A design method of holographic metasurface for on-chip and free-space multiplexing

Abstract

The application discloses a design method of a holographic metasurface for on-chip and free-space multiplexing, comprising the following steps: designing a structure of the metasurface; calculating resonance phase response distribution and transmittance distribution of silicon nanoblocks in the metasurface under on-chip input light sources and free-space input light sources; designing different holographic patterns, inputting the holographic patterns into a GS algorithm to obtain phase distribution of the metasurface corresponding to the holographic images, and generating phase distribution of three metasurfaces; determining geometric sizes and relative positions of all silicon nanoblocks on the metasurface according to the resonance phase response distribution, the transmittance distribution and the phase distribution of the metasurface obtained in the step 3), determining a structure of the holographic metasurface for on-chip and free-space multiplexing, and completing the design. The application can solve the problem of limited application of on-chip metasurfaces, and provides a new holographic display multiplexing idea of metasurfaces, and three-channel holographic multiplexing at the same wavelength can be realized by combining the detour phase and the resonance phase principles.

Description

Technical Field

[0001] This invention relates to the fields of optical technology, integrated photonics technology, and optical communication information encryption, specifically to a design method for an on-chip and free-space multiplexed holographic metasurface. Background Technology

[0002] Most metasurfaces are driven by free-space light, utilizing the degrees of freedom of free-space light such as wavelength, polarization, and incident angle to achieve functions like beam deflection, focusing, and holography. This makes further integration on-chip difficult. Meanwhile, the ongoing trend towards miniaturization and universal photonic systems demands more complex device functions in a more compact, multifunctional, configurable, and CMOS-compatible manner, posing new challenges to traditional photonic integrated circuits. However, continuous advancements in nanofabrication technology provide technical support for on-chip metasurfaces. Combining metasurfaces with waveguides can inject new degrees of freedom into photonic integrated devices, enabling more complex functions.

[0003] Current research on on-chip metasurfaces mainly utilizes the detour phase modulation mechanism for on-chip holographic display, or combines the detour phase with the geometric phase (PB phase) to achieve on-chip reuse. However, these modulation mechanisms are relatively simple, and the geometric phase determines that the metasurface can only work under circularly polarized light, which to some extent limits the application of on-chip metasurfaces. Summary of the Invention

[0004] This invention provides a design method for on-chip and free-space multiplexed holographic metasurfaces. Based on the holographic display method of on-chip and free-space metasurfaces, it can solve the problem of limited application of on-chip metasurfaces and provide a new idea for metasurface holographic display multiplexing. By combining the principles of detour phase and resonant phase, it can realize three-channel holographic multiplexing under the same wavelength.

[0005] This invention provides a design method for on-chip and off-chip metasurfaces. The structural parameters of the nanoblock unit structure constituting the metasurface are used as the first phase influencing factor, and the relative distance between the nanoblock unit structures of the metasurface is used as the second phase influencing factor. The geometric structure and relative position of the nanoblock unit of the metasurface are arranged by combining the two phase influencing factors, so that the metasurface can generate different target holograms at a specified position in free space under waveguide incident light and free space incident light.

[0006] A design method for on-chip and free-space multiplexed holographic metasurfaces includes the following steps:

[0007] 1) Design the structure of the metasurface;

[0008] 2) Calculate the resonant phase response distribution and transmittance distribution of silicon nanobulks in the metasurface under on-chip input light source and free space input light source;

[0009] In step 2), the resonant phase response distribution and transmittance distribution of the silicon nanobulk in the metasurface under on-chip input light source and free-space input light source are calculated, specifically including:

[0010] Under the same operating wavelength of on-chip input light source and free space input light source, the resonant phase response spectrum, transmittance distribution of optical waveguide layer in metasurface, and resonant phase response spectrum and transmittance distribution of silicon nanoblocks with different geometric dimensions under on-chip input light source, as well as under free space input light source with different polarization, are calculated by the finite-difference time-domain method.

[0011] 3) Design different holographic patterns and input them into the GS (Gerchberg-Saxton) algorithm to obtain the phase distribution of the metasurface corresponding to the holographic image, and generate the phase distribution of the three metasurfaces;

[0012] 4) Determine the geometric dimensions and relative positions of all silicon nanoblocks on the metasurface based on the resonant phase response distribution and transmittance distribution obtained in step 2) and the phase distribution of the metasurface obtained in step 3).

