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Journal of the Optical Society of America B

Arbitrary delay-time control of ultraslow matched pulses

Akihiko Fujisawa, Nobuhito Hayashi, Ken-ichi Takahashi, Hiroaki Kido, Masaharu Mitsunaga

Published May 21, 2010• 1 citation

Abstract

We have theoretically and experimentally analyzed the ultraslow matched-pulse propagation in a parametric amplifying medium, especially focusing on the delay-time behaviors of the input probe pulse (signal) and the generated Stokes pulse (idler) under a strong coupling beam. Theoretically, it is predicted that the Stokes pulse always propagates with a definite group velocity with some initial delay time. The probe pulse, on the other hand, propagates at a fixed delay from Stokes after some transient period. This additional delay time is positive (probe is behind Stokes) if the medium is opaque for probe and transparent for Stokes, or it can be negative (probe is ahead of Stokes), if otherwise. It is also shown that the delay times of probe and Stokes, including the order of the pulses, can be arbitrarily and independently controlled by external parameters. The theoretical predictions and numerical simulations have been compared with experimental observations using a hot sodium vapor and they had a very nice agreement.

DOI: 10.1364/josab.27.001280

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We have theoretically and experimentally analyzed the ultraslow matched-pulse propagation in a parametric amplifying medium, especially focusing on the delay-time behaviors of the input probe pulse (signal) and the generated Stokes pulse (idler) under a strong coupling beam. Theoretically, it is predicted that the Stokes pulse always propagates with a definite group velocity with some initial delay time. The probe pulse, on the other hand, propagates at a fixed delay from Stokes after some transient period. This additional delay time is positive (probe is behind Stokes) if the medium is opaque for probe and transparent for Stokes, or it can be negative (probe is ahead of Stokes), if otherwise. It is also shown that the delay times of probe and Stokes, including the order of the pulses, can be arbitrarily and independently controlled by external parameters. The theoretical predictions and numerical simulations have been compared with experimental observations using a hot sodium vapor and they had a very nice agreement.

We have observed the ultraslow propagation of matched pulses in nondegenerate four-wave mixing in a hot atomic vapor. Probe pulses as short as 70 ns can be delayed by a tunable time of up to 40 ns with little broadening or distortion. During the propagation, a probe pulse is amplified and generates a conjugate pulse which is faster and separates from the probe pulse before getting locked to it at a fixed delay. The precise timing of this process allows us to determine the key coefficients of the susceptibility tensor. The fact that the same configuration has been shown to generate quantum correlations makes this system very promising in the context of quantum information processing.

In an ordinary electromagnetically-induced-transparency (EIT) experiment using a hot sodium vapor, we have observed a probe gain up to 32 in the presence of a coupling beam in an otherwise totally opaque (transmissivity $\ensuremath{\sim}{10}^{\ensuremath{-}5}$) situation, increasing the transmissivity by 6 orders of magnitude. This gain is attributable to parametric amplification (PA), accompanying the Stokes (or anti-Stokes) wave with similar output intensity, and is critically dependent upon the atomic density and the coupling beam power. Experimental and theoretical analyses have revealed that, while the EIT picture is valid for relatively low atomic densities, the PA picture becomes dominant for high atomic densities and high coupling powers. We have also found that the parametric coupling between the probe and Stokes waves can be nullified by the proper choice of the coupling detuning frequency, due to destructive interference of two elementary processes.

Group-velocity mismatch between the pump and Stokes pulses and the inertia of optical nonlinearity are shown to modify scenarios of slow-light evolution of laser pulses accompanying stimulated Raman and Brillouin scattering (SRS and SBS) of light. In the transient regime of SRS and SBS, the inertia of nonlinear polarization of a medium shifts the peak of an amplified Stokes field envelope relative to the input Stokes pulse. This shift is sensitive to the shape of input laser pulses and is controlled by the SRS and/or SBS gain, the damping time of the optical phonon wave, and the delay time between the input pump and Stokes pulses. In the regime where both the inertia of nonlinear polarization of an SRS medium and group-delay effects are significant, stationary modes of the Stokes field may exist in the frame of reference propagating with the group velocity of the pump field. The maximum group delay attainable for such modes of the Stokes field is limited, similar to the steady-state regime, by the ratio of the steady-state SRS gain to the Raman gain bandwidth.