Compact reconfigurable polarization converter based on sub-wavelength inclined phase change grating

Abstract

The compact reconfigurable polarization converter is characterized in that the compact reconfigurable polarization converter comprises a ridge-type silicon waveguide, and the upper surface of the ridge part of the ridge-type silicon waveguide is provided with a periodic sub-wavelength nano grating structure which is used for regulating and controlling the effective refractive index of transverse electric wave and transverse magnetic wave modes and has an inclination angle; the nanometer grating structure is made of Sb2Sb3 or Sb2S3 phase change materials, the nanometer grating structure is composed of a plurality of nanometer antennas which are the same in size and shape and parallel to one another, gaps of the nanometer grating structure are filled with SiO2 materials, an Al2O layer and a graphene heating layer are sequentially arranged on the upper surface of the nanometer grating structure and the upper surface of the SiO2 filling materials from bottom to top, and the graphene heating layer is arranged on the lower surface of the nanometer grating structure. And metal electrode palladium gold is arranged on the upper surface of the graphene heating layer and located on the two sides of the flat plate part of the ridge-type silicon waveguide. The device has the advantages that the function is reconfigurable, the size is smaller, the broadband is wider, and the power consumption is lower.

Description

Technical Field

[0001] The invention relates to the field of microelectronics, and in particular to a compact reconfigurable polarization converter based on a sub-wavelength tilted phase-change grating. Background Art

[0002] Photonic integrated circuits (PICs) are expected to break through the bottleneck of traditional integrated circuit transmission speed. PICs have the advantages of fast transmission speed, low signal loss, and wide operating bandwidth. With these advantages, PICs have great development potential in high-speed data computing and processing and ultra-large capacity communication network applications. Most photonic integrated devices are based on four types of platforms: silicon on insulator (SOI), III-V materials, lithium niobate, and silicon nitride, among which the SOI platform is the first choice for large-scale integration. However, due to the birefringence effect of SOI waveguides, light in different polarization states exhibits different characteristics during propagation, resulting in silicon-based photonic devices being very sensitive to the polarization state of the propagating signal. A typical solution to this problem is to use a polarization diversity circuit consisting of a polarization beam splitter and a polarization converter. This configuration ensures that the mode in the waveguide can operate in a single polarization state, which can well solve the problem of the device's sensitivity to the polarization state. The polarization converter is also a key part of polarization multiplexing. Currently, polarization converters mostly break the waveguide symmetry and achieve polarization conversion through mode evolution. However, the asymmetric structure limits the free conversion between modes, and the device function is not adjustable. At the same time, a larger size is required to complete the mode conversion, which is not conducive to integration. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a compact reconfigurable polarization converter based on a subwavelength tilted phase change grating, which has reconfigurable functions, smaller size, wider bandwidth and lower power consumption.

[0004] The technical solution adopted by the present invention to solve the above technical problems is: a compact reconfigurable polarization converter based on a subwavelength tilted phase change grating, comprising a ridged silicon waveguide, the upper surface of the ridge portion of the ridged silicon waveguide is provided with a periodic subwavelength nano-grating structure with a tilt angle for regulating the effective refractive index of the transverse electric wave (TE) and transverse magnetic wave (TM) modes, and the nano-grating structure is made of Sb2Sb3 or Sb2S3 phase change material. The phase change material has at least two stable states, namely, a crystalline state and an amorphous state. In these two states, the refractive index of the phase change material is significantly different, which is the key to the device being able to realize the reconfigurable function. The grating is set at a tilt angle to achieve a zero difference in the effective refractive index of the TE mode and the TM mode.

[0005] Furthermore, the nano-grating structure is composed of a plurality of nano-antennas of the same size and shape and parallel to each other, the nano-antennas are parallelograms, the inclination angle of the nano-antennas is 30 to 70 degrees, and the two ends of the nano-antennas are flush with the edges of the ridge portion of the ridge-type silicon waveguide.

[0006] Furthermore, the gaps in the nano-grating structure are filled with SiO2 material.

[0007] Furthermore, the upper surfaces of the nano-grating structure and the SiO2 filling material are sequentially provided with an Al2O layer for heat transfer and a graphene heating layer for heating the nano-grating structure from bottom to top, so that the phase change material undergoes a reversible phase change between a crystalline state and an amorphous state.

[0008] Furthermore, metal electrodes palladium (Pd) are provided on the upper surface of the graphene heating layer and on both sides of the flat plate portion of the ridge-type silicon waveguide.

[0009] Furthermore, the ridge silicon waveguide is located on a SiO2 substrate, allowing TE mode and TM mode transmission.

