High-efficiency broadband all-dielectric reflector based on metasurface

Abstract

The utility model relates to the technical field of photodynamics, and discloses a metasurface-based efficient broadband all-dielectric reflector, the all-dielectric reflector is 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 circular holes, and the silicon layer is provided with a plurality of through holes. 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 reflector is of a double-layer film structure and is formed by stacking the silicon nitride photonic crystals and the ultrathin silicon films, broadband reflection higher than 70% can be achieved within the range of 1,300 nm to 1,550 nm, the peak reflectivity at the position of 1,330 nm exceeds 90%, the structural reliability is high, and the application prospect is large; 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] This invention belongs to the field of photodynamics, specifically relating to a high-efficiency broadband all-dielectric reflector based on a metasurface. Background Technology

[0002] The Sun's gravitational lensing has an optimal focal distance of over 548 astronomical units (AU) from Earth; therefore, to fully utilize its extraordinary imaging capabilities, a solar sail is indispensable. At such a great distance, a solar sail is needed for precise positioning.

[0003] Because there is no air resistance in space, even the extremely small thrust generated by a light sail can accumulate almost without loss. Without an external power source or the chemical energy of fuel, this accumulated thrust can provide considerable propulsion for small spacecraft. Therefore, light sails are considered a propulsion technology with enormous future potential. However, traditional light sails, due to their poor stability, cannot effectively capture high-resolution images of exoplanets at high speeds.

[0004] Therefore, this invention designs a high-efficiency broadband all-dielectric reflector based on metasurface, which can achieve ultra-high speed under the impetus of a laser beam and can effectively solve the problems existing in the prior art. Utility Model Content

[0005] (a) Technical problems to be solved

[0006] To address the shortcomings of existing technologies, this invention provides a high-efficiency broadband all-dielectric reflector based on metasurfaces. The high-efficiency broadband all-dielectric reflector optical sail based on metasurfaces can achieve a broadband reflectivity of over 70% in the 1300nm to nearly 1550nm wavelength band, and its bandwidth can cover the Doppler spectrum when accelerated. It has the advantages of high reflectivity, broadband response, large receiving area, and light weight, solving the problem of poor stability of traditional optical sails and their inability to effectively capture high-resolution images of exoplanets at high speeds.

[0007] (II) Technical Solution

[0008] To achieve the aforementioned goal of achieving a broadband reflectivity of over 70% in the 1300nm to near 1550nm band, and having a bandwidth covering the Doppler spectrum during acceleration, this invention provides the following technical solution:

[0009] A high-efficiency broadband all-dielectric reflector based on metasurfaces is disclosed. The all-dielectric reflector is a double-layer film structure formed by periodically extending several optical sail units. Each optical sail unit includes a silicon nitride layer and a silicon layer. Several circular holes are provided on the silicon nitride layer, and the circular holes are arranged in an array on the silicon nitride layer. Circular openings are provided on the silicon layer.

[0010] In some implementations, the silicon nitride thickness is 400 nm and the radius of the circular aperture is 500 nm.

[0011] In some implementations, the silicon layer is 321 nm thick.

[0012] In some implementations, the circular opening is located at the center of the solar sail unit, and the diameter of the circular opening is 1.5 mm.

[0013] (III) Beneficial Effects

[0014] Compared with the prior art, this utility model provides a high-efficiency broadband all-dielectric reflector based on metasurface, which has the following beneficial effects:

[0015] This invention designs a high-efficiency broadband all-dielectric reflector based on metasurfaces. The reflector has a double-layer dielectric structure and can achieve broadband reflectivity of over 70% in the range of 1300nm to 1550nm, with a peak reflectivity of over 90% at 1330nm. It has strong structural reliability and great application prospects. It achieves the goal of achieving a broadband reflectivity of over 70% in the 1300nm to near 1550nm band and covering the Doppler spectrum when accelerated. Attached Figure Description

[0016] Figure 1 The diagram shows the simulation and analysis of the equivalent wave impedance of the multilayer dielectric in the SiN photonic crystal and Si bilayer film structure of this utility model.

[0017] Figure 2 This is a diagram of the photonic crystal structure of this invention;

[0018] Figure 3 This is a graph showing the shift in the position of the optical sail reflection peak of this utility model.

