Terahertz Airy beam generation method based on electrically-controlled liquid crystal cubic phase plate

Abstract

The invention discloses a terahertz Airy beam generation method based on an electric control liquid crystal cubic phase plate, and belongs to the technical field of terahertz signal modulation, and the method comprises the following steps: S1, calculating a structural electrode pattern required for manufacturing the cubic phase plate; s2, carrying out photoetching on the DMSO doped PEDOT: PSS organic thin film by using a stripping process, and manufacturing a structural electrode with a cubic phase pattern; s3, manufacturing a liquid crystal cubic phase plate required for generating Airy beams by using a method of loading a vertical electric field to drive liquid crystal through a structural electrode; and S4, generating Airy beams by using the terahertz liquid crystal cubic phase plate. According to the terahertz Airy beam generation method based on the electrically-controlled liquid crystal cubic phase plate, a manufactured terahertz Airy beam generation device is short in finished product period and low in cost, has good broadband optical anisotropy and electro-optical and magneto-optical modulation characteristics, can meet the requirements for large working bandwidth and tunable performance, and has wide application prospects. And the practical application process of the terahertz imaging technology can be accelerated.

Description

Technical Field

[0001] The invention relates to the technical field of terahertz signal modulation, and in particular to a terahertz Airy beam generation method based on an electrically controlled liquid crystal cubic phase plate. Background Art

[0002] Terahertz waves are located in the transition zone between macroscopic electronics and microscopic optoelectronics. They have some advantages of both microwaves and light waves, and also have excellent properties different from electromagnetic waves in other bands. They have broad application prospects in wireless communications, security inspections, nondestructive testing, medical diagnosis and other fields.

[0003] In traditional terahertz imaging technology, the beam incident on the sample is a Gaussian beam, which faces the difficult problem of balancing long focal depth and high lateral resolution. In recent years, structured beams represented by Airy beams and Bessel beams have gradually become one of the research hotspots in the international optical field. These structured beams have many excellent characteristics and provide an effective solution to this problem. Among them, the terahertz accelerated Airy beam has a non-diffraction characteristic and can keep the main lobe width almost unchanged over a long propagation distance. Therefore, these two terahertz structured beams can be introduced into the terahertz imaging system to expand its depth of field, improve resolution, contrast and image quality, and realize three-dimensional reconstruction and super-resolution imaging. In addition, the self-acceleration characteristics of the Airy beam enable it to propagate along a curved path around obstacles, maintain the integrity of the signal, and help achieve faster and more stable wireless communications.

[0004] At present, the research on Airy beams is mainly focused on microwave and visible light bands, while there are few studies on the terahertz band due to the lack of effective control devices similar to spatial light modulators in optical bands. In the terahertz band, the generation of Airy beams is mainly based on metamaterials or 3D printing. 3D printing photosensitive resins have the advantages of high precision, easy printing and good surface quality, but there are also problems with hardening and chemical safety, and resin materials are easily affected by ultraviolet rays, and it is necessary to prevent ultraviolet rays from causing performance degradation. Metamaterials have special structures and properties and can work effectively at specific frequencies, but there are also some disadvantages and challenges. On the one hand, the properties and effects of metamaterials usually perform well in a specific frequency range, but they have the problem of narrow bandwidth, especially for Airy beams. This means that the application of metamaterials may be limited to a specific spectral range or frequency range. In addition, while some metamaterials perform well in a specific frequency range, they may introduce large energy losses. Metamaterials are usually based on microstructures and periodically arranged elements, the size of which is much smaller than the wavelength of electromagnetic waves. The special structure of metamaterials enables them to exhibit selective responses to electromagnetic waves, such as guiding, suppressing or extending specific frequencies of electromagnetic waves. However, this selective response may cause energy to be absorbed, scattered, or dissipated in the metamaterial. This energy loss may limit the efficiency and feasibility of metamaterials in certain applications, especially when efficient energy transmission or low-loss applications are required. In addition, the functions of 3D printed devices and metamaterial devices in the optical path are fixed. In order to achieve the switching of Airy beams and Gaussian beams in the optical path, the device needs to be physically placed in the optical path or removed, that is, it is impossible to achieve direct switching between the modulated state and the non-modulated state without changing the optical path structure. Therefore, the performance and scenario applicability of the devices currently reported in this field need to be improved urgently. Summary of the invention

[0005] The purpose of the present invention is to provide a terahertz Airy beam generation method based on an electrically controlled liquid crystal cubic phase plate. The manufactured terahertz Airy beam generation device has a short production cycle and low cost, and has good broadband optical anisotropy and electro-optical and magneto-optical modulation characteristics, can achieve large working bandwidth and tunable performance requirements, and is helpful to accelerate the practical application process of terahertz imaging technology.

