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Physical Review A

Amplification of laser beams counterpropagating through a potassium vapor: The effects of atomic coherence

William J. Brown, Jeff R. Gardner, Daniel J. Gauthier, Ramon A. Vilaseca

Published Oct 1, 1997• 19 citations
Publisher

Abstract

We observe amplification of a linearly polarized laser beam propagating through a Doppler-broadened atomic potassium vapor driven by an intense, linear and orthogonally polarized counterpropagating laser beam. The observed gain spectra are well explained by a model that incoporates only the effects of the coherent driving of the atomic dipole moment and not atomic recoil. Our results suggest that the recently reported observations of amplification and lasing using a laser-driven sodium vapor may not arise from the effects of collective atomic recoil.

DOI
10.1103/physreva.56.3255

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We observe amplification of a linearly polarized laser beam propagating through a Doppler-broadened atomic potassium vapor driven by an intense, linear and orthogonally polarized counterpropagating laser beam. The observed gain spectra are well explained by a model that incoporates only the effects of the coherent driving of the atomic dipole moment and not atomic recoil. Our results suggest that the recently reported observations of amplification and lasing using a laser-driven sodium vapor may not arise from the effects of collective atomic recoil.
We investigate theoretically the amplification of a laser beam propagating through a collection of Doppler-broadened two-level atoms driven by an intense counterpropagating laser beam. Large amplification of the beam is predicted when the pump-beam Rabi frequency is comparable to the Doppler width of the atomic transition, even without including the effects of atomic recoil. The microscopic origin of the gain can be attributed to the coherent driving of the atomic dipole moment, suggesting that amplification and lasing due to collective atomic recoil may be influenced by this process.
We develop a theoretical model of two-photon amplification in laser-driven potassium atoms and use it to analyze the recent experiments reported by Pfister et al. [Phys. Rev. A 60, R4249 (1999)]. The model takes into account most of the essential factors influencing the amplification process, including the atomic hyperfine structure (which makes multiphoton emission possible) and the simultaneous interaction with intense drive and probe beams with arbitrary detunings. We determine the origin and analyze the properties of different multiphoton gain resonances that appear in the light-matter interaction. In particular, the influence of the drive and probe field amplitudes and detunings on the strength and frequency of the two-photon amplification resonance is studied in detail, showing clearly the differences with respect to the behavior of single-photon or other multiphoton amplification processes. In addition, we investigate interferences between different quantum pathways originating from the hyperfine structure and determine the conditions under which they can enhance or suppress multiphoton resonances. The predictions of the model are in good agreement with the observations, indicating that it can be used to understand recent experiments on two-photon lasing reported by Pfister et al. [Phys. Rev. Lett. 86, 4512 (2001)].
Two-photon amplification and lasing in laser-driven potassium atoms is investigated theoretically using a rigorous model that takes into account the hyperfine level structure of the atoms for arbitrary intensity drive and lasing fields. The model correctly predicts most features of the recent experiments, such as the characteristics of the multiphoton gain resonances and the two-photon laser emission intensity and line shape. In addition, it constitutes a new tool for exploring behaviors that are difficult to address experimentally, such as interference effects between different quantum pathways. In contrast to single-photon lasers, the laser emission line shape is frequency pushed and has the form of a closed curve composed of a stable and an unstable branch.
When an atomic vapor interacts with an intense nearly resonant laser field, the ac Stark effect causes a profound modification of the weak-field absorption spectrum. Two of the induced spectral features can lead to amplification of a weak probe wave. We have previously measured in atomic sodium vapor high probe-wave gain (G = 38) due to the three-photon effect and a somewhat lower gain (G = 4) when the probe wave is detuned from the pump laser frequency by the inverse of the atomic response time.1 We present the results of an experimental and theoretical study of the energy coupling between two intense laser fields of comparable intensity through their interaction with an atomic vapor. In our experiments, a tunable alexandrite laser provides microsecond laser pulses of the required intensity that are nearly resonant with the 4s → 4p transition in potassium. We observed large energy transfer (50%) between the two waves at the nearly degenerate resonance and measured the intensity dependence of the coupling efficiency. Theoretical modeling of the interaction is based on a continued-fraction solution of the density-matrix equations of motion.