High-efficiency broadband all-dielectric reflector based on metasurface and design and preparation method thereof

Abstract

The invention relates to the technical field of photodynamics, and discloses a metasurface-based efficient broadband all-dielectric reflector and a design and preparation method thereof.The all-dielectric reflector is of a double-layer film structure formed by periodic continuation of a plurality of light sail units, each light sail unit comprises a silicon nitride layer and a silicon layer, the silicon nitride layer is provided with a plurality of round holes, and the round holes are formed in the silicon layer. The plurality of circular holes are distributed on the silicon nitride layer in an array, and the silicon layer is provided with a circular opening. The broadband reflection of more than 70% can be realized in the range of 1300 nm to 1550 nm when the broadband reflection antenna is used, the peak reflectivity at 1330 nm exceeds 90%, the bandwidth can cover the Doppler spectrum when the broadband reflection antenna is accelerated, and the broadband reflection antenna has the advantages of high reflectivity, broadband response, large receiving area and light weight; the problems that a traditional light sail is poor in stability, and high-resolution images of extrasolar planets cannot be effectively captured under the condition of high-speed driving are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of photodynamics, and in particular relates to a high-efficiency broadband all-dielectric reflector based on a metasurface and a design and preparation method thereof. Background Art

[0002] In the field of interstellar exploration, the dream of launching spacecraft to neighboring galaxies has lasted for decades. In this grand exploration, a project called Breakthrough Starshot came into being, with the goal of sending a spacecraft to our nearest star, Proxima Centauri, within human lifetimes. The core issue of the goal of sending a spacecraft to the star Proxima Centauri is to use lasers to drive light sails. Once achieved, this technology will also promote more groundbreaking space exploration missions. For example, the optimal focal distance of the solar gravitational lens is more than 548 astronomical units (AU) from the earth. Therefore, in order to give full play to the extraordinary imaging capabilities, light sails are indispensable. And at such a long distance, light sails are needed to accurately position it.

[0003] Since there is no air resistance in space, even the extremely small propulsion generated by light sails can be accumulated with almost no loss, and without the need for an external power source or the chemical energy of fuel, it can be accumulated to provide a very impressive propulsion speed for small spacecraft. Therefore, light sails are considered to be a propulsion technology with great application potential in the future. However, traditional light sails cannot effectively capture high-resolution images of exoplanets at high speeds due to their poor stability.

[0004] Therefore, the core of the present invention lies in the development of a high-efficiency broadband all-dielectric reflector based on a metasurface, which can achieve ultra-high speed under the impetus of a laser beam and can effectively solve the problems existing in the above-mentioned prior art. Summary of the invention

[0005] 1. Technical issues to be solved

[0006] In view of the shortcomings of the prior art, the present invention provides a high-efficiency broadband all-dielectric reflector based on a metasurface and a design and preparation method thereof. The high-efficiency broadband all-dielectric reflector based on a metasurface can achieve a broadband reflectivity of more than 70% in the 1300nm to nearly 1550nm band, and the bandwidth can cover the Doppler spectrum when accelerated. It has the advantages of high reflectivity, broadband response, large receiving area and light weight, and solves the problem that traditional light sails have poor stability and cannot effectively capture high-resolution images of exoplanets when traveling at high speeds.

[0007] (II) Technical solution

[0008] In order to achieve the above-mentioned purpose of being able to achieve a broadband reflectivity of more than 70% in the 1300nm to nearly 1550nm band and having a bandwidth that can cover the Doppler spectrum when accelerated, the present invention provides the following technical solutions:

[0009] The present invention provides a first solution: a design method for a high-efficiency broadband all-dielectric reflector based on a metasurface, comprising:

[0010] Step 1: Analyze the power source of the high reflector according to the mechanical and electromagnetic theories, and establish the power model of the reflector;

[0011] Step 2: Based on the dynamic model of the reflector, establish the frequency and wavelength model of the reflector;

[0012] Step 3: Design a metasurface broadband dielectric reflector based on the multilayer dielectric equivalent wave impedance theory and determine the reflector structure;

[0013] Step 4: Design the double-layer membrane structure parameters of the reflector based on the deep learning method.

[0014] Preferably, the process of establishing the dynamic model of the high reflector in step 1 includes:

[0015] Step 1.1: Calculate the radiation pressure of the high reflector;

[0016] Step 1.2: Calculate the radiation pressure from an ideal perfect mirror

[0017]

[0018] Among them, I f-in I is the incident wave radiation pressure per unit area on the surface of the object, f-re is the reflected wave radiation pressure, S is the energy flux density, and c is the speed of light in vacuum;

[0019] Step 1.3: Calculate the radiation pressure of a photon. A photon always moves at the speed of light and its energy is:

[0020]

[0021] Among them, h is Planck's constant, ν is the vibration frequency of the photon, λ is the corresponding wavelength, and c is the speed of light in vacuum.

[0022] Preferably, the process of calculating the radiation pressure of the high reflector in step 1.1 includes:

[0023] The energy flux density of electromagnetic waves is calculated by the following formula:

[0024]

[0025] Among them, ΔE is the total energy or energy flow of electromagnetic waves, ΔA is the effective action area, ΔF is the radiation pressure of electromagnetic waves acting on the surface of the object, and ΔX is the effective work distance of the pressure;

[0026] Considering the force per unit time,

[0027] X = Δt × c = 1 × c = c

[0028] Therefore, the calculation formula of the energy flux density of electromagnetic waves is simplified to

[0029]

[0030] Among them, I f It is the radiation pressure of the mirror sail per unit area of ​​the object's surface.

