A design method of an on-chip all-optical nonlinear activation unit based on a vanadium dioxide volatile thermal phase change process

Abstract

The application discloses a design method of an on-chip all-optical nonlinear activation unit based on a vanadium dioxide volatile thermal phase change process, and comprises the following steps: designing a waveguide composite structure; determining a working environment temperature interval of the composite structure; designing a mapping between an optical input intensity and a required working environment temperature of the structure and a material model of vanadium dioxide; calculating a transmittance of vanadium dioxide material under different phase change processes of single-mode light under different environment temperatures, and constructing a nonlinear mapping between inputs and outputs of the unit as an activation function; and optimizing geometric structure parameters such as length, width and height of vanadium dioxide through an algorithm to complete the design of the nonlinear activation unit. The application can overcome the problem of lacking nonlinearity in traditional optical calculation, avoid high energy consumption caused by electro-optical conversion, and realize the function of the on-chip all-optical nonlinear activation unit.

Description

Technical Field

[0001] This invention relates to the fields of optical technology, integrated photonics technology, and nonlinear photonic computing units, specifically to a design method for an on-chip all-optical nonlinear activation unit based on the volatile thermal phase transition process of vanadium dioxide. Background Technology

[0002] Optical computing is a computing approach that uses photons as information carriers and leverages the propagation, interference, diffraction, modulation, and detection of light to perform computations, thus overcoming the bottlenecks of traditional electronic computing in terms of speed, bandwidth, and power consumption. Compared to diffraction neural networks that perform matrix calculations through light diffraction, on-chip optical computing units have the advantage of easy packaging and integration, and can be applied to fields such as machine learning and facial recognition. However, due to the inherent properties of light, optical computing lacks nonlinear processes, thus limiting computational efficiency and performance in advanced tasks. Therefore, there is an urgent need to introduce nonlinear computing units. Traditional solutions typically introduce photoelectric conversion stages to achieve nonlinear operations using electrons as signal carriers, and then convert the processed electrical signals back into optical signals for subsequent calculations. However, photoelectric-to-optical conversion relies on photodiodes and lasers, which significantly increases energy consumption.

[0003] Nonlinear modulation can be achieved by utilizing the unique thermo-optic or electro-optic effects of materials, but the nonlinear coefficient is limited by the material's inherent properties, making it difficult to maintain a high nonlinear coefficient in material systems with low input energy. Vanadium dioxide is a non-volatile phase transition material that undergoes a reversible phase transition at approximately 68°C, transforming from a low-temperature insulating monoclinic phase (M1 phase) to a high-temperature metallic rutile phase (R phase). This transformation is accompanied by drastic changes in electrical conductivity and optical properties, specifically manifested as a cyclic hysteresis loop, exhibiting significant nonlinear characteristics. Summary of the Invention

[0004] This invention provides an on-chip all-optical nonlinear activation unit based on the volatile thermal phase transition process of vanadium dioxide. A specific "loop"-shaped nonlinear activation function is achieved by employing a composite structure created by depositing vanadium dioxide (VO2) on a silicon (Si) substrate of a conventional optical waveguide, and a temperature-controlled plate below. Specifically, the transmittance of the mode light input to the composite structure is calculated using the finite-difference time-domain method. By setting a material model, vanadium dioxide with different refractive indices under different ambient temperatures and crystal states is simulated to obtain the response relationship between input and output under heating and cooling conditions, i.e., the desired "loop"-shaped nonlinear activation function.

