Tunable metasurface, super lens and optical tweezers

Abstract

The utility model belongs to the technical field of metasurfaces, and particularly relates to a tunable metasurface, a super lens and optical tweezers, which are formed by periodically arranging basic units, each basic unit comprises a dielectric layer and a phase change layer which are overlapped up and down, the dielectric layer is provided with a phase change nano-column, the phase change layer is provided with a nano-column, and the phase change nano-column is arranged on the phase change nano-column. The cross section of the phase-change nano-column is rectangular, the side lengths of the rectangle are 0.5-2 microns and 0.5-2 microns respectively, and the height of the phase-change nano-column is 1-3 microns; according to the utility model, the phase regulation and control of two phases can be realized, the space utilization rate and the diversity of light modulation are improved, phase contours required in two modes are met at the same time, and the stability of particle capture is improved.

Description

Technical Field

[0001] The utility model belongs to the technical field of metasurfaces, and particularly relates to a tunable metasurface, a metalens and an optical tweezer. Background Art

[0002] Traditional lenses cannot meet the new requirements of modern optical systems such as high performance, low loss and easy integration. Metasurfaces provide a new way to solve this problem. A metasurface is a two-dimensional artificial structure composed of sub-wavelength units arranged in an orderly manner. By regulating its unit structure, electromagnetic waves can be accurately regulated. In recent years, related applications based on metasurfaces have been proposed one after another, such as metalenses, holographic imaging, electromagnetic stealth, optical computing, vortex generators, and so on. As one of the most practical devices among many applications, a metalens can flexibly generate single-focus, multi-focus and vortex beams, reducing the size, complexity and manufacturing cost of the optical system.

[0003] With the in-depth development of micro-nano optical technology in various application fields, it has been difficult for a metalens with a single fixed function to meet the requirements of practical applications. Therefore, the development of tunable metalenses has become a new research hotspot.

[0004] Publication No. CN 116165731 A discloses an intensity-tunable planar metalens in the infrared band and its adjustment method, including a top layer, a reflective layer and a substrate. The top layer is composed of a cylindrical unit array with symmetric numbers of left and right units. The radii r of multiple cylindrical units decrease from the center to both sides. The reflective layer is composed of a phase change material VO 2 and the substrate is composed of a dielectric.

[0005] Publication No. CN 117608009 A discloses an electrically controlled infrared spectral tunable zoom metasurface lens and its imaging method. The device includes a substrate, a twisted nematic liquid crystal, a substrate, a metasurface lens and a detector arranged in sequence along the optical axis direction. The metasurface lens is composed of a series of dielectric nanocolumns arranged on the substrate; the phase of the metasurface lens is determined by the length and width parameters of the dielectric nanocolumns at different positions. Summary of the Utility Model

[0006] The technical problem to be solved by the utility model is to provide a tunable metasurface, a metalens and an optical tweezer, which can realize dual-phase phase regulation, improve space utilization rate and the diversity of optical modulation, and at the same time meet the phase profiles required in two modes, and improve the stability of particle capture.

[0007] An embodiment of the present utility model provides a tunable metasurface, which is composed of periodically arranged basic units. The basic unit includes a dielectric layer and a phase change layer stacked on top of each other. A phase change nanocolumn is arranged on the dielectric layer, and a nanocolumn is arranged on the phase change layer. The cross-section of the phase change nanocolumn is rectangular, and the side lengths of the rectangle are 0.5 - 2 μm and 0.5 - 2 μm respectively, and the height of the phase change nanocolumn is 1 - 3 μm.

[0008] Preferably, the cross-section of the phase change nanocolumn is rectangular, and the side lengths of the rectangle are 1.6 μm and 0.6 μm respectively, and the height of the phase change nanocolumn is 1.9 μm.

[0009] The material of the phase change layer is VO 2 , and the material of the phase change nanocolumn is VO 2 , and the material of the dielectric layer is SiO 2 or calcium fluoride (preferably SiO 2 ), and the material of the nanocolumn is Si.

[0010] The periodic arrangement of the present utility model means that the basic units are arranged by lateral expansion, that is, it is obtained by expanding and arranging in the plane where the dielectric layer and the phase change layer are stacked on top of each other.

[0011] The so-called dielectric layer refers to a layer composed of a dielectric material, and the so-called phase change layer refers to a layer composed of a phase change material. In the same basic unit, the cross-sectional dimensions of the dielectric layer and the phase change layer are the same, and the heights can be different.

