CN110333560B - Broadband achromatic device based on medium super surface - Google Patents

Abstract

The invention discloses a broadband achromatic device based on a dielectric metasurface, and relates to the field of novel artificial composite material metasurfaces. The analysis includes the following steps: using a combination of geometric phase and propagation phase to control the wavefront of the incident left-circularly polarized light and eliminate chromatic aberration, respectively, and then, tailoring the size to construct a metasurface array, and by measuring the right-circularly polarized energy of the detection surface. Extraction and processing to achieve achromatic analysis of the device body. On the one hand, the broadband achromatic device in the present invention utilizes the method of combining two phases, and on the other hand, realizes achromatic achromatic in a continuous bandwidth range, which greatly reduces the volume and cost compared with the previous devices.

Description

A Broadband Achromatic Device Based on Dielectric Metasurface

technical field

The invention relates to the field of novel artificial composite material metasurfaces, in particular to a broadband achromatic device based on dielectric metasurfaces.

Background technique

Wavelength dispersion is an important property of optical materials and has always played an important role in the design of optical components and systems. In most media, like glass, the refractive index decreases with increasing wavelength, which is called normal dispersion. With this material, the refractive lens will have a larger focal length at longer wavelengths than at shorter wavelengths, and the prism will deflect at a smaller angle. This chromatic aberration severely degrades the performance of panchromatic optical applications such as communications, detection, imaging, display, and more.

Traditionally, chromatic aberration has been eliminated by the integration of several different materials whose refractive indices are inversely related to wavelength. Such refractive optical elements are usually bulky, high in cost, high in manufacturing precision, and time-consuming, and have gradually been unable to meet increasing device performance requirements. Therefore, metasurface optical devices with miniature, ultra-thin and integration-friendly characteristics are gradually replacing traditional optical devices. Optical devices realized based on metasurfaces are mainly composed of two materials, metal and dielectric. Due to the inherent ohmic loss of metal structures, the transmission efficiency of metal metasurfaces is extremely low. Although reflective metal metasurfaces can achieve 80% efficiency, most mainstream optical devices are transmissive devices, and reflective metasurfaces are not applicable. Huygens metasurfaces can achieve high transmittance, but are very sensitive to structure size, posing great challenges to modern processes. Replacing the metal structure with a dielectric structure and optimizing the structural parameters can eliminate the ohmic loss, thereby further improving the efficiency of the reflective metasurface. At the same time, a relatively mature semiconductor fabrication process can be used to fabricate a dielectric metasurface, so it is beneficial to realize optical devices with high transmission, low loss and compatibility.

At present, there are also metalens that use a multi-layer structure to achieve the elimination of two-wavelength and three-wavelength chromatic aberrations. While this strategy has been successful, it adds weight, complexity, and cost to optical systems, greatly limiting their use. Therefore, it is of great significance to implement a broadband achromatic device with a lightweight, simple structure and low cost method.

SUMMARY OF THE INVENTION

The purpose of the present invention is to propose a working band in the mid-infrared band of 3.7-4.5 μm, which mainly focuses (deflects) left-hand circularly polarized light (LCP) on the same focal plane (same deflection angle) in a focusing (deflecting) manner. ), thus realizing a broadband achromatic device based on a dielectric metasurface.

In order to achieve the above object, the present invention adopts the following technical solutions:

A broadband achromatic device based on a dielectric metasurface, comprising a device body, the device body comprising a dielectric rectangular block on the upper layer and a substrate on the lower layer, and the achromatic analysis of the device includes the following steps:

The combination of geometric phase and propagation phase is used to control the wavefront of incident left-circularly polarized light and eliminate chromatic aberration, respectively. After that, the size of the dielectric rectangular block is cut to construct a metasurface array. By extracting the right-circularly polarized energy of the detection surface and processing to achieve achromatic analysis of the device body.

Preferably, the combination of the geometric phase and the propagation phase is specifically: using the principle of geometric phase to rotate the dielectric rectangular block to control the wavefront of the incident left-handed circularly polarized light, so that it satisfies the hyperbolic phase or gradient phase profile, so that at the selected central wavelength to achieve focus or deflection;

Using the propagation phase as the compensation phase, the required different compensation phase values are obtained by controlling the length and width of the dielectric rectangular block to ensure that within the selected wavelength range, the propagation phase has a linear relationship with the reciprocal of the wavelength. Chromatic aberration effects at wavelengths.

