CN111880347A - Terahertz lens with adjustable focal length and preparation method and application thereof - Google Patents
Terahertz lens with adjustable focal length and preparation method and application thereof Download PDFInfo
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- CN111880347A CN111880347A CN202010881883.7A CN202010881883A CN111880347A CN 111880347 A CN111880347 A CN 111880347A CN 202010881883 A CN202010881883 A CN 202010881883A CN 111880347 A CN111880347 A CN 111880347A
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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
- G02F1/137—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 characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering
- G02F1/139—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 characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent
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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
- G02F1/137—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 characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering
- G02F1/139—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 characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent
- G02F1/1393—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 characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering based on orientation effects in which the liquid crystal remains transparent the birefringence of the liquid crystal being electrically controlled, e.g. ECB-, DAP-, HAN-, PI-LC cells
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Abstract
The invention discloses a terahertz lens with adjustable focal length, a preparation method and application thereof, wherein the terahertz lens comprises a first substrate, a second substrate and a liquid crystal layer which are oppositely arranged, one side of the second substrate, which is close to the first substrate, is provided with a medium super-structure surface layer, and the liquid crystal layer is arranged between a first orientation layer and a second orientation layer; the first alignment layer and the second alignment layer have the same alignment direction and are uniformly aligned along the polarization direction of incident terahertz waves; 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 liquid crystal is used as an environment medium of the dielectric super-structure surface lens, the crystal axis pointing direction of the liquid crystal is adjusted by electrifying, so that the environment refractive index of the dielectric super-structure surface lens is changed, the dynamic continuous change of the focal length of the lens is realized, the dynamic function can realize the multifunctional adjustable terahertz super-structure lens, and the technical problem that the terahertz device in the prior art is single in function and application is solved.
Description
Technical Field
The invention relates to an optical lens and a preparation method and application thereof, in particular to a terahertz lens with adjustable focal length and a preparation method and application thereof.
Background
In the electromagnetic spectrum, the terahertz (THz) band is located between the microwave and infrared bands, and is a transition frequency band between electronics and photonics. This band has become the most spectrally elusive "terahertz gap", and the last band that has not been fully developed, once due to the limitations of the corresponding generation and detection techniques. Until the appearance of high-power terahertz sources and sensitive terahertz detectors, a series of potential application researches of terahertz, such as material diagnosis, semiconductor characterization, tomography and the like, are started, so that the terahertz researches become an attractive hot spot from the cold. This band has the following unique properties: the terahertz wave has low photon energy, only one millionth of X-ray, and cannot generate ionization damage to biological tissues; characteristic absorption of many materials and biomacromolecules and intermolecular weak interaction absorption are in a terahertz frequency band, which is called terahertz fingerprint spectrum; terahertz waves have low absorption rate and high transmittance in many nonmetallic and nonpolar materials, and therefore, terahertz waves can be used for detecting information in the materials. The terahertz imaging technology has wide application prospect in the fields of non-contact human body security inspection, nondestructive inspection of industrial products and the like due to the characteristics.
The lens is an indispensable basic element in the terahertz imaging system. The common terahertz lens is composed of a crystal or a polymer with a curved surface, realizes a focusing effect by depending on the accumulation of the phase of terahertz waves propagating in the lens, and is a refractive optical element. The device is heavy and large in size, and cannot meet the trend of integration and miniaturization of the terahertz system. Unlike the conventional phase accumulation method, the recent emerging super-structured lens (metalens) can introduce an abrupt phase for wavefront manipulation by designing a sub-wavelength metal or dielectric resonant cell. The super-structure lens reported so far has realized multiple functions such as super-resolution focusing, broadband achromatic focusing, spin selective 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 and control devices, such as a zoom lens with adjustable external field, has great practical significance for terahertz imaging application.
However, the existing terahertz lens with adjustable focal length is only dynamically switched between two focuses in function, and cannot continuously tune the focal length; the liquid crystal wave plate and the super-surface lens are cascaded in principle, so that the size is large, and the integration of a terahertz optical system is not facilitated. Therefore, a terahertz lens with continuously adjustable focal length and high integration is urgently needed.
Disclosure of Invention
The purpose of the invention is as follows: the invention aims to provide a terahertz lens with adjustable focal length, which integrates liquid crystal and a super-structured surface, and can realize continuous dynamic tuning of different focal lengths; the second purpose of the invention is to provide a preparation method of the terahertz lens with adjustable focal length; the invention further aims to provide an application of the terahertz lens with the adjustable focal length.
