We report pulse modulation caused by the exciton quantum beat in a GaAs/AlAs multiple quantum well.The modulation was evaluated by measuring the cross-correlation signal which is the second harmonic light generated by the probe pulse reflected from the sample and the gate pulse.The intensity of the correlationsignal decreases owing to the generation of the exciton quantum beat, and recovers with dephasing of the quantum beat oscillation.Moreover, we found that the decrease caused by the quantum beat is larger than that by changing the refractive index due to the exciton generation.These results indicate that quantum beats can be a potential mechanism to enable low power operation of ultrafast optical switches.
Photon-echo modulation spectroscopy is extended into a new regime to study coherences between states that are widely separated in frequency compared with the bandwidth of the exciting laser pulses. Time-delayed four-wave mixing (TDFWM) is observed on the Na
D doublet by use of 7-nsec pulses of light spectrally peaked at each of the
D-line transitions. As the two double-frequency, 7-nsec light pulses are delayed with respect to each other a 1.9-psec beating of the integrated TDFWM signal is observed, even for delays greater than the excitation pulse width.
Recently some novel two-photon interference experiments have been performed, where quantities such as the field coherence time and the beat spacing between light beams with different frequencies were measured with high time resolution1. These experiments have overcome the traditional limits on time resolution by combining a Mach-Zehnder interferometer configuration with the two-photon correlation technique. All of these experiments, however, have been performed with cw light sources, and no attempt has yet been made to measure the temporal variation of the field intensity.