CN114421129A - A Circular Trajectory Photoconductive Antenna Array for Terahertz Polarization Imaging System - Google Patents

Abstract

The invention discloses a circular track photoconductive antenna array for a terahertz polarization imaging system, comprising M antenna array units fixed on a substrate, the M antenna array units are uniformly arranged in a circle, and The consecutive 1st to M/2th antenna array units are arranged horizontally, and the (M+1)/2th to Mth antenna array units are arranged in a vertical manner to the first antenna array unit; the present invention also further Another structure is disclosed, comprising the first to Nth semicircular antenna arrays fixed on the substrate, the centers of the first to Nth semicircular antenna arrays are the same, and the first to Nth semicircular antennas The radius of the array is increasing. By changing the arrangement structure of the antenna array units on the circular track photoconductive antenna array and combining with the non-polarized imaging system, the invention can obtain the polarization information and light intensity information of the measured object under different polarization states, which is beneficial to improve the imaging system's ability to detect the target object. detection and identification capabilities.

Description

A Circular Trajectory Photoconductive Antenna Array for Terahertz Polarization Imaging System

technical field

The invention relates to the field of imaging, in particular to a circular track photoconductive antenna array used in a terahertz polarization imaging system.

Background technique

Terahertz waves refer to electromagnetic waves with frequencies in the range of 0.1-10THz, corresponding to wavelengths of 0.03-3mm, and are located between infrared and microwaves in the electromagnetic spectrum. Terahertz waves have many unique advantages. For example, compared with X-rays, terahertz photons have very low energy and will not generate ionizing radiation to damage the sample to be tested; compared with infrared light, terahertz waves can penetrate many non-polar In addition, the vibrational and rotational energy levels of most organic molecules such as polar molecules and biological macromolecules are located in the terahertz band, and the fingerprint characteristic spectrum of these molecules can be detected by using broadband terahertz spectroscopy. Based on the above characteristics, terahertz plays an important role in many fields such as food safety, biomedicine, and safety inspection.

Polarization imaging is an imaging technology that detects the polarization state of light waves of an object. It can obtain the polarization information and light intensity information of the object to be measured, and improve the detection and recognition ability of the imaging system for the target object. Target camouflage recognition and other aspects have great advantages. Terahertz polarization imaging can provide richer sample structure and optical information, and is very sensitive to subwavelength microstructural changes. It not only takes advantage of the advantages of non-destructive imaging of terahertz waves, but also effectively improves the recognition efficiency of targets. It can be applied to detect hidden or camouflaged targets, distinguish target objects from lures, and so on.

The current terahertz polarization imaging is mostly based on the ZnTe crystal method instead of the photoconductive antenna method, because the photoconductive antenna used as a detector in the photoconductive antenna-based time-domain terahertz system generally has only one electrode unit, and the direction of the electrode unit is fixed and single. , data about the polarization information of the object under test cannot be obtained.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a circular track photoconductive antenna array for a terahertz polarization imaging system. By changing the arrangement structure of the dipole electrodes on the photoconductive antenna, different The optical information of the measured object in the polarization state, through which the relevant information of the measured object can be obtained.

The present invention adopts the following technical solutions for solving the above-mentioned technical problems:

A circular track photoconductive antenna array for a terahertz polarization imaging system proposed according to the present invention includes a substrate and M antenna array units fixed on the substrate, where M is an integer greater than 5 and an even number , the M antenna array units are uniformly arranged in a circle, and the consecutive 1st to M/2th antenna array units are arranged horizontally, and the arrangement of (M+1)/2th to Mth antenna array units The pattern is an arrangement pattern perpendicular to the first antenna array element.

As a further optimization solution of the circular track photoconductive antenna array used in the terahertz polarization imaging system according to the present invention, the antenna array unit is a dipole structure.

As a further optimization solution of the circular track photoconductive antenna array used in the terahertz polarization imaging system according to the present invention, the antenna array unit is an H-shaped electrode.

