Quantum interference of virtual and real amplitudes in a semiconductor exciton system.

By two-color pulse shaping, we simultaneously create virtual and real amplitudes for excitons in GaAs quantum wells, and monitor population and amplitude by pump-probe and four-wave mixing spectroscopies. Excited-state probability amplitude can be induced by the off-resonant, virtual excitations as well as by the resonant, real excitations. Population modulation in time-domain results from the interference between the virtual and real amplitudes, and the modulation depth reveals the relative contributions of these two amplitudes. The fact that virtual and real amplitudes have a phase difference of 90 degrees is demonstrated directly in time-domain.