Lightwave Localization In Disordered Media And Random Lasing

The random laser represents a non-conventional laser, whose basic property is that the lasing phenomeon can appear in random systerms with gain. In conventional laser systerms, the feedback mechanism comes from the light confinement by the cavity. While in random lasers, the multiple scattering origined from the randomness is the actual lasing feedback mechanism for such systerms. Ramdom lasing phenomeon are studied widely not only for their interesting physical properties, but also for their importance in technological applications. The theoretical investigation on the random lasers is based on the localization theory and laser physics.In this thesis, by use of the time-dependment theory of random lasers, we study the emission properties in such systerms as follows:(1) We present the physical model in short-pulse pumping regime based on the time-dependent theory of random lasers. Results show that this model can represent the emission properties more accurately in short-pulse regime.(2) Based on the physical model of random laser in short-pulse pumping regime, we study the relationship between the parameters of the pumping pulse and the emission light properties. Results show that the peak intensity and width of the pumping pulse have much influence on the emission pulse in temporal regime, while the shape of the pumping pulse can hardly influence the emission light. When the random media is pumped by a pulse train with high repetition rate, the pumping process has an accumulate effect. This will allow us to use the pulse train with low peak intensity as the effective pumping source of the random laser systerms.(3) We study how the sample parameters of the random media will influence the emission light properties by use of the physical model of random laser in short-pulse pumping regime. Results show that with the increase of the surface-filling fraction of the random media, the intensity of the emission light becomes higher, at the same time the pulse width becomes narrower and the delay time becomes shorter. The refractive index of the scatterer will also affect the emission light property. The lightwave in the random media behave the transition from diffusion to localization state with the increase the of the refractive index, accompanied by the intensive stimulate emission process. (4) The light localization phenomenon originated from the anisotropic uniaxial scatterer is discussed based on the time-dependent theory for the anisotropic regime. There is a characteristic value for the ratio of two principal refractive indexes. The lightwave in the random media can be localized when this ratio exceed a specific value.(5) Based on the theoretical model of the anisotropic random laser, we study the relationship between the sample parameters and the lightwave localization. The threshold curves for different conditions are presented. Results show that with the increase of the surface-filling fraction, the localization level becomes higher and the emission light will become more intensive. When the sample size is enlarged, it can support more lasing modes and the lasing threshold will be decreased simultaneously.(6) The competition and threshold property between two polarization states in two-dimensional random medium are studied. Results show that the transverse magnetic state has a lower lasing threshold than transverse electric state when they do not share the inverted population. If these two states share the inverted population, the transverse electric state dominates the lasing process while the transverse magnetic state is strongly suppressed.(7) By use of the competition model of two polarization states which share the inverted population, we discuss the sample structure dependence of the lasing properties in this two-dimensional medium. Numerical results show that by increasing the surface-filling fraction of the sample, one can hardly change the situation that the transverse magnetic state was suppressed. However, we can decrease the lasing threshold of the transverse magnetic state by enlarging the sample size, which will achieve the coexist of the two polarization states.