Terahertz wave band zooming super-structure lens and preparation method thereof

Abstract

The invention discloses a terahertz wave band zooming super-structure lens and a preparation method, the super-structure lens is composed of a substrate and a super-structure unit located above the substrate, the shape of the substrate is a cylinder, and COP is used as a structural material; the super-structure unit is of a micro-structure size, the upper surface of the substrate is provided with a plurality of rectangular units with the same size, the side length of each rectangular unit is 0.25 mm, the super-structure unit is a cross-shaped elliptic cylinder, and each super-structure unit is located at the center of the corresponding rectangular unit. The wavelength of the incident light source covers the terahertz wave band, the polarization state is arbitrary polarization, the super-structure lens is positively correlated and negatively correlated to the focal length when the environment refractive index is 1-1.6 and the incident wavelength is 0.7-0.8 mm, rapid zooming can be achieved by changing the working medium or the incident wavelength, the detection efficiency of the optical system is effectively improved, and the detection cost is reduced. And the optical zoom system is promoted to develop towards integration and miniaturization directions.

Description

Technical Field

[0001] The present invention belongs to the field of micro-nano optics, and relates to a metasurface lens structure design. In particular, it relates to a metasurface lens designed based on transmission phase in the terahertz band. It has a simple manufacturing process and can achieve fast and repeatable zooming by changing the working medium or the incident wavelength. Background Art

[0002] The terahertz (THz) band corresponds to a frequency range mainly concentrated between 0.1 and 10 THz, which is located between microwaves and far-infrared rays and is in the transition stage between electronics and photonics. Terahertz waves have low photon energy, strong penetrability, and good penetration ability for non-polar metallic materials, and have broad application prospects in fields such as high-speed communication, imaging, sensing, and biomedicine. Among many research fields, terahertz imaging technology has always been a hot topic in scientific research. By combining with terahertz time-domain spectroscopy and terahertz frequency-domain spectroscopy, high-resolution imaging and detection of small targets can be achieved.

[0003] Metasurfaces are two-dimensional array structures composed of sub-wavelength microstructural units, which can precisely control the phase, polarization, amplitude, etc. of the beam. As one of the specific applications of metasurfaces, metalenses achieve the focusing function of traditional lenses in a thin, planar, and miniaturized manner. Traditional terahertz imaging systems include large-sized lens groups, which limit their development in integration and miniaturization. Existing terahertz band metalenses have made certain progress in the zoom function, but still face problems such as chromatic aberration, diffraction limit, manufacturing process complexity, and material loss. The Chinese patent application with the patent number CN202310173571.4 discloses a structure of an achromatic metalens based on the principle of nonlinear phase compensation. This structure includes metasurface units composed of titanium dioxide materials with eight different cross-sectional shapes and cylinders composed of silica materials as the substrate. The multi-shaped metasurface units of this invention require more complex geometric designs and calculation methods, which increase the difficulty of design and optimization, and may also extend the R & D cycle. In addition, multi-shaped metasurface units require higher manufacturing precision, which may lead to an increase in processing costs, especially in large-scale production, and this cost problem may be more obvious. Summary of the Invention

[0004] In order to improve the focusing ability of the metalens in the terahertz band, have the characteristic of being insensitive to incident polarization, and can achieve fast zooming by changing the environmental medium or the incident wavelength, the present invention provides a transmissive metalens composed of dielectric materials and designed based on transmission phase.

[0005] A terahertz-band zoom meta-lens, wherein the meta-lens is composed of a substrate and meta-units located above the substrate. The substrate is in the shape of a cylinder and is made of COP as the structural material. The meta-units are of microstructural dimensions. The upper surface of the substrate has a number of rectangular units of the same size, and the side length of each rectangular unit is 0.25 mm. The meta-units are cross-shaped elliptical cylinders, and each meta-unit is located at the center of the corresponding rectangular unit.

[0006] Further, the meta-units are made of Si as the structural material; the lower surface of the meta-units is attached to the upper surface of the substrate.

