CN111399261A - Focal length-adjustable terahertz super-structured lens and preparation method and application thereof - Google Patents
Focal length-adjustable terahertz super-structured lens and preparation method and application thereof Download PDFInfo
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Abstract
The invention discloses a terahertz super-structure lens with adjustable focal length and a preparation method and application thereof, and the terahertz super-structure lens comprises a substrate, a dielectric 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 a dielectric super-structure surface layer is arranged on one side of the dielectric layer, which is far away from the substrate, and the liquid crystal layer is arranged between the first orientation layer and the second orientation layer; the first alignment layer and the second alignment layer have the same alignment direction, are uniformly aligned in a plane, and form an angle of 45 degrees with the horizontal 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. According to the invention, the dynamic switching of the focal length of the lens can be realized through the integration of the polarization multiplexing super-structured lens and the electrically-tunable liquid crystal wave plate, the dynamic switching of the function realizes the multifunctional tunable terahertz super-structured lens, and the technical problem that the function and the application of a terahertz device are single in the prior art is solved.
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 adjustable focal length and a preparation method and application thereof.
Background
Currently, fifth generation (5G) wireless networks have been gradually popularized around the world, and there is a need to accelerate the fundamental research of next generation (6G) communications. The requirement of the 6G system on channel capacity far exceeds the requirement of the current, so that the terahertz (THz) frequency band with higher carrier frequency than a radio frequency band has great application prospect. The terahertz band is the last band in the electromagnetic spectrum that has not been fully explored, the so-called "terahertz gap", and many terahertz devices are far from mature.
Lenses are important components for beam coupling, focusing and collimation in communication systems. Conventional terahertz lenses are generally large in size, such as single-convex spherical lenses. Unlike the conventional phase accumulation method, the meta-lens (metalens) can introduce an abrupt phase for wavefront manipulation by designing a sub-wavelength metallic or dielectric resonant cell. The super-structure lens reported so far has realized a plurality of functions such as spin selection focusing, broadband achromatic focusing, super-resolution focusing and the like. Compared with a metal super-structure surface, the medium super-structure surface has higher modulation efficiency and manufacturing performance compatible with CMOS, so that the medium super-structure surface has wider application prospect. The function of a common super-structured lens is static and cannot meet the requirements of various practical applications. In recent years, the active tuning of a super-structured surface by combining the super-structured surface with functional materials such as semiconductors, graphene, phase change materials, superconductors and the like has become a great hot point of research. However, the structural elements of most tunable superstructure surfaces are uniform, so that their function is limited to spectral tuning, rather than dynamic wavefront control; further developing tunable wavefront regulation devices, such as adjustable lenses, dynamic beam deflectors and the like, has important practical significance.
Disclosure of Invention
The purpose of the invention is as follows: one of the purposes of the invention is to provide a terahertz super-structure lens with an adjustable focal length, which can realize dynamic tuning of different focal lengths under a power-up condition so as to solve the technical problem that the terahertz lens in the prior art is single in function and application; 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 aims to provide application of the terahertz metamaterial lens with the adjustable focal length.
The technical scheme is as follows: the terahertz metamaterial 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 arranged oppositely, a dielectric metamaterial 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, a second electrode layer and a second orientation layer are sequentially arranged on one side of the substrate, which faces the dielectric layer, and the liquid crystal layer is arranged between the first orientation layer and the second orientation layer;
the medium super-structure surface layer comprises an anisotropic medium column structure, medium columns are distributed in an annular array, the medium columns have the length of a transverse shaft and the length of a longitudinal shaft, the length of the transverse shaft and the length of the longitudinal shaft of different medium columns in the same radius ring are the same along the angular direction, and the length of the transverse shaft and the length of the longitudinal shaft of the medium 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 uniformly aligned in a plane and are distributed at 45 degrees with the direction of a transverse axis or a longitudinal axis to induce liquid crystal molecules to be aligned;
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 in plane and is 45 degrees with the horizontal axis or the vertical axis.
The length and width parameters of the medium column are determined according to the lens phase required by different positions, and the height and the period of the medium column are not changed at different positions.
Further, when the incident terahertz wave is transmitted through the dielectric pillars, a resonance phase is generated by each of the dielectric pillars
The following lens phase formula requirements are satisfied:
wherein x is the coordinate of the position in the horizontal axis direction, y is the coordinate of the position in the vertical axis direction, and fiIs the focal length of the lens when incident x-polarized terahertz waves, fjThe focal length of the lens when the y-polarization terahertz waves are incident, and lambda is the wavelength of the terahertz waves; f. ofi，jDenotes fiOr fjI.e., each dielectric cylinder satisfies both the x-polarization phase and the y-polarization phase.
