Interference pulses in optical free induction decay

Abstract

A new feature of optical free induction decay (FID) is observed in a coherently prepared sample of CH13F3 by the method of Stark switching. The preparative phase is due to the resonant steady-state excitation by a cw laser beam, and is followed by FID upon sudden application of a dc Stark field that switches the molecular sample out of resonance. The emission is not observed as a simple decay but instead appears as a train of sharp pulses regularly spaced in time as a result of a repetitive interference. This situation arises because an entire set of infrared transitions within the Stark split manifold are initially prepared, in contrast to our previous study of a nondegenerate transition. The emission, which beats with the laser, produces a heterodyne beat spectrum, consisting of a set of regularly spaced frequencies, that is the Fourier transform of the slowly decaying pulse progression observed. We thus demonstrate what is the optical analog of the well-known NMR method of high-resolution pulse Fourier spectroscopy. The detailed behavior of the pulse train agrees well with an FID theory that assumes the transitions to be uncoupled. The subtle behavior of FID near the time origin is explored also by approximate analytic expressions that reveal either a near zero amplitude or sizable amplitude depending on the degree of saturation in the preparative stage. The experimental technique discussed offers an attractive way for obtaining high-resolution optical spectra without Doppler broadening, and for generating an optical pulse train whose time scale can be compressed by simply increasing the Stark field. © 1974 The American Physical Society.