The Influence Of Nanoparticles On Random Laser From Dye Doped Liquid Crystal

Liquid crystal has optical anisotropy, which can be used as scattering medium in dye doped liquid crystal random laser. The huge optical anisotropy of liquid crystals can be tuned by the external temperature, magnetic field and electric field, which could be realized wavelength tunable, electric field controllable and magnetic field controllable random laser. This kind of random laser will be potential applications in the field of liquid crystal display, integrated photonics and ambient light.In this paper, the influence of Ag nanoparticles (NPs) on random laser from dye doped nematic liquid crystal (DDNLC) has been investigated. And then, the proprieties of random laser, such as threshold energy and degree of polarization (DOP), are studied in dye doped Polyvinyl Alcohol (PVA) dispersed liquid crystals (DDPDLC) film.The threshold energy of random laser from DDNLC solution without applied electric field was reduced by addition of Ag NPs. When the Ag NPs concentration increased to 2.246x1014/mL, the threshold energy of random laser decreased to 1.7uJ/pulse, which is half of the threshold energy (3.9uJ/pulse) in the DDNLC solution without addition of Ag NPs. This is because the localized surface plasmon resonance (LSPR) of Ag NPs can improve pumping efficiency and effectively enhance the excitation probability of a dye molecule. When the concentration of Ag NPs in DDNLC solution with applied electric field increased from 0 to 1.123×1015/mL, the turn off voltage of random laser was reduced from 1.88V to 1.50V. The reason is that the electro-optical characteristic of liquid crystal was enhanced due to the presence of Ag NPs. When the concentration of Ag NPs in DDNLC solution is 0,2.246×1014/mL and 1.123×1015/mL, respectively, the turn-on time is 46s,35s,31s, and the turn-off time is 140s,136s,130s. The total response time in DDNLC without Ag NPs is 186s. When the concentration of Ag NPs increased to 2.246×1014/mL and 1.123×1015/mL, the total response time reduced 15s and 25s. These experimental results indicates the energy consumption of random laser from DDNLC can be reduced by doping Ag NPs, which could provide valuable reference for the potential applications of random laser in the field of liquid crystal display and ambient light.The influence of the factors, such as the film thickness, the concentration of dye doped liquid crystal, drawing operation, on the threshold, wavelength, polarization of random laser from DDPDLC film has been investigated. The DDPDLC film with 32.7wt% of dye doped liquid crystal (DDLC) were made, and the thickness were 15um,25um,38um,52um,61 um and 72um, respectively. When the thickness of film was 38 um, the lowest threshold energy (4.6uJ/pulse) of random laser was obtained. The reason is that the increase of thickness will lead to the increase of scattering strength and improve the pumping efficiency. When the thickness exceeds 38um, the scattering strength and the pumping efficiency reach saturation. With increasing the percentage of DDLC from 19.7wt% to 49.3wt% in the 38 um film, the threshold energy decreased from 6.9uJ/pulse to 3.4uJ/pulse and the peak wavelength showed about 5nm red shift. The decrease of threshold energy is because the increase of scattering strength and the improvement of pumping efficiency due to the increase of DDLC concentration. The red shift is due to the reabsorption of dye. In the DDPDLC film with 32.7wt% of DDLC and the thickness of 38um, the alignment direction of liquid crystal molecules, as well as dye molecules, could be reoriented along the stretching direction by stretching operations of the DDPDLC film. These results lead to the enhancement of scattering strength in parallel plane and the weaken strength of scattering strength in vertical plane. This behavior leads that the parallel polarization components of random laser intensity increases and the vertical polarization components of random laser intensity decreases. When the DDPDLC film isn't stretched, the DOP of random laser is 0.02, nearly zero. So the DOP of random laser could be adjusted by stretching operations of the DDPDLC film. When the draw ratio of DDPDLC film reaches 4.5 and 5, the DOP of random laser reaches 0.81 and 0.82, respectively. These experimental results could provide valuable reference for the potential applications of dye doped liquid random laser in the field of liquid crystal display, polarization imaging and sensor.