CN110018537A - A kind of super surface device of high efficiency for realizing big view field imaging based on medium continuous structure - Google Patents

Abstract

The invention provides a high-efficiency metasurface device based on a medium continuous structure to realize imaging with a large field of view, comprising a medium grating structure (1), a medium substrate (2) and a medium continuous structure (3), and a single-piece imaging half field of view to 88 degrees. The continuous structure designed by the present invention has the advantages of high efficiency, large bandwidth, etc., and has angle insensitivity, and still has high efficiency under large angle incidence. The present invention realizes single-chip, flat-panel, light, thin, and integrated large-field imaging.

Description

A high-efficiency metasurface device for large-field imaging based on a dielectric continuum structure

technical field

The invention relates to the technical field of electromagnetic wave phase regulation, in particular to a high-efficiency metasurface device that realizes imaging with a large field of view based on a medium continuous structure.

Background technique

With the development of optoelectronic imaging technology, in order to obtain target image information with a larger spatial range and more spatial details, the optical system is gradually developing in the direction of large field of view, light weight and even integration. Lightweight optical imaging systems with large fields of view have broad application prospects in military fields such as aerospace remote sensing, aerial reconnaissance, seekers, and space situational awareness. A typical design method for large-field imaging devices is the multi-piece large-field fisheye lens, which has a large number of lenses and a large length, which makes it difficult to develop towards light weight and integration. In order to achieve large field of view imaging, domestic and foreign researchers have proposed a variety of new imaging structures and special solutions based on advanced optical, mechanical and electronic technologies. At present, there are mainly four methods: the first is to obtain a large-scale pixel focal plane array by mechanical splicing of multiple small-scale detectors. This method is difficult to achieve seamless splicing, and faces the focal plane array after the splicing is completed. At the same time, it requires expensive and huge optical systems. This technology is mainly used in large astronomical telescopes; the second is scanning and splicing imaging through a single high-resolution lens. This method is relatively mature and can be realized by engineering. It is relatively easy, but the time delay between the spliced fields of view caused by the field of view scanning determines that this imaging technology is only suitable for observing static or quasi-static scenes, and the application scope of this technology is limited; Two high-resolution lenses take images at the same time, and obtain high-pixel images of the full field of view through post-image stitching. This method solves the time delay problem of single-lens scanning imaging, and does not require a rotating mechanism, but the entire system consists of multiple lenses. The volume is huge and the cost is high; the fourth is a multi-scale optical system, which consists of an objective lens at the front and a micro-camera array at the back, which integrates the large-field collection capability of the objective lens and the local field-of-view correction capability of the micro-camera. The system structure It is compact and is an effective means to achieve high-resolution imaging with a large field of view. Based on the above demand background and technical status, it can be seen that the current large field of view imaging is still under the design paradigm dominated by traditional geometric optical systems, and there are many limitations such as a large number of lenses, complex lenses, and limited real-time performance.

SUMMARY OF THE INVENTION

In order to solve the above technical problems, the present invention is based on an all-dielectric continuous structure, and designs and realizes a plane, single-piece large field of view imaging metasurface device, which gets rid of the dependence on technologies such as complex fisheye lens and compound eye lens, and greatly compresses the length of the lens. , easy to integrate with detectors and equipment. The invention designs a metasurface lens based on geometric phase, which has a wide working bandwidth because the geometric phase is independent of wavelength. At the same time, based on the quadratic phase The designed lens can realize the imaging of the target with a large angle of incidence.

The technical solution adopted by the present invention to solve the technical problem is as follows: a high-efficiency metasurface device based on a dielectric continuous structure to realize large field of view imaging, comprising a dielectric grating structure 1 , a dielectric substrate 2 and a dielectric continuous structure 3 . The thickness of the dielectric grating structure 1 is h, the radial period is P _x , the tangential period is P _y , and the width is w ₂ ; the thickness of the dielectric substrate 2 is t; the radial span of the dielectric continuous structure 3 is l, and the width is is w ₁ , and the thickness is h.

Wherein, the thickness of the dielectric substrate is t, and its value range is t<λ ₀ , the thickness of the equal-width dielectric grating structure is h, and its value range is h<λ ₀ , and λ ₀ is the center wavelength.

Wherein, the radial period of the equal-width dielectric grating structure is P _x , the value range is unlimited, the tangential period is P _y , the value range is P _y <λ ₀ /2, and λ ₀ is the center wavelength.

