Dual-wavelength confocal super lens and design method thereof, and photoacoustic imaging method and system

Abstract

The invention provides a dual-wavelength confocal super lens, a design method thereof, a photoacoustic imaging method and a photoacoustic imaging system, and belongs to the technical field of optical lenses. The invention provides a design method of a dual-wavelength confocal super lens, cancer cells and normal tissues can be developed by utilizing light with two different wavelengths, the normal cells and the cancer cells are heated and expanded by absorbing energy with two different wavelengths, ultrasonic waves are emitted after cooling, sound signals are collected through an ultrasonic transducer, and the super lens can be used for imaging the cancer cells and the normal tissues. And finally, photoacoustic imaging is realized through calculation. The invention aims to enhance the photoacoustic imaging contrast while avoiding the traumatic operation risk. The dual-wavelength design can provide multiple comparison mechanisms, can obtain the comparison of various tissue characteristics in the same imaging process, enhances the detection capability of lesion, and also can effectively reduce the noise influence in photoacoustic signals. The method can be applied to early detection of cancers, and also can be used for monitoring the response of tumors in the treatment process and evaluating the treatment effect.

Description

Technical Field

[0001] The present invention relates to the field of optical lens technology, and in particular to a dual-wavelength confocal superlens and a design method thereof, and a photoacoustic imaging method and system. Background Art

[0002] Metamaterials have attracted widespread attention because they can achieve electromagnetic responses that do not exist in nature. These materials are composed of sub-wavelength-scale nano-units. By regulating the geometric size, material selection and lattice constant of the unit, the polarization, phase, amplitude and frequency of electromagnetic waves can be precisely controlled. Based on this principle, superlenses use artificially designed two-dimensional nanostructures to regulate the propagation path and focal imaging of light, breaking through the physical limitations of traditional lenses. Superlenses have the advantages of being light, compact, and highly integrated, especially in terms of flexibility and adjustability. It is compatible with a variety of materials, suitable for different bands, and can exhibit different optical properties at different optical wavelengths according to design requirements, realizing multi-wavelength confocal imaging.

[0003] Cancer is the second leading cause of death in the world, second only to heart disease. Due to the complexity of cancer types, there is no completely comprehensive treatment method. The main means of treatment are chemotherapy, radiotherapy and surgery. For cancer monitoring and postoperative rehabilitation testing, imaging (such as x-rays, ultrasound detection, etc.), tumor markers, blood tests, tissue biopsies and other means are also used. Although the technology is still being improved, it will cause secondary damage to the human body, including radiation risks and possible bleeding and infection during surgery, which may damage normal healthy cells or increase the chance of secondary cancer. Photoacoustic technology integrates the fields of optics, acoustics, biology and computer vision. Its basic principle is that laser irradiation of biological tissue causes local temperature rise through light energy absorption, causing tissue expansion and generating ultrasonic signals. By detecting these ultrasonic signals, the spatial distribution of tissues can be reconstructed, high-resolution imaging can be achieved, and clinical diagnosis and treatment can be assisted. As a non-invasive technology, photoacoustic imaging can obtain tumor information through surface or shallow optical signals, which is of great value in the early diagnosis of cancer. Compared with traditional pathological examination, photoacoustic imaging avoids the risk of traumatic operation. By selecting a specific wavelength of laser, the biomarkers of cancer cells, especially the vascular structure and oxygenated hemoglobin distribution in the tumor area, can be sensitively monitored, thereby improving the sensitivity and specificity of diagnosis.

[0004] Single-wavelength photoacoustic imaging in related technologies has the following defects: 1. Overlapping tissue light absorption spectra limits the system's ability to distinguish different tissues or substances; 2. Lack of multimodal information, only single contrast information can be provided; 3. Spatial resolution is limited, and sufficient resolution may not be obtained for imaging of fine structures or tiny lesions. Summary of the invention

[0005] In view of this, the purpose of the present invention is to provide a dual-wavelength confocal superlens and its design method, photoacoustic imaging method and system. The dual-wavelength confocal superlens design method of the present invention can realize high-contrast photoacoustic imaging for cancer cell monitoring and treatment, and with enhanced imaging contrast, it can obtain the contrast of multiple tissue features in the same imaging process, thereby enhancing the detection ability of lesions.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a design method of a dual-wavelength confocal superlens, comprising the following steps:

[0008] S1. Define the design goal, determine the dual working wavelengths to achieve confocalization, clarify the optical function that the metasurface should achieve, so that it can respond to two different wavelengths of light and achieve confocalization. The metasurface includes a focusing metasurface and a filtering metasurface;

[0009] S2. Select materials and parameters, find the dual-wavelength metalens material and substrate respectively, and test the optical properties at different wavelengths, wherein the optical properties include refractive index and transmittance;

[0010] S3. Design of multi-scale structure of metasurface, adjust its response to light of different wavelengths by adjusting the size of unit structure, optimize the geometric parameters of unit structure, analyze the influence on the phase change of outgoing light wavefront, and establish a quantitative relationship database between unit structure parameters and wavefront phase;

[0011] S4. According to the quantitative relationship database, the propagation phase distribution formula derived by the designed wavefront phase formula is used to place unit structures of different materials and sizes at specific positions on the two surfaces of the substrate to achieve filtering or focusing functions;

[0012] S5. Repeat S1 to S4, so that different materials are placed at specific positions of the substrate according to the propagation phase formula, and the focusing metasurface and the filtering metasurface are obtained respectively to form the dual-wavelength confocal superlens.