[0031] Preferably, the process of establishing the frequency and wavelength model of the reflector in step 2 includes:

[0032] Step 2.1: Establish the frequency model of the wavefront reaching the receiver in the working frequency band of the high reflector

[0033]

[0034] Among them, f r,s is the frequency at which the wavefront arrives at the receiver;

[0035]

[0036] Where β = vc, f s is the frequency, v is the receiver speed, and c is the speed of light in vacuum;

[0037] Step 2.2: Build a wavelength model for the wavefront reaching the receiver in the high reflector operating band

[0038]

[0039] Among them, λ r is the wavelength of the wave received by the receiver, λ s is the wavelength of the wave emitted by the source.

[0040] The present invention provides a second solution: a high-efficiency broadband all-dielectric reflector based on a metasurface, wherein the all-dielectric reflector is a double-layer membrane structure formed by periodically extending a number of light sail units, wherein the light sail unit includes a silicon nitride layer and a silicon layer, wherein a number of circular holes are arranged on the silicon nitride layer, wherein the number of circular holes are distributed in an array on the silicon nitride layer, and a circular opening is arranged on the silicon layer.

[0041] Preferably, the single thickness of the silicon nitride layer is 400 nm, the opening radius of the circular hole is 500 nm; and the thickness of the silicon layer is 321 nm.

[0042] Preferably, the all-dielectric reflector achieves a broadband reflection higher than 70% in the range of 1300 nm to 1550 nm, and a peak reflectivity of more than 90% at 1330 nm.

[0043] The present invention provides a third solution: a method for preparing a high-efficiency broadband all-dielectric reflector based on a metasurface, comprising:

[0044] Step 1: Use SOI wafer as the base layer;

[0045] Step 2: Depositing a high stress silicon nitride layer on both sides of the SOI wafer;

[0046] Step 3: Spin-coat the photoresist S1813 on the front surface of the chip as a protective layer, and spin-coat the photoresist AZ10XT on the back surface of the chip;

[0047] Step 4: Expose the back side of the chip with AZ10XT resist and develop on the back side of the chip;

[0048] Step 5: The chip is then removed from the silicon carrier wafer and cleaned in hot acetone and isopropyl alcohol, leaving behind a clean SiLi-Con nitride / silicon bilayer membrane device.

[0049] Preferably, the process of depositing a high stress silicon nitride layer on both sides of the SOI wafer in step 2 includes:

[0050] Step 2.1: Deposit a 400 nm high stress silicon nitride layer on both sides of the SOI wafer by low pressure chemical vapor deposition, and determine the thickness of the grown silicon nitride layer by ellipsometry;

[0051] Step 2.2: After the LPCVD process, the SOI wafer was cut into chips of equal volume; individual chips were selected and their front sides were spin-coated with an electron beam AR-P 6200, and then the front side of the samples was subjected to electron beam lithography using a Raith EBPG 5200 at an accelerating voltage of 100 keV;

[0052] Step 2.3: After etching the silicon nitride plasma using a CH3F / O2 mixed gas, the photonic crystal circular hole pattern is transferred to the silicon nitride layer on the front side of the sample.

[0053] Preferably, the process of exposing the back side of the chip having the AZ10XT resist and developing on the back side of the chip in step 4 includes:

[0054] Step 4.1: Expose the back side of the chip with AZ10XT resist using a Heidelberg Instruments Laserwriter and develop to form a circular opening with a diameter of approximately 3 mm;

[0055] Step 4.2: Afterwards, the same CH3F / O2 plasma etch is used to first remove 400nm of silicon nitride on the back of the chip, and then a deep reactive ion silicon etch is used to remove 500μm of silicon;

[0056] Step 4.3: When the silicon etching is completed, the color contrast between silicon and silicon dioxide is confirmed by optical microscopy observation, and finally the BOX layer is removed using isotropic silicon etching with SF6 gas.

[0057] (III) Beneficial effects

[0058] Compared with the prior art, the present invention provides a high-efficiency broadband all-dielectric reflector based on a metasurface and a design and preparation method thereof, which has the following beneficial effects:

[0059] 1. The present invention designs a high-efficiency broadband all-dielectric reflector based on a metasurface. The reflector is a double-layer film structure, which is composed of a stack of silicon nitride photonic crystals and ultra-thin silicon films. It can achieve a broadband reflection of more than 70% in the range of 1300nm to 1550nm, and a peak reflectivity of more than 90% at 1330nm. It has strong structural reliability and great application prospects.

[0060] 2. The present invention designs a design method for a high-efficiency broadband all-dielectric reflector based on a metasurface. The method analyzes the possibility that the structure used can exhibit a wider reflection bandwidth based on the multilayer dielectric equivalent wave impedance theory, and optimizes the theoretical analysis results into specific structural parameters through a deep learning method. The design method has a mature theoretical basis, the optimization process is short in time, and the performance of the designed high-efficiency broadband all-dielectric reflector based on a metasurface is effectively guaranteed;

[0061] 3. The present invention designs a method for preparing a high-efficiency broadband all-dielectric reflector based on a metasurface. The method uses a commercial SOI wafer as a substrate, and sequentially obtains an all-dielectric metasurface broadband reflector through low-pressure chemical vapor deposition, double-sided spin coating, Si3N4 etching, wafer flipping, laser writing, SF6 isotropic silicon etching, deep silicon etching, and hot acetone and isopropyl acetone cleaning. The processing technology is precise and controllable, and can effectively guarantee the performance of the obtained high-efficiency broadband all-dielectric reflector based on the metasurface. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 A conceptual model diagram of the all-dielectric reflector of the present invention;

