CN111399261B - Terahertz super-structured lens with adjustable focal length and preparation method and application thereof - Google Patents
Terahertz super-structured lens with adjustable focal length and preparation method and application thereof Download PDFInfo
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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
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- 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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- 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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- 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
- G02F1/1343—Electrodes
- G02F1/134309—Electrodes characterised by their geometrical arrangement
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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
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- G02F2203/13—Function characteristic involving THZ radiation
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Abstract
The invention discloses a terahertz super-structure lens with an adjustable focal length, a preparation method and application thereof, and the terahertz super-structure lens comprises a substrate, a medium layer, a first electrode layer, a second electrode layer, a first orientation layer, a second orientation layer and a liquid crystal layer which are oppositely arranged, wherein one side of the medium layer, which is far away from the substrate, is provided with the medium super-structure surface layer, and the liquid crystal layer is arranged between the first orientation layer and the second orientation layer; the first orientation layer and the second orientation layer have the same orientation direction, are uniformly oriented in a plane and form 45 degrees with the transverse axis or the longitudinal axis; the alignment direction of the liquid crystal layer is induced by the alignment layer and is the same as the alignment direction of the alignment layer. The invention can realize the dynamic switching of the focal length of the lens through the integration of the polarization multiplexing super-structured lens and the electric-tuning liquid crystal wave plate, and the dynamic switching of the function realizes the multifunctional adjustable terahertz super-structured lens, thereby solving the technical problems of single function and application of terahertz devices in the prior art.
Description
Technical Field
The invention relates to an optical lens and a preparation method and application thereof, in particular to a terahertz super-structure lens with an adjustable focal length and a preparation method and application thereof.
Background
Currently, fifth generation (5G) wireless networks have been gradually promoted around the world, and it is necessary to accelerate the basic research of next generation (6G) communications. The requirement of the 6G system on the channel capacity is far beyond the current requirement, so that the terahertz (THz) frequency band with higher carrier frequency than the radio frequency band has great application prospect. The terahertz band is the last insufficiently explored band of the electromagnetic spectrum, the so-called "terahertz gap", and many terahertz devices are still far from being mature.
Lenses are important components of beam coupling, focusing and collimation in communication systems. Conventional terahertz lenses are typically bulky, such as Shan Tu spherical lenses. Unlike conventional phase accumulation methods, the super-structured lens (metalens) can introduce abrupt phases for wavefront manipulation by designing sub-wavelength metallic or dielectric resonating units. The super-structured lens reported so far has realized various functions such as spin selection focusing, broadband achromatic focusing, super-resolution focusing and the like. Compared with the metal super-structured surface, the dielectric super-structured surface has higher modulation efficiency and manufacturing performance compatible with CMOS, so that the dielectric super-structured surface has wider application prospect. The function of the common super-structure lens is static and cannot meet the requirements of various practical applications. In recent years, the combination of a super-structured surface with a semiconductor, graphene, a phase change material, a superconductor and other functional materials, and the realization of active tuning of the super-structured surface have become a big hot spot for research. However, most of the structural elements of the tunable super-structured surface are uniform, such that their function is limited to spectral tuning, rather than dynamic wavefront control; further develop tunable wave front regulation and control device, for example adjustable lens, dynamic beam deflector, etc. have important practical meaning.
Disclosure of Invention
The invention aims to: the invention aims to provide a terahertz super-constructed lens with an adjustable focal length, which can realize dynamic tuning of different focal lengths under the condition of power-on so as to solve the technical problems of terahertz lens functions and single application in the prior art; the second purpose of the invention is to provide a preparation method of the terahertz super-structure lens with adjustable focal length; the invention further provides an application of the terahertz super-structure lens with the adjustable focal length.