[0006] To achieve the above object, the present invention provides a method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate, comprising the following steps:

[0007] S1. Calculate the structural electrode pattern required to make a cubic phase plate;

[0008] S2, using a lift-off process to photolithograph the DMSO-doped PEDOT:PSS organic film to fabricate a structured electrode with a cubic phase pattern;

[0009] S3, using a method of using a structured electrode to load a vertical electric field to drive the liquid crystal to produce a liquid crystal cubic phase plate required to generate an Airy beam;

[0010] S4. Use terahertz liquid crystal cubic phase plate to generate Airy beam.

[0011] Preferably, in S1, the structured electrode pattern is calculated using the principle that the Fourier spectrum of the Fourier transform of a one-dimensional accelerated Airy beam with finite energy is proportional to a Gaussian distribution carrying a cubic phase.

[0012] Preferably, a cubic phase factor is introduced into the initial Gaussian beam, and the cubic phase factor is binarized, and the phase delay value between -π and π is set to the minimum value 0, and 0 to π is set to π.

[0013] Preferably, in S2, the upper and lower substrates of the structure electrode are made of a quartz substrate with a thickness of 500 microns.

[0014] Preferably, in S2, the photoresist used is a negative lift off UV photoresist.

[0015] Preferably, in S3, the specific steps are: cleaning the substrate; making a uniform transparent electrode, spin coating a DMSO-doped PEDOT:PSS organic solution into a film; making an external electrode, coating a low-temperature silver paste on the substrate to facilitate the application of an external voltage; making an orientation layer, using polyimide as the orientation layer, spin coating it on the substrate, and using a flannel cloth to slowly rub it in one direction as a liquid crystal orientation layer; making a box, two substrates with orientation layers have been prepared, one of which uses a uniform transparent electrode and the other uses a structured transparent electrode with a cubic phase pattern, the films are placed opposite to each other, the parts with the silver paste electrode are staggered and leaked out, and a Teflon gasket is used to control the box thickness; the liquid crystal is poured and sealed to complete the production.

[0016] Preferably, the material of the liquid crystal is nematic liquid crystal LDn-LC.

[0017] Preferably, in S4, specifically: first use a polarizer and a half-wave plate to make the polarization direction of the light beam perpendicular to the friction direction of the liquid crystal; then use a gold concave reflector to collimate the terahertz wave so that the terahertz wave is incident on the liquid crystal cubic phase plate, and the terahertz wave is effectively modulated after an external voltage is applied, and a terahertz lens with a focal length of f=50mm is placed on a plane 50mm behind the liquid crystal cubic phase plate to obtain the Fourier transform of the phase-modulated Gaussian beam, and the direction of the focal line is parallel to the x-direction; finally, the Airy beam is detected at the rear focal plane of the terahertz lens, and its changes are tested and recorded using a Pyrocam IV beam analysis camera.

[0018] Therefore, the beneficial effects of the terahertz Airy beam generation method based on the electrically controlled liquid crystal cubic phase plate of the present invention are as follows:

[0019] (1) The method provided by the present invention uses ultraviolet lithography technology to make a structured transparent electrode and combines it with a liquid crystal box to make a cubic phase plate required to generate a terahertz Airy beam. It has low cost, is easy to make, and has a short production cycle.

[0020] (2) The terahertz Airy beam generating device provided by the present invention can realize the switching between the Airy beam and the Gaussian beam through electrical control without changing the optical path structure, which has important potential for accelerating the development of terahertz Airy beam in multi-dimensional synchronous measurement.

[0021] (3) The invention uses liquid crystal materials to make terahertz Airy beam generating devices, which have good broadband optical anisotropy and electro-optical and magneto-optical modulation characteristics, and can achieve large working bandwidth and tunable performance requirements. It can solve the problem that the application of metamaterials may be limited to a specific spectral range or frequency range, and is an important solution to achieve broadband operation and tunable working mode.