[0007] Further, there are 13 rows and 13 columns of meta-units, which are distributed in an array on the surface of the substrate. The cross-section of the meta-units is a cross-shaped ellipse, and the ratio of the major axis to the minor axis of each ellipse is 3:1.

[0008] Further, the above-mentioned meta-lens operates in the terahertz band, the terahertz wavelength is 0.7 mm - 0.8 mm, the refractive index of the working medium is 1 - 1.6, the working mode is transmissive, and the polarization state of the incident light is arbitrary polarization.

[0009] Further, the height of the substrate is 0.5 mm. The height of the meta-units is 0.85 mm.

[0010] Further, the distance from each meta-unit to the center of the substrate is less than the radius (R) of the substrate. The distance from the center of each meta-unit to the center of the substrate is r, and the cross-sectional dimensions of the meta-units with equal distance (r is a constant value) from the center of the substrate are also equal.

[0011] Further, the diameter D of the meta-lens is 11 mm, the focal length f is 11 mm, the transmittance of the meta-lens is 85%, and the average focusing efficiency is 51%; the operating temperature range is -30 - 60 °C.

[0012] A preparation method of a terahertz-band zoom meta-lens:

[0013] S1. Prepare the substrate:

[0014] The COP is processed into a substrate in the shape of a cylinder with a thickness of 0.5 mm by methods such as injection molding or laser cutting. This process needs to be carried out under low-temperature conditions to avoid the curing of COP above 80 °C.

[0015] S2. Process the microstructure profile on the upper surface of the substrate

[0016] Use photolithography and etching methods to process the corresponding microstructure profile on the upper surface of the substrate.

[0017] S3. Material deposition

[0018] A layer of silicon material is deposited on the top of the microstructure profile by chemical vapor deposition method to change the refractive index and dispersion characteristics of light, so as to obtain a metasurface lens.

[0019] S4. Integration and testing

[0020] Test the optical performance, mechanical performance and durability of the metasurface lens, and adjust the design according to the test results.

[0021] Compared with the prior art, the advantages of the present invention are:

[0022] 1. The present invention provides a transmissive metasurface lens operating in the terahertz band. The metasurface lens includes a substrate with COP as the structural material at the bottom and metasurface units with Si as the structural material at the top. The substrate is in the shape of a cylinder, and the model of the metasurface unit is two vertically orthogonal elliptical cylinders with the same shape parameters. This makes its design and manufacturing relatively simple, with lower processing costs, suitable for large-scale production, and has high consistency and stability in optical performance, which is particularly important for optical systems that require precise control. The lower surface of the metasurface unit fits with the upper surface of the substrate, and the metasurface units are distributed in an array on the substrate. The distance from the metasurface unit to the center of the metasurface lens is less than the radius of the substrate. The metasurface units at the same radius position in the array have the same cross-section and size, while the metasurface units at different radius positions have different cross-sections or sizes. The present invention is used for focusing in the terahertz band. The metasurface units based on the transmission phase principle have equal heights, and the center spacing between adjacent two groups of metasurface units is 0.25 mm both horizontally and vertically. Only by changing the size of the metasurface unit can the focusing of the transmitted light field be controlled, reducing the processing difficulty in the micro-nano manufacturing process and reducing the manufacturing cost. At the same time, the function of continuous zoom can be realized by changing the environmental medium or the incident wavelength.

[0023] 2. The terahertz metasurface lens described in the present invention is composed of sub-wavelength-sized microstructural units, which can significantly reduce the lens size and make it possible to miniaturize the terahertz imaging system. It not only promotes the development of the terahertz imaging zoom system towards integration and miniaturization, but also shows application potential in multiple fields such as sensing technology, non-invasive detection, and multi-focus imaging.

