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.

• Received 5 January 2011

DOI:https://doi.org/10.1103/PhysRevLett.107.037402