A Study On The Interrelated Questions For The Physical Mechanism Of Lasing In Random Media

In this dissertation, based on Laser Physics and the electro-magnetic theory on light, we study the interrelated questions for the physical mechanism of laseing in random media and found the whole scattering effect physical model on lasing in active random media independently. We study theoretically and numerically the physical mechanism of random lasers, the local modes in photonic crystals with many structural defects or the disorder of dielectric layer's thickness, and random media with uniaxial scatterers that are assumed to be order in spatial location but disorder in spatial orientation of their optical axis.Firstly, based on the experimental phenomena of lasing in random media, we think there could exist some structure which are in between perfectly disorder and perfectly order in the local of active random media, called local aperiodic quasi-structure, and it results in complex localized modes. As the localized modes spatially overlap the localized gain region of active random media, light is amplified in the gain media through stimulated emission. Lasing in active random media is a whole collective effect, and it is due to the interaction of the complex localized modes in active random media with local aperiodic quasi-structure with appropriate pump light. The typical transmission through the active random media with local aperiodic quasi-structure is calculated, and it's localized mode is discussed. The effect of pump area on lasing modes and pump power on the lasing of localized modes in active random media is investigated. Considered the structure characteristics in real experimental system, the random media is divided into two regions, i.e., pump and non-pump region. Dependence of lasing modes on pump area is qualitatively explained by means of the model that lasing is due to the interaction of the complex localized modes in active random media with local aperiodic quasi-structure with appropriate pump light. There exists different pump size for lasing for different modes. As the pump size decrease in this random system, the pump threshold of lasing modes increases inversely. There are different lasing modes for different excitation area in this random system. The threshold of pump power for the lasing of localized modes is qualitatively explained by means of the model which lasing is ascribed to the interaction of the complex localized modes in active random media with local aperiodic quasi-structure with appropriate pump light. The frequency of localized modes does not change with the loss and gain of active random media, the shape of the relative intensity distribution of the light is independent on the loss and gain, the only change is its amplitude, the mode which has the largest Q has the smallest threshold for lasing, the mode which has the smallest Q has the largest threshold for lasing, and the pump power threshold for different lasing modes is different.Secondly, the transmission spectrum in one-dimensional medium with uniaxial scatterers that are assumed to be order in spatial location but disorder in spatial orientation of their optical axis has been investigated by use of the transfer matrix method. Results show that the medium disorder plays an important role in determining the lightwave state, the localized state appears when the medium disorder is strange enough, a new mechanism creating random laser phenomenon is revealed.Thirdly, using the tight-binding method, defect modes in photonic crystals with many structural defects are investigated. We obtained an equation for the Defect modes in photonic crystals with many structural defects. Using the transfer-matrix-method simulation, the transmission spectrum in such one dimensional(1-D) systems including three defects is calculated, the influence of the interaction between defects on defect modes is discussed, and it is compared with the case of positive refractive index structural defects。Finally, the influence of the disorder of dielectric layer's thickness on the light propagation in 1D photonic crystals is investigated. The transmission spectrum is calculated using transfer-matrix method. The result shows that the transmission property is related to the random degree, frequency and the number of dielectric layers, and there exists band-gap extension due to the non-relating disorder of dielectric layer's thickness. The transmissivity spectrum of light in one-dimensional photonic crystals with random change of dielectric layer's thickness is investigated using transfer-matrix method. Compared with periodical structure, the band gap fade away and the transmissivity property is related to the random degree, frequency and the number of dielectric layers.