A circular track photoconductive antenna array for a terahertz polarization imaging system, comprising a substrate, including first to Nth semicircular antenna arrays fixed on the substrate, each semicircular antenna array is composed of multiple The antenna array elements are uniformly composed, N is an even number greater than or equal to 2, the centers of the 1st to Nth semicircular antenna arrays are the same, the radii of the 1st to Nth semicircular antenna arrays are increasing, and the ith The semicircular antenna array is distributed on the same side, the jth semicircular antenna array is distributed on the same side, the ith semicircular antenna array and the jth semicircular antenna array are distributed on different sides, i is an odd number, j is an even number, 0<i<N, 1<j<N, there are two ways to arrange the antenna array units in the semicircular antenna array: the first way: the antenna array units in the i-th semicircular antenna array are arranged horizontally, The arrangement of the antenna array elements in the jth semicircular antenna array is perpendicular to the arrangement of the first antenna array elements; the second method: the antenna array elements in the jth semicircular antenna array are arranged horizontally, and the ith semicircular antenna array is arranged horizontally. The arrangement of the antenna array elements in the circular antenna array is perpendicular to the arrangement of the second antenna array elements.

As a further optimization solution of the circular track photoconductive antenna array used in the terahertz polarization imaging system according to the present invention, the antenna array unit is a dipole structure.

As a further optimization solution of the circular track photoconductive antenna array used in the terahertz polarization imaging system according to the present invention, the antenna array unit is an H-shaped electrode.

Compared with the prior art, the present invention adopts the above technical scheme, and has the following technical effects:

(1) By changing the arrangement structure of the dipole electrodes on the circular track photoconductive antenna array, combined with the non-polarization imaging system, the polarization information and light intensity information of the measured object under different polarization states can be obtained, which can make the imaging system provide more abundant The sample structure and optical information are beneficial to improve the detection and recognition ability of the imaging system to the target object;

(2) Because the photoconductive antenna array with circular trajectories has symmetry, it is only necessary to sample the semicircular trajectories, which reduces the number of samples and is conducive to improving the imaging speed;

(3) Using the circular track photoconductive antenna array proposed by the present invention as a terahertz detector, and replacing mechanical scanning by decoding the sample space information carried by the broadband detection wave spectrum, a single excitation can realize full Fourier space sampling, Improve the imaging speed of the system;

(4) The technology provided by the present invention can realize fast terahertz polarization imaging, so it has a very wide range of applications, such as: detecting hidden or camouflaged targets; effectively distinguishing metals and insulators or distinguishing real targets from lures; Features (such as fingerprints, etc.) for identification.

Description of drawings

1 is a schematic diagram of a circular track photoconductive antenna array with the same two semicircular radii;

2 is a schematic diagram of a circular track photoconductive antenna array with different semicircular radii;

FIG. 3 is a schematic diagram of a multi-layer circular track conductance antenna array with different radii.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, but not to limit the present invention.

The polarization imaging part, namely the circular track photoconductive antenna array with special structure, is combined with the terahertz time-domain spectroscopy system to form a terahertz polarization imaging system. The pulse generated by the femtosecond laser is divided into two paths after passing through the beam splitter, one path is the pump light and the other path is the probe light, and these two paths finally terminate on the detection array. The pump light is incident on the terahertz transmitting antenna after passing through the optical delay line. The most common antenna is the dipole structure, which consists of two long metal electrodes separated by a small gap. The pump light is focused on the small gap between the electrodes, and the photogenerated carriers (electron-hole pairs) generated in the light-absorbing region of the photoconductive antenna are accelerated by the bias electric field to generate a terahertz pulse, which passes through a pair of parabolic mirrors Then focus on the sample to be tested. The probe light is split, modulated and focused by the spatial light modulator to the gap of the circular track photoconductive antenna array element without bias voltage, and the element is gated to excite and generate free carriers. At this time, the terahertz pulse passes through the sample to be tested and is incident on the circular track detection array. As a bias electric field applied to the antenna electrode, it drives the movement of free carriers and forms a photocurrent in the detection array unit. After the transient electric field signal collected by the circular trajectory photoconductive antenna array passes through the low-noise current amplifier, it is collected in parallel by the multi-channel lock-in amplifier, which can speed up the data collection speed, and finally the sample image is reconstructed through an inverse transform algorithm.