[0024] 3. The wavelength of the incident light source covers the terahertz band, and the polarization state is arbitrary polarization. The focal length of the metasurface lens shows a positive correlation and a negative correlation with the environmental refractive index of 1 - 1.6 and the incident wavelength of 0.7 - 0.8 mm respectively. Fast zoom can be achieved by changing the working medium or the incident wavelength, effectively improving the detection efficiency of the optical system and promoting the development of the optical zoom system towards integration and miniaturization. It not only plays an important role in the imaging system, but also shows great application potential in multiple fields such as sensing technology, non-invasive detection, optical regulation, and multi-focus imaging. Description of the Drawings

[0025] Figure 1 is a schematic structural diagram of the transmissive dielectric metasurface lens provided by the present invention (13*13 array);

[0026] Figure 2 is the front view of the structural diagram of the transmissive dielectric metasurface lens provided by the present invention (13*13 array);

[0027] Figure 3 is the front view of the substrate after lithography and etching;

[0028] Figure 4 is the top view of the cross-sectional shape of the metasurface unit structure provided by the present invention. Detailed Embodiments

[0029] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not constitute a limitation to the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0030] A terahertz-band zoom metasurface lens, the metasurface lens is composed of a substrate and metasurface units located above the substrate. The substrate is in the shape of a cylinder and is made of COP as the structural material; the metasurface units are of microstructural dimensions. The upper surface of the substrate has a number of rectangular units with the same size, and the side length of each rectangular unit is 0.25 mm. The metasurface units are cross-shaped elliptical cylinders, and each metasurface unit is located at the center of the corresponding rectangular unit.

[0031] The metasurface units are made of Si as the structural material; the lower surface of the metasurface units is attached to the upper surface of the substrate.

[0032] There are 13 rows and 13 columns of metasurface units, which are arrayed on the surface of the substrate. The cross-section of the metasurface unit is a cross-shaped ellipse, and the ratio of the major axis to the minor axis of each ellipse is 3:1.

[0033] The above metasurface lens operates in the terahertz band, the terahertz wavelength is 0.7 mm - 0.8 mm, the refractive index of the working medium is 1 - 1.6, the working mode is transmissive, and the polarization state of the incident light is arbitrary polarization.

[0034] The height of the substrate is 0.5 mm. The height of the metasurface unit is 0.85 mm.

[0035] The distance from each meta - unit to the center of the substrate is less than the radius (R) of the substrate.

[0036] The distance from the center of each meta - unit to the center of the substrate is r. Meta - units with equal distances (constant r) from the center of the substrate have equal cross - sectional dimensions.

[0037] The diameter D of the meta - lens is 11 mm, the focal length f is 11 mm, the transmittance of the meta - lens is 85%, and the average focusing efficiency is 51%;

[0038] The operating temperature range is - 30 - 60 °C.

[0039] For the meta - lens to produce a beam focusing function, the meta - units at different positions need to generate a phase of the transmitted beam. Here, r is the displacement from the center of the meta - unit to the center of the meta - lens, f is the focal length of the meta - lens, and is the incident wavelength. The model of the meta - unit is two perpendicularly orthogonal elliptical cylinders with the same shape parameters. Its symmetric structure can achieve polarization insensitivity to the incident light source. By setting the cross - section of the elliptical cylinder and setting the ratio of the major axis to the minor axis of the ellipse to 3:1, and adjusting the sizes of the major and minor axes of the two ellipses proportionally, the effective refractive index of the meta - unit can be changed, thereby changing the phase of the transmitted light. Meta - units with the same cross - sectional shape form a meta - unit library according to different cross - sectional dimensions. Select the meta - units that meet the requirements from the meta - unit library and set them at the corresponding positions of the meta - lens to achieve the focusing function at an incident wavelength of 0.75 mm. At the same time, the meta - units set at the same radius position in the meta - lens have cross - sectional shapes of the same size. The focal length of the meta - lens is positively correlated with the environmental refractive index in the range of 1 - 1.6 and negatively correlated with the incident wavelength in the range of 0.7 - 0.8 mm. Fast zoom can be achieved by changing the environmental medium or the incident wavelength, effectively improving the detection efficiency of the optical system.

