Ultrahigh-Q Fano resonance using topological corner modes in second-order pseudospin-Hall photonic systems

Abstract

Topological corner states have significantly narrow spectral widths due to their nature as the states with single-valued eigenfrequencies independent on the wavenumber. This report reveals that ultrahigh-Q Fano resonances can be obtained in photonic crystal structures supporting topologically protected corner states by their coherent coupling, taking pseudospin-Hall second-order photonic topological insulators as examples. For observing such resonances by external excitation, we additionally use a topological edge waveguide coupled to the cavity supporting the corner states. The resultant Fano resonances have Q-factors in the order of 104 ∼ 105. The system exhibits the index sensitivity of about 180 nm/RIU and the figure-of-merit in the order of 104, enabling applications for index sensing with ultrahigh sensitivity. The results also show that all-optical switching with near unity contrast can be realized by using the pump with moderate intensity lower than 20 GW/cm2 in pseudospin-Hall photonic silicon-on-insulator slab structures. Due to the ultrahigh Q-factor and topologically protected nature of all photonic states, the Fano resonances using topological states have a great potential for index sensing and all-optical switching, which are almost not affected by structural disorder and defects inevitably created in fabrication processes.