The substrate is made of a material with high transmittance in a terahertz wave band for enhancing the modulation efficiency of terahertz waves and an ultraviolet wave band for irradiating an alignment layer on the inner side of the substrate with ultraviolet illumination alignment energy; preferably, the material of the substrate includes quartz, polyimide, or the like. The material used by the dielectric layer is a material with high refractive index and high transmittance in the terahertz waveband, and preferably high-resistance silicon. Wherein the resistivity of the high-resistance silicon is 5000-8000 omega-cm.
The super-structured surface is formed by periodically arranging artificially prepared sub-wavelength basic structure units, and can regulate and control the phase, amplitude and polarization of incident electromagnetic waves. The structural units of the dielectric super-structure surface layer are anisotropic columns, such as elliptical dielectric columns, rectangular dielectric columns and the like; and parameters of length, width and height of the medium columns at different positions in the super-structure surface structure unit and periodic parameters of the structure unit can be determined through the early-stage simulation design. The orientation layer can adopt a light control orientation layer or a friction orientation layer.
Preferably, the medium column structure on the medium super-structure surface is a rectangular medium column; when the length and width parameters of the dielectric cylinder are different, the dielectric cylinder has different effective refractive indexes for incident terahertz waves with electric field vectors along the long direction (x linear polarization) and the electric field vectors along the wide direction (y linear polarization), so that the resonance phases of x and y polarization emergent waves are different, and the metamaterial surface has polarization-dependent characteristics. Structural parameters of the surface of the super structure can be optimized according to the terahertz frequency of the target, so that the regulation efficiency is optimized, and under 1THz, the optimal parameters of the dielectric column array are as follows: the length is 20-120 μm, the width is 20-120 μm, the height is 150-250 μm, and the period is 100-200 μm; by using the design principle of the polarization-dependent super-structured surface, lenses with different focal lengths under x polarization and y polarization can be designed and prepared.
The material of the first electrode layer and the material of the second electrode layer are materials with high transmittance and high conductivity in a terahertz waveband; the materials of the first electrode layer and the second electrode layer are few-layer graphene, PEDOT, ITO nanowhiskers and the like, and few-layer graphene is preferable. The first orientation layer and the second orientation layer are both light-operated orientation layers, control patterns of the light-operated orientation layers are erasable, and the light-operated orientation layers are made of azo dyes.
Preferably, the liquid crystal material of the liquid crystal layer is a birefringence material, and has a first refractive index and a second refractive index; when the frequency range of incident light to the terahertz metamaterial lens is 0.5-2.5 THz, the difference value (birefringence index delta n) between the first refractive index and the second refractive index is more than or equal to 0.2 and less than or equal to 0.4; the liquid crystal layer was oriented at 45 ° to the x-axis direction. The larger the birefringence, the better, because a sufficiently large liquid crystal layer thickness is required to satisfy the 1THz half-wave condition, the smaller the thickness required for the larger the birefringence, and the faster the threshold voltage and response speed of energization.
Further, the terahertz metamaterial lens also comprises spacing particles positioned between the substrate and the dielectric layer, wherein the spacing particles are used for supporting the substrate and the dielectric layer to form a filling space of the liquid crystal layer; preferably, when the incident light frequency range is 0.5 to 2.5THz, the thickness of the liquid crystal layer is 300 to 500 μm; under the condition that 1THz half-wave conditions are met, the thickness of the liquid crystal layer is preferably selected according to the 1THz half-wave conditions, and when the conditions are met, the linear polarized light in the x direction can be completely converted into y polarization only when the 1THz incident light is incident.
The invention also provides a preparation method of the terahertz metamaterial 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 photo-alignment layer on the surface of one side, which is not provided with the super-structure surface layer, of the dielectric layer;
sequentially preparing a second electrode layer and a second photoalignment layer on one side surface of the substrate;
arranging spacing particles on one side of a second photoalignment layer of the substrate, and then packaging the dielectric layer and the substrate to enable the substrate and the dielectric layer to be oppositely arranged, wherein the super-structure surface is positioned on one side far away from the substrate, the first photoalignment layer faces the substrate, and the second photoalignment layer faces the dielectric layer;
carrying out ultraviolet polarization exposure on the light control orientation layer to form a control pattern with a molecular director forming 45 degrees with the direction of a transverse axis or the direction of a longitudinal axis; the exposure treatment is carried out on both the first photoalignment layer and the second photoalignment layer.
Liquid crystal material is poured between the substrate and the dielectric layer, and the control pattern controls the liquid crystal molecular director to form 45 degrees with the horizontal axis direction or the longitudinal axis direction.
