;
performing multi-step overlapped exposure on the photo-control orientation layer to form a control pattern with a periodic 0-180-degree continuous gradual change distribution of molecular directors in the radial direction;
and filling a liquid crystal material between the substrate and the medium layer, wherein the control pattern controls the liquid crystal molecular directors to be in periodical 0-180-degree continuous gradual change distribution in the radial direction. Namely: the directors of liquid crystal molecules in the liquid crystal layer are periodically distributed in a gradual change way of 0-180 degrees along the radial direction.
The preparation method of the dielectric super-structured surface layer comprises the following steps: and cleaning the high-resistance silicon wafer, transferring the pattern on the mask plate onto the silicon wafer by utilizing a photoetching process, etching the exposed silicon by utilizing plasma until the etching reaches a target depth, stopping etching, and finally washing off residual photoresist.
Wherein, the preparation of the photo-alignment layer on the substrate and the dielectric layer further comprises the pretreatment of the surfaces of the substrate and the dielectric layer before the preparation, and the pretreatment steps comprise: ultrasonically cleaning a substrate and a dielectric layer (such as a silicon wafer) for 20-40 minutes by using a cleaning solution, ultrasonically cleaning the substrate and the dielectric layer twice by using ultrapure water for 8-10 minutes each time, drying the substrate and the dielectric layer in a drying oven at 100-120 ℃ for 40-60 minutes, and finally performing ultraviolet light ozone cleaning for 30-45 minutes.
The invention also provides application of the adjustable terahertz lens in spectral imaging. The achromatic terahertz lens can be used for THz hyperspectral imaging under the condition of no power-up, and different biological tissues can be identified by combining the characteristic THz fingerprint spectrum of biological molecules; the terahertz lens can be applied to a spectrometer or tomography under the condition of power-up, and THz energy with different frequencies can be spatially separated and utilized independently.
The beneficial effects are that: in contrast to the prior art, the method has the advantages that,
(1) The invention generates the required resonance phase by designing the structural parameters of the super-structured surface, generates the required geometric phase by designing the spatial orientation of the liquid crystal, and can realize the achromatic focusing effect in a broadband by superposing the two phases; when the saturated voltage is added to the graphene electrodes on the upper substrate and the lower substrate of the liquid crystal layer, the liquid crystal molecules are aligned completely perpendicular to the substrates, so that the geometric phase type modulation disappears, only the resonance phase of the super-structured surface of the medium is left to act, the dispersive focusing effect is displayed, and the focal length is reduced along with the increase of the frequency. The dynamic switching of the two functions ensures the realization of a multifunctional adjustable terahertz super-structured lens, and solves the technical problems of single functions and application of an adjustable terahertz function device in the prior art.
(2) The adjustable terahertz lens can realize the functions of achromatic focusing and dispersive focusing by electrically-powered dynamic switching; the device has the characteristics of wide-band applicability, miniaturization, easy integration, light weight and thinness, can dynamically switch chromatic aberration or achromatic lenses according to different applicable scenes, and has great application potential in the aspects of terahertz communication, imaging, sensing and the like.