[0012] The material of the phase change nanocolumn is a phase change material, which is preferably a square column, that is, the cross-section is rectangular. The material of the nanocolumn is preferably Si, and its cross-section is preferably rectangular. The cross-sectional dimensions of the phase change nanocolumn and the nanocolumn are preferably the same, and the heights can be different.

[0013] Both the phase change nanocolumn and the nanocolumn belong to a kind of nanocolumn.

[0014] In one embodiment, the cross-sections of both the phase change nanocolumn and the nanocolumn are rectangular.

[0015] In one embodiment, both the dielectric layer and the phase change layer stacked on top of each other are square, and the sides of the rectangle are neither parallel nor perpendicular to the sides of the square.

[0016] Preferably, the cross-section of the phase change nanocolumn is rectangular, and the side lengths of the rectangle are 1.6 μm and 0.6 μm respectively, and the height of the phase change nanocolumn is 1.9 μm.

[0017] In one embodiment, both the dielectric layer and the phase change layer stacked on top of each other are square, and the side length of the square pThe thickness of the bottom electrode is 3 μm, the thickness of the dielectric layer is 0.8 μm, and the thickness of the phase change layer is 0.4 μm.

[0018] The cross-section of the nanorod is the same as that of the phase change nanorod. Preferably, the cross-section of the nanorod is rectangular, with side lengths of 1.6 μm and 0.6 μm respectively, and the height of the nanorod is 3.5 μm.

[0019] An embodiment of the present invention provides a metalens, including the tunable metasurface described above.

[0020] An embodiment of the present invention provides an optical tweezer, including the metalens described above.

[0021] An embodiment of the present invention provides the use of a tunable metasurface in integrated optical circuits, temperature sensors, optical manipulation, biological imaging or optical storage devices.

[0022] The beneficial effect of the present invention is that the present invention is mainly a double-layer switchable optical vortex metalens operating at a wavelength of 4.6 μm, using the reversible phase change of the phase change material VO 2 to respectively control the transmission field and the reflection field. At room temperature, VO 2 is in the dielectric state, and the incident light converges on one side of the lens after passing through the lens; when the temperature rises to 355 K, VO 2 transforms into the metallic state, and the incident light is reflected by the lens and converges on the other side of the lens. The present invention uses the commercial software FDTD Solutions to verify the scheme, and simulates the phenomenon that a vortex metalens with a numerical aperture of 0.88 converts circularly polarized light into vortex beams with different topological charges (0, 1, and 2) and realizes transmission and reflection focusing. The high numerical aperture of the lens itself and the orbital angular momentum carried by the vortex beam both contribute to the efficient capture and manipulation of particles. The present invention attempts to use this series of lenses for the controllable three-dimensional capture of SiO 2 particles, and the results prove that the nanoparticles can be stably captured on both the transmission surface and the reflection surface. Since the tunable metalens of the present invention can well achieve two-way phase regulation at the transmission end and the reflection end, that is, it can work in the whole space, it will show great application value in the fields of integrated optical circuits, temperature sensing, optical manipulation, biological imaging, optical storage devices, etc.

[0023] The present invention proposes a method based on the phase change material VO 2Design scheme of a double-layer switchable optical vortex metalens. The designed lens can work independently in transmission and reflection modes, and realizes reversible switching between modes by controlling the ambient temperature, greatly improving the space utilization rate and the diversity of optical modulation. To simultaneously meet the required phase profiles in both modes, two layers of metasurfaces are adopted, and the overall required phase is obtained through layer-by-layer phase accumulation. Subsequently, by simply adjusting the temperature to trigger the phase transition of VO 2 can achieve transmission and reflection focusing vortices without changing the lens structure and the polarization state of the incident light. Based on this, the present utility model first simulates three metalenses carrying different topological charges. Under the irradiation of the same incident circularly polarized light, the metalenses with topological charges of 0, 1, and 2 can all generate vortex focal fields with corresponding orbital angular momenta in the transmission space and the reflection space, and are close to the optical diffraction limit. In addition, when studying the aberration characteristics of the designed lens represented by the metalens with a topological charge of 0, it is found that in the mid-infrared band, the lens can converge the incident light in different modes and has good broadband performance. To expand the application of the lens, we also study the optical characteristics of dielectric particles on the focal planes of the metalenses with topological charges of =0 and =2, and simulate and analyze the optical force exerted on the particles by the full-space vortex field with a specific orbital angular momentum. The results show that these two lenses can not only well realize the function of the optical tweezer, but also can achieve three-dimensional stable trapping of particles at different positions by actively adjusting the working mode of the lens, improving the flexibility and practicality of the metalens optical tweezer system. Considering the duplex attribute and reversible temperature tuning characteristics of the designed lens, it is not difficult to foresee that it will play a huge role in the fields of optical communication, optical storage, optical manipulation, sensing, and biological imaging. Description of the Drawings