[0063] Figure 2 A flow chart of the design and preparation method of the all-dielectric reflector proposed in the present invention;

[0064] Figure 3 The equivalent wave impedance simulation and analysis diagram of the multilayer medium of the SiN photonic crystal and Si double-layer film structure of the present invention;

[0065] Figure 4 This is a diagram of the photonic crystal structure of the present invention;

[0066] Figure 5 is a moving curve diagram of the reflection peak position of the light sail of the present invention;

[0067] Figure 6 A flow chart of manufacturing silicon nitride photonic crystal and silicon bilayer film from SOI wafer according to the present invention;

[0068] Figure 7 This is an enlarged view of the all-dielectric reflector (light sail unit); (a) front; (b) back; where 1-silicon nitride layer; 101-circular hole; 2-silicon layer; 201-circular opening;

[0069] Figure 8 Characterization diagram of the light sail sample made for the present invention.

[0070] exist Figure 3 In the figure, Figure (a) is the equivalent model of a single-layer photonic crystal structure, and Figure (b) is the equivalent impedance curve of the equivalent model of a single-layer photonic crystal structure; Figure (c) is the equivalent model of a double-layer superphotonic crystal structure, and Figure (d) is the equivalent impedance curve of the equivalent model of a double-layer superphotonic crystal structure;

[0071] exist Figure 4 In the figure, Figure (a) is a top view of the photonic crystal structure, and Figure (b) is a side view of the photonic crystal structure; Figure (c) is a top view of the superphotonic crystal, and Figure (d) is a side view of the superphotonic crystal; Figure (e) is a top view of the double-layer dielectric structure (without holes), and Figure (f) is a side view of the double-layer dielectric structure (without holes); Figure (g) is a comparison of the reflection coefficient curves of various structures, and Figure (h) is a comparison of the power curves of various structures; Figure (i) shows the change of the observed wavelength with the speed of the light sail, and Figure (j) is the curve of the reflectivity changing with the wavelength after adjusting the structural parameters;

[0072] exist Figure 5 In the figure, (a) is a diagram of the Doppler frequency shift effect of the moving light sail from low speed to 20% of the speed of light, and (b) is a parameter scanning curve diagram of the photonic crystal period value from 1.15 to 1.3μm to show the movement of the light sail reflection peak position;

[0073] exist Figure 6 In the figure, Figure (a) is a flow chart for the preparation of an all-dielectric mirror; Figure (b) is a typical optical image of the front side (4×4 mm) of the all-dielectric mirror; Figure (c) is the back side of the all-dielectric mirror; Figures (e) and (f) show optical microscope images; Figure (g) is a successfully prepared 4-inch wafer-level all-dielectric mirror.

[0074] exist Figure 8In the figure, (a) is a diagram of the optical measurement device, and (b) is an optical measurement diagram of a single and SiN PhC / Si sample. DETAILED DESCRIPTION

[0075] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0076] Example 1: Please refer to Figure 1-7 The present invention provides a technical solution: a design method for a high-efficiency broadband all-dielectric reflector based on a metasurface, comprising:

[0077] Step 1: Analyze the power source of the high reflector based on mechanical and electromagnetic theories and establish the power model of the reflector

[0078] The high reflector designed in this embodiment uses radiation pressure as a power source, and can reach ultra-high speed under the impetus of a laser beam; radiation pressure (also called light pressure) is a mechanical pressure exerted on any surface due to the exchange of momentum between an object and an electromagnetic field; this includes the momentum of any wavelength of light or electromagnetic radiation absorbed, reflected, or otherwise emitted (such as blackbody radiation) by any scale of matter (from macroscopic objects to dust particles to gas molecules), and the force generated in this process is called radiation pressure, or light force for short. The pressure generated by radiation is usually too small to be easily perceived in everyday situations. However, they are important in certain physical processes and technologies, especially objects in outer space, where, in addition to gravity, radiation pressure is usually the main force acting on objects, and tiny forces can have a large cumulative effect over a long period of time. For example, if the effect of solar radiation pressure on a spacecraft is ignored, the spacecraft will deviate from its fixed orbit. The radiation pressure of starlight is also crucial in many astrophysical processes. Therefore, based on this principle, light sails can operate in space driven by large lasers. Radiation pressure can also be explained by considering the momentum of classical electromagnetic fields or the momentum of photons. When electromagnetic waves or photons interact with matter, momentum is exchanged. Due to the law of conservation of momentum, any change in the total momentum of the wave or photon must cause an equal and opposite change in the momentum of the matter it interacts with (Newton's third law of motion), such as when light is perfectly reflected by a mirror. Based on the law of conservation of momentum, we can give a general explanation of light sails.

[0079] Step 1.1: This example calculates the radiation pressure of the reflector using electromagnetic theory and the momentum of electromagnetic waves.

