We investigate photoluminescence (PL) spectra and dynamics of clean silicon photonic crystal nanocavities at 10 K. A sharp emission peak due to the nanocavity mode has the largest intensity when the energy of the nanocavity mode is equal to the emission energy of the electron-hole droplets (EHDs). Time-resolved PL spectroscopy indicates that the PL lifetime of the EHD is reduced to as short as 1.2 ns by the nanocavity mode. A careful analysis of the lifetimes indicates that the radiative recombination rate for EHD is enhanced by a factor of larger than 5 by the Purcell effect.

doi:10.1103/PhysRevB.96.035303