A Terahertz Single Imaging Method and System Based on Metasurface Structure

Abstract

A terahertz single-shot imaging method and system based on a metasurface structure is disclosed. The method includes: placing the metasurface structure above or in front of an imaging object, wherein the metasurface structure comprises a pixel array consisting of several pixel units, and the pixel units correspond one-to-one with the imaging units of the imaging object and have the same parameters; perpendicularly penetrating the metasurface structure and the imaging object with a terahertz beam to obtain the metasurface terahertz transmission frequency domain spectrum and the imaging object terahertz transmission frequency domain spectrum, respectively; constructing a resonance intensity detection limit model to reconstruct the pixel distribution of the imaging object, thereby realizing terahertz single-shot imaging of the imaging object. This application eliminates the need for adaptive correction algorithms and complex external modulation and decoding equipment, reconstructs the image of the object under test using the resonant frequency, reduces imaging time, and increases sampling speed, solving the problems of slow sampling speed, low compatibility, and inability to perform real-time imaging in existing terahertz single-shot imaging technologies.

Description

Technical Field

[0001] This application relates to the field of terahertz imaging technology, and in particular to a terahertz single-shot imaging method and system based on metasurface structures. Background Technology

[0002] Terahertz waves have a frequency of 1×10⁻⁶. 12 Terahertz electromagnetic radiation is on the order of Hertz. Terahertz technology encompasses various aspects of military, security, and civilian life, including secure communications, satellite communications, security inspection, biopharmaceuticals, early detection of cancer cells, and industrial quality inspection. Terahertz imaging, as one of the key applications of terahertz technology, is functionally similar to visible light or infrared spectral imaging. However, the photon energy (meV) of terahertz waves is relatively low, and only a few semiconductor materials have sufficiently small bandgap energies to achieve effective terahertz photon detection. Field-effect terahertz sensors, pyroelectric detectors, and terahertz micro-radiometry calorimeters can operate at room temperature, but it is difficult to balance sensitivity, detection wavelength range, and imaging speed. Furthermore, the aforementioned terahertz array sensors can only resolve the intensity response of the imaged object, lacking phase information. In contrast, terahertz time-domain scanning imaging can provide amplitude, phase, and ultrafast time and spectral information of the object under inspection, but its point-by-point scanning imaging method limits its application in large-area imaging or real-time in-situ detection. How to reduce redundant operations and achieve single-shot imaging through non-point-by-point scanning to improve the efficiency of terahertz imaging is a current technical challenge.

[0003] Existing terahertz single-shot imaging mainly falls into two categories: pump pulse-coded terahertz detection and coded terahertz illumination. Encoding and modulating terahertz or probe light generally requires the use of metallic masks, artificial metamaterials, or digital micromirrors. Current technologies suffer from several drawbacks: spatial encoding of terahertz or probe light requires continuous mask changes, resulting in long sampling times for computational imaging, slow imaging speeds, and an inability to achieve real-time imaging. Furthermore, they require external modulation and decoding equipment, leading to low compatibility.

[0004] Therefore, existing technologies still need to be improved and developed. Summary of the Invention

[0005] In view of the shortcomings of the prior art, this application provides a terahertz single-shot imaging method and system based on metasurface structure. It does not require adaptive correction algorithms and complex external modulation and decoding equipment. It can reconstruct the image of the object under test by utilizing the resonant frequency and intensity parameters in the terahertz transmission radio frequency domain spectrum, reducing imaging time and thus improving sampling speed. It can achieve near real-time or even real-time imaging frame rate. Furthermore, the independence of each pixel unit in this application enables the method to be compatible with imaging and near-field sensing applications, solving the problems of slow sampling speed, low compatibility, and inability to perform real-time imaging in the prior art.

[0006] This application is achieved through the following technical solution:

[0007] The first aspect of this application is to provide a terahertz single-shot imaging method based on metasurface structures, comprising the following steps:

[0008] A metasurface structure is placed above or in front of the imaging object. The metasurface structure includes a pixel array composed of several pixel units, and the pixel units correspond one-to-one with the imaging units of the imaging object and have the same parameters.

[0009] A terahertz beam is perpendicularly passed through the metasurface structure and the imaging object to obtain the terahertz transmission radio frequency domain spectrum of the metasurface and the terahertz transmission radio frequency domain spectrum of the imaging object, respectively.

[0010] A resonance intensity detection limit model is constructed based on the terahertz transmission frequency domain spectrum of the metasurface and the terahertz transmission frequency domain spectrum of the imaging object to reconstruct the pixel distribution of the imaging object and realize terahertz single imaging of the imaging object.

[0011] Specifically, the pixel unit is composed of several microstructure blocks, and each microstructure block includes a set of tilted elliptical silicon pillars with a two-dimensional structure based on symmetry breaking; the tilt angle of the tilted elliptical silicon pillars is 8° to 12°.

[0012] Specifically, any pixel unit of the metasurface structure generates only one resonant frequency in the terahertz spectrum range of 0.55THz to 0.75THz.

