Atom-Light Interaction in High-Pressure Optical Waveguides

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The aim of this project is the study of nonlinear optics and collective photonic quantum effects with light confined in optical hollow core waveguides filled with an atomic gas. A buffer gas pressure of several hundreds of bars leads to a wide pressure-broadened bandwidth of the system, interpolating between the usual sharp atomic physics spectral lines and the band structure of solid state physics systems.In previous works, we have demonstrated atomic spectroscopy on a rubidium cell filled with 500 bar of argon and helium buffer gas, respectively. Pressure and saturation broadening in a tightly focused optical beam geometry leads to a spectral bandwidth of tens of nanometers. The room temperature linewidth of the well controlled quantum optical system in energy units approaches the thermal energy, and we have observed evidence for thermal equilibrium of the strongly coupled dressed state (coupled atom-light Eigenstate) system. While our interaction length is presently limited by the spatial depth of the Gaussian focus of the driving light beam, we expect that with appropriate light confinement in a waveguide the optically dense regime can be reached, allowing for novel, high bandwidth nonlinear optical systems and collective photonic quantum effects. Taking advantage of the existing expertise of the Bonn focused Research Unit, we plan to investigate both hollow core photonic crystal fibers and metallic waveguides. The latter ones are well-developed for the mid-infrared spectral region, but the fabrication and the exploration of plasmon-induced effects of metal waveguides in the optical regime are an active field of research. Clearly, a considerable further increase of the interaction length is possible with gas-filled hollow core photonic crystal fibres, which will be studied subsequently within the program.We expect to demonstrate wideband frequency mixing, taking advantage of the possible exponential suppression of absorption in the pressure broadened waveguide system. The increased interaction length in a resonantly pumped sample allows for intriguing thermal equilibrium photonic quantum effects, as a Bose-Einstein-like phase transition of polaritons (i.e. coupled hybrid atom-light quasiparticles) between a thermal state and a condensed, ordered polariton state. An intriguing perspective of such densely filled optical waveguides, interpolating between usual gas phase and solid state conditions, lies in the exploration of novel ways to tailor the state of light.