[0005] A design method for an on-chip all-optical nonlinear activation unit based on the volatile thermal phase transition process of vanadium dioxide includes the following steps: 1) Design waveguide composite structure; 2) Based on the phase transition characteristics of the vanadium dioxide layer and the nonlinear activation function requirements of different waveguide composite structures, determine the range of geometric structural parameters and the operating temperature range of the waveguide composite structure. 3) Based on the range of geometric parameters and the range of operating temperature obtained in step 2), design the mapping between the light input intensity and the required operating temperature of the waveguide composite structure, as well as the material model of vanadium dioxide under different phase transition processes. 4) Based on the mapping between the light input intensity and the required operating temperature of the waveguide composite structure obtained in step 3) and the material model of vanadium dioxide under different phase transition processes, the transmittance of the waveguide composite structure under different phase transition processes of the vanadium dioxide layer at different ambient temperatures is calculated using the finite difference time-domain method. Thus, a nonlinear mapping between the input and output of the waveguide composite structure is constructed as the activation function. 5) Describe the activation function obtained in step 4) using the Bouc-Wen model and compare it with the Bouc-Wen model corresponding to the design requirements of the waveguide composite structure. Determine the optimization interval based on the geometric parameters of the activation function obtained in step 4). Optimize the geometric parameters of the vanadium dioxide layer in the optimization interval using the finite-difference time-domain method until a nonlinear activation function that meets the requirements described by the Bouc-Wen model is completed. This completes the optimized design of the waveguide composite structure and yields the on-chip all-optical nonlinear activation unit.

[0006] In step 1), the waveguide composite structure includes a silicon (Si) substrate, a vanadium dioxide (VO2) layer disposed on the silicon substrate, and a silicon dioxide (SiO2) cladding layer.

[0007] In step 2), the phase transition characteristics of vanadium dioxide include: the transformation of vanadium dioxide from a low-temperature insulating monoclinic phase (M1 phase) to a high-temperature metallic rutile phase (R phase), specifically the complex refractive index of input light with wavelengths in the 500-1550nm range within an ambient temperature range of 290K to 360K. The nonlinear activation function requires the width of the "loop" function and the input segment of the "loop" action. Based on the phase transition characteristics of vanadium dioxide material and the nonlinear activation function requirements for different shapes of waveguide devices, the geometric parameter range and operating ambient temperature range of the composite structure are determined.

[0008] The nonlinear activation function requirements for different shapes of the waveguide composite structure include: all parameters required to describe the nonlinear activation function using the Bouc-Wen model. Specifically, the geometric parameter range and operating temperature range of the waveguide composite structure include: for the silicon substrate conforming to single-mode conduction geometry, the vanadium dioxide layer has a length in the range of 600-1500 nm, a width in the range of 200-700 nm, and a height in the range of 200-300 nm; the operating temperature range is 300-360 K.

[0009] In step 3), the geometric parameter range and operating temperature range of the waveguide composite structure specifically include: the length of the vanadium dioxide layer conforming to single-mode conduction on the silicon substrate is in the range of 600-1500 nm, the width is in the range of 200-700 nm, and the height is in the range of 200-300 nm; the operating temperature range is from 300-360 K. The mapping between the light input intensity and the required operating temperature of the structure is: generally, a linear relationship is satisfied between the structure's operating temperature and the light input intensity. The material model of vanadium dioxide under different phase transition processes is: specifically, for wavelength input light in the 500-1550 nm range within the ambient temperature range of 290 K to 360 K, the complex refractive index during the two processes of vanadium dioxide transforming from a low-temperature insulating monoclinic phase (M1 phase) to a high-temperature metallic rutile phase (R phase) and vanadium dioxide transforming from a high-temperature metallic rutile phase (R phase) to a low-temperature insulating monoclinic phase (M1 phase).

[0010] In step 4), based on the mapping between the light input intensity and the required operating temperature of the structure obtained in step 3), and the vanadium dioxide material model, the transmittance of single-mode light under different phase transition processes of vanadium dioxide at different ambient temperatures is calculated using the finite-difference time-domain method. This constructs a nonlinear mapping between the unit's input and output as the activation function, specifically including: 4.1) Based on the target wavelength of 1497nm and the requirement of the desired "loop"-shaped nonlinear activation function, determine the geometric parameter range and operating temperature range of the composite structure according to step 2). Specifically, the structured silicon substrate conforms to the geometric parameters of single-mode conduction, the length of the vanadium dioxide structure is between 600-1500nm, the width is between 0-700nm, the height is between 200-300nm, and the operating temperature range is between 300-360K.

