CN117169850A - Phase gradient super-surface and preparation method and application thereof - Google Patents
Phase gradient super-surface and preparation method and application thereof Download PDFInfo
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Abstract
The invention discloses a phase gradient super-surface and a preparation method and application thereof, belonging to the field of nano photonics. The phase gradient super-surface comprises three layers of materials, and the bottom layer is a thick metal layer, so that incident light can be totally reflected, and transmission loss is avoided; the middle layer is a transparent medium layer for isolating the upper layer and the lower layer of metal; the top layer is a planar array made of metal materials and is formed by periodically arranging composite trapezoid pair units, the composite trapezoid pair units are formed by mutually sleeving two trapezoids, and the planar array comprises a homodromous trapezoid pair structure, a butt trapezoid pair structure and a butt trapezoid pair structure of the two trapezoids arranged in the x direction and a staggered trapezoid pair structure formed by the two trapezoids staggered in the y direction.
Description
Technical Field
The invention relates to nano photonics, surface plasmon photonics, a laser principle, an optical communication technology, a frequency modulation continuous wave laser radar technology and a nano processing technology, in particular to a phase gradient super surface capable of realizing scanning of a frequency modulation continuous wave laser Lei Daqie and a preparation method thereof.
Background
In the last 60 th century, the world has been on the rise for laser research with the successful development of the first laser in the world. Without exception, lidar technology has also gradually evolved as an emerging technology. Compared with the traditional microwave radar, the laser radar has the advantages of small volume, light weight, high collimation, strong anti-interference capability, high safety, higher resolution and suitability for low-altitude detection, and is widely applied to the fields of ranging, 3D modeling and mapping.
Since the new century, the rapid development of the autopilot industry has placed higher demands on lidar, as it is an integral component of the autopilot vehicle, determining the level of development of the autopilot industry. The existing mechanical laser radar has mature technology and can scan in 360 degrees in all directions, but also has the problems of complex configuration, high price, difficult integration and the like. In the current integrated trend, the concept of hybrid/all-solid-state laser radar is gradually developed, and MEMS micro-galvanometer, optical phased array, flash and frequency modulation continuous wave method are four main technical routes: the MEMS micro-vibrating mirror technology is relatively mature, small in size, low in cost and high in reliability, but short in service life, short in effective distance and low in signal-to-noise ratio; the laser radar based on the optical phased array is durable, good in controllability, high in scanning speed and capable of monitoring multiple targets, but has very high process requirements, so that the cost is high, and the scanning angle is limited; the working principle of the Flash laser radar is similar to that of a camera, environmental information can be captured only by Flash once, the environment is prevented from being scanned, but the view field angle is limited, the exploration distance is short, and the detection precision is low; the frequency modulation continuous wave (FMCW, frequency Modulated Continuous Wave) laser radar can measure distance and speed by utilizing the Doppler principle, has high sensitivity, strong anti-interference capability, good confidentiality and no crosstalk problem, but can not detect tangential moving targets, and has high system cost and high power consumption. In addition, due to the traction required by autopilot, the wave band of the lidar is concentrated in near infrared waves, such as 905nm and 1550nm, and the lidar can only be applied on land, but not under water because water has strong absorption to infrared light.
Through overall analysis, the performance of the FMCW lidar is better than that of the other three schemes, and the FMCW lidar is very likely to become the next generation lidar technology. In the business industry, large companies have noted the potential of FMCW technology and have gradually begun to expand, for example, in 2019, autopilot Aurora will develop Blackmore company acquisitions for fm continuous wave lidar.
The super surface is developed on the basis of the metamaterial, is a two-dimensional material which is artificially synthesized and is formed by periodically arranging nano antennas with sub-wavelength sizes, can realize functions of focusing, imaging, holography, abnormal reflection and the like, and greatly improves the control capability of people on light. Compared with traditional optical elements such as gratings and lenses, the ultra-surface optical phase control method does not depend on phase accumulation generated by light propagation in space, but can change the phase within a sub-wavelength range by introducing phase mutation, so that the ultra-surface optical phase control method is small in size, light in weight and convenient to integrate. If the super surface is applied to the FMCW laser radar, the super surface has the characteristics of simple structure, passivity, convenient integration, high precision and the like, and the miniaturization, energy conservation and commercialization of the FMCW laser radar are promoted to a great extent.
