Plasmonic Random Lasing With Silver Nanoparticles Embedded In Certain Gain Media

With the rapid development of nanofabrication techniques,increasing attention focuses on the design of ever-diminishing laser systems,even at a scale where both their mode size and device dimension are less than the diffraction limit in free space.Of these,various mirrorless lasers utilizing metallic nanostructures,such as metal-particle-based random lasers and plasmon lasers,might be the very hopeful candidates in the future applications.The localized plasmon-polariton fields of Ag nanostructures exhibit very competitive capabilities to improve and tailor the stimulated emission characteristics of the surrounding gain media.Moreover the localized plasmon-polariton can be easily controlled by engineering the particle size and shape,changing the dielectric and chemical environments,and modulating the electromagnetic coupling with other adjacent particle partners.Since the advent of the first random laser several decades ago,the interest in random lasers has grown very rapidly because of their extensive application prospect in the fields of display,target identification,biomedicine and photonic devices.The necessary condition for achieving a typical random lasing is that the disordered laser material is required to support coherent or incoherent feedback and meantime provide a sufficient optical gain for the seed photons to be lasing.In this thesis we aim to incorporate the merits of localized plasmon-polariton and random lasers by introducing silver nanosparticles into some gain media to realize plasmonic random lasers.We experimentally and theoretically studied the optic properties of several new laser structures containing silver nanoparticles,well understood the physics behind the laser emissions of these structures,and elaborately demonstrated an electrical modulation over the plasmonic random lasing resonances of silver nanoparticles in a liquid crystal environment.The majority of investigation results in this thesis are summarized as follows:1.Random lasing from dye-doped Ag nanoparticle suspensions in capillary fibers.In order to ascertain the riddle of the random lasing observed in a general class of metalnanoparticle-based gain media,we created a series of dye solutions containing monodisperse Ag nanoparticles of different concentrations and infiltrated them into capillary fibers to form random laser samples.After optically exciting these samples,we found that the low concentrations of Ag nanoparticles support noncoherent emission,while the higher ones contribute to coherent random lasing.There is a critical value of particle's concentration which indicates the transition of the coherence of the laser field.Furthermore,we are impressed by the pronounced red shift of the lasing frequencies with increasing the concentration of Ag nanoparticles.These observations strongly suggest that the random lasing induced by metal nanoparticles in gain media is by no mean an individual particle effect,and the coupled plasmonic resonances of Ag nanoparticles together with the enhancement effects of localized plasmon-polaritons should account for the prominent lasing properties of our disordered samples.2.Electrically controlled plasmonic lasing resonances with Ag nanoparticles embedded in amplifying nematic liquid crystals.We demonstrate the first electric control of coherent plasmonic random lasing with very dilute Ag nanoparticles dispersed in a dye-doped nematic liquid crystal(NLC),in which external electric field dependent emission intensity and frequency-splitting are recorded.A modified rate equation model is proposed to interpret the observed coherent lasing that is a manifestation of double enhancements,caused by the plasmon-polariton near-fields of Ag particles,on the population inversion of laser dye molecules and on the optical energy density of lasing modes.The noticeable quenching of lasing resonances in a weak applied field is due to the dynamic light scattering by irregular director fluctuations of the NLC host,which wash out the constructive interference among different particle plasmon-polariton fields.This provides a direct proof to support that the present lasing resonances are very sensitive to the dielectric perturbations in the host medium and thus are likely associated with some coupled plasmonic oscillations of metal nanoparticles.3.Lasing governed by core-shell nanostructures(metal spheres covered by an amplifying shell).By developing an analytic method able to treat core-shell laser systems,which incorporates both the inevitable gain saturation mechanism of any realistic amplifying medium and the complete vector properties of laser field,we find that the lasing from an amplifying spherical shell with a metallic core can be physically understood as nonlinear resonances of the intrinsic transverse modes of this kind of composite nanostructures.We established a compact form of nonlinear wave equation for laser field under the framework of semiclassical laser theory,obtained the approximate solutions for all TM and TE lasing modes of such coreshell structures,and derived out the characteristic equations determining the threshold lasing frequencies.The nonlinear laser equation we suggested starts from the basic electromagnetic laws and the density matrix theory of laser atoms without any other additional hypotheses on laser modes,and thus might provide an united picture for various lasing phenomena based on the stimulated emission of excited atoms or something.