[0011] 4.2) Based on the mapping between the light input intensity and the required operating temperature of the structure obtained in step 3) and the vanadium dioxide material model, select the geometric parameters of the initial waveguide device, and use the finite-difference time-domain method to calculate the transmittance of single-mode light under different phase transition processes of vanadium dioxide material at different ambient temperatures. This allows the transmittance of the input light to show a large difference under different phase transition processes at the same operating temperature. Thus, a nonlinear mapping between the input and output of the unit is constructed as the activation function, thereby realizing the "loop"-shaped nonlinear activation function.

[0012] In step 5), the finite-difference time-domain method optimizes the geometric parameters of the vanadium dioxide layer within the optimization interval, completing the nonlinear activation function described by the Bouc-Wen model that meets the requirements, thus completing the optimized design of the waveguide composite structure. Specifically, this includes: using the algorithm to traverse the length, width, and height within the preset range of geometric parameters of the vanadium dioxide layer, completing the nonlinear activation function described by the Bouc-Wen model that meets the requirements, and thus completing the optimized design of the waveguide composite structure. By optimizing the length, width, and height geometric parameters of the vanadium dioxide layer through the algorithm, the "loop"-shaped nonlinear activation function obtained in step 4) meets the requirements, thus completing the design of the nonlinear activation unit.

[0013] Compared with the prior art, the present invention has the following advantages: I. This discovery is innovative and proposes a novel structure. It adopts a composite structure formed by depositing vanadium dioxide (VO2) on a silicon (Si) substrate, which is a traditional optical waveguide. Through the coupling effect of the upper and lower layers, a "loop"-shaped nonlinear activation function is generated, which solves the problem of nonlinearity deficiency and insufficient effective nonlinear structure in traditional optical computing.

[0014] Second, this invention optimizes the geometric structure of the upper vanadium dioxide layer and adjusts the nonlinear activation function by adjusting the ambient temperature within a small range, thereby realizing the computational programming control of the nonlinear unit and breaking through the bottleneck of nonlinear integrated computation in all-optical computing.

[0015] Third, this invention adopts a planar waveguide design, utilizing silicon and silicon dioxide to coat a vanadium dioxide layer. The waveguide structure thickness is less than 500nm, avoiding the space occupation problem of traditional nonlinear path photoelectric conversion devices, making it easy to integrate and providing core device support for integrated all-optical computing chips.

[0016] Fourth, the process of this invention is simple and stable, compatible with existing semiconductor manufacturing processes, and can reduce production costs and process complexity. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the composite waveguide structure provided in an embodiment of the present invention.

[0018] Figure 2The composite waveguide structure parameters obtained by numerical simulation in this embodiment of the invention at a working wavelength of 1459nm are as follows: silicon substrate width 700nm, height 220nm, vanadium dioxide width 300nm, height 200nm, length 1200nm, the transmission amplitude response of light extracted by vanadium dioxide in the low-temperature insulating monoclinic phase (M1 phase) during single-mode light incidence (a), the transmission amplitude response of light extracted by vanadium dioxide in the amorphous phase during phase transition (b), and the transmission amplitude response of light extracted by vanadium dioxide in the high-temperature metallic rutile phase (R phase) (c).

[0019] Figure 3 This is the transmittance distribution of the structure provided in this embodiment of the invention at different operating temperatures and different phase transition processes at a working wavelength of 1459 nm.

[0020] Figure 4 This is a schematic diagram of the "loop"-shaped nonlinear activation function implemented at a working wavelength of 1459 nm according to an embodiment of the present invention. Detailed Implementation

[0021] 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.