Wherein, the radial span of the medium in the medium continuous structure is l, its value range is P _x /2<l<P _x , the medium width is w ₁ , and its variation range is P _y /10<w ₁ <P _y /2.

Wherein, the width of the dielectric grating structure is w ₂ , and its value range is P _y /10<w ₂ <P _y /2, the number of dielectric grating structures between two continuous dielectric structures is m, and its value range is is (P _x -l)/P _y ≤m≤2(P _x -l)/P _y .

The principle of the present invention is as follows: firstly, high-efficiency regulation of incident light waves is realized by adopting a medium continuous structure. Secondly, the rotational symmetry of the object space is transformed into the translational symmetry of the image space by using the quadratic phase, so as to realize the imaging of a large field of view.

The beneficial effects that the present invention has are:

First of all, the device provided by the present invention can realize single-piece large-field imaging, and solve many limitations of traditional geometrical optical systems, such as a large number of lenses, complex lenses, and limited real-time performance. phase, has ultra-broadband characteristics, and can realize polarization detection. Finally, the medium continuous structure proposed by the present invention has the characteristics of high diffraction and angle insensitivity.

Description of drawings

1 is a schematic diagram of a high-efficiency metasurface device for realizing large-field imaging according to the present invention, wherein FIG. 1(a) is a partial three-dimensional schematic diagram of the device, FIG. 1(b) is a partial schematic diagram of the device, and FIG. 1(c) is a top view of the device .

Figure 2 shows the simulation results of the diffraction efficiency of the designed metasurface device unit structure at different incident angles, in which Figure 2(a) is the absolute efficiency of each diffraction order at normal incidence, and Figure 2(b) is at 30° oblique incidence. The absolute efficiency of each diffraction order, Figure 2(c) is the absolute efficiency of each diffraction order at 60-degree oblique incidence, and Figure 2(d) is the absolute efficiency of each diffraction order at 88-degree oblique incidence.

Figure 3 shows the CST simulation results of a focusing lens with a radius R of 90um and a focal length of 72um at 0°, 30°, -60°, and 88° incidents, of which, Figure 3(a) shows different incidences in the design focal plane The normalized light intensity distribution under the angle and the light intensity change curve on the X axis; Fig. 3(b) is the normalized light intensity distribution under different incident angles on the XZ plane; Fig. 3(c) is the XZ plane Light intensity curve along the Z axis. Figures 3(d)-(f) are theoretical results corresponding to Figures 3(a)-(c).

Figure 4 shows the MATLAB simulation analysis results of the focusing lens with a radius R of 600um and a focal length of 512um at 0°, 30°, -60°, and 88° incident, among which, Figure 4(a) shows the difference in the design focal plane The light intensity distribution under the incident angle and the light intensity change curve on the X axis; Figure 4(b) is the light intensity distribution on the XZ plane under different incident angles; Figure 4(c) is the light intensity along the Z axis on the XZ plane Curve.

Figure 5 shows the imaging simulation results of a focusing lens with a radius R of 600um and a focal length of 512um on a 1.2mm*1.2mm US Air Force target at different incident angles; among them, Figure 5(a)~(d) are the incident angles respectively are the simulation results of imaging at 0°, 30°, 60° and 88°. Figure 5(e)-(f) are the light intensity distribution curves on the horizontal and vertical midline of the No.

The meanings of the serial numbers in the figure are: 1 is a dielectric grating structure, 2 is a dielectric substrate, and 3 is a dielectric continuous structure.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited to the following examples, and should include all the contents in the claims. Moreover, those skilled in the art can realize all the contents in the claims from the following embodiment.

As shown in FIG. 1 , a high-efficiency metasurface device that realizes large-field imaging based on a dielectric continuous structure includes a dielectric grating structure 1 , a dielectric substrate 2 and a dielectric continuous structure 3 . The thickness of the dielectric grating structure 1 is h, the radial period is P _x , the tangential period is P _y , and the width is w ₂ ; the thickness of the dielectric substrate 2 is t; the radial span of the dielectric continuous structure 3 is l, and the width is is w ₁ , and the thickness is h.

Combined with the above structure, in order to facilitate the analysis, the analytical modeling is carried out from the metasurface lens, as follows:

For a quadratic phase lens with a focal length f, the quadratic phase distribution of its exit pupil surface can be expressed as:

φ(x)=k ₀ x ² /2f (1)

Among them, k ₀ is the wave number corresponding to the working wavelength, and x is the position coordinate on the metasurface lens. When the incident light is obliquely incident in the meridional plane XZ plane at the incident angle θ, the phase distribution of the outgoing light is:

where k ₀ x sinθ is the additional phase introduced by oblique incidence, It is irrelevant to the position and thus can be ignored; it can be seen from equation (2) that when the incident is oblique, the outgoing phase only introduces a translation fsinθ in the X direction. Therefore, a metasurface focusing lens with a secondary phase can theoretically achieve the same imaging effect under any angle of oblique incidence.

