AQP Seminar: Stimulated thermalization and long-range spatial coherence of Bose-Einstein condensates in plasmonic lattices
We study Bose–Einstein condensation of surface plasmon polaritons in strongly coupled lattice modes of a nanoparticle array covered with fluorescent molecules at room temperature. The plasmonic condensate is a manifestation of macroscopic quantum coherence in unprecedented, sub-picosecond, timescales. We study the dynamics in an experiment that utilizes the open cavity character of the system. Spatially resolved spectra and momentum space measurements reveal three distinct regimes as a function of excitation pump fluence: lasing, (incomplete) thermalization, and Bose-Einstein condensation . By varying the lattice size, we show that the thermalization occurs through a stimulated process. Experiments with Michelson interferometer in a retroreflector configuration show that the condensation is accompanied by extended spatial coherence in two dimensions. Above the condensation threshold, the first-order spatial correlation function is nearly constant over array sizes up to half a millimeter . We find that both spatial and temporal coherence display non-exponential decay; the results suggest power-law or stretched exponential behaviour with different exponents for spatial and temporal correlation decays.
 Väkeväinen, A. I., Moilanen, A. J., Nečada, M., Hakala, T. K., Daskalakis, K. S., and Törmä, P., Sub-picosecond Thermalization Dynamics in Condensation of Strongly Coupled Lattice Plasmons, Nature Communications 11, 3139 (2020)
 Moilanen, A. J., Daskalakis, K. S., Taskinen, J., and Törmä, P., Spatial and Temporal Coherence of Strongly Coupled Plasmonic Bose-Einstein Condensates, preprint in arXiv:2103.10397v1 [cond-mat.quant-gas] (2021) https://arxiv.org/abs/2103.10397