[0019] Figure 4 Enlarged views of the all-dielectric mirror (optical sail unit); (a) front view; (b) back view;

[0020] Figure 5 This is a flowchart illustrating the process of fabricating a silicon nitride photonic crystal and a silicon bilayer film from an SOI wafer according to this invention.

[0021] Among them: Figure 1 (a) is an equivalent model of a single-layer photonic crystal structure. Figure 1 (b) shows the equivalent impedance curve of the equivalent model of a single-layer photonic crystal structure; Figure 1 (c) is an equivalent model of a two-layer superphotonic crystal structure. Figure 1 (d) is the equivalent impedance curve of the equivalent model of the double-layer superphotonic crystal structure;

[0022] exist Figure 2 (a) is a top view of the photonic crystal structure. Figure 2 (b) is a side view of the photonic crystal structure; Figure 2 Image (c) is a top view of the superphotonic crystal. Figure 2 Image d is a side view of the superphotonic crystal; Figure 2 Image (e) is a top view of the double-layer dielectric structure (without holes). Figure 2 (f) is a side view of the double-layer dielectric structure (without holes); Figure 2 (g) shows a comparison of the reflection coefficient curves for various structures. Figure 2 The middle (h) section compares the power curves of various structures; Figure 2 In the middle (i), the observed wavelength changes with the speed of the optical sail. Figure 2 (j) represents the curve of reflectivity as a function of wavelength when the structural parameter is adjusted.

[0023] exist Figure 3 (a) shows the Doppler frequency shift effect of the moving optical sail from low speed to 20% of the speed of light. Figure 3 (b) is a parameter scan curve of the photonic crystal period value from 1.15 to 1.3 μm to show the shift of the position of the optical sail reflection peak;

[0024] exist Figure 4 In the middle, 1-silicon nitride layer; 101-circular hole; 2-silicon layer; 201-circular opening;

[0025] exist Figure 5 (a) is a flowchart of the fabrication process of an all-dielectric mirror; Figure 5 (b) shows a typical optical image of the front of the all-dielectric mirror (4×4mm); Figure 5 (c) shows the back of the all-dielectric mirror; Figure 5 The optical microscope images shown in (e) and (f) are shown in the middle. Figure 5 (g) is a 4-inch wafer-level all-dielectric reflector that was successfully fabricated. Detailed Implementation

[0026] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of the present utility model.

[0027] Example 1: This utility model provides a technical solution:

[0028] A high-efficiency broadband all-dielectric mirror based on metasurfaces is disclosed. The all-dielectric mirror is a double-layer dielectric structure formed by periodically extending several optical sail units. Each optical sail unit includes a silicon nitride layer 1 and a silicon layer 2. The double-layer dielectric structure is parameterized as follows: period p, silicon nitride 1 thickness h1, silicon 2 thickness h2, and upper layer aperture radius r.

[0029] Table 1: Optimal parameters of the bilayer membrane structure

[0030] p / nm <![CDATA[h1 / nm]]> <![CDATA[h2 / nm]]> r / nm Material Top membrane 1200 - 400 500 Silicon nitride (SiN) bottom membrane 1200 321 - - Silicon (Si)

[0031] As a preferred embodiment, the all-dielectric reflector can achieve broadband reflectivity of over 70% in the range of 1300nm to 1550nm; and has a peak reflectivity of over 90% at 1330nm.

[0032] Example 2: Please refer to Figure 1-4 The design and functional verification process of the high-efficiency broadband all-dielectric mirror based on metasurfaces, as described in Example 1, is shown in this example. The design method of the high-efficiency broadband all-dielectric mirror based on metasurfaces includes...

[0033] Step 1: Analyze the power source of the high-reflectivity mirror based on mechanical and electromagnetic theories, and establish a dynamic model of the mirror.

[0034] This embodiment designs a high-reflectivity mirror powered by radiation pressure, enabling it to reach ultra-high speeds driven by a laser beam. Radiation pressure (also known as light pressure) is the mechanical pressure exerted on any surface due to the exchange of momentum between an object and an electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength absorbed, reflected, or otherwise emitted (e.g., blackbody radiation) by matter of any scale (from macroscopic objects to dust particles to gas molecules). The force generated in this process is called radiation pressure, or simply light force. The pressure generated by radiation is usually too small to be noticeable in everyday situations. However, it is important in certain physical processes and technologies, especially for objects in outer space, where, in addition to gravity, radiation pressure is often the dominant force acting on objects, and small forces can have a large cumulative effect over long periods. For example, if the effect of solar radiation pressure on a spacecraft were ignored, the spacecraft would deviate from its fixed orbit. The radiation pressure of starlight is also crucial in many astrophysical processes. Therefore, light sails can operate in space based on this principle, 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 exchange is involved. 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 interacting matter (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 provide a general explanation for light sails.