The invention realizes the polarization conversion of circular polarization through the medium continuous structure, that is, the wavefront regulation is performed by the geometric phase generated by the rotation of the medium continuous structure. We know that the value of the geometric phase is equal to twice the rotation angle β of the continuous structure of the medium. Due to the rotational symmetry of the lens, in order to facilitate the analysis, taking the positive x direction as an example, in order to realize the phase control of φ(x)=k ₀ x ² /2f, the rotation angle distribution of the medium continuous structure β(x)=φ(x )/2=k ₀ x ² /4f, the slope distribution of the central contour curve of the continuous structure of the medium can be obtained as follows:

k(x)=tan(β(x)) (3)

Then the numerical integration operation is performed on formula (3) to obtain the central contour curve of the continuous structure of the medium as:

y=∫tan(k ₀ x ² /4f)dx (4)

In order to avoid the infinite value of formula (4), it needs to be intercepted, and the interception range is: m is an integer, the curve obtained by interception is shown as the curved dotted line in Fig. 1(b), and the intercepted part is replaced by a vertical line. Then, take equal width w1/2 along the normal direction on both sides of the curve to form a continuous dielectric structure of equal width, and at the same time, extend w2/2 along the positive and negative directions of X to form a dielectric grating structure of equal width; Then translate a single dielectric continuous structure to the Y direction by n*py (n is an integer); the thickness of the dielectric grating structure and the dielectric continuous structure is h, and they are placed on a dielectric substrate with a thickness of t. The specific structure is shown in Figure 1(a) ) and shown in Figure 1(b).

Assuming that the initial phase of the center is zero, in order to obtain a two-dimensional large field of view imaging lens, it is necessary to superimpose a helical phase on the basis of the initial phase to cancel the additional geometric phase introduced by the continuous structure of the rotating medium. Azimuth The central contour curve of the medium continuous structure at is:

in, In two-dimensional metalens, the intercepted part is replaced by a dielectric grating structure. In the numerical integration, y' _m is an uncertain value, so the central contour curve of the continuous structure of the medium is determined by the following formula:

The resulting dielectric continuous structure is then rotated around the center of the 2D metalens while the rotation angle interval is given by Decide. The local period of the dielectric grating structure is: in The two-dimensional metalens is finally composed of a series of dielectric grating structures and dielectric continuous structures, as shown in Fig. 1(c).

For a better understanding of the present invention, further explanations are given below in conjunction with the embodiments.

Without loss of generality, this embodiment designs a high-efficiency metasurface device for the mid-infrared band, and the invention is also applicable to the optical band, the terahertz band and the microwave band. As shown in FIG. 1 , the unit structure includes: a dielectric grating structure 1 , a dielectric continuous structure 3 and a dielectric substrate 2 . Among them, the material of the dielectric grating structure 1 and the dielectric continuous structure 3 is selected from silicon; the material of the dielectric substrate 2 is selected from magnesium fluoride. The thickness h of the dielectric grating structure 1 is 4.9 μm, the tangential period P _y =5 μm, the radial span l=0.75 P _x , and the width w ₂ is 1 μm. In this embodiment, CST electromagnetic simulation software is used to simulate and test the performance of the device. During the simulation, the dielectric constant of silicon is 11.36, and the dielectric constant of magnesium fluoride is 1.33.

Figures 2(a)-2(d) show the simulation results of the diffraction efficiency of the one-dimensional diffraction grating constructed by this method under different incident angles. The incident angles are 0°, 30°, 60° and 88°, respectively. During the simulation, we use left-handed circularly polarized light incident from the substrate. The period of the one-dimensional grating is 15 μm, ie P _x =15 μm. It can be seen that the energy is almost all in the -1 order. The -1st-order diffraction efficiencies at the working wavelength of 10.6 μm are 82.67%, 87.34%, 79.01% and 69.14%, respectively, under the incident angles of 0°, 30°, 60° and 88°. The diffraction efficiency is defined as the ratio of the energy of each diffraction order to the incident energy. It can be seen that the diffraction efficiency of the structure is still very high under large incident angles. In addition, we also calculated the average efficiencies in the 9-13 μm band, which were 71.66%, 73.23%, 67.78% and 65.29%, respectively.

