A terahertz long-focus, deep-depth dual-mode imaging system based on metasurfaces

Abstract

The present invention discloses a metasurface-based terahertz (THz) dual-mode imaging system with a long focal depth, comprising a THz signal transceiver module and a spatial filtering module. The THz signal transceiver module is used to transmit THz signals and receive THz signals on an imaging plane. The spatial filtering module is used to perform multifunctional filtering on an imaging object illuminated by the THz signal using the metasurface, and then emit the THz signal to the imaging plane. The spatial filtering module simultaneously presents a bright-field image and an edge-enhanced image on the imaging plane, thereby achieving THz dual-mode imaging with a long focal depth. The multifunctional filtering includes diffraction-limited imaging and vortex filtering based on the metasurface. The system uses a single metasurface to achieve THz dual-mode imaging with a long focal depth, eliminating the need for a complex 4f filtering system. The entire imaging system is simple and compact, and offers higher resolution and better imaging quality than traditional THz imaging.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz imaging, and in particular relates to a terahertz long-focus-depth dual-mode imaging system based on a metasurface. Background Art

[0002] Terahertz waves have excellent penetrating properties, while their single-photon energy is low, far below the damage threshold of biological tissue, making them suitable for biomedical imaging. In microscopy, traditional brightfield imaging relies primarily on the intensity change of light waves passing through an object to detect amplitude-dependent objects. However, when light waves pass through phase-dependent objects, only the phase changes, while the intensity remains largely unchanged, making brightfield imaging ineffective.

[0003] To detect such transparent objects, researchers have proposed a variety of techniques, among which dark-field and phase contrast methods are widely used. Spiral phase contrast imaging, in particular, is a key optical image processing method. It convolves the imaging target with a phase factor exp(-jlφ), causing flat areas in the target image to weaken due to destructive interference, while areas with significant phase or amplitude changes receive amplification, significantly improving imaging contrast and making the object's boundaries clearer.

[0004] In the field of image processing, brightfield imaging and spiral phase contrast imaging are used to reveal the complete morphological information and edge contour information of an object, respectively, and are indispensable for image processing and pattern recognition. Especially when faced with complex object recognition scenarios, real-time and detailed sample information is crucial. Therefore, the simultaneous realization of brightfield imaging and edge detection of objects has become a key to technological development.

[0005] In 2023, Y. Zhang et al. proposed a metasurface-based synchronous dual-mode imaging system in Dielectric Metasurface for Synchronously Spiral Phase Contrast and Bright-Field Imaging. This system cleverly integrates multifunctional spatial filtering onto a single metasurface, supporting not only diffraction-limited focused imaging but also edge detection through a spatial light modulator with a spiral phase, thereby simultaneously displaying bright-field images and edge-enhanced images in the same imaging plane. However, due to the characteristics of focused Gaussian beams and OAM beams, the system has a limited focal depth and can only support fixed-position 2D / 3D imaging. Summary of the Invention

[0006] In view of the above, the present invention aims to provide a metasurface-based terahertz long-focus-depth dual-mode imaging system. This system does not require a complex 4f filter system and utilizes only a single metasurface to achieve long-focus-depth, simultaneous bright-field imaging and spiral phase-contrast imaging.

[0007] To achieve the above-mentioned purpose of the invention, the embodiment provides a terahertz long-focus-depth dual-mode imaging system based on a metasurface, comprising a terahertz signal transceiver module and a spatial filtering module;

[0008] The terahertz signal transceiver module is used to transmit terahertz signals and receive terahertz signals in the imaging plane;

[0009] The spatial filtering module is used for the metasurface to perform multifunctional filtering on the imaging object illuminated by the terahertz signal, and then emit the terahertz signal to the imaging plane, and simultaneously present a bright field image and an edge-enhanced image on the imaging plane, thereby realizing terahertz long-focus-depth dual-mode imaging;

[0010] Among them, multifunctional filtering includes diffraction-limited imaging and vortex filtering based on metasurfaces.

[0011] Preferably, the expression of the equivalent phase function φ(x,y) achieved by the designed metasurface during multifunctional filtering is:

[0012]

[0013] Where angle{·} is the phase acquisition function, x is the horizontal coordinate of the metasurface, and i is the imaginary unit. and r are the angular and radial coordinates of the metasurface, respectively, k0 is the free space wave number, α is the cone angle of the axicon, and β is the deflection angle of the beam.

[0014] Preferably, the cone angle α of the axicon is less than or equal to 20°.

[0015] Preferably, the deflection angle β of the beam is selected in such a manner that the deflection angle is selected with the goal of minimizing crosstalk between the two images and avoiding off-axis aberrations during the imaging process.