[0013] Preferably, the dual working wavelengths include a visible light band and a near-infrared light band, and the optical functions that the metasurface should achieve include imaging, focusing, transmission, refraction and filtering.

[0014] Preferably, the optical band gap of the substrate in the visible light range is greater than 3.1 eV, and the optical band gap in the near-infrared light range is greater than 1.5 eV; the refractive index of the unit structure in the visible light range is 2.0-4.0, and the optical band gap is greater than 3.1 eV, and the refractive index in the near-infrared light range is 1.8-3.5, and the optical band gap is greater than 1.5 eV.

[0015] Preferably, the unit structure is a centrosymmetric structure.

[0016] Preferably, the surface shape of the unit structure is circular, square or regular hexagonal.

[0017] Preferably, the geometric parameters include one or more of lattice constant, cross-sectional radius and height.

[0018] The present invention also provides a dual-wavelength confocal superlens made by the design method described in the above technical solution, comprising a substrate, a focusing supersurface on the surface of one side of the substrate, and a filtering supersurface on the surface of the other side of the substrate.

[0019] Preferably, the focusing metasurface uses 532nm and 785nm as working wavelengths, uses TiO2 as a unit structure for focusing 532nm light, and uses Si as a unit structure for focusing 785nm light; the filtering metasurface uses TiO2 as a unit structure.

[0020] The present invention also provides a photoacoustic imaging method, which utilizes the dual-wavelength confocal superlens described in the above technical solution.

[0021] The present invention also provides a photoacoustic imaging system, comprising the dual-wavelength confocal superlens and ultrasonic transducer described in the above technical solution.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention provides a design method for a dual-wavelength confocal superlens, which realizes high-contrast photoacoustic imaging based on the dual-wavelength confocal superlens and is applied to the method of monitoring and treating cancer cells. With enhanced imaging contrast, the contrast of multiple tissue features can be obtained in the same imaging process, thereby enhancing the detection capability of lesions. The combination of dual wavelengths can also effectively reduce the influence of noise in the photoacoustic signal and improve the resolution of the imaging system. By controlling the geometric dimensions, material selection, and lattice constant of its sub-wavelength unit structure, precise control of the polarization, phase, amplitude, and frequency of electromagnetic waves by different mechanisms can be achieved, so that different optical properties can be exhibited at different optical wavelengths, thereby realizing multi-wavelength confocal imaging function.

[0024] Specifically, the present invention has the following advantages:

[0025] 1. Enhanced imaging contrast: Using two light sources with different wavelengths can simultaneously obtain different light absorption characteristics and provide more tissue information, which can significantly improve the imaging contrast.

[0026] 2. Multiple contrast mechanisms: regulating two different wavelengths of light to act on different components of the tissue respectively to obtain a contrast of multiple tissue characteristics, thereby enhancing the ability to detect lesions.

[0027] 3. Improve the signal-to-noise ratio and effectively reduce the impact of noise in photoacoustic signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a design flow chart of the dual-wavelength confocal metalens of the present invention;

[0029] Figure 2 Design a flow chart for the photoacoustic imaging of the present invention;

[0030] Figure 3 It is a schematic diagram of the structure of the dual-wavelength confocal super lens unit of the present invention;

[0031] Figure 4 This is a scanning data diagram of the focusing metasurface unit structure with a wavelength of 532nm of the present invention;

[0032] Figure 5 This is a scanning data diagram of the focusing metasurface unit structure with a wavelength of 785nm of the present invention;

[0033] Figure 6 It is a schematic diagram of the dual-wavelength confocal superlens structure of the present invention;

[0034] Figure 7 It is a dual-wavelength light field focusing diagram of the dual-wavelength confocal superlens of the present invention;

[0035] Figure 8 It is a schematic diagram of the dual-wavelength focal plane light field of the dual-wavelength confocal superlens of the present invention;

[0036] Fig. 9 It is the photoacoustic imaging sound pressure reconstruction map of the present invention;

[0037] In the figure, 1 is the unit structure, 2 is the substrate, 3 is TiO2, and 4 is Si. DETAILED DESCRIPTION

[0038] The present invention provides a design method of a dual-wavelength confocal superlens, comprising the following steps:

[0039] S1. Define the design goal, determine the dual working wavelengths to achieve confocalization, clarify the optical function that the metasurface should achieve, so that it can respond to two different wavelengths of light and achieve confocalization. The metasurface includes a focusing metasurface and a filtering metasurface;

