Resonant size enhancement of degenerate four-wave mixing signal of weakly confined excitons

The peculiar size enhancement of the degenerate four-wave mixing (DFWM) signal of weakly confined excitons is elucidated by the use of the microscopic nonlocal theory of the nonlinear response. It is clarified that the high coherence of excitons plays an important role to enhance the internal field, which is a source of the large nonlinear signal. This result suggests that the enhancement of the DFWM signal reflecting the level structure of the quantized center-of-mass motion of excitons can be observed in high quality semiconductor thin films with very small non-radiative damping constants.