Design and implementation method of near-infrared light-splitting super lens

Abstract

The invention discloses a design and implementation method of a near-infrared light-splitting super lens, and relates to the technical field of super lenses. The super lens is composed of a rotational symmetric structure, so that polarization insensitivity is achieved, incident broadband light can be dispersed to different positions of the same focal plane according to different wavelengths through ingenious optical design, an effective light splitting effect is achieved, and the polarization insensitivity of the super lens is improved by adjusting geometric parameters and shapes of the super lens nano structure units. The positions of light with different wavelengths on a focal plane can be accurately controlled; in the optimization process, a particle swarm optimization algorithm is adopted to reduce errors, and performance attenuation and optical distortion caused by dispersion can be effectively reduced by adjusting dispersion characteristics and geometric parameters of the nano-structure units, so that the precision and reliability of light splitting are improved, and the light splitting precision is improved. The identification capability of the laser detection system is greatly improved, and integration and miniaturization are realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of superlenses, and more specifically, relates to a design method and an implementation method for a near-infrared spectroscopic superlens. Background Art

[0002] Traditional laser detection systems use narrow-band filters as spectral recognition devices. In principle, they will inevitably cause a certain loss of laser intensity, thus causing losses to the accuracy of laser detection and posing higher requirements for the sensitivity and response ability of detectors. Moreover, spectral recognition is achieved by means of discrete optical elements, that is, multiple optical elements are included in the system. Therefore, restricted by the optical elements, the system structure is relatively complex and bulky, the processing accuracy requirements are high, resulting in difficult processing, and the bulky volume makes it difficult to integrate the device. With the development of intelligent devices, various devices in optical systems are developing towards miniaturization, integration, multi-function, and high performance. In recent years, a metasurface composed of sub-wavelength-sized and spaced structures arranged in a two-dimensional plane has been proposed to regulate electromagnetic wave parameters. By reasonably designing the shape, size, position, and direction of the structure, the metasurface can achieve arbitrary regulation of all parameters of light, such as phase, amplitude, polarization, and frequency. Coupled with the ultrathin and ultra-flat characteristics of the metasurface, redesigning traditional optical elements into thin, light, and multi-functional new elements is expected to provide a new solution for reducing the complexity of optical systems.

[0003] Traditional military laser detection systems have limitations in that it is difficult to balance a large field of view and high precision, and they are large in volume and weight, and it is increasingly difficult to meet the requirements of integration, lightweight, and high performance of modern military equipment. Domestic and foreign researchers have successively carried out active explorations in the field of spectral detection and imaging technology with planar metasurfaces composed of sub-wavelength structures as physical carriers, and have achieved some basic theoretical innovation results. However, most of the research focuses on the visible light band, and its application in infrared detection equipment is still restricted by band mismatch, low transmittance, etc. So far, there are few reports on spectral detection and imaging technology based on split-focus superlenses, and they all remain at the stage of paper reports, and there is no publicly reported conversion application. Therefore, it is of great significance to conduct research on the application transformation of infrared laser spectral detection and imaging technology based on split-focus superlenses. The present invention images incident light to different spatial positions according to information such as wavelength and viewing angle, subverts the architecture of an imaging device plus a filter element in a traditional laser detection imaging system, integrates the spectral splitting and imaging functions into a single element, and breaks through the efficiency upper limit of traditional spectral imaging, thereby greatly improving the recognition ability of the laser detection system and achieving integration and miniaturization. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a design and implementation method for a near-infrared spectroscopic metalens. The obtained metalens can image incident light to different spatial positions according to information such as wavelength and viewing angle, subverting the architecture of an imaging device plus a filter element in a traditional laser detection imaging system, and integrating the spectroscopic and imaging functions into a single element, breaking through the efficiency limit of traditional spectral imaging, thereby greatly improving the recognition ability of the laser detection system and realizing integration and miniaturization.