[0040] S2. Select materials and parameters, find the dual-wavelength metalens material and substrate respectively, and test the optical properties at different wavelengths, wherein the optical properties include refractive index and transmittance;

[0041] S3. Design of multi-scale structure of metasurface, adjust its response to light of different wavelengths by adjusting the size of unit structure, optimize the geometric parameters of unit structure, analyze the influence on the phase change of outgoing light wavefront, and establish a quantitative relationship database between unit structure parameters and wavefront phase;

[0042] S4. According to the quantitative relationship database, the propagation phase distribution formula derived by the designed wavefront phase formula is used to place unit structures of different materials and sizes at specific positions on the two surfaces of the substrate to achieve filtering or focusing functions;

[0043] S5. Repeat S1 to S4, so that different materials are placed at specific positions of the substrate according to the propagation phase formula, and the focusing metasurface and the filtering metasurface are obtained respectively to form the dual-wavelength confocal superlens.

[0044] Figure 1 It is a design flow chart of the dual-wavelength confocal superlens of the present invention.

[0045] The present invention defines the design goal, determines the dual working wavelengths to achieve confocality, clarifies the optical function that the metasurface should achieve, enables it to respond to two different wavelengths of light and achieve confocality, and the metasurface includes a focusing metasurface and a filtering metasurface.

[0046] In the present invention, the dual working wavelengths preferably have obvious differences in the absorption spectra of materials or tissues so as to obtain rich information from the photoacoustic signals.

[0047] In the present invention, the dual working wavelengths preferably include a visible light band and a near-infrared light band, and the optical functions that the metasurface should achieve preferably include imaging, focusing, transmission, refraction and filtering.

[0048] The present invention preferably fully understands the light absorption characteristics of endogenous substances in the human body in photoacoustic imaging, and selects appropriate targeted imaging agents to improve the detection ability of photoacoustic imaging, enhance the signals of cancer cells and tumor-specific molecules, and thereby determine the working wavelength to achieve confocal. And the optical function that the metasurface should achieve is clearly designed so that it can respond to two different wavelengths of light and achieve confocal.

[0049] In a specific embodiment of the present invention, the working wavelengths are preferably determined based on the light absorption characteristics of blood cells and contrast agents in the human body, specifically 532 nm (green light) and 785 nm (infrared light), respectively.

[0050] The present invention selects materials and parameters, finds the dual-working wavelength metalens material and substrate respectively, and tests the optical properties at different wavelengths, wherein the optical properties include refractive index and transmittance.

[0051] In the present invention, the different wavelengths preferably include 532 nm and 785 nm.

[0052] In the present invention, the optical properties preferably also include absorptivity, loss and phase change.

[0053] In the present invention, the substrate preferably has a wider optical band gap, has excellent transparency, can effectively transmit light waves within these bands without producing significant absorption, and exhibits lower optical loss.

[0054] In the present invention, the optical band gap of the substrate in the visible light range (preferably 400-700 nm) is preferably greater than 3.1 eV, and the optical band gap in the near-infrared light range (preferably 700-2500 nm) is preferably greater than 1.5 eV.

[0055] In the present invention, the material of the substrate preferably includes but is not limited to metal materials, dielectric materials, semiconductor materials, composite materials, two-dimensional materials or photonic crystal materials; the metal material more preferably includes but is not limited to silver, gold, copper, and aluminum; the dielectric material more preferably includes but is not limited to silicon, silicon nitride, silicon dioxide, calcium fluoride, and barium titanate; the semiconductor material more preferably includes but is not limited to gallium arsenide and indium phosphide; the composite material more preferably includes but is not limited to silicon-based composite materials; the photonic crystal material more preferably includes but is not limited to silicon photonic crystals and silicon nitride photonic crystals. The photonic crystal material is used in the visible light to infrared band and has a high refractive index and good optical properties.

[0056] In the present invention, the two-dimensional material preferably includes graphene and transition metal disulfide, and the transition metal disulfide more preferably includes MoS2. The graphene has adjustable optical absorption and transmission properties in a wide spectral range, and is very suitable for designing new superlenses. The transition metal disulfide exhibits excellent optical response in the visible light to near-infrared band, and is suitable for superlens design.

[0057] In the present invention, the silicon-based composite material preferably includes one or more of a silicon oxide composite material, a silicon nitride composite material and a silicon carbide composite material.

[0058] In a specific embodiment of the present invention, the substrate is preferably silicon dioxide (SiO2).

[0059] In the present invention, the unit structure preferably has a higher refractive index, can effectively regulate the propagation of light, has a higher nonlinear optical effect or has a specific optical band gap.

[0060] In the present invention, the refractive index of the unit structure in the visible light range (preferably 400-700nm) is preferably 2.0-4.0, and the optical band gap is preferably greater than 3.1eV. The refractive index in the near-infrared light range (preferably 700-2500nm) is preferably 1.8-3.5, and the optical band gap is preferably greater than 1.5eV.