[0013] 5) Based on the geometric dimensions of all silicon nanoblocks on the metasurface obtained in step 4.1) and the relative positions of all silicon nanoblocks on the metasurface obtained in step 4.4), determine the structure of the on-chip and free space multiplexed holographic metasurface to complete the design.

[0014] In step 1), the metasurface includes a silicon dioxide substrate, an optical waveguide layer disposed on the silicon dioxide substrate, and a single-atom amorphous silicon nanoblock structure array disposed on the optical waveguide layer. The single-atom amorphous silicon nanoblock structure array includes a plurality of silicon nanoblocks with the same height but different arrangement positions on the chip.

[0015] In step 1), the optical waveguide layer is a silicon nitride optical waveguide layer or a lithium niobate optical waveguide layer.

[0016] In step 2), the different polarizations are X linear polarization and Y linear polarization.

[0017] In step 4), the geometric dimensions and relative positions of all silicon nanoblocks on the metasurface are determined based on the resonant phase response distribution and transmittance distribution obtained in step 2) and the phase distribution of the metasurface obtained in step 3). Specifically, this includes:

[0018] 4.1) First, using the principle of resonance phase, two of the phase distributions of the three metasurfaces obtained in step 3) are compared with the resonance phase response spectra of the free space input light source with different polarizations obtained in step 2), and the transmittance of the silicon nanoblocks obtained in the end is greater than 80%, and the geometric dimensions of all silicon nanoblocks on the metasurface are determined.

[0019] 4.2) The geometric dimensions of all silicon nanoblocks on the metasurface obtained in step 4.1) correspond to the resonance phase response spectra of silicon nanoblocks with different geometric dimensions obtained in step 2) under on-chip input light source, thus obtaining the metasurface resonance phase distribution under on-chip input light source.

[0020] 4.3) Subtract the remaining one of the three metasurface phase distributions obtained in step 3) from the metasurface resonant phase distribution under the on-chip input light source obtained in step 4.2) to obtain the metasurface tortuous phase distribution under the on-chip input light source.

[0021] 4.4) Using the principle of detour phase, the relative positions of all silicon nanoblocks on the metasurface are calculated by inputting the detour phase distribution of the metasurface under the on-chip input light source obtained in step 4.3).

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] In this invention, the structural parameters of the nanoblock units constituting the metasurface are used as the first phase influencing factor, and the relative distance between the nanoblock units of the metasurface is used as the second phase influencing factor. The geometric structure and phase position of the nanoblock units of the metasurface are arranged and designed by combining the two phase influencing factors, so that the metasurface generates different target holographic phases at a specified position in free space under waveguide incident light and free space incident light, thereby realizing on-chip and off-chip three-channel holographic multiplexing.

[0024] Second, this invention innovatively proposes a method for designing metasurfaces by combining resonant phase and detour phase, which can achieve three-channel holographic multiplexing effect under waveguide and free space incident at the same wavelength.

[0025] Second, the single-atom nanoblock placed above the waveguide in this invention expands the degree of freedom of optical field manipulation. The designed period and unit geometry are both subwavelength scale, which can ensure high-resolution holographic imaging in the far field.

[0026] Third, in this invention, both the waveguide and the free space incident light are of the same wavelength, using fewer degrees of freedom to achieve multi-channel holographic multiplexing, and achieving almost no crosstalk effect, resulting in clear imaging. It is expected to be applied in fields such as optical information encryption.

[0027] Fourth, the process is simple, the equipment is miniaturized, and it is easy to integrate. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the metasurface structure provided in an embodiment of the present invention.

[0029] Figure 2 This is a schematic diagram of the single-atom nanoblock unit structure in the metasurface provided in the embodiments of the present invention.

[0030] Figure 3 These are the phase response spectrum and transmittance distribution extracted from the waveguide TEO mode light source incident when the nanobulk unit structure parameters change from 100nm to 400nm at a working wavelength of 1310nm, obtained through numerical simulation in this embodiment of the invention. Figure 3 (a); and the phase response spectrum and transmittance distribution extracted from x-polarized and y-polarized light incident in free space, respectively, such as Figure 3 (b) and (c).