[0005] To solve the above technical problems, the technical solution adopted by the present invention is: a design method for a near-infrared spectroscopic metalens, comprising the following steps:

[0006] Step 1: Design the shape and size of the nanocolumns; select a square, a circle, a cross, or other isotropic shapes as the cross-section of the nanocolumns to achieve polarization insensitivity, determine the unit period according to the Nyquist sampling theorem, and determine the height and size of the nanocolumns according to the transmission phase principle;

[0007] Step 2: Construct a database; explore the resonance phenomenon of electromagnetic waves based on theories such as Mie resonance and equivalent medium, calculate the equivalent waveguide modes presented by the polarization-insensitive nano-unit structure of the nanocolumns under incident light of different wavelengths by combining electromagnetic wave numerical simulation algorithms such as finite-difference time-domain and finite element, further obtain its amplitude and phase responses, and correlate the optical response with the structural degrees of freedom to construct a database;

[0008] Step 3: Phase matching; combine the off-axis focusing phase distributions at multiple wavelengths into the ideal phase distribution of the metasurface lens, and use an optimization algorithm such as a particle swarm algorithm to determine the position information and specific size of the nanocolumns of the metasurface lens to achieve complete phase matching;

[0009] Step 4: Performance evaluation; respectively simulate and calculate the far fields of the micrometer-scale and centimeter-scale metasurface lenses through a vector diffraction simulation tool FDTD and a scalar diffraction algorithm to evaluate the performance of the metasurface lens; if the design target is not met, return to Step 3 for iterative optimization, otherwise, proceed to the next step;

[0010] Step 5: Layout output; perform layout output according to the position information and structural dimensions of the metasurface lens provided in Step 1 and Step 3.

[0011] Preferably, in Step 1, according to the equivalent medium theory, the dielectric subwavelength structure unit can be equivalent to a waveguide, and the expression of its propagation phase is:

[0012]

[0013] where η effis the equivalent refractive index of the nanostructure, which is determined by the refractive index of the material of the nanostructure itself and the equivalent waveguide modes of different structures. h is the height of the nanocolumn. Therefore, by controlling the sizes of the individual sub-wavelength nanostructure units, the desired phase can be obtained.

[0014] Preferably, in step two, taking a square as an example, the computer FDTD software can be used to simulate the phase change generated by the nanocolumns with a certain height and sizes in the range of 50 nm to 500 nm under the incident light within the wavelength range, so as to establish a database.

[0015] Preferably, in step three, for the specific functional design of multi-wavelength split foci, the metasurface lens needs to be given the function of focusing light beams of different wavelengths to different regions of the detector, that is, the metasurface lens with split foci. The ways to transform the target function into the desired ideal optical response at different wavelengths include accumulating or iteratively optimizing the off-axis focusing lens formula for different wavelengths through the lens analysis form. The iterative optimization algorithms include the gradient descent method, the GS algorithm, and the particle swarm optimization algorithm. The analysis form calculates multiple sets of phase distributions at different wavelengths according to the off-axis focusing lens formula. The phase distribution that generates a single-focus off-axis focus at the desired position at each individual wavelength can be calculated according to Fermat's principle:

[0016]

[0017] In the formula, λ0 is the working wavelength, f is the position of the focal plane, (x0, y0) is the coordinate of the focus, and f off-axis is the focus offset, that is, the distance from the target focus to the center of the lens. Finally, taking the obtained multi-wavelength split-focus off-axis focusing phase distribution as the target distribution, search for the structural parameters that meet the optimal match through optimization algorithms such as the heuristic optimization algorithm in the structural phase response database, and place the unit structures that meet the target phase requirements on the metasurface through spatial arrangement to achieve the target functions of focusing to different regions and different relative positions of the detector when incident with multi-wavelength and multi-angle light.

[0018] Preferably, in step five, the specific implementation of the layout output is as follows: First, calculate the specific point position data of each micro-nano structure unit on the metasurface lens in the Cartesian coordinate system in Matlab, and write the point position data to form a closed loop. Then store the data as a txt file, and then convert it into a GDSⅡ file through the linkCAD software to obtain the layout of the metasurface lens to be processed and used.