[0061] In the present invention, the material of the unit structure preferably includes but is not limited to metal materials, dielectric materials, semiconductor materials, two-dimensional materials or photonic crystal materials; the metal materials more preferably include but are not limited to silver, gold, copper, and aluminum; the dielectric materials more preferably include but are not limited to silicon, silicon nitride, silicon dioxide, calcium fluoride, and barium titanate; the semiconductor materials more preferably include but are not limited to gallium arsenide and indium phosphide; the photonic crystal materials more preferably include but are not limited to silicon photonic crystals and silicon nitride photonic crystals.

[0062] In the present invention, the two-dimensional material preferably includes graphene and transition metal disulfide, and the transition metal disulfide more preferably includes MoS2.

[0063] In the present invention, the material of the unit structure preferably has a higher refractive index at 532 nm and 785 nm, and a higher transmittance, and is more preferably transparent.

[0064] In a specific embodiment of the present invention, the material of the unit structure is preferably TiO2 or Si, the TiO2 is suitable for use in the 532nm band, the Si is suitable for use in the 785nm band, the refractive index of the TiO2 is 2.66 (532nm), and the refractive index of the Si is 3.70 (785nm).

[0065] In a specific embodiment of the present invention, the material of the focusing metasurface is preferably TiO2 or Si, and the material of the filtering metasurface is preferably TiO2.

[0066] The present invention designs a metasurface multi-scale structure, adjusts its response to light of different wavelengths by regulating the size of the unit structure, optimizes the geometric parameters of the unit structure, analyzes the impact on the change of the outgoing light wavefront phase, and establishes a quantitative relationship database between the unit structure parameters and the wavefront phase.

[0067] In the present invention, it is preferred to optimize the geometric parameters of the unit structure using a numerical simulation method, and the numerical simulation method preferably includes geometric optics method, wave optics method, finite difference time domain method, finite element method, boundary element method or discrete Fourier transform method.

[0068] In the present invention, after the optimization, it is preferred that the phase range of the emitted light covers 0 to 2π, and the light transmittance is preferably greater than 85%.

[0069] In the present invention, the unit structure is preferably a centrosymmetric structure.

[0070] In the present invention, the surface shape of the unit structure is preferably circular, square or regular hexagonal.

[0071] In the present invention, the geometric parameters preferably include one or more of a lattice constant (P), a cross-sectional radius (r) and a height (H).

[0072] In the present invention, the optimization preferably includes the following steps: first, the height of the unit structure is fixed, and then the lattice constant and the cross-sectional radius of the unit structure are nested and scanned with a fixed step length to obtain the incident light wavefront phase change and transmittance parameters, and then the lattice constant is fixed, and the height and the cross-sectional radius of the unit structure are nested and scanned with a fixed step length, and the above steps are repeated several times, and finally the phase change gradient of the outgoing light is achieved to cover 0 to 2π, while maintaining a high light transmittance. In order to ensure the superiority of optical performance, the lattice constant must comply with the Nyquist sampling theorem to avoid the generation of optical artifacts.

[0073] In a specific embodiment of the present invention, the geometric parameters of the unit structure of the focusing metasurface include: the lattice constant P = 0.31 μm, height H = 0.8 μm, and cross-sectional radius r = 0.05-0.09 μm of the unit structure of a wavelength of 532 nm, the lattice constant P = 0.3 μm, height H = 1.2 μm, and cross-sectional radius r = 0.01-0.08 μm of the unit structure of a wavelength of 785 nm; the geometric parameters of the unit structure of the filtering metasurface include: the lattice constant P = 0.5 μm, height H = 1 μm, and cross-sectional radius r = 0.08 μm of the unit structure of a filtering wavelength of 532 nm, the lattice constant P = 0.7 μm, height H = 1 μm, and cross-sectional radius r = 0.18 μm of the unit structure of a filtering wavelength of 785 nm.

[0074] After obtaining the quantitative relationship database, the present invention places unit structures of different materials and sizes at specific positions on the two surfaces of the substrate according to the quantitative relationship database and the propagation phase distribution formula derived by the designed wavefront phase formula to achieve filtering or focusing functions.

[0075] The present invention repeats S1 to S4, so that different materials are placed at specific positions of the substrate according to the propagation phase formula, and the focusing supersurface and the filtering supersurface are respectively obtained to form the dual-wavelength confocal superlens.

[0076] The present invention preferably takes the substrate as the center and divides the substrate into a number of grids according to the lattice constant as the step size. The unit structures of different materials and sizes are placed on specific grid points on the two surfaces of the substrate through the propagation phase distribution formula derived from the designed wavefront phase formula, so that the incident light conforms to the propagation phase curve after passing through the metalens to achieve filtering or focusing functions.

[0077] After obtaining the dual-wavelength confocal metalens, the present invention uses a numerical simulation method to perform far-field simulation to obtain light field data, and then designs a three-layer skin model, and uses a numerical simulation method to perform light absorption and thermal effect simulation, sound wave generation and propagation simulation, sound wave detection simulation, and image reconstruction and processing.