Figure 3 shows the CST simulation results of a focusing lens with a radius R of 90um and a focal length of 72um at 0°, 30°, -60°, and 88° incidents, of which, Figure 3(a) shows different incidences in the design focal plane The normalized light intensity distribution under the angle and the light intensity change curve on the X axis; Fig. 3(b) is the normalized light intensity distribution under different incident angles on the XZ plane; Fig. 3(c) is the XZ plane Light intensity curve along the Z axis. Figures 3(d) to 3(f) are the theoretical results corresponding to Figures 3(a) to 3(c). The comparison results show that the simulation results of CST are not much different from the theoretical results. Among them, the small deviation is mainly caused by the fact that the lens structure is too small and the proportion of the middle blank area is relatively large in the simulation. In practical applications, the central blank area accounts for a very small proportion, which hardly affects the lens performance.

Because it is difficult for CST software to simulate large-diameter metasurfaces, the MATLAB numerical simulation is carried out on the metasurface with a radius R of 600um and a focal length of 512um using the vector angular spectrum theory. Figure 4 shows the MATLAB simulation analysis results of the lens at 0°, 30°, -60° and 88° incidence, respectively. Figure 4(a) shows the light intensity distribution and the X-axis at different incident angles in the design focal plane. Figure 4(b) is the light intensity distribution under different incident angles on the XZ surface; Figure 4(c) is the light intensity change curve along the Z axis on the XZ surface. Comparing the results in Figure 3, it can be seen that when the lens size is enlarged, the focusing effect of the lens is significantly improved.

Figure 5 shows the imaging simulation results of a focusing lens with a radius R of 600um and a focal length of 512um on a 1.2mm*1.2mm US Air Force target at different incident angles; among them, Figures 5(a) to 5(d) are respectively Simulation results of imaging at incident angles of 0°, 30°, 60°, and 88°. 5(e) to 5(f) are the light intensity distribution curves on the horizontal and vertical midline of the No. 5 target in the No. 4 area, respectively. It is obvious that under different angles of incidence, the resolution of the lens is almost equal, reaching 50lp/mm.

Therefore, the embodiments of the present invention are described above with reference to the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation manners, which are merely illustrative rather than restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many forms without departing from the spirit of the present invention and the scope protected by the claims, which all belong to the protection of the present invention. Parts not described in detail in the present invention belong to the well-known technologies of those skilled in the art.

Claims (6)

1. A high-efficiency metasurface device for realizing large field of view imaging based on a dielectric continuous structure, characterized in that: comprising a dielectric grating structure (1), a dielectric substrate (2) and a dielectric continuous structure (3), wherein the dielectric grating structure The thickness of (1) is h, the radial period is P _x , the tangential period is P _y , and the width is w ₂ ; the thickness of the dielectric substrate (2) is t; the radial span of the dielectric continuous structure (3) is l , the width is w ₁ , and the thickness is h. 2. A high-efficiency metasurface device based on a continuous dielectric structure to realize large field of view imaging according to claim 1, characterized in that: the thickness of the dielectric substrate is t, and its value range is t<λ ₀ ; The thickness of the dielectric grating structure is h, its value range is h<λ ₀ , and λ ₀ is the center wavelength. 3. A high-efficiency metasurface device for realizing large-field imaging based on a continuous dielectric structure according to claim 1, wherein the radial period of the dielectric grating structure is P _x , and the value range is unlimited, The tangential period is P _y , its value range is P _y <λ ₀ /2, and λ ₀ is the central wavelength. 4 . The high-efficiency metasurface device for realizing large-field imaging based on a continuous medium structure according to claim 1 , wherein the radial span of the continuous medium structure is 1, and its value range is P _x 4 . /2<l<P _x ; the medium width is w ₁ , and its variation range is P _y /10<w ₁ <P _y /2. 5 . The high-efficiency metasurface device based on a continuous dielectric structure to realize imaging with a large field of view according to claim 1 , wherein the width of the dielectric grating structure is w ₂ , and its value range is P _y . 6 . /10<w ₂ <P _y /2, the number of dielectric gratings between two continuous dielectric structures is m, and its value range is (P _x -l)/P _y ≦m≦2(P _x -l)/ _Py . 6 . The high-efficiency metasurface device for realizing large-field imaging based on a continuous medium structure according to claim 1 , wherein the designed structure is simple, and a single flat lens can realize large-field imaging. 7 .