[0010] Furthermore, the thickness of the SiO2 substrate is 2-5μm; the width of the ridge portion of the ridge silicon waveguide is 0.6-1.2μm, the thickness of the ridge portion of the ridge silicon waveguide is 170-340nm, and the thickness of the slab portion of the ridge silicon waveguide is 50-200nm; the period of the phase change grating structure is 200-500nm and the thickness is 100-200nm, the ratio of the lateral length of the ridge silicon waveguide to the ridge portion width of the ridge silicon waveguide is in the range of 0.5-1, and the duty cycle is 0.4-0.7; the thickness of the Al2O layer is 10-20nm; the thickness of the graphene heating layer is 2-3nm.

[0011] Compared with the prior art, the advantages of the present invention are: the present invention is a compact reconfigurable polarization converter based on a sub-wavelength tilted phase change grating, which can flexibly produce different control effects on the TE mode and the TM mode by using a tilted grating, and compared with the Bragg grating, the tilted grating greatly reduces the Bragg reflection and achieves low loss. The structure can control the temperature during the crystallization process, realize the uniform reversal of the crystal phase of the phase change material, thereby avoiding the power loss problem caused by partial crystallization, and the device uses a graphene heater. Since graphene has high thermal conductivity and low specific heat capacity, the phase change material heats up quickly, which speeds up the switching speed of the device, making the crystallization time 100ns and the amorphization time 55ns, so that the phase change material can quickly achieve a reversible phase change between the crystalline state and the amorphous state under heating.

[0012] In summary, the present invention proposes a reconfigurable polarization converter based on Sb2Se3 and Sb2S3 tilted gratings. By designing a grating with a tilt angle on the upper surface of the waveguide, the effective refractive index of the TM mode can be freely adjusted, so that the TE mode and the TM mode can achieve equal effective refractive index. In addition, the loss of the device in all states is less than 1.5dB, achieving a working bandwidth of 100nm. The device size is designed within 18um, with the advantages of small size, high polarization extinction ratio, low loss and wide working bandwidth. Reconfigurable and multifunctional optoelectronic devices with ultra-compact size, fast switching speed and low power consumption can be developed. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 Schematic diagram of the structure of a compact reconfigurable polarization converter based on a subwavelength tilted phase-change grating, where (a) is a schematic diagram of the three-dimensional structure, (b) is a schematic diagram of the position of the phase-change grating on the waveguide, and the graphene heating layer above is not drawn for the convenience of representing the grating, (c) is a schematic diagram of the cross-section at the position Ⅰ-Ⅲ in the corresponding figure (b), and (d) is a top view of the device without the graphene heating layer;

[0014] Figure 2 The cross-sectional electric field intensity distribution of the polarization converter at the input port, the center of the device, and the output port in the input TE0 mode, where (a) is the crystalline state and (b) is the amorphous state;

[0015] Figure 3 In the figure, (a) is the device electric field component in the "OFF" state when TE0 is input in the amorphous state, (b) is the device electric field component in the "OFF" state when TM0 is input in the amorphous state; (c) is the device electric field component in the "ON" state when TE0 is input in the crystalline state, (d) is the device electric field component in the "ON" state when TM0 is input in the crystalline state;

[0016] Figure 4 In the figure, (a) is the polarization extinction ratio of the crystalline and amorphous states at room temperature, and (b) is the additional loss of the crystalline and amorphous states at room temperature;

[0017] Figure 5 Among them, (a) is the temperature response of the crystallization process of Sb2Se3 under 1V~170ns electric pulse, (b) is the temperature response of the amorphization process of Sb2Se3 under 3V~55ns electric pulse, (c) is the two-dimensional cross-sectional temperature distribution of Sb2Se3 crystalline state simulating Joule heating, (d) is the two-dimensional cross-sectional temperature distribution of Sb2Se3 amorphous state simulating Joule heating;

[0018] Figure 6 are the polarization conversion efficiencies of the polarization converter when the input modes are TE0 and TM0 respectively. DETAILED DESCRIPTION

[0019] The present invention is further described in detail below with reference to the accompanying drawings. Specific embodiment 1

[0021] like Figure 1 As shown in (a)-(d), a ridge Si waveguide is constructed on a SiO2 substrate. A subwavelength phase change grating with an inclined angle is arranged on the upper surface of the ridge Si waveguide. The temperature of the device can be controlled by applying an electric pulse to make the grating undergo a reversible phase change from crystalline to amorphous. The material of the phase change grating structure is selected as Sb2Se3. The nano-grating structure is composed of 48 nano-antennas of the same size and shape and parallel to each other. The nano-antennas are parallelograms, and the two ends of the nano-antennas are flush with the edges of the ridge part of the ridge silicon waveguide. The gap of the nano-grating structure is filled with SiO2 material. The upper surface of the nano-grating structure and the SiO2 filling material is sequentially provided with an Al2O layer for heat transfer and a graphene heating layer for heating the nano-grating structure from bottom to top, so that the phase change material undergoes a reversible phase change from crystalline to amorphous. Metal electrodes palladium (Pd) are arranged on the upper surface of the graphene heating layer and on both sides of the flat plate part of the ridge silicon waveguide.

