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Physical Review Letters

Realization of a continuous-wave, two-photon optical laser

Daniel J. Gauthier, Qilin Wu, S. Morin, THOMAS W. MOSSBERG

Published Jan 27, 1992• 172 citations
Publisher

Abstract

We report the first observation of continuous-wave two-photon lasing in the optical regime, and demonstrate that its initiation requires the injection of a trigger pulse into the laser resonator. Successful operation of the two-photon laser relies on the use of a novel gain medium consisting of laser-driven, two-level atoms and the use of a high-finesse optical cavity to isolate the two-photon gain from competing processes. Threshold conditions for laser action are in good agreement with recent theoretical predictions.

DOI
10.1103/physrevlett.68.464

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We report the first observation of continuous-wave two-photon lasing in the optical regime, and demonstrate that its initiation requires the injection of a trigger pulse into the laser resonator. Successful operation of the two-photon laser relies on the use of a novel gain medium consisting of laser-driven, two-level atoms and the use of a high-finesse optical cavity to isolate the two-photon gain from competing processes. Threshold conditions for laser action are in good agreement with recent theoretical predictions.
We observe 30% two-photon optical amplification of a probe laser-field propagating through a laser-pumped potassium vapor. This amplification is spectrally isolated and substantially larger than that of previously reported continuous-wave two-photon amplifiers. The combination of large amplification and spectral isolation of the two-photon gain feature will greatly facilitate precise studies of the photon statistics of this highly nonlinear quantum amplifier and the development and characterization of a two-photon laser based on this gain medium. We also observe spectrally-distinct three-photon amplification $(\ensuremath{\sim}5$%) in the same system under different experimental conditions. We present a simple model of the interaction that gives qualitative agreement with our observations and explains the dependence of the two-photon gain on the various system parameters. This model predicts that the size of the two-photon gain is quite sensitive to an interference between two different quantum pathways.
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.
Driven two-level atoms are shown to exhibit strong, resonantly enhanced, two-photon gain. This gain, unlike that observed in nonlinear mixing processes, is not constrained by a phase-matching condition involving the pump beam. Physically, the gain can be associated with inverted two-photon transitions between dressed states or multiphoton stimulated Raman scattering, and it may be useful in the realization of a two-photon laser. Comparison with the well-known single-photon gain found in the same system is made.
Although the possibility of two-photon laser action1,2 was first discussed over 25 years ago, it has been realized only under a very limited set of circumstances3,4 and never under cw optical conditions. This is unfortunate because the two- photon laser has been predicted to display a number of interesting properties,5,6 such as bistable operation, novel threshold behavior, and in some cases squeezing. This situation is primarily a consequence of the presence of competing one-photon processes that occur in the previously considered two-photon gain media.
We observe and analyze a two-photon continuous-wave optical gain mechanism designed for building a two-photon laser. The two-photon stimulated emission is spectrally isolated and resonantly enhanced using the multilevel structure of 39K,{}^{39}\mathrm{K},39K, in conjunction with an alternative interaction geometry involving orthogonal beams and polarizations. The observed two-photon laser beam amplification increases linearly with low input laser beam intensity, as expected, and saturates at a gain of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ at high intensity. A theoretical analysis of the observations is outlined.