Research On The Fabrication Of Polymer Nanofibers And Their Applications In Optics

Electrospinning is a simple and easy-controlling method to make nanofibers which evolves from the electronic spraying technique.The diameter of fiber is adjustable from dozens of nanometer to several micrometers.When the viscosity of the solution is highly enough,the solvent will evaporate and the spraying droplets will be stretched to fibers because of the electric field force.After the commencement of this technology,many kinds of materials were prepared to be nanofibers.These functional nanofibers were applied to a dozens of fields,such as sewage treatment,electrode,air purity,etc.A conventional laser facility mainly contains two sections,including a cavity that is used to provide feedback and the gain medium that acts to amplify the light.When the gain surpasses the loss,the light will be amplified to form laser after reaching a threshold.The basic principle of random laser is quite similar to a conventional laser but instead of the cavity,scattering is the main source that provides feedback.When gain is larger than loss,the light will undergo a multiple scattering steps due to the random materials and lase can occur,which is defined as random laser.Since the random laser was firstly reported in 1994,a growing kinds of materials were attempted as gain and scatterers to generate random lasers,such as semiconductors,biomaterials,polymer,metals,etc.In recent decades,polymer attracted many attentions to realize random lasers which were usually fabricated by a drop method or an electrospinning technology.It was reported that the light would propagate in the fiber that acted as a waveguide,and then it was amplified by the gain materials in fibers,finally laser emission can be achieved.Recently,some researchers reported random laser in the polymer fibers,but the components were always polymethyl methacrylate and laser dyes.The diameter of these fiber that can induced random laser was normally larger than 1 micrometer.In this thesis,polyvinyl alcohol nanofibers were fabricated through an electrospinning method.The random laser was confirmed to be closed related to the diameter of nanofiber.Only when the diameter of fibers was larger than 255 nm,light with a wavelength of 570 nm could be propagated in the fibers and generate random laser.Nanofibers with diameters of 300 nm and more than 500 nm were fabricated in this study,and single mode and multi-modes random lase were realized which indicated the number of modes of the fiber was determined by the diameter of fibers.These results suggested that the random fiber lasers could be applied in the light communications and light-based cryptography.The electrospun fibers were randomly distributed,leading to the fiber acting as a role of scatterers to emit random lasers.Polyvinyl alcohol nanofibers with a diameter of 150 nm were fabricated,but only photoluminescence of laser dye was observed which indicated that the scattering of the fibers was not strong enough to yield random laser.To enhance the scattering,morphological modification and nanoparticle doping were proposed and experimentally measured,by which random laser with a single mode and double mode was realized,indicating that it was possible to produce random lasers based on nanofibers with a small diameter.Light scattering and propagation in fibers were numerically analyzed through a FDTD method and the results indicated that the scattering of laser dye could be ignored,but the scattering of nanoparticles played a key role for random laser generation.The number of guided modes would increase when the diameter of fibers was expanded.Owing to the scattering of nanoparticles,light could propagate in a small nanofiber.As for morphological modified nanofibers,light could propagate in the additional mass made it possible to generate multiple modes of light.The numerical studies confirmed the number of modes was determined by the diameter of nanofibers and accounted for the random lasing actions in scattering-enhanced fibers.