[0031] Figure 4 These are three target holographic input images and target holographic phase distribution images obtained using the improved GS algorithm in this embodiment of the invention.

[0032] Figure 5 Based on the embodiments of the present invention Figure 3 and Figure 4 The flowchart shown illustrates the principle design for implementing three-channel holographic multiplexing.

[0033] Figure 6 This is a simulation diagram of the three-channel holographic result in the holographic display method based on on-chip and off-chip metasurfaces provided in the embodiments of the present invention.

[0034] Figure 7 This is a schematic diagram of the metasurface function provided in an embodiment of the present invention. Detailed Implementation

[0035] The implementation method, principle design, and technical effects of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] The design methodology for metasurfaces includes the following steps:

[0037] 1) Design of the metasurface structure: The metasurface comprises a silicon dioxide substrate, a silicon nitride optical waveguide layer on the substrate, and an array of single-atom amorphous silicon nanoblock structures on the optical waveguide layer. The nanoblock structure array includes several nanoblock unit structures with the same height but different arrangements on the chip. The nanoblock unit structures in the nanoblock structure array include designable subwavelength geometry of the nanoblocks and the relative positions of the nanoblocks within the subwavelength period calculated from the detour phase. The single-atom nanoblocks can extract guided waves from within the waveguide to free space.

[0038] 2) Calculate the phase response and transmittance distributions of the nanobulk unit structure under different input light sources. Using the finite-difference time-domain method (commercial Lumerical FDTD software), calculate the resonant phase response spectra and transmittance distributions of nanobulk unit structures with different geometric dimensions under waveguide-induced specific mode light sources at the same operating wavelength, as well as the resonant phase response spectra and corresponding transmittance distributions under free-space incident light sources of different polarizations.

[0039] 3) Design different holographic patterns and obtain the phase distribution of the target holographic image. Using the GS algorithm, three phase-only holograms are generated to reconstruct the far-field target image.

[0040] 4) Determine the geometric dimensions and relative positions of all nanoblock units on the metasurface. First, using the principle of resonant phase, the two target holographic phases are correlated with the phase response spectra of the metasurface units under different polarization light sources incident in free space, obtained in step 2, ensuring that the transmittance of the final nanoblocks is greater than 80%. This determines the geometric dimensions of all nanoblocks on the metasurface. The nanoblock geometric dimensions are then mapped to the phase response spectra under waveguide incidence in step 2 to obtain the resonant phase under waveguide light incidence. The relative positions of the nanoblocks within the subwavelength period are obtained by combining the detour phase and the resonant phase. This ensures that the metasurface corresponds to the target holographic phase under waveguide incidence and under different polarization light incident in free space, respectively. Thus, the metasurface unit nanoblock structure and the positional arrangement of the unit nanoblocks are determined.

[0041] 5) By combining resonant phase and detour phase, the final on-chip and off-chip metasurface two-dimensional arrays with waveguide input light source and free space input light source under the same wavelength are obtained.

[0042] 6) Using the same wavelength, three-channel multiplexed holograms can be obtained by passing the waveguide and free space incident signals through the metasurface array above the waveguide.

[0043] Furthermore, the specific steps are as follows:

[0044] In step 1: The wavelength λ of the guided wave propagating along the x-direction is selected. The thicknesses of silicon nitride and silicon are determined through parameter simulation optimization, ensuring that the geometrical variations of the metasurface unit structure satisfy the phase coverage of 0-2π under the incident wavelength λ in free space. The corresponding period P is calculated from the corresponding propagation constant and diffraction angle. X P Y P X =P Y .