[0040] A preparation method for a terahertz - band zoom meta - lens:

[0041] S1. Prepare the substrate:

[0042] Process the COP into a cylindrical substrate with a thickness of 0.5 mm by injection molding or laser cutting. This process needs to be carried out under low - temperature conditions to avoid the curing of COP above 80 °C.

[0043] S2. Process the micro - structure profile on the upper surface of the substrate

[0044] Use photolithography and etching methods to process the corresponding micro - structure profile on the upper surface of the substrate, as Figure 3 shown.

[0045] S3. Material deposition

[0046] A layer of silicon material is deposited on the top of the microstructure profile by chemical vapor deposition to change the refractive index and dispersion characteristics of light, thereby obtaining a metasurface lens. As Figure 1 and Figure 2 shown.

[0047] S4. Integration and testing

[0048] Test the optical performance, mechanical performance and durability of the metasurface lens, and adjust the design according to the test results.

[0049] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention. Each component not clearly defined in this embodiment can be realized by existing technologies.

Claims

1. A terahertz-band zoom meta-lens, characterized in that: The meta-lens consists of a substrate and a meta-unit located above the substrate. The substrate is in the shape of a cylinder and uses COP as a structural material. The meta-unit is of microstructure size. The upper surface of the substrate has a plurality of rectangular units of the same size, each of which has a side length of 0.25 mm. The meta-unit is a cross-shaped elliptical column, and each meta-unit is located at the center of the corresponding rectangular unit.

2. The terahertz-band zoom meta-lens according to claim 1, characterized in that: The super-structure unit uses Si as a structural material; the lower surface of the super-structure unit is bonded to the upper surface of the substrate.

3. The terahertz-band zoom meta-lens according to claim 2, characterized in that: There are 13 rows and 13 columns of superstructure units distributed in an array on the substrate surface. The cross-section of the superstructure unit is a cross-shaped ellipse, and the ratio of the major semi-axis to the minor semi-axis of each ellipse is 3:

1.

4. The terahertz-band zoom meta-lens according to claim 3, characterized in that: The above-mentioned meta-lens works in the terahertz band, the terahertz wavelength is 0.7mm-0.8mm, the refractive index of the working medium is 1-1.6, the working mode is transmission, and the polarization state of the incident light is arbitrary polarization.

5. The terahertz-band zoom meta-lens according to claim 4, characterized in that: The height of the substrate is 0.5 mm, and the height of the superstructure unit is 0.85 mm.

6. The terahertz-band zoom meta-lens according to claim 5, characterized in that: The distance from each superstructure unit to the center of the substrate is less than the radius of the substrate (R). The distance from the center of each superstructure unit to the center of the substrate is r. Superstructure units with equal distances to the center of the substrate (r constant) have equal cross-sectional dimensions.

7. The terahertz-band zoom meta-lens according to claim 6, characterized in that: The diameter of the meta-lens is D=11mm, the focal length is f=11mm, the transmittance of the meta-lens is 85%, and the focusing efficiency is 51% on average; the operating temperature range is -30-60℃.

8. A method for preparing a terahertz-band zoom meta-lens according to any one of claims 1 to 7, characterized in that: The following steps are included S1. Preparation of substrate: The COP is processed into a 0.5 mm thick cylindrical substrate by injection molding or laser cutting. This process needs to be carried out under low temperature conditions to avoid solidification of the COP above 80°C. S2. Processing microstructure profile on the substrate surface Using photolithography and etching methods, a corresponding microstructure profile is processed on the upper surface of the substrate; S3. Material Deposition A layer of silicon material is deposited on top of the microstructure profile using chemical vapor deposition to form a metastructure unit, which is used to change the refractive index and dispersion characteristics of light, thereby obtaining a metastructure lens; S4. Integration and testing The optical performance, mechanical properties and durability of the metalens are tested, and the design is adjusted based on the test results.

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