Electric tuning silicon nitride waveguide multimode interference coupler based on two-dimensional material covering layer

Abstract

The invention belongs to the technical field of integrated photonic devices, and particularly relates to an electric tuning silicon nitride waveguide multimode interference coupler based on a two-dimensional material covering layer. In order to solve the problem that an existing silicon nitride photonic device lacks effective electro-optical regulation and control capacity, a two-dimensional material covering layer (such as molybdenum disulfide and tungsten disulfide) is integrated on the surface of a silicon nitride waveguide, and an up-and-down structure of a metal electrode-two-dimensional material-silicon dioxide cladding-silicon nitride waveguide is constructed. When external bias voltage is applied, the effective refractive index of the waveguide mode is changed through evanescent field coupling by means of the electrical refraction effect of the two-dimensional material, and active regulation and control of the optical path of the silicon nitride waveguide without the thermo-optic effect are achieved. Compared with a lithium niobate-silicon nitride heterogeneous integration scheme, the two-dimensional material with the atomic layer thickness is compatible with the CMOS technology, a complex heterogeneous bonding technology is not needed, and the advantage of monolithic integration is achieved. The device is particularly suitable for a reconfigurable optical signal processing device in a silicon-based photon integrated circuit.

Description

Technical Field

[0001] The present invention relates to the technical field of integrated photonic devices, and specifically to an electrically tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material coating layer. Background Art

[0002] As a core device in integrated photonics, the multimode interference coupler (MMI) has important application values in the fields of optical communication, optical computing, and sensing due to its compact structure, high process tolerance, and uniform beam splitting characteristics. Traditional MMI devices are based on material platforms such as silicon, indium phosphide, or silicon nitride, and achieve static beam splitting functions through fixed geometric parameters, but cannot meet the requirements of the development of reconfigurable photonic integrated circuits (PICs). The development of tunable multimode interferometers has gone through three stages: in the early stage, the thermo-optic and carrier dispersion tuning technologies based on silicon waveguides were limited by millisecond-level response and milliwatt-level power consumption; in the middle stage, although lithium niobate and polymer materials improved the tuning speed, they faced problems such as high loss, insufficient stability, and high difficulty in heterogeneous integration; until the rise of two-dimensional materials in recent years, their atomic-level thickness and strong electro-optic response characteristics have provided a new path for high-speed and low-power dynamic tuning.

[0003] Silicon nitride waveguides, with ultra-low transmission loss (<0.1 dB / cm) and ultra-wideband optical transparency (0.4 μm - 2.35 μm), have become ideal carriers for high-performance MMIs, especially suitable for high-power photon integration and broadband sensing fields. However, its intrinsic non-tunability severely restricts the development of dynamically reconfigurable photonic integrated circuits: on the one hand, as a centrosymmetric crystal, silicon nitride lacks the asymmetric polarization response required for the linear electro-optic effect and cannot achieve dynamic refractive index regulation through direct electro-optic modulation; on the other hand, its wide bandgap property (~5 eV) results in an extremely low intrinsic carrier concentration, and it cannot achieve high-speed carrier injection modulation through the plasma dispersion effect like silicon-based materials. Although silicon nitride can be finely tuned in refractive index through the thermo-optic effect, the scheme relying on integrated micro heaters is limited by the thermal conductivity of the material, and the response speed only reaches the microsecond level. The milliwatt-level power consumption is prone to thermal crosstalk and device degradation, and it is difficult to meet the stringent requirements of optical communication and optical computing. In addition, the alternative scheme of heterogeneous integration of silicon nitride - lithium niobate faces multiple challenges: the lattice mismatch between lithium niobate and silicon nitride reaches 8%, and high-temperature bonding is likely to cause waveguide stress cracking; the mode field diameters of the two are significantly different, and even through tapered waveguide adaptation, additional insertion losses are introduced, and the evanescent field coupling efficiency is less than 20%, resulting in an effective electro-optic coefficient far lower than the theoretical value of lithium niobate. These defects make traditional silicon nitride MMIs only able to be used as static beam splitters and cannot support the dynamic routing of software-defined optical networks or the real-time reconstruction of photonic neural networks.

[0004] To this end, this patent proposes an electrically tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material coating layer. Through the Dirac electron or exciton effect of materials such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2), the refractive index of the waveguide evanescent field is dynamically regulated under an externally applied electric field. Combined with the wet transfer technology, large-area lossless integration is achieved. While retaining the ultra-low loss characteristics of silicon nitride, breakthroughs are made in achieving nanosecond-level response and microwatt-level power consumption for dynamic beam splitting ratio regulation, solving the application bottlenecks of traditional silicon nitride MMI in scenarios such as optical communication dynamic routing, photonic neural network reconstruction, and high-sensitivity sensing, and providing key technical support for the next-generation intelligent photonic integration system. Summary of the Invention

[0005] The present invention provides an electrically tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material coating layer. By innovatively introducing a two-dimensional material coating layer, the technical problem that the traditional silicon nitride waveguide multimode interference coupler cannot be dynamically tuned due to the lack of effective electro-optic regulation ability is solved, providing a high-performance core device for reconfigurable photonic integrated circuits.