[0035] Step 1.1: Calculate the radiation pressure of the reflector using electromagnetic theory and the momentum of electromagnetic waves.

[0036] The energy flux density of electromagnetic waves is calculated using the Poynting vector as follows:

[0037]

[0038] The energy flux density can be calculated using the following formula:

[0039]

[0040] Where ΔE is the total energy or the energy flux of the electromagnetic wave, ΔA is the effective area, ΔF is the radiation pressure of the electromagnetic wave acting on the surface of the object, and ΔX is the effective work distance of the pressure.

[0041] Because we are considering the forces acting per unit time, therefore

[0042] X=Δt×c=1×c=c (3)

[0043] Therefore, formula (2) can be simplified to

[0044]

[0045] Among them, I f The radiation pressure per unit area of ​​the object's surface;

[0046] Therefore, when electromagnetic waves are incident perpendicularly on the surface of an object, the relationship between the radiation pressure per unit area and the energy flux density is:

[0047]

[0048] Note that the momentum direction of electromagnetic waves is consistent with the energy flux direction, where S is the energy flux density;

[0049] Step 1.2: Next, we calculate the radiation pressure of a perfect reflector under ideal conditions.

[0050] When an electromagnetic wave is incident perpendicularly on a perfect reflector, the incident wave generates radiation pressure. The reflected wave further affects this radiation pressure. Because the mirror perfectly reflects the incident wave, the radiation pressure generated by the reflected wave is equal to that generated by the incident wave.

[0051]

[0052] Therefore, the total radiation pressure on the surface of the object is

[0053]

[0054] For imperfect reflectors, the second term must be multiplied by the reflectivity, thus reducing the total radiation pressure. For optical sail designs, we should maximize the reflectivity to increase surface radiation pressure and thereby improve the radiation efficiency of the laser source.

[0055] Step 1.3: Next, we calculate the radiation pressure of photons. Here, electromagnetic radiation is viewed from the perspective of particles, since photons always travel at the speed of light. According to quantum mechanics, their energy is given by the following formula:

[0056]

[0057] Where h is Planck's constant, ν is the vibration frequency of the photon, and λ is the corresponding wavelength;

[0058] When a photon is incident perpendicularly 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 the reflected photon is similar.

[0059] Step 2: Based on the relativistic analysis of the wide-band operating requirements of the high-reflectivity mirror, establish its frequency and wavelength model.

[0060] The Doppler shift refers to the change in frequency observed by an observer relative to the frequency of the wave source as the observer moves relative to the source. A common example of the Doppler shift is the change in pitch heard when a vehicle horn approaches and moves away from an observer. The Doppler effect occurs because as the wave source moves towards the observer, each successive wave crest is emitted closer to the observer than the previous wave crest. Therefore, each wave takes less time to reach the observer than the previous wave. This reduced time between successive wave crests leads to an increase in frequency. As the wave propagates, the distance between the wave crests decreases, so the wave "clusters together." Conversely, if the wave source moves away from the observer, each wave crest is emitted further away from the observer than the previous wave crest, thus increasing the time between wave crests and lowering the frequency. The increased distance between the wave crests then causes the wave to "spread out."

[0061] For waves propagating in a medium, the velocities of the observer and source are relative to the medium in which the wave propagates. Therefore, the total Doppler shift can be generated 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 arises. Therefore, in this project, we need to further investigate the relativistic Doppler effect caused by the velocity generated by the optical sail, where the observer is the optical sail and the wave source is the radiating laser.

[0062] The longitudinal relativistic Doppler shift, i.e., the movement of the source and receiver directly toward or away from each other, can usually be derived like a classical phenomenon, but requires modification by adding a time dilation term.