Disclosure of Invention
Aiming at the analysis, the invention designs a super surface based on a trapezoid pair composite unit structure, which can realize multi-path light beam reflection scanning and regulate and control the energy proportion among light beams in order to solve the problems that an FMCW laser radar cannot tangentially detect and has high energy consumption.
The technical scheme provided by the invention is as follows:
the phase gradient super-surface based on the trapezoid-pair composite structure comprises three layers of materials, wherein the bottom layer is a thick metal layer, so that incident light can be totally reflected, and transmission loss is avoided; the middle layer is a transparent medium layer for isolating the upper layer and the lower layer of metal; the top layer is a planar array made of metal materials and is formed by periodically arranging two trapezoid pair units, wherein the two trapezoid pair units are mutually sleeved and formed by two trapezoid pair structures which are arranged in the x direction and are in the same direction, a pair tail trapezoid pair structure (the lower bottom edges of the two trapezoids are opposite), a pair head trapezoid pair structure (the upper bottom edges of the two trapezoids are opposite) and a staggered trapezoid pair structure in which the two trapezoids are arranged in the y direction. w (w) 1 Is the first ladderUpper bottom edge of shape, w 2 Is the lower base (w) 1 ＜w 2 ) H is the height of the first trapezoid, and similarly, w' 1 Is the upper bottom edge of the second trapezoid, w' 2 Is the lower base of the second trapezoid (w' 1 ＜w′ 2 ) H' is the height of the second trapezoid, and the geometric parameters of the two trapezoids may be the same or different. P (P) x Is the period of the compound trapezoid pair unit in the x direction, P y Is the period of the composite trapezoidal pair unit in the y direction, and l represents the interval between two trapezoids in the composite trapezoidal pair unit. For the same-direction trapezoid pair structure, the opposite tail trapezoid pair structure and the opposite head trapezoid pair structure which are arranged in the x direction, the requirement P is that x /2 is greater than the operating wavelength and remainsP y Is smaller than the working wavelength, and is not smaller than 0 but not larger than l (P) x -h-h')/2. For the staggered trapezoid pair structure arranged in the y direction, P x Is greater than the operating wavelength and remains->P y And/2 is smaller than the working wavelength, and is 0 ll-min { (P) y -w 1 -w′ 2 )/2，(P y -w′ 1 -w 2 )/2}。
The super surface provided by the invention is passive, so that the energy consumption of the system can be reduced, and the characteristics of small volume, light weight, simple structure and the like inherent to the super surface are introduced into the FMCW laser radar system, thereby promoting the miniaturization of the whole system. The invention further provides application of the phase gradient super-surface in an FMCW laser radar system, namely, for a composite trapezoidal pair unit, the super-surface reflected light is symmetrically distributed at +1 level and +2 level; the composite trapezoid pair unit is of a pair tail trapezoid pair structure, the pair tail trapezoid pair structure is symmetrical in the x direction, and the super-surface reflected light is symmetrically distributed between +1 level and-1 level by means of strong coupling among trapezoids. For the compound trapezoid pair unit is of a butt trapezoid pair structure, by means of coupling among trapezoids, single Shu Zheng incident light can generate 4 beams of abnormal reflected light with equal energy, the abnormal reflected light is distributed in +/-1 level and +/-2 level, and compared with two beams of detected light, a larger range can be scanned, and the analysis and test results are more comprehensive. For the composite trapezoidal pair unit is of a staggered trapezoidal pair structure, two trapezoids are arranged in the longitudinal direction in an opposite direction, and the imaging center is symmetrical, so that the reflected light of the super surface is symmetrically distributed at +2 level and-2 level. By adjusting the distance l between two trapezoids in the compound trapezoid pair unit, the light intensity of the target scanning range is increased, and the signal-to-noise ratio and the detection precision are improved.