The technical scheme is as follows: the terahertz super-structure lens with the adjustable focal length comprises a substrate, a dielectric layer and a liquid crystal layer, wherein the substrate and the dielectric layer are oppositely arranged, the dielectric super-structure surface layer is arranged on one side, far away from the substrate, of the dielectric layer, a first electrode layer and a first orientation layer are sequentially arranged on one side, facing the substrate, of the dielectric layer, a second electrode layer and a second orientation layer are sequentially arranged on one side, facing the dielectric layer, of the substrate, and the liquid crystal layer is arranged between the first orientation layer and the second orientation layer;
the dielectric super-structure surface layer comprises an anisotropic dielectric column structure, dielectric columns are distributed in an annular array, each dielectric column has a transverse axis length and a longitudinal axis length, the transverse axis lengths and the longitudinal axis lengths of different dielectric columns positioned in the same radius ring along the angular direction are the same, and the transverse axis lengths and the longitudinal axis lengths of the dielectric columns positioned in different radius rings are different;
the first alignment layer and the second alignment layer have the same alignment direction, the alignment directions are all aligned uniformly in a plane and are distributed at 45 degrees with the transverse axis or longitudinal axis direction so as to induce the alignment of liquid crystal molecules;
the liquid crystal layer is arranged between the first alignment layer and the second alignment layer, and the alignment direction of liquid crystal molecules in the liquid crystal layer is uniform alignment in a plane and is 45 degrees with the direction of a transverse axis or a longitudinal axis.
The length and width parameters of the dielectric pillars are determined according to the lens phases required by different positions, and the heights and periods of the dielectric pillars at different positions are unchanged.
Further, when the incident terahertz wave passes through the dielectric pillars, the resonance phase generated by each dielectric pillarMeets the following lens phase formula requirements:
wherein x is the position coordinate in the horizontal axis direction, y is the position coordinate in the vertical axis direction, f i Focal length f of lens for incident x-polarized terahertz wave j The focal length of the lens when the terahertz wave is incident is y polarized, and lambda is the wavelength of the terahertz wave; f (f) i，j Represents f i Or f j I.e. each dielectric pillar satisfies both x-polarization phase and y-polarization phase.
The substrate is made of a material with high transmittance in a terahertz wave band and an ultraviolet wave band, the material is used for enhancing the modulation efficiency of terahertz waves, and the material is used for irradiating an orientation layer on the inner side of the substrate by ultraviolet irradiation orientation; preferably, the material of the substrate includes quartz, polyimide, 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.
The super-structured surface is formed by periodically arranging artificially prepared sub-wavelength basic structural units, and can regulate and control the phase, amplitude and polarization of incident electromagnetic waves. The structural unit of the dielectric super-structured surface layer in the invention is an anisotropic column, such as an elliptic dielectric column, a rectangular dielectric column and the like; and the length, width and height parameters of the dielectric pillars at different positions in the super-structured surface structure unit and the period parameters of the structure unit can be determined through the early simulation design. The alignment layer may be a photoalignment layer or a rubbed alignment layer.
Preferably, the dielectric pillar structure of the dielectric super-structured surface is a rectangular dielectric pillar; when the length and width parameters of the dielectric pillars are different, the dielectric pillars have different effective refractive indexes on incident terahertz waves with electric field vectors along a long direction (x-ray polarization) and electric field vectors along a wide direction (y-ray polarization), so that the resonant phases of x-polarization and y-polarization emergent are different, and therefore the super-structured surface has polarization dependence characteristics. The structural parameters of the super-structured surface can be optimized according to the terahertz frequency of the target, so that the regulation and control efficiency is optimized, and under 1THz, the preferred parameters of the dielectric pillar array are as follows: the length is 20-120 mu m, the width is 20-120 mu m, the height is 150-250 mu m, and the period is 100-200 mu m; lenses with different focal lengths in x-polarization and y-polarization can be designed and prepared by using the design principle of the polarization dependent super-structured surface.
The material of the first electrode layer and the material of the second electrode layer are materials with high transmittance and high conductivity in terahertz wave bands; the materials of the first electrode layer and the second electrode layer are 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, the 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 terahertz super-structure lens is 0.5-2.5 THz, the difference (double refractive index delta n) between the first refractive index and the second refractive index is 0.2-delta n-0.4; the liquid crystal layer is oriented at 45 ° to the x-axis direction. The greater the birefringence, the better, the smaller the thickness required for greater birefringence, and the faster the threshold voltage and response speed of the power up, because a sufficiently large liquid crystal layer thickness is required to meet the 1THz half-wave condition.