The preparation method of the dielectric ultra-structure surface layer comprises the following steps: and cleaning the high-resistance silicon wafer, transferring the pattern on the mask onto the silicon wafer by utilizing a photoetching process, etching the exposed silicon by utilizing plasma until the etching is finished to the target depth, stopping etching, and finally washing off the residual photoresist.
The preparation of the photoalignment 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 comprises the following steps: the substrate and the dielectric layer (such as a silicon wafer) are ultrasonically cleaned for 10-30 minutes by using a cleaning solution, then ultrasonically cleaned twice by using ultrapure water, each time lasts for 8-10 minutes, then dried in an oven at 100-120 ℃ for 40-60 minutes, and finally cleaned for 30-45 minutes by using ultraviolet ozone.
The invention also provides application of the terahertz metamaterial lens in terahertz polarization imaging. The lens focal length of the terahertz metamaterial lens is related to the applied voltage and the incident linear polarization direction, so that the terahertz metamaterial lens can be applied to THz polarization imaging, for example, two objects emitting THz waves in different polarization states can be imaged by 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 0.1-10THz (corresponding to a wavelength of 30-3000 μm). Terahertz waves have some unique properties: 1) the low photon energy is suitable for biopsy of biological tissues; 2) skeleton vibration and rotation energy levels of a plurality of biological molecules and condensed substances and intermolecular interaction energy levels (such as hydrogen bonds) are in a terahertz frequency band; 3) many non-metal and non-polar materials have small absorption to terahertz waves and high transmittance; 4) compared with visible light and infrared light, the terahertz wave has extremely high directivity and strong cloud penetration capacity, and can realize wireless transmission rate of more than Gbit/s. The terahertz technology has wide application prospect in various fields such as safety inspection, biomedicine, high-speed wireless communication and the like. Terahertz lenses have wide application in these fields, and they are generally composed of crystals and polymers, have large volume and no tunability, and limit the integration of terahertz systems.
The metamaterial is an artificial electromagnetic medium, and can realize some unique performances which natural materials do not have, such as artificial magnetism, negative index materials, electromagnetic stealth and the like, by artificially designing a unit structure. The super-structure surface is a two-dimensional form of a super-structure material, and the design and the processing are more convenient. In recent years, the nanostructured surface has started to be increasingly used in the field of modulation of terahertz. However, once the surface of the super-structure is prepared, the structure is fixed, the function is fixed, and dynamic regulation and control cannot be performed, so that the search for tunability of the surface of the super-structure becomes a great hotspot in the research field. In recent years, liquid crystal materials have been widely used for development of tunable terahertz modulators, such as phase shifters, wave plates, vortex light generators, and the like, due to their broadband electrically controlled birefringence characteristics. The directional distribution of the liquid crystal can be arbitrarily controlled by a photo-alignment technology, so that the terahertz wave plate is very suitable for design and manufacture of terahertz wave plates. Transparent electrode materials in the terahertz wave band, such as graphene, PEDOT, ITO nanowhiskers, and the like, have also been developed, and provide necessary conditions for electric field control of liquid crystal elements.
Liquid crystals have wide-band birefringence from visible light to microwave and excellent electro-optic response characteristics, and are widely used in various optical fields other than display, including special light beam generation, tunable filters, spatial light modulators, and the like. At present, two main obstacles limiting the application of liquid crystals in the terahertz frequency band have been addressed, namely: the terahertz liquid crystal device is developed greatly by adopting a transparent terahertz electrode material and a very thick liquid crystal layer (hundreds of microns) photo-alignment technology. The uniformly aligned 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 can be obtained, such as vortex light generators, beam splitters, etc. These devices can be dynamically tuned using electric fields. If the terahertz liquid crystal element is integrated with the super-structure surface, the terahertz super-structure lens with tunable focal length can be realized, and the practical application development of terahertz spectrum and imaging system can be greatly promoted.
The invention principle is as follows: the terahertz wave plate can be divided into two parts in principle, the first part is a polarization-dependent dielectric metamaterial surface and realizes different lens focal lengths under the incidence of x polarization and y polarization, the second part is a liquid crystal terahertz wave plate and realizes the dynamic switching of the incidence x polarization and y polarization under the power-up condition, and the two parts are integrated to realize the function. The dynamic switching of the focal length of the lens can be realized through the integration of the polarization multiplexing super-structured lens and the electrically-adjusted liquid crystal wave plate. The invention generates the required polarization-dependent resonance phase by designing the structural parameters of the surface of the super-structure, realizes different focusing focal lengths under the incident linear polarization terahertz waves in the x direction and the y direction, and simultaneously superposes the electric control liquid crystal half-wave plate to realize polarization orthogonal conversion, thereby realizing the change of different focal lengths under the condition of not adding electricity and adding saturation voltage. The dynamic switching of the function ensures that the multifunctional adjustable terahertz super-structural lens is realized, and the technical problem that the function and the application of an adjustable terahertz functional device in the prior art are single is solved.