[0024] Figure 1 is a schematic diagram of the full-space thermally tunable optical vortex metalens of the present utility model under temperature change. At room temperature, VO 2 is in the dielectric state, and the lens realizes transmission focusing vortex (left); when the temperature rises to 355K, VO 2 becomes the metallic state, and the lens realizes reflection focusing vortex (right).

[0025] Figure 2 is a schematic diagram of the structure of the basic unit of the present utility model. (a) Three-dimensional structure diagram. The incident light is perpendicularly incident along the negative direction of the axis. When VO 2 is in the dielectric state, the metalens works in the transmission mode; when VO 2 becomes the metallic state, the metalens switches to the reflection working mode. (b) Front view. (c) Top view, which is the angle between the unit structure and the axis. Among them, H 1 =1.9μm, H 2= 3.5μm, T 1 = 0.8μm, T 2 = 0.4μm, P = 3μm, L = 1.6μm, W = 0.6μm.

[0026] Figure 3 For a metalens with a topological charge of 0 in the transmission mode, the intensity distributions on the (a) x - y plane, (b) x - z plane, and the normalized intensity distribution along the x axis in the focal plane.

[0027] Figure 4 For a metalens with a topological charge of 0 in the reflection mode, the intensity distributions on the (a) x - y plane, (b) x - z plane, and the normalized intensity distribution along the x axis in the focal plane.

[0028] Figure 5 For a metalens with a topological charge of 1 operating in the transmission and reflection modes, the intensity distributions on the (a,b) x - y plane, (c,d) x- z plane, and the normalized intensity distribution along the x axis in the focal plane.

[0029] Figure 6 For a metalens with a topological charge of 2 operating in the transmission and reflection modes, the intensity distributions on the (a,b) x - y plane, (c,d) x- z plane, and the normalized intensity distribution along the x axis in the focal plane.

[0030] Figure 7 For a switchable lens with a topological charge of 0 in the incident wavelength range of 4 to 5μm, the (a) reflection and (b) transmission focal field intensity distributions on the x - z plane.

[0031] Figure 8 For the optical force exerted on the SiO x particles when moving along the (a) z axis and (b) 2 axis in the transmission focal field with a topological charge of 0, and the corresponding potential well depths along the (c) x axis and (d) z axis.

[0032] Figure 9 For the optical force exerted on the SiO x particles when moving along the (a) zOptical force exerted on SiO 2 particles during axial movement, and the corresponding potential well depths along the (c) x axis and (d) z axis.

[0033] Figure 10 For the (a) x axis and (b) z axis during axial movement of the transmitted focal field with a topological charge of 2, the optical force exerted on SiO 2 particles, and the corresponding potential well depths along the (c) x axis and (d) z axis.

[0034] Figure 11 For the (a) x axis and (b) z axis during axial movement of the reflected focal field with a topological charge of 2, the optical force exerted on SiO 2 particles, and the corresponding potential well depths along the (c) x axis and (d) z axis. Detailed implementation manners

[0035] Example 1

[0036] Switchable vortex metalens design

[0037] Figure 1 Shows the working principle diagram of the designed lens. This metalens is actually composed of two cascaded metasurfaces, the upper and the lower. Align and fix them, and ensure that the interlayer gap is less than the Talbot length, which is defined as , where in the formula P represents the period of the lens unit structure, and is the wavelength of the electromagnetic wave in free space.

[0038] When the ambient temperature is lower than 355 K, VO 2 is in the dielectric state, and at this time a fully dielectric transmission-type metasurface can be formed. According to the dielectric waveguide theory, the incident light is converted into vortex light and converged after passing through the entire metalens; when the ambient temperature reaches 355 K, VO 2 phase-transforms into the metallic state, forming a MIM structure. The incident light is also converted into vortex light and converged after being reflected by the VO 2 thin film in the lens. In the same lens, we can easily achieve mode switching only through temperature control, breaking the limitation that conventional lenses generally only work in one-sided space.