[0045] In step 4: the geometric dimensions of the nanoblock are determined by minimizing the difference between its phase response spectrum under free-space light input and the phase of the target hologram. The resonant phase distribution under waveguided illumination is then determined by the correspondence between the nanoblock's geometric dimensions and its phase response spectrum under waveguided illumination. By employing a combination of detour phase and resonant phase methods, the positional information of the nanoblock is confirmed, ensuring that its phase satisfies:

[0046]

[0047]

[0048] In formula (1) β is the detour phase corresponding to the nanobulk unit, and β is the propagation constant corresponding to the incident waveguide light. n eff λ and λ0 are the effective refractive index and free-space wavelength of the waveguide, respectively. x is the abscissa position of the nanobulk along the x-direction. The phase of the target hologram is given by equation (2), where P is the period and Δx is the phase position of the nanoblock along the x-direction within the period. Therefore, the structural parameters of the nanoblock determine the phase distribution extracted into free space under free-space light incidence. The structural parameters and relative position of the nanoblock together determine the phase distribution extracted into free space under waveguide incidence, satisfying a phase change of 0-2π. This allows for the shaping of arbitrary wavefronts from the waveguide to the off-chip and from free space to the off-chip, thereby achieving on-chip and off-chip three-channel holographic multiplexing. This function cannot be achieved by using only resonant phase or detour phase. To a certain extent, this invention combines resonant phase and detour phase control methods to increase the number of multiplexing channels.

[0049] It is particularly noteworthy that under waveguided radiation, the change in free space transmittance corresponding to the change in the geometric size of the nanoblock is very small, and it is weak radiation, with the maximum outward radiation intensity of a single atom not exceeding 0.3% to ensure that the detour phase can work.

[0050] The principle of detour phase modulation of on-chip single-atom nanoblocks in this invention is as follows: For the same nanoblock unit, when a guided wave propagating within a waveguide passes through a single-atom nanostructure, it will be extracted and decoupled into free space, and its phase distribution can be designed by the phase accumulation of the propagating guided wave. For a guided wave with a propagation constant of β propagating along the x-direction, to achieve a phase accumulation of 2π, the required propagation distance is... Where λ0 is the free space wavelength, and n eff This is the effective refractive index of the waveguide. The design period P = D in this invention ensures that the relative position change of the nanobulb throughout the entire period satisfies a phase modulation of 0-2π. The detour phase of the light extracted by the nanobulb can be expressed as:

[0051]

[0052] in, and Δx i Let P be the extracted phase and corresponding displacement of the i-th nanoblock along the x-direction within one period. Here, P is the subwavelength period. Therefore, by adjusting the position of the nanoblock above the waveguide and combining it with resonant phase changes, arbitrary wavefront manipulation from the waveguide to off-chip can be achieved, thereby realizing on-chip holographic functionality.

[0053] This embodiment describes an on-chip and off-chip three-channel holographic multiplexed metasurface utilizing waveguides and free-space light incidence. Its structure is as follows: Figure 1 As shown. Figure 2 This is a schematic diagram of the unit structure. The metasurface consists of single-atom amorphous silicon nanobulbs (1000 nm high) situated on a silicon nitride (Si3N4) waveguide (other materials, such as lithium niobate waveguides, can also be used). The waveguide thickness is 900 nm (to ensure single-mode conditions and meet TE0 input), and the refractive index is approximately 2.05. A 500 μm thick silicon dioxide layer serves as the substrate. Electromagnetic simulation software FDTD solutions was used to simulate the structure. Figure 2 Numerical simulations of the unit structure shown yielded the phase response spectrum and transmittance distribution under the same wavelength of 1310 nm (the wavelength can be extended to other optical communication bands and the visible light band), with TEO incident light and free space x-linearly polarized and y-linearly polarized light inputs, respectively. Figure 3 As shown. The period P is fixed at 650 nm, and the length lx and width ly of the nanobulb vary from 100 nm to 400 nm. Using the improved GS algorithm, the target holographic phase map corresponding to the three channels is obtained by inputting three target holograms, as shown. Figure 4 As shown. The final required geometry of the nanoblock is determined by minimizing the difference between the phase response spectrum corresponding to the nanoblock's geometric dimensions and the target holographic phase map, ensuring a free-space incident transmittance greater than 80%. The resonant phase under waveguide illumination is obtained by correlating the determined nanoblock geometry with the phase distribution under waveguide illumination. The waveguide target holographic phase is determined by the sum of the waveguide resonant phase and the waveguide detour phase. This allows for the determination of the waveguide detour phase and ultimately the relative position of the nanoblock. Thus, the determination... Figure 1 The metasurface structure. The design principle of this metasurface is as follows: Figure 5 As shown, the target hologram of the waveguide is Z, and the target holograms of free-space incident x-linear polarization and y-linear polarization are J and U, respectively. This is the input. Through the combined effect of the detour phase and the resonant phase, on-chip and off-chip three-channel holographic multiplexing can be finally realized, as shown in the figure. Figure 6 The image shows the simulation results of the three channels obtained using the Lumercal FDTD solution, and it can be seen that there is almost no crosstalk between the channels.