The electrodes on a circular track photoconductive antenna array for polarized imaging (taking H-type electrodes as an example) are arranged in different directions on different semicircles. Figure 1 shows a circular track photoconductive antenna array with the same radius of the two semicircles. The electrodes on the two semicircles are arranged vertically and horizontally. Figure 2 shows two circular track photoconductive antenna arrays with different semicircular radii, wherein electrodes on one semicircular track are arranged in a vertical direction, and electrodes on the other semicircular track are arranged in a horizontal direction. Figure 3 is a multi-layer circular track photoconductive antenna array with different radii. The radius of the semicircular track increases from the inside to the outside. The electrodes on one side of the vertical symmetry axis are arranged in the vertical direction, and the electrodes on the other side are arranged in the horizontal direction. .

By decoding the broadband terahertz wave carrying the spatial information of the sample, the two-dimensional raster scan sampling is simplified to the broadband signal sampling along the circular trajectory. Since the dipole electrodes on the circular trajectory are arranged in different directions, it is possible to obtain different polarization states. Polarization information and light intensity information of the measured object. Because of the symmetry of the photoconductive antenna array with circular trajectories, only the semicircular trajectories in it need to be sampled. Taking the photoconductive antenna array of FIG. 2 as an example, since it is symmetrical up and down with respect to the horizontal symmetry axis, only the upper half of the array needs to be sampled in practice, thereby reducing the number of samples and improving the imaging speed. By changing and controlling the radius of the circular trajectory, the required polarization distribution can be generated for different colors, and the color and intensity information can be simultaneously encoded into the wavelength-dependent polarization distribution of the beam, which is expected to realize color image imaging of the object to be measured. The light intensity information can also get the details of the image.

According to the Fourier optics theory, the light field distribution S(x, y, v) of the object in the front focal plane of the lens can be obtained after the inverse Fourier transform of the light field distribution U(ξ, η, ν) in the Fourier domain, as shown in the formula (1). where v is the incident light frequency, F is the lens focal length, and c is the speed of light.

The spatial frequencies k _ξ and k _η of the Fourier domain are related to the coordinate positions (ξ, η) as follows:

Use a broadband light source (v∈[v _min ,v _max ]) to illuminate the sample to be tested and record the signal at a specific position (ξ ₀ , η ₀ ), and then change the k-space ratio η ₀ /ξ ₀ to achieve k Spatial sampling, as shown in formula (3):

为极坐标系下透镜前焦平面(即待测物体)的光场分布，

为极坐标系下透镜傅立叶面的光场分布。The easiest way to change the k-space ratio η ₀ /ξ ₀ is to measure specific data points along a certain circular trajectory. In polar coordinates, equation (1) can be rewritten as equation (4). in,

is the light field distribution of the front focal plane of the lens (that is, the object to be measured) in the polar coordinate system,

is the light field distribution of the Fourier surface of the lens in the polar coordinate system.