Drawings
FIG. 1 is a schematic cross-sectional structure of a tunable terahertz lens of the present invention;
FIG. 2 is a schematic illustration of structural elements of a media super-structured surface of the present invention; in the figure, l represents length, w represents width, h represents height and T represents period;
FIG. 3 is a simulation of resonant phase and polarization conversion efficiency for structural elements (50 μm long, 36 μm wide, 200 μm high, 60 μm periodic) of a dielectric superstructural surface of the present invention;
FIG. 4 is a graph of length and width parameters of structural elements of the media super-structured surface of the present invention from center to edge locations;
FIG. 5 is a geometric phase distribution and corresponding liquid crystal director profile of the liquid crystal layer of the tunable terahertz lens of the present invention;
FIG. 6 is a photograph of a localized area of a media super-structured surface of the present invention under a scanning electron microscope;
FIG. 7 is a photograph showing the alignment of the liquid crystal layer of the tunable terahertz lens of the present invention under an orthogonal polarization microscope;
FIG. 8 is a simulated view of the focusing effect of the tunable terahertz lens of the present invention under an unpowered condition, showing terahertz far field intensity profiles of 0.9THz,1.0THz,1.1THz,1.2THz,1.3THz,1.4THz, in order from left to right;
FIG. 9 is a graph of experimental test of the focusing effect of the tunable terahertz lens of the present invention under an unpowered condition, from left to right, showing the terahertz far field intensity profiles at 0.9THz,1.0THz,1.1THz,1.2THz,1.3THz,1.4THz, with the upper surface being a plot of the light spot on the wavefront propagation surface, and the lower surface being a plot of the intensity in the wavefront propagation direction;
FIG. 10 is a simulated view of the focusing effect of the tunable terahertz lens of the present invention at saturation voltage, from left to right, for terahertz far field intensity profiles of 0.9THz,1.0THz,1.1THz,1.2THz,1.3THz,1.4THz in sequence;
FIG. 11 is a graph of experimental test of the focusing effect of the tunable terahertz lens of the present invention at saturation voltage, from left to right, for terahertz far field intensity profiles at 0.9THz,1.0THz,1.1THz,1.2THz,1.3THz,1.4THz, with the upper being a plot of the light spot on the wavefront propagation surface and the lower being a plot of the intensity in the wavefront propagation direction;
FIG. 12 is a graph of the focusing efficiency of the tunable terahertz lens of the present invention at unpowered and saturated voltages;
FIG. 13 is a schematic flow chart of a method of fabricating a tunable terahertz lens of the present invention;
FIG. 14 is a schematic diagram of steps in a method of manufacturing a tunable terahertz lens of the present invention;
fig. 15 is a functional schematic of the tunable terahertz lens of the present invention.
Detailed Description
The present invention will be described in further detail with reference to examples.
The tunable terahertz lens of this embodiment integrates a liquid crystal and a super-structured surface, and the cross section of the lens is shown in fig. 1, and includes a substrate 6 and a dielectric layer 1 that are disposed opposite to each other, and a first electrode layer 2, a second electrode layer 5, a first alignment layer 3, a second alignment layer 4, and a liquid crystal layer 7. Wherein, one side of the dielectric layer 1 far away from the substrate 6 is provided with a dielectric super-structure surface layer 8, namely the upper surface of the dielectric layer 1 shown in fig. 1; the dielectric super-structure surface layer 8 comprises a rectangular dielectric column array with continuously changed radial structural parameters, a first electrode layer 2 and a first orientation layer 3 are sequentially arranged on one side, facing the substrate 6, of the dielectric layer 1, the substrate 6 is positioned below the dielectric layer 1, a second electrode layer 5 and a second orientation layer 4 are sequentially arranged on one side, facing the dielectric layer 1, of the substrate 6, the first orientation layer 3 and the second orientation layer 4 have the same orientation direction, and the orientation is periodically distributed in a gradual change mode at 0-180 degrees along the radial direction to induce liquid crystal molecules to orient.
The liquid crystal layer 7 is disposed between the first alignment layer 3 and the second alignment layer 4, and the liquid crystal layer 7 includes liquid crystal molecules 9 and spacers 10 disposed at two ends of the liquid crystal molecules 9, and circular structures on both sides of the liquid crystal molecules as shown in fig. 1 are spacers, so as to form a liquid crystal cell with a fixed thickness. Wherein, the orientation direction of the liquid crystal molecules 9 is periodically and gradually oriented from 0 degrees to 180 degrees along the radial direction in the orientation layer surface.
In the embodiment, the material of the dielectric layer 1 is a high-resistance silicon wafer, the material of the substrate 6 is quartz, the electrode layers are all made of graphene materials, and the orientation layers are all made of azo dye materials. The structural unit of the dielectric super-structure surface layer 8 is a rectangular silicon column, and the length, width and height parameters of the silicon column at different positions in the super-structure surface structural unit and the period parameters of the structural unit can be determined through the early simulation design; in this embodiment, the period of the structural unit is set to 60 μm, the height of the silicon column is set to 200 μm, and the length and width parameters of the silicon column are determined according to the resonance phases required for different positions. The thickness of the liquid crystal layer was 200. Mu.m.