[0016] Preferably, the designed metasurface generates a zero-order Bessel beam transmitted in the (0, β) direction and a high-order Bessel beam transmitted in the (0, -β) direction under the excitation of a single feed terahertz signal. These two beams have the characteristics of diffraction-free propagation and can achieve high-resolution imaging in long-focus and deep-depth dual modes.

[0017] Preferably, the terahertz signal transceiver module includes a vector network analyzer equipped with a pair of spectrum spreading modules, a terahertz cable, a terahertz horn antenna and a probe antenna;

[0018] The S1 port of the vector network analyzer is connected to a spectrum spreading module and then connected to a terahertz horn antenna through a terahertz cable to form the transmitting part of the terahertz signal;

[0019] The S2 port of the vector network analyzer is connected to another spectrum spreading module and then connected to the terahertz probe antenna through another terahertz cable to form the receiving part of the terahertz signal.

[0020] Preferably, the terahertz signal transceiver module further includes a three-dimensional scanning platform and a control system;

[0021] The terahertz probe antenna is arranged on a three-dimensional scanning platform, and the step-controlled scanning of the terahertz probe antenna is realized through the control system to complete the reception of the terahertz signal in the imaging plane.

[0022] Preferably, the system further comprises a collimating lens, and the terahertz signal emitted by the terahertz signal transceiver module is collimated by the collimating lens and then irradiated onto the imaging object.

[0023] The vector network analyzer (VNA) serves as both the signal source for generating terahertz signals and the transceiver link for transmitting and receiving terahertz signals. The terahertz signal generated by the VNA is output from the S1 port and transmitted via a terahertz cable to the terahertz horn antenna. After collimation by a lens, it radiates toward the imaging object. After spatial filtering by the metasurface, the transmitted terahertz signal simultaneously produces a bright-field image and an edge-enhanced image on the imaging plane. The axicon phase of the metasurface enables the imaging system to achieve long-depth-of-focus imaging. The terahertz signal on the imaging plane is received by the receiving terahertz probe antenna and transmitted via a terahertz cable to the S2 port of the VNA. To receive signals across the entire imaging plane, the probe antenna is placed on a 3D scanning platform, where precise step-by-step scanning is achieved via PC control. Finally, the S21 parameter, measured by the VNA, completes terahertz signal transmission and reception.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The present invention uses a single metasurface to achieve long-focus-depth terahertz dual-mode imaging without the need for a complex 4f filter system. The entire imaging system is simple and compact, and has higher resolution and better imaging quality than traditional terahertz imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0027] Figure 1 1 is a schematic structural diagram of a metasurface-based terahertz long-focus-depth dual-mode imaging system provided in an embodiment;

[0028] Figure 2 is a phase distribution diagram of the metasurface device provided in the embodiment;

[0029] Figure 3 This is a diagram of the terahertz long-focus depth dual-mode imaging results based on the metasurface in an embodiment of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is 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 and do not limit the scope of protection of the present invention.

[0031] like Figure 1 As shown in the figure, the metasurface-based terahertz long-focus-depth dual-mode imaging system provided in the embodiment includes a terahertz signal transceiver module and a spatial filtering module. The terahertz signal transceiver module includes a vector network analyzer equipped with a pair of 75-110 GHz spectrum spreader modules, a terahertz cable, a terahertz horn antenna and a probe antenna, and a 3D scanning platform. The spatial filtering module includes a lens, an imaging object, and a metasurface.

[0032] The S1 port of the vector network analyzer is connected to the terahertz horn antenna through a terahertz cable to form the terahertz signal transmission part, and the S2 port of the vector network analyzer is connected to the terahertz probe antenna through another terahertz cable to form the terahertz signal receiving part; the vector network analyzer is used as a signal source for generating terahertz signals and a transceiver link for transmitting and receiving terahertz signals; the terahertz signal generated by the vector network analyzer is output from the S1 port, transmitted to the terahertz horn antenna via the terahertz cable, collimated by the lens, and radiated toward the imaging object. The transmitted terahertz signal passes through the lens and then through the metasurface for spatial filtering, where the multifunctional filtering includes diffraction-limited imaging and vortex filtering based on the metasurface. The terahertz signal of the transmitted wave is received by the receiving terahertz probe antenna and transmitted to the S2 port of the vector network analyzer via the terahertz cable; finally, the S21 parameter measured by the vector network analyzer completes the terahertz signal transmission and reception.

