WO2023216472A1 - Method for regulating focal length range of zoom super-lens by introducing additional phase - Google Patents

Abstract

Disclosed in the present invention is a method for regulating a focal length range of a zoom super-lens by introducing an additional phase. A super-lens comprises: two layers of glass serving as substrates, and two layers of dielectric pillar layer arrays respectively arranged on the glass substrates. When the super-lens works, incident light vertically irradiates the lower surface of the glass substrate at the first layer, focusing is implemented by means of transmission of the glass and phase modulation of the dielectric silicon nanopillar arrays, a super-surface at the second layer is rotated, the focal length is changed, and a focal length range can be regulated by means of an additional deflection phase. According to the double-layer zoom super-lens additionally provided with the deflection phase put forward in the present invention, the contradiction between large rotation angles and efficiency of conventional zoom lenses is solved; in the present invention, a micro-nano structure is used to improve conventional zooming modes, the size and the weight of the lens are reduced as much as possible, and the trend of miniaturization and integration of modern optical devices or photoelectric devices is met.

Description

Method for controlling the focal length range of zoom hyperlenses by introducing additional phases Technical field

The invention belongs to the field of optical zoom, and in particular relates to a method for controlling the focal length range of a zoom hyperlens by introducing an additional phase.

Background technique

Tunable optics, especially tunable lenses with variable refractive power, have indispensable applications in many optical systems, such as imaging systems for simulating the functions of the human eye, or for tunable illumination and beam control. There are many ways to achieve adjustable focal length, including liquid polymer containers and liquid crystal lens groups. However, these focusing methods require complex optical systems composed of many optical devices to complete. They are large in size and are not conducive to integration. Metasurfaces, ultrathin two-dimensional artificial structures composed of metallic or dielectric optical subwavelength antennas for wavefront modulation, have attracted widespread research interest in recent years. By designing appropriate sub-wavelength structures, metasurfaces can achieve arbitrary control of the phase, amplitude, and polarization state of incident light. Therefore, various metasurface-based devices have been proposed for structural design, such as holograms, deflectors, and wave plates. Due to their advantages of small size, good integration with semiconductor devices, and high degree of design freedom, metasurfaces are used to solve problems of traditional geometric optical components, such as chromatic aberration and monochromatic aberration. Metasurfaces are also widely used in adjusting the focal length of lenses.

At present, the main metalenses that realize focus adjustment are mainly divided into three categories: thermal modulation, electric tuning, and mechanical modulation. Thermal modulation of metalens exploits the response to temperature by changing the complex refractive index that affects the phase shift and its phase distribution. A thermally tunable metalens uses a spiral gold heater to adjust the temperature increase; the refractive index changes with temperature, depending on the thermal coefficient dn/dT. Such tunable lenses that exploit both electrically and thermally mediated phenomena are classified as electrothermal optical systems. In electrically tunable metalens, the phase shift is obtained via bias voltage. The design of field-effect-induced tunable metallic materials takes advantage of the proportional increase in the complex permittivity of elementary atoms with carrier concentration. A lens with tunable NA and focal length is proposed, which exploits the change in carrier density under the action of an electric field; the advantage of this lens is that this change does not require a change in shape and the tuning speed is faster than that of a thermally tunable lens quick. Mechanical stimulation of the optical system induces structural changes that affect the position, size, and shape of the focal point. These stimuli include electrostatic actuation, rotation, and stretching. However, thermal modulation and electrical tuning require additional control systems. In contrast, mechanically modulated metalens devices have fast tuning speed and simple modulation methods, making them more suitable for imaging applications in mobile electronic devices. However, the above-mentioned polarization-insensitive zoom metalens has low light utilization efficiency when the relative angle of the doublet metasurface is large, so the focusing efficiency is low.

Contents of the invention

The object of the present invention is to provide a method for controlling the focal length range of a zoom hyperlens by introducing additional phases.

The technical solution to achieve the object of the present invention is: a method for regulating the focal length range of a zoom hyperlens by introducing an additional phase. The zoom hyperlens includes two layers of glass as a base, and a lens for adjusting the phase provided on the upper surface of the glass. The two-layer dielectric cylindrical layer array, the zoom metalens, constructs a metasurface according to the relationship between the micro-nano dielectric cylindrical unit structure radius and phase change. Its phase formula is superimposed by the actual lens phase and the additional phase. The additional phase is added by adding a constant phase Compensation is used to control the focal length range.