Further, the terahertz super-structure lens further comprises spacer particles positioned between the substrate and the dielectric layer, wherein the spacer particles are used 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 300 to 500 μm when the frequency of the incident light is in the range of 0.5 to 2.5 THz; under the condition of meeting 1THz half-wave condition, the thickness of the liquid crystal layer is preferable according to the 1THz half-wave condition, and when the condition is met, the linearly polarized light in the incident x direction of 1THz can be completely converted into y polarization.
The invention also provides a preparation method of the terahertz super-structured lens with the adjustable focal length, 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, and enabling the super-structure surface to be positioned on one side far away from the substrate, wherein the first photo-alignment layer faces the substrate, and the second photo-alignment layer faces the dielectric layer;
exposing the photo-alignment layer to ultraviolet polarization to form a control pattern having a molecular director at 45 ° to the transverse or longitudinal axis; the exposure treatment is to expose both the first photoalignment layer and the second photoalignment layer.
Liquid crystal material is poured between the substrate and the dielectric layer, and the director of the control pattern control liquid crystal molecules is 45 degrees with the transverse axis direction or the longitudinal axis 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 10-30 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 terahertz super-structure lens in terahertz polarization imaging. The lens focal length of the terahertz super-constructed lens is related to the applied voltage and the incident linear polarization direction, so that the terahertz super-constructed lens can be applied to THz polarization imaging, for example, two objects emitting THz waves with different polarization states can be imaged by using the lens, and the positions of the objects can be judged through the depth of field.
Terahertz waves refer to electromagnetic waves having a frequency between 0.1 and 10THz (corresponding wavelength of 30-3000 μm). Terahertz waves have some of 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. In recent years, the super-structured surface is beginning to be increasingly used in the field of terahertz modulation. However, once the super-structured surface is prepared, the structure is fixed, the functions are fixed, and dynamic regulation and control cannot be performed, so that the searching for the tunability of the super-structured surface becomes a big hot spot in the research field. In recent years, liquid crystal materials have been widely used for the development of tunable terahertz modulators, such as phase shifters, wave plates, vortex light generators, and the like, due to their broad-band electrically controlled birefringence characteristics. The orientation distribution of the liquid crystal can be arbitrarily controlled by a photo-orientation technology, so that the method is very suitable for designing and manufacturing terahertz wave plates. Transparent electrode materials in terahertz wave bands, such as graphene, PEDOT, ITO nanowhiskers and the like, have also been developed, and necessary conditions are provided for electric field regulation of liquid crystal elements.
Liquid crystals have broadband birefringence from visible light to microwaves and excellent electro-optic response characteristics, and are widely used in various optical fields other than display, including special beam generation, tunable filters, spatial light modulators, and the like. Currently, two major obstacles limiting the application of liquid crystals in the terahertz frequency band have been solved, namely: transparent terahertz electrode materials and extremely thick liquid crystal layers (up to hundreds of micrometers) are adopted, so that terahertz liquid crystal devices are greatly developed. The uniformly oriented liquid crystal can be used as a terahertz phase shifter and a wave plate. By encoding the geometric phase into the liquid crystal director distribution, more functions such as vortex light generators, beam splitters, etc. can be obtained. These devices can be dynamically tuned using an electric field. If the terahertz liquid crystal element is integrated with the super-structured surface, the terahertz super-structured lens with tunable focal length can be realized, and the actual application development of terahertz spectrum and an imaging system is greatly promoted.
The principle of the invention: the invention can be divided into two parts in principle, wherein the first part is the polarization-dependent dielectric super-structure surface for realizing different lens focal lengths under the incidence of x polarization and y polarization, the second part is the liquid crystal terahertz wave plate for realizing the dynamic switching of the incidence of the x polarization and the y polarization under the condition of power-on, and the functions can be realized by integrating the two parts. The dynamic switching of the focal length of the lens can be realized through the integration of the polarization multiplexing super-structure lens and the electroalignment liquid crystal wave plate. According to the invention, by designing the structural parameters of the super-structured surface to generate the needed polarization dependent resonance phase, different focusing focal lengths under the x-direction and y-direction incident ray polarization terahertz waves are realized, and simultaneously, the electric control liquid crystal half wave plate is overlapped to realize polarization orthogonal conversion, so that the change of different focal lengths under the conditions of no power-up and saturation voltage is realized. The dynamic switching of the functions ensures that the multifunctional adjustable terahertz super-structured lens is realized, and solves the technical problems of single functions and application of the adjustable terahertz functional device in the prior art.
