A grating coupler based on a sodium-based hybrid plasmonic waveguide

Abstract

The application discloses a grating coupler based on a sodium-based hybrid plasmonic waveguide, and relates to the field of photonic integrated devices, which comprises, from bottom to top, a sapphire substrate, a silicon waveguide layer, a SOG layer and a sodium layer; the sodium layer is prepared by using a sodium spin coating process; the grating coupler adopts bottom coupling, a single-mode optical fiber is arranged below the sapphire substrate, the included angle between the axis of the single-mode optical fiber and the normal line of the sapphire substrate is 0-15°, and the roughness of the bottom surface of the sapphire substrate is less than 10 nm. The existing substrate slotted SOI is improved by using the planar and slot-free sapphire substrate, the interface irregular scattering caused by the substrate slotted SOI is reduced, and the alignment deviation of the incident angle is reduced; the high flatness of the slot-free substrate and the high surface quality of the spin-coated sodium film form a synergistic effect, interface scattering loss is inhibited, and the overall structure has excellent robustness to preparation deviation.

Description

Technical Field

[0001] This invention relates to the field of photonic integrated devices, and more specifically to a grating coupler based on a sodium-based hybrid plasma waveguide. Background Technology

[0002] On-chip optical coupling devices can use traditional silicon waveguide mode and hybrid waveguide mode to realize on-chip optical path transmission. Traditional silicon waveguides rely on total internal reflection to confine the light field. Due to the diffraction limit, metal layers are usually used to increase reflection, but hybrid modes are not formed. In contrast, in hybrid waveguide mode, the gap between the metal layer and the silicon layer is small, and the light field is formed by the coupling of the dielectric mode and the metal surface plasmon mode. This can break the diffraction limit, but the transmission loss is generally higher.

[0003] With the rapid development of low-loss plasma waveguides, the dual mismatch of mode field and wave vector between standard single-mode fiber and subwavelength plasma waveguides has become increasingly prominent, leading to a significant decrease in coupling efficiency. Sodium, due to its significantly lower transmission loss in the 1200–1700 nm communication band compared to traditional noble metals, has emerged as a promising low-loss plasma material for achieving subwavelength-scale optical mode field confinement. However, sodium's low melting point (97.8℃) and extreme sensitivity to air and water vapor have long prevented its compatibility with standard silicon photonics fabrication processes, and its fabrication methods have been extremely limited.

[0004] US Patent 11761093B2 discloses a method for preparing a sodium interface and a method for preparing sodium-based optical structures. The method involves melting solid sodium metal into a liquid state by heating, obtaining molten sodium metal, and then peeling off solid oxides and impurities from the surface of the molten sodium metal to obtain liquid sodium with a metallic luster. This liquid sodium is free of solid oxides and impurities. The liquid sodium is then used to apply a spin coating to a dielectric substrate, allowing the sodium interface to adhere tightly to the transparent substrate. This method effectively isolates the sodium interface from air and prevents it from reacting with sodium. Currently, almost all sodium metal layers in grating couplers using sodium-based hybrid plasmonic waveguides are prepared using this method. This not only severely limits their large-scale application in photonic integrated circuits but also restricts grating couplers using sodium-based hybrid plasmonic waveguides to bottom coupling schemes. However, most existing bottom coupling methods use SOI substrate trenching, which inevitably leads to flatness issues: etching trenches on the SOI substrate will damage the optical flatness of the substrate bottom surface, and the roughness of the trench structure is difficult to control precisely, which will cause random deviations in the incident angle. Furthermore, the incident light undergoes irregular refraction and scattering at the substrate interface, introducing additional optical path loss and reducing the device's process robustness.

[0005] In summary, although sodium films prepared by spin coating possess excellent surface flatness, there is currently no suitable bottom coupling scheme for grooveless substrates, which fails to fully utilize the material advantages of sodium films, such as low loss and high flatness. How to achieve efficient, broadband fiber-to-chip coupling in hybrid waveguide mode, while leveraging sodium's low-loss characteristics and subwavelength mode field confinement capabilities, has become a pressing technical problem to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to propose a grating coupler based on a sodium-based hybrid plasma waveguide, comprising, from bottom to top, a transparent substrate layer, a silicon waveguide layer, a low-refractive-index spacer layer, and a sodium layer; the sodium layer is prepared by sodium spin coating; the coupling method of the grating coupler is bottom-incident vertical coupling, with a single-mode fiber disposed below the transparent substrate layer, the angle between the axis of the single-mode fiber and the normal of the transparent substrate layer being 0-15°; the bottom surface roughness of the transparent substrate layer is less than 10 nm.

[0007] Furthermore, the transparent substrate is selected as a sapphire substrate, and a silicon thin film is epitaxially grown on the surface of the sapphire substrate using the sapphire-on-silicon epitaxial process, wherein the thickness of the sapphire substrate is 500-5000 μm.