Preferably, the underlying substrate is made of low-refractive index material calcium fluoride, the dielectric rectangular block is made of high-refractive index material silicon, and the incident wavelength range of the device body is 3.7-4.5 μm.

Preferably, when the incident wavelength of the device body is 4.5 μm, the thickness of the dielectric rectangular block is 4 μm, the lattice constant is 1.8 μm, the thickness of the substrate is 3 μm, and the device body passes through the right side of the detection surface. Extraction and processing of circularly polarized energy, analyzing the focusing performance of the device.

Preferably, when the incident wavelength of the device body is not equal to 4.5 μm, the focal plane of the device body to achieve focusing remains unchanged, and the achromatic characteristics of the device are analyzed by extracting and processing the right-handed circularly polarized energy of the detection surface. .

In the present invention, a dielectric is used as a structural unit, and a high-refractive index material is used as a modulation structure, and there is no material absorption in the wavelength band, which greatly enhances transmission and improves efficiency.

The beneficial effects of the present invention are:

1. The present invention is suitable for a large frequency range, which can be realized from the visible light band to the terahertz band. In addition, the designed metasurface realizes achromatic achromaticity within the continuous bandwidth range. Besides the metasurface can be used to realize achromatic achromatic, it can also be realized. Combined with traditional optics, it can improve the performance and size of metasurface achromatic devices and expand the range of applications.

2. The low cost and compatibility of semiconductor fabrication make the achromatic device in the present invention suitable for application in nanophotonics and integrated optics.

3. The incident left-handed circularly polarized light used in the present invention vertically irradiates the metasurface structure. According to the selected incident center wavelength, the structural parameters of the dielectric rectangular block are scanned and optimized, and the required compensation phase can be obtained by changing the size of the rectangular silicon. , and achieve high transmittance. Moreover, the propagation phase will not be affected in the process of using the PB phase, which ensures that the method of combining the PB phase and the propagation phase can independently control the wavefront of the incident light and eliminate the chromatic aberration.

4. On the one hand, the broadband achromatic device in the present invention utilizes a method of combining two phases, and on the other hand, realizes achromatic achromaticity in a continuous bandwidth range. Compared with previous devices, the volume and cost are greatly reduced.

Description of drawings

Figure 1 is a design diagram of a single rectangular block antenna.

Figure 2 is the effective refraction of the left-handed circularly polarized light incident to change the length and width of the rectangular block, and the propagation phase and geometric phase diagrams of two cell structures of different sizes.

Figure 3 is a schematic diagram and phase distribution diagram of an achromatic metalens.

Figure 4 is a schematic diagram of the results of the broadband achromatic metalens and the chromatic lens after incident left-handed circular polarization.

Figure 5 is a schematic diagram of the result of the broadband achromatic deflector and the chromatic deflector after incident left-handed circular polarization.

Reference numerals in the figure: 1 device body, 11 substrate, 12 dielectric rectangular block.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments.

1-5, a broadband achromatic device based on a dielectric metasurface includes a device body 1, and the device body 1 includes a dielectric rectangular block 12 located on the upper layer and a substrate 11 located on the lower layer, and its achromatic analysis includes the following steps: using The geometric phase and the propagation phase are combined to control the wavefront of the incident left-handed circularly polarized light and eliminate chromatic aberration respectively. After that, the size of the dielectric rectangular block 12 is cut to construct a metasurface array, and the right-handed circularly polarized energy is extracted from the detection surface. and processing to realize the analysis of the achromatic characteristics of the device body 1 .

In this embodiment, the combination of the geometric phase and the propagation phase is specifically: using the principle of geometric phase to rotate the dielectric rectangular block 12 to control the wavefront of the incident left-handed circularly polarized light, so that it satisfies the hyperbolic phase or gradient phase profile, so that the selected Focusing or deflection can be achieved at the center wavelength of the sensor; using the propagation phase as the compensation phase, the required different compensation phase values are obtained by controlling the length and width of the dielectric rectangular block 12 to ensure that within the selected wavelength range, the propagation phase and the reciprocal of the wavelength are linear relationship to eliminate chromatic aberration effects at wavelengths other than the center wavelength.

