"Observation of polarization instabilities in a two-photon laser."

M. D. Stenner, W.J. Brown, O. Pfister, D. J. Gauthier, Duke University

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Abstract

The two-photon laser is a novel quantum oscillator based on the nonlinear process of two-photon stimulated emission. It has received much theoretical attention, predicting such phenomena as polarization instabilities, squeezing, and photon entanglement. However, very little experimental work has been done to confirm or deny the theory. We observe and analyze polarization instabilities in a two-photon Raman laser based on two-photon optical amplification in laser driven 39K surrounded by a high finesse cavity. In this system, there exist multiple frequency-degenerate quantum pathways between the initial and final states, giving rise to amplification for different states of polarization of the field. This tensor nature of the interaction, together with the extreme nonlinear behavior of two-photon lasers, open up the possibility of polarization instabilities. We observe that the state of polarization of the beam generated by the laser exhibits fast periodic and non-periodic oscillation for a variety of experimental parameters.

Summary

The two-photon laser is a new type of quantum oscillator that has potential applications in the areas of quantum computation and communication, and optical metrology. It is based on two-photon stimulated emission process whereby an excited atom interacting simultaneously with two incident photons is stimulated to the lower laser level and four photons are scattered by the atom, giving rise to nonlinear amplification of the incident light. As in the case of one-photon stimulated emission, the new scattered photons are identical copies of the original incident photons, imparting novel coherence properties to the beam of light generated by the laser.

In this presentation, we focus on the nonlinear properties of the two-photon laser that arise from the nature of the stimulated emission process. The two-photon gain medium on which the laser is based is highly nonlinear in that the amplification of a beam of light is proportional to the intensity of the incident light until saturation sets in; there is no amplification for a weak beam of light. Hence, two-photon oscillation commences when the round-trip gain equals the round-trip losses, which can only occur when there is a critical inversion atomic number density in the cavity and a critical photon number density in the cavity. The laser starts discontinuously, only after a minimum number of photons are injected into the cavity from an auxiliary source.

Once oscillation sets in, there is a run-away effect in the photon number. As the intensity of the beam circulating in the laser resonator grows, the single-pass amplification grows, thereby accelerating the rate of growth of the circulating beam. This process continues until the atomic transition is saturated. Therefore, even at threshold, the laser operates in the saturated regime. Since saturation is a source of nonlinear coupling between the atoms and the electromagnetic field, it is expected that the two-photon laser may display complex dynamical instabilities.

Our two-photon laser is based on two-photon optical amplification in laser driven potassium atoms surrounded by a high finesse cavity. The amplification arises from a multi-photon Raman scattering process between the hyperfine levels of the 39K 4S1/2 ground state. In order to suppress unwanted nonlinear optical effects that can prevent two-photon lasing, our system is realized using state-of-the-art cavity quantum electrodynamics methods.

We have confirmed that the oscillator is a true two-photon laser by measuring the threshold characteristics. We find that the laser will turn on only after a pulse of light is injected into the cavity and shuts off and remains off once the pump mechanism is momentarily removed. Interestingly, we find that the total intensity of the laser is essentially constant in time, whereas the state of polarization of the light can fluctuate in a periodic or chaotic manner. We believe that the origin of the instability is competition between multiple frequency-degenerate quantum pathways connecting the initial and final laser levels.