[0063] The following derives the longitudinal relativistic Doppler effect, assuming that the receiver and the source are moving away from each other at a relative velocity v, which is measured by an observer on the receiver or the source (the notation used here is that v is negative if the receiver and the source are moving toward each other), with the source as the frame of reference.

[0064] Step 2.1: Establish the frequency model of the wavefront arriving at the receiver in the high-reflectivity mirror's operating band.

[0065] (1) Assume that a wavefront of the source arrives at the receiver, and the next wavefront is located at a distance λ from the receiver. s At, λ s =c / f s , where λ s f is the wavelength of the emitted wave from the source. s Let c be the corresponding frequency, c be the speed of light in a vacuum, c be the velocity of the wavefront, and t be the time interval between the arrival of the next wavefront at the receiver. r,s During the same time period, the receiver moves away from the source at a velocity v, and the following can be observed at the source location:

[0066] λ s +vt r,s =ct r,s (9)

[0067] Transforming equation (9), we can obtain

[0068]

[0069] Where β = v / c, the corresponding frequency at which the wavefront arrives at the receiver is...

[0070]

[0071] So far, the equation is the same as the equation for the classical Doppler effect;

[0072] Step 2.2: Establish a wavelength model for the wavefront of the high-reflectivity mirror reaching the receiver.

[0073] Due to relativistic effects, the clock on the receiver has a time dilation relative to the clock on the source, t r =t r,s / γ, where γ is the Lorentz factor. Therefore, the frequency received by the receiver is

[0074]

[0075] The above is the relativistic Doppler frequency shift equation; written in wavelength form as follows:

[0076]

[0077] Where, λ r The wavelength of the wave received by the receiver.

[0078] An optical sail moves at near the speed of light, and a light source emits laser light at a fixed wavelength. As the speed of the sail gradually increases, the wavelength observed at the sail gradually increases, meaning the relativistic Doppler effect becomes more pronounced. To accelerate the sail to even higher speeds, it is essential to ensure that the sail has a high reflectivity response to the single-frequency light emitted by the laser when stationary. Secondly, as the sail accelerates, its corresponding observation wavelength changes; therefore, it is even more necessary to achieve high reflectivity over a wide wavelength range.

[0079] Step 3: Design a metasurface broadband dielectric mirror based on the multilayer dielectric equivalent wave impedance theory and determine the mirror structure.

[0080] To theoretically explain why the SiN(PhC)-Si bilayer structure exhibits a wider reflection bandwidth than a single-layer SiN photonic crystal, this step analyzes the problem based on the equivalent wave impedance theory of multilayer dielectrics. The results are as follows:

[0081] Step 3.1: As Figure 1 As shown in Figure (a), there are three different lossless media with two parallel interfaces. The normalized impedances of media 1, 2, and 3 are Z1, Z2, and Z3, respectively, and the thickness of media 2 is d. When an electromagnetic wave is incident perpendicularly from media 1 along the positive z-axis, reflection and transmission occur at the interfaces z = 0 and z = d. Therefore, in media 1 and media 2, there are incident waves propagating in the +z direction and reflected waves propagating in the -z direction, while in media 3, there are only transmitted waves propagating in the +z direction.

[0082] According to the equivalent wave impedance theory of multilayer media, the reflection coefficient at the interface z=0 is:

[0083]

[0084] Among them, Z ef23 Let be the equivalent wave impedance of medium 2 and medium 3 at z = 0, and

[0085]

[0086] Where j is the imaginary part, Let be the propagation constant of electromagnetic waves in medium 2;

[0087] Step 3.2: Returning to our classic single-layer photonic crystal model, the photonic crystal is medium 2, and its forward (medium 1) and backward (medium 3) sides are both air layers. Therefore, Z1 = Z3 = Z0; according to Figure 2 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, which, due to its two-layer medium, can be equivalent to a four-layer medium model, as follows: Figure 1 As shown in (c); the matching layer (gray) Si is medium 2, and the silicon nitride layer is medium 3, with normalized impedances of respectively. and Z 23 Its forward (medium 1) and backward (medium 4) directions are both air layers; therefore, Z 21 =Z 24 =Z0; Equivalent wave impedance Z of media 3 and 4 at z = d1 ef2 =Z ef23 ,Right now Figure 1 The result in (b) is used; then, according to formula (15), the equivalent wave impedance of media 2, 3, and 4 at z = 0 can be calculated as follows:

[0088]

[0089] Finally, the reflection coefficient is calculated according to formula (14).

