Sampling a terahertz dipole transition with subcycle time resolution

me present a time-resolved technique to measure optical excitation processe s with a time resolution shorter than the oscillation period of the excitin g light. Our terahertz (THz) experiments fully resolve the polarization dyn amics of electrons in semiconductor heterostructures when they are excited by a THz pulse. The time resolution of the polarization enables us to deduc e the population dynamics of the excited state, which includes the dynamics of a virtual population in the case of off-resonant excitation. (C) 2000 O ptical Society of America OCIS codes: 320.7150, 320.7130, 300.6240, 270.167 0.