Has the advantages that: compared with the prior art, the method has the advantages that,
(1) the invention solves the defects of single function and tuning incapability of single-layer super-surface wavefront modulation, and has higher modulation efficiency;
(2) according to the terahertz lens with the adjustable focal length and the preparation method thereof, the terahertz metamaterial lens with the adjustable focal length can be realized by integrating the uniformly-oriented liquid crystal wave plate and the dielectric metamaterial surface based on the resonance phase. When no voltage is applied, the liquid crystal wave plate converts incident x-direction linearly polarized terahertz waves into y-direction linearly polarized terahertz waves, and the linearly polarized terahertz waves are focused to a focal distance through the super-structure surface lens; when saturation voltage is applied to graphene electrodes on the upper substrate and the lower substrate of the liquid crystal layer, liquid crystal molecules are oriented to be completely vertical to the substrates and arranged, so that wave plate modulation disappears, X-direction linearly polarized terahertz waves are emitted from the wave plates, and due to polarization dependence of the surfaces of the super-structures, the focusing focal length of the super-structure lens is changed. The dynamic switching of the two functions ensures that the multifunctional adjustable terahertz metamaterial lens is realized, and the technical problem that the function and the application of an adjustable terahertz functional device in the prior art are single is solved.
(3) The terahertz super-structure lens has the characteristics of miniaturization, easy integration and lightness, can dynamically switch different focal lengths according to different applicable scenes, and has great application potential in terahertz communication, imaging, sensing and the like.
Drawings
FIG. 1 is a schematic cross-sectional structure diagram of a tunable terahertz lens of the present invention;
FIG. 2 is a schematic representation of the structural elements of the dielectric nanostructured surface of the present invention; wherein, l in the figure represents length, w represents width, h represents height, and T represents period; the direction of the transverse axis is the direction of the x axis, and the direction of the longitudinal axis is the direction of the y axis;
FIG. 3 is a graph of resonance phase variation with frequency for x-and y-linear polarization incidence of structural elements (length 80 μm, width 60 μm, height 200 μm, period 150 μm) of a dielectric nanostructured surface and a cross section of normalized magnetic field intensity distribution in respective silicon columns according to an embodiment of the present invention;
fig. 4 is a distribution diagram of an outgoing terahertz phase of a structural unit of a dielectric metamaterial surface according to an embodiment of the present invention, the distribution diagram is along with length and width parameter changes, the frequency is 1THz, and the incident polarization is x-direction linear polarization;
FIG. 5 is a distribution diagram of the outgoing terahertz transmittance of a structural unit of a dielectric metamaterial surface according to the embodiment of the present invention, which varies with length and width parameters, with a frequency of 1THz and incident polarization of x-direction linear polarization;
FIG. 6 is a diagram of a phase template of a designed lens with a dielectric nanostructured surface at x-direction linearly polarized incidence, with a focal length of 12.0mm, according to an embodiment of the present invention;
FIG. 7 is a diagram of a phase template of a designed lens with a focal length of 16.0mm under a linearly polarized incident in the y-direction on a surface of a dielectric superstructure according to an embodiment of the present invention;
FIG. 8 is an overall photomicrograph of a dielectric microstructured surface provided in an embodiment of the present invention;
FIG. 9 is a photograph under a scanning electron microscope of a localized area of a dielectric microstructured surface provided in an embodiment of the invention;
fig. 10 is a simulation diagram of a focusing effect of a dielectric nanostructured surface at 1THz, where the left and right are distribution diagrams of terahertz far-field intensity incident by x-linear polarization and y-linear polarization, respectively;
FIG. 11 is a diagram showing the variation of phase retardation with frequency of an electrically-tunable liquid crystal wave plate provided in an embodiment of the present invention under the condition of no power-on;
fig. 12 is a graph of conversion efficiency of exit y-linear polarization of an electrically-tunable liquid crystal wave plate under the condition of incident x-linear polarization terahertz waves provided by an embodiment of the present invention, and a black line and a red line respectively represent the conditions under the condition that no voltage is applied to the wave plate and a saturation voltage is applied to the wave plate;
fig. 13 is a focusing effect experimental measurement diagram of the focal length adjustable super-structured lens at 1THz, where incident polarization is x-linear polarization, and the left and right are terahertz far-field intensity distribution diagrams under the condition that no voltage is applied to a wave plate and a saturation voltage is applied to the wave plate, respectively;
fig. 14 is a schematic diagram of steps in a method for manufacturing a terahertz lens with adjustable focal length according to an embodiment of the present invention;
fig. 15 is a schematic flow chart of a method for manufacturing a terahertz lens with an adjustable focal length according to an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to examples.