The technical scheme is as follows: the terahertz lens with the adjustable focal length comprises a first substrate, a second substrate and a liquid crystal layer, wherein the first substrate and the second substrate are arranged oppositely, and the liquid crystal layer is positioned between the first substrate and the second substrate; a first orientation layer is arranged on one side of the first substrate, which faces the second substrate, a second orientation layer is arranged on one side of the second substrate, which faces the first substrate, and a medium super-structure surface layer is arranged between the second orientation layer and the second substrate;
the first alignment layer and the second alignment layer have the same alignment direction and are uniformly aligned along the polarization direction of incident terahertz waves; 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 along the polarization direction of incident terahertz waves.
When the polarization direction of incident terahertz waves is the x direction, the first alignment layer and the second alignment layer are uniformly aligned along the x direction, and then liquid crystal molecules are induced to be aligned; and the alignment direction of the liquid crystal molecules in the liquid crystal layer is uniform alignment along the x-direction.
When the polarization direction of incident terahertz waves is the y direction, the first alignment layer and the second alignment layer are uniformly aligned along the y direction, and then liquid crystal molecules are induced to be aligned; and the alignment direction of the liquid crystal molecules in the liquid crystal layer is uniform alignment along the y-direction.
The dielectric super-structure surface layer comprises a dielectric column structure, the dielectric columns are distributed in an array shape, the dielectric columns at different array positions have different length parameters (along the x direction) and width parameters (along the y direction), and the design basis of the parameters is that the required lens phase and high generation efficiency of the corresponding position can be met; 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.
The first substrate and the second substrate are made of materials with high refractive index, high transmittance and high conductivity in a terahertz wave band, and the materials can be selected from intrinsic silicon or doped silicon; the first alignment layer and the second alignment layer are both photo-alignment layers, and the photo-alignment material can be selected from azo dye materials SD 1. The liquid crystal layer further comprises spacing particles located between the first substrate and the second substrate, the spacing particles are used for supporting the first substrate and the second substrate to form a filling space of the liquid crystal layer, and when the incident light frequency range is 0.6-1.4 THz, the thickness of the liquid crystal layer is 200-300 mu m.
Preferably, the dielectric surface layer comprises an array of dielectric pillar structures, and the dielectric pillars at different array positions have the same height and period and different lengths and widths.
Preferably, when the terahertz wave is incident on the dielectric super-structure surface layer coated by the liquid crystal, the resonance phase generated by the dielectric column structure satisfies:
wherein x is the position coordinate of the length direction of the dielectric column, y is the position coordinate of the width direction of the dielectric column, f is the focal length of the lens, and lambda is the wavelength of the terahertz wave.
The basic structural unit of the terahertz lens provided by the invention is a super-structured surface coated by a liquid crystal layer with a certain thickness. The super-structured surface is formed by periodically arranging artificially prepared sub-wavelength structural 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 cylinders, 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.
Wherein, the cross section of the medium column is oval or rectangular. 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 column are different, the dielectric column coated by the liquid crystal has different effective refractive indexes for incident terahertz waves of an electric field vector along the long direction (x-ray polarization), so that the emergent resonance phases are different, and the convergence phase of the lens can be realized by arranging structural units with different length and width parameters at different positions of the lens; for a certain structural unit with a specific length and width parameter, the deflection direction of the liquid crystal axis can be adjusted by electrifying the first substrate and the second substrate, so that the change of the environmental refractive index of the dielectric column is realized, the resonance phase can be correspondingly changed, and the adjustable characteristic of the focal length of the lens is realized.
Structural parameters of the super-structure surface can be optimized according to the target terahertz frequency so as to optimize regulation efficiency, the cross section of the medium column is rectangular, the wavelength of incident terahertz waves is lambda, the length of the medium column is l, the width of the medium column is w, the height of the medium column is h, the array period is T, wherein l is less than lambda/2, w is less than 2/2, h is approximately equal to lambda, and lambda/3 is less than T and less than lambda/2.
If the frequency of the incident terahertz wave is 1THz, the preferable parameters of the dielectric column array are as follows: the length is 10-120 μm, the width is 10-120 μm, the height is 200-250 μm, and the array period is 100-150 μm.
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 material of the light-operated orientation layers is azo dye.
Preferably, the liquid crystal of the liquid crystal layer is a birefringence material with a first refractive index and a second refractive index, when the frequency range of incident light entering the terahertz lens is 0.6-1.4 THz, the difference value between the first refractive index and the second refractive index is delta n, and delta n is greater than or equal to 0.2 and less than or equal to 0.4; the liquid crystal layer is aligned in a uniform direction along the x-axis direction.