The beneficial effects are that: in contrast to the prior art, the method has the advantages that,
(1) The invention solves the defects of single-layer super-surface wave front modulation function and incapability of tuning, and has higher modulation efficiency;
(2) According to the terahertz lens with the adjustable focal length and the preparation method thereof, the terahertz super-structured lens with the adjustable focal length can be realized by integrating the liquid crystal wave plate with uniform orientation and the dielectric super-structured surface based on the resonance phase. When no voltage is applied, the liquid crystal wave plate converts incident linear polarization terahertz waves in the x direction into linear polarization in the y direction, and then focuses to a focal length after passing through the super-structured surface lens; 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 wave plate modulation disappears, the terahertz waves still polarized in the x direction are emitted from the wave plate, and the focusing focal length of the super-structure lens is changed due to the polarization dependence of the super-structure surface. 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.
(3) The terahertz super-structured lens has the characteristics of miniaturization, easiness in integration and thinness, can dynamically switch different focal lengths 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 focus-adjustable 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; the horizontal axis direction is the x-axis direction, and the vertical axis direction is the y-axis direction;
FIG. 3 is a graph of the change of resonant phase with frequency under incidence of x-ray polarization and y-ray polarization and a cross-sectional view of normalized magnetic field intensity distribution in a respective silicon pillar for a structural unit (80 μm long, 60 μm wide, 200 μm high, 150 μm periodic) of a dielectric superstructural surface provided by an embodiment of the present invention;
FIG. 4 is a graph showing the distribution of the terahertz phase of the outgoing of the structural unit of the super-structured surface of the medium according to the embodiment of the invention, the frequency is 1THz, and the incident polarization is x-direction linear polarization;
FIG. 5 is a graph showing the distribution of the terahertz transmittance of the structural unit of the super-structured surface of the medium according to the embodiment of the invention, wherein the frequency is 1THz, and the incident polarization is x-direction linear polarization;
FIG. 6 is a diagram of a design lens phase template with a dielectric super-structured surface with a focal length of 12.0mm under x-direction linear polarization incidence provided in an embodiment of the present invention;
FIG. 7 is a diagram of a design lens phase template with a focal length of 16.0mm for a dielectric superstructural surface with linear polarization incidence in the y-direction provided in an embodiment of the invention;
FIG. 8 is an overall photomicrograph of a media super-structured surface provided in an embodiment of the invention;
FIG. 9 is a photograph of a localized area of a media super-structured surface under a scanning electron microscope provided in an embodiment of the present invention;
FIG. 10 is a schematic diagram of a focusing effect of a dielectric super-structured surface at 1THz, with left and right terahertz far-field intensity distribution diagrams for x-ray polarization and y-ray polarization incidence, respectively, provided by an embodiment of the present invention;
FIG. 11 is a graph showing the phase retardation of an electrically tunable liquid crystal wave plate with frequency without power up according to an embodiment of the present invention;
FIG. 12 is a graph showing conversion efficiency of outgoing y-ray polarization of an electrically tunable liquid crystal wave plate under the condition of incident x-ray polarized terahertz waves, where black lines and red lines represent the condition of no voltage and saturated voltage applied to the wave plate, respectively;
FIG. 13 is a graph of experimental measurement of focusing effect of an ultra-structured lens with adjustable focal length at 1THz, wherein incident polarization is x-ray polarization, and the left and right are terahertz far-field intensity distribution diagrams under the condition of no voltage and saturated voltage on a wave plate respectively;
fig. 14 is a schematic diagram of steps in a preparation method of a focal length-adjustable terahertz lens according to an embodiment of the present invention;
fig. 15 is a schematic flow chart of a preparation method of a terahertz lens with adjustable focal length according to an embodiment of the invention.
Detailed Description
The present invention will be described in further detail with reference to examples.