Compared with the existing technology, the significant advantages of the present invention are: 1) The zoom hyperlens used arranges an array of micro-nano dielectric cylindrical layers on a planar substrate to realize the function of a traditional zoom hyperlens, and can minimize its thickness to the wavelength order of magnitude, making it easy to integrate into complex systems or chip-based micro systems; 2) The complex beam modulation of the zoom metalens can be achieved by the phase superposition principle. This processing can simplify the optical system originally composed of multiple optical elements. to a metasurface thin sheet, further reducing the volume of the optical system; 3) The zoom hyperlens used has polarization insensitivity and can change the focal length for incident light of different polarizations, which breaks away from the previously common geometric phase hyperlens. The surface has specific requirements on the polarization of the incident light, and can effectively reduce the efficiency loss caused by the polarization conversion of the geometric phase metasurface; 4) The present invention can realize a zoom hyperlens with adjustable focal length and has good flexibility; 5 ) The focal length change is achieved by rotating two metasurfaces. This method provides a direct wavefront control method based on metasurfaces for lens zoom, and provides a new possibility for the application of structured beams; 6) Above guarantee In the case of the above capabilities, an additional phase is added, which can change the focus control range and make the focus control more flexible.

The present invention will be described in further detail below in conjunction with the accompanying drawings.

Description of the drawings

Figure 1 is a schematic structural diagram of an optical zoom metalens based on a polarization-insensitive metasurface in one embodiment.

Figure 2 is a schematic diagram of the structure of a metasurface unit in one embodiment.

Figure 3 is a normalized magnetic energy density diagram of two adjacent nanopillars with a radius of 80 nm in one embodiment. The white lines in the figure represent the boundaries of the structure.

Figure 4 is a diagram showing the relationship between the radius of the metasurface unit structure, the phase change amount and the efficiency under x-polarization, y-polarization, left-handed circular polarization and right-handed circular polarization obtained by performing a simulation scan on the metasurface unit structure in Figure 2 in one embodiment. .

Figure 5 is a diagram of the simulation results after applying a round function to the optical zoom metalens of the polarization-insensitive metasurface in an embodiment, wherein Figures 5(a) to (d) are 40°, 80°, 120° and 160° focus diagram, Figure 5 (e) ~ (h) are their phase distribution diagrams respectively.

Figure 6 is a graph of focus at different angles in one embodiment.

Figure 7 is a schematic diagram of focus points under different polarized lights in one embodiment. Figures 7(a) to (d) are respectively the results of focusing using linearly polarized light, X-polarized light, Y-polarized light and circularly polarized light at a rotation angle of 100°.

FIG. 8 is a schematic diagram of the focus and phase corresponding to the four angles in FIG. 5 after adding an additional phase of 120° in an embodiment. Figures 8(a)~(d) are focus diagrams of -80°, -40°, 0° and 40° respectively, and Figures 8(e)~(h) are their phase distribution diagrams respectively.

Figure 9 is a schematic diagram of focusing efficiency, which is divided into four situations, with or without round function and whether to add 120° additional phase, and a comparison chart of the efficiency in various situations at three different focal lengths of 16.5um, 23um, and 34um.

Detailed ways

As shown in Figure 1, a method for regulating the focal length range of a zoom hyperlens by introducing an additional phase. The zoom hyperlens includes two layers of glass as a base, and a two-layer dielectric cylinder disposed on the upper surface of the glass for adjusting the phase. Layer array, characterized in that the zoom metalens constructs a metasurface according to the relationship between the micro-nano dielectric cylindrical unit structure radius and phase change, and its phase formula is characterized in that it can be superimposed by the actual lens phase and the additional phase; the additional phase is added by adding a constant Phase compensation is used to control the focus range, making focus control more flexible.

In one embodiment, the dielectric material used in the cylindrical structure should meet the condition that the real part of the refractive index is large (reaching more than 1.5 times the base refractive index) and the imaginary part is close to 0 within the working wavelength. By analyzing the micro-nano dielectric cylindrical unit structure FDTD simulation calculation can obtain the relationship between radius, phase change and transmittance curve.

In one embodiment, the micro-nano cylindrical structure layer is an array of nano-cylinders with different radii. In order to ensure high efficiency, the geometric parameters of the cylindrical unit structure (as shown in Figure 2) need to meet the following restrictions: According to Nyquist sampling Law, the lattice constant p should be less than λ/2NA, λ is the wavelength, NA is the numerical aperture of the hyperlens, and at the same time, in order to ensure that there is only 0th order diffraction at normal incidence, p should be less than the equivalent working wavelength of the incident light in the substrate (λ/n _L1 ) is greater than the diffraction condition (λ/2n _L1 ), n _L1 is the substrate refractive index.