[0008] Furthermore, a grating structure is etched on the top of the silicon waveguide layer, with the grating teeth having a consistent etching depth.

[0009] Furthermore, the apodization method of the grating structure is as follows: the duty cycle of the grating etching is configured to decrease linearly along the coupling direction, in order to control the local diffraction intensity of the grating and make it match the Gaussian mode field profile of the single-mode fiber.

[0010] Furthermore, the grating structure also employs a linear chirped period, with the grating period gradually increasing along the coupling direction, so that grating segments at different positions satisfy the local Bragg phase matching condition at different wavelengths.

[0011] Furthermore, the grating period varies in the range of 200-800 nm.

[0012] Furthermore, the low refractive index spacer layer is made of spin-coated glass SOG or plexiglass PMMA.

[0013] Furthermore, the etching depth of the grating teeth is 50-100 nm.

[0014] Furthermore, the thickness of the sodium layer is 50-50000 nm.

[0015] Furthermore, the transparent substrate layer is selected from silicon dioxide substrates.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] (1) This invention utilizes sodium as a low-loss plasma material, which effectively reduces communication transmission loss. It also improves the existing substrate slotted SOI by using a planar slotless transparent substrate, which not only greatly reduces the process difficulty, but also reduces the interface irregular scattering and incident angle alignment deviation caused by substrate slotting from the root. Furthermore, by controlling the roughness of the substrate bottom surface within the range of 10 nanometers, it maintains high optical flatness, making the incident angle stable and controllable. The high flatness of the slotless substrate and the high surface quality of the spin-coated sodium film form a synergistic effect, further suppressing interface scattering loss and widening the incident angle tolerance of the device to ±2°. The overall structure has excellent robustness to fabrication deviations and can still maintain a peak coupling efficiency of more than 70% even when the incident angle deviates from the optimal value, which significantly improves the actual reliability of the device.

[0018] (2) The grating coupler provided by the present invention is built on a sapphire-silicon platform. It utilizes the full transparency of the sapphire substrate in the visible to near-infrared band to achieve efficient alignment between the grating and the optical fiber, thereby reducing the packaging difficulty and performance loss caused by alignment error.

[0019] (3) This invention significantly improves the mode matching degree between optical fiber and hybrid plasma waveguide through the synergistic effect of chirped structure and duty cycle apodization. Broadband phase matching is achieved by using chirped grating, and side lobes and back reflections are effectively suppressed by duty cycle apodization. The 3dB working bandwidth reaches 50nm. The SOG layer is used to strengthen mode field confinement, and the advantages of sodium film in low-band absorption in the near-infrared band are utilized to achieve subwavelength mode field confinement of less than λ / 10 and low-loss transmission. Attached Figure Description

[0020] Figure 1 This is a cross-sectional view of the grating coupler provided in Embodiment 1 of the present invention;

[0021] Figure 2 This is a schematic diagram of the overall structure of the grating coupler provided in Embodiment 1 of the present invention;

[0022] Figure 3 This is a schematic diagram of the bottom incidence of a single-mode optical fiber provided in Embodiment 1 of the present invention;

[0023] Figure 4 This is the optimized steady-state |E|² field distribution diagram provided in Embodiment 2 of the present invention;

[0024] In the figure, 1 is the sapphire substrate; 2 is the silicon waveguide layer (SOG); 3 is the spin-coated glass; and 4 is the sodium layer. Detailed Implementation

[0025] To better understand the present invention, the following description, in conjunction with the accompanying drawings of the embodiments of the present invention, will further illustrate the present invention, but this is not intended to limit the present invention; various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the design concept of the present invention should fall within the protection scope of the present invention.

[0026] It should be noted that this invention is only proposed for hybrid waveguide modes, in order to solve the problems of high transmission loss and complex existing fabrication processes in hybrid waveguide modes.

[0027] Example 1

[0028] refer to Figure 1 and Figure 2 This embodiment discloses a grating coupler based on a sodium-based hybrid plasma waveguide, comprising a transparent substrate layer, a silicon waveguide layer, a low-refractive-index spacer layer and a sodium layer arranged sequentially from bottom to top, wherein the bottom surface roughness of the transparent substrate layer is less than 10 nm.

[0029] like Figure 1 As shown, in one embodiment, a sapphire substrate is selected as the transparent substrate layer, and a silicon thin film is epitaxially grown on the surface of the sapphire substrate using the sapphire-on-silicon epitaxial process, wherein the thickness of the sapphire substrate is 500-5000μm.

[0030] As another implementation method, a silicon dioxide glass substrate is selected as the transparent substrate layer.

[0031] As is well known, the metal layers in existing grating coupler structures mostly use traditional precious metals such as gold and silver. Although there are mature coating processes available and the processing difficulty is low, it is impossible to achieve low transmission loss.