[0039] Example 2

[0040] A tunable metasurface is composed of periodically arranged basic units. The basic unit includes a dielectric layer and a phase change layer stacked on top of each other. A phase change nanocolumn is disposed on the dielectric layer, and a nanocolumn is disposed on the phase change layer.

[0041] The material of the dielectric layer 2 is SiO 2 , and the material of the phase change layer 3 is VO 2 , the material of the phase change nanocolumn 1 is VO 2 , and the material of the nanocolumn 4 is Si.

[0042] As Figure 2 shown, the top layer is a VO 2 square column, the second layer is a SiO 2 dielectric layer, the third layer is a VO 2 thin layer, and the bottom layer is a Si square column. The top layer VO 2 nanocolumn and the bottom layer Si nanocolumn rotate simultaneously, so that the phase distribution required at the corresponding position can be satisfied by changing the rotation angle of the nanocolumn.

[0043] The nanocolumns at any position ( x , y ) on the lens must satisfy the following phase formula:

[0044] (1)

[0045] where, ( x , y ) is the position coordinate of any nanocolumn on the lens plane, is the incident wavelength, is the focal length, is the topological charge number. Accordingly, the secondary wave emitted by the metalens can produce constructive interference on the focal plane, thus generating a focused optical vortex with a specific topological charge. Let the angle between the unit structure and the x axis be θ , the abrupt phase generated by the basic unit and the angle θ have a variation relationship, so the rotation angle of the basic unit needs to satisfy the formula:

[0046] (2)

[0047] In the mid-infrared band, VO 2 has significant refractive index tuning ability and high optical contrast. When VO 2 is in the dielectric state, it has a high transmittance. As the temperature increases, the transmittance of VO 2 decreases accordingly; after the phase change is completed, the VO 2Then it has a relatively high reflectivity. In addition, it is also considered that Si and SiO 2 also have the excellent property of lossless refractive index in this wavelength band. Therefore, 4.6μm is selected as the working wavelength of the metalens, and the incident light is right-handed circularly polarized light.

[0048] To achieve all the functions of the designed lens, the key lies in the design and optimization of the unit structure. The present utility model selects the finite-difference time-domain (FDTD) method for numerical calculation, and conducts modeling and simulation in the software FDTD Solutions. The size of the unit structure is optimized through parameter scanning to obtain VO 2 The length of the square pillar is L = 1.6μm, the width is W = 0.6μm (the length and width are defined according to "front long side wide"), and the height is H 1 = 1.9μm. The length and width of the Si square pillar are the same as those of the VO 2 square pillar, and the height is H 2 = 3.5μm, ensuring that it can not only meet the 2π phase coverage in both modes, but also have a relatively high polarization conversion efficiency. The middle two layers are square units with a certain thickness. The thickness of the SiO 2 dielectric layer is T 1 = 0.8μm, and the thickness of the VO 2 thin layer is T 2 = 0.4μm, ensuring that substantial light reflection can be achieved when VO 2 is in the metallic state, and low-loss transmission can be achieved when it is in the dielectric state. To avoid the coupling between adjacent waveguides, we set the period P to 3μm.

[0049] When VO 2 is in the dielectric state, the metalens realizes the transmission function, and the VO 2 nanopillars and Si nanopillars play a synergistic role in the transmission mode; when VO 2 is in the metallic state, the metalens realizes the reflection function, and the VO 2 nanopillars play a major role in the reflection mode. Subsequently, the optimized unit structures are arranged according to formula (1) to obtain a complete lens with an aperture of 120μm. The focal lengths in both the transmission and reflection modes are set to 32μm, then NA = 0.88.

[0050] Among them, the specific parameters are H 1 = 1.9μm, H 2 = 3.5μm, T 1 = 0.8μm, T 2 = 0.4μm, P = 3μm, L = 1.6μm, W = 0.6μm.

[0051] Example 3

[0052] Under this parameter, a tunable single - focus metalens with a topological charge number = 0 was first designed and simulated. A right - hand circularly polarized light with a wavelength of 4.6 μm was irradiated downward along the z axis onto the lens, and its working performance in the transmission mode and the reflection mode was studied respectively.

