Using the nonperturbative many-body time-dependent approach, we investigate the nonequilibrium dynamics of the coherent longitudinal optical phonon-plasmon coupled ͑LOPC͒ modes in a polar semiconductor and explore their coherent optical control and eventually the carrier mobility of the semiconductor. The basic idea for a control of the carrier mobility is to manipulate the ultrafast dephasing of the coherent carrier-relevant LOPC mode. We theoretically propose two possible options to realize the idea and reach the final goal. One is to optimize a semiconductor by finely balancing two kinds of carrier densities by chemical doping and optical doping ͑or photodoping͒, where the relaxation of the coherent carrier-relevant LOPC mode would respond in a ͑weak͒ singular way. It is found that, in this way, the carrier mobility could be enhanced by a few tens of percent. The other is to optimize the optical pumping laser. In this option, the pulse train creating pure virtual carriers through the below-band-gap excitation would be incorporated for an optical pumping, which can make possible the dephasing-free dynamics of the coherent carrier-relevant LOPC mode. The carrier mobility can then be efficiently controlled and dramatically enhanced by synchronizing the pulse train with its coherent oscillation. This might imply one of ultimate ways to control the carrier mobility of the semiconductor.

We demonstrate optical control of the LO phonon-plasmon coupled (LOPC) modes in GaAs by using a femtosecond pump-pulse pair. The relaxation time of the plasmonlike LOPC mode significantly depends on the separation time (Dt) of the pump-pulse pair. Especially it is maximized when Dt becomes simultaneously comparable to the half period of the longitudinal optical (LO) phonon oscillation and resonant to the 3/4 period of the plasmonlike LOPC oscillation. We attribute these observations to the modification of carrier-LO phonon scattering and ballistic motion of the plasmonlike LOPC mode.