Theoretical Study of Ultra-Fast Dephasing by Four-Wave Mixing
Microscopic dynamics of materials can be studied by time delayed four-wave mixing spectroscopy. Dephasing constants can be measured by such experiments. Experimental results are easily analyzed when the correlation time τc of the exciting field is much shorter than the dephasing time, T2. However, when these quantities are on the same time scale, numerical fits are necessary. In the present work, we analyzed theoretically a time delayed four-wave mixing experiment. The material is described by an ensemble of two-level systems. Using the third order term of the density matrix, we establish a relation between T2 τc and τm the time delay for which the maximum of the diffracted light intensity is observed. The condition to obtain an estimate of T2 from the measurement of τm are discussed.