[0019] A method for realizing the metasurface of a near-infrared spectroscopic metalens, fabricating the metalens according to the layout of the metalens obtained in step five, and the specific steps are as follows:

[0020] Step 6: Deposit a silicon film with a specified thickness on the glass substrate by plasma-enhanced chemical vapor deposition;

[0021] Step 7: Spin-coat a layer of PMMA electron beam photoresist on the glass substrate coated with the silicon film of the specified thickness, and perform electron beam lithography;

[0022] Step 8: After lithography, perform development and fixation, that is, dissolve the photoresist in the patterned area;

[0023] Step 9: Subsequently, perform ion beam sputtering to deposit a layer of metal chromium film on the upper surface, and the thickness of the metal chromium film is determined according to the thickness of the silicon film;

[0024] Step 10: Then, tear off the photoresist in the non-patterned area by dry stripping to leave a metal chromium mask;

[0025] Step 11: Immediately perform ICP etching. Under the protection of the metal chromium mask, etch to leave a patterned structure of metal chromium and nanorod structure;

[0026] Step 12: Finally, soak the wafer in a metal chromium stripping solution to remove the residual chromium mask to obtain the final experimental sample.

[0027] Preferably, the material of the nanorod structure can be TiO2, HfO2, ZrO2, GaN, Si2N3 or Si.

[0028] Preferably, the height range of the nanorod structure is 600nm - 1000nm, and the size of the nanorod structure on the surface of the dielectric substrate is 50nm - 500nm.

[0029] The beneficial effects produced by adopting the above technical solutions are as follows:

[0030] 1. The present invention utilizes a metasurface to realize a new architecture integrating spectral splitting and positioning functions. Without changing the detector, the target is focused to different positions of the detector according to different wavelength information through a monolithic metasurface lens, subverting the complex discrete architecture of the focusing system plus filter elements in traditional multi-information spectral splitting technology, and monolithically integrating the spectral splitting and focusing functions, greatly reducing the volume, weight and cost of the multi-information spectral splitting system.

[0031] 2. Since the present invention utilizes the principle of spectral splitting rather than filtering, this achievement breaks through the efficiency upper limit of traditional multi-information spectral splitting devices. Traditional monolithic spectral splitting based on filter films is mostly based on the principle of filtering, resulting in a large loss of the optical energy of the incident light. However, the metasurface of the present invention uses the principle of spectral splitting to divide the components of different wavelength lights on the entire surface into different positions, which can greatly improve the energy utilization rate.

[0032] 3. The present invention combines the new principle of multi-parameter regulation under multiple wavelengths and the forward design algorithm to facilitate the design of metasurface components for achieving various high-performance spectral regulations. Currently, the design of metasurface components still remains at the design of single-wavelength multi-parameter regulation, and the design method is relatively simple. Through in-depth research on the multi-parameter regulation mechanism under a wide wavelength band, the present invention makes full use of the shape freedom of micro-nano structures and combines the forward algorithm to realize the combination of multiple functions such as large-area, tunable, ultra-wideband achromatic, polarization, three-dimensional, spectral detection, and optical computing to meet the usage in different scenarios. Description of the Drawings

[0033] Figure 1 is the flowchart of design and fabrication;

[0034] Figure 2 is the cross-sectional shape of the nano-column structure;

[0035] Figure 3 is the phase distribution diagram of the structure at different wavelengths;

[0036] Figure 4 is the schematic diagram of the sub-focus focusing of the superlens;

[0037] Figure 5 is the schematic diagram of the light splitting principle of the superlens;

[0038] Figure 6 is the hard mask etching process;

[0039] Figure 7 is the MATLAB scalar simulation result of the superlens at four wavelengths;

[0040] Figure 8 is the spectral analysis diagram corresponding to the four target focal positions of the superlens;

[0041] Figure 9 is the test result of the superlens after processing at four wavelengths; Detailed Embodiments

[0042] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present application.

[0043] To make the principle and advantages of the present invention clearer, taking the simple design of a nanostructured surface with a square cross-section as an example (the shape is not unique in actual design), and taking four discrete wavelengths as the design wavelengths, including 808 nm, 1064 nm, 1310 nm, and 1550 nm, and taking 600 nm as the height of the nanostructure, the present invention will be further described in detail. It should be understood that the description herein is only used to explain the present invention and is not used to limit the present invention.

[0044] Figure 3 It is the phase distribution diagram with respect to the change of the structural size at different wavelengths. The abscissa is the structural size, and the ordinate is the corresponding phase value. It can be seen that a phase of 0 - 2π can be provided at all four wavelengths.