Further, the terahertz lens also comprises a spacer particle positioned between the first substrate and the second substrate, wherein the spacer particle is used for supporting the substrates and the medium layer to form a filling space of the liquid crystal layer; preferably, the thickness of the liquid crystal layer is 200 to 300 μm when the incident light frequency is in the range of 0.6 to 1.4 THz.
The invention also provides a preparation method of the terahertz lens with the adjustable focal length, which comprises the following steps:
providing a first substrate and a second substrate provided with a medium super-structure surface layer;
preparing a first photoalignment layer on one side surface of a first substrate;
preparing a second photoalignment layer on one side surface of the second substrate medium super-structure surface layer;
carrying out ultraviolet polarization exposure on the light-controlled orientation layer to form a control pattern with oriented arrangement of molecular directors, wherein the control pattern is arranged along the direction of an x axis or the direction of a y axis;
arranging spacing particles on one side of a first light control orientation layer of a first substrate, and then packaging the first substrate and a second substrate to enable the first substrate and the second substrate to be oppositely arranged, wherein a super-structure surface is positioned on one side close to the first substrate, the first light control orientation layer faces a medium super-structure surface layer, and the second light control orientation layer faces the first substrate;
liquid crystal material is poured between the first substrate and the second substrate, and the control graph controls the director of the liquid crystal molecules to be arranged along the orientation; the liquid crystal molecular directors are aligned along the x-axis direction or the y-direction.
The preparation method of the dielectric ultra-structure surface layer comprises the following steps: and cleaning the silicon wafer, transferring the pattern on the mask plate to the silicon wafer by utilizing a photoetching process, etching the exposed silicon by utilizing plasma until the etching reaches the target depth, stopping etching, and finally washing off the residual photoresist.
Further, the preparation of the photoalignment layer on the first substrate and the second substrate further comprises pretreatment of the surfaces of the first substrate and the second substrate before preparation; wherein the pretreatment step comprises: the first substrate and the second substrate are ultrasonically cleaned for 5-10 minutes by using a cleaning solution, then ultrasonically cleaned for two times by using ultrapure water, each time lasts for 5-10 minutes, then dried in an oven at 100-120 ℃ for 20-30 minutes, and finally cleaned for 40-60 minutes by using ultraviolet ozone.
The invention also provides application of the terahertz lens in terahertz imaging. The lens focal length of the terahertz lens is related to the applied voltage, and when the voltage is gradually increased, the focal length is gradually reduced. The terahertz imaging system can flexibly perform terahertz imaging on objects with different depths of field, realizes real-time capture of target distance and resolution of shape and size, and can be applied to application scenes of terahertz human body security inspection instruments, industrial product nondestructive flaw detectors and the like.
The invention principle is as follows: the structural unit of the adjustable terahertz lens is a super-structural surface coated by a liquid crystal layer with a certain thickness. The super-structure surface is formed by periodically arranging artificially prepared sub-wavelength structural 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 cylinders, such as elliptical dielectric columns, rectangular dielectric columns and the like; preferably, the medium column structure of the medium super-structure surface is a rectangular medium column. When the length and width parameters of the dielectric column are different, the dielectric column coated by the liquid crystal has different effective refractive indexes for incident terahertz waves of an electric field vector along the long direction (x-ray polarization), so that the emergent resonance phases are different, and the convergence phase of the lens can be realized by arranging structural units with different length and width parameters at different positions of the lens; for a certain structural unit with a specific length and width parameter, the deflection direction of the liquid crystal axis can be adjusted by electrifying the first substrate and the second substrate, so that the change of the environmental refractive index of the dielectric column is realized, the resonance phase can be correspondingly changed, and the adjustable characteristic of the focal length of the lens is realized. In the electromagnetic field simulation design, firstly, determining height parameters and period parameters of a rectangular dielectric column, setting a liquid crystal director direction as an x-axis direction (in an unpowered state), and then scanning length and width parameters to obtain transmission phase matrixes with different length and width parameters; changing the direction of the liquid crystal director to be the z-axis direction (in an electrified saturation state), and scanning once again to obtain the transmission phase matrixes with different length and width parameters. Lens phases with different focal lengths in the non-electrified and electrified saturated states are respectively designed according to the maximum phase difference which can be achieved, and then appropriate structural parameters are screened to meet the corresponding lens phases, so that the dynamic function of continuously changing the focal lengths under the continuous electrified condition is realized. The dynamic function can realize a multifunctional adjustable terahertz lens, and the technical problem that the function and the application of an adjustable terahertz functional device in the prior art are single is solved.
