Theoretical Study On The Emission Of Random Lasing In Disordered Media

Random lasers, which have many features, e.g. threshold for lasing action and frequency narrowing, in common with more conventional lasers based on an optical gain medium enclosed in a cavity with two mirrors to enhance stimulated emission, are strongly scattering media with optical gain. Eventually, the random lasers have many different optical properties, the most important one of which is in absence of well-defined cavity mode, from the conventional lasers. Randomness, which is the actual lasing feedback mechanism for the random lasers, is the essential element in such system. The theoretical investigation on the random lasers is based on the localization and laser physics.In this dissertation we study the emission properties in random lasers as follows:(1) We study the dynamical evolution of the localized states of the emission by use of the quasi-state modes theory of random lasers in one-dimension random media with four-level atomic system. Results show that the larger the pump rate is, the shorter the formation time of the localized states is.(2) The saturation effects of quasi-state modes in one-dimension random lasers are numerically investigated based on the time-dependent theory. The results show that many properties of quasi-state modes, such as the localization length, will influence the saturation effects of the random laser system. By investigating these effects, one can choose the best quasi-state mode, which has a low threshold and high saturation intensity, as a steady output in such system with one single frequency pump.(3) We study the relationship between the emission of random lasers and quasistate modes by use of the finite difference time domain (FDTD) method. The spatial profile and the spectra of qusaistate modes are obtained for the passive and active random media. The results show that random lasing modes directly originate from the localized modes inside random media. In the presence of gain, any one of the localized modes can be amplified and serves as random lasing modes.(4) The pumping rate dependence of the peak intensity of the individual lasing mode in two-dimensional active media is investigated. The results show that each of these modes has a typical threshold gain behavior and different threshold pumping rate. The relationship between the mode's threshold and lifetime, which is that the longer the lifetime is, the lower the threshold is, is obtained.(5) The spectral width of transverse magnetic (TM) lasing modes in two-dimensional random media is theoretically investigated by using the time-dependent theory. The evolution curve, in which one can calculate some characteristic parameters of the emission, e.g. the threshold, of the spectral width of TM-polarization modes versus the pump intensity is obtained. The results show that the emission, from the view of spectral width, of the TM-polarization lasing modes have better out-put property than that, which was obtained previously, of TE-polarization.(6) The power spectra from a set of two-dimensional passive random media with the same randomness and different shapes have been investigated by numerically solving the Maxwell equations. The results show the power spectra of such set of passive random system are morphological-dependent. This character is promising for proposing a new mode-selecting technique for random lasers.(7) Based the diffusion model, we study the temporal evolution and the saturation of the emission from two-photon pumping random lasers. Numerical results show how the intensity of emitted light varies from the spontaneous radiation, which undergoes amplification by small-signal gain, to the steady output due to the saturation effect. From the analysis of normalized emitted intensity and saturation intensity, one can obtain a method to control steady output-intensity of two-photon pumping random lasing by adjusting some parameters of the random media.