The terahertz super-structured lens with adjustable focal length of the embodiment integrates liquid crystal and super-structured surface, and the cross section of the lens is shown in fig. 1, and the lens comprises a substrate 6, a dielectric layer 1, a first electrode layer 2, a second electrode layer 5, a first orientation layer 3, a second orientation layer 4 and a liquid crystal layer 7 which are oppositely arranged; 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 layer 1 is provided with the first electrode layer 2 and the first orientation layer 3 in turn towards one side of the substrate 6, the substrate 6 is positioned below the dielectric layer 1, the substrate 6 is provided with the second electrode layer 5 and the second orientation layer 4 in turn towards one side of the dielectric layer 1, and the liquid crystal layer 7 is arranged between the first orientation layer 3 and the second orientation layer 4. The liquid crystal layer 7 includes liquid crystal molecules 9 and spacers 10 at both ends of the liquid crystal molecules 9, and circular structures of both sides of the liquid crystal molecules as shown in fig. 1 are spacers to form a liquid crystal cell of a fixed thickness.
In this 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 graphene materials, and the orientation layers are 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 an embodiment, the period of the structural unit is set to 150 μm, the height of the silicon column is set to 200 μm, and the length-width parameters of the silicon column are determined according to the resonance phases required for different positions.
The terahertz super-structured lens of the embodiment can be divided into two parts in principle, wherein the first part is a polarization-dependent dielectric super-structured surface for realizing different lens focal lengths under the incidence of x polarization and y polarization; the second part is a liquid crystal terahertz wave plate which can realize the dynamic switching of incident x polarization and y polarization under the condition of power-on, and the functions can be realized by integrating the incident x polarization and the y polarization.
First analyze the first part: 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 super-structured surface of the medium in the embodiment is a rectangular silicon cylinder, the length is 20-120 mu m, the width is 20-120 mu m, the height is 200 mu m, and the period is 150 mu m. When terahertz waves are incident in a direction perpendicular to the super-structured surface of the medium, each dielectric pillar can be regarded as one dielectric waveguide. The phase of the transmitted wave can be given by the following formula:wherein λ is the wavelength, n eff And h are the effective refractive index and propagation distance (height of the dielectric pillar) of the waveguide mode, respectively. Based on this model, the transmission phase increases linearly with frequency (c/λ). Due to asymmetry of rectangular column length and width parameters, n under incidence of x-ray polarization and y-ray polarization eff Slightly different, which enables polarization multiplexed wavefront phase modulation.
Waveguide mode responses of structural units (80 μm long, 60 μm wide, 200 μm high, 150 μm periodic) of one specific structural parameter were simulated using commercial software, lumerical FDTD, as shown in FIG. 3. It can be seen that the resonant phase varies approximately linearly with frequency at x-and y-polarization incidence, and that there is a certain phase difference at 1THz due to the difference in length and width parameters. Also shown is a cross-sectional view of normalized magnetic field intensity distribution within a silicon column at x-ray and y-ray polarized incidence, revealing different resonant modes within the column. In order to realize terahertz wave front phase regulation and control related to arbitrary polarization at 1THz, the height of a fixed dielectric column is 200 mu m, the period is 150 mu m, the length and width parameters are changed to be 20-120 mu m respectively, the normalized phase distribution of the emergent terahertz wave under the condition of incident x-ray polarization is obtained through simulation, and the range of the structural parameters completely covers the phase of 0-2 pi as shown in figure 4. The normalized phase distribution of the outgoing terahertz wave under the incident y-linear polarization can be obtained by symmetrically operating fig. 4 along a diagonal line of length=width. The phase distribution diagram of the X-ray polarization and the Y-ray polarization can be designed independently by using the emergent phase distribution diagram of different length and width parameters, and the ultra-structured surface of different length and width parameters at different positions can be constructed by searching the length and width parameters at specific positions in the phase distribution diagram to minimize the phase difference required by each position of the phase distribution diagram, so that the polarization independent phase regulation and control effect can be realized. To optimize efficiency, we also simulated the corresponding exit terahertz transmittance profile, as shown in fig. 5. In matlab codes for screening structural parameters, the light transmittance criterion of each dielectric column is made to be larger than 0.7, so that a high-efficiency terahertz phase regulating device can be realized.
