Lasing and Bose-Einstein condensation in plasmonic lattices at weak and strong coupling regimes

Plasmonics takes advantage of the coupling of light to charge oscillations in metals, which enables breaking the diffraction limit and confining the optical fields in sub-wavelength volumes. The extreme field confinement can greatly enhance the light-matter interaction of photons and quantum emitters, such as atoms and molecules. In this dissertation, I have studied metal nanoparticles arranged in regular two-dimensional lattices, combined with organic dye molecules. The aim is to study strong light-matter interaction as well as lasing and condensation phenomena in these plasmonic lattices. I have fabricated the nanoparticle arrays with electron beam lithography that allows precise control of the shape and size of the nanoparticle, and periodicity and geometry of the lattice. Optical modes of the plasmonic lattices are characterized by white light transmission and reflection measurements. Angular and spatial distribution of the photoluminescence intensity is measured, together with the corresponding spectra, under optical excitation of 50 fs laser pulses. I have developed a rate-equation model to understand the dynamics of stimulated processes. In Publication I, we show strong-coupling between the lattice modes and the molecular excitations. Lasing action in both bright and dark modes of the plasmonic lattice is demonstrated in Publication II. In Publication III, we probe the dynamics of ultrafast laser pulse generation and observe a modulation speed of more than 100 GHz. The pulse build-up time and pulse duration is measured with a double-pump spectroscopy technique that utilizes the non-linearity of photoluminescence at the lasing threshold.In Publication IV, we demonstrate Bose-Einstein condensation of excitations in the plasmonic lattice, the first realization of condensation in plasmonic systems. We establish a measurement scheme that provides a direct access for observing thermalization and the accumulation of macroscopic population to the energy ground state. The scheme utilizes open-cavity character and propagating modes in the plasmonic lattice. Thermalization is explained with recurrent absorption-emission cycles in the dye molecules forming a thermal bath, in the weak coupling regime. In Publication V, we achieve a Bose-Einstein condensate of strongly-coupled lattice plasmons. This condensate is hundred-thousand fold brighter in luminescence intensity compared to the first one. We observe a distinct thermalized population distribution extending over one decade, in time integrated luminescence signal. Multiple condensation peaks are observed at the lowest energy states with a thermal tail at higher energies that follows Maxwell-Boltzmann distribution at room temperature (333 K). The thermalization occurs in a 200 fs timescale, which is explained with stimulated processes and strong coupling. Room-temperature operation, significant robustness and high luminescence of the samples provide an excellent platform for future studies of luminous driven-dissipative condensates, non-equilibrium quantum dynamics and topological photonics.