No abstract

Lasing at the nanometre scale promises strong light-matter interactions and ultrafast operation. Plasmonic resonances supported by metallic nanoparticles have extremely small mode volumes and high field enhancements, making them an ideal platform for studying nanoscale lasing. At visible frequencies, however, the applicability of plasmon resonances is limited due to strong ohmic and radiative losses. Intriguingly, plasmonic nanoparticle arrays support non-radiative dark modes that offer longer life-times but are inaccessible to far-field radiation. Here, we show lasing both in dark and bright modes of an array of silver nanoparticles combined with optically pumped dye molecules. Linewidths of 0.2 nm at visible wavelengths and room temperature are observed. Access to the dark modes is provided by a coherent out-coupling mechanism based on the finite size of the array. The results open a route to utilize all modes of plasmonic lattices, also the high-Q ones, for studies of strong light-matter interactions, condensation and photon fluids.

We show strong coupling involving three different types of resonances in plasmonic nanoarrays: surface lattice resonances (SLRs), localized surface plasmon resonances on single nanoparticles, and excitations of organic dye molecules. The measured transmission spectra show splittings that depend on the molecule concentration. The results are analyzed using finite-difference time-domain simulations, a coupled-dipole approximation, coupled-modes models, and Fano theory. The delocalized nature of the collective SLR modes suggests that in the strong coupling regime molecules near distant nanoparticles are coherently coupled.

Bose-Einstein condensation is a remarkable manifestation of quantum statistics and macroscopic quantum coherence. Superconductivity and superfluidity have their origin in Bose-Einstein condensation. Ultracold quantum gases have provided condensates close to the original ideas of Bose and Einstein, while condensation of polaritons and magnons have introduced novel concepts of non-equilibrium condensation. Here, we demonstrate a Bose-Einstein condensate (BEC) of surface plasmon polaritons in lattice modes of a metal nanoparticle array. Interaction of the nanoscale-confined surface plasmons with a room-temperature bath of dye molecules enables thermalization and condensation in picoseconds. The ultrafast thermalization and condensation dynamics are revealed by an experiment that exploits thermalization under propagation and the open cavity character of the system. A crossover from BEC to usual lasing is realized by tailoring the band structure. This new condensate of surface plasmon lattice excitations has promise for future technologies due to its ultrafast, room-temperature and on-chip nature.Bosonic quantum statistics imply that below a certain critical temperature or above a critical density the occupation of excited states is strictly limited, and consequently, a macroscopic population of bosons accumulates on the ground state 1 . This phenomenon is known as Bose-Einstein condensation (BEC). Superconductivity of metals and high-temperature superconducting materials are understood as BEC of Cooper pairs 2, 3 . The BEC phenomenon is central in superfluidity of helium although the condensate constitutes a small fraction of the particles 4 . Textbook Bose-Einstein condensates with large condensate fractions and weak interactions were created with ultracold alkali atoms 5-7 , and the fundamental connection between the superfluidity of Cooper pairs and the Bose-Einstein condensation was confirmed by experiments with ultracold Fermi gases 3 . While all these condensates allow essentially equilibrium description, as was the original one by Bose and Einstein, the phenomenology has expanded to non-equilibrium systems [8][9][10][11][12] . Hybrid particles of semiconductor excitons and cavity photons, called exciton-polaritons, have shown condensation and interaction effects [13][14][15][16][17][18][19] , creating coherent light output that deviates from usual laser light. Magnons, that is, spin-wave excitations in magnetic materials 20, 21 , and photons in microcavities 22, 23 form condensates as well. The most technologically groundbreaking manifestation of macroscopic population due to bosonic statistics has so far been laser light, which is a highly non-equilibrium state not thermalized to a temperature of any reservoir. As the BEC phenomenon has been observed only in a limited number of systems, new ones are needed for pushing the time, temperature and spatial scales where a BEC can exist, as well as for opening viable routes to technological applications of BEC.Here we report the observation of BEC for bosonic quasip...

Bosonic condensates offer exciting prospects for studies of non-equilibrium quantum dynamics. Understanding the dynamics is particularly challenging in the sub-picosecond timescales typical for room temperature luminous driven-dissipative condensates. Here we combine a lattice of plasmonic nanoparticles with dye molecule solution at the strong coupling regime, and pump the molecules optically. The emitted light reveals three distinct regimes: one-dimensional lasing, incomplete stimulated thermalization, and two-dimensional multimode condensation. The condensate is achieved by matching the thermalization rate with the lattice size and occurs only for pump pulse durations below a critical value. Our results give access to control and monitoring of thermalization processes and condensate formation at sub-picosecond timescale.