The terahertz super-structure lens with the adjustable focal length integrates liquid crystal and a super-structure 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 alignment layer 3, a second alignment 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 liquid crystal display panel comprises a medium layer 1, a substrate 6, a first electrode layer 2, a first orientation layer 3, a second electrode layer 5, a second orientation layer 4 and a liquid crystal layer 7, wherein the first electrode layer 2 and the first orientation layer 3 are sequentially arranged on one side of the medium layer 1 facing the substrate 6, the substrate 6 is located below the medium layer 1, the second electrode layer 5 and the second orientation layer 4 are sequentially arranged on one side of the substrate 6 facing the medium layer 1, and the liquid crystal layer 7. The liquid crystal layer 7 includes liquid crystal molecules 9 and spacers 10 at both ends of the liquid crystal molecules 9, and the circular structures on the left and right sides of the liquid crystal molecules as shown in fig. 1 are the spacers, so as to form a liquid crystal cell with a fixed thickness.
In this embodiment, the dielectric layer 1 is made of a high-resistance silicon wafer, the substrate 6 is made of quartz, the electrode layers are made of graphene, and the orientation layers are made of azo dye.
The structural units of the dielectric super-structure surface layer 8 are rectangular silicon columns, and length, width and height parameters of the silicon columns at different positions in the super-structure surface structural units and the period parameters of the structural units can be determined through early-stage simulation design; in the embodiment, the period of the structural unit is set to 150 μm, the height of the silicon pillar is set to 200 μm, and the length and width parameters of the silicon pillar are determined according to the resonance phases required at different positions.
The terahertz metamaterial lens can be divided into two parts in principle, wherein the first part is a polarization-dependent dielectric metamaterial surface and realizes different lens focal lengths under the incidence of x polarization and y polarization; the second part is a liquid crystal terahertz wave plate which realizes the dynamic switching of incident x polarization and y polarization under the condition of power-up, and the functions can be realized by integrating the two parts.
First, the first part is analyzed: FIG. 2 is a schematic diagram of the structural elements of a dielectric nanostructured surface, where l in FIG. 2 denotes length, w denotes width, h denotes height, and T denotes period; the structural unit of the dielectric super-structure surface of the embodiment is a rectangular silicon cylinder, the length is 20-120 μm, the width is 20-120 μm, the height is 200 μm, and the period is 150 μm. When terahertz waves are incident along the direction vertical to the surface of the dielectric superstructure, each dielectric columnMay be considered a dielectric waveguide. The phase of the transmitted wave can be given by the following equation:
where λ is the wavelength, neffAnd h are the effective index of refraction and the propagation distance (height of the dielectric cylinder) of the waveguide mode, respectively. On the basis of this model, the transmission phase increases linearly with frequency (c/λ). Due to the asymmetry of the length and width parameters of the rectangular prism, n is incident under x-linear polarization and y-linear polarizationeffSlightly different, this makes polarization multiplexed wavefront phase modulation possible.
The waveguide mode response of a structural unit (length 80 μm, width 60 μm, height 200 μm, period 150 μm) of a specific structural parameter is simulated using the commercial software L umetical FDTD, as shown in fig. 3. it can be seen that the resonant phase changes approximately linearly with frequency at the incidence of x-and y-polarization and that there is a certain phase difference at 1THz due to the difference of the length and width parameters. the normalized magnetic field strength distribution cross-section inside the silicon column at the incidence of x-and y-polarization is also shown, revealing the different resonant modes inside the column, in order to achieve terahertz wavefront phase modulation with any polarization dependence at 1THz, we fix the height of the dielectric column at 200 μm and the period at 150 μm, changing the length and width parameters from 20-120 μm, respectively, simulating the normalized phase distribution of the outgoing terahertz wave under the incidence x-linear polarization, as shown in fig. 4, this structural parameter range completely covers the phase distribution under the phase of 0-2 polarization pi, the normalized phase distribution of the phase at the incidence x-linear polarization, simulating the normalized phase distribution of the outgoing terahertz wave under the condition of the incidence x-linear polarization, which we can obtain the normalized phase distribution of the normalized terahertz wave at the same position of the terahertz polarization, which we can obtain the normalized phase distribution of the terahertz wave, which is obtained by the normalized phase distribution of the normalized terahertz polarization, which is obtained by the normalized phase distribution of the model, which is obtained by the normalized phase at the model, which is equal to be found by the normalized linear polarization, which is equal to the.