[0078] Figure 2 The photoacoustic imaging design flow chart of the present invention includes the following steps: first define the calculation grid attribute properties, then define the medium properties, define the initial pressure distribution, define the sensor mask, then set the photoacoustic reconstruction method, and then perform the photoacoustic reconstruction simulation.

[0079] In the present invention, the three-layer skin model is preferably respectively an epidermis layer, a dermis layer and a subcutaneous fat layer, and each layer is assigned different optical and acoustic properties to reflect the heterogeneity of the skin.

[0080] In the present invention, the designing of the three-layer skin model preferably includes defining calculation grid properties, defining medium properties, defining initial pressure distribution, and defining a sensor mask.

[0081] In the present invention, in order to restore the response of light passing through tissue as much as possible, the defined medium properties preferably include but are not limited to optical properties, acoustic properties and thickness, the optical properties preferably include but are not limited to absorption coefficient, scattering coefficient, dispersion coefficient and refractive index, and the acoustic properties preferably include but are not limited to density, sound speed and impedance.

[0082] In the present invention, the numerical simulation method of light absorption and thermal effect simulation preferably includes but is not limited to calculating the light energy absorption amount of each tissue point according to the light absorption coefficient, scattering coefficient and incident light intensity of the tissue, and obtaining the initial pressure according to the photothermal conversion formula.

[0083] In the present invention, the method of simulating the generation and propagation of acoustic waves preferably includes but is not limited to the wave equation method, the finite element method or the boundary element method.

[0084] In the present invention, the method of acoustic wave detection simulation and image reconstruction preferably includes but is not limited to an inversion reconstruction algorithm, a back projection algorithm or an iterative reconstruction algorithm for acoustic wave signals. The present invention preferably converts the detected acoustic wave signals into images and reconstructs the light absorption distribution in the tissue, thereby realizing photoacoustic imaging.

[0085] In a specific embodiment of the present invention, it is preferred to use a numerical simulation method to simulate the light absorption and thermal effect of the light field data to obtain the total light energy that passes through three layers of skin and reaches the pathological tissue, and then perform sound wave generation and propagation simulation, and use the photothermal conversion formula to obtain the corresponding sound pressure. Thereafter, the normal and pathological tissue sound signals are detected by the set ultrasonic sensor, and the image reconstruction is obtained by performing an inversion algorithm through the sensor data.

[0086] The present invention also provides a dual-wavelength confocal superlens made by the design method described in the above technical solution, comprising a substrate, a focusing supersurface on the surface of one side of the substrate, and a filtering supersurface on the surface of the other side of the substrate, wherein the light first passes through the filtering supersurface, then passes through the substrate, and then passes through the focusing supersurface.

[0087] The present invention provides a dual-wavelength confocal superlens to achieve high-contrast photoacoustic imaging, which can be applied to cancer cell monitoring and treatment. With enhanced imaging contrast, a comparison of multiple tissue features can be obtained in the same imaging process, thereby enhancing the detection capability of lesions. The combination of dual wavelengths can also effectively reduce the influence of noise in the photoacoustic signal and improve the resolution of the imaging system (up to micrometer level). The dual-wavelength confocal superlens achieves precise control of the polarization, phase, amplitude and frequency of electromagnetic waves by different mechanisms by controlling the geometric dimensions, material selection and lattice constant of its sub-wavelength unit structure, thereby exhibiting different optical properties at different optical wavelengths and realizing multi-wavelength confocal imaging function.

[0088] In the present invention, the focusing metasurface preferably uses 532nm and 785nm as the working wavelengths, preferably uses TiO2 as the unit structure for focusing 532nm light, and preferably uses Si as the unit structure for focusing 785nm light; the filtering metasurface preferably uses TiO2 as the unit structure.

[0089] In the present invention, the geometric parameters of the unit structure are preferably consistent with the above-mentioned solution, which will not be described in detail here.

[0090] The present invention also provides the application of the dual-wavelength confocal superlens described in the above technical solution in the field of photoacoustic imaging.

[0091] The present invention also provides a photoacoustic imaging method, which uses the dual-wavelength confocal superlens described in the above technical solution to perform photoacoustic imaging.

[0092] The present invention also provides a photoacoustic imaging system, comprising the dual-wavelength confocal superlens and ultrasonic transducer described in the above technical solution.

[0093] In view of the problems of spatial resolution limitation of single-wavelength photoacoustic imaging, overlapping of tissue light absorption spectra, lack of multimodal information, etc., the present invention provides a method for realizing high-contrast photoacoustic imaging based on dual-wavelength confocal superlens for application in cancer cell monitoring and treatment.

[0094] The present invention can use a double-supersurface structure superlens to achieve dual-wavelength confocalization, so as to improve the spatial resolution of photoacoustic imaging, enhance imaging contrast, and improve imaging signal-to-noise ratio.