In one embodiment, the minimum value of the radius r of the cylindrical unit structure depends on the manufacturing process, and the maximum value thereof cannot be greater than half of the lattice constant.

In one embodiment, the height h of the cylindrical unit structure should be greater than λ/(n _M1 -1), where n _M1 is the refractive index of the nanocolumn material.

In one embodiment, when the radius of the cylindrical unit structure changes within a certain range, the phase change should cover 0 to 2π.

In one of the embodiments, the radius-phase change and radius-transmission efficiency change curves of the cylindrical unit structure obtained by scanning the radius should be polarization insensitive, that is, the radius-phase change curves measured under different polarizations should be consistent. The radius-transmission efficiency change curve measured under polarization should be consistent.

Scattering from zoom metalensing nanostructures should be a local effect, with no coupling between nanostructures.

The zoom hyperlens is polarization insensitive, that is, for incident light of different polarization states, the zoom hyperlens can focus without changing the focal position.

In order to facilitate understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. There is shown in the drawing a preferred embodiment of the invention. However, the invention may be embodied in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

It should be noted that if the embodiments of the present invention involve directional indications such as up, down, left, right, front, back, etc., then the directional indications are only used to explain each position in a specific posture as shown in the drawings. The relative positional relationship between components, movement conditions, etc., if the specific posture changes, the directional indication will also change accordingly.

In addition, if there are descriptions involving “first”, “second”, etc. in the embodiments of the present invention, the descriptions of “first”, “second”, etc. are only for descriptive purposes and shall not be understood as indications or implications. Its relative importance or implicit indication of the number of technical features indicated. Therefore, features defined as "first" and "second" may explicitly or implicitly include at least one of these features. In addition, the technical solutions in various embodiments can be combined with each other, but it must be based on the realization by those of ordinary skill in the art. When the combination of technical solutions is contradictory or cannot be realized, it should be considered that such a combination of technical solutions does not exist. , nor within the protection scope required by the present invention.

1 , in one embodiment, a polarization-insensitive zoom metalens based on a double-layer metasurface is provided. The zoom metalens includes double-layer glass L1 and L2 as a base, and is disposed on the upper surface of the glass. The two-layer micro-nano dielectric cylindrical layer arrays M1 and M2 are used to adjust the phase.

Here, the micro-nano dielectric cylindrical structure layers M1 and M2 are used to realize the phase control function and are respectively arranged on the glasses L1 and L2.

When the super lens is working, the incident light is vertically irradiated on the lower surface of the first layer of L2 glass substrate. After the transmission of the glass and the phase modulation of the dielectric silicon nanopillar array, it is focused, and the second layer of L1 metasurface is rotated, and the focal length changes. The focal length range can be adjusted by adding a deflection phase.

In one of the embodiments, TiO ₂ is selected as the cylindrical material. The reason is that in the visible light band, the refractive index of TiO ₂ has a large real part and a negligible imaginary part. Therefore, it has a high refractive index and is almost Negligible absorption losses. Here, preferably, the height h of the TiO ₂ cylinder is selected to be 600 nm, and the lattice constant p is 300 nm.

Furthermore, the relationship between the phase and transmission efficiency of the TiO ₂ cylindrical unit structure and the radius of the TiO ₂ cylinder can be obtained by performing FDTD simulation scanning on the TiO ₂ cylindrical unit structure (as shown in Figure 2). Since the coupling between adjacent nanopillars is extremely weak (as shown in Figure 3), they can be considered to independently perform phase control, so the changes in radius and transmission efficiency can be obtained by scanning the radius of the nanopillars. In the simulation, periodic boundary conditions are used in the x and y directions, and PML boundary conditions are used in the z direction. A point monitor is set up at 80 μm on the upper surface of the substrate to obtain changes in phase and transmission efficiency. The relationship between phase, transmission efficiency and radius is measured under the incidence of x-polarized, y-polarized, left-handed circularly polarized, and right-handed circularly polarized plane waves as shown in the figure below 4 shown. It can be seen that for different polarizations, the relationship between phase, transmission efficiency and radius is consistent, which illustrates that the unit structure has polarization insensitivity. At the same time, when the nanocolumn changes in the range of 40nm to 110nm, the phase change covers 0 to 2π. Moreover, within this range, the transmission efficiency is above 80%. The above results show that by changing the radius of the nanopillar in the range of 40nm to 110nm, high-efficiency phase control of arbitrary polarization can be achieved.