[0090]

[0091] Therefore, by adding a layer of Si and rationally designing its thickness and the parameters of the photonic crystal, efficient and broadband reflection can be ultimately achieved. This step uses three different lossless media to derive the reflectivity expression. The two interfaces are parallel to each other, and the normalized impedances of media 1, 2, and 3 are Z1, Z2, and Z3, respectively. The thickness of media 2 is d. When an electromagnetic wave is incident perpendicularly along the positive z-axis from media 1, reflection and transmission occur at the interfaces z = 0 and z = d. Therefore, in media 1 and media 2, there are incident waves propagating in the +z direction and reflected waves propagating in the -z direction, while in media 3, there are only transmitted waves propagating in the +z direction. According to the equivalent wave impedance theory of multilayer media, the double-layer film structure has more efficient and broadband reflection performance than the classical single-layer photonic crystal model.

[0092] Step 4: Design the parameters of the bilayer membrane structure based on deep learning methods to achieve broadband high reflectivity.

[0093] Based on the derivation in step 3, a double-layer film structure was selected for the optimized design of the high-reflectivity mirror. The double-layer film structure was parameterized as follows: period p, upper silicon nitride thickness h1, lower silicon thickness h2, and upper aperture radius r. A large number of initial samples were generated using the Monte Carlo method and numerical simulations were performed in the commercial electromagnetic field simulation software CST STUDIO SUITE. The correspondence between reflectivity and each parameter was recorded to form a basic data sample set. A multilayer error backtracking neural network was used to optimize the optimal structural parameters. The data in the basic data sample set was used for neural network training. A fast optimization program was written using Matlab software. Finally, broadband reflectivity was used as the objective function, and the double-layer film structure was designed through the neural network.

[0094] As a preferred embodiment, the specific process of generating a large number of initial samples using the Monte Carlo method and performing numerical simulations in the commercial electromagnetic field simulation software CST STUDIO SUITE, recording the correspondence between reflectivity and various parameters, and forming a basic data sample set includes:

[0095] Step 4.1: First, the refractive indices of silicon (Si) and high-stress silicon nitride (SiN) were accurately measured using an ellipsometer, and the values ​​obtained were Si = 3.4 and SiN = 2.0, respectively.

[0096] from Figure 2 Starting with the standard monolayer photonic crystal periodic structure shown in (a) and (b), SiN is used as the dielectric material with parameters p1 = 1200 nm, h1 = 400 nm, and r = 500 nm.

[0097] Step 4.2: Using the Finite-Difference Time-Domain (FDTD) algorithm, simulate the electromagnetic wave incident along the -z direction (electric field along the x direction) under periodic boundary conditions; for example... Figure 2 The simulation results shown by the green curve in (g) indicate that, in the wavelength spectrum covering 1334 nm to 1385 nm, |S 11 | indicates a reflectance greater than 90%; this result can be converted to high reflectance, such as Figure 2 The green curve shown in (h), |S 11 | 2 Over 80%, with a bandwidth of 51nm;

[0098] Step 4.3: However, the above analysis reveals an inherent limitation of photonic crystals: their reflection bandwidth is finite due to their resonant properties. To further improve the device bandwidth, a matching layer is introduced beneath the SiN photonic crystal layer, resulting in a new structure called a superphotonic crystal, such as... Figure 2As shown in (c) and (d), the matching layer is made of silicon (red portion) with parameters p2, h2, and r2 consistent with the initial structure, where h2 = 321 nm, determined based on the specific properties of the SOI wafer used in this work. Subsequent simulations using the same method yielded significant results. Specifically, |S 11 The reflectivity |S| exceeds 90% in the wavelength range from 1308 nm to 1449 nm, contributing to a reflectivity of |S|. 11 | 2 Over 80%, with extended bandwidth reaching 141nm ( Figure 2 (The red curves in (g) and (h)). Compared to the original structure, its bandwidth can be increased by nearly 3 times, and it is expected to be applied to fields such as optical sails with significant Doppler frequency shift characteristics.

