Investigation of coherent emission of plasmonic nanolasers

In the last two decades, a considerable amount of research has been done in the field of nanolasers, with the purpose of building coherent light sources with limited size and low pump thresholds. This work deals with the topic of plasmonic nanolasers, with the main purpose of analyzing their emission properties. Plasmonic nanolasers are devices in which light is coupled to matter at the nanoscale, providing a source of coherent light without the need for macroscopic feedback cavities. This opens the door for numerous applications, mainly in the field of integrated photonics and sensing. After a first part, dedicated to a historical and theoretical introduction to nanolasers, some experimental data are reported. Two samples (both produced using Electron Beam Lithography) have been analyzed: a 2D square Au nanodisk array emitting in the near-infrared region and a 2D hexagonal Al nanocone array with emission in the visible range. The first sample is made of Au nanodisks arranged in a 2D square lattice with a spacing of 600 nm. Its morphology was first analyzed with imaging techniques, such as Atomic Force Microscopy and Scanning Electron Microscopy. Finite-elements simulations with COMSOL Multiphysics allowed to study the extinction map and to select a suitable emitter (IR-140) to be coupled with the nanostructure. A layer of a liquid solution of IR-140 in Dimethyl sulfoxide (DMSO) has been placed over the structure and its emission properties have been studied with photoluminescence techniques. Lasing was observed at the Γ point of the reciprocal lattice, corresponding to the normal direction with respect to the array surface. The emission map and the lasing properties were also investigated and analyzed. After this characterization, a ~150-nm-thick silica layer was deposited on the array with the purpose of creating a spacer between the nanoparticles and the emitting molecules. Lasing emission was observed, but with a higher threshold and larger linewidth with respect to the original sample, meaning that plasmonic near-field properties play a role in the nanolasing process, but the lattice dispersion properties of the array still make it possible to achieve lasing. The second sample is made of Al truncated cones, arranged in a 2D hexagonal array, with a lattice spacing of 475 nm and a solid layer of Poly(methyl methacrylate) doped with an organic dye (Lumogen F305 Red) of about 300 nm thickness. Its emission properties have been analyzed using the same techniques as for the square array. In this sample the polarization and temporal coherence of the emitted light have also been studied. The coherent beam was proven not to be linearly polarized, as expected due to symmetry reasons from emission at the Γ point of the reciprocal lattice. Moreover, the coherence length resulted on the order of millimeters, a very promising value that opens possibilities to new technological applications.