Optically reconfigurable quantum spin-valley Hall effect of light in coupled nonlinear ring resonator lattice

Scattering immune propagation of light in topological photonic systems may revolutionarize the design of integrated photonic circuits for information processing and communications. In optics, various photonic topological circuits have been developed, which were based on classical emulation of either quantum spin Hall effect or quantum valley Hall effect. On the other hand, the combination of both the valley and spin degrees of freedom can lead to a new kind of topological transport phenomenon, dubbed quantum spin valley Hall effect (QSVH), which can further expand the number of topologically protected edge channels and would be useful for information multiplexing. However, it is challenging to realize QSVH in most known material platforms, due to the requirement of breaking both the (pseudo-)fermionic time-reversal (T) and parity symmetries (P) individually, but leaving the combined symmetry S=TP intact. Here, we propose an experimentally feasible platform to realize QSVH for light, based on coupled ring resonators mediated by optical Kerr nonlinearity. Thanks to the inherent flexibility of cross-mode modulation (XMM), the coupling between the probe light can be engineered in a controllable way such that spin-dependent staggered sublattice potential emerges in the effective Hamiltonian. With delicate yet experimentally feasible pump conditions, we show the existence of spin valley Hall induced topological edge states. We further demonstrate that both degrees of freedom, i.e., spin and valley, can be manipulated simultaneously in a reconfigurable manner to realize spin-valley photonics, doubling the degrees of freedom for enhancing the information capacity in optical communication systems.