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. A coherent longitudinal optical phonon and plasmon coupling mode only at 447cm−1 can be resolved in the spectrum. Its frequency shows no dependence on the photoinjected carrier density up to 1.5×1019cm−3. This phenomenon is attributed to the hybridization of a coherent A1(LO) phonon with the intrinsic cold plasma accumulated in the near-surface region of InN, where the plasma density could reach on the order of 1020cm−3, much higher than the bulk carrier concentration 1×1018cm−3 determined by Hall effect measurement.