[0053] The simulation results in the transmission mode are as Figure 3 shown. Through far - field detection, we can obtain the x - y plane (Figure a) and the x - z plane (Figure b) of the focal - field intensity distribution. It can be clearly seen from the figure that at z = - 36 μm, that is, about 31.3 μm away from the lens, a solid circular focal spot appears. The actually obtained focal length is slightly smaller than the designed focal length. The possible reasons are errors in the simulation calculation process and the loss of phase information. After normalization, the intensity distribution of the focus along the x direction in the focal plane is as Figure 3 shown in (c). Then the full - width at half - maximum of this transmission lens is about 2.78 μm, which is close to the diffraction limit , and it can well achieve sub - wavelength resolution.

[0054] Next, the working condition of the lens in the reflection mode was verified. Figure 4 The simulated focused - electric - field distribution in the reflection mode is shown. It can be seen that there is obvious strong focusing near the designed focal length at the reflection end, which is basically consistent with the design. In the normalized two - dimensional field - strength distribution, as Figure 4 shown in (c), the full - width at half - maximum of the focus in the x direction can be intuitively shown to be 2.84 μm, which is slightly larger than the diffraction limit, indicating that the lens has ultra - high resolution. Thus, it can be seen that the proposed all - space - tunable single - focus metalens has good focusing performance in both working states. Generally, the focusing efficiency is defined as the ratio of the optical power in a circular region with a radius of 3 times the full - width at half - maximum on the focal plane to the incident optical power. Therefore, we can calculate that the transmission focusing efficiency and the reflection focusing efficiency of this lens at the working wavelength are 28.9% and 29.6% respectively.

[0055] Example 4

[0056] The present utility model designs a tunable focusing vortex metalens that works at the same wavelength but carries different topological charges ( = 1, 2). Through simulation calculation, two focal fields with topological charges of 1 and 2 are obtained respectively as Figure 5 , 6As shown. Both of these two metalenses can convert most of the incident right-handed circularly polarized light into vortex light carrying orbital angular momentum (OAM) and focus it on the set focal plane, forming a doughnut-shaped focal spot in the figure. Due to the existence of the phase singularity, the center of the vortex beam shows a dark region with zero intensity.

[0057] When the number of carried topological charges is 1, the spot intensities of the metalens in the transmission mode and the reflection mode on the x - y plane are as shown in Figure 5 (a) and Figure 5 (b). It can be seen that the vortex beam can have good focusing effects on both the x - y planes and both form an annular aperture. Figure 5 (c) and Figure 5 (d) are the optical field diagrams of the metalens in the transmission mode and the reflection mode on the x - z plane respectively. We can see that in both modes, the metalens can focus on a specific focal plane, and the focal length is basically consistent with the designed value. Figure 5 (e) and (f) are the normalized intensity distributions along the x axis of the focal plane when the metalens works in the transmission mode and the reflection mode respectively. It is speculated that due to the interaction of errors in the unit structure and the phase modulation process, the uneven intensity distribution of the vortex light appears. According to Figure 5 (e) and Figure 5 (f), the full width at half maximum of the metalens in two working states is calculated, and the full width at half maximum in the transmission mode and the reflection mode are 6.44 μm and 6.17 μm respectively.

[0058] When the number of carried topological charges is 2, the focusing effect of the metalens is as shown in Figure 6 . Figure 6 (a) and Figure 6 (b) respectively show the intensity distributions of the lens in the transmission mode and the reflection mode on the x - y plane. The results show that the lens can still convert the normally incident circularly polarized light into a focused vortex light carrying a higher OAM value. It is observed that when the topological charge increases from = 0 to = 2, the corresponding spot expands, and the annular radius of the vortex intensity profile also increases. Figure 6 (c) and Figure 6 (d) respectively show the intensity distributions of the lens in the transmission mode and the reflection mode on the x - z plane. As seen in the figure, the actual focal lengths in both modes are about 32 μm. It can be seen that when When it increases from 0 to 2, the actual focal length of the metalens for generating focused vortex light is not much different from the designed focal length and remains basically unchanged, demonstrating the flexibility of the metalens design. We can also see that there are some sidelobes near the focus, which is caused by the interaction between unit structures. The unit structure can be further optimized later to improve this phenomenon. From Figure 6 (e) and Figure 6 In the two normalized field strength distribution diagrams of x - y the full width at half maximum (FWHM) of the annular focal spot on the focal plane in the transmission mode is 8.96 μm, and in the reflection mode is 8.78 μm.