The invention can realize the functions of achromatic focusing and chromatic dispersion focusing by electrically switching, and under the unpowered state, the geometrical phase of liquid crystal and the dynamic phase of the super-structured surface of the medium jointly provide the phase of the achromatic lens, and the principle is as follows:
for an achromatic lens, its phase can be written as:
where F represents the frequency, c is the speed of light in vacuum, F represents the focal length, r is the radius of the lens,is an additionally introduced phase factor, which is related to f only. At->And f are introduced into a linear relationship, i.e.)>(k and d are both constant), then achromatic lens phase +.>Can be deformed into:
in the course of this formula (ii) the formula,and f are linear. Such linear phase dispersion can be achieved by introducing waveguide resonance modes at the super-structured surface of the medium. The latter term d is a constant term, except for the former linear dispersion term in the formula, and is not changed with frequency, and can be achieved by introducing the geometric phase of the liquid crystal. This embodiment exemplifies an achromatic lens with a focal length of 15mm and an operating frequency bandwidth of from 0.9 to 1.4 THz.
FIG. 2 is a schematic diagram of structural elements of a super-structured surface of a medium, where l in FIG. 2 represents length, w represents width, h represents height, and T represents period; the structural unit of the dielectric super-structure surface layer 8 in this embodiment is a rectangular silicon column, and fig. 3 is a simulation diagram of the resonance phase and polarization conversion efficiency of the structural unit (length 50 μm and width 36 μm) of the dielectric super-structure surface provided in this embodiment, and it can be seen that the resonance phase of this structural unit shows a linear increase within 0.9 to 1.4THz, so that the requirement of linear phase dispersion in the achromatic lens formula can be satisfied, and meanwhile, it can be seen that the efficiency of different frequencies is slightly different. The slope of the linear phase dispersion required at different r-positions of the lens is different, so the length-width parameters of the structural unit need to be changed to meet this condition. In addition, in order to achieve a good focusing effect even when the geometric phase of the liquid crystal is lost, it is also necessary to consider whether the resonance phase of the structural unit can form a lens phase. In combination with the two requirements, the structural parameters are optimized through electromagnetic field simulation. Fig. 4 is a diagram of length and width parameters of a structural unit of a dielectric super-structured surface from center to edge according to an embodiment of the present invention. The overall trend is that the length remains substantially constant from center to edge and the width gradually decreases, since larger structures provide greater linear phase dispersion and are therefore placed in the middle.
The residual frequency-independent phase term d can be realized by the liquid crystal geometrical phase. The geometric phase is the phase which is only related to the geometric direction of the optical axis and is introduced in the process of rotating the optical axis by anisotropic materials such as liquid crystal, crystal and the like, and the geometric phase is 2 times of the rotation angle. Fig. 5 shows a geometric phase distribution of a liquid crystal layer and a corresponding liquid crystal director distribution diagram of a tunable terahertz lens according to an embodiment of the present invention. It can be seen that the phase distribution varies continuously in any radial direction and therefore the director direction of the liquid crystal also varies continuously in the radial direction. Such spatially diverse liquid crystal alignment distribution can induce liquid crystal alignment by zonal alignment of the alignment agent by polarized light.
Fig. 6 is a photograph of a localized area of a sample of the super-structured surface of the medium in this example under a scanning electron microscope, the microstructure fitting well with the ideal parameters. Fig. 7 is an alignment photograph of a liquid crystal layer of an adjustable terahertz lens according to an embodiment of the present invention under an orthogonal polarization microscope. Since the structural unit period of the lens in this embodiment is 60 μm, the corresponding geometric phase is pixelated as well.
