Metallic nanostructures provide a toolkit for the generation of coherent light below the diffraction limit. Plasmonicbased lasing relies on the population inversion of emitters (such as organic fluorophores) along with feedback provided by plasmonic resonances. In this regime, known as weak light-matter coupling, the radiative characteristics of the system can be described by the Purcell effect. Strong light-matter coupling between the molecular excitons and electromagnetic field generated by the plasmonic structures leads to the formation of hybrid quasi-particles known as plasmon-exciton-polaritons (PEPs). Due to the bosonic character of these quasi-particles, exciton-polariton condensation can lead to laser-like emission at much lower threshold powers than in conventional photon lasers. Here, we observe PEP lasing through a dark plasmonic mode in an array of metallic nanoparticles with a low threshold in an optically pumped organic system. Interestingly, the threshold power of the lasing is reduced by increasing the degree of light-matter coupling in spite of the degradation of the quantum efficiency of the active material, highlighting the ultrafast dynamic responsible for the lasing, i.e., stimulated scattering. These results demonstrate a unique roomtemperature platform for exploring the physics of exciton-polaritons in an open-cavity architecture and pave the road toward the integration of this on-chip lasing device with the current photonics and active metamaterial planar technologies.

Lighting applications require directional and polarization control of the emitted light, which is currently achieved by bulky optical components such as lenses, parabolic mirrors, and polarizers. Ideally, this control would be achieved without any external optics, but at the nanoscale, during the generation of light. Semiconductor nanowires are promising candidates for lighting devices due to their efficient light outcoupling and synthesis flexibility. In this work, we demonstrate a precise control of both the directionality and the polarization of the nanowire array emission by changing the nanowire diameter. We change the angular emission pattern from a large-angle doughnut shape to a narrow-angle beaming along the nanowire axis. In addition, we tune the polarization from unpolarized to either p- or s-polarized. Both the far-field emission pattern and its polarization are controlled by the number and type of guided or leaky modes supported by the nanowire, which are determined by the nanowire diameter.

We present a theoretical framework that allows us to investigate the scattering of terahertz surface plasmon polaritons ͑SPP's͒ by arrays of subwavelength grooves and ridges on semiconductors. The formulation is based on the reduced Rayleigh equation resulting upon imposing an impedance boundary condition. Guided by approximate estimations of the broadening with temperature of the first gap in the SPP dispersion relation in the case of indium antimonide samples with rectangular grooves, numerical calculations are carried out to determine the spectral dependence of all the SPP scattering channels ͑reflection, transmission, and radiation͒ in the immediate vicinity of that gap. The thermally induced switching of the SPP reflection and transmission nearby the lower SPP band edge is investigated as a function of groove parameters ͑size and number͒; near-field intensity maps are also presented. We thus shed light on the SPP scattering and switching physical mechanisms, thereby providing the most suitable experimental configurations.