Femtosecond buildup of phonon-plasmon coupling in photoexcited InP observed by ultrabroadband THz probing

We study the ultrafast transition of a pure longitudinal optical phonon resonance to a coupled phonon-plasmon system. Following 10-fs photoexcitation of intrinsic indium phosphide, ultrabroadband THz opto-electronics monitors the buildup of coherent beats of the emerging hybrid modes directly in the time domain with sub-cycle resolution. Mutual repulsion and redistribution of the oscillator strength of the interacting phonons and plasmons are seen to emerge on a delayed femtosecond time scale. Both branches of the mixed modes are monitored for various excitation densities N . We observe a pronounced anticrossing of the coupled resonances as a function of N . The characteristic formation time for phonon-plasmon coupling exhibits density dependence. The time is approximately set by one oscillation cycle of the upper branch of the mixed modes.