[0006] The specific technical solution of the present invention is as follows:

[0007] Figure 1 An electrically tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material coating layer, whose core structure includes: silicon nitride input / output waveguides (① in the figure), tapered silicon nitride waveguide structures (② in the figure), multimode interference silicon nitride waveguides (③ in the figure), two-dimensional material coating layers (④ in the figure), metal modulation electrodes (⑤ in the figure), silica lower cladding layers (⑥ in the figure), and silica upper cladding layers (⑦ in the figure). To ensure that the silicon nitride input / output waveguides strictly meet the single-mode transmission conditions in a wide spectral range, the single-mode transmission conditions of the silicon nitride waveguides are first simulated. Figure 2 (a) shows the transverse optical field distribution at different waveguide widths, Figure 2 (b) is the corresponding simulation result of the single-mode transmission conditions. The simulation data shows that when the thickness of the silicon nitride waveguide is fixed, the number of modes increases significantly with the increase of the waveguide width. When the waveguide width exceeds the cut-off width, higher-order modes are excited. Subsequently, the optical signal is transmitted through the single-mode input waveguide and then coupled into the tapered waveguide structure. This tapered waveguide is designed with an adiabatic gradient to effectively expand the input single-mode optical field into a multimode optical field and guide it to the multimode interference region. To quantitatively analyze the influence of key geometric parameters on the device performance, this study used FDTD and COMSOL simulation software to study the regulation rules of the tapered waveguide length and the multimode interference waveguide length on the output optical field distribution, as shown in Figure 3 (a) and (b), and the optical field distribution diagram of the final MMI structure is as shown in Figure 4As shown, the uniform distribution of the output optical field in both arms can be seen. On this basis, the thickness of the silica upper cladding is optimized by COMSOL simulation software. While ensuring the low transmission loss of the silicon nitride waveguide, the effective mode coverage rate of the two-dimensional material and the evanescent field of the waveguide is increased, achieving a balance between low loss and high modulation efficiency. The cross-section of the waveguide optical field is as shown in Figure 5 . Further, a push-pull differential drive metal electrode is fabricated on the surface of the two-dimensional material by electron beam evaporation. The simulation shows that ( Figure 6 ), when an external static bias voltage is applied, the electrorefractive effect of the two-dimensional material can significantly change the effective refractive index of the silicon nitride waveguide, realizing efficient regulation of the optical path, and thus changing the light intensity distribution at the output ends of both arms. This design provides a new idea for high-speed and low-power photonic integrated devices through the synergistic effect of the electric field and the optical field.

[0008] Based on the above process, compared with the existing waveguide resonators, an electro-tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material cladding layer provided by the present invention has the following advantages:

[0009] (1) Aiming at the problems of slow response speed and high power consumption of the traditional silicon nitride tunable MMI based on the thermo-optic effect, an electro-tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material cladding layer provided by the present invention utilizes the strong electro-optic response characteristics of the two-dimensional material to achieve nanosecond-level refractive index regulation through an externally applied electric field, meeting the requirements of high-speed optical communication and real-time photonic computing; the electrorefractive effect of the two-dimensional material does not rely on the thermo-optic effect or carrier injection, and the power consumption can be reduced to the micro-watt level, effectively avoiding thermal crosstalk.

[0010] (2) Aiming at the problems of high process difficulty, large transmission loss, and mode field mismatch of the traditional tunable MMI based on lithium niobate-silicon nitride heterogeneous integration, an electro-tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material cladding layer provided by the present invention utilizes the characteristics of the atomic-level thickness of the two-dimensional material being compatible with the CMOS process, without the need for lithium niobate hetero-bonding or complex epitaxial growth, simplifies the manufacturing process, reduces the process cost, and at the same time supports high-density photonic integration; in addition, the intrinsic ultra-low loss and wide spectral transparency of the silicon nitride waveguide are retained, which is suitable for high-power laser transmission, multi-wavelength optical sensing, and quantum optical applications.

[0011] (3) An electro-tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material cladding layer provided by the present invention adopts a silicon nitride waveguide structure with an extremely low aspect ratio, effectively expanding the effective mode coverage rate of the two-dimensional material cladding layer and the evanescent field, enhancing the optical field interaction, combined with the push-pull differential electrode design, realizing efficient optical path regulation, having a wide dynamic range of the splitting ratio, and supporting reconfigurable optical routing and photon neural network weight reconstruction. Description of the Drawings

[0012] Figure 1Schematic diagram of an electro-tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material coating layer proposed by the present invention. The figure includes silicon nitride input / output waveguides (① in the figure), tapered silicon nitride waveguide structures (② in the figure), multimode interference silicon nitride waveguides (③ in the figure), two-dimensional material coating layers (④ in the figure), metal modulation electrodes (⑤ in the figure), silica bottom cladding (⑥ in the figure), and silica top cladding (⑦ in the figure).