[0080] The energy flux density of electromagnetic waves is calculated by the Poynting vector as follows:

[0081]

[0082] The energy flux density can be calculated by the following formula:

[0083]

[0084] Among them, ΔE is the total energy or energy flow of electromagnetic waves, ΔA is the effective action area, ΔF is the radiation pressure of electromagnetic waves acting on the surface of the object, and ΔX is the effective work distance of the pressure;

[0085] Because we consider the force per unit time,

[0086] X=Δt×c=1×c=c (3)

[0087] Therefore, formula (2) can be simplified to

[0088]

[0089] Among them, I f It is the radiation pressure per unit area on the surface of the object;

[0090] Therefore, when the electromagnetic wave is incident vertically on the surface of an object, the relationship between the radiation pressure per unit area and the energy flux density is:

[0091]

[0092] Note that the momentum direction of the electromagnetic wave is consistent with the direction of the energy flow, where S is the energy flow density;

[0093] Step 1.2: Next, we calculate the radiation pressure of an ideal perfect mirror:

[0094] When an electromagnetic wave is incident vertically on a perfect reflector, the incident wave will generate radiation pressure, and the reflected wave will further affect the radiation pressure. The incident wave is perfectly reflected by the reflector, so the radiation pressure generated by the reflected wave is equal to the radiation pressure generated by the incident wave, that is,

[0095]

[0096] Therefore, the total radiation pressure on the surface of the object is

[0097]

[0098] For imperfect reflectors, the second term must be multiplied by the reflectivity, so the total radiation pressure will decrease. For light sail design, we should increase its reflectivity as much as possible to increase the surface radiation pressure, thereby improving the radiation efficiency of the laser light source.

[0099] Step 1.3: Next, we calculate the radiation pressure of photons. At this point, electromagnetic radiation is viewed from the perspective of particles, because photons always move at the speed of light. According to the theory of quantum mechanics, their energy is given by the following formula:

[0100]

[0101] Among them, h is Planck's constant, ν is the vibration frequency of the photon, and λ is the corresponding wavelength;

[0102] When photons are incident vertically on the surface of an object, the radiation pressure generated by the incident light can still be calculated using formulas (2) and (5). Similarly, the calculation method for the radiation pressure generated by reflected photons is similar.

[0103] Step 2: Analyze the wide-band requirements of high-reflection mirrors based on the theory of relativity and establish their frequency and wavelength models

[0104] Doppler shift refers to the change in the frequency observed by an observer relative to the frequency of the wave source as the observer moves relative to the wave source. A common example of Doppler shift is the change in pitch heard when a vehicle honks its horn closer and farther away from an observer. The cause of the Doppler effect is that when the wave source moves toward the observer, each successive wave crest is emitted closer to the observer's location than the crest of the previous wave. Therefore, each wave takes slightly less time to reach the observer than the previous wave. Therefore, the time between successive wave crests arriving at the observer decreases, causing the frequency to increase. As the waves propagate, the distance between the crests decreases, so the waves "bunch together." Conversely, if the wave source moves away from the observer, each wave crest is emitted farther from the observer's location than the previous crest, so the arrival time between the crests increases, decreasing the frequency. The distance between the crests then increases, so the waves "spread out."

[0105] For waves propagating in a medium, the velocities of the observer and the source are relative to the medium in which the wave propagates. Therefore, the total Doppler shift can be caused by the motion of the source, the motion of the observer, the motion of the medium, or any combination thereof. For waves propagating in a vacuum, such as electromagnetic waves, only the velocity difference between the observer and the source needs to be considered. If this relative velocity is not negligible compared to the speed of light, a complex relativistic Doppler effect will occur. Therefore, in this project, we need to further study the relativistic Doppler effect brought about by the light sail when it generates velocity, where the observer is the light sail and the wave source is the radiating laser.

[0106] Longitudinal relativistic Doppler shifts, where the source and receiver move directly toward or away from each other, can generally be derived like the classical phenomenon, but need to be modified by adding a time dilation term.

[0107] The longitudinal relativistic Doppler effect is derived below, assuming that the receiver and source are moving away from each other at a relative velocity v, which is measured by an observer at the receiver or the source. (The sign convention used here is that if the receiver and source are moving towards each other, v is negative) The problem is considered with the source as the reference frame.

[0108] Step 2.1: Establish the frequency model of the wavefront reaching the receiver in the working frequency band of the high reflector

[0109] (1) Assume that a wavefront from the source reaches the receiver and the next wavefront is located at a distance λ from the receiver. s At s =cf s , where λ s is the wavelength of the wave emitted by the source, f s is the corresponding frequency, c is the speed of light in a vacuum; the speed of the wavefront is c, and the time interval for the next wavefront to reach the receiver is t r,s , in the same time period, the receiver moves away from the source at a speed v, and the observation at the source position is

[0110] λ s +vt r,s =ct r,s (9)

[0111] By transforming equation (9), we can get

[0112]

[0113] Where β = vc, the corresponding wavefront frequency reaching the receiver is

[0114]

[0115] So far, the equation is the same as that for the classical Doppler effect;

[0116] Step 2.2: Build a wavelength model for the wavefront reaching the receiver in the high reflector operating band

[0117] Due to relativistic effects, the clock at the receiver is time-dilated relative to the clock at the source, t r =t r,s γ / , where γ is the Lorentz factor, Therefore, the frequency received by the receiver is

[0118]

[0119] The above is the relativistic Doppler shift equation; written in wavelength form as follows

[0120]

[0121] Among them, λr is the wavelength of the wave received by the receiver.

[0122] The light sail moves at a high speed close to the speed of light, and the light source emits laser light at a fixed wavelength. As the speed of the light sail gradually increases, the wavelength observed at the light sail will gradually increase, that is, the relativistic Doppler effect becomes increasingly obvious. If you want the light sail to be accelerated to a higher speed, first of all, it is necessary to ensure that the light sail can respond to the single frequency light emitted by the laser with high reflectivity when it is stationary. Secondly, when the light sail is accelerated, its corresponding observation wavelength changes. Therefore, it is even more necessary to achieve high reflectivity over a wide wavelength range.

