A method for generating terahertz Airy beam based on electrically controlled liquid crystal cubic phase plate

Abstract

The present invention discloses a method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate, belonging to the field of terahertz signal modulation technology, and includes the following steps: S1. calculating and manufacturing the structural electrode pattern required for the cubic phase plate; S2. photolithography of a DMSO-doped PEDOT:PSS organic thin film using a lift-off process to manufacture a structural electrode with a cubic phase pattern; S3. utilizing the structural electrode to load a vertical electric field to drive the liquid crystal to manufacture the liquid crystal cubic phase plate required for generating the Airy beam; S4. utilizing the terahertz liquid crystal cubic phase plate to achieve Airy beam generation. The present invention utilizes the aforementioned method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate to manufacture a terahertz Airy beam generating device with a short production cycle and low cost, and exhibits excellent broadband optical anisotropy and electro-optical and magneto-optical modulation properties. It can achieve large operating bandwidth and tunable performance requirements, helping to accelerate the practical application of terahertz imaging technology.

Description

Technical Field

[0001] The present 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 lie in the transition zone between macroscopic electronics and microscopic optoelectronics. They combine some of the advantages of microwaves and light waves, while also possessing excellent properties different from electromagnetic waves in other bands. They have broad application prospects in wireless communications, security inspections, nondestructive testing, and medical diagnosis.

[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 properties 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 types of 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 property of the Airy beam enables it to propagate along a curved path around obstacles, maintain signal integrity, and help achieve faster and more stable wireless communication.

[0004] Currently, research on Airy beams has primarily focused on the microwave and visible light bands. Due to the lack of effective control devices similar to spatial light modulators in optical bands, relatively little research has been conducted in the terahertz band. In the terahertz band, Airy beam generation is primarily based on metamaterials or 3D printing. 3D printing with photosensitive resins offers advantages such as high precision, easy printing, and good surface quality, but also presents challenges with curing and chemical safety. Furthermore, the resins are susceptible to UV radiation, requiring protection against UV-induced performance degradation. Metamaterials possess specialized structures and properties that allow them to operate effectively at specific frequencies, but they also present drawbacks and challenges. While their properties and effects typically excel within a specific frequency range, they suffer from a narrow bandwidth, particularly for Airy beams. This means that their applications may be limited to specific spectral or frequency ranges. Furthermore, while some metamaterials excel in specific frequency ranges, they may also introduce significant energy losses. Metamaterials are typically based on microstructures and periodically arranged elements whose dimensions are significantly smaller than the wavelength of electromagnetic waves. Their specialized structures enable them to exhibit selective responses to electromagnetic waves, such as guiding, suppressing, or broadening specific frequencies. 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 switch between Airy beams and Gaussian beams in the optical path, the device needs to be physically placed in or removed from the optical path. In other words, it is impossible to directly switch 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 urgently need to be improved. 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 terahertz Airy beam generation device produced has a short production cycle and low cost, and has good broadband optical anisotropy and electro-optical and magneto-optical modulation characteristics. It can achieve large working bandwidth and tunable performance requirements, which will help accelerate the practical application 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. Photolithography is performed on the DMSO-doped PEDOT:PSS organic film using a lift-off process to produce a structured electrode with a cubic phase pattern;

[0009] S3, using a method of applying a vertical electric field to a structured electrode to drive a liquid crystal to produce a liquid crystal cubic phase plate required for generating 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 structural 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 a film on the substrate, and using a flannel cloth to slowly rub 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, and a Teflon gasket is used to control the box thickness; pouring liquid crystal and sealing to complete the production.

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

[0017] Preferably, in S4, specifically: first, a polarizer and a half-wave plate are 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.

[0018] Therefore, the beneficial effects of the present invention using the above-mentioned method for generating a terahertz Airy beam based on an electrically controlled liquid crystal cubic phase plate are as follows:

[0019] (1) The method provided by the present invention uses ultraviolet lithography technology to produce a structured transparent electrode combined with a liquid crystal box to produce a cubic phase plate required for generating a terahertz Airy wave beam. The method has low cost, is easy to produce, 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 produce a terahertz Airy beam generating device with excellent broadband optical anisotropy and electro-optical and magneto-optical modulation properties, which can achieve large operating bandwidth and tunable performance requirements. This 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 and tunable operation.