[0022] like Figure 1 As shown in (b) and (d), the width of the SiO2 substrate is 4μm and the thickness is 4μm; the width of the ridge part of the ridge Si waveguide is 0.72μm, the thickness of the ridge part of the ridge Si waveguide is 220nm, and the height of the slab part of the ridge Si waveguide is 154nm; the period of the phase change grating structure is 347nm, the height is 180nm, the grating width is 110nm, and the grating tilt angle is 57°; the thickness of the Al2O layer is 20nm, and the thickness of the graphene heating layer is 3nm. The ratio of the lateral length of the ridge silicon waveguide to the width of the ridge part of the ridge silicon waveguide ranges from 0.5 to 1, and the duty cycle is 0.4 to 0.7. The phase change grating structure has at least two stable states, namely, crystalline and amorphous states. Under the action of the graphene heater, a reversible phase change from crystalline to amorphous state occurs, completing the phase reversal. In addition, by precisely controlling the phase change temperature to achieve different crystallinity, different mode purities can be output, and multi-level adjustability can be achieved.

[0023] Table 1 Physical parameters of each material

[0024]

[0025] As shown in Table 1, the high refractive index contrast of Sb2Se3 in the crystalline state (aSb2Se3) and the amorphous state (cSb2Se3) makes the device transmit signals in the waveguide differently in the two states. In the crystalline state, due to the large refractive index contrast between Sb2Se3 and Si, the grating will have a significant impact on light transmission, while in the amorphous state, the refractive index contrast between Sb2Se3 and Si is small, and the impact on light transmission is negligible. Combining phase change materials with tilted gratings reduces the size of the device and achieves reconfigurability. This tilted grating structure is completely placed on the silicon waveguide.

[0026] 2. Results Analysis

[0027] Figure 1 (a) is a schematic diagram of the three-dimensional structure of the reconfigurable polarization converter with a graphene heater. Figure 1 (b) is a schematic diagram of the three-dimensional structure of the reconfigurable polarization converter without the graphene heater, which is convenient for indicating the position of the tilted grating. The simulation software Finite-Difference Time-Domain (FDTD) from Lumerical Solutions was used to construct a three-dimensional device model for simulation optimization. Figure 1 (b) It can be seen that the grating is composed of multiple nanoantennas, and the two ends of the antenna are flush with the edge of the waveguide. The device can achieve polarization state conversion mainly because the tilted grating can achieve regulation of the finite refractive index of the mode by changing the tilt angle. In the present invention, the tilted grating is set above the waveguide, which will change the propagation path of the optical signal in the y direction. Therefore, the effective refractive index of the TE mode with polarization direction in the y direction will be significantly affected, while the effective refractive index of the TM mode with polarization direction in the z direction will not change significantly with the change of angle. According to this principle, the grating tilt angle can be changed to achieve the effective refractive index difference between the TE mode and the TM mode is 0, and the two modes meet the conditions for mode coupling. In addition, the subwavelength phase change grating introduces periodic medium disturbances, which promotes energy exchange between modes.

[0028] Figure 2 The electric field distribution at each position on the waveguide and the polarization direction of the mode are shown. Figure 2 As shown in (a), in the crystalline state, mode conversion has an evolutionary process. Due to the introduction of perturbations, mode coupling occurs in the grating loading part. As the propagation distance increases, energy is continuously exchanged between the TE0 mode and the TM0 mode, and finally the conversion is completed and the TM0 mode is output. On the contrary, Figure 2 As shown in (b), in the amorphous state, there is little coupling between the two modes, and the conversion ratio of TE0 to TM0 is extremely small. The modal distribution is not affected, and TE0 is still the output mode.

[0029] Figure 3 The electric field components of the phase change material in the amorphous and crystalline states when the TE0 and TM0 modes are input are shown. The black dashed line is the grating position. In the crystalline state ("ON" state) of Sb2Se3, the function of TE0-TM0 (TM0-TE0) polarization state conversion is realized. In the amorphous state (OFF state) of Sb2Se3, the waveguide is in a normal working state (TE0-TE0 / TM0-TM0). The primary electric field component in the TE mode is E y , the primary electric field component in TM mode is E z , so we can judge the mode conversion situation by observing the two electric field components. Figure 3 As shown in (a) and (b), the phase change material is in an amorphous state, the device is in the "OFF" state, and the electric field component remains unchanged. Figure 3 As shown in (c) and (d), the phase change material is in a crystalline state and the device is in the "ON" state to complete the mode conversion. Under input TE0, E y The electric field component is significantly reduced when passing through the grating area, which is converted into E z Electric field component. Under input TM0, E z The electric field component is significantly reduced when passing through the grating area and is converted into E under input TM0. y Electric field component. Phase change materials achieve effective mode conversion in the crystalline state.