[0022] The design method for nonlinear activation units includes the following steps: 1) Design of waveguide composite structure: The waveguide composite structure includes: a silicon substrate, a vanadium dioxide layer disposed on the silicon substrate, and a silicon dioxide cladding layer.

[0023] 2) Based on the phase transition characteristics of vanadium dioxide material and the nonlinear activation function requirements of waveguide devices with different shapes, determine the range of geometric parameters and the operating temperature range of the composite structure.

[0024] 3) Based on the range of geometric parameters and the range of operating temperature obtained in step 2, design the mapping between light input intensity and the required operating temperature of the structure, as well as the material model of vanadium dioxide under different phase transition processes.

[0025] 4) Based on the mapping between the light input intensity and the required operating temperature of the structure obtained in step 3, and the vanadium dioxide material model, the transmittance of single-mode light under different phase transition processes of vanadium dioxide material at different ambient temperatures is calculated using the finite-difference time-domain method (commercial Lumerical FDTD software), thereby constructing a nonlinear mapping between the input and output of the unit as the activation function.

[0026] 5) Based on the calculation method in step 4, the algorithm iterates through the geometric parameters of vanadium dioxide, such as length, width, and height, so that the transmittance of the input light can show a large difference under different phase transition processes at the same operating temperature, while ensuring a certain transmittance to control the loss of the control unit. Thus, the "loop"-shaped nonlinear activation function obtained in step 4 meets the requirements, and the design of the nonlinear activation unit is completed.

[0027] Furthermore, the specific steps are as follows: In step 1: The operating wavelength λ of the device is selected based on the required signal carrier wavelength. The thickness and width of the silicon (Si) substrate and vanadium dioxide (SiO2) cladding layer are determined through simulation optimization under single-mode waveguide conditions. Based on the substrate width and thickness, the thickness and width range of the vanadium dioxide (VO2) layer are determined according to the optical coupling conditions. Generally, to meet the single-mode conditions, the width w > 200 nm, while also being greater than the width of the silicon substrate, and the thickness of the vanadium dioxide layer needs to be less than the thickness of the silicon substrate. The length range of the vanadium dioxide (VO2) layer is determined based on the optical propagation loss conditions, with the following specific conditions: The structural transmittance includes: Transmission loss needs to be controlled within a certain range, which can be expressed as: In formula (1) The transmission loss coefficient of this structure is related to the width and thickness of the silicon (Si) substrate and the vanadium dioxide (VO2) layer, where L is the length of the vanadium dioxide (VO2) layer. To realize equation (1), different lengths of the vanadium dioxide (VO2) layer need to be designed according to different widths and thicknesses of the silicon (Si) substrate and the vanadium dioxide (VO2) layer.

