Lasing boosted with plasmonic nanostructures

Plasmonics, a technique to tightly concentrate light down to the nanoscaleby coupling photons to surface plasmons,has been employed to downscalelasersto sub-wavelength dimensions. The resultant device is called a spaser, short for surface plasmon amplification by stimulated emission of radiation, or more generally plasmonic nanolaser, which is being explored for applicationsin areas such as sensing and biomedical imaging. In this talk we will first make an overview of the major advances achieved thus far in this emerging field. Then, we will present our work addressing two major challenges regarding wavelength tunablility and lasing directionality of plasmonic nanolasers. To this date, single plasmonic nanoparticles, two-dimensional arrays of nanoapertures in plasmonic metasurfaces, and bulk plasmonic metamaterials have been designed and utilized as plasmonicnanocavities. Lasing wavelength tunability and directionality have been achieved with these nanocavities. Interesting effects have been also observed in other geometries. For instance, the 3-D random nanostructures bring in a possibility to control the lasing resonance by simply tuning the polarization of the pump laser, which is out of reach of conventional dielectric-cavity based lasers. By engineering the absorption and scattering properties, the laser can operate without degradation of lasing performance in the presence of densely packed metal nanostructures; consequently, spatial confinement of lasing modes at micron-scales has been obtained. We also present our two approaches to numerically model realistic spasers and nanolaser arrays. In the frequency domain, the spasers that utilize plasmonic modes of metallic nano-particles are modeled within the classical electrodynamics scattering framework using an intensity-dependent Lorentzian dielectric function, which rigorously accounts for quantum-mechanical saturation effects. In the time domain, we have developed a systematic approach to study lasing in plasmonic nanostructures using a finite difference model coupled to the rate equations of a multi-level gain system (4- and 6-level system). The modeling results show good agreement with experimental data.