[0045] Figure 4 It is a schematic diagram of the spectral splitting and focusing of the superlens. Focus 1 corresponds to the incident wavelength of 808 nm, Focus 2 corresponds to the incident wavelength of 1064 nm, Focus 3 corresponds to the incident wavelength of 1310 nm, Focus 4 corresponds to the incident wavelength of 1550 nm. 1 is the Si nanorod, 2 is the SiO2 substrate, and each focus is on the same focal plane.

[0046] Figure 5 It is the schematic diagram of the spectral splitting principle of the superlens. Figure 6 It is the hard mask etching process adopted for fabricating the present invention. After the light passes through the nanostructured surface, its phase is composed of two parts in total, namely the phase of the structure response on the nanostructured surface and the propagation phase to an arbitrary wavefront. Its phase composition can be expressed by the following formula.

[0047]

[0048] In the formula, is the total phase, is the propagation phase from the superlens to the focus, is the phase of the structure response on the nanostructured surface. According to Figure 5 the geometric relationship, the following formula can be further obtained:

[0049]

[0050] In the formula, λ i is the incident light wavelength, (x, y) is the coordinate from the center of the nanostructure unit to the plane of the superlens, and f is the focal length of the designed superlens.

[0051] To make the light incident from different positions converge at the same point, from the geometric relationship, it can be known that it is necessary to make That is, it is necessary to satisfy:

[0052]

[0053] In addition, since there is a positive correlation between the wave number and the response phase of the nanostructure, and from the above-mentioned phase distribution relationship of the superlens, it can be seen that the relationship between the phase distribution of the superlens and the wave number does not remain constant. It shows a positive correlation in some position ranges and a negative correlation in other position ranges, which results in the inability to achieve spectral focusing of multiple wavelengths simultaneously. Because the dispersion relationship of the structure itself cannot be changed, the phase response of the superlens is selected to be regulated here. After correction, the phase distribution of the superlens is as follows:

[0054]

[0055] In the formula, C i is the compensation phase value at different wavelengths, and its value is determined by optimization algorithms such as the particle swarm algorithm.

[0056] Figure 7 are the MATLAB scalar simulation results of the superlens with a diameter of 1 mm and a focal length of 2.56 mm designed by the above method at four wavelengths. Obviously, it can be seen from the simulation results that there is an obvious spectral splitting effect at the four wavelengths. Figure 8 is the spectral analysis diagram corresponding to the four target focal positions of the superlens. Figure 9 are the test results of the superlens with a diameter of 1 mm and a focal length of 2.56 mm designed by the above method after processing at four wavelengths. Obviously, it can be seen from the test results that the spectral splitting effect of the superlens is basically consistent with the simulation results.

[0057] The metasurface lens in the near-infrared band provided by the present invention has the characteristics of efficient spectral splitting function, polarization insensitivity of the rotationally symmetric structure, and reduction of errors through the particle swarm optimization algorithm. Such a metasurface lens has a wide range of potential applications in optical applications, including spectral analysis, optical imaging, optical communication, and biomedicine and other fields.

[0058] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution of the present invention and its inventive concept, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. A design method for a near-infrared spectroscopic metalens, characterized in that: The following steps are involved: Step 1: Design the shape and size of the nanopillars; select square, circular, cross or other isotropic shapes as the cross-section of the nanopillars to achieve polarization insensitivity, determine the unit period according to the Nyquist sampling theorem, and determine the height and size of the nanopillars according to the transmission phase principle; Step 2: Construct a database; explore the resonance phenomenon of electromagnetic waves based on Mie resonance and equivalent medium theory, and combine finite time-domain difference and finite element electromagnetic wave numerical simulation algorithms to calculate the equivalent waveguide mode of the polarization-insensitive nanostructure of the nanocolumn under incident light of different wavelengths, and further obtain its amplitude and phase response, and associate the optical response with the structural degree of freedom to construct a database; Step 3: Phase matching: Combine the off-axis focusing phase distributions at multiple wavelengths into the ideal phase distribution of the metalens, and use the particle swarm optimization algorithm to determine the position information and specific size of the nanopillars of the metalens to achieve complete phase matching; Step 4: Performance evaluation: Use the vector diffraction simulation tool FDTD and the scalar diffraction algorithm to simulate and calculate the far field of the micron-scale and centimeter-scale metalenses, respectively, to evaluate the performance of the metalens; if it does not meet the design goal, return to step 3 for iterative optimization; if it meets the design goal, proceed to the next step; Step 5: Layout output: Output the layout according to the meta-lens position information and structure size provided in steps 1 and 3.

