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Physical Review A

Two-photon stimulated emission in laser-driven alkali-metal atoms using an orthogonal pump-probe geometry

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

Published Dec 1, 1999• 19 citations
Publisher

Abstract

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 ${}^{39}\mathrm{K},$ 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.

DOI
10.1103/physreva.60.r4249

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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×\else\texttimes\fi10\ensuremath−42\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}2\ifmmode×\else\texttimes\fi10\ensuremath−4 at high intensity. A theoretical analysis of the observations is outlined.
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.
Summary form only given.A less well understood regime of quantum optical systems is the case when they are far from equilibrium and highly nonlinear. One device that inherently operates in this regime is the two-photon laser that is based on the two-photon-stimulated emission process, whereby two photons stimulate the emission of four photons. Such lasers are predicted to have highly unusual nonlinear dynamic and quantum statistical behaviors. Recently, our group demonstrated large continuous-wave two-photon amplification in a laser-driven potassium vapor based on a multiphoton Raman scattering process. Subsequent studies have revealed that destructive interference between multiple quantum pathways significantly reduces the two-photon-stimulated emission rate for this process. To overcome this problem, we have devised a new mechanism for two-photon amplification that suppresses the quantum interference using electromagnetically induced transparency in conjunction with a novel geometry.
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.
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 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)].
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.