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Journal of Applied Physics

Controlling pulse propagation speed in CH3F far-infrared amplifier

Kijoon Lee, N. M. Lawandy

Published Apr 1, 2002• 0 citations
Publisher

Abstract

We calculated the group velocity of a probe pulse located at the gain center of five-level CH3F far-infrared molecular gas amplifier as a function of pump beam intensity and pressure. The results indicate this large class of optically pumped molecular system can exhibit significant group velocity reduction over a broad range of the electromagnetic spectrum. The group velocity can be further controlled by using the rotational sublevel splitting in a Stark static electric field in conjunction with optical pumpings.

DOI
10.1063/1.1455152

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We calculated the group velocity of a probe pulse located at the gain center of five-level CH3F far-infrared molecular gas amplifier as a function of pump beam intensity and pressure. The results indicate this large class of optically pumped molecular system can exhibit significant group velocity reduction over a broad range of the electromagnetic spectrum. The group velocity can be further controlled by using the rotational sublevel splitting in a Stark static electric field in conjunction with optical pumpings.
We analyse the group velocity of a laser pulse in an optically dressed atomic system. A Λ system with an additional close upper level and an incoherent pump is specially investigated. The group velocity of a pulse is strongly dependent on the optical properties of the system which can be modified by changing the amplitude or detuning of a strong coupling field or of an incoherent pump between the two lower levels of the system. Depending on the parameters, the medium may have alternatively absorptive or gain properties and the dispersion can be changed from anomalous into a normal one. The ranges of parameters are especially investigated in which absorption (gain) is not too strong, with dispersion being not too steep. The group velocity of a pulse propagating through a sample with such optical properties can be switched from the subinto superluminal regime. The dynamics of propagation in the case of negative group velocities of a small absolute value is especially interesting. In such a regime the peak of the transmitted pulse exits the sample before the peak of the incoming pulse reaches the medium. The transmitted pulse splits into two pulses - one of them propagates forward behind the medium and the other propagates backward and is cancelled at the entrance of the sample by the incoming pulse. Ranges of parameters are seeked in which the shape of the pulse is changed and the group velocity as a single parameter is not sufficient to describe the pulse propagation.
We investigate an active Raman gain scheme for significant group velocity reduction. This scheme, which is fundamentally different from the electromagnetically induced transparency scheme, is capable of achieving ultraslow and distortion-free propagation of a pulsed probe field. The group velocity behavior is drastically different from the conventional electromagnetically induced transparency scheme, and the new scheme can be used to accurately determine the decoherence rate of a long-lived state. In addition, the Raman gain scheme has the advantage of being broadly tunable, an important feature that may have potential applications.
We investigate an active Raman gain scheme for significant group velocity reduction. We show that this scheme, which is fundamentally different from the electromagnetically induced transparency scheme, is capable of achieving ultraslow and distortion-free propagation of a pulsed probe field. We demonstrate the group velocity behavior that is drastically different from the conventional electromagnetically induced transparency scheme, and we show that the new scheme can be used to accurately determine the decoherence rate of a long-lived state. In addition, the Raman gain scheme has the advantage of being broadly tunable, an important feature that may have potential applications.
Abstract We study light-pulse propagation in a dynamically controllable periodic structure (grating) resulting from Raman interaction of a weak probe pulse with a standing-wave pump and a second control laser field in N-type four-level atomic media. The grating is induced due to periodic spatial modulation of the Raman gain in a standing pump field (Raman gain grating). We show that it is possible to control both the probe pulse amplitude and the group velocity of the pulse from subluminal to superluminal by varying the pump or control field. Such a grating is of interest for all-optical switches and transistors.
Several polar molecules possessing strong far-infrared (FIR) transitions have been optically pumped by a mode-locked CO2TEA laser in a single-pass configuration. All the resulting FIR pulses are shorter than the sub-T2pump pulses; subnanosecond FIR pulses are observed. These short FIR pulses exhibit lethargic gain, stimulated Raman emission, and multiple-pulse superradiance. The experimental results are modeled by numerical solution of the Maxwell-density matrix equations for two fields interacting with a three-level system. Level degeneracy is included, and the pump is described by a multimode power spectrum with a parabolic envelope, resulting in a periodic train of pulses. Variation of the detuning of the pump from resonance allows simulation of the superradiant or Raman cases. This fully coherent model, which is the major contribution of this paper, explains key experimental observations such as the noncoincidence of the Raman peak with the pump modulation minimum and the enhancement of multiple-pulse superradiance due to remanent polarization.