Cryogenic optical bistability in 1D topological photonic structures for reconfigurable THz devices

Topological photonics offers robust light guiding, but achieving ultra-low-power nonlinear functionality in the terahertz band remains a challenge. We theoretically demonstrate optical bistability and hysteresis in a one-dimensional topological photonic crystal made from alternating layers of moderately doped silicon and the strongly Kerr-nonlinear polymer polydiacetylene. Two constituent photonic crystals with opposite Zak phases are interfaced to generate a topologically protected edge state that amplifies light–matter interaction and drastically lowers the nonlinear threshold. Using a transfer-matrix framework that incorporates the temperature-dependent Drude permittivity of m-Si and the third-order susceptibility of 9-BCMU, we identify a TES at ≈0.973THz. This mode exhibits a polarization-selective blueshift (TE:0.973→1.061THz, TM:0.973→1.164THz) with angle of incidence, a thermal blueshift from 0.966 to 0.977 THz as the device is warmed from 20 to 70 K, and a Kerr-induced redshift to 0.956 THz when the input power increases from 0.1 to 9 mW. These frequency shifts are accompanied by pronounced hysteresis loops in the reflection spectrum and power-dependent field localization, confirming bistable operation at milliwatt-level thresholds. The interplay of topology, thermo-optic dispersion, and Kerr nonlinearity enables dynamic, all-optical switching and memory while preserving topological resilience against disorder. Our findings demonstrate that this one-dimensional heterostructure serves as a scalable, cryo-compatible platform for reconfigurable THz photonics, enabling energy-efficient modulators, logic components, sensors, and quantum-enabled devices that harness the distinct benefits of topological edge states.