Photonic crystal nanocavities that simultaneously possess small modal volumes and high quality (Q) factors have opened up novel research areas in photonics during this decade. Here, we present an important key for the increase of Q factors to ranges beyond ten million. A systematic investigation on photon lifetimes of air-bridge-type heterostructure nanocavities fabricated from silicon on insulator (SOI) substrates indicated the importance of cleaning the bottom side (buried oxide side) of the nanaocavites. Repeated thermal oxidation and an oxide removal process applied after the removal of the buried oxide layer underneath the nanocavities realized an experimental Q factor greater than eleven million, which is the highest experimental Q ever recorded. The results provide important information not only for Si PC nanocavities but also for general Si nanophotonic devices and photonic electronic convergence systems.

The optical gain properties of InGaN-based green (512 nm) laser diodes fabricated on (0001) GaN substrates are investigated. Fitting simulations to the experimental gain spectra provides a Gaussian inhomogeneous broadening of 95 meV, an optical confinement factor of 0.006, and an internal loss as low as 10/cm. The remarkable suppression of inhomogeneous broadening and internal loss compensate for the low optical confinement, leading to a low threshold current density of 2.75 kA/cm 2 . The suppressed inhomogeneity contributes to the highly linear gain increase with the injection carrier, while the low optical confinement results in a relatively low differential mode gain.