Conventional light-confinement mechanisms such as reflection and total internal reflection degrade dramatically with decreasing device size, resulting in an exponential drop in the quality (Q) factor. This is a major obstacle to realizing wavelength- and subwavelength-scale photonic systems. Yet recent advances in meta-photonics and materials science can overcome this problem.

In general, resonances in open systems lose energy when coupled to external radiative channels. Once waves along different channels interact one another, the radiative leakage can be destructively suppressed and the Q factor is enhanced dramatically. This corresponds to a well-known phenomenon, bound states in the continuum (BICs).

Topology—the branch of mathematics concerned with quantities that are preserved under continuous deformations—has emerged as a new perspective animating various fields of study. In the photonic regime, topological properties can be described by band dispersion in the 2D Brillouin zone. Topological insulators can explain awide range of materials with a variety of symmetries, including time-reversal symmetry, spatial symmetries such as reflection and inversion, and crystalline symmetry. 

A topological deformation is the next stage. The deformation of the host lattice characterized by real-space topology cannot be eliminated by local continuous transformation. Dislocation, disclination, moiré lattice, and Dirac vortex structures are representative topological deformations of the host lattice. Topological deformations can presents a exotic beam shape, such as vector vortex lasing in a photonic disclination, polarization vortex modes in a Dirac vortex structure, and magic-angle lasing in a moiré photonic lattice.

The ultimate smallest possible laser has long been a topic of central interest in laser and quantum optics. Recently gained renewed attention with the aid of the photonic crystal that enables strong photon confinement. The photonic crystal cavity has a lot of merit as an efficient light emitter compared with the other types of microcavities: one can easily control distinct lasing properties such as wavelength, radiation direction, and near/far mode shapes by slight modification of lattice parameters. Also, one can obtain a high quality factor and small modal volume simultaneously, which are the two main conditions of low-threshold lasers. Since this small cavity supports only a few resonant modes, the ultimate thresholdless laser with a large spontaneous emission factor near unity can be actualized. In addition, once the issue of coupling with a photonic crystal waveguide and/or a commercial tapered fiber is solved, photons extracted from the extremely small cavity can be efficiently utilized. Furthermore, as a strong candidate of a single photon generator on chip, this ultrasmall cavity with a high Purcell factor has attracted many research groups.