[0095] The technical solutions in the present invention will be described clearly and completely below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0096] Example 1

[0097] The dual-wavelength superlens adopts a dual supersurface structure, which is divided into three layers. The bottom is the filtering supersurface, the middle is the substrate, and the top is the focusing supersurface. The filtering supersurface only allows light of a specific wavelength to pass through and filters out light of other wavelengths. The focusing supersurface focuses the incident light to the designed focal length. Among them, the design of the focusing supersurface is the most important. The focusing supersurface in this embodiment is composed of nanostructures that can be attached with different optical paths. The design method includes the following steps:

[0098] Step S1: According to the light absorption characteristics of endogenous substances in human body in photoacoustic imaging, gold nanoparticles are selected as the targeted imaging agent of tumor cells, and 532nm and 785nm are determined as the working wavelengths. TiO2 is selected as the unit structure for focusing 532nm light, and Si is selected as the unit structure for focusing 785nm light. The lattice constant P range and shape of the unit structure are set according to the incident light band.

[0099] In step S1:

[0100] In order to make the unit structure produce the same response to different polarization states of incident light, its cross-sectional shape needs to be a centrally symmetrical structure, with a cylinder as the initial shape of the unit structure, as shown in the schematic diagram. Figure 3 shown.

[0101] The lattice constant P of the unit structure is defined as the minimum distance between adjacent units in the periodic structure. Its size determines the optical properties of the material. In order to avoid the existence of multi-order diffraction light that reduces the optical efficiency and thus reduces the effective transmission efficiency of the superlens for the target wavelength, its lattice constant needs to be smaller than the equivalent wavelength in the substrate. The equivalent wavelength can be calculated by dividing the incident wavelength by the refractive index of the substrate material. At the same time, the lattice constant should also satisfy the Nyquist sampling theorem, and its size must be smaller than the ratio of the wavelength to twice the numerical aperture;

[0102]

[0103] Step S2: Different geometric parameters will change the effective refractive index of the material, and according to Formula 1:

[0104]

[0105] In formula 1 represents the change in phase, the unit is rad, λ represents the wavelength, the unit is nm, n eff represents the effective refractive index, with a dimension of 1, and H represents the optical path length, which is also the height of the unit structure, in nm;

[0106] Different effective refractive indices will cause different optical paths of the outgoing light, thus deflecting at different angles. According to the lattice constant range set by S1, the wavefront phase changes corresponding to different geometric parameters are calculated through parameter scanning and numerical simulation.

[0107] The parameter scanning method is as follows: first, the height of the unit structure is fixed, and the lattice constant and cross-sectional radius of the unit structure are scanned in a nested manner with increasing steps to obtain key optical parameters such as the phase change and transmittance of the incident light wavefront. Next, the lattice constant P is kept unchanged, and the height H and cross-sectional radius r of the unit structure are further scanned in a nested manner with increasing steps. The process is as follows: Figures 4-5 As shown. Repeat the above steps several times, and finally determine the parameters of the two unit structures: the unit structure with a wavelength of 532nm has a lattice constant P = 0.31μm, a height H = 0.8μm, and a cross-sectional radius r = 0.05-0.09μm; the unit structure with a wavelength of 785nm has a lattice constant P = 0.3μm, a height H = 1.2μm, and a cross-sectional radius r = 0.01-0.08μm; the phase change gradient of the emitted light covers 0-2π, and at the same time maintains a high light transmittance, with the minimum transmittance exceeding 85%. Establish a quantitative relationship database between structural geometric parameters and wavefront phase.

[0108] Step S3: With the substrate origin as the center and the lattice constant as the step length, the substrate is divided into several grids. The propagation phase formula 2 can be derived from the generalized Snell's law:

[0109]

[0110] In formula 2, ψ(x, y) is the phase magnitude of the point with coordinates (x, y), the unit is rad, x and y both represent the length from the origin, f is the focal length, and the units of x, y and f are nm;

[0111] The unit structures of different materials and sizes are placed on specific grid points of the substrate according to the propagation phase formula (the substrate is divided into several grids with the substrate origin as the center and the lattice constant as the step size. The grid will have grid points, and each grid point has its coordinate value (x, y)), so that the incident light conforms to the propagation phase curve after passing through the superlens to achieve the focusing function.

