Ultrafast Nonlinear Absorption in an Exciton Bragg Reflector
Absorption saturation associated with quantum well excitons periodically distributed in a Bragg reflector was investigated by pump-probe reflectance spectroscopy. Femtosecond spectrogram of reflected probe pulses yielded frequency chirping due to cross-phase modulation. The frequency chirping, which was obtained as 3.2 THz for a time interval of 370 fs, gives rise to an optical phase shift as large as 1.6π. The phase shift evidences the enhancement of excitonic nonlinearity by photon confinement in the Bragg reflector.
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