The key technology of the invention is that the super-structure surface is combined with a liquid crystal layer, and the electric field tuning liquid crystal material is used as the environment medium of the super-structure lens for dynamic tuning; and the optimal size parameter of the dielectric column is obtained by adopting electromagnetic field simulation, and the required structural parameter is reasonably selected according to the required focal length tuning range, so that the high-efficiency focal length tuning effect is realized.
The directional distribution of the liquid crystal can be arbitrarily controlled by a rubbing orientation or photo-orientation technology, so that the liquid crystal is very suitable for the design and manufacture of the terahertz modulator. Under the action of an external field (e.g., an electric field, a magnetic field, etc.), the liquid crystal director is reconfigured, resulting in a change in the refractive index of the liquid crystal itself. The electric field tuning liquid crystal material is used as the environment medium of the super-structure lens for dynamic tuning, so that the variable focal length lens with the adjustable external field is realized.
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) compared with the terahertz lens in the prior art, the terahertz lens disclosed by CN111399261A is in cascade connection with the liquid crystal wave plate and the super-surface lens in principle, has larger volume and is not beneficial to the integration of a terahertz optical system; the terahertz lens has the characteristics of miniaturization, easy integration and lightness and thinness, can dynamically change different focal lengths according to different applicable scenes, and has great application potential in the aspects of terahertz communication, imaging, sensing and the like;
(3) the terahertz lens in the prior art is only dynamically switched between double focuses in function and cannot be used for continuously tuning the focal length; the adjustable terahertz lens integrates liquid crystal and a super-structure surface, the medium column structure unit on the super-structure surface is coated by the liquid crystal, and different phase responses are formed under different power-on states, so that the effect of the focal length continuously adjustable lens is realized.
Drawings
FIG. 1 is a schematic diagram of a cross-sectional structure of a tunable terahertz lens of the present invention;
FIG. 2 is a schematic diagram of the structural unit of the tunable terahertz lens of the present invention in unpowered and powered saturated states; wherein, fig. 2(a) is an unpowered state, and fig. 2(b) is a powered saturation state; in the figure, l represents the length of the dielectric column, w represents the width of the dielectric column, h represents the height of the dielectric column, and T represents the period of the structural unit;
FIG. 3 is a diagram of variation of outgoing phase with frequency of a structural unit (dielectric column length 40 μm, width 120 μm, height 235 μm, liquid crystal layer thickness 250 μm, structural unit period 130 μm) of a dielectric super-structure surface according to an embodiment of the present invention under x-ray polarization incidence, where a dotted line is an unpowered state and a solid line is an energized state;
FIG. 4 is a graph showing the variation of the outgoing phase of a structural unit (dielectric column length 40 μm, width 120 μm, height 235 μm, liquid crystal layer thickness 250 μm, structural unit period 130 μm) on the dielectric superstructure surface with the direction of the liquid crystal director (i.e. the applied voltage) under the incidence of x-ray polarization according to an embodiment of the present invention;
fig. 5 is a distribution diagram of an outgoing phase of a structural unit of the tunable terahertz lens provided by the embodiment of the invention along with changes of length and width parameters under a condition that a liquid crystal layer is not electrified, the frequency is 1THz, and incident polarization is linear polarization in the x direction;
fig. 6 is a distribution diagram of an outgoing phase of a structural unit of the tunable terahertz lens provided by the embodiment of the present invention changing with length and width parameters under the condition of applying saturation voltage to a liquid crystal layer, where the frequency is 1THz, and the incident polarization is linear polarization in the x direction;
fig. 7 is a distribution diagram of the outgoing transmission efficiency of the structural unit of the tunable terahertz lens provided by the embodiment of the invention under the condition that the liquid crystal layer is not electrified, which varies with the length and width parameters, the frequency is 1THz, and the incident polarization is x-direction linear polarization;
fig. 8 is a distribution diagram of the outgoing transmission efficiency of the structural unit of the tunable terahertz lens under the condition of applying saturation voltage to the liquid crystal layer, which varies with length and width parameters, where the frequency is 1THz, and the incident polarization is linear polarization in the x direction;
fig. 9 is a length and width parameter distribution diagram of a structural unit dielectric column of a tunable terahertz lens provided by an embodiment of the present invention on a diameter;
FIG. 10 shows lens phases (solid and dashed lines) of two different focal lengths and lens phases (square and triangular data points) of a tunable terahertz lens designed in two power-up states (unpowered and saturated voltage) according to an embodiment of the present invention;
FIG. 11 is a simulation diagram of the focusing effect at 1THz of a tunable terahertz lens provided by the embodiment of the invention under two power-up states (unpowered and saturated voltage); wherein, diagram (a) is a distribution diagram of the intensity of an unpowered terahertz far field when an x-ray polarized terahertz wave is incident; the graph (b) is a terahertz far-field intensity distribution graph of saturated voltage when the x-ray polarization terahertz waves are incident;
fig. 12 is a schematic diagram illustrating steps in a method for manufacturing a tunable terahertz lens according to an embodiment of the present invention;
fig. 13 is a schematic flow chart of a method for manufacturing a tunable terahertz lens according to an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to examples.