This embodiment designs lens phase masks with focal distances of 12.0mm and 16.0mm for x-and y-linear polarizations, respectively. The required lens phase is shown in the following equation:
FIGS. 6 and 7 are normalized phase templates of a 12.0mm and 16.0mm lens designed, respectively, with a periodic phase change of 0-2 π becoming progressively denser radially from the center to the edge, with the size of the entire phase region being 1cm × 1 cm.
This example prepared a dielectric superstructural lens sample according to the above design. The sample photograph is shown in fig. 8, and it can be seen that the whole structure array presents a ring distribution; further, the local morphology of the sample is observed by using a scanning electron microscope, as shown in fig. 9, the rectangular silicon pillar array is clearly visible, the length and width parameters of different areas are different according to the design, the edge of each silicon pillar presents a certain rounded corner and has a certain error with the designed perfect rectangle, and thus, the measurement error is inevitably brought in the subsequent measurement process.
For the terahertz super-structured lens, the focusing effect can be simulated by using FDTD electromagnetic field simulation. Because the lens phases are symmetrically distributed along each diameter, only one diameter structure needs to be simulated, which can greatly reduce the simulation time. Fig. 10 is a graph showing a focusing effect simulation of a dielectric metamaterial surface at 1THz, where the left and right distribution diagrams of the incident terahertz far-field intensity of x-ray polarization and y-ray polarization respectively have obvious focusing effects. When the linear polarization is incident in the x direction, the intensity of the emergent terahertz electric field in the x direction is detected, the focusing focal length is about 12.5mm, and a slight error exists between the focusing focal length and the designed 12.0 mm; when the linear polarization is incident in the y direction, the intensity of the emergent terahertz electric field in the y direction is detected, the focusing focal length is about 15.7mm, and a slight error exists between the focusing focal length and the designed 16.0 mm. Such errors may be caused by different phase modulation efficiencies (see fig. 5) due to different structural parameters of the silicon pillars at different positions.
The second fraction is then analyzed: in order to endow the prepared polarization multiplexing super-structure surface with electro-optic adjustable characteristic, an electrically-adjustable liquid crystal wave plate is introduced. In order to achieve complete conversion of the x-and y-polarizations at 1THz, a half-wave plate of liquid crystal at 1THz is required. The liquid crystal wave plate is composed of a layer of uniformly-oriented liquid crystal layer and terahertz transparent electrode layers on an upper substrate and a lower substrate. Due to the high conductivity and the high terahertz transmittance, graphene is selected as the transparent electrode material in the embodiment. The orientation direction of the liquid crystal layer is 45 ° to the x-direction. When incident X-ray polarization terahertz waves pass through the liquid crystal wave plate, the e light and o photoelectric field components of the terahertz waves respectively feel two refractive indexes n of the liquid crystaleAnd no. A phase delay is generated between these two components
Where Δ n denotes a birefringence (a difference between an extraordinary refractive index and an ordinary refractive index) of the liquid crystal, and d is a thickness of the liquid crystal layer. When at 1THz
The half-wave condition is satisfied, and the orthogonal transformation of linear polarization can be realized, so that the thickness of the liquid crystal layer needs to be designed reasonably according to the applicable wavelength and the birefringence of the liquid crystal. In this embodiment, 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 electricity is applied to the electrodes on both sides of the liquid crystal layer, the orientation direction of the liquid crystal gradually deflects towards the electricity application direction, the difference of the refractive indexes sensed by the e light and o photoelectric field components is gradually reduced, and the modulation effect of the linear polarization orthogonal deflection is gradually weakened. When the voltage is greatly up to the saturation voltage, the pointing direction of the liquid crystal layer is completely arranged along the direction of the electric field (perpendicular to the direction of the substrate), the wave plate modulation effect is completely lost, and incident x-ray polarization terahertz waves directly penetrate without modulation.
This example systemThe liquid crystal wave plate is prepared, and the performance of the liquid crystal wave plate is represented by a terahertz time-domain spectroscopy system (THz TDS). Fig. 11 is a graph of the phase retardation of the electrically-tunable liquid crystal wave plate according to the variation with frequency under the condition of no power supply, and it can be seen that the phase retardation gradually increases linearly with frequency, and is about pi at 1THz, which well meets the design requirements. Fig. 12 is a graph of conversion efficiency of outgoing y-linear polarization of an electrically-tunable liquid crystal wave plate under the condition of incident x-linear polarization terahertz waves, where a black line and a red line represent the conditions under the condition of no voltage and saturated voltage applied to the wave plate, respectively. When not powered, the x-ray polarization incident at 1THz is completely converted into y-ray polarization; while applying a saturation voltage (150V)rms1kHz square wave ac electrical signal), the polarization conversion efficiency at 1THz is 0, indicating that all the outgoing polarization components are incident x-ray polarization.