The focusing effect of the tunable terahertz lens is simulated by using FDTD electromagnetic field simulation. Fig. 8 is a schematic diagram of a focusing effect simulation of the tunable terahertz lens provided in this embodiment under the condition of no voltage, and the terahertz far field intensity distribution diagrams of the tunable terahertz lens are sequentially 0.9thz,1.0thz,1.1thz,1.2thz,1.3thz, and 1.4thz from left to right. It can be seen that the terahertz far field exhibits a very good achromatic focus in this band, with a focal length of 12mm. Note that there is an error with the designed focal length of 15mm, which is due to the difference in the plasma etching rate caused by the difference in the length and width parameters of the structural units at different r positions in the process of preparing the super-structured surface, and finally, the etching depth in the middle of the sample is smaller and the etching depth in the edge area is larger, thereby generating a focusing error.
The focusing effect of the tunable terahertz lens is characterized by a method for generating and detecting terahertz waves based on a photoconductive antenna. Fig. 9 is a diagram for experimental test of focusing effect of the tunable terahertz lens under the condition of no voltage application, from left to right, the terahertz far field intensity distribution diagrams under the conditions of 0.9thz,1.0thz,1.1thz,1.2thz,1.3thz, and 1.4thz are sequentially shown, the light spot diagram on the wavefront propagation surface is shown above, and the intensity diagram in the wavefront propagation direction is shown below. The experimental result and the simulation result can be basically matched, and the focal spot is slightly larger than the diffraction limit, which indicates that the focusing effect is better.
When a saturation voltage is applied to the liquid crystal layer, the geometric phase modulation disappears, and the lens exhibits a dispersive focusing effect. Fig. 10 is a diagram showing a simulation of the focusing effect of the tunable terahertz lens of this embodiment under saturation voltage, and the terahertz far-field intensity distribution diagrams under 0.9thz,1.0thz,1.1thz,1.2thz,1.3thz, and 1.4thz in this order from left to right. Fig. 11 is a diagram for testing the focusing effect of the tunable terahertz lens under the saturation voltage, which shows the terahertz far field intensity distribution diagrams of 0.9thz,1.0thz,1.1thz,1.2thz,1.3thz and 1.4thz in sequence from left to right, where the upper part is a light spot diagram on the wavefront propagation surface, and the lower part is an intensity diagram in the wavefront propagation direction. It can be seen that the focal length gradually decreases as the frequency increases in the range from 0.9 to 1.4THz in the results of the simulation and experiment.
Fig. 12 is a graph of the focusing efficiency of the tunable terahertz lens of this embodiment at unpowered and saturated voltages. It can be seen that the focusing efficiency is generally lower when unpowered than when powered, since there is one efficiency of the geometric phase when unpowered, and the efficiency of the geometric phase when powered is not considered. The overall efficiency is determined by geometric factors: polarization conversion efficiency of the super-structured surface structure unit, efficiency of geometric phase of liquid crystal, absorption and scattering loss of all structural layers and imperfections of sample preparation. These factors can be improved by optimizing the structural parameters, thickness of the liquid crystal layer, introduction of low loss substrates, and the like.
Fig. 13 is a schematic diagram of a preparation flow of the tunable terahertz lens of this embodiment, and fig. 14 is a schematic diagram of each step of the preparation method; the preparation method comprises the following steps:
step 120, providing a substrate and a dielectric layer with a dielectric super-structure surface layer on one side;
the preparation process of the dielectric super-structured surface layer comprises the following steps: and cleaning the high-resistance silicon wafer, transferring the pattern on the mask plate onto the silicon wafer by utilizing a photoetching process, etching the exposed silicon by utilizing plasma until the etching reaches a target depth, stopping etching, and finally washing off residual photoresist by using acetone.
Step 121, forming graphene electrode layers on adjacent sides of the substrate and the dielectric layer respectively, wherein the graphene electrode layers are a first electrode layer and a second electrode layer respectively; namely, a graphene electrode layer is formed on the surface of one side of the dielectric layer, on which the super-structured surface is not arranged, and a graphene electrode layer is formed on the surface of one side of the substrate, which faces the dielectric layer;
step 122, forming photo-alignment layers on adjacent sides of the substrate and the dielectric layer respectively, namely preparing a first alignment layer on the first electrode layer and preparing a second alignment layer on the second electrode layer;
before the photo-alignment film is prepared on the substrate and the dielectric layer, the method further comprises the step of preprocessing the substrate and the dielectric layer, and the preprocessing step comprises the following steps: the substrate and the dielectric layer are ultrasonically cleaned by using a cleaning solution for 10 to 30 minutes, then are ultrasonically cleaned by using ultrapure water for two times, each time lasts for 8 to 10 minutes, then are dried in a baking oven at 100 to 120 ℃ for 40 to 60 minutes, and finally are cleaned by using ultraviolet light and ozone for 30 to 45 minutes.