Example 1:
as shown in fig. 1, a schematic cross-sectional view of a terahertz lens of the present invention is shown in fig. 1, and includes a first substrate 1 and a second substrate 4, a first alignment layer 2, a second alignment layer 3, a dielectric metamaterial surface layer 5, and a liquid crystal layer 6, which are oppositely disposed. Wherein, the first orientation layer 2 is arranged on the side of the first substrate 1 facing the second substrate 4, the second orientation layer 3 is arranged on the side of the second substrate 4 facing the first substrate 1, and the dielectric super structure surface layer 5 is arranged between the second orientation layer 3 and the second substrate 4, namely a rectangular column on the upper surface of the second substrate 4 shown in fig. 1; the liquid crystal layer 6 is disposed between the first alignment layer 2 and the second alignment layer 3. The liquid crystal layer 6 includes liquid crystal molecules 7 and spacers 8 disposed at both ends of the liquid crystal molecules 7 to form a liquid crystal cell having a fixed thickness, and the circular structures on the left and right sides of the liquid crystal molecules as shown in fig. 1 are the spacers. In this embodiment, the first substrate 1 and the second substrate 4 are made of high-resistance intrinsic silicon wafers, and the alignment layers are made of an azo dye material SD 1.
The structural units of the dielectric super-structure surface layer 5 are silicon columns with rectangular cross sections, and length, width and height parameters of the silicon columns at different positions in the super-structure surface structural units and 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 130 μm, the height of the silicon pillar is set to 235 μm, the thickness of the liquid crystal layer is set to 250 μm, and the length and width parameters of the silicon pillar are determined according to the transmission phase required at different positions.
As shown in fig. 2, the structural unit of the terahertz lens is in a schematic diagram in an unpowered (left) state and a powered saturated (right) state; in the figure, l represents the length of the dielectric column, w represents the width of the dielectric column, h represents the height of the dielectric column, and T represents the period of the structural unit. Liquid crystals can be seen as long rod-like dielectric anisotropic materials. When not electrified, the liquid crystal is anchored by the orientation layers on the surfaces of the first substrate 1 and the second substrate 4 and is oriented along the x axis (left in FIG. 2); when a saturation voltage is applied to the first and second substrates 1, 4, the liquid crystals are reoriented along the direction of the electric field, i.e. along the z-axis (fig. 2 right).
The terahertz lens of the embodiment has the following principle: as shown in fig. 2, the structural unit of the lens is a rectangular dielectric column coated with a liquid crystal layer of a certain thickness. When the length and width parameters of the dielectric column are different, the dielectric column coated by the liquid crystal has different effective refractive indexes for incident terahertz waves of an electric field vector along the long direction (x-ray polarization), so that the emergent resonance phases are different, and the convergence phase of the lens can be realized by arranging structural units with different length and width parameters at different positions of the lens; for a certain structural unit with a specific length and width parameter, the deflection direction of the liquid crystal axis can be adjusted by electrifying the first substrate and the second substrate, so that the change of the environmental refractive index of the dielectric column is realized, the resonance phase can be correspondingly changed, and the adjustable characteristic of the focal length of the lens is realized. In the electromagnetic field simulation design, firstly, determining height parameters and period parameters of a rectangular dielectric column, setting a liquid crystal director direction as an x-axis direction (in an unpowered state), and then scanning length and width parameters to obtain transmission phase matrixes with different length and width parameters; changing the direction of the liquid crystal director to be the z-axis direction (in an electrified saturation state), and scanning once again to obtain the transmission phase matrixes with different length and width parameters. Lens phases with different focal lengths in the non-electrified and electrified saturated states are respectively designed according to the maximum phase difference which can be achieved, and then appropriate structural parameters are screened to meet the corresponding lens phases, so that the dynamic function of continuously changing the focal lengths under the continuous electrified condition is realized.
