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EXPERIMENTAL REALIZATION OF A TWO-PHOTON RAMAN LASER

William J. Brown

Published Nov 1, 1999• 2 citations
OpenAlex

Abstract

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

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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
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.
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.
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 describe the operating characteristics of a new type of quantum oscillator that is based on a two-photon stimulated emission process. This two-photon laser consists of spin-polarized and laser-driven 39K atoms placed in a high-finesse transverse-mode-degenerate optical resonator and produces a beam with a power of approximately 0.2 microW at a wavelength of 770 nm. We observe complex dynamical instabilities of the state of polarization of the two-photon laser, which are made possible by the atomic Zeeman degeneracy. We conjecture that the laser could emit polarization-entangled twin beams if this degeneracy is lifted.
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.