The polarization multiplexing super-structure lens and the electrically-adjusted liquid crystal wave plate are integrated to realize the dynamic switching of the focal length of the lens; and the focusing effect of the liquid crystal integrated super-structure lens can be represented by a method for generating and detecting terahertz waves based on a photoconductive antenna. Fig. 13 is a focusing effect experimental measurement diagram of the focal length adjustable super-structured lens at 1THz, where incident polarization is x-linear polarization, and the left and right distributions are terahertz far-field intensity distribution diagrams under the condition that no voltage is applied to the wave plate and a saturation voltage is applied to the wave plate, respectively, and a significant focal length change can be seen. At 0VrmsAnd 150VrmsThe lower focal lengths are 14.9mm and 11.4mm, respectively, with a large error from the designed 16.0mm and 12.0 mm. This is mainly due to the ultrastructural surface preparation errors (structural unit fillet errors, etch depth errors).
Fig. 14 is a schematic diagram illustrating a manufacturing flow of the terahertz metamaterial lens according to the present embodiment, and fig. 15 is a schematic diagram illustrating steps of a manufacturing method; the preparation method comprises the following steps:
the process for preparing the dielectric nanostructured surface comprises the following steps: and cleaning the high-resistance silicon wafer, transferring the pattern on the mask onto the silicon wafer by utilizing a photoetching process, etching the exposed silicon by utilizing plasma until the etching is finished to the target depth, stopping etching, and finally washing the residual photoresist by utilizing acetone.
the method comprises the following steps of preparing a light-operated orientation film on a substrate, preparing a dielectric layer, and carrying out pretreatment on the substrate and the dielectric layer, wherein the pretreatment comprises the following steps: the substrate and the dielectric layer are ultrasonically cleaned for 10-30 minutes by using a cleaning solution, ultrasonically cleaned twice by using ultrapure water, each time lasts for 8-10 minutes, then dried in an oven at 100-120 ℃ for 40-60 minutes, and finally cleaned for 30-45 minutes by using ultraviolet light and ozone.
and 124, carrying out ultraviolet polarization exposure on the photoalignment layer to form a control pattern with a molecular director forming 45 degrees with the x direction.
And step 125, pouring the liquid crystal material between the substrate and the dielectric layer, and controlling the liquid crystal molecular director to be uniformly distributed at an angle of 45 degrees with the x direction by the control graph. Wherein, the horizontal axis direction is the x-axis direction, and the vertical axis direction is the y-axis direction.
The lens prepared in this example was mainly composed of a layer of dielectric superstructured surface and a layer of liquid crystal layer superimposed. The dielectric superstructure surface is composed of a rectangular silicon column array of sub-wavelength structural units, the length and width of the silicon column units are different at different positions, and the silicon column units have polarization dependence on phase control of incident terahertz waves, so that the incident linearly polarized terahertz waves in the x direction and the y direction have different focusing focal lengths after passing through the superstructure surface. Liquid crystal molecule director in the liquid crystal layer is uniformly distributed in a plane vertical to the incident direction of the terahertz wave and forms 45 degrees with the incident x-direction linear polarization, and the liquid crystal molecule director can be used as a half-wave plate under specific terahertz wavelength to realize orthogonal conversion of incident linear polarization. When the incident X-ray polarization is carried out, the incident X-ray polarization is converted into y-ray polarization through the liquid crystal layer, and the y-ray polarization is focused through the surface of the dielectric super-structure; when electricity is applied to the electrodes on the two sides of the liquid crystal layer, the liquid crystal director is arranged perpendicular to the substrate, the half-wave plate modulation effect disappears, the emergent polarization is still the incident x-ray polarization and is focused to another position through the super-structure surface, and therefore the dynamic change of the focal length is achieved.
The lens focal length of the terahertz super-structure lens is related to the applied voltage and the incident linear polarization direction, and the terahertz super-structure lens is applied to THz polarization imaging, can be used for imaging two objects emitting THz waves in different polarization states, and can judge the positions of the objects through the depth of field.
