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Quantum Electronics and Laser Science Conference

Experimental realization of a two-photon laser in strongly driven potassium atoms

Olivier Pfister, William J. Brown, Michael D. Stenner, Daniel J. Gauthier

Published Jan 20, 2003• 1 citation
Publisher

Abstract

Summary form only given. We present experimental evidence for novel quantum oscillators consisting of strongly driven potassium atoms contained in a high finesse optical resonator. These oscillators are based on two-photon stimulated emission, making their behavior and properties significantly different from conventional lasers, but making them harder to realize. The multi-level structure of alkali atoms is ideally suited for realizing such an oscillator since it offers a wealth of scattering processes that display strong resonant enhancement. Two-photon amplification arises from laser-driven resonant scattering between magnetic hyperfine levels. A novel configuration using different states of polarization and an orthogonal beam geometry allows discrimination between the numerous gain mechanisms.

DOI
10.1109/qels.1999.807469

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Physical Review A•Oct 24, 2003•3 citations
Summary form only given. We present experimental evidence for novel quantum oscillators consisting of strongly driven potassium atoms contained in a high finesse optical resonator. These oscillators are based on two-photon stimulated emission, making their behavior and properties significantly different from conventional lasers, but making them harder to realize. The multi-level structure of alkali atoms is ideally suited for realizing such an oscillator since it offers a wealth of scattering processes that display strong resonant enhancement. Two-photon amplification arises from laser-driven resonant scattering between magnetic hyperfine levels. A novel configuration using different states of polarization and an orthogonal beam geometry allows discrimination between the numerous gain mechanisms.
This thesis describes the development of a novel quantum oscillator known as a two-photon Raman laser. It is based on two-photon stimulated emission in strongly driven potassium atoms. Two-photon lasers were Þrst proposed in the 1960s, but only recently have such devices been experimentally realized. The two-photon Raman laser is an important step forward as it provides the Þrst oppurtunity to study the turn-on behavior, instabilities, and noise properties of a pure two-photon optical device. The necessary ingredients for a two-photon laser are a medium displaying two-photon laser beam ampliÞcation and an optical resonator. In my two-photon Raman laser the ampliÞcation arises from a multi-photon process in which state-prepared potassium atoms undergo two-photon Raman transitions. This ampliÞcation process was studied using a high-density, small-Doppler-width potassium atomic beam that was driven by a strong pump laser and probed by a weak probe laser. I observed two-photon Raman ampliÞcation for a range of pump laser frequencies, atomic beam
Summary form only given. The two-photon laser is a new type of quantum oscillator based on the two-photon stimulated emission process. The laser is intrinsically nonlinear due to the nature of the stimulated emission and is thus expected to display novel nonlinear phenomena. While there has been continued interest in two-photon lasers since the original conception significant progress has been hampered by the difficulty in achieving two-photon lasing. In this contribution we summarize a model of the interaction between two intense fields and a collection of potassium atoms and use it to explain features of the recent two-photon experiments and to make predictions for future experiments.
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.
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.
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.
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)].