[0054] The on-chip and off-chip three-channel holographic multiplexed metasurface proposed in this invention has good imaging intensity and clarity. The proposed combination of on-chip detour phase and resonant phase is compatible with photonic integrated circuits (PICs) and is expected to be used in optical information encryption technology, AR holographic display and wearable device projection.

Claims

1. A design method for an on-chip and free-space multiplexed holographic metasurface, characterized in that, Includes the following steps: 1) Design the structure of the metasurface; 2) Calculate the resonant phase response distribution and transmittance distribution of silicon nanobulks in the metasurface under on-chip input light source and free-space input light source, specifically including: Under the same operating wavelength of on-chip input light source and free space input light source, the resonant phase response spectrum, transmittance distribution of optical waveguide layer in metasurface, and resonant phase response spectrum and transmittance distribution of silicon nanoblocks with different geometric dimensions under on-chip input light source, as well as under free space input light source with different polarization, are calculated by the finite-difference time-domain method. 3) Design different holographic patterns, input them into the GS algorithm, obtain the phase distribution of the metasurface corresponding to the holographic image, and generate the phase distribution of 3 metasurfaces; 4) Based on the resonant phase response distribution and transmittance distribution obtained in step 2) and the phase distribution of the metasurface obtained in step 3), determine the geometric dimensions and relative positions of all silicon nanoblocks on the metasurface, determine the structure of the on-chip and free space multiplexed holographic metasurface, and complete the design; Based on the resonant phase response distribution and transmittance distribution obtained in step 2) and the phase distribution of the metasurface obtained in step 3), the geometric dimensions and relative positions of all silicon nanoblocks on the metasurface are determined, specifically including: 4.1) First, using the principle of resonance phase, two of the phase distributions of the three metasurfaces obtained in step 3) are compared with the resonance phase response spectra of the free space input light source with different polarizations obtained in step 2), and the transmittance of the silicon nanoblocks obtained in the end is greater than 80%, and the geometric dimensions of all silicon nanoblocks on the metasurface are determined. 4.2) The geometric dimensions of all silicon nanoblocks on the metasurface obtained in step 4.1) correspond to the resonance phase response spectra of silicon nanoblocks with different geometric dimensions obtained in step 2) under on-chip input light source, thus obtaining the metasurface resonance phase distribution under on-chip input light source. 4.3) Subtract the remaining one of the three metasurface phase distributions obtained in step 3) from the metasurface resonant phase distribution under the on-chip input light source obtained in step 4.2) to obtain the metasurface tortuous phase distribution under the on-chip input light source; 4.4) Using the principle of detour phase, the relative positions of all silicon nanoblocks on the metasurface are calculated by inputting the detour phase distribution of the metasurface under the on-chip input light source obtained in step 4.3).

2. The design method for on-chip and free-space multiplexed holographic metasurfaces according to claim 1, characterized in that, In step 1), the metasurface includes a silicon dioxide substrate, an optical waveguide layer disposed on the silicon dioxide substrate, and a single-atom amorphous silicon nanoblock structure array disposed on the optical waveguide layer. The single-atom amorphous silicon nanoblock structure array includes a plurality of silicon nanoblocks with the same height but different arrangement positions on the chip.

3. The design method for on-chip and free-space multiplexed holographic metasurfaces according to claim 2, characterized in that, In step 1), the optical waveguide layer is a silicon nitride optical waveguide layer or a lithium niobate optical waveguide layer.

4. The design method for on-chip and free-space multiplexed holographic metasurfaces according to claim 1, characterized in that, In step 2), the different polarizations are X linear polarization and Y linear polarization.

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