[0123] Step 3: Design a metasurface broadband dielectric reflector based on the multilayer dielectric equivalent wave impedance theory and determine the reflector structure

[0124] In order to theoretically explain why the SiN(PhC)-Si double-layer film structure exhibits a wider reflection bandwidth than the single-layer SiN photonic crystal, this step is analyzed based on the multilayer dielectric equivalent wave impedance theory, and the results are as follows:

[0125] Step 3.1: If Figure 3 As shown in (a), there are three layers of different lossless media, two interfaces are parallel to each other, the normalized impedances of media 1, 2, and 3 are Z1, Z2, and Z3, and the thickness of medium 2 is d. When an electromagnetic wave is incident vertically along the positive z-axis from medium 1, reflection and transmission occur at the interfaces z=0 and z=d. Therefore, there are incident waves propagating along the +z direction and reflected waves propagating along the -z direction in both media 1 and 2, while there is only a transmitted wave propagating along the +z direction in medium 3;

[0126] According to the theory of equivalent wave impedance of multilayer media, the reflection coefficient at the interface z = 0 is

[0127]

[0128] Among them, Z ef23 is the equivalent wave impedance of medium 2 and medium 3 at z = 0, and

[0129]

[0130] Among them, j is the imaginary part, is the propagation constant of electromagnetic waves in medium 2;

[0131] Step 3.2: Back to our classic single-layer photonic crystal model, the photonic crystal is medium 2, and its front (medium 1) and back (medium 3) are both air layers, so Z1 = Z3 = Z0; according to Figure 4 The reflection coefficient of the photonic crystal in (g) and formula (14) are used to calculate the normalized equivalent impedance Z ef23; Then consider the superphotonic crystal model. Since it has two layers of medium, it is equivalent to a four-layer medium model as follows Figure 3 (c) As shown; the matching layer (gray) Si is medium 2, and the silicon nitride layer is medium 3, and their normalized impedances are and Z 23 ; Its front (medium 1) and back (medium 4) are both air layers, so Z 21 =Z 24 = Z0; the equivalent wave impedance Z of medium 3 and medium 4 at z = d1 ef2 =Z ef23 ,Right now Figure 3 (b) According to formula (15), the equivalent wave impedance of media 2, 3, and 4 at z = 0 can be calculated as

[0132]

[0133] Finally, according to formula (14), the reflection coefficient is calculated:

[0134]

[0135] Therefore, a layer of Si is added and its thickness and the parameters of the photonic crystal are reasonably designed to ultimately achieve efficient and broadband reflection; this step uses three layers of different lossless media to derive the reflectivity expression, the two interfaces are parallel to each other, the normalized impedances of media 1, 2, and 3 are Z1, Z2, and Z3, the thickness of medium 2 is d, and when the electromagnetic wave is incident vertically from medium 1 along the positive z-axis, reflection and transmission occur at the interfaces z=0 and z=d. Therefore, there are incident waves propagating along the +z direction and reflected waves propagating along the -z direction in both media 1 and 2, while there are only transmitted waves propagating along the +z direction in medium 3. According to the theory of equivalent wave impedance of multilayer media, the double-layer membrane structure has more efficient and broadband reflection performance than the classical single-layer photonic crystal model.

[0136] Step 4: Design double-layer membrane structure parameters based on deep learning method to achieve broadband high reflectivity

[0137] According to the derivation of step 3, it is determined that a double-layer membrane structure is used to optimize the design of a high-reflection mirror, and the double-layer membrane structure is parameterized as: period p, upper silicon nitride thickness h1, lower silicon thickness h2, upper opening radius r; a large number of initial samples are generated by the Monte Carlo method and numerical simulation is performed in the commercial electromagnetic field simulation software CST STUDIO SUITE, and the corresponding relationship between the reflectivity and each parameter is recorded to form a basic data sample set; a multi-layer error backtracking neural network is used to optimize the optimal structural parameters, and the data in the basic data sample set is used for neural network training. A fast optimization program is written using Matlab software, and finally the broadband reflectivity is used as the objective function to design the double-layer membrane structure through a neural network.

[0138] As a preferred embodiment, the specific process of using the Monte Carlo method to generate a large number of initial samples and performing numerical simulation in the commercial electromagnetic field simulation software CST STUDIO SUITE to record the corresponding relationship between reflectivity and various parameters to form a basic data sample set includes:

[0139] Step 4.1: First, the refractive index of silicon (Si) and high stress silicon nitride (SiN) is accurately measured using an ellipsometer, and the values ​​obtained are Si=3.4 and SiN=2.0 respectively;

[0140] from Figure 4 Starting from the standard single-layer photonic crystal periodic structure shown in (a) and (b), SiN is used as the dielectric material, and the parameters are p1 = 1200nm, h1 = 400nm, r1 = 500nm;

[0141] Step 4.2: Use the finite-difference time-domain (FDTD) algorithm to simulate the electromagnetic wave incident along the -z direction (the electric field is along the x direction) under periodic boundary conditions; Figure 4 The simulation results shown in the green curve in (g) show that over the wavelength spectrum covering 1334nm to 1385nm, |S 11 | indicates a reflection coefficient greater than 90%; this result is converted into a strong reflectivity, such as Figure 4 The green curve shown in (h), |S 11 | 2 More than 80%, bandwidth is 51nm;

[0142] Step 4.3: However, the above analysis reveals a limitation inherent to photonic crystals, namely, their limited reflection bandwidth due to their resonant nature. To further increase the bandwidth of the device, a matching layer is introduced below the SiN photonic crystal layer, resulting in a new structure called a superphotonic crystal, such as Figure 4 (c) and (d); the matching layer is made of silicon (red part), and its parameters are p2, h2 and r2, which are consistent with the initial structure, and h2 = 321nm, which is determined according to the specific properties of the SOI wafer used in this work. The same simulation method was then used to achieve remarkable results. Specifically, |S 11 | value exceeds 90% in the wavelength range from 1308nm to 1449nm, which makes the reflectivity |S 11 | 2 More than 80%, extended bandwidth up to 141nm ( Figure 4 The red curves in (g) and (h) show that the bandwidth of the structure can be increased by nearly 3 times compared with the original structure, and it is expected to be applied to fields such as light sails with significant Doppler frequency shift characteristics.

