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INPEC Sintesi della relazione

Project ID: 320917
Finanziato nell'ambito di: FP7-IDEAS-ERC
Paese: Germany

Mid-Term Report Summary - INPEC (Interacting Photon Bose-Einstein Condensates in Variable Potentials)

The INPEC project studies the physics of interacting photon condensates in variable potentials. In the first 30 months of the project, we have confined optical condensates in variable potentials generated by thermo-optic imprinting within a dye-filled microcavity. In this way, both spatially periodic optical lattice potentials as well as arbitrarily shaped potentials for the quantum gas were created. We have observed the tunneling of photons between two microsites as well as the influence of effective photon interactions in the individual microsites. Our results pave the way for a study of the Josephson dynamics for the photon gas, the Mott transition, and more complex entangled manybody states of light that can be directly populated in a thermal equilibrium process.
At the core of this research is the study of thermal equilibrium effects for light in the microcavity filled with dye solution. Photons are frequently absorbed and re-emitted by the dye molecules, leading to a thermalization of the photon gas to the dye rovibrational temperature. The spacing between the cavity mirrors is in the wavelength regime, and the short cavity imprints a low-frequency cutoff well above the thermal energy in frequency units. The longitudinal quantum number is frozen and the two transverse modal quantum numbers thermalize in a (average) number conserving way to the dye temperature. In earlier work, we have in this system observed the first photon Bose-Einstein condensate, using a single, harmonic trapping potential for the photon gas imprinted by mirror curvature.
The present project has started by building up a new dye-filled microcavity set-up to study periodic potentials for the light condensate. By thermo-optic patterning within the cavity using radiation of an auxiliary heating laser beam, controllable modifications of the refractive index and correspondingly variable potentials for the photon gas were induced. We successfully realize two-dimensional lattices of the photonic gas with a spacing of 5-10 microns between microsites. Such a spacing is sufficiently small that a tunneling between microsites occurs, as has been directly monitored by spectrally analyzing the emission of two such nearby sites showing the spectral splitting of modes due to the tunnel coupling. Spectral analysis of the cavity emission also allows us to quantity effective photon interactions within the microsites.
In a separate experimental set-up, the thermalization dynamics of photons in the dye-filled microcavity system was studied in detail, using pulsed laser excitation and streak-camera based analysis of the cavity emission. When the thermalization by dye absorption and re-emission is faster than the cavity loss rate, photons accumulate near the low-frequency cutoff and finally form a Bose-Einstein condensate of photons. On the other hand for a small reabsorption with respect to the photon loss, the state remains laser-like. We observe a crossover between a laser-like state and condensation, which can be controlled by adjusting the ratio of dye reabsorption and cavity loss rate.
In further work, the Kennard–Stepanov law, a Boltzmann-type scaling between absorption and emission was verified for a gas mixture of rubidium atoms and argon buffer gas atoms at high pressure. Additionally, ultraviolet absorption spectra of Xenon under high pressure have been measured, a first step towards Kennard-Stepanov measurements in the UV. This part of the work holds prospect for thermalization of ultraviolet spectral range photons in a cavity filled with high pressure noble gas.
We have moreover determined the degrees of first and second order coherence of a photon Bose-Einstein condensate. Interestingly, non-vanishing Hanbury Brown-Twiss correlations in the condensed state give evidence for Bose-Einstein condensation in the grand canonical statistical limit for a large relative size of the dye reservoir, as understood from effective particle exchange with dye electronic excitations.

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