2. The design method of a near-infrared spectroscopic metalens according to claim 1, characterized in that: In step 1, according to the equivalent medium theory, the dielectric subwavelength structural unit can be equivalent to a waveguide, and the expression of its propagation phase is: Among them, η eff is the equivalent refractive index of the nanostructure, which is determined by the refractive index of the material of the nanostructure itself and the equivalent waveguide mode of different structures, and h is the height of the nanocolumn. The required phase is obtained by controlling the size of each subwavelength nanostructure unit.

3. The design method of a near-infrared spectroscopic metalens according to claim 1, characterized in that: In step 2, the nanocolumns of the polarization-insensitive nanounit structure are square, and computer FDTD software is used to simulate the phase changes of nanocolumns with a certain height and a size ranging from 50nm to 500nm under incident light within a wavelength range, thereby constructing a database.

4. The design method of a near-infrared spectroscopic metalens according to claim 1, characterized in that: In step 3, for the specific functional design of multi-wavelength focus separation, it is necessary to give the meta-lens the function of focusing light beams of different wavelengths to different areas of the detector, that is, the meta-lens of focus separation; the method of converting the target function into the ideal optical response required at different wavelengths includes accumulating the off-axis focusing lens formulas of different wavelengths through the lens analytical form or iterative optimization algorithm, and the iterative optimization algorithm includes the gradient descent method, the GS algorithm, and the particle swarm optimization algorithm; the analytical form calculates multiple sets of phase distributions at different wavelengths according to the off-axis focusing lens formula, and the phase distribution of the single focus off-axis focusing at the desired position at each individual wavelength can be calculated according to the Fermat principle: Where λ0 is the operating wavelength, f is the focal plane position, (x0, y0) is the coordinate of the focus, and f off-axis is the focus offset, i.e. the distance from the target focus to the center of the lens, Finally, the obtained multi-wavelength off-axis focusing phase distribution is used as the target distribution, and the structural parameters that meet the optimal match are searched in the structural phase response database through heuristic optimization algorithms and other optimization algorithms. The unit structures that meet the target phase requirements are placed on the meta-surface through spatial arrangement to achieve the target function of focusing on different areas of the detector and different relative positions when multi-wavelength and multi-angle incident.

5. The design method of a near-infrared spectroscopic metalens according to claim 1, characterized in that: In step five, the specific implementation of the layout output is: first, calculate the specific point data of each micro-nano structure unit on the meta-lens in the Cartesian coordinate system in Matlab, and write the point data to form a closed loop; then store the data as a txt file, and then convert it into a GDSⅡ file through linkCAD software to obtain the meta-lens layout to be processed.

6. The design method of a near-infrared spectroscopic metalens according to claim 1, characterized in that: The metalens is fabricated according to the metalens pattern obtained in step 5. The specific steps are as follows: Step 6: Plating a silicon film of a specified thickness on the glass substrate by plasma enhanced chemical vapor deposition; Step 7, spin-coating a layer of PMMA electron beam photoresist on a glass substrate coated with a silicon film of a specified thickness, and performing electron beam lithography; Step 8: After photolithography, development and fixing are performed, that is, the photoresist in the patterned area is dissolved; Step 9: Then, ion beam sputtering is performed to deposit a layer of metal chromium film on the upper surface, and the thickness of the metal chromium film is determined according to the thickness of the silicon film; Step 10: Then, the photoresist in the non-patterned area is removed by dry stripping, leaving a metal chrome mask; Step 11: ICP etching is then performed to etch away the patterned structure of metal chromium and nano-pillar structure under the protection of the metal chromium mask; Step 12: Finally, the wafer is immersed in a metal chromium stripping solution to remove the remaining chromium mask to obtain the final experimental sample.

7. The design method of the near-infrared spectroscopic metalens according to claim 6, characterized in that: The material of the nanocolumn structure is TiO2, HfO2, ZrO2, GaN, Si2N3 or Si.

8. The method for designing a near-infrared spectroscopic metalens according to claim 6, characterized in that: The height of the nano-pillar structure ranges from 600nm to 1000nm, and the size of the nano-pillar structure on the surface of the dielectric substrate is from 50nm to 500nm.

Images