[0143] Step 4.4: As an intermediate step, a double-layer dielectric structure is formed by eliminating the circular hole structure on the SiN layer while retaining the underlying Si layer. This simplified structure also has advantages in nanofabrication process while retaining parameters such as layer thickness. The final design is selected Figure 4 The structures shown in (c) and (d) have dimensions as shown in Table 1.

[0144] Table 1: Optimal parameters of double-layer membrane structure

[0145] p / nm <![CDATA[h2 / nm]]> <![CDATA[h2 / nm]]> <![CDATA[r2 / nm]]> Material Top film 1200 - 400 500 Silicon Nitride (SiN) Base film 1200 321 - - Silicon (Si)

[0146] The specific design effects are as follows:

[0147] The simulation results show that Figure 4 As shown in (g), within the wavelength range of 1321nm to 1553nm, the reflectivity always exceeds 80%; correspondingly, within the wavelength range of 70%, the reflectivity exceeds 1300nm to 1550nm ( Figure 4 (blue curves in (g) and (h)). Although this double-layer dielectric structure has a wider bandwidth, it has a lower peak reflection and larger mass compared to the other two designs, so it was decided to omit this particular design in the fabrication process.

[0148] The above design ultimately achieves a broadband reflector that seamlessly covers the entire wavelength range of the Doppler-shifted laser light during the acceleration phase of the light sail. To provide concrete context, imagine a laser with an initial wavelength of 1300nm whose task is to accelerate the light sail to about 20% of the speed of light. Under these conditions, Figure 5 As shown in (a), the wavelength of the laser will shift significantly, from 1300nm to about 1550nm. It is worth mentioning that the peak reflection position of the light sail can be easily adjusted by changing the parameters of the photonic crystal layer (such as the period). Figure 5 As shown in (b), by increasing the period from 1.15 to 1.3 μm, the peak of the reflection changes from below 1300 nm to about 1420 nm. This provides a high degree of flexibility in designing light sails or other optical elements with different optical responses, such as metalenses, on-chip integrated photonics, or free-space optics.

[0149] Embodiment 2: Different from the above-mentioned embodiment 1, this embodiment provides a method for processing and manufacturing a high-efficiency broadband all-dielectric reflector based on a metasurface, comprising:

[0150] Step 1: For a double-layer structure device, such as Figure 6 As shown in (a), we use a commercial SOI wafer and process it to have a 321nm thick silicon on the SiO2 buried oxide (BOX) layer and a silicon dioxide thickness of about 500μm on the other side.

[0151] Step 2: Then, a 400nm high-stress silicon nitride layer was deposited on both sides of the SOI wafer by low-pressure chemical vapor deposition (LPCVD); the thickness of the grown silicon nitride layer was determined by ellipsometry. After the LPCVD process, the 4-inch wafer was cut into 1×1 cm chips. Individual chips were selected and their front sides (silicon nitride / silicon double-layer) were spin-coated with an electron beam AR-P 6200 (CSAR 62), and then the front side of the sample was electron-beam lithography using a Raith EBPG 5200 with an acceleration voltage of 100keV; after etching the silicon nitride plasma using a CH3F / O2 mixed gas, the photonic crystal circular hole pattern was transferred to the silicon nitride layer on the front side of the sample.

[0152] Step 3: Then, photoresist S1813 from Microresist was spin-coated on the front side of the chip as a protective layer, and photoresist AZ10XT from MicroChemicals was spin-coated on the back side of the chip for the next step of processing.

[0153] Step 4: The back of the chip with AZ10XT resist is then exposed with a Heidelberg Instruments Laserwriter (μMLA) and developed to form a circular opening with a diameter of about 3 mm. A circular opening is used here because the success rate of the final double-layer film is higher. After that, the same CH3F / O2 plasma etching is used to first remove 400nm of silicon nitride on the back of the chip, and then deep reactive ion silicon etching (Bosch process) is used to remove 500μm of silicon. When the silicon etching is completed, the color contrast between silicon and silicon dioxide can be confirmed by observing the results with an optical microscope, and finally the isotropic silicon etching with SF6 gas is used to remove the BOX layer.

[0154] Step 5: The chip is then removed from the silicon carrier wafer and cleaned in hot acetone and isopropyl alcohol, leaving Figure 6 (b) Cleaned SiLi-Con nitride / silicon bilayer device shown. Notably, the 400nm high-stress silicon nitride and 321nm silicon bilayers exhibited remarkable mechanical resilience, resulting in remarkable membrane survival when separated from the silicon carrier wafer. In addition, the use of a thick and soft thermal composite adhesive between the chip and the silicon carrier wafer, as well as the pre-application of a spin-coated S1813 resist throughout the etching process, ensured device yield.