[0030] Figure 4 The polarization extinction ratio and excess loss of the crystalline and amorphous states at room temperature are shown. Figure 4 As shown in (a), the device has the largest polarization extinction ratio near 1550nm. Figure 4 As shown in (b), the additional loss of the converter is less than 1.5 dB in the range of 1500 nm-1600 nm, indicating that the present invention achieves low loss.

[0031] Figure 5 (a) is the temperature change curve of the Sb2Se3 crystallization process under the action of an electric pulse with an amplitude of 1V and a duration of 170ns. Figure 5 (b) is the temperature change curve of the amorphization process of Sb2Se3 under the action of an electric pulse with an amplitude of 3V and a duration of 55ns. Through thermal simulation, we can find that the highest temperature (<1200K) generated during the phase change of Sb2Se3 is significantly lower than the melting temperature of silicon, silicon dioxide, graphene and Al2O3, ensuring that the device is not damaged during the phase change process and the required power consumption is also very low.

[0032] Figure 5(c) and (d) show the heat distribution on the waveguide cross section at 170ns during the crystallization process and 55ns during the amorphization process. It can be seen from the figure that during the heating process, the temperature of the phase change material rises faster than that of the waveguide.

[0033] Figure 6 The polarization conversion efficiency of the device is described when the input modes are TE0 and TM0 respectively, and the highest conversion efficiency can reach 99.99%.

[0034] In summary, the device of the present invention combines phase change material with tilted grating, realizes the controllability of device function, improves the utilization rate of the device, and at the same time, the tilted grating alleviates the reflection effect of the sub-wavelength grating and reduces the loss of the device. Applying electric pulses to control the multi-level phase state of phase change materials through Joule heating is expected to be applied to optical convolution calculations.

[0035] The above description is not intended to limit the present invention, and the present invention is not limited to the above examples. Any changes, modifications, additions or substitutions made by a person skilled in the art within the spirit and scope of the present invention shall also fall within the protection scope of the present invention.

Claims

1. A compact reconfigurable polarization converter based on a subwavelength tilted phase-change grating, comprising a ridged silicon waveguide, characterized in that: The upper surface of the ridge portion of the ridge-type silicon waveguide is provided with a periodic subwavelength nano-grating structure with an inclined angle for regulating the effective refractive index of transverse electric wave and transverse magnetic wave modes. The nano-grating structure is made of Sb2Sb3 or Sb2S3 phase change material.

2. A compact reconfigurable polarization converter based on a sub-wavelength tilted phase-change grating according to claim 1, characterized in that: The nano grating structure is composed of a plurality of nano antennas of the same size and shape and parallel to each other. The nano antennas are parallelograms, the inclination angle of the nano antennas is 30 to 70 degrees, and the two ends of the nano antennas are flush with the edges of the ridge portion of the ridge silicon waveguide.

3. The compact reconfigurable polarization converter based on sub-wavelength tilted phase-change grating according to claim 1, characterized in that: The gaps in the nano-grating structure are filled with SiO2 material.

4. The compact reconfigurable polarization converter based on sub-wavelength tilted phase change grating according to claim 1, characterized in that: The upper surfaces of the nano-grating structure and the SiO2 filling material are sequentially provided with an Al2O layer for heat transfer and a graphene heating layer for heating the nano-grating structure from bottom to top.

5. The compact reconfigurable polarization converter based on sub-wavelength tilted phase change grating according to claim 1, characterized in that: Metal electrodes palladium gold are arranged on the upper surface of the graphene heating layer and on both sides of the flat plate portion of the ridge-type silicon waveguide.

6. A compact reconfigurable polarization converter based on a sub-wavelength tilted phase-change grating according to any one of claims 1 to 5, characterized in that: The ridge-type silicon waveguide is located on a SiO2 substrate.

7. The compact reconfigurable polarization converter based on sub-wavelength tilted phase-change grating according to claim 6, characterized in that: The thickness of the SiO2 substrate is 2-5 μm; the width of the ridge portion of the ridge silicon waveguide is 0.6-1.2 μm, the thickness of the ridge portion of the ridge silicon waveguide is 170-340 nm, and the thickness of the slab portion of the ridge silicon waveguide is 50-200 nm; the period of the phase change grating structure is 200-500 nm and the thickness is 100-200 nm, the ratio of the lateral length of the ridge silicon waveguide to the ridge portion width of the ridge silicon waveguide is in the range of 0.5-1, and the duty cycle is 0.4-0.7; the thickness of the Al2O layer is 10-20 nm; the thickness of the graphene heating layer is 2-3 nm.

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