[0013] Specifically, the parameter consistency includes that any corresponding set of imaging units and pixel units have the same size and the same spatial position.

[0014] Specifically, the terahertz beam is obtained through the following steps:

[0015] A femtosecond pulsed laser excites a terahertz source to generate a diverging terahertz wave;

[0016] The terahertz wave is focused by several off-axis parabolic mirrors to form a collimated terahertz beam.

[0017] Specifically, the plurality of off-axis parabolic mirrors include a first mirror located facing the emission direction of the terahertz source, a third mirror located above the metasurface structure, and a second mirror located between the first mirror and the third mirror;

[0018] The terahertz wave is collimated into a terahertz wave after passing through the first mirror;

[0019] The collimated terahertz wave is focused by the second and third mirrors to form a collimated terahertz beam with a smaller spot diameter.

[0020] Specifically, the terahertz beam penetrates perpendicularly through the metasurface structure and the imaging object to obtain the terahertz transmission frequency domain spectrum of the metasurface and the terahertz transmission frequency domain spectrum of the imaging object, respectively, including the following steps:

[0021] The terahertz beam is incident perpendicularly and penetrates the metasurface structure to obtain the terahertz transmission signal of the metasurface structure. The terahertz transmission signal of the metasurface structure is converted into the terahertz transmission radio frequency domain spectrum of the metasurface structure through Fourier transform. Each pixel unit has a corresponding metasurface structure resonant frequency in the terahertz transmission radio frequency domain spectrum of the metasurface structure.

[0022] The terahertz beam passing through the metasurface structure is incident perpendicularly and penetrates the light-transmitting region of the imaging object, generating a terahertz transmission signal of the imaging object. The terahertz transmission signal of the imaging object is then converted into the terahertz transmission radio frequency domain spectrum of the imaging object through a Fourier transform; wherein, each imaging unit has corresponding spatial information.

[0023] Specifically, the step of constructing a resonance intensity detection limit model based on the terahertz transmission frequency domain spectrum of the metasurface and the terahertz transmission frequency domain spectrum of the imaging object to reconstruct the pixel distribution of the imaging object and realize terahertz single imaging of the imaging object includes the following steps:

[0024] By comparing the difference in resonant frequency intensity between the imaging unit and the corresponding pixel unit, a resonant intensity detection limit model is constructed.

[0025] Quantify the differences in the resonance intensity detection limit models and establish judgment criteria;

[0026] The pixel distribution of the imaging object is reconstructed according to the judgment criteria to restore the image of the imaging object and complete the terahertz single imaging.

[0027] A second aspect of this application is to provide a terahertz single-shot imaging system based on metasurface structures, including an excitation device, a transmission device, and an imaging processing device.

[0028] Excitation device, used to excite terahertz beams;

[0029] A transmission device is used to focus and transmit the terahertz beam, so that the terahertz beam is incident perpendicularly and passes through the metasurface structure and the imaging object in sequence, and outputs a terahertz transmission signal.

[0030] An imaging processing device is used to receive the terahertz transmission signal and convert it into a time-domain signal, convert the time-domain signal into a terahertz transmission radio frequency domain spectrum through Fourier transform, obtain the metasurface terahertz transmission radio frequency domain spectrum and the imaging object terahertz transmission radio frequency domain spectrum, and construct a resonance intensity detection limit model to reconstruct the pixel distribution of the imaging object, thereby realizing terahertz single imaging of the imaging object.

[0031] Specifically, the imaging processing device includes a signal receiving and processing module and an imaging module.

[0032] Specifically, the signal receiving and processing module includes a terahertz receiving antenna for receiving the terahertz transmitted signal and converting it into a time-domain signal, and converting the time-domain signal into the terahertz transmission radio frequency domain spectrum via Fourier transform, thereby generating the metasurface terahertz transmission radio frequency domain spectrum and the imaging object terahertz transmission radio frequency domain spectrum.

[0033] Specifically, the imaging module is used to construct a resonance intensity detection limit model based on the terahertz transmission frequency domain spectrum of the metasurface and the terahertz transmission frequency domain spectrum of the imaging object, so as to reconstruct the pixel distribution of the imaging object, realize the image restoration of the imaging object, and complete the terahertz single imaging.

[0034] Compared with existing technologies, this application has the following advantages: This application provides a terahertz single-shot imaging method and system based on metasurface structure, which does not require adaptive correction algorithms and complex external modulation and decoding equipment. It can reconstruct the image of the object under test by utilizing the resonant frequency and intensity parameters in the terahertz transmission radio frequency domain spectrum, reducing imaging time, increasing sampling speed, and achieving near real-time or even real-time imaging frame rates. Furthermore, the independence of each pixel unit in this application enables the method to be compatible with imaging and near-field sensing applications, solving the problems of slow sampling speed, low compatibility, and inability to perform real-time imaging in existing terahertz single-shot imaging technologies. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a flowchart illustrating a terahertz single-shot imaging method based on metasurface structures, as an embodiment of this application.