The exit phase versus frequency for a building block of a particular design parameter was simulated using commercial software, commercial FDTD. Fig. 3 is a graph of variation of outgoing phase with frequency of a structural unit (dielectric column length 40 μm, width 120 μm, height 235 μm, liquid crystal layer thickness 250 μm, structural unit period 130 μm) of a dielectric super-structure surface provided by an embodiment of the present invention under x-ray polarization incidence, where a dotted line is an unpowered condition, and a solid line is an energized saturation condition. When the x-ray polarized terahertz waves are incident along the direction vertical to the surface of the dielectric metamaterial, the dielectric cylinder can be regarded as 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 linear increase of the transmission phase with frequency (c/λ) is consistent with the linear relationship of the emergent phase with frequency shown in fig. 3. Since the liquid crystal axes in the non-energized and energized saturated states have different directions of orientation, they can be regarded as uniaxial crystals, and therefore the refractive index of the uniaxial crystals changes, thereby affecting the outgoing phase. This effect is mainly reflected in two aspects: firstly, the change of the environmental refractive index affects the resonant mode of the dielectric waveguide, resulting in the change of the resonant phase; and secondly, the liquid crystal layer in the edge area of the dielectric cylinder can be regarded as a uniform wave plate, and the phase accumulation of the terahertz waves in the wave plate is changed due to the change of the refractive index, so that the change of the emergent phase is caused. The combined action of the two different mechanisms results in a change in the emergent phase. As shown in fig. 3, the outgoing phase in the energized saturation state is somewhat smaller at each frequency than the outgoing phase in the unpowered state.
Fig. 4 is a graph of the variation of the outgoing phase of the structural unit (the dielectric column length is 40 μm, the width is 120 μm, the height is 235 μm, the liquid crystal layer thickness is 250 μm, and the structural unit period is 130 μm) on the dielectric superstructure surface with the liquid crystal director direction (i.e. the applied voltage) under the incidence of x-ray polarization according to the embodiment of the present invention. When the voltage is gradually increased, the direction of the liquid crystal axis rises gradually from the direction along the x-axis, and finally, is arranged along the z-axis under the saturation voltage. The gradual change of the crystal axis pointing direction leads to continuous change of the emergent phase. In practical operation, the phase can be changed continuously when the applied voltage is increased gradually, so that the focal length of the lens can be changed continuously under two critical conditions (the focal length under non-power and the focal length under saturation voltage).
If the structural parameters are changed and the applied voltage is kept unchanged, the phase of the outgoing terahertz wave is also changed. Fig. 5 is a distribution diagram of an outgoing phase of a structural unit of the tunable terahertz lens provided by the embodiment of the present invention under a condition that a liquid crystal layer is not powered on, along with changes of length and width parameters, where a frequency is 1THz, an incident polarization is x-direction linear polarization, and length and width parameters are scanned from 10 μm to 120 μm; fig. 6 is a distribution diagram of an outgoing phase of a structural unit of the tunable terahertz lens provided by the embodiment of the invention along with changes of length and width parameters under the condition that a saturation voltage is applied to a liquid crystal layer. As can be seen from fig. 5 and 6, in the unpowered or powered saturation state, the phase of the outgoing terahertz wave increases with the increase of the length-width parameter. When the aspect parameter is gradually increased from (10 μm ) to (120 μm, 120 μm), the transmission phase in the unpowered state is gradually increased from 23.9rad to 30.3 rad; the transmission phase at power-up saturation gradually increases from 22.4rad to 30.2 rad. The increase of the outgoing phase along with the length and width parameters in the power-on saturation state is not larger than that in the non-power-on state. Based on this, the present embodiment designs two lens phases with different focal lengths, corresponding to the unpowered and powered saturated states, respectively. The structural units at all positions on the lens can respectively meet the designed two lens phases with different focal lengths under the non-power-up and power-up saturation states, and the effect of the focal length adjustable lens can be achieved.
Example 2:
this embodiment designs lens phase masks with focal lengths of 10.5mm and 8.3mm in unpowered and powered saturation states, respectively. The required lens phase is shown in the following equation:
wherein f isiAnd fiTwo focal lengths as designed. In fig. 10 we show the lens phases (solid and dashed) for two different focal lengths of the design in two power-up states (unpowered and saturated voltage), the lens phases being parabolic phases of a rounded curve, the slope of the parabolic phase curve for a focal length of 8.3mm being slightly larger than the parabolic phase for a focal length of 10.5 mm.
