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Physical Review A

Two-color lasing in cold atoms

Jin‐Hui Wu, M. Artoni, Giuseppe Carlo La Rocca

Published Oct 14, 2013• 13 citations
Publisher

Abstract

Elaborating versatile strategies for new lasing schemes is important for both fundamental research and realistic applications. By using atomic photonic crystals, two-color distributed feedback lasing can be shown to occur in a nonlinear regime of balanced optical gain. Within the context of a simple model based on coupled-wave theory and applied to alkali-metal-atom crystals, we find that the two-color lasing threshold in cold atoms is within state-of-the-art experimental reach.

DOI
10.1103/physreva.88.043823

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Elaborating versatile strategies for new lasing schemes is important for both fundamental research and realistic applications. By using atomic photonic crystals, two-color distributed feedback lasing can be shown to occur in a nonlinear regime of balanced optical gain. Within the context of a simple model based on coupled-wave theory and applied to alkali-metal-atom crystals, we find that the two-color lasing threshold in cold atoms is within state-of-the-art experimental reach.
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Generating nonclassical light offers a benchmark tool for fundamental research and potential applications in quantum optics. Conventionally, it has become a standard technique to produce nonclassical light through the nonlinear optical processes occurring in nonlinear crystals. We describe this process using cold atomic-gas media to generate such nonclassical light, especially focusing on narrowband biphoton generation. Compared with the standard procedure the new biphoton source has such properties as long coherence time, long coherence length, high spectral brightness, and high conversion efficiency. Although there exist two methodologies describing the physical process, we concentrate on the theoretical aspect of the entangled two-photon state produced from the four-wave mixing in a multilevel atomic ensemble using perturbation theory. We show that both linear and nonlinear optical responses to the generated fields play an important role in determining the biphoton waveform and, consequently, on the two-photon temporal correlation. There are two characteristic regimes determined by whether the linear or nonlinear coherence time is dominant. In addition, our model provides a clear physical picture that brings insight into understanding biphoton optics with this new source. We apply our model to recent work on generating narrowband (and even subnatural linewidth) paired photons using the technique of electromagnetically induced transparency and slow-light effect in cold atoms and find good agreement with experimental results.
Atomic systems that display a linear or weakly nonlinear interaction with light are well known and well understood. However, when the interaction between light and matter becomes highly nonlinear and the light and matter strongly couple, the systems become much more difficult to understand both theoretically and experimentally. One example of a strongly coupled, highly nonlinear system is the two-photon laser that is based on the two-photon stimulated emission process. This laser has intrigued theorists and experimentalists alike. Research has been hindered, however, by the difficulties in constructing such a laser. Most two-photon gain media prove unsuitable due to small gain and the occurrence of destructive competing nonlinear effects. I have developed a new two-photon gain medium that overcomes these difficulties. It consists of a laser-driven potassium vapor in which the origin of the gain is due to the two-photon Raman scattering process. The two-photon gain feature is identified by performing spectroscopy of the laser-driven potassium vapor. To complement the experimental observations, I have developed a theoretical model of the two-photon Raman gain medium using the semi-classical density-matrix formalism. The predictions of the model are in qualitative agreement with the experimentally observed frequency- and intensity-dependence of the two-photon gain. I also describe a simplified rate-equation model of two-photon lasers through which I explore their steady-state and transient behavior. The model highlights the novel threshold behavior of two-photon lasers and the need to inject an external field to initiate lasing. This work, both theoretical and experimental, provides the first step toward a robust experimental realization of a two-photon laser.