[0037] Figure 2 This is a schematic diagram of a two-dimensional structure of a tilted elliptical silicon pillar based on symmetry breaking, which is an embodiment of the metasurface structure of this application.

[0038] Figure 3 This is a schematic diagram of the composition structure of a metasurface according to an embodiment of this application.

[0039] Figure 4 This is a schematic diagram of a terahertz single-shot imaging method based on metasurface structures, which is an embodiment of this application.

[0040] Figure 5 This is a schematic diagram of a terahertz single-shot imaging system based on a metasurface structure, as an embodiment of this application.

[0041] Figure 6 This is a structural diagram of a terahertz single-shot imaging system based on a metasurface structure, according to an embodiment of this application. Detailed Implementation

[0042] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0043] It should be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the embodiments of this application. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0044] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0045] Terahertz single-shot imaging is a type of correlated computational imaging that replaces point-by-point scanning-based imaging by combining a single-pixel detector with spatial encoding of the probe light field. Existing terahertz single-shot imaging technologies are mainly divided into two categories: pump pulse-coded terahertz detection and coded terahertz illumination. Encoding and modulating the terahertz or probe light generally requires the use of metallic masks, artificial metamaterials, or digital micromirrors. The basic process of terahertz single-shot imaging is as follows: First, the terahertz or probe light is spatially encoded; then, the spatial distribution of the perturbation light field is transmitted onto a metasurface device, thereby using the spatially modulated metasurface to sample the terahertz scattering field of the object being imaged; finally, an intensity detector reads the terahertz field intensity values ​​corresponding to different spatial codes. After acquiring a series of data, the image of the object is reconstructed by combining the Hadmar matrix and a compressed imaging algorithm, and parameters such as imaging resolution and imaging frame rate are calculated. This spatial modulation of the perturbation light field avoids the addressing process of the functional primitives in electronic control, greatly reducing the complexity of the spatial light modulation device; the modulation spatial resolution of the device is only limited by the diffraction limit of short-wavelength perturbation light, thus enabling extremely high imaging resolution for terahertz waves.

[0046] However, in existing technologies, spatial encoding of terahertz or probe light requires continuous mask changes (according to the Nyquist sampling theorem, the number of masks must be at least equal to the number of pixels), resulting in a longer sampling time for computational imaging compared to focal plane imaging schemes. Improving the sampling speed to achieve near-real-time or even real-time imaging frame rates (>26 frames / s) while maintaining the image signal-to-noise ratio is a problem that needs to be addressed in the development of terahertz single-pixel imaging.

[0047] In view of this, this application proposes a terahertz single-shot imaging method based on pixelated metasurfaces with bound states in the continuum (BICs). Each pixel unit generates a high-quality-factor BIC resonance spectrum in a parallel terahertz incident light field, and the resonance spectra of the pixel units in the metasurface structure do not interfere with each other. Utilizing the spatial correspondence between the metasurface pixel units and the imaged object, a resonance intensity detection model is constructed by analyzing the terahertz resonance spectrum transmitted through the imaged object. By performing a single terahertz spectral detection on the imaged object, the image of the object can be reconstructed, achieving terahertz single-shot imaging.

[0048] Bound states (BICs) are a concept in quantum mechanics, referring to electronic states confined within an infinite potential well. When applied to photons, they refer to photon eigenstates below the light ray in the band structure diagram, a typical example being eigenmodes in waveguides. Bound states also exist when considering modes coupled to free space (above the light ray), i.e., bound states within a continuous state. Theoretically, such BIC modes can completely confine photons within an optical microcavity without radiating into free space, thus achieving an infinitely high radiation quality factor. To further reduce the non-radiative loss of photon oscillations, constructing BIC modes using dielectric materials can significantly improve the overall quality factor of the BIC modes. These BIC modes are closely related to the functional unitary symmetries of metasurfaces.

[0049] The proposed terahertz single-shot imaging technique based on BIC pixelated metasurfaces eliminates the need for adaptive correction algorithms and complex external modulation and coding equipment. It reconstructs the image of the object under test using resonant frequencies and intensity parameters in the terahertz transmission radio frequency domain spectrum, reducing imaging time. Furthermore, the pixelated metasurface array proposed in this invention allows for independent operation of each pixel unit, making it compatible with both imaging and near-field sensing applications, thus providing a novel spectral detection method in the terahertz field.

[0050] like Figure 1 As shown, the first aspect of this application is to provide a terahertz single-shot imaging method based on metasurface structures, comprising the following steps:

[0051] S1. Place the metasurface structure above or in front of the imaging object. The metasurface structure includes a pixel array composed of several pixel units. The pixel units correspond one-to-one with the imaging units of the imaging object and have the same parameters.

[0052] S2. A terahertz beam is incident perpendicularly onto the surface of the metasurface structure to obtain the metasurface terahertz transmission radio frequency domain spectrum and the imaging object terahertz transmission radio frequency domain spectrum, respectively.