For optimization of the lens efficiency, FIGS. 7 and 8 show the distribution of the exit transmittance of the building block as a function of the length and width parameters with a frequency of 1THz and the incident polarization as x-direction linear polarization, under unpowered and saturated voltage applied to the liquid crystal layer, respectively. It can be seen that the transmittance is generally high, but some parameters correspond to the transmittance of the structural unit of less than 50%. In the matlab code for screening the structural parameters, the light transmittance criterion of each medium column is greater than 0.7, so that the high-efficiency terahertz lens can be realized.
According to the above design, the present embodiment provides a tunable terahertz lens with a number of structural units of 40 × 40 (the size of the whole lens region is 5.2mm × 5.2 mm). The length and width structural parameters of 40 media columns along the diameter direction are shown in fig. 8. It can be observed that the structural parameters differ at different radii.
The transmission phases of the 40 building blocks under unpowered and powered saturated conditions were compared to the designed lens phase, and the results are shown in fig. 10. The square points are the transmission phases that can be generated by the respective structures in the unpowered state, and the triangular points are the transmission phases that can be generated by the respective structures in the powered saturated state. It can be observed from fig. 10 that the resulting phases coincide with the phases of the lenses having focal lengths of 10.5mm and 8.3mm, respectively, indicating that there is no problem with the parametric design.
For the designed tunable terahertz lens, the focusing effect under the unpowered and powered saturation conditions can be simulated by using a Lumerical FDTD electromagnetic field simulation. Since the lens phases are symmetrically distributed along each diameter direction, only one diameter structure needs to be simulated, which can greatly reduce the simulation time. Fig. 11 is a simulation diagram of a focusing effect of the tunable terahertz lens provided by the embodiment of the present invention at 1THz under two power-on states (unpowered and saturated voltage), where the left and right are distributions of terahertz far-field intensities of the unpowered and saturated voltage respectively when an x-ray polarized terahertz wave is incident. Significant focusing effects were observed both in the unpowered and the saturated voltage regime. The simulated focal length under the non-electrified state is 10.6mm, and the simulated focal length under the electrified saturated state is 8.4mm, which are consistent with the designed focal length of the lens.
Example 3:
fig. 12 is a schematic view of a manufacturing process of the tunable terahertz lens, and fig. 13 is a schematic view of steps of the manufacturing method. The preparation method comprises the following steps:
the process for preparing the dielectric nanostructured surface comprises the following steps: cleaning the silicon wafer, transferring the pattern on the mask plate to the silicon wafer by utilizing a photoetching process, etching the exposed silicon by utilizing plasma until the etching reaches the target depth, stopping etching, and finally washing the residual photoresist by utilizing acetone.
the method comprises the following steps of preparing a light control orientation film on a first substrate and a second substrate, and pretreating the first substrate and the second substrate, wherein the pretreatment process comprises the following steps: and ultrasonically cleaning the first substrate and the second substrate for 10-30 minutes by using a cleaning solution, ultrasonically cleaning the first substrate and the second substrate by using ultrapure water twice, wherein each time lasts for 8-10 minutes, drying the substrates in an oven at 100-120 ℃ for 40-60 minutes, and finally carrying out ultraviolet ozone cleaning for 30-45 minutes.
and step 124, pouring the liquid crystal material between the first substrate and the dielectric metamaterial surface, wherein the control pattern of the photoalignment layer controls the liquid crystal molecular directors to be uniformly distributed along the x direction.
The adjustable terahertz lens is mainly formed by superposing a layer of dielectric metamaterial surface and a layer of liquid crystal layer. The dielectric super-structure surface is composed of a rectangular silicon column array of sub-wavelength structural units, the length and width dimensions of the silicon column units are different at different positions, and phase control is provided for incident terahertz waves. The liquid crystal molecule director in the liquid crystal layer is distributed along the x-axis direction and covers the whole rectangular silicon column array. The liquid crystal is used as an environment medium of the dielectric super-structure surface lens, the crystal axis pointing direction of the liquid crystal is adjusted by electrifying, so that the environment refractive index of the dielectric super-structure surface lens is changed, the dynamic change of the focal length of the lens is realized, the dynamic function can realize the multifunctional adjustable terahertz super-structure lens, and the technical problem that the terahertz device in the prior art is single in function and application is solved.
