Non-Markovian Dynamics of Spectral Narrowing for Excitons in the Layered Semiconductor GaSe Observed Using Optical Four-Wave Mixing Spectroscopy

We have investigated non-markovian dephasing by using time-resolved and spectrally resolved four-wave mixing measurements in a layered semiconductor GaSe. When the time interval between the first and second excitation pulses is increased, photon echo spectra exhibit narrowing only in a regime of a few picoseconds. In the initial dephasing of these signals, fast damping is observed. The narrowing of the spectrally resolved signal is consistent with the Fourier transformation of the time-resolved signals. Spectral narrowing is crucial evidence of the transition from non-markovian to markovian dynamics. By comparing experimental data with calculation results based on the non-markovian theory, we have found that the correlation time of the exciton-phonon interaction is 1.1 ps.