This embodiment designs lens phase templates with focal lengths of 12.0mm and 16.0mm for x-ray polarization and y-ray polarization, respectively. The required lens phase is shown in the following equation:
where i and j represent x-and y-linearly polarized terahertz incidence, respectively, and f is the designed focal length. Fig. 6 and 7 are normalized phase templates of designed 12.0mm and 16.0mm lenses, respectively, with periodic phase variations of 0-2 pi gradually narrowing from center to edge along the radial direction, the entire phase region being 1cm x 1cm in size.
According to the design above, a dielectric super-structured lens sample was prepared in this example. The sample photograph is shown in fig. 8, and the whole structure array can be seen to be annularly distributed; further, the local morphology of the sample is observed by a scanning electron microscope, as shown in fig. 9, the rectangular silicon column array is clearly visible, the length and width parameters of different areas are different according to the design, each silicon column edge presents a certain round angle, and a certain error exists with the designed perfect rectangle, which inevitably causes measurement errors in the subsequent measurement process.
For the terahertz super-structured lens, the focusing effect can be simulated by utilizing FDTD electromagnetic field simulation. Since the lens phases are symmetrically distributed along each diameter, only one diametric configuration needs to be simulated, which can greatly reduce the simulation time. As shown in FIG. 10, the simulation graph of the focusing effect of the super-structured surface of the medium at 1THz provided by the embodiment of the invention has obvious focusing effect when the terahertz far-field intensity distribution graphs of the incident x-ray polarization and y-ray polarization are respectively on the left and right sides. When the linear polarization in the x direction is incident, the intensity of the terahertz electric field in the x direction is detected, the focusing focal length is about 12.5mm, and the focusing focal length is slightly different from the designed 12.0mm; when linear polarization in the y direction is incident, the intensity of the terahertz electric field in the y direction is detected, the focusing focal length is about 15.7mm, and the focusing focal length is slightly different from the designed 16.0 mm. This error may be due to the different structural parameters of the silicon pillars at different locations, and the different phase modulation efficiencies (see fig. 5).
The second part is then analyzed: in order to endow the prepared polarization multiplexing super-structured surface with electro-optical adjustable characteristics, an electroalignment liquid crystal wave plate is introduced. In order to achieve complete conversion of the x-polarization and y-polarization at 1THz, a 1THz liquid crystal half wave plate is required. The liquid crystal wave plate is composed of a layer of uniformly oriented liquid crystal layer and terahertz transparent electrode layers on the upper substrate and the lower substrate. Thanks to high electrical conductivityThe graphene is selected as a transparent electrode material in the embodiment. The alignment direction of the liquid crystal layer was 45 ° to the x-direction. When the incident x-ray polarized terahertz wave passes through the liquid crystal wave plate, the terahertz e-light and the o-photoelectric field component respectively sense the two refractive indexes n of the liquid crystal e And n o . A phase delay is generated between the two componentsWhere Δn represents the birefringence (difference between extraordinary refractive index and ordinary refractive index) of the liquid crystal, and d is the thickness of the liquid crystal layer. When at 1 THz->When the half-wave condition is met, orthogonal conversion of linear polarization can be realized, so that the thickness of the liquid crystal layer is required to be reasonably designed according to the applicable wavelength and the birefringence of the liquid crystal. In this example, the applicable wavelength is 1THz, the birefringence of the liquid crystal layer is about 0.32, and the thickness d of the liquid crystal layer is set to 450 μm.
When the electrodes on both sides of the liquid crystal layer are energized, the alignment direction of the liquid crystal is gradually deflected toward the energized direction, the refractive index difference perceived by the e-light and o-field components is gradually reduced, and the modulating effect on the orthogonal linear polarization deflection is gradually weakened. When the voltage reaches the saturation voltage greatly, the direction of the liquid crystal layer is completely aligned along the electric field direction (vertical to the direction of the substrate), the wave plate modulation effect is completely lost, and the incident x-ray polarization terahertz wave directly penetrates without modulation.