[0033] The metasurface used to achieve spatial filtering is an artificial layered material with a thickness less than the wavelength. It can control the polarization, phase, amplitude, frequency and other characteristics of electromagnetic waves through sub-wavelength microstructures. The phase diagram of the metasurface is as follows: Figure 2 As shown, the horizontal and vertical coordinates represent the position coordinates of the metasurface in the x and y directions. The expression of the equivalent phase function φ(x,y) of the metasurface device is:

[0034]

[0035] where angle{·} represents the phase acquisition function, and r are the angular and radial coordinates of the metasurface, respectively, k0 is the free space wave number, α=10 is the cone angle of the axicon, and β=15° is the deflection angle of the beam. The deflection angle should be selected to minimize the crosstalk between the two images as much as possible while avoiding off-axis aberrations during the imaging process.

[0036] When excited by a single feed horn antenna, the metasurface generates a zero-order Bessel beam propagating toward (0°, 15°) and a high-order Bessel beam propagating toward (0°, -15°). These two beams exhibit non-diffraction propagation, enabling the system to achieve high-resolution imaging over a long range.

[0037] After the spatial filtering is achieved by the metasurface, the probe antenna is precisely stepped and scanned on the imaging plane by controlling the PC until the terahertz signal is received on the entire plane. The imaging results at different positions are obtained by adjusting the distance from the two-dimensional plane to the metasurface, such as Figure 3 As shown, on the left side of each imaging plane, spiral phase contrast imaging effectively filters out low-frequency information and enhances the edge contour of the object, while the bright field image on the right side presents the complete morphology of the object.

[0038] The specific implementation methods described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A terahertz long-focus-depth dual-mode imaging system based on a metasurface, characterized in that: Including terahertz signal transceiver module and spatial filtering module; The terahertz signal transceiver module is used to transmit terahertz signals and receive terahertz signals in the imaging plane; The spatial filtering module is used for the metasurface to perform multifunctional filtering on the imaging object illuminated by the terahertz signal, and then emit the terahertz signal to the imaging plane, and simultaneously present a bright field image and an edge-enhanced image on the imaging plane, thereby realizing terahertz long-focus-depth dual-mode imaging; Among them, multifunctional filtering includes diffraction-limited imaging and vortex filtering based on metasurfaces; The expression of the equivalent phase function φ(x,y) achieved by the designed metasurface during multifunctional filtering is: Where angle{·} is the phase acquisition function, x is the horizontal coordinate of the metasurface, and i is the imaginary unit. and r are the angular and radial coordinates of the metasurface, respectively, k0 is the free space wave number, α is the cone angle of the axicon, and β is the deflection angle of the beam.

2. The metasurface-based terahertz long-focus-depth dual-mode imaging system according to claim 1, characterized in that: The cone angle α of the axicon is less than or equal to 20°.

3. The metasurface-based terahertz long-focus-depth dual-mode imaging system according to claim 1 or 2, characterized in that: The deflection angle β of the beam is selected in such a manner that the crosstalk between the two images is reduced as much as possible and the off-axis aberration during the imaging process is avoided.

4. The metasurface-based terahertz long-focus-depth dual-mode imaging system according to claim 1, characterized in that: Under the excitation of a single feed terahertz signal, the designed metasurface generates a zero-order Bessel beam transmitted in the (0, β) direction and a high-order Bessel beam transmitted in the (0, -β) direction. These two beams have the characteristics of diffraction-free propagation and can achieve long-focus-depth dual-mode high-resolution imaging.

5. The metasurface-based terahertz long-focus-depth dual-mode imaging system according to claim 1, characterized in that: The terahertz signal transceiver module includes a vector network analyzer equipped with a pair of spectrum spreading modules, a terahertz cable, a terahertz horn antenna and a probe antenna; The S1 port of the vector network analyzer is connected to a spectrum spreading module and then connected to a terahertz horn antenna through a terahertz cable to form the transmitting part of the terahertz signal; The S2 port of the vector network analyzer is connected to another spectrum spreading module and then connected to the terahertz probe antenna through another terahertz cable to form the receiving part of the terahertz signal.

6. The metasurface-based terahertz long-focus-depth dual-mode imaging system according to claim 5, characterized in that: The terahertz signal transceiver module also includes a three-dimensional scanning platform and a control system; The terahertz probe antenna is arranged on a three-dimensional scanning platform, and the step-controlled scanning of the terahertz probe antenna is realized through the control system to complete the reception of the terahertz signal in the imaging plane.

7. The metasurface-based terahertz long-focus-depth dual-mode imaging system according to claim 1, characterized in that: It also includes a collimating lens. The terahertz signal emitted by the terahertz signal transceiver module is collimated by the collimating lens and then irradiated onto the imaging object.

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