[0028] This embodiment describes an on-chip all-optical nonlinear activation unit based on the volatile thermal phase transition process of vanadium dioxide, and its structure is as follows: Figure 1As shown. To ensure single-mode transmission conditions, the substrate is silicon (Si) with a thickness of H1 = 220 nm and a width of W1 = 700 nm. The upper vanadium dioxide (VO2) layer has a thickness of H2 = 200 nm, a width of W2 = 300 nm, and a length of L = 1200 nm, used for input modulation using optical coupling and thermo-optical effects. The silicon substrate with structural parameters H1 and W1 exhibits good single-mode effects, allowing coupling of a large component of the TEO wave with the vanadium dioxide layer. The thickness and width of this vanadium dioxide layer can achieve 50% optical coupling, realizing the nonlinear effect caused by structural absorption. Due to the volatile phase transition process of vanadium dioxide in the temperature range of 300-360 K, the complex refractive index of the material model varies greatly. A reasonable geometric structure is key to ensuring the normal signal transmission function and nonlinear activation function of the structure. Under this geometric structure, sufficient transmittance is ensured to provide signal strength, while also ensuring a large variation in transmittance for different phase transition processes at the same operating temperature. The electromagnetic simulation software Lumercal FDTD solutions was used to simulate the signal transmission. Figure 1 Numerical simulations were performed on the unit structure shown. At the same wavelength of 1459 nm (a commonly used optical communication band), to simulate single-mode light incidence, the ambient temperature varied from 300 K to 360 K. This simulated the transformation of vanadium dioxide from a low-temperature insulating monoclinic phase (M1 phase), through an amorphous phase, to a high-temperature metallic rutile phase (R phase). The refractive index parameters of the vanadium dioxide material model needed to be adjusted, specifically from n=3.27, k=0.33 to n=1.7, k=3.15. Simultaneously, the refractive index of vanadium dioxide also needed to be adjusted according to different phase transformation processes. Specifically, for vanadium dioxide at 330 K, n=3.21, k=0.42 during heating, and n=2.7, k=1.75 during cooling. After setting the structural and material models, the electromagnetic simulation software Lumercal FDTD solutions was used to simulate... Figure 1 Numerical simulation of the unit structure shown can simulate the amplitude response distribution of light propagation in the waveguide under different temperatures and phase transition processes. For example... Figure 2 Figures (a), (b), and (c) show the transmission response of the yz cross section during heating at 300K, 315K, and 330K, respectively. This demonstrates the response process with different transmittances during the phase transition, yielding transmittances of 0.42, 0.25, and 0.36, which are represented in the figures as input light loss and the coupling ratio to the upper layer. Similarly, by uniformly sampling the temperature range from 300K to 360K and mapping the material parameters at the corresponding sampling points, the corresponding transmittance is obtained through FDTD simulation. Spline interpolation is then used to construct the relationship curve between the operating temperature and the transmittance of the corresponding vanadium dioxide material under different phase transition processes, as shown in the figure. Figure 3 As shown.

[0029] Next, based on the mapping between light input intensity and the required operating temperature of the structure, it is generally modeled as a linear mapping, that is, expressed as: In formula (2), T represents the operating temperature of the waveguide composite structure under light input, I represents the light input intensity, k represents the heating coefficient, and T0 represents the ambient operating temperature. Based on the light input intensity and the transmittance of vanadium dioxide material under different phase transition processes at operating temperatures mapped by different input intensities, the mapping relationship between the input and output of the structure is obtained, i.e., the "loop"-shaped nonlinear activation function implemented by this unit. This nonlinear activation function can be described by the Bouc-Wen model: Where A is the amplitude of the control function, and The shape of the function is controlled. When a sine function is chosen as the fixed input state function, different hysteresis loop-shaped functions can be described by the parameters of the Bouc-Wen model. By describing the obtained nonlinear activation function with the Bouc-Wen model and comparing it with the Bouc-Wen model parameters of the required nonlinear activation function, the optimization interval is set to 20nm with the above geometric structure parameters as the center. That is, the optimization interval of the upper vanadium dioxide (VO2) layer is H2=180-220nm, and the width is W2=280-320nm. Next, a traversal scan is performed with a step size of 1nm to obtain different nonlinear activation functions and corresponding Bouc-Wen model parameters for different geometric structures. The geometric parameters whose model parameters are closest to the requirements are selected as the final optimization result, and the corresponding nonlinear activation function is obtained, such as... Figure 4 As shown. The optimization result is the desired nonlinear activation unit.

[0030] The present invention proposes a design method for an on-chip all-optical nonlinear activation unit based on the volatile thermal phase transition process of vanadium dioxide. The proposed unit structure, which is a composite of a single-mode silicon waveguide and a vanadium dioxide layer, overcomes the problem of lack of nonlinearity in traditional optical computing, avoids the high energy consumption caused by electro-optic conversion, and realizes the function of an on-chip all-optical nonlinear activation unit. It is expected to be used in fields such as machine learning and autonomous driving that require high real-time performance and high computing power.

