Bose-Einstein Condensation and stimulated thermalization of polaritons in plasmonic lattices

Bose-Einstein condensation has been realized for various particles or quasi-particles, such as atoms, molecules, photons, magnons and semiconductor exciton polaritons. We have experimentally realized a new type of condensate: a BEC of hybrids of surface plasmons and light in a nanoparticle array [1]. The condensate forms at room temperature and shows ultrafast dynamics. We utilized a special measurement technique, based on formation of the condensate under propagation of the plasmonic excitations, to monitor the sub-picosecond thermalization dynamics of the system. Recently, we have achieved such Bose-Einstein condensation also at the strong coupling regime, and shown by varying the lattice size that the thermalization in these systems is a simulated process that occurs in 100 femtosecond scale [2]. This new platform is ideal for studies of differences and connections between BEC and lasing [3,4,5]. While usually lasing in nanoparticle arrays occurs at the centre of the Brillouin zone, we have now demonstrated lasing also at the K-point [6]. The lasing mode can be identified with the help of group theory. Clear lasing is observed despite a narrow band gap at the K-point, which is promising considering future studies of topological photonics.
References
[1] T.K. Hakala, A.J. Moilanen, A.I. Väkeväinen, R. Guo, J.-P. Martikainen, K.S. Daskalakis, H.T. Rekola, A. Julku, P. Törmä, Nature Physics, 2018, 14, 739.
[2] A.I. Väkeväinen, A.J. Moilanen, M. Necada, T.K. Hakala, K.S. Daskalakis, P. Törmä, arxiv:1905.07609, 2019.
[3] T.K. Hakala, H.T. Rekola, A.I. Väkeväinen, J.-P. Martikainen, M. Necada, A.J. Moilanen, P. Törmä, Nature Communications, 2017, 8, 13687.
[4] H.T. Rekola, T.K. Hakala, and P. Törmä, ACS Photonics 2018, 5, 1822.
[5] K.S. Daskalakis, A.I. Väkeväinen, J.-P. Martikainen, T.K. Hakala, P. Törmä, Nano Letters, 2018, 18, 2658.
[6] R. Guo, M. Necada, T.K. Hakala, A.I. Väkeväinen, P. Törmä, Physical Review Letters, 2019, 122, 013901.