[0053] S3. Construct a resonance intensity detection limit model based on the terahertz transmission frequency domain spectrum of the metasurface and the terahertz transmission frequency domain spectrum of the imaging object to reconstruct the pixel distribution of the imaging object and realize terahertz single imaging of the imaging object.

[0054] In the specific operation process, a metasurface structure is first fabricated according to the size and shape of the pixels of the imaging object, i.e., the imaging unit. During the imaging process, after the terahertz beam is excited, the metasurface structure is placed in the device, allowing the terahertz beam to penetrate the metasurface structure and form a terahertz transmission signal of the metasurface structure. After processing, the terahertz transmission radio frequency spectrum of the metasurface is obtained. Then, the imaging object is placed below or behind the metasurface structure to obtain the terahertz transmission radio frequency spectrum of the imaging object. Therefore, the above steps S1 and S2 can also be described as:

[0055] A terahertz beam is incident perpendicularly onto the surface of a metasurface structure to obtain a metasurface terahertz transmission radio frequency spectrum. The metasurface structure includes a pixel array composed of several pixel units.

[0056] Keeping the terahertz beam constant, the imaging object is placed below or behind the metasurface structure to obtain the terahertz transmission radio frequency spectrum of the imaging object. The imaging unit and pixel unit of the imaging object correspond one-to-one and have the same parameters.

[0057] like Figure 2 Figure (a) shows a schematic diagram of the two-dimensional structure of the BIC metasurface proposed in this application, based on a tilted elliptical silicon pillar with symmetry breaking. Its corresponding terahertz transmission frequency spectrum is shown below. Figure 2 As shown in (b) of this application, the metasurface structure designed in this application generates only one high-quality resonant frequency over a relatively wide terahertz frequency range, which is crucial for single-shot imaging. The uniqueness of the resonant frequency of each pixel unit of the metasurface determines the uniqueness of the spatial modulation of parallel terahertz light. If more than one resonant frequency appears in the transmission radio frequency domain spectrum of a pixel array within a selected wavelength range, the transmission radio frequency domain spectra of multiple pixel arrays will overlap to form an envelope, making it difficult to distinguish the resonances of the BICs corresponding to each pixel array. The BIC-based metasurface structure proposed in this application can adjust the frequency range by modifying the structural parameters, as long as it ensures that each pixel unit of the metasurface structure generates only one high-quality resonant frequency over a relatively wide terahertz frequency range.

[0058] Structural parameters refer to the structural parameters of the tilted elliptical silicon pillars that make up the pixel unit. For example... Figure 3 The diagram shown is a schematic representation of the composition of the pixelated metasurface structure of this application. Figure 3 (a) is a schematic diagram of the planar structure of a metasurface structure including four pixel units i, ii, iii, and iv. The four pixel units are arranged in pairs with a certain interval, which is determined according to the arrangement of the imaging units of the imaging object, ensuring that the size and spatial position of each pixel unit are consistent with the corresponding imaging unit. The consistency of spatial position can be understood as the imaging unit and pixel unit being on the same vertical axis when the metasurface structure is above the imaging object. Figure 3 (b) is Figure 3 The diagram shows an enlarged view of one pixel unit iv of the metasurface structure in (a). Pixel unit iv consists of 3x3 adjacent microstructure blocks, each of which includes a set of tilted elliptical silicon pillars with broken symmetry. Figure 3 As shown in (c), Figure 3The magnified three-dimensional structure diagram of the first pixel in the third row of pixel unit iv in (b). Preferably, any pixel unit may include several microstructure blocks containing tilted elliptical silicon pillars, with adjacent microstructure blocks closely connected to ensure that the pixel unit completely corresponds to the corresponding imaging unit.

[0059] Specifically, the structural parameters of the tilted elliptical silicon cylinder include the major axis r of the elliptical silicon cylinder. a minor axis r b The thickness h and the tilt angle α in the two-dimensional direction (xy direction), as well as the length Px and width P of the microstructure block containing the tilted elliptical silicon pillar. y The parameters of several microstructure blocks in each pixel unit are the same. By adjusting these structural parameters, each pixel unit generates only one high-quality resonant frequency within a relatively wide terahertz frequency range. The resonant frequencies of adjacent pixel units are different. This ensures that the resonant frequencies generated by several pixel units of the same metasurface structure are different, thus guaranteeing the uniqueness of the metasurface structure in spatial modulation of parallel terahertz light.

[0060] Preferably, any pixel unit is composed of several microstructure blocks containing tilted elliptical silicon pillars. Adjacent microstructure blocks are tightly connected without gaps. Each microstructure block includes at least one set of tilted elliptical silicon pillars. The number of microstructure blocks that make up the pixel unit can be determined according to the size of the imaging unit of the imaging object, the size of the silicon pillars, and actual requirements.

