Room-temperature lasing from nanophotonic topological cavities.

The study of topological phases of light underpins a promising paradigm for engineering disorder-immune compact photonic devices with unusual properties. Combined with an optical gain, topological photonic structures provide a novel platform for micro- and nanoscale lasers, which could benefit from nontrivial band topology and spatially localized gap states. Here, we propose and demonstrate experimentally active nanophotonic topological cavities incorporating III–V semiconductor quantum wells as a gain medium in the structure. We observe room-temperature lasing with a narrow spectrum, high coherence, and threshold behaviour. The emitted beam hosts a singularity encoded by a triade cavity mode that resides in the bandgap of two interfaced valley-Hall periodic photonic lattices with opposite parity breaking. Our findings make a step towards topologically controlled ultrasmall light sources with nontrivial radiation characteristics. Active topological cavities that can lase at room temperature could bring new opportunities for controlling light in integrated nanophotonic circuits. Smirnova et al. etched a special pattern of nanoscale holes into a 250 nm thick slab of the compound semiconductor InGaAsP to create a triangle-shaped cavity with optical behaviour governed by the band topology. The presence of quantum wells in the slab provides the cavity with optical gain allowing it to lase in the near-infrared when excited with nanosecond pump pulses at 980 nm. The laser emission is observed to be spectrally narrow with high coherence and hosts a donut-shaped singularity in Fourier space. The achievement opens the way to a new type of nanophotonic light source which exhibits unusual radiation characteristics and suits integration with metasurfaces.