Second harmonics conversion and primary and second harmonics echoes from a resonant two-photon three-level atom system

Abstract

We investigate the second harmonics direct conversion and the echoes produced at the primary and second harmonics frequencies from a Doppler broadened three-level system, when excited by two collinear delta pulses which center frequency is two-photon resonant with the ground to upper level transition. We show that the primary frequency echo is modulated by the detuning between the frequencies of the incident radiation and that of the transition from ground to intermediate level. We show that the directly converted pulses and the echoes at the second harmonics frequency can be comparable in magnitude. We also show that the second harmonics first echo is centered, in the time-domain, half-way between the second pulse and the primary frequency first echo, that its width is half that of the primary frequency echo's envelope and that the magnitudes of the echoes at ω and 2ω decrease at different rates as function of the exciting pulses' seperation. Finally the effects of finite excitation pulse width and of propagation in optically thick samples on the results are investigated numerically and are shown to be appreciable.