[0061] Specifically, such as Figure 2 As shown, the tilt angle α of the tilted elliptical silicon pillar is 8° to 12°, preferably 10°. Optionally, the microstructure block can also have other two-dimensional tilted silicon pillars, such as circular or rectangular ones. The silicon pillars can be made of high-resistivity silicon with a conductivity greater than 10000 ohm / cm.

[0062] Specifically, in one embodiment, any pixel unit of the metasurface structure generates only one resonant frequency in the terahertz spectrum range of 0.55THz to 0.75THz, and the resonant frequency is unique.

[0063] Specifically, the one-to-one correspondence between the pixel units and the imaging units of the imaging object mentioned in step S1, and the consistency of their parameters, means that, while ensuring that the number of pixel units and imaging units is consistent, the size and spatial position of any corresponding set of imaging units and pixel units are also consistent. For example, in one embodiment, the imaging object includes N pixels (pixel units), and it is necessary to ensure that the metasurface structure also has N pixel units, with one-to-one spatial correspondence and equal size.

[0064] Specifically, S2, the terahertz beam is perpendicularly transmitted through the metasurface structure and the imaging object to obtain the terahertz transmission frequency domain spectrum of the metasurface and the terahertz transmission frequency domain spectrum of the imaging object, including the following steps:

[0065] S21. The terahertz beam is incident perpendicularly and penetrates the metasurface structure to generate a metasurface structure terahertz signal. The metasurface structure terahertz transmission signal is converted into the metasurface terahertz transmission radio frequency domain spectrum by Fourier transform. Each pixel unit has a corresponding metasurface structure resonant frequency in the metasurface terahertz transmission radio frequency domain spectrum.

[0066] S22. The terahertz beam passing through the metasurface structure is incident perpendicularly and penetrates the light-transmitting region of the imaging object, generating a terahertz transmission signal of the imaging object. The terahertz transmission signal of the imaging object is converted into the terahertz transmission radio frequency domain spectrum of the imaging object by Fourier transform; wherein, each imaging unit has corresponding spatial information.

[0067] Specifically, S3, the step of constructing a resonance intensity detection limit model based on the terahertz transmission frequency domain spectrum of the metasurface and the terahertz transmission frequency domain spectrum of the imaging object to reconstruct the pixel distribution of the imaging object and realize terahertz single imaging of the imaging object, includes the following steps:

[0068] S31. Compare the resonant frequency intensity difference between the imaging unit and the corresponding pixel unit to obtain resonance information at different spatial locations, and construct the resonance intensity detection limit model.

[0069] S32. Quantify the differences in the resonance intensity detection limit models and establish judgment criteria;

[0070] S33. Reconstruct the pixel distribution of the imaging object according to the judgment criteria to restore the image of the imaging object and complete the terahertz single imaging.

[0071] like Figure 4 The diagram shown is a schematic of a terahertz single-shot imaging method based on metasurface structures according to this application. In this embodiment, as... Figure 4 As shown in (c), the imaging object includes four square imaging units. Black represents opaque imaging units that terahertz light cannot penetrate, and white represents imaging units that terahertz light can penetrate. They are arranged in pairs, and the corresponding metasurface structure has four pixel units i, ii, iii, and iv, as shown in (c). Figure 4 As shown in (b), in the vertical direction, the imaging units and pixel units correspond one-to-one and have the same size. The number and size of the pixel units of the metasurface structure can be set according to the number and size of the imaging units of the object being imaged.

[0072] First, the pixelated metasurface structure is placed in a terahertz collimated optical path. Linearly polarized parallel terahertz light field ( Figure 4 (a) is incident perpendicularly on the pixelated metasurface ( Figure 4 (b)), and covers the entire metasurface. After terahertz light is incident perpendicularly through the metasurface, the resulting terahertz transmission signal is a time-domain signal. After Fourier transform, it is converted into a frequency-domain signal. The four pixel units i, ii, iii, and iv arranged in two dimensions are excited to generate four independent high-Q resonance peaks in the spectrum (e.g., Figure 4 (e) The resonant frequencies 1, 2, 3, and 4 correspond to the four pixel units i, ii, iii, and iv, respectively. Based on this, the resonance peaks in the spectrum carry two-dimensional spatial information. The resonant frequencies in the broadband spectrum precisely correspond to the spatial positions of the pixel units, thus establishing a one-to-one mapping between spectral information and spatial information. Therefore, the pixelated metasurface design achieves modulation of the terahertz light field, using frequency information to encode terahertz spatial information.