The invention further provides a preparation method of the super surface, comprising the following steps:
(1) Preparing a bottom metal: if the wavelength of the incident laser is in the visible light range, silver and aluminum materials are generally selected, and if the wavelength is in the infrared band, gold materials are generally selected, and the thickness d of the metal substrate is required 1 ≥100nm。
(2) Preparing an intermediate layer transparent medium: the dielectric layer can be made of common transparent dielectric materials such as silicon dioxide, magnesium fluoride, calcium fluoride and the like. The thickness of the dielectric isolation layer is 30nm less than or equal to d 1 ≤60nm。
(3) Imaging: the designed periodically arranged compound trapezoid pair units are obtained through the steps of photoresist throwing (positive photoresist), pre-baking, electron beam exposure, development, fixation, post-baking and the like. Determining the upper bottom w of two trapezoids in the compound trapezoid pair unit through layout design 1 ，w 1 ' two trapezoidal bottom w 2 ，w 2 'height h, h' of two trapezoids, spacing l between the two trapezoids, and cell period P in x-direction and y-direction x ，P y 。
(4) Pattern transfer: the top metal is finally obtained through electron beam evaporation and stripping, and the thickness of the top metal is not more than 20nm and not more than d 3 Silver and aluminum are selected as materials of less than or equal to 50nm in the visible light wave band, and can be different from the bottom metal materials.
(5) Focused ion beam etching: when l is more than 0nm and less than 60nm, the gap cannot be sensitive to the small gap in the process of electron beam evaporation, so that the preset gap is still covered by metal, and the superfluous metal is etched by utilizing a focused ion beam at the moment, so that the gap between the composite trapezoids and the diameters is accurately controlled. When l.gtoreq.60 nm or l=0nm, the spacing is sufficiently wide or no spacing and no focused ion beam is required for etching.
The invention has at least the following technical advantages:
(1) And the power consumption is low. The designed super surface belongs to a passive device, does not need to provide energy additionally, does not need an additional control system, and has low cost and less material consumption.
(2) The process is simple. The invention has simple and universal process flow, is compatible with the modern Si-based CMOS process, has high processing efficiency and is convenient for mass production.
(3) And the integration is convenient. The super-surface has small volume, light weight, simple structure and no complex system.
(4) Quantitative light splitting. By adjusting the size of the beam I and controlling the energy ratio of the multipath reflected beams, the emergent energy can be quantitatively controlled.
(5) And (5) directional deflection. By adjusting the period of the structure, a specific angular deflection of a specific wavelength can be achieved.
(6) The scanning range is large. If a proper laser source exists, two or more paths of abnormal reflected light can scan the space at a large angle, so that the disadvantage that the FMCW laser radar cannot detect a transverse object is solved.
(7) The application range is wide. After the structural parameters are determined through numerical simulation, the super-surface provided by the invention can be suitable for visible light wave bands and infrared wave bands and can be applied to land and underwater.
(8) The measurement accuracy is high. The multipath abnormal reflected light not only increases the scanning range, but also can be mutually referenced and compared, and the measurement result is obtained through comprehensive analysis.
Drawings
FIG. 1 is a schematic diagram of the use of a phase gradient subsurface of the present invention in an FMCW lidar system, where α is the plane of incidence, w, and the structure and function of the subsurface 1 ，w′ 1 Respectively two trapezoidal upper bottoms w 2 ，w′ 2 The lower bottoms of the two trapezoids are respectively h and h' are respectively the heights of the two trapezoids, l is the interval between the two trapezoids, and d 1 ，d 2 ，d 3 The thickness of the bottom metal, the middle medium and the top trapezoid metal, P x ，P y Respectively x squareThe unit period in the direction of y and the direction of y, E is the incident light electric field, k is the incident light wave vector, theta r1 Is the first-order abnormal reflection angle theta r2 Is the second-order abnormal reflected light reflection angle.