Furthermore, the metasurface can be designed according to the phase formula through the above method. Formulas (1) and (2) respectively represent the transmission functions of the two phase plates. Formula (3) is the transmission of the phase plate corresponding to formula (2) after being rotated by an angle θ. function. Formula (4) is the total transmission function after superposition of two phase plates. Compared with the transmission function of a standard spherical lens, the focal length of the designed zoom lens can be obtained as formula (5). It can be seen that the focal length f is a function of θ, and the focal length can be changed by changing the rotation angle of the second super lens, thereby achieving the zoom function. Equations (6) and (7) are respectively the transmission functions of two phase plates after adding additional phases, which can add the function of additional phases. By adding a constant phase compensation, the contradiction between focusing energy and focusing range can be solved, allowing it to maintain high efficiency within the desired focal length range. Among them, the Moiré superlens structure can be obtained by formula (1) (2) (3) (4), the focal length can be obtained by formula (5), and the Moiré after adding additional phase can be obtained by formula (6) (7). Metalens structure.

T _com =T ₁ (r, φ)T _{2, rot} (r, φ) = exp[iθround(ar ² ) (4)

f＝π/θaλ (5)

In the formula, λ represents the working wavelength, which is 532nm in this embodiment, r is the radius from any point to the center of the phase plate, φ is the angle between the x-axis and the axis at the center of the point, f is the focal length, and θ is the lens The rotation angle, a and b are constants, are limited by the resolution of the metalens, and depend on the pixel size and the maximum radius of the metalens, C is the unit cell size of the nanobrick structure, and rmax is the radius of the GEMS. According to the corresponding phase formula, a hyperlens for zoom can be made.

The production process of the optical zoom metalens based on the polarization-insensitive metasurface of the present invention is as follows: coating a flat silicon dioxide surface with hexamethyldisilazane to promote resist adhesion, and then rotating and plating undiluted Positive electron beam anti-corrosion agent ZEP-520A film to obtain an anti-corrosion material coating with a thickness of 600nm. The anti-corrosion agent is then baked at 180°C for 5 minutes. The samples were then coated with 10 nm of chromium via electron beam evaporation to avoid charging effects during the writing process. The inverted metasurface pattern was etched into the anti-corrosion coating by exposing the pattern to an accelerating voltage of 125 kV and gently stirring in o-xylene for 60 seconds to visualize the pattern. The system is then placed in a N flow and TiO ₂ is deposited by atomic layering technology at 90°C using tetrakis(dimethylamino)titanium as the precursor. The film thickness produced by complete deposition of TiO ₂ should be greater than the maximum feature size width _. half, that is, 600nm. After subsequent reactive ion etching with a mixture of _Cl2 and _BCl3 ions, the top of the _TiO2 metasurface and residual corrosion inhibitor are exposed. Finally, the sample was exposed to UV radiation and ozone, and soaked in a remover for 24 hours to obtain the final dielectric metasurface.

In one embodiment, FDTD simulation verification is performed on the optical zoom metalens based on the polarization-insensitive metasurface of the present invention. The process is as follows:

1) Set the metasurface structure parameters. The setting parameters are: silica glass layer, specification is 20μm*20μm, thickness 0.5μm; simulation wavelength 532nm; metasurface unit cylinder, thickness 0.6μm, lattice constant 0.3μm.

2) Simulate the established model. With x-polarized plane light incident, two different monitors are set up. One monitor is set up on the xz plane to monitor the phase; a monitor is set up 100nm away from the plane to monitor the entire light field. As can be seen from Figure 5, a schematic diagram of the focus and focus effect on the focal plane under four different rotation angles θ. As expected, the focal length decreases with increasing angle. The phase distribution shows the characteristics of a Fresnel lens. Figure 6 shows the relationship curve between the rotation angle θ and the focal length f. The two curves respectively represent the relationship between the theoretical focus and the simulated focus and the rotation angle. It can be seen that as the angle increases, the focal length gradually decreases from infinity and gradually levels off. As the angle increases, the difference between the two curves gradually decreases. The reason for the difference in completion is the limited size of the metasurface.

