Coherent longitudinal optical phonon and plasmon coupling in the near-surface region of InN

Coherent phonon spectroscopy of a high-quality InN epitaxial layer is carried out using time-resolved second-harmonic generation. Only coherent longitudinal optical phonon and plasmon coupling mode at 447 cm$^{-1}$ can be resolved in the spectrum. Its frequency shows no dependence on the photoinjected carrier density up to 1.5$\times $10$^{19}$ cm$^{- 3}$. This phenomenon is attributed to the hybridization of coherent A$_{1} $(LO) phonon with the intrinsic cold plasma accumulated in the near-surface region of InN, where the plasma density could reach the order of 10$^{20} $ cm$^{-3}$, much higher than the bulk carrier concentration, 1$\times $10$^ {18}$ cm$^{-3}$, determined by Hall effect measurement..