Step 123, spacer particles are arranged on the second orientation layer of the substrate and are encapsulated with the dielectric layer, so that the substrate and the dielectric layer are oppositely arranged, the super-structured surface is positioned at one side far away from the substrate, the first photo-alignment layer faces the substrate, and the second photo-alignment layer faces the dielectric layer;
step 124, performing multi-step overlapping ultraviolet polarization exposure on the photo-alignment film to form a control pattern with a continuously gradient radial distribution of directors.
And step 125, pouring the liquid crystal material between the substrate and the medium layer, wherein the control pattern of the photo-alignment film controls the liquid crystal molecular directors of the liquid crystal layer to continuously and gradually distribute in the radial direction.
As shown in fig. 15, the function of the tunable terahertz lens is schematically shown, all THz waves in the frequency range have the same focal length under the condition of no power-up, and the liquid crystal molecules stand up when saturated voltage is applied, at this time, the focal length of the lens changes with the frequency, and the larger the frequency is, the smaller the focal length is.
The terahertz lens is mainly formed by superposing a liquid crystal layer and a dielectric super-structured surface; the directors of liquid crystal molecules in the liquid crystal layer are periodically distributed in a gradual change way of 0-180 degrees along the radial direction so as to form the required geometric phase type regulation and control for focusing terahertz wave fronts; the dielectric super-structure surface is composed of a rectangular silicon column array, and the length and width dimensions of the silicon column units are gradually reduced from the center to the edge of the array so as to form the required resonant phase type regulation and control of terahertz wave fronts. The superposition of the two phase regulation and control realizes the achromatic focusing effect of the device in a broadband; when the liquid crystal layer is vertically electrified, the geometric phase modulation disappears, only the resonance phase of the super-structured surface of the medium acts, the device forms a chromatic aberration focusing effect in a broadband, and the focal length gradually decreases along with the increase of the frequency.
The achromatic terahertz lens can be used for THz hyperspectral imaging under the condition of no power-up, and different biological tissues can be identified by combining the characteristic THz fingerprint spectrum of biological molecules; the terahertz lens can be applied to a spectrometer or tomography under the condition of power-up, and THz energy with different frequencies can be spatially separated and utilized independently.
Claims (9)
1. An adjustable terahertz lens, characterized in that: the liquid crystal display comprises a substrate and a medium layer which are oppositely arranged, and a liquid crystal layer positioned between the substrate and the medium layer; a dielectric super-structure surface layer is arranged on one side of the dielectric layer, which is far away from the substrate, and comprises an anisotropic dielectric column array structure, wherein the width of the dielectric column is gradually reduced from the center of the super-structure surface to the edge along the radial direction;
the inner side surfaces of the substrate and the dielectric layer, which are opposite, are respectively provided with a first electrode layer and a second electrode layer, and the inner side surfaces of the second electrode layer and the first electrode layer, which are opposite, are respectively provided with an orientation layer, which is marked as a first orientation layer and a second orientation layer; the first orientation layer and the second orientation layer have the same orientation direction and are in periodic 0-180-degree gradual change distribution orientation along the radial direction so as to induce liquid crystal molecules to orient; the liquid crystal layer is arranged between the first alignment layer and the second alignment layer, the directors of liquid crystal molecules in the liquid crystal layer are periodically and gradually aligned at 0-180 degrees along the radial direction in the alignment layer,
the change of the resonant phase of the tunable terahertz lens with frequency satisfies the following formula:
；
in the method, in the process of the invention,fis the frequency at which the frequency is to be determined,cis the speed of light in the vacuum,Frepresenting the focal length of the lens,ris the radius of the lens and,kanddare all constant and are used for the preparation of the high-voltage power supply,din order to introduce the geometric phase of the liquid crystal,for the phase of the lens,
when a saturation voltage is applied across the second electrode layer and the first electrode layer, the liquid crystal molecules are aligned completely perpendicular to the substrates, resulting in the disappearance of the geometric phase modulation.