3) In Figure 7, the amplitude and phase distributions are basically consistent under different polarization incidences, which shows that the metalens has polarization insensitivity, and the slight rotational asymmetry it displays does not have a significant impact on the overall polarization insensitivity. , Based on this characteristic, we use metasurfaces to achieve changes in focus and phase at different angles in the following.

4) Figure 8 is a schematic diagram of the focus and phase after adding an additional phase of 120°. It can be seen that the focus position and phase are not affected at the corresponding angle, but a large angle can be converted into a small angle to flexibly control the focus range.

5) Figure 9 is a schematic diagram of focal length and focusing efficiency. Focusing efficiency is defined as the total power in a circular area with a diameter of 2×FWHM and the total power in a circular area with a diameter of 1mm in the focal plane. The center of the two circular areas is the point of maximum intensity. It can be seen from Figure 9 that the additional phase has a certain effect on improving the focusing efficiency and can maintain a relatively high efficiency within the desired focal length range.

The optical zoom metalens used in the present invention is a two-dimensional array metasurface made of micro-nano dielectric cylinders on a substrate. It can control the optical wavefront at sub-wavelength spatial resolution and has a complete phase control function to achieve lens zoom. Its polarization-insensitive feature allows it to achieve zooming of the lens for incident light of any polarization, which is very different from the previous common PB phase-based incident light that requires a specific polarization state. The designed zoom metalens has potential applications in the field of super-resolution imaging. This optical zoom metalens based on polarization-insensitive metasurfaces can be easily combined into chips to form composite devices with expanded functionality.

The basic principles, main features and advantages of the present invention have been shown and described above. Those skilled in the industry should understand that the present invention is not limited by the above embodiments. The above embodiments and descriptions only illustrate the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will also have other aspects. Various changes and modifications are possible, which fall within the scope of the claimed invention. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims (9)

1. A method for regulating the focal length range of a zoom hyperlens by introducing an additional phase. The zoom hyperlens includes two layers of glass as a base, and a two-layer dielectric cylindrical layer array provided on the upper surface of the glass for adjusting the phase, characterized by: , the zoom metalens, the metasurface is constructed according to the relationship between the structural radius and phase change of the micro-nano dielectric cylindrical unit. Its phase formula is superimposed by the actual lens phase and the additional phase. The additional phase realizes the focus range by adding a constant phase compensation. control.
2. The method of controlling the focal length range of a zoom hyperlens by introducing an additional phase according to claim 1, characterized in that the dielectric material used in the cylindrical structure should meet the condition that the real part of the refractive index reaches more than 1.5 times the base refractive index within the working wavelength, by The FDTD simulation calculation of the micro-nano dielectric cylindrical unit structure obtains the relationship between radius, phase change, and transmittance curve.
3. The method of controlling the focal length range of a zoom hyperlens by introducing additional phases according to claim 1, characterized in that the dielectric cylindrical layer array is a nanocylindrical array of different radii, and the geometric parameters of the cylindrical unit structure satisfy the following conditions: According to Nyquist According to the special sampling law, the lattice constant p should be less than λ/2NA. At the same time, p should be less than the equivalent working wavelength of the incident light in the substrate λ/n _L1 and greater than the diffraction condition λ/2n _L1 , where λ is the wavelength and NA is the super lens. Numerical aperture, n _L1 is the substrate refractive index.
4. The method of controlling the focal length range of a zoom hyperlens by introducing additional phases according to claim 3, characterized in that the maximum value of the cylindrical unit structural radius is not greater than half of the lattice constant.
5. The method of controlling the focal length range of a zoom hyperlens by introducing additional phases according to claim 4, characterized in that when the structural radius of the cylindrical unit changes within a certain range, the phase change should cover 0 to 2π.
6. The method of controlling the focal length range of a zoom hyperlens by introducing additional phases according to claim 4, characterized in that the height h of the cylindrical unit structure is greater than λ/(n _M1 -1), and n _M1 is the refractive index of the nanocolumn material.
7. The method of controlling the focal length range of a zoom hyperlens by introducing additional phases according to claim 4, characterized in that the material of the cylindrical unit structure is _TiO2 .
8. The method of controlling the focal length range of a zoom hyperlens by introducing additional phases according to claim 1, characterized in that the radius-phase change curve and the radius-transmission efficiency change curve obtained by scanning the radius of the cylindrical unit structure are measured under different polarizations. The curves should be consistent.
9. The method of controlling the focal length range of a zoom hyperlens by introducing an additional phase according to claim 1, wherein the zoom hyperlens can focus with a constant focus position for incident light of different polarization states.

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