Degenerate four-wave mixing of femtosecond pulses in an absorbing dye jet

Abstract

Intracavity measurements of degenerate four-wave mixing have been performed with femtosecond laser pulses to determine the phase relaxation time of the absorbing dye, diethyloxadicarbocyanine iodide (DODCI), in ethylene glycol. The influence of various broadening mechanisms (homogeneous, inhomogeneous, cross relaxation) and pulse parameters (frequency, pulse shape, phase modulation, and energy) is investigated. The dependence of the signal energy on probe delay is found to be strongly affected by the phase modulation of the pulse; hence determination of the phase relaxation time requires accurate diagnostics of the laser source. The rise of the signal provides an upper limit of 10 fsec for the phase relaxation time of the main absorption band of the dye DODCI. The decay of the signal is found to be related to the phase relaxation time of a two-photon absorption in the same dye. Accurate fitting of the data with the theory yields 50 ± 10 fsec for the dephasing time of the two-photon excitation in DODCI.