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European Quantum Electronics Conference (EQEC)

Control of group velocity of light by phase-changing collisions

A. D. Wilson‐Gordon, Chana Goren, M. Rosenbluh, Harry Friedmann

Published Jan 18, 2006• 0 citations
Publisher

Abstract

This investigation discusses the influence of phase-changing collisions on the probe group velocity in Doppler-broadened, cycling, degenerate two-level systems, with F/sub e/ = F/sub g/ +1 and F/sub g/ > 0, interacting with an intense pump laser and a weak probe laser. In addition, This study demonstrates that a change in sign of the group velocity due to phase-changing also be obtained in realistic atomic systems.

DOI
10.1109/eqec.2005.1567262

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Physical Review A•Oct 21, 2004•34 citations
This investigation discusses the influence of phase-changing collisions on the probe group velocity in Doppler-broadened, cycling, degenerate two-level systems, with F/sub e/ = F/sub g/ +1 and F/sub g/ > 0, interacting with an intense pump laser and a weak probe laser. In addition, This study demonstrates that a change in sign of the group velocity due to phase-changing also be obtained in realistic atomic systems.
We discuss the influence of phase-changing collisions on the group velocities in Doppler-broadened, cycling, degenerate two-level systems where Fe=Fg+1{F}_{e}={F}_{g}+1Fe​=Fg​+1 and Fg>0{F}_{g}>0Fg​>0, interacting with pump and probe lasers, that exhibit electromagnetically induced absorption (EIA). Two model systems are considered: the NNN system where the pump and probe are polarized perpendicularly, and EIA is due to transfer of coherence (TOC), and the double two-level system (TLS) where both lasers have the same polarization, and EIA is due to transfer of population (TOP). For the case of Doppler-broadened EIA TOC, which occurs at low pump intensity, there is a switch from positive to negative dispersion and group velocity, as the rate of phase-changing collisions is increased. For the case of EIA TOP at low pump intensity, the dispersion and group velocity remain negative even when the collision rate is increased. Pressure-induced narrowing, accompanied by an increase in the magnitude of the negative dispersion and a decrease in the magnitude of the negative group velocity, occurs in both EIA TOC and EIA TOP, at low pump intensity. When the pump intensity is increased, a switch from negative to positive dispersion and group velocity, with increasing collision rate, also occurs in the double TLS system. However, the effect is far smaller than in the case of the NNN system at low pump intensity.
Under weak probe field, rotating wave and electric dipole approximations, we have derived analytic expressions for absorption and dispersion coefficients, group index and group velocity in a four-level cascade atomic medium interacting with the probe and pump laser fields. The model is applied to 85Rb atomic system to study the control of the fast and slow light propagation of the probe beam according to the intensity and frequency of the pump laser beam. With this excitation model, we find two transparent domains and thus the light group velocity is also controlled at these two frequency domains. In particular, the amplitude and the sign of the group index are easily changed by adjusting the intensity or frequency of the pump laser. That is, the probe beam propagation can be changed between fast and slow light modes by adjusting the intensity or frequency of the pump laser. Our analytical results can be useful for related experimental studies.
The propagation of a weak probe field in a laser-driven four-level atomic system is investigated. We choose mercury as our model system, where the probe transition is in the ultraviolet region. A high-resolution peak appears in the optical spectra due to the presence of interacting dark resonances. We show that this narrow peak leads to the superluminal light propagation with strong absorption, and thus by itself is only of limited interest. But if in addition a weak incoherent pump field is applied to the probe transition, then the peak structure can be changed such that both sub- and superluminal light propagation or a negative group velocity can be achieved without absorption, controlled by the incoherent pumping strength. A suitable choice of laser propagation directions allows us to preserve these results under Doppler averaging.
We predict the occurrence of electromagnetically induced absorption (EIA) in cycling degenerate two-level transitions where Fe=Fg+1{F}_{e}={F}_{g}+1Fe​=Fg​+1 and Fg>0{F}_{g}>0Fg​>0, interacting with pump and probe lasers with the same polarization. The EIA is due to transfer of population (TOP) between the Zeeman levels of the ground hyperfine state, rather than transfer of coherence (TOC) which occurs for perpendicularly polarized lasers. We model EIA-TOP using a double two-level system (TLS) which we compare with the four-level NNN system, which models EIA-TOC. When the pump intensity is low, both models give an EIA peak at line center. The effect of introducing phase-changing collisions is studied, in the presence and absence of Doppler broadening, for both the double TLS and NNN systems. In the presence of phase-changing collisions, the central EIA peaks are narrowed in both models and persist to higher pump Rabi frequencies than in the absence of collisions. In the double TLS, in the presence of Doppler broadening, the central EIA-TOP peak becomes narrower and does not develop a dip in its center, in contrast to the NNN system. The central dip that appears in the Doppler-broadened EIA-TOC spectrum can be wiped out by adding phase-changing collisions. We demonstrate that EIA-TOP can be obtained for realistic atomic transitions interacting with lasers that have the same polarization.