[0155] The high-efficiency broadband all-dielectric reflector based on the metasurface prepared by the above steps is a double-layer film structure (such as Figure 7As shown in FIG. 1 , the double-layer membrane structure is a double-layer dielectric structure formed by periodically extending a number of light sail units. The light sail unit includes a silicon nitride layer 1 and a silicon layer 2. The thickness of the silicon nitride layer 1 is 400 nm, and a number of circular holes 101 distributed in an array are arranged on the silicon nitride 1. The radius of the circular hole 101 is 500 nm. The thickness of the silicon layer 2 is 321 nm, and a circular opening 201 is arranged on the silicon layer 2. The circular opening 201 is located at the center of the light sail unit, and its radius is 1.5 mm.

[0156] To show more details, Figure 6 A typical optical image of a 1×1 cm chip with a 4×4 mm photonic crystal area (rainbow colors) is shown in (b). Figure 6 (c) shows the back side of the chip with a circular opening. As mentioned above, after the deep silicon etching is completed, the Figure 6 Optical microscope images shown in (e) and (f) confirm that the photonic crystal structure on the front side of the chip can be observed from the edge of the opening, and then immediately switched to isotropic silicon etching with SF6 gas to remove the last few microns of the BOX layer. Scanning electron microscope (SEM) images confirm that it has a perfect periodic photonic crystal structure, such as Figure 6 (f) is shown. It is worth noting that Figure 6 In (g), photonic crystal fabrication at near 4-inch wafer scale is successfully demonstrated. The rainbow-colored region in the middle of the wafer is an electron-beam patterned and plasma-etched photonic crystal, the rainbow color originating from sunlight scattered when the wafer was placed in sunlight. In order to pattern such a large-area device with a large number of holes using electron-beam lithography, a large amount of pattern data was generated using the TeXLib program provided by Raith. By using a relatively large beam spot size of about 100nm, the total electron-beam write time for a 4-inch wafer sample is about 5-7 hours, which is typically achieved during an overnight electron-beam exposure. It is worth mentioning that deep silicon was skipped for this 4-inch sample because dry etching is very challenging for such a large device due to practical limitations of the plasma etcher, such as etching uniformity. This challenge can be overcome by adopting wet etching to obtain large-scale light sail devices.

[0157] Step 6: Optical Measurements

[0158] use Figure 8(a) The setup shown analyzes the fabricated sample. A tunable laser emitting light in the wavelength range of 1280-1600 nm is used as the light source. After the laser, a polarization controller (PC) is placed. By adjusting the PC, the light power detected by the photodetector 1 (PD1) after the polarization beam splitter (PBS) can be minimized, ensuring that the input light is completely linearly polarized (p-polarized). Subsequently, the transmitted light passes through a quarter-wavelength wave plate, which converts its polarization from linear to circular polarization before reaching the light sail sample. The light that passes through the light sail is captured by PD2 and recorded as P trans On the contrary, the light reflected by the light sail undergoes a 180-degree polarization change and passes through the quarter-wavelength wave plate again to become s-polarized; the PBS finally guides the s-polarized light to PD3 to obtain P refl reflection.

[0159] The power detected at PD1 is negligible (<0.5%) compared to the initial input power after the laser pin. trans / P in and P refl / P in The ratio calculates the transmission and reflection of the light sail; due to inherent losses in the optical setup caused by factors such as laser beam misalignment or the influence of optical components, a small fraction of the total input laser power is lost; therefore, P refl +P trans Slightly lower than P in ; To quantify these losses, the system was calibrated using commercial metal mirrors, which, as confirmed in previous studies, have near-perfect reflection over a wide range of wavelengths.

[0160] like Figure 8 As shown by the light pink dots in (b), a silicon nitride photonic crystal coupled to a silicon bilayer structure (denoted as SiNPhC+Si) achieves broadband reflection (>70%, as shown by the black dashed line) from 1300 to nearly 1550 nm. The peak reflection exceeds 90% at 1330 nm with a bandwidth of 200 nm, and the corresponding transmission is represented by the light blue dots. Taking into account the above-mentioned system losses, the calibrated reflection (depicted by the purple dots) shows similar broad performance, reaching a peak reflection of more than 96% at 1330 nm. The red and blue curves represent the simulated reflection and transmission of the light sail, respectively. The differences between the simulation and measurement may arise from manufacturing deviations or imperfections in the measurement setup. In contrast, a typical single-layer silicon nitride photonic crystal called "sinusoidal PhC" only achieves a reflection of >70% between 1525 nm and 1575 nm with a bandwidth of 50 nm. This bandwidth is significantly narrower than the double-layer light sail design, highlighting the excellent performance of the all-dielectric mirror designed in this invention.

[0161] Result analysis: According to theoretical calculations, numerical simulations and physical test results, the all-dielectric reflector designed in the present invention can achieve efficient reflection with a reflectivity of ≥70% in the expected broadband frequency band of 1300nm to 1550nm, thus providing a feasible basis for generating radiation pressure.

[0162] At the same time, it also confirms the effectiveness of the above-mentioned deep learning-based all-dielectric mirror design method.

[0163] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0164] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A design method for a high-efficiency broadband all-dielectric reflector based on a metasurface, characterized by: include Step 1: Analyze the power source of the high reflector according to the mechanical and electromagnetic theories, and establish the power model of the reflector; Step 2: Based on the dynamic model of the reflector, establish the frequency and wavelength model of the reflector; Step 3: Design a metasurface broadband dielectric reflector based on the multilayer dielectric equivalent wave impedance theory and determine the reflector structure; Step 4: Design the double-layer membrane structure parameters of the reflector based on the deep learning method.

