A Photonic Crystal Waveguide Intersection Using Phase Change Material for Optical Neuromorphic Synapses

This paper introduces an innovative photonic crystal device that amalgamates a low cross-talk GaAs waveguide intersection with a ring employing phase change material (PCM) for application in all-optical neuromorphic synapses. The device uses a degenerate optical cavity housing Germanium-Antimony-Telluride (GST) phasechange material whose optical properties can be manipulated via a control signal. The proposed design facilitates the development of a non-volatile synapse for photonic neural networks and optical neuromorphic circuits. Numerical simulations using the finite difference time domain method (FDTD) exhibit a notably high-quality factor of 900 and a minimal cross-talk level of -60 dB at the intersection of two waveguides. To validate the FDTD results, the structure undergoes further simulations using the finite element method (FEM), confirming the accuracy of the initial calculations. The resultant structure achieves transmissions of 81 % and 13 % in the amorphous and fully crystalline states of PCM, respectively, enabling low output power in the crystalline state and high output power in the amorphous state. Moreover, modulation of the transmission coefficient between 13 % and 81 % is feasible by manipulating the crystallization coefficient of GST materials. To reduce the effect of the imaginary part of the refractive index of the GST material, the evaluation of the output transmission results for the GSST material (Ge2Sb2Se4Te1) has been performed. The results revealed that, despite having a lower imaginary part of the refractive index compared to GST, GSST exhibits lower absorption in both amorphous and crystalline states. However, the use of GSST materials shows a 27 % reduction in the figure of merit (FOM), indicating a lower effective range and fewer degrees of freedom in the different crystallization levels of the PCM ring. To investigate the effect of fabrication defects and structural sensitivity, the radius of photonic crystal rods and the rings of GaAs and GST have been changed, and the results indicate that the radius of the photonic crystal rods and the GST ring have negligible effects on the output transmission and resonant wavelength. However, the thickness of the GaAs ring shows a high sensitivity, which leads to a change in the resonance wavelength that is utilized as an effective tool for tuning the pass wavelength through the structure within the 171 nm bandgap range. The transmission loss in the amorphous state is -0.6 dB and in the fully crystalline state -7.5 dB, which is attributed to the absorption of GST material. This proposed photonic crystal synapse structure with an area of less than 30 mu m2 significantly minimizes the required space compared to similar silicon photonic structures and increases the prospects for integration and scalability. The compatibility of the materials employed with optical fibre communications at the 1310 nm wavelength further bolsters the practical feasibility of this design. In summary, this paper presents a significant advancement in the domain of neuromorphic photonics by introducing a promising structure with substantial potential for neural network synapses.