Strong coupling of photons and materials 1 in semiconductor nanocavity systems has been investigated because of its potentials in quantum information processing 2 and related applications, and has been testbeds for cavity quantum electrodynamics (QED) 3,4 . Interesting phenomena such as coherent exchange of a single quantum between a single quantum dot and an optical cavity, called vacuum Rabi oscillation 5-9 , and highly efficient cavity QED lasers 10-18 have been reported thus far.The coexistence of vacuum Rabi oscillation and laser oscillation appears to be contradictory in nature, because the fragile reversible process may not survive in laser oscillation. However, recently, it has been theoretically predicted that the strong-coupling effect could be sustained in laser oscillation in properly designed semiconductor systems 19 . Nevertheless, the experimental realization of this phenomenon has remained difficult since the first demonstration of the strong-coupling 5, 6 , because an extremely high cavity quality factor and strong light-matter coupling are both required for this purpose. Here, we demonstrate the onset of laser oscillation in the strong-coupling regime in a single quantum dot (SQD)-cavity system. A high-quality semiconductor optical nanocavity and strong SQD-field coupling enabled to the onset of lasing while maintaining the fragile coherent exchange of quanta between the SQD and the cavity. In addition to the interesting physical features, this device is seen as a prototype of an ultimate solid state light source with an SQD gain, which operates at ultra-low power, with expected applications in future nanophotonic integrated systems and monolithic quantum information devices.

Topological phonics has emerged as a novel approach to engineer the flow of light and provides unprecedented means for developing diverse photonic elements, including robust optical waveguides immune to structural imperfections. However, the development of nanoscale standing-wave cavities in topological photonics is rather slow, despite its importance when building densely-integrated photonic integrated circuits. In this Letter, we report a photonic crystal nanocavity based on a topological corner state, supported at a 90-degrees-angled rim of a two dimensional photonic crystal.A combination of the bulk-edge and edge-corner correspondences guarantees the presence of the higher-order topological state in a hierarchical manner. We experimentally observed a corner mode that is tightly localized in space while supporting a high Q factor over 2,000, verifying its promise as a nanocavity. These results cast new light on the way to introduce nanocavities in topological photonics platforms.

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Topological edge states exist at the interfaces between two topologically-distinct materials.The presence and number of such modes are deterministically predicted from the bulk-band topologies, known as the bulk-edge correspondence 1 . This principle is highly useful for predictably controlling optical modes 2-5 in resonators made of photonic crystals (PhCs), leading to the recent demonstrations of micro-scale topological lasers 6-10 .Meanwhile, zero-dimensional topological trapped states in the nanoscale remained unexplored, despite its importance for enhancing light-matter interactions and for wide applications including single-mode nanolasers. Here, we report a topological PhC nanocavity with a near-diffraction-limited mode volume and its application to single-mode lasing. The topological origin of the nanocavity, formed at the interface between two topologically-distinct PhCs, guarantees the existence of only one mode within its photonic bandgap 11 . The observed lasing accompanies a high spontaneous emission coupling factor stemming from the nanoscale confinement [12][13][14][15] . These results encompass a way to greatly downscale topological photonics 2-5 .