Claims (10)
1. The utility model provides a focus adjustable terahertz is super constructs lens which characterized in that: the liquid crystal display panel comprises a substrate, a dielectric layer and a liquid crystal layer, wherein the substrate and the dielectric layer are oppositely arranged, and the liquid crystal layer is positioned between the substrate and the dielectric layer; a dielectric super-structure surface layer is arranged on one side of the dielectric layer away from the substrate, a first electrode layer and a first orientation layer are sequentially arranged on one side of the dielectric layer facing the substrate, and a second electrode layer and a second orientation layer are sequentially arranged on one side of the substrate facing the dielectric layer;
the medium super-structure surface layer comprises an anisotropic medium column structure, the medium columns are distributed in an annular array, the medium columns have the length of a transverse shaft and the length of a longitudinal shaft, the lengths of the transverse shaft and the longitudinal shaft of the medium columns in the same radius ring are the same, and the lengths of the transverse shaft and the longitudinal shaft of the medium 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 uniformly aligned in a plane and are distributed at 45 degrees with the direction of a transverse axis or a longitudinal axis to induce liquid crystal molecules to be aligned; 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 in plane and is 45 degrees with the horizontal axis or the vertical axis.
2. The terahertz metamaterial lens with adjustable focal length as claimed in claim 1, wherein: when an incident terahertz wave transmits through the dielectric column, the resonance phase generated by the dielectric column satisfies the following formula:
wherein x is the coordinate of the position in the horizontal axis direction, y is the coordinate of the position in the vertical axis direction, and fiIs the focal length of the lens when incident x-polarized terahertz waves, fjThe focal length of the lens when the terahertz waves with y polarization are incident, and lambda is the wavelength of the terahertz waves.
3. The terahertz metamaterial lens with adjustable focal length as claimed in claim 1, wherein: the dielectric column structure is a rectangular dielectric column, when the frequency of incident terahertz waves is 1THz, the length of the dielectric column is 20-120 mu m, the width of the dielectric column is 20-120 mu m, the height of the dielectric column is 150-250 mu m, and the array period is 100-200 mu m.
4. The terahertz metamaterial lens with adjustable focal length as claimed in 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-layer graphene, PEDOT or ITO nanowhiskers, and the first orientation layer and the second orientation layer are both photo-control orientation layers.
5. The terahertz metamaterial lens with adjustable focal length as claimed in claim 1, wherein: the liquid crystal material of the liquid crystal layer is a birefringence material and has a first refractive index and a second refractive index; when the frequency range of incident light to the terahertz metamaterial lens is 0.5-2.5 THz, the difference value between the first refractive index and the second refractive index is delta n, and delta n is larger than or equal to 0.2 and smaller than or equal to 0.4.
6. The terahertz metamaterial lens with adjustable focal length as claimed in claim 1, wherein: the liquid crystal display panel further comprises spacing particles positioned between the substrate and the dielectric layer, the spacing particles are used for supporting the substrate and the dielectric 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.
7. A preparation method of the terahertz metamaterial lens as claimed in any one of claims 1 to 6, 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 photo-alignment layer on the surface of one side, which is not provided with the super-structure surface layer, of the dielectric layer;
sequentially preparing a second electrode layer and a second photoalignment layer on one side surface of the substrate;
arranging spacing particles on one side of a second photoalignment layer of the substrate, and then packaging the dielectric layer and the substrate to enable the substrate and the dielectric layer to be oppositely arranged, wherein the super-structure surface is positioned on one side far away from the substrate, the first photoalignment layer faces the substrate, and the second photoalignment layer faces the dielectric layer;
carrying out ultraviolet polarization exposure on the light-controlled orientation layer to form a control pattern with a molecular director forming 45 degrees with the direction of the x axis;
liquid crystal material is poured between the substrate and the dielectric layer, and the control graph controls the liquid crystal molecular director to form 45 degrees with the x-axis direction.
8. The method for preparing the terahertz metamaterial lens with adjustable focal length as claimed in claim 7, wherein the step of preparing the dielectric metamaterial surface layer comprises: and cleaning the high-resistance silicon wafer, transferring the pattern on the mask onto the silicon wafer by utilizing a photoetching process, etching the exposed silicon by utilizing plasma until the etching is finished to the target depth, stopping etching, and finally washing off the residual photoresist.
9. The method for preparing the terahertz metamaterial lens with the adjustable focal length as claimed in claim 7, wherein the method comprises the following steps: preparing the photoalignment layer on the substrate and the dielectric layer further comprises pretreating the surfaces of the substrate and the dielectric layer before preparation; wherein the pretreatment step comprises: the substrate and the dielectric layer are ultrasonically cleaned for 10-30 minutes by using a cleaning solution, then ultrasonically cleaned for two times by using ultrapure water, each time lasts for 8-10 minutes, then dried in an oven at 100-120 ℃ for 40-60 minutes, and finally cleaned for 30-45 minutes by using ultraviolet light and ozone.
10. Use of the terahertz metamaterial lens as claimed in any one of claims 1 to 6 in terahertz polarization imaging.
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