In this embodiment, the lower substrate 11 is made of low-refractive index material calcium fluoride, the dielectric rectangular block 12 is made of high-refractive index material silicon, and the incident wavelength range of the device body 1 is 3.7-4.5 μm.

In this embodiment, when the incident wavelength of the device body 1 is 4.5 μm, the thickness of the dielectric rectangular block 12 is 4 μm, the lattice constant is 1.8 μm, the thickness of the substrate 11 is 3 μm, and the device body 1 passes through the right-handed circle on the detection surface. Extraction and processing of polarized energy to analyze the focusing performance of the device.

In this embodiment, when the incident wavelength of the device body 1 is not equal to 4.5 μm, the focal plane of the device body 1 to achieve focusing remains unchanged. By extracting and processing the right-handed circularly polarized energy of the detection surface, the achromatic characteristics of the device are analyzed. .

Parameter optimization and simulation test of broadband achromatic metasurface device:

Determine and optimize element structure size parameters. Taking the selected rectangular column element structure as an example, the base thickness is h ₁ ; the height of the rectangular column is h ₂ ; the length along the x-axis is a, the width along the y-axis is b, and the lattice constant is p. Modeling in comsol, changing the length and width at the same time, the phase response of the rectangular block unit to different polarizations is obtained by simulation, and then the appropriate structural unit is selected according to the phase required for the designed metasurface to achieve the corresponding function. Taking the incident center wavelength of 4.5 μm as an example, the lattice constant p is preferably 1.8 μm, and the height of the rectangular block is 4 μm. Scanning the structural parameters of a single rectangular block, that is, changing the dimensions _a and b at the same time, can obtain different propagation phase values. This is because the propagation phase of the rectangular block at a given coordinate position x is φ(x,λ)=2π/λn _eff h ₂ , where n _eff represents the effective refractive index of the rectangular block, and n _eff is known from the waveguide theory It is closely related to the length and width of the rectangular block, as shown in Figure 2a.

Metasurface element structure selection. In order to realize the broadband achromatic function, each structure on the metasurface needs to satisfy the focusing phase and the compensation phase at the same time, and these two phases are independent of each other. The focusing phase is realized by the PB phase, which is only related to the rotation angle of the unit structure, that is, when the rotation angle of the unit structure is α, the phase response corresponding to the unit structure is ± 2α, and the sign is related to the rotation of the incident circularly polarized light. The left-handed circular polarization incident is a positive sign; and the compensation phase is realized by the propagation phase. Within the selected bandwidth of 3.7-4.5 μm, the difference between the phase response corresponding to the maximum wavelength and the minimum wavelength is the phase compensation value realized by the unit structure, As shown in Figure 2b. Moreover, in order to prove that the PB phase does not affect the propagation phase response, the present invention studies the relationship between α and wavelength, as shown in Figure 2c. It can be seen from the figure that the value of the propagation phase difference remains unchanged when α is changed.

Design of broadband achromatic metalens. Taking a two-dimensional surface as an example for specific simulation, the focal plane is in the XOZ plane, and the required phase of the transmitted light is described as follows:

To realize broadband achromatic metalens, equation (1) can be rewritten as:

compensation phase

和补偿相位

的大小，消色差超透镜示意图和所需相位分布图如图3。对于中心波长4.5μm的入射光，焦距设置为f＝27μm，超单元由43个共振器构成，总长度L_x＝77.4μ^m。由公式求出所需聚焦相位与补偿相位，再由步骤(1)和(2)优化和选取合适的超表面单元结构，从而仿真得到本发明的宽带消色差超透镜，采用5个样品波长入射，结果如图 4a。为更好地说明消色差效果，本发明还设计了与消色差透镜大小相同的色差透镜做对比，结果如图4b。where f is the focal length, x is the horizontal distance between each structure and the focal point, and λ _max is the maximum wavelength within the selected bandwidth. According to the position of each subunit structure, determine the value of the horizontal distance x, and then substitute other parameters into the above formula to obtain the corresponding focusing phase

and compensation phase

The size, schematic diagram of the achromatic metalens and the required phase distribution are shown in Figure 3. For incident light with a central wavelength of 4.5 μm, the focal length is set to f=27 μm, and the supercell is composed of 43 resonators with a total length of L _x =77.4 ^μm . The required focusing phase and compensation phase are obtained by the formula, and then the appropriate metasurface unit structure is optimized and selected by steps (1) and (2), so as to simulate the broadband achromatic superlens of the present invention, using 5 sample wavelengths to be incident , the result is shown in Figure 4a. In order to better illustrate the achromatic effect, the present invention also designs a chromatic lens with the same size as the achromatic lens for comparison, and the result is shown in Figure 4b.