FIG. 2 is a schematic illustration of a composite trapezoidal pair unit of the present invention constructed from two trapezoids nested within each other.
Fig. 3 is a simulation result of the phase of the co-directional trapezoidal pair structure and a simulation result of the far field distribution of the reflected light in the embodiment of the present invention.
FIG. 4 is a graph showing the ratio of the measured field strength relative to the two-stage beam field strength at 580nm for a co-directional trapezoidal pair structure with different spacing l in an embodiment of the present invention.
Fig. 5 is a scanning electron micrograph of a sample of a tail trapezoid pair structure and its measured far field intensity distribution in an embodiment of the present invention.
Fig. 6 is a scanning electron micrograph of a sample of a butt trapezoid structure and its measured far field intensity distribution in an embodiment of the present invention.
FIG. 7 is a schematic diagram of the structure of a staggered trapezoid pair of super-surfaces and the results of simulating far-field light intensity distribution.
Fig. 8 is a schematic diagram of focused ion beam processing.
Detailed Description
The present invention will be described in detail with reference to the accompanying drawings and examples. The examples are illustrative only and are not intended to limit the design and fabrication methods according to the present invention to the materials, patterns, conditions, or process parameters described in the examples.
The phase gradient super surface plays a role in scanning in a large range in an FMCW laser radar system, and the structure and the function are shown in figure 1. The phase gradient super surface of the invention is a trapezoid pair composite structure, which comprises three layers of materials, wherein the bottom layer is a thick metal layer, the middle layer is a transparent medium layer, the top layer is a planar array formed by periodically arranging trapezoid pair units made of metal materials, and the trapezoid pair units are shown in fig. 2, and the phase gradient super surface comprises: the trapezium pair structure formed by two trapezium arranged in x direction is the same direction trapezium pair structure, tail trapezium pair structure (lower bottom edge of two trapeziumOpposite), a butt trapezoid pair structure (the upper bottom edges of the two trapezoids are opposite), and a staggered trapezoid pair structure formed by the two trapezoids arranged in the y direction, w 1 Is the upper bottom edge of the first trapezoid, w 2 Is the lower base (w) 1 ＜w 2 ) H is the height of the first trapezoid, and similarly, w' 1 Is the upper bottom edge of the second trapezoid, w' 2 Is the lower base of the second trapezoid (w' 1 ＜w′ 2 ) H' is the height of the second trapezoid, and the geometric parameters of the two trapezoids may be the same or different. P (P) x Is the period of the compound trapezoid pair unit in the x direction, P y Is the period of the composite trapezoidal pair unit in the y direction, and l represents the interval between two trapezoids in the composite trapezoidal pair unit. For the same-direction trapezoid pair structure, the opposite tail trapezoid pair structure and the opposite head trapezoid pair structure which are arranged in the x direction, the requirement P is that x /2 is greater than the operating wavelength and remainsP y Is smaller than the working wavelength, and is not smaller than 0 but not larger than l (P) x -h-h')/2. For the staggered trapezoid pair structure arranged in the y direction, P x Is greater than the operating wavelength and remainsP y And/2 is smaller than the working wavelength, and l is more than or equal to 0 and less than or equal to min { (P) y -w 1 -w′ 2 )/2，(P y -w′ 1 -w 2 )/ 2}。
Taking the same-direction trapezoid pair structure as an example, P x ＝1440nm，P y ＝240nm，l＝0nm，w 1 ＝w′ 1 ＝60nm， w 2 ＝w′ 2 ＝180nm，h＝h′＝500nm，d 1 ＝150nm，d 2 ＝40nm，d 3 =20 nm. The s-polarized (direction of vertical incidence plane of electric field) normal incidence laser interacts with structure, each trapezoid can make phase change linearly and cover 2 pi, in which a part of light phase is simply overlapped, phase change is 4 pi in whole period, another part of light is not overlapped due to mutual coupling between two trapezoids, and their phases are directly passed through whole period, i.e. in whole periodThe phase change in each cycle is 2 pi, and the angle of the abnormal reflection is smaller than that of the former, which is called primary abnormal reflection light, and the former is secondary abnormal reflection light. According to the generalized Snell's theorem:
wherein θ is i For incident angle, theta r Is the reflection angle, lambda is the operating wavelength, n is the ambient refractive index,is the phase value along the x-axis, +.>Is the phase gradient along the x-axis. For first order abnormal reflected light->For second order abnormal reflected light->In the case of normal incidence: />The two-stage light can realize the scanning of the light beam by changing the frequency, and compared with the single-beam light, the two-beam light has wider scanning range, can mutually refer to each other, and comprehensively obtains the measurement result. As shown in FIG. 3, the left graph shows the simulated phase +.>The right graph shows the reflection angle and the relative intensity of the first-order abnormal reflection and the second-order abnormal reflection obtained by simulation. The distance l is a new degree of freedom, the energy ratio of the two abnormal reflected lights can be regulated by adjusting the size of l, as shown in figure 4, taking 580nm incident light as an example, as l increases, the energy of the primary light gradually decreases, and the energy of the secondary light gradually increasesDefining χ as the ratio of the primary light to the secondary field intensity, it can be seen that the primary light energy is rapidly transferred to the secondary light and slowly transferred from 60nm to 220nm until vanishing at a distance l of 0nm to 60 nm. In the FMCW laser radar system, the light intensity of the target scanning range can be properly increased by controlling the laser radar system, and the signal-to-noise ratio and the detection precision are improved.
For the example of a tail-to-tail trapezoid structure, P x ＝1440nm，P y ＝240nm，l＝0nm，w 1 ＝w′ 1 ＝60nm， w 2 ＝w′ 2 ＝180nm，h＝h′＝500nm，d 1 ＝150nm，d 2 ＝40nm，d 3 =20 nm. The lower bottom edges of the two trapezoid structures are opposite. Under the irradiation of s-polarized (normal incidence plane direction of electric field), due to the strong near field coupling effect between the two trapezoids, the two abnormal reflected light beams are symmetrically distributed in + -1 level, as shown in figure 5, and the applicable wave band is 400nm-650 nm.
For the butt ladder structure example, P x ＝1440nm，P y ＝240nm，l＝0nm，w 1 ＝w′ 1 ＝60nm， w 2 ＝w′ 2 ＝180nm，h＝h′＝500nm，d 1 ＝150nm，d 2 ＝40nm，d 3 =20 nm. The upper base of the two trapezoids are opposite as shown in figure 6. Under the irradiation of normal incidence laser light with s polarization (the direction of the normal incidence plane of the electric field), the reflected light is symmetrically distributed on both sides of the normal line. Because the upper bottom edges of the two trapezoids are shorter, the near field coupling effect between the two trapezoids is weaker than the coupling strength between the structural units of the tail trapezoids, so that part of light is symmetrically distributed at +/-1 level due to near field coupling, the other part of light is not coupled, the phases are simply superposed, the light is symmetrically distributed at +/-2 level, the final result is that four beams of abnormal reflected light are symmetrically distributed at +/-1 level and +/-2 level, the band of 400-650 nm is applicable, and compared with two beams of detected light, the band of the two trapezoids can scan a larger range and more comprehensively analyze and test results.
For the example of staggered trapezoidal structure, P x ＝720nm，P y ＝480nm，l＝240nm，w 1 ＝w′ 1 ＝60nm， w 2 ＝w′ 2 ＝180nm，h＝h′＝500nm，d 1 ＝150nm，d 2 ＝40nm，d 3 =20 nm. The two trapezoids are staggered in the y direction, the two reflected lights are symmetrically distributed at + -2 levels, and the applicable wave bands are 500nm-600nm, as shown in figure 7.
Compared with the prior art, the process flow of the invention has the advantages that the focused ion beam etching is added besides the different patterns exposed during patterning, namely, when the l is more than 0nm and less than 60nm, the focused ion beam etching cannot be sensitive to such small intervals in the process of electron beam evaporation, so that the preset intervals are still covered by metal, redundant metal is etched by utilizing the focused ion beam at the moment, and the intervals of the diameters of the composite trapezoids are accurately controlled, as shown in figure 8. When l.gtoreq.60 nm or l=0nm, the spacing is sufficiently wide or no spacing and no focused ion beam is required for etching.