[0099] Step 4.4: As an intermediate step, the circular holes on the SiN layer are eliminated while retaining the underlying Si layer, forming a double-layer dielectric structure. This simplified structure retains parameters such as layer thickness and also offers advantages in nanofabrication processes. The final design uses... Figure 2 The structures shown in (c) and (d) have dimensions as shown in Table 1 below.

[0100] Table 1: Optimal parameters of the bilayer membrane structure

[0101] p / nm <![CDATA[h2 / nm]]> <![CDATA[h1 / nm]]> <![CDATA[r2 / nm]]> Material Top membrane 1200 - 400 500 Silicon nitride (SiN) bottom membrane 1200 321 - - Silicon (Si)

[0102] The specific design effect is as follows:

[0103] Simulation results show that, Figure 2 As shown in (g), the reflectivity consistently exceeds 80% in the wavelength range of 1321 nm to 1553 nm; correspondingly, the reflectivity exceeds 70% in the wavelength range of 1300 nm to 1550 nm. Figure 2 (Blue curves in (g) and (h)). Although this double-layer dielectric structure has a wide bandwidth, it has a lower peak reflection and a larger mass compared to the other two designs, so it was decided to omit this particular design during manufacturing.

[0104] The above design ultimately achieves a wideband reflector that seamlessly covers the entire wavelength range of the Doppler-shifted laser during the acceleration phase of the optical sail. To provide specific context, imagine a laser with an initial wavelength of 1300 nm whose task is to accelerate the optical sail to approximately 20% of the speed of light. Under these conditions, such as Figure 3 As shown in (a), the laser wavelength undergoes a significant shift, transitioning from 1300 nm to approximately 1550 nm. It is worth noting that the peak reflection position of the optical sail can be easily adjusted by changing the parameters of the photonic crystal layer (such as the period). Figure 3As shown in (b), by increasing the period from 1.15 to 1.3 μm, the peak of reflection changes from below 1300 nm to approximately 1420 nm. This provides a high degree of flexibility for designing light sails or other optical elements with different optical responses, such as superlenses, on-chip integrated photonics, or free-space optics.

[0105] The high-efficiency broadband all-dielectric mirror based on metasurfaces designed above is a double-layer film structure (e.g. Figure 4 As shown, the double-layer film structure is a double-layer dielectric structure formed by periodically extending several optical sail units. The optical sail unit includes a silicon nitride layer 1 and a silicon layer 2. The silicon nitride layer 1 has a thickness of 400 nm and has several arrayed circular holes 101 with a radius of 500 nm. The silicon layer 2 has a thickness of 321 nm and has a circular opening 201 located at the center of the optical sail unit with a radius of 1.5 mm.

[0106] Example 3

[0107] This embodiment provides a fabrication process for a high-efficiency broadband all-dielectric mirror based on metasurfaces, as follows:

[0108] Step 1: For double-layer structure devices, such as Figure 5 As shown in (a), we used a commercial SOI wafer (one side is a silicon dioxide layer and the other side is a silicon layer) and processed it to have a silicon layer with a thickness of 321 nm on the SiO2 buried oxide (BOX) layer and a silicon dioxide thickness of about 500 μm on the other side.

[0109] Step 2: Next, a 400 nm high-stress silicon nitride layer was deposited on both sides of the SOI wafer using 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 diced into 1 × 1 cm chips. Individual chips were selected, and their front sides (silicon nitride / silicon bilayer) were spin-coated using an electron beam AR-P 6200 (CSAR 62). Then, electron beam lithography was performed on the front side of the samples using a Raith EBPG 5200 at an accelerating voltage of 100 keV. After silicon nitride plasma etching using a CH3F / O2 mixed gas, the photonic crystal circular aperture pattern was transferred to the silicon nitride layer on the front side of the samples.

[0110] Step 3: Next, spin-coat the front side of the chip with photoresist S1813 from Microresist as a protective layer, and spin-coat the back side of the chip with photoresist AZ10XT from MicroChemicals for the next step of processing.

[0111] Step 4: The back side of the chip with AZ10XT resist was then exposed using a Heidelberg Instruments Laserwriter (μMLA) and developed to form an annular opening approximately 3 mm in diameter. A circular opening was used here because it resulted in a higher success rate for the final bilayer film. Next, the same CH3F / O2 plasma etching was used to first remove the 400 nm silicon nitride on the back side of the chip, followed by deep reactive ion silicon etching (Bosch process) to remove the 500 μm silicon dioxide. When the silicon dioxide etching was complete, the color contrast between silicon and silicon dioxide was confirmed by observation under an optical microscope. Finally, isotropic silicon etching with SF6 gas was used to remove the BOX layer.