[0112] The design of the filtering metasurface follows the above steps, and TiO2 is used as the unit structure for both wavelengths of light. However, in step S2, by systematically scanning the geometric parameters of the unit structure, the design result is intended to allow a specific wavelength to be transmitted while another wavelength is effectively filtered out, so that the unit structure that filters the 532nm wave has a transmittance of nearly 0% for 532nm light, and a transmittance of more than 80% for 785nm light, and the higher the better, preferably close to 100%, and the unit structure that filters the 785nm wave has a transmittance of more than 80% for 532nm light, and the higher the better, preferably close to 100%, and the transmittance of nearly 0% for 785nm light. By optimizing the geometric parameters (first, fix the height of the unit structure, and perform nested scanning of the lattice constant and cross-sectional radius of the unit structure with increasing steps to obtain key optical parameters such as the phase change and transmittance of the incident light wavefront. Then, keep the lattice constant P unchanged, further perform nested scanning of the height H and cross-sectional radius r of the unit structure with increasing steps, and repeat the above steps several times until the parameters that meet the design result of "allowing a specific wavelength to pass through while another wavelength is effectively filtered out" are found), the transmission characteristics of the metasurface for different wavelengths can be regulated, thereby achieving a wavelength-selective filtering effect. The parameters of the two unit structures finally determined are: the lattice constant P = 0.5μm, height H = 1μm, and cross-sectional radius r = 0.08μm of the unit structure for filtering a wavelength of 532nm; the lattice constant P = 0.7μm, height H = 1μm, and cross-sectional radius r = 0.18μm of the unit structure for filtering a wavelength of 785nm.

[0113] The final realized superlens structure is shown in Figure 6 middle.

[0114] Example 2

[0115] Photoacoustic imaging also requires a skin model as a support to fit the light energy entering the tissue cells. The present invention establishes a three-layer skin model, corresponding to the three-layer structure of the skin epidermis, dermis, and subcutaneous fat layer. The specific steps are as follows:

[0116] Step S1: Set the properties of the computational grid properties. The properties of the computational grid properties include the number of discrete points Nx, Ny, and Nz in three-dimensional space, which determine the spatial resolution of the simulation area. The increase in the number of grid points can improve the resolution of the simulation, but it will also increase the complexity and amount of calculation. In addition, the physical size of the grid needs to be set, and the spatial scale of each grid unit is defined by the step parameters dx, dy, and dz. The reciprocal of these steps is the spatial resolution, which determines the spatial accuracy of each grid in the simulation. Fine grids and smaller steps help to capture the details of the sound wave propagation process more accurately, but they will also significantly increase the computational burden. Therefore, choosing the right grid resolution and step size is the key to achieving a balance between accurate simulation and reasonable computing resource consumption.

[0117] In step S1, in theory, the number of discrete points N and the step size parameter d can be set extremely finely if there are sufficient computing resources, that is, N is extremely large and d is extremely small. Considering that actual computing resources are limited, the actual number of calculation grid points can be set between 10 and 1000. In this embodiment, the number of grid points set is 450.

[0118] Step S2: Define medium properties. The characteristics of wave propagation in a medium are significantly affected by the physical properties of the medium, mainly including the speed of sound, density and absorption coefficient. These physical parameters directly determine the propagation speed, attenuation process and reflection characteristics of sound waves in the medium. Its acoustic properties are shown in Table 1.

[0119] Table 1 Acoustic properties of the skin model

[0120]

[0121]

[0122] Step S3: define the initial pressure distribution. In photoacoustic imaging, the generation of sound waves is due to the physical expansion caused by the temperature change of the local area, which will cause the initial pressure distribution. For cancer cell detection, normal tissue cells and cancer cells are the initial pressure sources in the human body. According to the light intensity data obtained in Example 1, according to the radiation transfer equation formula 3:

[0123]

[0124] In formula 3 is the position r, direction and the light intensity at time t, c is the speed of light, μ t =μa +μ8, is the total attenuation coefficient, μ a is the absorption coefficient, μ8 is the scattering coefficient, is the scattering phase function, describing the direction of the photon Scattering direction The probability of is the light source term;

[0125] The light energy that reaches the inside of the cells after passing through three layers of skin tissue can be obtained. According to the light-to-heat conversion formula 4, the initial value of the ultrasonic wave emitted by the cells after absorbing the light energy due to heat expansion and then cooling deformation can be obtained.

[0126]

[0127] In formula 4, p0(x) represents the initial pressure distribution, in units of Pa, μ a (x) represents the light absorption coefficient distribution function, the unit is m -1 , Γ(x) represents the Grunesen coefficient, dimensionless, L(x) is the light distribution function, unit is joule;

[0128] Step S4: Define the sensor mask. In photoacoustic imaging, the sensor array is used to receive the acoustic wave signals that propagate through the medium and are reflected by the tissue. By collecting these signals, imaging reconstruction can be performed to obtain the internal structure information of the medium. In order to be practical, the sensor mask should be established outside the epidermis.

[0129] Example 3

[0130] By using numerical simulation methods, we can obtain the focusing light path of the superlens and the focal plane light field data. The result is shown in the figure. Figure 7 and Figure 8 After obtaining the light field data and establishing the skin model, computational verification is required to verify whether high-contrast cancer cell photoacoustic imaging can be achieved. Cancer cells are not arranged in a regular pattern in the actual body, so the random distribution of cancer cells during the calculation process can more realistically reflect the diversity and complexity of the tumor. In addition, the random distribution of cancer cells at different locations can simulate the various challenges that may be encountered during photoacoustic imaging, such as uneven signal intensity and complexity of the imaging area. The propagation of its acoustic wave signal follows the wave equation as shown in Formula 5:

[0131]