Example 1:
(1) Parameters are determined. Gradually optimizing each parameter through numerical simulation: w (w) 1 ＝w′ 1 ＝60nm，w 2 ＝w′ 2 ＝ 180nm，h＝h′＝500nm，l＝0nm，P x ＝1440nm，P y ＝240nm，d 1 ＝150nm，d 2 ＝ 40nm，d 3 =20 nm. The trapezoids are all isosceles trapezoids.
(2) Depositing an underlying metal. By the magnetron sputtering method, the Al target is sputtered for 50min under the power of 100W, and the 150nm aluminum film can be deposited on the Si sheet.
(3) And depositing an intermediate layer. Sputtering SiO under 120W power by magnetron sputtering 2 The target material is 60min, and SiO with the thickness of 40nm can be deposited on the Si sheet 2 A film;
(4) And (5) imaging. The composite trapezoid pair unit in the exposure layout is of a pair tail trapezoid pair structure, AR-P6200.09 positive photoresist is spin-coated at 6000r/s, the film thickness is about 100nm, the photoresist is baked for 1min on a hot plate at 150 ℃, and 180 mu C/cm is adopted in a Voyager electron beam exposure system 2 Is exposed to light and then soaked in AR 600-546 developing solution for 1min, soaking in AR 600-60 fixing solution for 30s, soaking in deionized water for 30s, and finally post-baking on a hot plate at 130 ℃ for 1min.
(5) Evaporating the top metal. Steaming 20nm aluminum film by an electron beam film plating instrument, soaking in AR 600-71 stripping solution for 30min, performing ultrasonic treatment for 5min, and finally cleaning with deionized water.
(6) Characterization and testing SEM photographs of the composite trapezoidal versus cell arrangement and test structures in this example are shown in figure 5.
Example 2:
(1) Parameters are determined. Gradually optimizing each parameter through numerical simulation: w (w) 1 ＝w′ 1 ＝40nm，w 2 ＝w′ 2 ＝ 170nm，h＝h′＝500nm，P x ＝1440nm，l＝0nm，P y ＝220nm，d 1 ＝180nm，d 2 ＝ 50nm，d 3 =30nm. The trapezoid is an isosceles trapezoid.
(2) Depositing an underlying metal. And depositing a 180nm aluminum film by an electron beam evaporation method.
(3) And depositing an intermediate layer. Deposition of 50nm SiO by PECVD 2 An insulating layer.
(4) And (5) imaging. The composite trapezoid pair unit in the exposure layout is of a head-to-head trapezoid pair structure, and the rest is the same as the step (4) in the embodiment 1.
(5) And (5) transferring the graph. Evaporating a 30nm aluminum film by an electron beam film plating instrument, soaking in AR 600-71 stripping solution for 30min, performing ultrasonic treatment for 5min, and finally cleaning with deionized water.
(6) Characterization and testing. SEM photographs of the structure and test structures are shown in fig. 6.
While embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that: many changes, modifications, substitutions and variations may be made to the embodiment without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.
Claims (10)
1. The phase gradient super surface comprises three layers of materials from bottom to top, wherein the bottom layer is a metal layer and is used for totally reflecting incident light; the top layer is a planar array of metal materials; the device is used for regulating and controlling light; the middle layer is a transparent dielectric layer and is used for isolating the bottom layer and the top layer metal; the planar array is characterized in that the planar array is formed by periodically arranging compound trapezoid pair units, each compound trapezoid pair unit is formed by mutually sleeving two trapezoids, specifically, a homodromous trapezoid pair structure, a butt trapezoid pair structure and a butt trapezoid pair structure which are formed by combining two trapezoids in the x direction, and a staggered trapezoid pair structure formed by staggered arrangement of two trapezoids in the y direction.