The terahertz lens can be applied to a terahertz imaging system. The lens focal length of the terahertz lens is related to the applied voltage, terahertz imaging can be flexibly performed on objects with different depths of field, real-time capture of target distance and resolution of shape and size are achieved, and the terahertz lens can be applied to application scenes such as terahertz human body security inspection instruments and industrial product nondestructive flaw detectors.
Claims (10)
1. The utility model provides a focus adjustable terahertz lens which characterized in that: the liquid crystal display panel comprises a first substrate, a second substrate and a liquid crystal layer, wherein the first substrate and the second substrate are oppositely arranged, and the liquid crystal layer is positioned between the first substrate and the second substrate; a first orientation layer is arranged on one side of the first substrate, which faces the second substrate, a second orientation layer is arranged on one side of the second substrate, which faces the first substrate, and a medium super-structure surface layer is arranged between the second orientation layer and the second substrate; the first alignment layer and the second alignment layer have the same alignment direction and are uniformly aligned along the polarization direction of incident terahertz waves; 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 along the polarization direction of incident terahertz waves.
2. The terahertz lens with adjustable focal length of claim 1, wherein: the dielectric super-structure surface layer comprises dielectric column structures distributed in an array, and dielectric columns located at different array positions have the same height and period and different lengths and widths.
3. The terahertz lens with adjustable focal length of claim 1, wherein: when terahertz waves are incident to the liquid crystal coated dielectric super-structure surface layer, the resonance phase generated by the dielectric column structure meets the following requirements:
wherein x is the position coordinate of the length direction of the dielectric column, y is the position coordinate of the width direction of the dielectric column, f is the focal length of the lens, and lambda is the wavelength of the terahertz wave.
4. The terahertz lens with adjustable focal length of claim 2, wherein: the cross section of the medium column is oval or rectangular.
5. The terahertz lens with adjustable focal length of claim 4, wherein: the wavelength of incident terahertz waves is lambda, the cross section of the dielectric column is rectangular, the length of the dielectric column is l, the width of the dielectric column is w, and the array period is T; wherein l is less than 2/2, w is less than lambda/2, and lambda/3 is less than T is less than lambda/2.
6. The terahertz lens with adjustable focal length of claim 1, wherein: the liquid crystal of the liquid crystal layer is made of a birefringence material with a first refractive index and a second refractive index, 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.
7. The terahertz lens with adjustable focal length of claim 1, wherein: the thickness of the liquid crystal layer is 200 to 300 μm.
8. The terahertz lens with adjustable focal length of claim 1, wherein: the first substrate and the second substrate are intrinsic silicon or doped silicon, and the first orientation layer and the second orientation layer are photoalignment layers.
9. A preparation method of the terahertz lens as claimed in any one of claims 1 to 8, characterized by comprising the following steps:
providing a first substrate and a second substrate provided with a medium super-structure surface layer;
preparing a first photoalignment layer on one side surface of a first substrate;
preparing a second photoalignment layer on one side surface of the second substrate medium super-structure surface layer;
carrying out ultraviolet polarization exposure on the light-controlled orientation layer to form a control pattern with oriented arrangement of molecular directors;
arranging spacing particles on one side of a first light control orientation layer of a first substrate, and then packaging the first substrate and a second substrate to enable the first substrate and the second substrate to be oppositely arranged, wherein a super-structure surface is positioned on one side close to the first substrate, the first light control orientation layer faces a medium super-structure surface layer, and the second light control orientation layer faces the first substrate;
and liquid crystal material is poured between the first substrate and the second substrate, and the control graph controls the director of the liquid crystal molecules to be arranged along the orientation.
10. Use of the terahertz lens according to any one of claims 1 to 8 in terahertz imaging.
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| CN113325496A (en) * | 2021-05-13 | 2021-08-31 | 中国科学院上海微系统与信息技术研究所 | Sub-wavelength antenna, wavelength-controllable superlens and superlens design method |
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| CN114859607A (en) * | 2022-04-25 | 2022-08-05 | 北京京东方技术开发有限公司 | Superlens, manufacturing method thereof and display device |
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| CN113346248A (en) * | 2021-04-16 | 2021-09-03 | 上海大学 | Terahertz wave beam scanning super-surface device, wave beam scanning antenna, system and method |
| CN113325496A (en) * | 2021-05-13 | 2021-08-31 | 中国科学院上海微系统与信息技术研究所 | Sub-wavelength antenna, wavelength-controllable superlens and superlens design method |
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