The liquid crystal wave plate is prepared and the performance of the liquid crystal wave plate is characterized by a terahertz time-domain spectroscopy system (THz TDS). Fig. 11 is a graph of the phase retardation with frequency under the condition of no power-up of the electrically tunable liquid crystal wave plate provided by the embodiment of the invention, and it can be seen that the phase retardation gradually increases linearly with frequency, and is about pi at 1THz, so that the electrically tunable liquid crystal wave plate well meets the design requirements. FIG. 12 is a graph showing conversion efficiency of emergent y-ray polarization of an electrically tunable liquid crystal wave plate under incident x-ray polarized terahertz wave, wherein black and red lines represent the non-applied voltage and saturated voltage of the wave plate, respectivelyIs the case in (a). When not powered, the x-ray polarization incident at 1THz is completely converted to y-ray polarization; while a saturation voltage (150V) rms 1kHz square wave ac signal), the polarization conversion efficiency at 1THz is 0, indicating that all polarization components exiting are incident x-ray polarization.
The dynamic switching of the focal length of the lens can be realized by integrating the polarization multiplexing super-structure lens and the electroalignment liquid crystal wave plate; and the focusing effect of the liquid crystal integrated super-structure lens can be characterized by utilizing a method for generating and detecting terahertz waves based on the photoconductive antenna. Fig. 13 is a graph of experimental measurement of focusing effect of an ultra-structured lens with adjustable focal length at 1THz, wherein incident polarization is x-ray polarization, and terahertz far field intensity distribution diagrams under the condition that no voltage is applied to a wave plate and saturated voltage is applied to the wave plate are left and right, so that obvious focal length change can be seen. At 0V rms And 150V rms The lower focal lengths are 14.9mm and 11.4mm respectively, and the designed 16.0mm and 12.0mm have larger errors. This is mainly due to the super-structured surface preparation errors (structural unit fillet errors, etch depth errors).
Fig. 14 is a schematic diagram of a preparation flow of the terahertz super-structured lens according to this embodiment, and fig. 15 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 the side surface;
the process for preparing the super-structured surface of the medium 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;
and 124, performing ultraviolet polarization exposure on the photo-alignment layer to form a control pattern with molecular directors at 45 degrees to the x direction.
And step 125, pouring the liquid crystal material between the substrate and the medium layer, and controlling the pattern to control the liquid crystal molecular directors to be uniformly distributed at 45 degrees with respect to the x direction. Wherein the horizontal axis direction is the x-axis direction, and the vertical axis direction is the y-axis direction.
The lens prepared by the embodiment is mainly formed by laminating a layer of medium super-structured surface and a layer of liquid crystal layer. The dielectric super-structure surface consists of rectangular silicon column arrays of sub-wavelength structural units, the length and width dimensions of the silicon column units are different at different positions, the polarization dependence on the phase regulation of incident terahertz waves is achieved, and the incident linear polarized terahertz waves in the x direction and the y direction have different focusing focal lengths after passing through the super-structure surface. The directors of liquid crystal molecules in the liquid crystal layer are uniformly distributed in a plane perpendicular to the incidence direction of the terahertz waves and linearly polarized to 45 degrees with the incidence x-direction, and the directors can be used as half-wave plates under specific terahertz wavelengths to realize orthogonal conversion of the incidence ray polarization. When the incident x-ray polarization is performed, the incident x-ray polarization is converted into y-ray polarization through the liquid crystal layer, and then focusing is performed through the super-structured surface of the medium; when the electrodes on the two sides of the liquid crystal layer are powered on, the liquid crystal directors are arranged perpendicular to the substrate, the half-wave plate modulation effect disappears, the emergent polarization is still the incident x-ray polarization, and the emergent polarization is focused to another position through the super-structure surface, so that the dynamic change of the focal length is realized.
The lens focal length of the terahertz super-constructed lens is related to the applied voltage and the incident linear polarization direction, and is applied to THz polarization imaging, two objects emitting THz waves with different polarization states can be imaged by the lens, and the positions of the objects can be judged through the depth of field.