[0112] Step 5: Then the chip is removed from the silicon carrier wafer and cleaned in hot acetone and isopropanol, leaving as follows Figure 5 The clean SiLi-Con nitride / silicon bilayer device is shown in (b). Notably, the 400 nm high-stress silicon nitride and 321 nm silicon bilayer exhibit significant mechanoelasticity, resulting in a high film survival rate upon separation from the silicon carrier wafer. Furthermore, the use of a thick, soft thermally bonded adhesive between the chip and the silicon carrier wafer throughout the etching process, along with the pre-application of spin-coated S1813 resist, ensured high device yield.

[0113] To show more details, Figure 5 Figure (b) shows a typical optical image of a 1×1cm chip with a 4×4mm photonic crystal area (iridescent). Similarly, Figure 5 Image (c) shows the back of the chip with a circular opening. As previously mentioned, once deep silicon etching is complete, it can be accessed via... Figure 5 The optical microscopy 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. This was immediately followed by isotropic silicon etching with SF6 gas to remove the final few micrometers of the BOX layer. Scanning electron microscopy (SEM) images confirm its perfectly periodic photonic crystal structure, as shown in (e) and (f). Figure 5 As shown in (f). It is worth noting that in Figure 5In section (g), near-4-inch wafer-scale photonic crystal fabrication was successfully demonstrated. The iridescent region in the center of the wafer is the photonic crystal formed by electron beam patterning and plasma etching; the iridescence originates from sunlight scattered when the wafer is placed in sunlight. To pattern such a large-area device with numerous 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 approximately 100 nm, the total electron beam writing time for the 4-inch wafer sample was approximately 5–7 hours, an effect typically achieved during overnight electron beam exposure. Notably, this 4-inch sample skipped deep silicon because dry etching is extremely challenging for such a large device due to practical limitations of plasma etching machines, such as etching uniformity. This challenge can be overcome by employing wet etching to obtain large-scale photonic sail devices.

[0114] A silicon nitride photonic crystal coupled to a silicon bilayer structure (denoted as SiN PhC+Si) achieves broadband reflection (>70%, as shown by the black dashed line) from 1300 to nearly 1550 nm. Peak reflection exceeds 90% at 1330 nm with a bandwidth of 200 nm, and the corresponding transmission is represented by light blue dots. Considering the aforementioned system losses, calibrated reflection (described by purple dots) exhibits similarly broad performance, reaching a peak reflection of over 96% at 1330 nm. The red and blue curves represent simulated reflection and transmission of the sail, respectively. The difference between simulation and measurement may stem from manufacturing deviations or defects in the measurement setup. In contrast, a typical monolayer silicon nitride photonic crystal, termed "Sinusoidal PhC," achieves >70% reflection only between 1525 nm and 1575 nm with a bandwidth of 50 nm. This bandwidth is significantly narrower than the bilayer sail design, highlighting the superior performance of the all-dielectric reflector designed in this invention.

[0115] Results Analysis: Based on theoretical calculations, numerical simulations, and physical test results, the novel all-dielectric reflector designed in this paper can achieve high-efficiency 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.

[0116] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0117] Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the present invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A high-efficiency broadband all-dielectric reflector based on metasurface, characterized in that: The all-dielectric reflector is a double-layer film structure formed by periodically extending several optical sail units. Each optical sail unit includes a silicon nitride layer and a silicon layer. Several circular holes are provided on the silicon nitride layer, and the several circular holes are distributed in an array on the silicon nitride layer. Circular openings are provided on the silicon layer.

2. The high-efficiency broadband all-dielectric reflector based on metasurface according to claim 1, characterized in that: The silicon nitride is 400 nm thick and the radius of the circular hole is 500 nm.

3. The high-efficiency broadband all-dielectric reflector based on metasurface according to claim 2, characterized in that: The thickness of the silicon layer is 321 nm.

4. The high-efficiency broadband all-dielectric reflector based on metasurface according to claim 3, characterized in that: The circular opening is located at the center of the light sail unit, and the diameter of the circular opening is 1.5 mm.

Images