Optical Coherent Transients in Cold Atoms: From Free-Induction Decay to Optical Precursors

J. F. Chen, M. M. T. Loy, George K. Wong +1 more

Shengwang Du, M. M. T. Loy, George K. Wong

Unyob Shim, S. B. Cahn, Anantharaman Kumarakrishnan +2 more

J. F. Chen, Heejeong Jeong, M. M. T. Loy +1 more

J. F. Chen, Shuyuan Wang, Dong Wei +3 more

We report theoretical and experimental studies of the propagation of a square-modulated laser pulse through a laser-cooled atomic ensemble of two-level absorbers, as well as through a three-level system with electromagnetically induced transparency. We find that the transmission characteristics over a wide range of optical depth can be fully accounted for as optical coherent transients excited by the wide spectral content of the square-modulated optical pulse. We show that both time-domain atom-field coupled equations and frequency-domain linear dispersion theory give precisely the same description of the optical transients at a weak power limit. At low optical depth with moderate absorption, resonant excitation dominates and free-induction decay (FID) contributes mostly to the transient field. At high optical depth when absorption and propagation effects become significant, lossless optical precursors start to dominate the transient response. By varying the optical depth from 0 to 45, we observe that optical transients evolve gradually from FID to optical precursors. We thus show that FID and optical precursors, which have been considered as two different optical transients for many decades, can be unified within a single theoretical frame and they are the manifestations of the same physical process in two different regimes.

J. F. Chen

The light wave propagating in a dispersive medium can be formularized as electromagnetic (EM) fields or quantized as a pair of Hermitian operators. The fascinating optical phenomena display classical and nonclassical properties, depending on whether they can be described by the classical EM field theory. A cold atomic ensemble with neligible inhomogenous Doppler broadening is ideal to explore the interaction between light and matter. A typical problem in classical pulse propagation is optical transients, which induced by rapid response of dispersive medium to the sudden change of the driving radiation fields. With cold atoms confined in the two-dimensional magneto-optical trap, whose optical depth is controllable in our system, optical precursor and free-induction decay both classified as optical transients are observed in a single experiment with different conditions. Moreover, the information velocity limit is verified in superluminal medium, with the observation and analysis of precursors. Characterized with narrow linewidth and long coherence time, paired photons generated from cold atomic ensemble in four-wave mixing process possess irreplaceable advantage in quantum optics area. Temporal modulation of narrow-band paired photons can be achieved by directly modulating the generated photons or by modulating the classical control fields. After successfully reshaping the temporal waveform of the paired photons, heralded single photon source is well prepared for studying the propagation of nonclassical single photon in dispersive medium. The speed limit imposed by Einstein Causality principle is firstly experimentally verified on single photon level.