[0032] To address this challenge, this embodiment uses sodium, an emerging low-loss plasma material, instead of traditional noble metals. The sodium layer exhibits significantly lower transmission loss in the communication band (1200–1700 nm) than traditional noble metals, especially with extremely low interband absorption at a wavelength of 1310 nm, enabling subwavelength mode field confinement.

[0033] To overcome the challenges in preparing the sodium layer, this embodiment employs a low-temperature sodium spin-coating process. It should be noted that the low-temperature sodium spin-coating process is prior art and is not within the scope of this invention.

[0034] In one implementation, the thickness of the sodium layer is 50-50000 nm.

[0035] Because the top sodium layer of the grating coupler provided in this embodiment is fabricated using an integral spin-coating process, it exhibits strong reflectivity in the near-infrared band, making it unsuitable for top-incidence coupling. Therefore, bottom-incidence coupling is the only viable coupling method. Existing SOI-based hybrid plasmonic waveguides mostly employ substrate grooving for bottom-incidence coupling. This not only compromises the optical flatness of the substrate's bottom surface but also makes precise control of surface roughness difficult, leading to random deviations in the incident angle. Furthermore, the incident light undergoes irregular refraction and scattering at the substrate interface, introducing additional optical path loss.

[0036] In contrast, this embodiment utilizes a sapphire substrate that is transparent in the visible to near-infrared band, using it as a matching substrate for a sodium-based hybrid plasma waveguide. This not only enables efficient coupling but also facilitates grating alignment under visible light illumination.

[0037] like Figure 3 As shown, a single-mode fiber (SMF) is placed beneath the sapphire substrate. The core diameter of the SMF is 9-10 μm, and the angle between the fiber axis and the normal to the sapphire substrate is 0-15°. The lateral offset between the fiber mode field center and the effective coupling origin of the grating is... .

[0038] like Figure 1 and Figure 2 As shown, the top of the silicon waveguide layer has a uniform etching depth of grating, with the grating teeth having an etching depth of 50-100 nm.

[0039] In this embodiment, the low refractive index spacer layer is a spin-coated glass SOG with a thickness of 320 nm (the distance between the bottom of the metal layer and the top of the silicon waveguide is 100 nm, and the silicon layer thickness is 220 nm). This serves two purposes: planarization and hybridization of the silicon waveguide mode and the plasmon mode, localized in the SOG layer.

[0040] In some implementations, the low-refractive-index spacer layer can also be made of PMMA (polymethyl methacrylate).

[0041] Example 2

[0042] In order to optimize the local distribution of the optical field and balance the field enhancement effect with the transmission loss, this embodiment proposes a preferred grating structure based on embodiment 1.

[0043] This embodiment employs a chirped duty cycle linear apodized grating with an etching depth of 70 nm; the duty cycle of the grating etching is configured to decrease linearly along the coupling direction, satisfying the formula:

[0044]

[0045] Where z is the distance from the grating position to the edge of the silicon waveguide. It is a linear apodilation factor. This represents the initial duty cycle.

[0046] In one embodiment, the initial duty cycle is 0.6-1.0, and the linear apodization factor is 0.01-0.1.

[0047] By using a linear apodization design, the local coupling strength of the grating changes smoothly along the length direction, achieving precise matching between the diffraction field distribution and the Gaussian mode field of the single-mode fiber.

[0048] Furthermore, a chirp period is introduced, and the grating period gradually increases along the coupling direction to compensate for the phase matching condition that varies along the coupling length direction, thereby broadening the working bandwidth.

[0049] In one embodiment, the grating period varies from 200 to 800 nm.

[0050] The working principle of this embodiment is as follows: When light with a wavelength of 1310 nm is incident from the bottom single-mode fiber at an 8° oblique angle, the beam first passes through the transparent sapphire substrate and reaches the silicon grating layer. The chirped duty cycle apodized grating diffracts the incident light and couples it into the silicon waveguide. The light field smoothly passes through the chirped apodized grating along the propagation direction and enters the SOG layer between the silicon waveguide and the sodium film, transferring the light energy to the mixed plasma mode near the SOG / sodium interface, thereby achieving efficient coupling.

[0051] The synergistic mechanism of each structural feature is as follows:

[0052] Chirped periodic structure: Gradually increase the grating period along the coupling length direction so that the grating segments at different positions satisfy the local Bragg phase matching condition at slightly different wavelengths, thereby converting the single-wavelength resonance into a continuous wavelength scanning response within the target band, effectively broadening the working bandwidth (3 dB bandwidth up to 50 nm).

[0053] Linear apodization of duty cycle: The duty cycle is gradually reduced along the coupling direction, so that the local diffraction intensity of the grating is approximately Gaussian distributed from the start end to the end end, which matches the Gaussian mode field profile of the single-mode fiber, thereby improving the mode field overlap efficiency and suppressing back reflection and side lobes at both ends of the grating.