[0022] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a flow chart of a method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate according to the present invention;

[0024] Figure 2 Schematic diagram of the structured electrode etching pattern of the liquid crystal cubic phase plate in the embodiment of the present invention (left) and a micrograph of the PEDOT:PSS structure electrode etched with a binary cubic phase pattern on a silicon substrate (right);

[0025] Figure 3 is a schematic structural diagram of a liquid crystal cubic phase plate in an embodiment of the present invention;

[0026] Figure 4 is a structural diagram of a system for generating a terahertz Airy beam in an embodiment of the present invention;

[0027] Figure 5 is a terahertz Airy beam spot morphology diagram produced by an experiment in an embodiment of the present invention;

[0028] Figure 6 is a diagram showing the dynamic changes of a one-dimensional terahertz Airy wave beam generated experimentally in an embodiment of the present invention during propagation from z=0 mm to z=100 mm;

[0029] Figure 7is a graph showing the relationship between the main lobe offset of the Airy beam generated experimentally and the change in voltage under different voltage conditions in an embodiment of the present invention;

[0030] Figure 8 : is the relationship between the local full width at half maximum (LWHM) of the main lobe of the Airy beam and the beam propagation distance, which is experimentally measured in an embodiment of the present invention. DETAILED DESCRIPTION

[0031] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0032] Unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0033] Embodiment 1

[0034] like Figure 1 As shown, the present invention provides a method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate, comprising the following steps:

[0035] S1. Calculate the structural electrode pattern required to make a cubic phase plate.

[0036] The structured electrode pattern is calculated based on the principle that the Fourier spectrum of the Fourier transform of a one-dimensional accelerated Airy beam with finite energy is proportional to a Gaussian distribution carrying a cubic phase.

[0037] A cubic phase factor is introduced into the initial Gaussian beam, and the cubic phase factor is binarized, and the phase delay value between -π and π is set to the minimum value 0, and 0 to π is set to π.

[0038] S2. Use the lift-off process to photolithograph the DMSO-doped PEDOT:PSS organic film to produce a structure electrode with a cubic phase pattern. The structure electrode etching pattern is the cubic phase factor introduced to the initial Gaussian beam: Where b is the scale factor. The cubic phase factor is binarized, and the phase delay values ​​between 0 and π are set to the minimum value 0, and the values ​​between π and 2π are set to π. The resulting binarized cubic phase template is as follows: Figure 2 As shown, black represents 0 and white represents π. In this embodiment, b=0.15 is set. The terahertz cubic phase plate required for generating Airy beam is manufactured by using a method of loading a vertical electric field on a structured electrode to drive a liquid crystal.

[0039] The upper and lower substrates of the structural electrode are made of a quartz substrate with a thickness of 500 microns, and the photoresist used is a negative liftoff ultraviolet photoresist.

[0040] S3. A method of using structured electrodes to load a vertical electric field to drive liquid crystal is used to produce a liquid crystal cubic phase plate required to generate an Airy beam.

[0041] The specific steps are as follows: cleaning the substrate; making a uniform transparent electrode, spin coating a DMSO-doped PEDOT:PSS organic solution into a film; making an external electrode, coating a low-temperature silver paste on the substrate to facilitate external voltage application; making an orientation layer, using polyimide as the orientation layer, spin coating it on the substrate, and using a flannel cloth to slowly rub it in one direction to serve as a liquid crystal orientation layer; making a box, using two substrates with an orientation layer, one of which uses a uniform transparent electrode and the other uses a structured transparent electrode with a cubic phase pattern, the films are placed opposite to each other, the parts with the silver paste electrode are staggered and leaked out, and a Teflon gasket is used to control the box thickness; injecting liquid crystal and sealing the box to complete the production.

[0042] In this embodiment, the size of the manufactured liquid crystal cubic phase plate is 2 cm×2 cm. Figure 2 As shown, it includes two quartz substrates arranged opposite to each other, and the transparent electrode is made of 15Vol% DMSO-doped PEDOT:PSS, covering the terahertz frequency band (0.1-10THz). One of the quartz substrates is etched with a PEDOT:PSS structure electrode, and the electrode on the other side is a fully spin-coated PEDOT:PSS electrode, which is led out through a low-temperature conductive silver paste and a silver wire. The liquid crystal layer is sandwiched between the electrodes and controlled by a polytetrafluoroethylene gasket, and the thickness is set to 200μm to achieve the required phase shift. The material of the liquid crystal used in this embodiment is a column phase liquid crystal LDn-LC, and its n e is 1.866, n o The power supply of the lead part is grounded, and an external voltage is applied to test and analyze the terahertz wave modulation characteristics of the device.