[0073] Continue to maintain perpendicular incidence of terahertz light, and then place the imaging object below or behind the metasurface. Figure 4 (c) The imaging unit, size, spatial position, and metasurface pixel unit of the imaging object are consistent. The black areas in the imaging object represent areas where terahertz light cannot pass through, and the white areas represent areas with high terahertz transmittance. After inserting the imaging object into the terahertz optical path, the terahertz transmission signal penetrating the imaging object is measured. The time-domain signal is then transformed using Fourier Transform (FFT) to obtain the terahertz transmission frequency-domain spectrum of the imaging object, as shown in Figure 1. Figure 4 As shown in (f). Because the spatial location of the imaging unit and the pixel unit of the metasurface are mutually mapped, when the terahertz structure light field cannot penetrate the location of the non-transparent imaging unit of the imaging object, the resonance corresponding to that spatial location in the terahertz transmission frequency domain spectrum of the imaging object disappears. For example... Figure 4 In (c), the black imaging unit positions corresponding to pixel units ii and iv are terahertz non-transparent regions, therefore Figure 4 In (f), the resonant frequencies 2 and 4 disappear. In contrast, the resonant frequencies 1 and 3 show almost no change. Based on this, the distribution of imaging cells of the imaged object in the terahertz light field is obtained through the resonant frequencies.

[0074] Obtaining the metasurface terahertz transmission radio frequency domain spectrum ( Figure 4 (e) and the terahertz transmission radio frequency spectrum of the imaged object ( Figure 4 After (f), a resonance intensity detection limit model was constructed. Figure 4 (g)) reconstructs the pixel distribution of the object to be imaged. Figure 4(d)). A criterion is established by quantifying the differences in the terahertz transmission frequency spectrum, i.e., ΔTi(ω)=|T0(ω)-Tr(ω)| / ΔIi, where Tr(ω) represents the reference spectrum, i.e., the terahertz transmission frequency spectrum of the metasurface structure measured without an imaging object, T0(ω) is the spectrum modulated by the imaging object, i.e., the terahertz transmission frequency spectrum of the imaging object, and ΔIi is defined as the peak intensity of the resonance peak of each individual pixel unit of the metasurface. By judging whether the difference is greater than or equal to a threshold (defined as 0.5), the spatial distribution of terahertz transparent and opaque regions can be directly visualized. Figure 4 (d)). This allows for real-time imaging without the need for external modulation and decoding equipment.

[0075] Specifically, a terahertz beam is obtained through the following steps:

[0076] A femtosecond pulsed laser excites a terahertz source to generate a diverging terahertz wave;

[0077] The terahertz wave is focused by several off-axis parabolic mirrors to form a collimated terahertz beam.

[0078] To realize a terahertz single-shot imaging method based on metasurface structures, a second aspect of this application is to provide a system for terahertz single-shot imaging based on metasurface structures, such as... Figure 6 The diagram shown is a schematic of a terahertz single-shot imaging system based on a metasurface structure according to this application, including:

[0079] Excitation device 10 is used to excite a terahertz beam;

[0080] Transmission device 20 is used to focus and transmit the terahertz beam, so that the terahertz beam is incident perpendicularly and passes through the metasurface structure and the imaging object in sequence, and outputs a terahertz transmission signal.

[0081] The imaging processing device 30 is used to receive the terahertz transmission signal and convert it into a time-domain signal, convert the time-domain signal into the terahertz transmission radio frequency domain spectrum through Fourier transform, obtain the metasurface terahertz transmission radio frequency domain spectrum and the imaging object terahertz transmission radio frequency domain spectrum, and construct a resonance intensity detection limit model to reconstruct the pixel distribution of the imaging object, thereby realizing terahertz single imaging of the imaging object.

[0082] Specifically, the excitation device 10 includes a femtosecond laser, a terahertz transmitting antenna, and an optical fiber connecting the femtosecond laser and the terahertz transmitting antenna. The laser is a 1550nm femtosecond pulsed laser.

[0083] Specifically, the transmission device 20 includes an off-axis parabolic mirror for transmitting terahertz waves and terahertz beams, and a metasurface structure and an imaging object located between the off-axis parabolic mirror. Specifically, when the imaging object is located below the metasurface structure, such as... Figure 5 As shown, the plurality of off-axis parabolic mirrors include a first mirror PM1 located facing the emission direction of the terahertz source, a third mirror PM3 located above the metasurface structure, a second mirror PM2 located between the first mirror PM1 and the third mirror PM3, and a fourth mirror PM4 located behind the solid object.

[0084] The terahertz wave emitted from the terahertz transmitting antenna Tx is collimated into a vertical terahertz wave by passing through the first mirror PM1;

[0085] The collimated terahertz wave is focused by the second mirror PM2 and the third mirror PM3 to form a collimated terahertz beam with a smaller spot diameter;

[0086] The terahertz beam passes through the pixelated metasurface and / or the imaging object, and after passing through the fourth mirror PM4, transmits the terahertz transmission signal (optical signal) to the terahertz receiving antenna (Rx), thereby converting it into an electrical signal. At this point, the terahertz transmission electrical signal is a frequency domain signal, which needs to be converted to a frequency domain signal using Fourier transform.

[0087] The imaging processing device 30 includes a signal receiving and processing module 31 and an imaging module 32;

[0088] The signal receiving and processing module 31 includes a terahertz receiving antenna (Rx) for receiving the optical signal of the terahertz transmission signal and converting it into an electrical signal. The electrical signal is a time-domain signal, which is converted into a frequency-domain signal by Fourier transform, that is, generating the metasurface terahertz transmission radio frequency domain spectrum and the imaging object terahertz transmission radio frequency domain spectrum.

