Research On The Plasmonic Enhanced Random Lasing In Liquid Crystals

Random lasers have many advantages over conventional lasers,such as low fabrication cost,small volume,flexible shape and omni-directional emission.Owing to these advantages,random lasers have broad application prospects in the fields of display device,target identification,medical diagnostics,etc.Random lasers do not have the fixed laser cavity,and the optical feedback comes from the multiple scattering of the random media.Liquid crystals are one kind of ideal host media for random lasers due to the anisotropic refractive indices and the high optical sensitivity to the external stimuli.Metal nanoparticles have important applications in random lasers because of their strong scattering and the localized surface p lasmon resonance(LSPR)properties.In this thesis we make a deep research on the optical properties of the noble metal nanoparticles,the electro-optical and plasmonic controllable nematic liquid crystal random lasers,and the blue phase liquid crystal random lasers.The main contents and inovation points of the thesis are summarized as follows:(1)We study the optical properties of the gold(Au)nanoparticles by means of the discrete dipole approximation method.We first investigate the LSPR of the nanosphere,nanoellipsoid,nanorod and nanocapsule.Then we analyze the plasmonic circular dichroism of both the helical Au nanosphere assemblies and the helical Au nanoellipsoid assemblies.(2)We systematically study the LSPR and the plasmon splitting(PS)of both the gold/nematic liquid crystal(Au/NLC)sphere core-shell nanostructures and Au/NLC ellipsoid core-shell nanostructures by means of the discrete dipole approximation method.We consider the effects of the anchoring condition of the NLC molecules,the radius of the Au core and the layer thickness of the NLC shell on the LSPR and on the PS.We find that the wavelength of the LSPR of the coll-shell nanostructure is different in the different configurations of the NLC director.For the Au/NLC sphere core-shell nanostructures,when the polarization of the incident light is parallel to the x-axis,the wavelength of the LSPR is longest in the weak anchoring condition.The surface alignment of the NLC director will lead to the splitting of the plasmon frequency.For the sphere core-shell nanostructure,the PS strength is largest in the weak anchoring condition.(3)We prepare the dye-doped polymer stabilized blue phase liquid crystals(PSBP-LCs)and investigate the coherent random lasing and band-edge lasing from these PSBP-LCs.From some samples that the photonic band edge is far away from the emission wavelength of the dye,we first observe the bichromatic coherent RL emission.The shorter-and longer wavelength lasing peak groups are centered at 612 nm and 652 nm,respectively,and the full width at half maximum of the peaks in the groups is about 0.3 nm.We prove that the optical feedback associated with the bichromatic emission originates from the multiple scattering of the emitted photons.We find that the relative intensity between the bichromatic coherent emission peak groups can be controlled by the polarization of the linearly polarized pump light.Besides,we study the band-edge lasing of the PSBP-LCs and analyze the formation mechanism of the random lasing and band-edge lasing in the blue phase liquid crystals.(4)We prepare the dye-doped nematic liquid crystal(DDNLC)containing the Au NPs and investigate the electrically controllable plasmonic enhanced coherent RL.We can only observe the amplified spontaneous emission from the DDNLC in absence of the Au NPs,but we can achieve the low threshold RL from the DDNLC when it has the Au NPs.The enhanced action of the Au NPs on the RL is mainly achieved through the enhancement of the localized electric field in the vicinity of the Au NPs.The lasing properties can be controlled by applying the direct current electric field on the liquid crystal cells.The electrically controllable RL is achieved by changing the orientation of the NLC and dye molecules,thereby changing the absorption and the spontaneous emission of the dye.To achieve the optimal control of the RL properties,the polarization of the pump light should be parallel to the rubbing direction of the cells.In addition,the lasing output intensity is direction-dependent.