2. The phase gradient super-surface of claim 1, wherein the composite trapezoidal pair units are of a homodromous trapezoidal pair structure, a tailed trapezoidal pair structure and a tailed trapezoidal pair structure, and an arrangement period of the composite trapezoidal pair units in an x direction is P x ，P x /2 is greater than the operating wavelength and remainsWherein h and h' respectively represent heights of two trapezoids in the compound trapezoid-to-unit, and the arrangement period in the y direction is P y ，P y Less than the operating wavelength.
3. The phase gradient subsurface of claim 1, wherein the composite trapezoidal pair units are of staggered trapezoidal pair structure, and the arrangement period of the composite trapezoidal pair units in the x direction is P x ，P x Is greater than the operating wavelength and remainsThe period of arrangement in the y direction is P y ，P y And/2 is less than the operating wavelength.
4. The phase gradient subsurface of claim 1, wherein in each composite trapezoidal pair unit, for a spacing l of two trapezoids, if the composite trapezoidal pair unit is of homodromous trapezoidal pair structure, tailed trapezoidal pair structure, and tailed trapezoidal pair structure, 0.ltoreq.l.ltoreq.p ( x -h-h')/2, if a composite ladderThe shape pair units are staggered trapezoid pair structures, and l is more than or equal to 0 and less than or equal to min { (P) y -w 1 -w′ 2 )/2，(P y -w′ 1 -w 2 )/2}。
5. The phase gradient subsurface of claim 1, wherein the bottom layer and top layer metals are silver or aluminum if the wavelength of the incident laser is in the visible range, and the bottom layer and top layer metals are gold materials if the wavelength of the incident laser is in the infrared range.
6. The phase gradient subsurface of claim 1, wherein the intermediate layer is selected from silicon dioxide, magnesium fluoride, or calcium fluoride.
7. The phase gradient subsurface of claim 1, wherein the underlayer has a thickness d 1 ，d 1 Not less than 100nm, the thickness d of the intermediate layer 2 ，30nm≤d 2 Thickness d of the top layer is less than or equal to 60nm 3 ，20nm≤d 2 ≤50nm。
8. A method of preparing a phase gradient subsurface as described in claim 1, comprising the steps of:
1) Preparing a bottom metal;
2) Preparing an intermediate layer transparent medium;
3) Imaging: the designed planar array is obtained through the layout design steps of spin coating, pre-baking, electron beam exposure, development, fixation, post-baking and the like, wherein the upper bottom w of two trapezoids is required to be determined for the composite trapezoids to the unit 1 ，w′ 1 Two trapezoidal lower bottoms w 2 ，w′ 2 The heights h, h' of the two trapezoids, the spacing l between the two trapezoids, and the cell period P of the composite trapezoids to the cells in the x-direction and the y-direction x ，P y ；
4) Pattern transfer: stripping through electron beam evaporation to finally obtain the top metal;
5) Focused ion beam etching: when l is more than or equal to 60nm or less than 0nm, the redundant metal is etched by utilizing the focused ion beam, the interval between the diameters of the compound trapezoids is precisely controlled, and when l is more than or equal to 60nm or l=0 nm, the focused ion beam is not needed for etching.
9. The use of the phase gradient super-surface according to claim 1 in an FMCW lidar system, wherein for a compound trapezoidal pair unit having a pair-tail trapezoidal pair structure, the reflected light of the phase gradient super-surface is symmetrically distributed between +1 and-1 stages; for the composite trapezoidal pair units with staggered trapezoidal pair structures, the phase gradient super-surface reflected light is symmetrically distributed at +2 level and-2 level; the phase gradient super-surface reflected light is distributed at +/-1 level and +/-2 level, and the phase gradient super-surface reflected light is symmetrically distributed at +1 level and +2 level for the same-direction trapezoidal pair structure.
10. The use of a phase gradient super surface in a FMCW lidar system according to claim 9, wherein the light intensity of the target scan range is increased by controlling the value of the spacing l between two trapezoids in the composite trapezoid pair unit to increase the signal to noise ratio and detection accuracy.
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