2. The tunable terahertz lens of claim 1, wherein: the structural unit of the dielectric column array is a rectangular dielectric column, when the incident terahertz wave frequency is 0.9-1.4 THz, the length of the dielectric column is 10-50 μm, the width is 10-50 μm, the height is 150-300 μm, and the period is 50-100 μm.
3. The tunable terahertz lens of claim 1, wherein: the substrate is made of quartz, polyimide or high-resistance silicon, and the dielectric layer is made of high-resistance silicon; the materials of the first electrode layer and the second electrode layer are few-layer graphene, PEDOT or ITO nanowhiskers; the orientation layers are all light-operated orientation layers.
4. The tunable terahertz lens of claim 1, wherein: the liquid crystal material of the liquid crystal layer is a double-refractive-index material and has a first refractive index and a second refractive index; when the frequency range of the incident light entering the adjustable terahertz lens is 0.9-1.4 THz, the difference between the first refractive index and the second refractive index is delta n, and delta n is more than or equal to 0.2 and less than or equal to 0.4.
5. The tunable terahertz lens of claim 1, wherein: the adjustable terahertz lens further comprises spacer particles located between the substrate and the medium layer, wherein the spacer particles are used for supporting the substrate and the medium layer to form a filling space of the liquid crystal layer, and when the incident light frequency range is 0.9-1.4 THz, the thickness of the liquid crystal layer is 200-300 mu m.
6. A method for manufacturing the tunable terahertz lens of any one of claims 1 to 5, comprising the steps of:
providing a substrate and a dielectric layer with a super-structure surface layer on one side;
sequentially preparing a first electrode layer and a first orientation layer on the surface of one side of the dielectric layer, on which the super-structured surface layer is not arranged;
sequentially preparing a second electrode layer and a second orientation layer on one side surface of the substrate;
arranging spacer particles on one side of a second orientation layer of the substrate, packaging the dielectric layer and the substrate, enabling the substrate and the dielectric layer to be arranged oppositely, enabling the super-structure surface layer to be located on one side far away from the substrate, enabling the first orientation layer to face the substrate, and enabling the second orientation layer to face the dielectric layer;
performing multi-step overlapped exposure on the orientation layer to form a control pattern with a periodic 0-180-degree continuous gradual change distribution of molecular directors in the radial direction;
and filling a liquid crystal material between the substrate and the medium layer, wherein the control pattern controls the liquid crystal molecular directors to be in periodical 0-180-degree continuous gradual change distribution in the radial direction.
7. The method for manufacturing a tunable terahertz lens according to claim 6, wherein: the preparation method of the dielectric super-structured surface layer comprises the following steps: and cleaning the high-resistance silicon wafer, transferring the pattern on the mask plate onto the silicon wafer by using a photoetching process, etching the exposed silicon by using plasma until the etching is stopped until the etching reaches the target depth, and finally washing off residual photoresist.
8. The method for manufacturing a tunable terahertz lens according to claim 6, wherein: preparing the orientation layer on the substrate and the dielectric layer further comprises pretreatment of the surfaces of the substrate and the dielectric layer before preparation; wherein the pretreatment step comprises the following steps: and ultrasonically cleaning the substrate and the dielectric layer for 20-40 minutes by using a cleaning solution, ultrasonically cleaning the substrate and the dielectric layer by using ultrapure water for two times, each time for 8-10 minutes, drying the substrate and the dielectric layer in a drying oven at 100-120 ℃ for 40-60 minutes, and finally performing ultraviolet ozone cleaning for 30-45 minutes.
9. Use of the tunable terahertz lens of any one of claims 1 to 5 in spectral imaging.
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