  Miniaturized and rationally assembled nanostructures exhibit extraordinarily
distinct physical properties beyond their individual units. This review will
focus on structured small-scale optical cavities that show unique
electromagnetic near fields and collective optical coupling. By harnessing
different material systems and structural designs, various light-matter
interactions can be engineered, such as nanoscale lasing, nonlinear optics,
exciton-polariton coupling, and energy harvesting. Key device performance of
nanoscale lasers, including low power threshold, optical multiplexing, and
electrical pump, will be discussed. This review will also cover emerging
applications of nanoscale optical cavities in quantum engineering and
topological photonics. Structured nanocavities can serve as a scalable platform
for integrated photonic circuits and hybrid quantum photonic systems.
