Active Tuning of Phonons and Surface-Phonon Polariton Resonances

The infrared spectra of many polar semiconductors are dominated by highly reflective reststhralen bands that occur between the transverse and longitudinal optical phonons. Through the LOPC effect, free carriers shift the reststrahlen band to higher frequencies. We have previously shown that photoinjected carriers transiently and reversibly modify the infrared reflectivity of bulk SiC. Within the reststrahlen band, SiC and InP nanostructures can exhibit surface-phonon polariton resonances. Here we report, for the first time, active tuning of SiC and InP surface-phonon polariton resonances via carrier photoinjection, achieving better modulation depths than active tuning in plasmonic systems. In SiC, ultraviolet excitation with femtosecond laser pulses induces >10 cm^-1 shifts in the transverse dipole resonance (width = 7 cm^-1). Time-resolved infrared reflection spectroscopy reveals that the photoinduced shifts decay in tens of ps, depending on the initial carrier density. Our results suggest that spatial redistribution of photoexcited carriers dominates the time dependence of the active tuning. We also report, for the first time, direct time-resolved infrared spectroscopy of the LO mode of GaN, made experimentally accessible through the Berreman effect.