Lasing action in dye-doped thin films coupled with 2D plasmonic nanoarrays

In the last few years, due to the great advances in nanotechnology the peculiar properties of the matter at the nanometric scale have been widely investigated. These properties can be exploited for many innovative applications involving several fields, and, in particular, in nanophotonics, where plasmonic nanolasers gather growing interest. These devices are coherent light sources that can support ultrafast dynamics and ultrasmall mode volume below the diffraction limit. Nowadays, the photonic band-edge lasers that are widely used for many applications have various drawbacks, such as low modulation speeds and diffraction-limited mode confinement. Plasmonic nanolasers can potentially overcome these limits and replace the actual light source technology in many fields from integrated photonic circuits and optical communications to high-performance biosensors. The purpose of the present Master thesis is the synthesis and characterisation of a novel plasmonic-based nanolaser device operating in the near infrared-region. In particular, the attention has been focused on plasmonic nanostructures composed of an array of 2D nanoparticles that act as field enhancer for the lasing of the gain medium. The fabrication of the plasmonic nanoarray was substantially different from the usual approach based on electron beam lithography (EBL). On the contrary, the NanoSphere Lithography (NSL) technique adopted in the present thesis is a highthroughput and cost-effective way to manufacture nanoparticle arrays with a highquality order, obtaining defectless domains of several hundreds of square micrometers. The nanostructure adopted in a this work is two-dimensional hexagonal lattice of nanodomes which is fabricated by gold deposition (carried out by magnetron sputtering) over an ordered array of polystyrene (PS) nanospheres (NSs) obtained using NSL. A gain medium layer was then deposited over the metallic array, and it consists of an organic dye, specifically Styryl 9M (LDS 821), embedded in a polymeric matrix. In order to maximise the efficiency and the throughput of the nanolaser, a match between the uorescence emission of the selected dye and the plasmonic resonance of the nanostructure is needed. To this aim, the domes geometry has been optimised by varying metal thickness and the PS nanosphere radius (while keeping the array lattice parameter) by reactive ion etching (RIE). The deposition process for the gain medium has been achieved by embedding the dye molecules in a PMMA matrix, which is then deposited upon the nanodome array to form a solid film of a hundred of micrometers. The nanostructures have been characterised morphologically by atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques and optically by re ection measurements. The absorption and uorescence spectra that characterise the dye were measured by a UV-VIS-NIR spectrophotometry. As a result, samples supporting lasing action were fabricated and characterised: the emission features of the obtained device were investigated by photoluminescence (PL) measurements to determine the main properties of the laser, including the lasing wavelength, linewidth, threshold, beam divergence and emission angle, showing how, when a coupling between the plasmonic resonance of the nanodome array and the dye emission wavelength is obtained, interesting lasing properties arise and the lasing threshold can be decreased by at least one order of magnitude.