[0089] The imaging module 32 is used to construct a resonance intensity detection limit model based on the terahertz transmission frequency domain spectrum of the metasurface and the terahertz transmission frequency domain spectrum of the imaging object, so as to reconstruct the pixel distribution of the imaging object, realize the image restoration of the imaging object, and complete the terahertz single imaging.

[0090] like Figure 4 In one specific embodiment, the imaging object includes four positively oriented imaging units with a side length of 4.5 mm, arranged in pairs. Correspondingly, the metasurface structure also has four positively oriented pixel units i, ii, iii, and iv with a side length of 4.5 mm. The structural parameters of the microstructure blocks of the four pixel units and the tilted elliptical silicon pillars are shown in Table 1.

[0091] Structural parameters <![CDATA[r a ]]> <![CDATA[r b ]]> <![CDATA[p x ]]> <![CDATA[p y ]]> α pixel unit i 206 62 231 205 12 Pixel unit ii 196 60 221 195 10 Pixel unit iii 186 57 210 186 9 Pixel unit iv 176 54 200 177 8

[0092] like Figure 5 In this process, a 1550nm femtosecond pulsed laser is split into two beams. One beam is transmitted through an optical fiber to excite a terahertz source, generating a diverging terahertz wave. This wave passes through off-axis parabolic mirrors PM1, PM2, and PM3 to form a terahertz beam. This beam vertically penetrates the metasurface structure, generating a terahertz transmission signal that is received by the terahertz receiving antenna Rx. Additionally, when an imaging object is placed below the metasurface structure, the resulting terahertz transmission signal is also received by the receiving antenna Rx. A first mirror PM1, equipped with a two-inch diameter, 50.8mm focal length off-axis parabolic mirror, is positioned behind the terahertz transmitting antenna (Tx) to convert the diverging terahertz source into a collimated beam. Subsequently, the beam is adjusted by a second mirror PM2 (two inches in diameter, 50.8mm focal length) and a third mirror PM3 (one inch in diameter, 12.7mm focal length), achieving a 4:1 beam reduction to generate a collimated terahertz beam with a diameter of approximately 10mm. Finally, the beam is focused onto the receiving antenna (Rx) through a fourth mirror PM4, which is one inch in diameter and has a focal length of 25.4 mm.

[0093] The receiving antenna Rx receives the terahertz transmission signal in the form of light waves, converts it into an electrical signal, amplifies it through a lock-in amplifier, and finally acquires it at the computer. The computer converts the time-domain terahertz transmission signal into a terahertz transmission radio frequency domain spectrum through Fourier transform, thus obtaining the metasurface terahertz transmission radio frequency domain spectrum and the imaging object's terahertz transmission radio frequency domain spectrum. Then, a resonance intensity detection limit model is constructed to reconstruct the pixel distribution of the imaging object, achieving image restoration and completing a single terahertz imaging. This application's terahertz single-imaging method based on metasurface structures eliminates the need for adaptive correction algorithms and complex external modulation and decoding equipment. It can reconstruct the image of the measured object using the resonant frequency and intensity parameters in the terahertz transmission radio frequency domain spectrum, reducing imaging time and thus increasing sampling speed to achieve near-real-time or even real-time imaging frame rates. Furthermore, the independence of each pixel unit in this application allows the method to be compatible with imaging and near-field sensing applications, solving the problems of slow sampling speed, low compatibility, and inability to achieve real-time imaging in existing terahertz single-imaging technologies.

[0094] In summary, this application provides a terahertz single-shot imaging method and system based on a metasurface structure. The method includes: placing a metasurface structure above or in front of an imaging object, the metasurface structure comprising a pixel array composed of several pixel units, each pixel unit corresponding one-to-one with the imaging unit of the imaging object and having the same parameters; perpendicularly penetrating a terahertz beam through the metasurface structure and the imaging object to obtain the metasurface terahertz transmission frequency domain spectrum and the imaging object terahertz transmission frequency domain spectrum, respectively; constructing a resonance intensity detection limit model based on the metasurface terahertz transmission frequency domain spectrum and the imaging object terahertz transmission frequency domain spectrum to reconstruct the pixel distribution of the imaging object, thereby achieving terahertz single-shot imaging of the imaging object. This application eliminates the need for adaptive correction algorithms and complex external modulation and decoding equipment, reconstructs the image of the object under test using the resonant frequency, reduces imaging time, and increases sampling speed, solving the problems of slow sampling speed, low compatibility, and inability to perform real-time imaging in existing terahertz single-shot imaging technologies.