[0059] Example 5

[0060] To characterize the performance of the lens and study its focusing performance within a certain broadband range, we randomly selected 4 wavelengths in the range of 4 - 5 μm and respectively simulated the vertical incidence of infrared beams of these 4 wavelengths onto the surface of the metalens with a topological charge of 0. Through far-field calculation, the electric field distribution of this lens on the x - z plane is as shown in Figure 7 . The simulation results show that within this wavelength band, incident light of any wavelength can be converged by the lens and form a solid focal spot, but the positions of the focal spots are different. It is not difficult to find after observation that as the wavelength of the incident light increases, the actual focal length of the lens shortens, which conforms to the focal shift formula of the diffractive lens . In addition, when the incident wavelength deviates from the operating wavelength of the lens, the focusing intensity will also decrease accordingly. We also calculated the full width at half maximum (FWHM) on the reflection and transmission focal planes respectively. When the incident wavelengths are 4 μm, 4.4 μm, 4.6 μm, and 5 μm respectively, the FWHM in the reflection mode corresponds to 2.6 μm, 2.73 μm, 2.84 μm, and 3.1 μm, and the FWHM in the transmission mode corresponds to 2.52 μm, 2.77 μm, 2.78 μm, and 2.96 μm. Thus, it can be seen that this metalens can achieve full-space convergence of the beam within a certain wavelength range in the mid-infrared, and its focal length changes with the incident wavelength. When the incident wavelength is less than the designed wavelength, the focal point position moves backward; conversely, the focal point position moves forward.

[0061] Example 6

[0062] The metasurface lens we designed achieved full-space focusing of vortex light in the mid-infrared band, and the high NA and good focusing effect of the lens itself will contribute to the three-dimensional capture and manipulation of particles. To verify the performance of the tunable optical tweezers in the designed lens, we need to calculate the optical forces exerted by the vortex light field on the dielectric particles in the transverse and axial directions. Since particles with sizes similar to the incident light wavelength are used, we choose to use the FDTD method to complete the modeling and numerically calculate the optical forces by combining the Maxwell stress tensor (MST) method. Assuming that the particle is confined in a closed cube, the average force it experiences can be written in the form of a time-averaged value. By integrating the MST over each surface of the cube, the average optical force acting on the particle can be obtained as follows:

[0063] (3)

[0064] where, is the unit vector perpendicular to the integration surface and pointing outwards, is the time-averaged MST, which can be expressed in terms of the complex amplitudes of the time-harmonic electromagnetic fields as follows:

[0065] (4)

[0066] where, and are the relative permittivity and permeability of the medium around the particle, respectively, E and H are the electric field component and magnetic field component on the cube surface, respectively, both directly obtained from the FDTD simulation data. E * and H * are their corresponding conjugate complex numbers, respectively. i and j can take x 、 y 、 z in three directions, representing different surfaces on the cube. is the Kronecker function. Therefore, can be expanded along 3 directions to obtain:

[0067] (5)

[0068] The forces acting on dielectric particles by a highly focused beam through a lens actually include the gradient force and the scattering force. The gradient force points towards the focal point and its magnitude is proportional to the transverse optical intensity gradient. The gradient force is the decisive factor in forming an optical trap. Under this action, theoretically, the particle will be trapped at the position with the strongest optical intensity. However, when the beam hits the particle, it will be scattered, generating a scattering force along the direction of the Poynting vector, resulting in the particle being finally confined near a position slightly deviated from the position with the strongest optical intensity. We integrate the MST on the particle surface to obtain the total optical force acting on the particle as follows:

[0069] (6)

[0070] To evaluate the stability of particle trapping, we calculate the trapping potential energy by integrating the optical force along the particle trapping direction, that is:

[0071] (7)

[0072] Here, represents the energy required to move the particle from infinity to the position, which is defined as the depth of the potential well. Generally, it is considered that when the depth of the potential well is greater than , the particle can be stably trapped. However, to overcome the interference of thermal effects and create a strong optical trap, a potential well depth greater than 10× is usually required. Where is the Boltzmann constant, T is the temperature.