2. The design method of a high-efficiency broadband all-dielectric reflector based on a metasurface as claimed in claim 1, characterized in that: The process of establishing the dynamic model of the high reflector in step 1 includes: Step 1.1: Calculate the radiation pressure of the high reflector; Step 1.2: Calculate the radiation pressure from an ideal perfect mirror Among them, I f-in I is the incident wave radiation pressure per unit area on the surface of the object, f-re is the reflected wave radiation pressure, S is the energy flux density, and c is the speed of light in vacuum; Step 1.3: Calculate the radiation pressure of a photon. A photon always moves at the speed of light and its energy is: Among them, h is Planck's constant, ν is the vibration frequency of the photon, λ is the corresponding wavelength, and c is the speed of light in vacuum.

3. The design method of a high-efficiency broadband all-dielectric reflector based on a metasurface as claimed in claim 2, characterized in that: The process of calculating the radiation pressure of a highly reflective mirror as described in step 1.1 includes The energy flux density of electromagnetic waves is calculated by the following formula: Among them, ΔE is the total energy or energy flow of electromagnetic waves, ΔA is the effective action area, ΔF is the radiation pressure of electromagnetic waves acting on the surface of the object, and ΔX is the effective work distance of the pressure; Considering the force per unit time, X = Δt × c = 1 × c = c Therefore, the calculation formula of the energy flux density of electromagnetic waves is simplified to Among them, I f It is the radiation pressure of the mirror sail per unit area of ​​the object's surface.

4. The design method of a high-efficiency broadband all-dielectric reflector based on a metasurface as claimed in claim 1, characterized in that: The process of establishing the frequency and wavelength model of the reflector in step 2 includes: Step 2.1: Establish the frequency model of the wavefront reaching the receiver in the working frequency band of the high reflector Among them, f r,s is the frequency at which the wavefront arrives at the receiver; Where β = vc, f s is the frequency, v is the receiver speed, and c is the speed of light in vacuum; Step 2.2: Build a wavelength model for the wavefront reaching the receiver in the high reflector operating band Among them, λ r is the wavelength of the wave received by the receiver, λ s is the wavelength of the wave emitted by the source.

5. A high-efficiency broadband all-dielectric reflector based on a metasurface designed by the method according to any one of claims 1 to 4, characterized in that: The all-dielectric reflector is a double-layer membrane structure formed by periodic extension of several light sail units. The light sail unit includes a silicon nitride layer and a silicon layer. The silicon nitride layer is provided with a number of circular holes, which are distributed in an array on the silicon nitride layer, and a circular opening is provided on the silicon layer.

6. The high-efficiency broadband all-dielectric reflector based on a metasurface as claimed in claim 5, characterized in that: The single thickness of the silicon nitride layer is 400 nm, and the opening radius of the circular hole is 500 nm; the thickness of the silicon layer is 321 nm.

7. The high-efficiency broadband all-dielectric reflector based on a metasurface as claimed in claim 5, characterized in that: The all-dielectric reflector achieves broadband reflection higher than 70% in the range of 1300nm to 1550nm, and the peak reflectivity exceeds 90% at 1330nm.

8. The method for preparing a high-efficiency broadband all-dielectric reflector based on a metasurface according to any one of claims 5 to 7, characterized in that: include Step 1: Use SOI wafer as the base layer; Step 2: Depositing a high stress silicon nitride layer on both sides of the SOI wafer; Step 3: Spin-coat the photoresist S1813 on the front surface of the chip as a protective layer, and spin-coat the photoresist AZ10XT on the back surface of the chip; Step 4: Expose the back side of the chip with AZ10XT resist and develop on the back side of the chip; Step 5: The chip is then removed from the silicon carrier wafer and cleaned in hot acetone and isopropyl alcohol, leaving behind a clean SiLi-Con nitride / silicon bilayer membrane device.

9. The method for preparing a high-efficiency broadband all-dielectric reflector based on a metasurface according to claim 8, characterized in that: The process of depositing a high stress silicon nitride layer on both sides of the SOI wafer in step 2 includes: Step 2.1: Deposit a 400 nm high stress silicon nitride layer on both sides of the SOI wafer by low pressure chemical vapor deposition, and determine the thickness of the grown silicon nitride layer by ellipsometry; Step 2.2: After the LPCVD process, the SOI wafer was cut into chips of equal volume; individual chips were selected and their front sides were spin-coated with an electron beam AR-P 6200, and then the front side of the samples was subjected to electron beam lithography using a Raith EBPG 5200 at an accelerating voltage of 100 keV; Step 2.3: After etching the silicon nitride plasma using a CH3F / O2 mixed gas, the photonic crystal circular hole pattern is transferred to the silicon nitride layer on the front side of the sample.

10. The method for preparing a high-efficiency broadband all-dielectric reflector based on a metasurface according to claim 8, characterized in that: The process of exposing the back side of the chip with AZ10XT resist and developing on the back side of the chip in step 4 includes: Step 4.1: Expose the back side of the chip with AZ10XT resist using a Heidelberg Instruments Laserwriter and develop to form a circular opening with a diameter of approximately 3 mm; Step 4.2: Afterwards, the same CH3F / O2 plasma etch is used to first remove 400nm of silicon nitride on the back of the chip, and then a deep reactive ion silicon etch is used to remove 500μm of silicon dioxide; Step 4.3: When the silicon etching is completed, the color contrast between silicon and silicon dioxide is confirmed by optical microscopy observation, and finally the BOX layer is removed using isotropic silicon etching with SF6 gas.

Images