[0043] S4. Use terahertz liquid crystal cubic phase plate to generate Airy beam.

[0044] Specifically, a polarizer and a half-wave plate are first used to make the polarization direction of the light beam perpendicular to the friction direction of the liquid crystal; then a gold concave reflector is used to collimate the terahertz wave so that the terahertz wave is incident on the liquid crystal cubic phase plate. After an external voltage is applied, the terahertz wave is effectively modulated, and a terahertz lens with a focal length of f = 50 mm is placed on a plane 50 mm behind the liquid crystal cubic phase plate to obtain the Fourier transform of the phase-modulated Gaussian beam, and the direction of the focal line is parallel to the x-direction; finally, the Airy beam is detected at the rear focal plane of the terahertz lens, and its changes are tested and recorded using a Pyrocam IV beam analysis camera.

[0045] Figure 4 The system used in this embodiment to generate the terahertz Airy beam is shown. A 2.54THz laser source (Coherent, SIFIR-50FPL) was used in the experiment. After the laser passes through the aperture and collimating lens, it is ensured that the beam matches the effective area. According to the distribution of the patterned electrodes, the terahertz wavefront is modulated spatially. After passing through a terahertz Fourier lens with a focal length of 50mm, the Airy beam is obtained at the focus (z=0) and detected by a pyroelectric array camera (Spricom, PY-IV). Figure 5 As shown, by applying an external voltage, the switching between Gaussian beam and Airy beam can be controlled. When no voltage is applied, the initial phase difference between the area with and without PEDOT electrode is zero, so the camera detects a Gaussian beam. When a voltage above 9V is applied, the initial phase difference between the area with and without PEDOT electrode reaches 180°. After the incident light passes through the liquid crystal cubic phase plate, the phase of the light wave is modulated by the liquid crystal layer, thereby forming a specific phase change in the outgoing light wave. Then, when passing through the terahertz lens, the lens performs Fourier transform on the modulated light wave and forms an Airy beam at the focus. Since the phase distribution mode of the liquid crystal cubic phase plate is a binary phase distribution from 0 to π, this method will simultaneously generate two symmetrically inverted Airy beams.

[0046] like Figure 6 The figure shows the propagation dynamics of the generated one-dimensional terahertz Airy beam in the xz plane. The experimentally generated one-dimensional terahertz Airy beam exhibits self-bending behavior, and its main lobe shows a trend of quadratic acceleration, that is, the bending trajectory of the beam along the propagation direction shows faster and faster changes. Self-bending propagation behavior is one of the main characteristics of the Airy beam, and this self-bending is caused by its unique phase distribution. This characteristic makes the Airy beam have a wide range of application potential in the precise control of light field distribution and manipulation of matter.

[0047] like Figure 7As shown in the figure, the lateral displacement of the maximum normalized intensity at different propagation distances z when different voltages are applied to the liquid crystal cubic phase plate is given. The lateral displacement in the figure is obtained by half the distance between the main lobes of the two Airy beams. The one-dimensional terahertz Airy beam generated experimentally exhibits self-bending phenomenon, and its main lobe shows a trend towards quadratic acceleration. After the applied voltage reaches the saturation voltage, the liquid crystal molecules are completely aligned along the direction of the applied electric field, resulting in a weakening effect of further increasing the voltage on the orientation of the liquid crystal molecules. Therefore, in Figure 7 In the experiment, it can be observed that the experimental results under three different high voltage conditions are very similar. This phenomenon shows that the liquid crystal device has a certain tolerance and robustness to high voltage, and can achieve dynamic modulation of the phase using the driving voltage within a certain range without significantly affecting the output result. This robustness makes the liquid crystal device more reliable and stable in practical applications, and can adapt to different electric field conditions and maintain good performance.