[0073] Based on the previously designed double-layer switchable metalens, we first simulate the controllable optical tweezers based on the metalens with a topological charge of 0 to verify its three-dimensional full-space trapping effect on dielectric particles. It is designed to place a SiO 2 particle with a radius of 1.2 μm around the focal field center of the metalens, and the incident optical power is kept at 100 mW. When the lens is in the transmission mode, the initial position of the particle is set to (0, 0, -36 μm), and the optical forces acting on the particle when it moves laterally and axially are as Figure 8 shown. Figure 8 (a) shows the force conditions of the SiO 2 particle at different positions on the x axis. Due to the symmetry of the metalens structure, the transverse optical force acting on the particle is also symmetrically distributed in the positive and negative directions of the x axis. When the particle is on the negative half-axis of the x axis, it is subjected to a force in the positive direction; when the particle is on the positive half-axis of the x axis, it is subjected to a force in the negative direction. After calculation, the maximum acting on the particle is about 6.06 pN. Figure 8(b) is SiO 2 The force on the particle at different positions along the z axis can be seen. It can be seen that the axial optical force is not symmetrically distributed in the z direction. The analysis shows that it is caused by the uneven intensity distribution of the focal spot in the z direction. The maximum positive force on the particle is about 0.9 pN, and the maximum negative force is about 5.54 pN. There is an equilibrium point at (0, 0, -36.7 μm), slightly deviating from the position of the peak of the focal field intensity. This is because the particle is inevitably affected by the double influence of scattering force and gravity. When the particle moves downward from z = -28 μm to the equilibrium point position, the direction of the force is negative; when the particle continues to move from the equilibrium point to z = -43 μm, the direction of the force is positive. Thus, it can be seen that the solid focal field generated at the transmission end of this lens can well achieve the three-dimensional trapping of SiO 2 particles.

[0074] To evaluate the stability of trapping particles at the equilibrium point, we further calculated the potential well depth of SiO 2 particles at different positions and normalized it with Here, the temperature T is taken as 300 K. Figure 8 (c) and (d) respectively show the potential well depth distributions along the transverse and axial directions, and the maximum potential well depth is obtained at the equilibrium point. On the x axis and the z axis, the maximum potential well depths are 303× and 753× , respectively, both far exceeding 10× , proving that the transmission focal field with topological charge of 0 can generate a stable optical trap, thus realizing the stable three-dimensional capture of SiO 2 particles.

[0075] When the lens is switched to the reflection mode, according to the change of the focal spot position, we set the initial position of the particle to (0, 0, 33.5 μm). At this time, the transverse optical force and the axial optical force on the particle around the center of the focal field are distributed as Figure 8 shown. As shown in Figure 9 (a) and (b), when the particle moves along the x axis, the maximum optical force it receives is close to 5.6 pN, and the maximum positive and negative They are close to 4.8 pN and 0.8 pN respectively. Since the focusing efficiency of the lens is different in different working modes, after the mode switch, the optical forces in all directions applied to the same particle also change accordingly. In addition, due to the non-negligible axial dispersion force, the total optical force is enhanced in the direction of the Poynting vector and reduced in the opposite direction, resulting in different distributions of with displacement in the two modes. The equilibrium point is located at (0, 0, 35.2 μm), and it also shifts slightly in the direction of the Poynting vector. Figure 9 (c) and (d) are respectively and The corresponding potential well depths. In the x direction and the z direction, the potential well depths at the equilibrium point are 292× and 697× respectively, both of which are much larger than 10× . It can be seen that the reflected focal field with a topological charge of 0 can also stably trap SiO 2 particles in three-dimensional space. In summary, with the mode switch of the lens, the controllable optical tweezers based on this lens respectively achieve the stable trapping of dielectric particles on the transmission focal plane and the reflection focal plane.

[0076] In addition, the present utility model also studies the trapping situation of a superlens with a topological charge of 2 for the same particle. First, we simulated the optical tweezers operating in the transmission mode. The distribution of the transverse optical force acting on the particle moving around the center of the focal field is shown in Figure 10 (a). The maximum acting on the particle is about 3 pN. It can be seen that this lens can generate a stable transverse trapping force to confine the particle under the highest intensity of the annular optical field. Figure 10 (c) is the distribution of the potential well depth corresponding to . Obviously, the potential well depths at the two equilibrium positions are much larger than 10× , indicating that the particle can be stably trapped transversely. Figure 10 (b) shows the optical force on the SiO 2 particle in the z axis direction. The equilibrium position is about -38 μm. The maximum positive acting on the particle is about 0.28 pN, and the maximum negative is about 1.55 pN. From Figure 10 (d), it can be seen that on the z axis, the potential well depth at the equilibrium position is 237× , indicating that the particle can also be stably trapped axially. Based on the trapping barriers established transversely and axially, the SiO 2 particle can be stably confined in the annular region of the optical vortex field.