Claims

1. A design method for an on-chip all-optical nonlinear activation unit based on the volatile thermal phase transition process of vanadium dioxide, characterized in that, Includes the following steps: 1) Design waveguide composite structure; 2) Based on the phase transition characteristics of the vanadium dioxide layer and the nonlinear activation function requirements of different waveguide composite structures, determine the range of geometric structural parameters and the operating temperature range of the waveguide composite structure. 3) Based on the range of geometric parameters and the range of operating temperature obtained in step 2), design the mapping between the light input intensity and the required operating temperature of the waveguide composite structure, as well as the material model of vanadium dioxide under different phase transition processes. 4) Based on the mapping between the light input intensity and the required operating temperature of the waveguide composite structure obtained in step 3) and the material model of vanadium dioxide under different phase transition processes, the transmittance of the waveguide composite structure under different phase transition processes of the vanadium dioxide layer at different ambient temperatures is calculated using the finite difference time-domain method. Thus, a nonlinear mapping between the input and output of the waveguide composite structure is constructed as the activation function. 5) Describe the activation function obtained in step 4) using the Bouc-Wen model and compare it with the Bouc-Wen model corresponding to the design requirements of the waveguide composite structure. Determine the optimization interval based on the geometric parameters of the activation function obtained in step 4). Optimize the geometric parameters of the vanadium dioxide layer in the optimization interval using the finite-difference time-domain method until a nonlinear activation function that meets the requirements described by the Bouc-Wen model is completed. This completes the optimized design of the waveguide composite structure and yields the on-chip all-optical nonlinear activation unit.

2. The design method of the on-chip all-optical nonlinear activation unit based on the volatile thermal phase transition process of vanadium dioxide according to claim 1, characterized in that, In step 1), the waveguide composite structure includes: A silicon substrate, a vanadium dioxide layer disposed on the silicon substrate, and a silicon dioxide layer covering the silicon substrate and the vanadium dioxide layer.

3. The design method for an on-chip all-optical nonlinear activation unit based on the volatile thermal phase transition process of vanadium dioxide according to claim 1, characterized in that, In step 2), the phase transformation characteristics of the vanadium dioxide layer include: vanadium dioxide transforming from an insulating monoclinic phase to a metallic rutile phase.

4. The design method of the on-chip all-optical nonlinear activation unit based on the volatile thermal phase transition process of vanadium dioxide according to claim 1, characterized in that, In step 2), the nonlinear activation function requirements for different shapes of the waveguide composite structure include: all parameters required to describe the nonlinear activation function using the Bouc-Wen model.

5. The design method of the on-chip all-optical nonlinear activation unit based on the volatile thermal phase transition process of vanadium dioxide according to claim 1, characterized in that, In step 2), the range of geometric parameters and the operating temperature range of the waveguide composite structure specifically include: The silicon substrate conforms to the geometric parameters for single-mode conduction. The vanadium dioxide layer has a length in the range of 600-1500nm, a width in the range of 200-700nm, and a height in the range of 200-300nm. The operating temperature range is from 300-360K.

6. The design method of the on-chip all-optical nonlinear activation unit based on the volatile thermal phase transition process of vanadium dioxide according to claim 1, characterized in that, In step 3), the mapping between the optical input intensity and the required operating temperature of the waveguide composite structure is as follows: the operating temperature of the waveguide composite structure and the optical input intensity satisfy a linear relationship.

7. The design method of the on-chip all-optical nonlinear activation unit based on the volatile thermal phase transition process of vanadium dioxide according to claim 1, characterized in that, In step 5), the finite-difference time-domain method optimizes the geometric parameters of the vanadium dioxide layer within the optimization interval until a nonlinear activation function that meets the requirements of the Bouc-Wen model is obtained, thus completing the optimized design of the waveguide composite structure. Specifically, this includes: The algorithm iterates through the length, width, and height within the range of the preset geometric parameters of the vanadium dioxide layer to complete the nonlinear activation function described by the Bouc-Wen model that meets the requirements, thus completing the optimized design of the waveguide composite structure.

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