[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 This 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 A schematic diagram of the structured electrode etching pattern of the liquid crystal cubic phase plate in an embodiment of the present invention (left) and a micrograph of a PEDOT:PSS structure electrode etched with a binary cubic phase pattern on a silicon substrate (right);

[0025] Figure 3 1 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 This is a terahertz Airy beam spot morphology diagram produced by an experiment in an embodiment of the present invention;

[0028] Figure 6 1 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 71 is a graph showing the relationship between the main lobe offset of the Airy beam and the voltage under different applied voltage conditions, which is experimentally generated in an embodiment of the present invention;

[0030] Figure 8 : This 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 with reference to the accompanying drawings and embodiments.

[0032] Unless otherwise defined, the technical or scientific terms used in the present invention shall have the usual meanings understood by persons of ordinary skill in the field to which the present invention belongs. The words "first", "second" and similar terms used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Words such as "include" or "comprise" mean that the elements or objects preceding the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Words such as "connect" or "connected" 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 object being described changes, the relative positional relationship may also change accordingly.

[0033] Example 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 with 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 the phase delay value between 0 and π is set to π.

[0038] S2. Use the lift-off process to photolithographically etch the DMSO-doped PEDOT:PSS organic film to produce a structured electrode with a cubic phase pattern. The structured electrode etching pattern is the cubic phase factor introduced by the initial Gaussian beam: , where b is the scale factor. Binarize the cubic phase factor, set the phase delay value between 0 and π to the minimum value 0, and set the phase delay value between π and 2π to π. The resulting binary cubic phase template is as follows Figure 2 As shown, black represents 0 and white represents π. In this embodiment, b is set to 0.15. The terahertz cubic phase plate required for generating the Airy beam is fabricated by using a method in which a structured electrode loads a vertical electric field to drive the 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 lift-off UV photoresist.

[0040] S3. Use the method of using structured electrodes to load a vertical electric field to drive liquid crystal to produce the liquid crystal cubic phase plate required to generate Airy beams.

[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 the application of an external voltage; making an orientation layer, using polyimide as the orientation layer, spin-coating a film on the substrate, and using a flannel cloth to slowly rub it in one direction to serve as the 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 each other, the parts with the silver paste electrode are staggered and leaked, and a Teflon gasket is used to control the box thickness; the liquid crystal is poured and sealed to complete the production.

[0042] In this embodiment, the size of the manufactured liquid crystal cubic phase plate is 2 cm×2 cm. Figure 3 As shown, it includes two quartz substrates arranged opposite to each other, and the transparent electrode is made of 15 Vol% DMSO-doped PEDOT:PSS, covering the terahertz frequency band (0.1-10 THz). 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. The thickness is set to 200μm to achieve the required phase shift. The liquid crystal material used in this embodiment is a columnar liquid crystal LDn-LC, which is 1.866, 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. A gold concave mirror is then used to collimate the terahertz wave so that it is incident on the liquid crystal cubic phase plate. After an external voltage is applied, the terahertz wave is effectively modulated. A terahertz lens with a focal length of f = 50 mm is placed 50 mm behind the liquid crystal cubic phase plate to obtain the Fourier transform of the phase-modulated Gaussian beam, with the focal line direction parallel to the x-direction. Finally, the Airy beam is detected at the back focal plane of the terahertz lens, and its changes are tested and recorded using a Pyrocam IV beam profiler.