[0077] Subsequently, we continued to simulate the reflective optical tweezers, and the generated reflective vortex field acted on the force on the SiO 2 particle and its corresponding potential well as Figure 10 shown. Figure 11 (a) shows the relationship between the and x direction displacements applied to the particle. The particle is also confined within the annular aperture and there are multiple equilibrium points. The maximum is approximately 2.49 pN. As can be seen from Figure 11 (c), the maximum potential well depth on the x axis is 112× , indicating that this focal field can generate a stable potential well in the transverse direction and provide a transverse force for trapping particles. Figure 11 (b) shows the relationship between the and z direction displacements applied to the particle. The maximum positive is nearly 1.79 pN, and the maximum negative is close to 0.27 pN. The equilibrium position is approximately 35.15 μm. As shown in Figure 11 (d), the potential well depth distribution in the z direction shows that the potential well depth here is also significantly greater than 10× , indicating that the particle can also be stably trapped axially. Given the optical performance of the particle in the x direction and the z direction, it verifies the three-dimensional trapping ability of this lens for SiO 2 particles, and can also well maintain the position and motion state of the particles. Combining the above discussions, the controllable optical tweezers based on the vortex superlens with a topological charge of 2 can achieve three-dimensional trapping of particles at different positions by actively adjusting the working state of the lens. According to the research in this paper, we can intuitively see that due to the difference in the light field intensity distribution, the transverse trapping of the solid focal field and the annular focal field on the particles also shows different trends. In addition, the motion behaviors of the particles at the centers of these two focal fields are also different. The solid focal field will firmly confine the particles near the light intensity peak, while the annular focal field carrying orbital angular momentum will force the particles to rotate around the light field center within the annular region. Nevertheless, the two optical tweezers described in this utility model can provide stable three-dimensional trapping forces in different working modes.

[0078] Those of ordinary skill in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of protection of this application is limited to these examples; under the concept of this application, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations in different aspects of one or more embodiments of this application as described above, and they are not provided in detail for the sake of brevity.

[0079] One or more embodiments of this application are intended to cover all such substitutions, modifications, and variations that fall within the broad scope of this application. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of one or more embodiments of this application shall be included within the scope of protection of this application.

Claims

1. A tunable metasurface, characterized in that: The invention is composed of a periodic arrangement of basic units, wherein the basic units include a dielectric layer and a phase change layer stacked up and down, the dielectric layer is provided with a phase change nanocolumn, the phase change layer is provided with a nanocolumn, the cross section of the phase change nanocolumn is a rectangle, the side lengths of the rectangle are 0.5-2μm and 0.5-2μm respectively, and the height of the phase change nanocolumn is 1-3μm.

2. The tunable metasurface according to claim 1, characterized in that: The cross section of the phase-change nanocolumn is a rectangle, the side lengths of the rectangle are 1.6 μm and 0.6 μm respectively, and the height of the phase-change nanocolumn is 1.9 μm.

3. The tunable metasurface according to claim 1, characterized in that: The material of the phase change layer is VO2, the material of the phase change nano-column is VO2, the material of the dielectric layer is SiO2 or calcium fluoride, and the material of the nano-column is Si.

4. The tunable metasurface according to claim 1, characterized in that: The cross sections of the phase-change nanorod and the nanorod are both rectangular.

5. The tunable metasurface according to claim 4, characterized in that: The dielectric layer and the phase change layer stacked up and down are both square, and the sides of the rectangle are not parallel and not perpendicular to the sides of the square.

6. The tunable metasurface according to any one of claims 1 to 5, characterized in that: The dielectric layer and the phase change layer stacked up and down are both square, with a side length of p The thickness of the dielectric layer is 0.8 μm, and the thickness of the phase change layer is 0.4 μm.

7. The tunable metasurface according to claim 6, characterized in that: The cross section of the nanorod is the same as the cross section of the phase-change nanorod.

8. The tunable metasurface according to claim 7, characterized in that: The cross section of the nanocolumn is a rectangle, the side lengths of the rectangle are 1.6 μm and 0.6 μm respectively, and the height of the nanocolumn is 3.5 μm.

9. A superlens, characterized in that: Comprising a tunable metasurface as described in any one of claims 1-8.

10. An optical tweezer, characterized in that: Comprising the superlens as claimed in claim 9.

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