Design of a broadband achromatic deflector. According to the generalized Snell's law, the required phase of deflection can be described as follows:

To achieve broadband achromatic deflection, equation (4) is rewritten as:

为中心波长处所需相位，

为补偿相位。在本发明的偏转器中，对于中心波长4.5μm的入射光，偏转角度设置为θ＝-18.2°，超单元由18个共振器构成，总长度 L_x＝32.4μm。由公式求出所需偏转相位与补偿相位，再由步骤(1)和(2) 优化和选取合适的超表面单元结构，从而仿真得到本发明的宽带消色差偏转器，采用5个样品波长入射，结果如图5a。为更好地说明消色差效果，本发明还设计了与消色差偏转器大小相同的色差偏转器做对比，仿真结果如图5b。where θ is the deflection angle of the transmitted light,

is the desired phase at the center wavelength,

to compensate the phase. In the deflector of the present invention, for incident light with a center wavelength of 4.5 μm, the deflection angle is set to θ=−18.2°, the superunit is composed of 18 resonators, and the total length L _x =32.4 μm. Calculate the required deflection phase and compensation phase by the formula, and then optimize and select a suitable metasurface element structure through steps (1) and (2), so as to simulate the broadband achromatic deflector of the present invention, using 5 sample wavelengths incident , the result is shown in Figure 5a. In order to better illustrate the achromatic effect, the present invention also designs a chromatic deflector with the same size as the achromatic deflector for comparison, and the simulation result is shown in Figure 5b.

The above description is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited to this. The equivalent replacement or change of the inventive concept thereof shall be included within the protection scope of the present invention.

Claims (3)

1. A broadband achromatic device based on a dielectric super-surface, comprising a device body (1), wherein said device body (1) comprises a rectangular dielectric block (12) on the upper layer and a substrate (11) on the lower layer, and the achromatization analysis comprises the following steps:

the wave front of incident left-handed circularly polarized light is controlled and chromatic aberration is eliminated by combining a geometric phase and a propagation phase, then a super-surface array is constructed by cutting the size of a dielectric rectangular block (12), and achromatic characteristic analysis of the device body (1) is realized by extracting and processing right-handed circularly polarized energy of a detection surface;

the combination of the geometric phase and the propagation phase is specifically as follows: the wave front of incident left-handed circularly polarized light is controlled by rotating a dielectric rectangular block (12) by utilizing the principle of geometric phase to enable the wave front to meet a hyperbolic phase or gradient phase profile, so that focusing or deflection is realized at a selected central wavelength;

the propagation phase is used as a compensation phase, different required compensation phase values are obtained through the length and the width of the dielectric rectangular block (12), and the propagation phase and the reciprocal of the wavelength are in a linear relation in a selected wavelength range and are used for eliminating the chromatic aberration effect at the wavelength outside the central wavelength;

the lower-layer substrate (11) is made of low-refractive-index material calcium fluoride, the dielectric rectangular block (12) is made of high-refractive-index material silicon, and the incident wavelength range of the device body (1) is 3.7-4.5 microns.

2. A dielectric-super-surface-based broadband achromatic device according to claim 1, wherein said device body (1) has an incident wavelength of 4.5 μm, said dielectric rectangular block (12) has a thickness of 4 μm, a lattice constant of 1.8 μm, said substrate (11) has a thickness of 3 μm, and said device body (1) analyzes the focusing performance of the device by extracting and processing right-hand circularly polarized energy of the detection plane.

3. A broadband achromatic device based on dielectric super-surface, according to claim 1, characterized in that, when the incident wavelength of said device body (1) is not equal to 4.5 μm, the focal plane of said device body (1) for focusing remains unchanged, and the achromatic characteristics of the device are analyzed by extracting and processing the right-handed circularly polarized energy of the detection plane.

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