空间的积分可以由固定半径ρ₀对频率的积分来代替。也就是说，可以通过解码宽带太赫兹波信号代替在固定频率下对傅立叶面进行二维光栅扫描，这样只需对k空间进行圆形轨迹采样，又因为圆形轨迹的光电导天线阵列具有对称性，所以只需对其中的半圆轨迹进行采样，最后通过一种逆变换算法重建样品图像。因此，式(4)中的傅立叶空间的二维积分可以替换为对一维圆形轨迹和光谱频率的积分，即：The integral in formula (4) contains the frequency v of the detection pulse and the position ρ of the detection unit. Therefore, at a fixed frequency v ₀ , for the Fourier surface

The integral over space can be replaced by the integral over frequency with a fixed radius _p0 . That is to say, the two-dimensional raster scanning of the Fourier surface at a fixed frequency can be replaced by decoding the broadband terahertz wave signal, so that only the circular trajectory sampling in k-space is required, and because the photoconductive antenna array of the circular trajectory has symmetry Therefore, it is only necessary to sample the semicircular trajectory, and finally reconstruct the sample image through an inverse transformation algorithm. Therefore, the two-dimensional integration in the Fourier space in Eq. (4) can be replaced by the integration of the one-dimensional circular locus and spectral frequency, namely:

Combining terahertz polarization imaging technology with a non-polarization imaging system can realize fast terahertz polarization imaging. The imaging system can provide more abundant sample structure and optical information. This method can not only take advantage of the advantages of non-destructive imaging of terahertz waves, but also Effectively improve the target recognition efficiency. The invention can obtain the polarization information and light intensity information of the measured object under different polarization states by changing the arrangement structure of the dipole electrodes on the circular track photoconductive antenna array, which is beneficial to improve the detection and identification ability of the imaging system on the target object; The photoconductive antenna array with circular trajectories has symmetry, so only the semicircular trajectories need to be sampled, which reduces the number of samples and improves the imaging speed. A single excitation can achieve full sampling in Fourier space, which improves the imaging speed of the system.

The above are only specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art who is familiar with the technical scope disclosed by the present invention can easily think of changes or substitutions. All should be covered within the protection scope of the present invention.

Claims (6)

1. A circular track photoconductive antenna array for a terahertz polarization imaging system comprises a substrate and is characterized by comprising M antenna array units fixed on the substrate, wherein M is an integer larger than 5 and is an even number, the M antenna array units are uniformly arranged into a circle, the 1 st to M/2 th continuous antenna array units are horizontally arranged, and the arrangement mode of the (M +1)/2 nd to M antenna array units is a mode vertical to the arrangement mode of the 1 st antenna array unit.

2. The circular track photoconductive antenna array for the terahertz polarization imaging system as claimed in claim 1, wherein the antenna array unit is a dipole structure.

3. The circular track photoconductive antenna array for the terahertz polarization imaging system as claimed in claim 1, wherein the antenna array elements are H-shaped electrodes.

4. A circular track photoconductive antenna array for a terahertz polarization imaging system comprises a substrate and is characterized by comprising 1 st to N th semicircular antenna arrays fixed on the substrate, wherein each semicircular antenna array is composed of a plurality of antenna array units uniformly, N is an even number which is more than or equal to 2, the circle centers of the 1 st to N th semicircular antenna arrays are the same, the radiuses of the 1 st to N th semicircular antenna arrays are increased progressively, the ith semicircular antenna array is distributed on the same side, the jth semicircular antenna array is distributed on the same side, the ith semicircular antenna array and the jth semicircular antenna array are distributed on different sides, i is an odd number, j is an even number, i is more than 0 and less than N and 1 is less than j and N, and the antenna array units in the semicircular antenna arrays are arranged in two modes: the first mode is as follows: the antenna array units in the ith semicircular antenna array are horizontally arranged, and the arrangement mode of the antenna array units in the jth semicircular antenna array is vertical to the arrangement mode of the 1 st antenna array unit; the second mode is as follows: the antenna array units in the jth semicircular antenna array are arranged horizontally, and the arrangement mode of the antenna array units in the ith semicircular antenna array is perpendicular to the arrangement mode of the 2 nd antenna array units.

5. The circular track photoconductive antenna array for the terahertz polarization imaging system as claimed in claim 4, wherein the antenna array unit is a dipole structure.

6. The circular track photoconductive antenna array for the terahertz polarization imaging system as claimed in claim 4, wherein the antenna array elements are H-shaped electrodes.

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