[0045] Figure 4 Figure 2 shows the system used in this example to generate the terahertz Airy beam. A 2.54 THz laser source (SIFIR-50 FPL, Coherent) was used in the experiment. The laser light passes through an aperture and a collimating lens to ensure that the beam matches the active area. The terahertz wavefront is spatially modulated according to the distribution of the patterned electrodes. After passing through a terahertz Fourier lens with a focal length of 50 mm, the Airy beam is obtained at the focus (z = 0) and detected by a pyroelectric array camera (PY-IV, Spricom). Figure 5 As shown, by applying an external voltage, the switching between Gaussian and Airy beams can be controlled. When no voltage is applied, the initial phase difference between the regions with and without PEDOT electrodes is zero, so the camera detects a Gaussian beam. When a voltage of 9V or higher is applied, the initial phase difference between the regions with and without PEDOT electrodes 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, resulting in a specific phase change in the outgoing light wave. Next, when passing through the terahertz lens, the lens performs a Fourier transform on the modulated light wave, forming an Airy beam at the focal point. Because the phase distribution pattern of the liquid crystal cubic phase plate is a binary phase distribution from 0 to π, this method simultaneously generates two symmetrically inverted Airy beams.

[0046] like Figure 6 Figure 2 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, with its main lobe exhibiting a quadratic acceleration trend, meaning the beam's curved trajectory along the propagation direction changes at an increasingly rapid rate. This self-bending propagation behavior is a key characteristic of Airy beams, arising from their unique phase distribution. This property holds great potential for broad applications in precisely controlling light field distribution and manipulating 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 by the experiment shows a 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, we observed very similar experimental results under three different high-voltage conditions. This phenomenon demonstrates that liquid crystal devices have a certain tolerance and robustness to high voltages, enabling dynamic phase modulation within a certain range of drive voltages without significantly affecting the output. This robustness makes liquid crystal devices more reliable and stable in practical applications, allowing them to adapt to different electric field conditions while maintaining 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 results in the beam forming an Airy function distribution in the y-direction, exhibiting non-diffraction and self-bending characteristics, while still maintaining the Gaussian distribution characteristics in the x-direction. Figure 8 As shown in the figure, as the propagation distance in the z direction varies 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 of ≈1 mm. This verifies that the generated one-dimensional terahertz Airy beam exhibits quasi-diffraction properties in the y direction. This non-diffraction property is a key characteristic of Airy beams, demonstrating their ability to maintain optical field stability during propagation. Simultaneously, in the x-direction, due to the lack of applied phase modulation, the beam naturally diffuses along the propagation direction, with its LWHM expanding by 4.6 mm from its original value over a propagation distance of 100 mm. This diffusion is consistent with the propagation characteristics of an ordinary Gaussian beam. This result validates the effectiveness of the one-dimensional cubic phase plate and demonstrates that the generated terahertz Airy beam exhibits quasi-diffraction properties in the y direction while retaining Gaussian beam characteristics in the x-direction. This unique characteristic of the beam is 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 produce a terahertz Airy beam generating device with a short production cycle and low cost, and has good broadband optical anisotropy and electro-optical and magneto-optical modulation characteristics. It can achieve large working bandwidth and tunable performance requirements, which helps 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 solutions of the present invention rather than to limit the same. 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 solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions 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. Photolithography is performed on the DMSO-doped PEDOT:PSS organic film using a lift-off process to produce a structured electrode with a cubic phase pattern; S3, using a method of applying a vertical electric field to a structured electrode to drive a liquid crystal to produce a liquid crystal cubic phase plate required for generating an Airy beam; S4. Use terahertz liquid crystal cubic phase plate to realize the generation of 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 the phase delay value between 0 and 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 structural electrodes are made of quartz substrates 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, wherein: 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 on the substrate to form a film to form a substrate; making an external electrode, applying a low-temperature silver paste on the edge of the substrate to form a film 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 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, and placing them opposite to each other, staggering the parts with the silver paste electrode so that the silver paste electrodes on both sides are exposed to the outside, and using a Teflon gasket to control the box thickness; pouring liquid crystal and sealing 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, wherein: In S4, specifically: first, a polarizer and a half-wave plate are used to make the polarization direction of the light beam perpendicular to the rubbing direction of the liquid crystal orientation layer; then, a gold concave mirror 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. 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. Cubic phase modulation is applied to the Gaussian beam only in the y-direction, while no modulation is performed in 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.

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