[0132] In formula 5, p(x,t) represents the pressure distribution in Pa, p0(x) represents the initial pressure distribution in Pa, v c represents the speed of sound in m / s, δ(t) is the pulse function, dimensionless;

[0133] Repeat the calculation many times, and the number and location of cancer cells and normal tissues are randomly distributed. Compare the difference between the initial value and the reconstructed value at the same location after each experiment to judge the imaging quality. The situation is shown as follows:

[0134] Case 1: Initial number: 5 normal tissue cells and 15 cancer cells. The photoacoustic imaging reconstruction results are 5 normal tissue cells and 15 cancer cells. The average sound pressure of normal tissue cells after reconstruction is: 21.70Pa, and the average sound pressure of cancer cells is: 207.15Pa. The initial sound pressure and the reconstructed sound pressure are compared at random positions, and the reconstructed sound pressure profile of the signal is highly fitted with the initial signal sound pressure profile.

[0135] Case 2: Initial number: 7 normal tissue cells and 18 cancer cells. The photoacoustic imaging reconstruction results are 7 normal tissue cells and 18 cancer cells. The average sound pressure of normal tissue cells after reconstruction is: 14.17Pa, and the average sound pressure of cancer cells is: 299.83Pa. The initial sound pressure and the reconstructed sound pressure are compared at random positions, and the reconstructed sound pressure profile of the signal is highly fitted with the initial signal sound pressure profile.

[0136] Case 3: Initial number: 14 normal tissue cells and 11 cancer cells. The photoacoustic imaging reconstruction results show 14 normal tissue cells and 11 cancer cells. The average sound pressure of normal tissue cells after reconstruction is 10.31 Pa, and the average sound pressure of cancer cells is 196.64 Pa. The initial sound pressure and the reconstructed sound pressure are compared at random positions, and the reconstructed sound pressure profile of the signal is highly consistent with the initial signal sound pressure profile.

[0137] The photoacoustically reconstructed pressure map is shown in Fig. 9 middle.

[0138] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A design method for a dual-wavelength confocal metalens, characterized in that: The following steps are involved: S1. Define the design goal, determine the dual working wavelengths to achieve confocalization, clarify the optical function that the metasurface should achieve, so that it can respond to two different wavelengths of light and achieve confocalization. The metasurface includes a focusing metasurface and a filtering metasurface; S2. Select materials and parameters, find the dual-wavelength metalens material and substrate respectively, and test the optical properties at different wavelengths, wherein the optical properties include refractive index and transmittance; S3. Design of multi-scale structure of metasurface, adjust its response to light of different wavelengths by adjusting the size of unit structure, optimize the geometric parameters of unit structure, analyze the influence on the phase change of outgoing light wavefront, and establish a quantitative relationship database between unit structure parameters and wavefront phase; S4. According to the quantitative relationship database, the propagation phase distribution formula derived by the designed wavefront phase formula is used to place unit structures of different materials and sizes at specific positions on the two surfaces of the substrate to achieve filtering or focusing functions; S5. Repeat S1 to S4, so that different materials are placed at specific positions of the substrate according to the propagation phase formula, and the focusing metasurface and the filtering metasurface are obtained respectively to form the dual-wavelength confocal superlens.

2. The design method according to claim 1, characterized in that: The dual working wavelengths include a visible light band and a near-infrared light band, and the optical functions that the metasurface should realize include imaging, focusing, transmission, refraction and filtering.

3. The design method according to claim 1, characterized in that: The optical band gap of the substrate in the visible light range is greater than 3.1eV, and the optical band gap in the near-infrared light range is greater than 1.5eV; the refractive index of the unit structure in the visible light range is 2.0-4.0, and the optical band gap is greater than 3.1eV, and the refractive index in the near-infrared light range is 1.8-3.5, and the optical band gap is greater than 1.5eV.

4. The design method according to claim 1, characterized in that: The unit structure is a centrally symmetrical structure.

5. The design method according to claim 1 or 4, characterized in that: The surface shape of the unit structure is circular, square or regular hexagonal.

6. The design method according to claim 1, characterized in that: The geometric parameters include one or more of a lattice constant, a cross-sectional radius and a height.

7. The dual-wavelength confocal metalens obtained by the design method according to any one of claims 1 to 6, characterized in that: It includes a substrate, a focusing metasurface on the surface of one side of the substrate, and a filtering metasurface on the surface of the other side of the substrate.

8. The dual-wavelength confocal superlens according to claim 7, characterized in that: The focusing metasurface uses 532nm and 785nm as working wavelengths, uses TiO2 as a unit structure for focusing 532nm light, and uses Si as a unit structure for focusing 785nm light; the filtering metasurface uses TiO2 as a unit structure.

9. A photoacoustic imaging method, characterized in that: Photoacoustic imaging is performed using the dual-wavelength confocal superlens described in claim 7 or 8.

10. A photoacoustic imaging system, characterized in that: It comprises the dual-wavelength confocal superlens and ultrasonic transducer as described in claim 7 or 8.

Images