[0054] Next, the performance of the grating coupler proposed in this embodiment is numerically simulated using the full-wave two-dimensional finite-difference time-domain method. The simulation results are as follows:

[0055] In terms of coupling efficiency, such as Figure 4As shown in (a), the peak coupling efficiency reaches 75% at a wavelength of 1310 nm. In contrast, a coupler using the same layer structure but with a uniform periodic grating (duty cycle of 0.5) achieves a peak efficiency of only 65% ​​at the same wavelength. It can be seen that this embodiment improves the efficiency by 10 percentage points through the chirped apodization design.

[0056] In terms of operating bandwidth, the 3dB bandwidth is 50 nm (1285 nm to 1335 nm). Figure 4 (b) shows the optimized steady-state |E|² field distribution, which demonstrates the smooth adiabatic transformation of the optical field along the coupling length.

[0057] Regarding process robustness, the coupling efficiency remains above 70% within the apodization factor R = 0.031 ± 0.002, corresponding to a tolerance of ±1.3% for the linear apodization factor R; within the initial duty cycle F0 = 0.93 ± 0.01, the coupling efficiency is still greater than 72%. This indicates that the grating coupler proposed in this embodiment has good tolerance to process deviations.

[0058] In terms of mode field confinement capability, the mode field width of the hybrid plasma mode is less than λ / 10, which is much smaller than the mode field size of traditional silicon waveguides, thus achieving subwavelength optical field confinement.

[0059] Regarding alignment tolerance: optimal coupling is achieved when the fiber lateral offset Δz = 0 μm and θ = 8°, and the peak efficiency remains ≥70% when the incident angle θ varies within the range of 8° ± 2°.

[0060] In summary, the grating coupler based on sodium-based hybrid plasmonic waveguide provided in this embodiment can achieve a peak coupling efficiency of 75% at a wavelength of 1310 nm, a 3dB bandwidth of 50 nm, subwavelength mode field confinement, and excellent process robustness, and the fabrication method is compatible with existing silicon photonics processes.

[0061] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be used to limit the scope of protection of the present invention. All equivalent transformations or modifications made in accordance with the spirit and essence of the present invention should be covered by the present invention.

Claims

1. A grating coupler based on a sodium-based hybrid plasmonic waveguide, characterized in that, It includes, from bottom to top, a transparent substrate layer, a silicon waveguide layer, a low-refractive-index spacer layer, and a sodium layer; The sodium layer was prepared using a sodium spin-coating process. The grating coupler is a bottom-incident vertical coupling, with a single-mode fiber disposed below the transparent substrate layer, and the angle between the axis of the single-mode fiber and the normal of the transparent substrate layer is 0-15°. The bottom surface roughness of the transparent substrate is less than 10 nm.

2. The sodium-based hybrid plasmonic waveguide grating coupler of claim 1, wherein, The transparent substrate is made of sapphire, and a silicon thin film is epitaxially grown on the surface of the sapphire substrate using the sapphire-on-silicon epitaxial process. The thickness of the sapphire substrate is 500-5000 μm.

3. The grating coupler based on a sodium-based hybrid plasmonic waveguide according to claim 1, characterized in that, The top of the silicon waveguide layer is etched with a grating structure, and the grating teeth are etched at a uniform depth.

4. The grating coupler based on a sodium-based hybrid plasmonic waveguide according to claim 3, characterized in that, The apodization method of the grating structure is as follows: the duty cycle of the grating etching is configured to decrease linearly along the coupling direction, which is used to control the local diffraction intensity of the grating so that it matches the Gaussian mode field profile of the single-mode fiber.

5. The grating coupler based on a sodium-based hybrid plasmonic waveguide according to claim 4, characterized in that, The grating structure also employs a linear chirped period, with the grating period gradually increasing along the coupling direction, so that grating segments at different positions satisfy the local Bragg phase matching condition at different wavelengths.

6. The grating coupler based on a sodium-based hybrid plasmonic waveguide according to claim 5, characterized in that, The grating period varies in the range of 200-800 nm.

7. The grating coupler based on a sodium-based hybrid plasmonic waveguide according to claim 3, characterized in that, The low-refractive-index spacer layer is made of spin-coated glass SOG or plexiglass PMMA.

8. The grating coupler based on a sodium-based hybrid plasmonic waveguide according to claim 3, characterized in that, The etching depth of the grating teeth is 50-100 nm.

9. The grating coupler based on a sodium-based hybrid plasmonic waveguide according to claim 1, characterized in that, The thickness of the sodium layer is 50-50000 nm.

10. The grating coupler based on a sodium-based hybrid plasmonic waveguide according to claim 1, characterized in that, The transparent substrate layer is made of silicon dioxide.

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