[0013] Figure 2 (a) is the mode field distribution diagram of the silicon nitride waveguide at different widths, Figure 2 (b) is the simulation diagram of the single-mode transmission condition at a fixed silicon nitride thickness.

[0014] Figure 3 (a) is the influence of the tapered waveguide length on the output end, Figure 3 (b) is the influence of the multimode interference waveguide length on the output end.

[0015] Figure 4 is the optical field distribution diagram of the multimode interference coupler proposed by the present invention.

[0016] Figure 5 is the optical field coupling distribution diagram between the two-dimensional material and the silicon nitride waveguide.

[0017] Figure 6 is the simulation diagram of the change in the light intensity distribution at the output ports of the two arms of the multimode interference coupler with the static voltage. Specific implementation scheme

[0018] First, a silicon nitride thin film with a thickness of 80 nm is grown on a silica lower cladding prepared by a thermal oxidation process using a low-stress chemical vapor deposition (LPCVD) process. Input / output single-mode waveguides, tapered waveguides, and multimode interference waveguides are fabricated through DUV lithography and reactive ion etching (RIE) techniques. The width of the input / output waveguides is designed to be 3 μm to ensure strict single-mode transmission conditions. The tapered waveguide adiabatically expands the single-mode optical field into the multimode interference region through a gradual design (with a length of approximately 110 μm). The width and length of the multimode interference waveguide are optimized by finite element method simulation, and the final dimensions are determined to be 87 μm × 12 μm to achieve a uniform beam splitting function. Subsequently, a silica upper cladding is grown on the waveguide surface through a TEOS-PECVD process, with the thickness controlled at around 1 μm to balance the contradiction between transmission loss and evanescent field coverage. After the silica upper cladding is prepared, a monolayer two-dimensional material such as molybdenum disulfide (MoS2) or tungsten disulfide (WS2) is precisely covered on the surface of the upper cladding in the multimode interference waveguide region using a wet transfer technique. During the transfer process, optical microscopy and Raman spectroscopy are used to ensure that the material has no wrinkles, no contamination, and fully overlaps with the waveguide evanescent field region. Then, a push-pull differential drive metal electrode (such as a Ti / Au composite layer) is prepared on the surface of the two-dimensional material using an electron beam evaporation process to ensure uniform distribution of the electric field on the surface of the two-dimensional material. The electrode pattern is aligned with high precision through a lift-off process to avoid the risks of short circuit or open circuit. After the device is fabricated, a static bias voltage is applied through an external voltage source, and the electro-optic effect of the two-dimensional material is used to dynamically regulate the effective refractive index of the waveguide evanescent field. When the voltage changes, the carrier concentration of the two-dimensional material changes, affecting the optical field distribution in the multimode interference region through evanescent field coupling, thereby achieving continuous tunability of the beam splitting ratio.

[0019] This design can achieve dynamic regulation of the optical path within a nanosecond response time, and the power consumption is only in the micro-watt range, significantly superior to traditional thermo-optical or carrier injection schemes. In addition, the entire process technology is compatible with CMOS technology, without the need for lithium niobate hetero-bonding or complex epitaxial growth, providing feasibility for large-scale monolithic integration of silicon nitride photonic integrated circuits. Finally, this device can be widely applied in fields such as optical communication dynamic routing, real-time reconstruction of photonic neural networks, and high-sensitivity biochemical sensing, with the advantages of high performance and process universality.

Claims

1. An electrically tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material coating, characterized in that: It includes a silicon nitride input / output waveguide (1), a tapered silicon nitride waveguide structure (2), a multimode interference silicon nitride waveguide (3), a two-dimensional material cladding layer (4), a metal modulation electrode (5), a silica upper cladding layer (6), and a silica lower cladding layer (7).

2. The electro-tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material coating layer according to claim (1), characterized in that: The input / output waveguide meets the single-mode transmission condition, and the light source is coupled into the wide-domain multimode interference waveguide through the single-mode input waveguide and the tapered waveguide structure. By optimizing the geometric parameters of the multimode interference waveguide, the uniform beam splitting function of the optical field distribution is achieved at the output end.

3. The electrically tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material coating layer according to claim (1), wherein: The two-dimensional material cladding layer is integrated on the surface of the upper cladding layer in the multimode interference waveguide region.

4. The electro-tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material covering layer according to claim (1), characterized in that: The silica upper cladding layer uses the TEOS-PECVD process.

5. The electro-tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material coating layer according to claim (1), characterized in that: The two-dimensional material cladding layer uses the wet transfer technology to achieve large-area defect-free transfer.

6. The electro-tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material coating layer according to claim (1), characterized in that: The metal modulation electrode uses a push-pull differential drive structure.

7. The electro-tunable silicon nitride waveguide multimode interference coupler based on a two-dimensional material coating layer according to claim (1), wherein: The silicon nitride waveguide uses a very low aspect ratio cross-section structure, and the multimode interference coupler is of a 1×2 beam splitter structure type.

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