Claims (9)
1. A terahertz super-structure lens with adjustable focal length is 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, a first electrode layer and a first orientation layer are sequentially arranged on one side of the dielectric layer, which faces the substrate, and a second electrode layer and a second orientation layer are sequentially arranged on one side of the substrate, which faces the dielectric layer;
the dielectric super-structure surface layer comprises an anisotropic dielectric column structure, dielectric columns are distributed in an annular array, the dielectric columns have a transverse axis length and a longitudinal axis length, the transverse axis length and the longitudinal axis length of the dielectric columns in the same radius ring are the same, and the transverse axis length and the longitudinal axis length of the dielectric columns in different radius rings are different;
the first alignment layer and the second alignment layer have the same alignment direction, the alignment directions are all aligned uniformly in a plane and are distributed at 45 degrees with the transverse axis or longitudinal axis direction so as to induce the alignment of liquid crystal molecules; the liquid crystal layer is arranged between the first alignment layer and the second alignment layer, and the alignment direction of liquid crystal molecules in the liquid crystal layer is uniform alignment in a plane and forms 45 degrees with the transverse axis or the longitudinal axis;
when the incident terahertz wave passes through the dielectric column, the resonance phase generated by the dielectric column satisfies the following formula:
wherein x is the position coordinate of the horizontal axis direction and y is the position of the vertical axis directionThe coordinates are set up and the position of the coordinates is changed,f i for the focal length of the lens upon incidence of the x-polarized terahertz wave,f j for the focal length of the lens upon incidence of the y-polarized terahertz wave,λis the wavelength of the terahertz wave,f i,j representation off i Or (b)f j Namely, each dielectric pillar simultaneously satisfies the x-polarization phase and the y-polarization phase;
the terahertz super-structure lens is divided into two parts, wherein the first part is a polarization-dependent dielectric super-structure surface for realizing different lens focal lengths under the incidence of x-polarized terahertz waves and y-polarized terahertz waves; the second part is a first electrode layer, a first orientation layer, a liquid crystal layer, a second electrode layer and a second orientation layer, and dynamic switching of incident x-polarized terahertz waves and y-polarized terahertz waves is realized under the condition of power-on.
2. The focal length adjustable terahertz super-structured lens according to claim 1, wherein: the dielectric column structure is a rectangular dielectric column, when the incident terahertz wave frequency is 1THz, the length of the dielectric column is 20-120 mu m, the width is 20-120 mu m, the height is 150-250 mu m, and the array period is 100-200 mu m.
3. The focal length adjustable terahertz super-structured lens according to claim 1, wherein: the substrate is made of quartz or polyimide, and the dielectric layer is made of high-resistance silicon; the first electrode layer and the second electrode layer are made of few layers of graphene, PEDOT or ITO nanowhiskers, and the first orientation layer and the second orientation layer are photo-control orientation layers.
4. The focal length adjustable terahertz super-structured lens according to 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 terahertz super-structure lens is 0.5-2.5 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 focal length adjustable terahertz super-structured lens according to claim 1, wherein: the liquid crystal display device further comprises spacer particles positioned 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 0.5-2.5 THz, the thickness of the liquid crystal layer is 300-500 mu m.
6. A method for preparing the terahertz super-structure lens in accordance with any one of claims 1 to 5, which is characterized by comprising the following steps:
providing a substrate and a dielectric layer with a super-structure surface layer on the side surface;
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, and enabling the super-structure surface to be positioned on one side far away from the substrate, wherein the first photo-alignment layer faces the substrate, and the second photo-alignment layer faces the dielectric layer;
exposing the photo-alignment layer to ultraviolet polarization to form a control pattern having a molecular director at 45 DEG to the x-axis direction;
liquid crystal material is poured between the substrate and the dielectric layer, and the director of the control pattern control liquid crystal molecules is 45 degrees with the x-axis direction.
7. The method for manufacturing a focus-adjustable terahertz super-structure lens according to claim 6, wherein the step of manufacturing a dielectric super-structure surface layer includes: 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.
8. The method for preparing the terahertz super-structured lens with adjustable focal length according to claim 6, which is characterized in that: preparing the light-operated orientation layer on the substrate and the medium layer further comprises pretreatment of the surfaces of the substrate and the medium layer before preparation; wherein the pretreatment step comprises the following steps: and ultrasonically cleaning the substrate and the dielectric layer for 10-30 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 terahertz super-structured lens of any one of claims 1 to 5 in terahertz polarization imaging.
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| CN114200669B (en) * | 2021-12-06 | 2023-07-21 | 清华大学深圳国际研究生院 | Multi-polarization-state multifunctional super-structure lens based on spatial multiplexing and manufacturing method thereof |
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| CN114397725A (en) * | 2022-01-13 | 2022-04-26 | 天津大学 | Terahertz metamaterial lens capable of directly converting unpolarized waves into circularly polarized waves |
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