[0095] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions are not in essence a departure from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A terahertz single-shot imaging method based on metasurface structures, characterized in that, Includes the following steps: A metasurface structure is placed above or in front of the imaging object. The metasurface structure includes a pixel array composed of several pixel units, and the pixel units correspond one-to-one with the imaging units of the imaging object and have the same parameters. A terahertz beam is perpendicularly passed through the metasurface structure and the imaging object to obtain the terahertz transmission radio frequency domain spectrum of the metasurface and the terahertz transmission radio frequency domain spectrum of the imaging object, respectively. A resonance intensity detection limit model is constructed based on the terahertz transmission frequency domain spectrum of the metasurface and the terahertz transmission frequency domain spectrum of the imaging object to reconstruct the pixel distribution of the imaging object and realize terahertz single imaging of the imaging object. The pixel unit is composed of several microstructure blocks, and any one of the microstructure blocks includes a set of tilted elliptical silicon pillars with two-dimensional structure based on symmetry breaking. The tilt angle of the tilted elliptical silicon pillar is 8°~12°; Any pixel unit of the metasurface structure generates only one resonant frequency in the terahertz spectrum range of 0.55THz to 0.75THz; The parameter consistency includes any set of corresponding imaging units and pixel units having the same size and the same spatial position; The terahertz beam penetrates perpendicularly through the metasurface structure and the imaging object to obtain the terahertz transmission frequency domain spectrum of the metasurface and the terahertz transmission frequency domain spectrum of the imaging object, respectively, including the following steps: The terahertz beam is incident perpendicularly and penetrates the metasurface structure to obtain the terahertz transmission signal of the metasurface structure. The terahertz transmission signal of the metasurface structure is converted into the terahertz transmission radio frequency domain spectrum of the metasurface structure through Fourier transform. Each pixel unit has a corresponding metasurface structure resonant frequency in the terahertz transmission radio frequency domain spectrum of the metasurface structure. The terahertz beam passing through the metasurface structure is incident perpendicularly and penetrates the light-transmitting region of the imaging object, generating a terahertz transmission signal of the imaging object. The terahertz transmission signal of the imaging object is then converted into the terahertz transmission radio frequency domain spectrum of the imaging object through a Fourier transform; wherein, each imaging unit has corresponding spatial information. The step of constructing a resonance intensity detection limit model based on the terahertz transmission frequency domain spectrum of the metasurface and the terahertz transmission frequency domain spectrum of the imaging object to reconstruct the pixel distribution of the imaging object and achieve terahertz single imaging of the imaging object includes the following steps: By comparing the difference in resonant frequency intensity between the imaging unit and the corresponding pixel unit, a resonant intensity detection limit model is constructed. Quantify the differences in the resonance intensity detection limit models and establish judgment criteria; The pixel distribution of the imaging object is reconstructed according to the judgment criteria to restore the image of the imaging object and complete the terahertz single imaging.

2. The terahertz single-shot imaging method based on metasurface structures according to claim 1, characterized in that, The terahertz beam is obtained through the following steps: A femtosecond pulsed laser excites a terahertz source to generate a diverging terahertz wave; The terahertz wave is focused by several off-axis parabolic mirrors to form a collimated terahertz beam.

3. The terahertz single-shot imaging method based on metasurface structures according to claim 2, characterized in that, The plurality of off-axis parabolic mirrors include a first mirror facing the emission direction of the terahertz source, a third mirror above the metasurface structure, and a second mirror between the first mirror and the third mirror; The terahertz wave is collimated into a terahertz wave after passing through the first mirror; The collimated terahertz wave is focused by the second and third mirrors to form a collimated terahertz beam with a smaller spot diameter.

4. A system for terahertz single-shot imaging based on metasurface structures, used to implement the terahertz single-shot imaging method based on metasurface structures as described in any one of claims 1-3, characterized in that, include: Excitation device, used to excite terahertz beams; A transmission device is used to focus and transmit the terahertz beam, so that the terahertz beam is incident perpendicularly and passes through the metasurface structure and the imaging object in sequence, and outputs a terahertz transmission signal. An imaging processing device is used to receive the terahertz transmission signal and convert it into a time-domain signal, convert the time-domain signal into a terahertz transmission radio frequency domain spectrum through Fourier transform, obtain the metasurface terahertz transmission radio frequency domain spectrum and the imaging object terahertz transmission radio frequency domain spectrum respectively, and construct a resonance intensity detection limit model to reconstruct the pixel distribution of the imaging object, thereby realizing terahertz single imaging of the imaging object. The imaging processing device includes a signal receiving and processing module and an imaging module; The signal receiving and processing module includes a terahertz receiving antenna for receiving the terahertz transmitted signal and converting it into a time-domain signal, and converting the time-domain signal into the terahertz transmission radio frequency domain spectrum through Fourier transform, thereby generating the metasurface terahertz transmission radio frequency domain spectrum and the imaging object terahertz transmission radio frequency domain spectrum. The imaging module is used to construct a resonance intensity detection limit model based on the terahertz transmission frequency domain spectrum of the metasurface and the terahertz transmission frequency domain spectrum of the imaging object, so as to reconstruct the pixel distribution of the imaging object, realize the image restoration of the imaging object, and complete the terahertz single imaging.

Images