[0048] The one-dimensional cubic phase plate used in this embodiment applies cubic phase modulation to the Gaussian beam only in the y direction, but does not modulate it in the x direction. This design causes the beam to form an Airy function distribution in the y direction, showing non-diffraction and self-bending characteristics, while the Gaussian distribution characteristics are still maintained in the x direction. Figure 8 As shown in the figure, when the propagation distance in the z direction changes from 0 mm to 100 mm, the local full width at half maximum (LWHM) of the main lobe in the y direction maintains its original value ≈1 mm. This verifies that the generated one-dimensional terahertz Airy beam has quasi-diffraction-free characteristics in the y direction. This non-diffraction characteristic is an important manifestation of the Airy beam, proving that it can maintain the stability of the light field during propagation. At the same time, in the x direction, since no phase modulation is applied, the beam naturally diffuses along the propagation direction, and its LWHM expands by 4.6 mm from the original value within a propagation distance of 100 mm. This diffusion is consistent with the propagation characteristics of an ordinary Gaussian beam. This result verifies the effectiveness of the one-dimensional cubic phase plate and proves that the generated terahertz Airy beam has quasi-diffraction-free characteristics in the y direction, while retaining the characteristics of a Gaussian beam in the x direction. The unique characteristics of this beam are of great value in light field manipulation and non-diffraction applications.

[0049] Therefore, the present invention adopts the above-mentioned terahertz Airy beam generation method based on the electrically-controlled liquid crystal cubic phase plate to manufacture a terahertz Airy beam generating device with short production cycle and low cost, and has good broadband optical anisotropy and electro-optical and magneto-optical modulation characteristics, which can achieve large working bandwidth and tunable performance requirements, and help to accelerate the practical application process of terahertz imaging technology.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate, characterized in that: The following steps are involved: S1. Calculate the structural electrode pattern required to make a cubic phase plate; S2, using a lift-off process to photolithograph the DMSO-doped PEDOT:PSS organic film to fabricate a structured electrode with a cubic phase pattern; S3, using a method of using a structured electrode to load a vertical electric field to drive the liquid crystal to produce a liquid crystal cubic phase plate required to generate an Airy beam; S4. Use terahertz liquid crystal cubic phase plate to generate Airy beam.

2. The method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate according to claim 1, characterized in that: In S1, the structured electrode pattern is calculated using the principle that the Fourier spectrum of the Fourier transform of a one-dimensional accelerated Airy beam with finite energy is proportional to a Gaussian distribution carrying a cubic phase.

3. The method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate according to claim 2, characterized in that: A cubic phase factor is introduced into the initial Gaussian beam, and the cubic phase factor is binarized, and the phase delay value between -π and π is set to the minimum value 0, and 0 to π is set to π.

4. The method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate according to claim 1, characterized in that: In S2, the upper and lower substrates of the structured electrodes are made of a quartz substrate with a thickness of 500 μm.

5. The method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate according to claim 1, characterized in that: In S2, the photoresist used is a negative lift-off UV photoresist.

6. The method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate according to claim 1, characterized in that: In S3, the specific steps are: cleaning the substrate; making a uniform transparent electrode, spin coating a DMSO-doped PEDOT:PSS organic solution into a film; making an external electrode, coating a low-temperature silver paste on the substrate to facilitate external voltage application; making an orientation layer, using polyimide as the orientation layer, spin coating it on the substrate, and using a flannel cloth to slowly rub it in one direction as a liquid crystal orientation layer; making a box, using two substrates with an orientation layer, one of which uses a uniform transparent electrode and the other uses a structured transparent electrode with a cubic phase pattern, the films are placed opposite to each other, the parts with the silver paste electrode are staggered and leaked out, and a Teflon gasket is used to control the box thickness; injecting liquid crystal and sealing the box to complete the production.

7. The method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate according to claim 6, characterized in that: The material of the liquid crystal is nematic liquid crystal LDn-LC.

8. The method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate according to claim 1, characterized in that: In S4, specifically: first use a polarizer and a half-wave plate to make the polarization direction of the light beam perpendicular to the friction direction of the liquid crystal; then use a gold concave reflector to collimate the terahertz wave so that the terahertz wave is incident on the liquid crystal cubic phase plate. After an external voltage is applied, the terahertz wave is effectively modulated, and a terahertz lens with a focal length of f = 50 mm is placed on a plane 50 mm behind the liquid crystal cubic phase plate to obtain the Fourier transform of the phase-modulated Gaussian beam, and the direction of the focal line is parallel to the x-direction; finally, the Airy beam is detected at the rear focal plane of the terahertz lens, and its changes are tested and recorded using a PyrocamIV beam analysis camera.

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