Theoretical And Simulated Study On The Radiation Characteristic Of Random Lasers

Random media usually refer to electromagnetic media with a random spatial variation of the refractive index in optical wavelength scale. Such a medium possesses much optical characteristics in which random lasing is the most interesting optical phenomenon. This kind of lasing phenomenon does not require a pre-defined physical cavity but originates from photon localization. The study on this phenomenon will open up land of photon localization and new field of laser physics, and expect to develop a kind of new-type micro-cavity laser. In this dissertation, Finite Difference Time Domain (FDTD) and Transfer Matrix Method (TMM) are developed to analyse the feedback mechanism and optical characteristic of random lasers. Based on laser physics, solid physics and conventional theory and algorithm of the FDTD method, several models are analyzed in detail,in which their application range and features are discused and compared with each other. The theory and algorithm of TMM and FDTD method are studied in detail and implemented in a unique and efficient approach. Several essential questions including excitation sources, detection techniques and Perfectly Matched Layer (PML) boundary conditions are analyzed to obtain the accurate information from simulations. The combination of the FDTD and TMM method provides a powerful tool for simulation and analysis of random lasers with high performance. Based on theory of photon localization, the effects of the dielectric constant fluctuation on the spatial distribution of light waves are investigated in detail in one-dimension and two-dimensional random media. Results show that the fluctuation on dielectric constant results in some special configurations forming in a random medium. Such configurations trap the lightwave and delay relaxation time of the photon in it, and play a role similar to an optical resonator, which results in lasing oscillation in a random medium with a pump beam. Such optical resonators distribute randomly in the random medium and their amount and distributing characteristics depend on the strength of the dielectric constant fluctuation. The quality factor is vital to understand the random lasers. Resulsts obtained from TMM method in one-dimensional random media, show that the strength of randomness affects the quality factor of quasi-state modes. It is possible for high quality factor of quasi-state mode to be formed in random medium with some degree of disorder strength. Based on theory of mode selection, the selectively excitation of quasi-state modes is studied in detail. Several mode-selecting techniques,such as the local pumping, adjusting gain curve and changing the shape of the random medium, are applied to mode selcetion. The results show that above techniques affect the excitation of quasi-state modes. Finally, on the basis of analysis of lasing threshold, the low-threshold random lasers are analyzed in detail. Results show that the quasi-state modes with a stronger spatial localization have lower threshold and can be amplified preferentially. The two-photon pump is an effective method to reduce threshold while partially random media serve as another material to realize the low-threshold random laser. Results show that the threshold is closely related to the strength of randomness and the lasing threshold reachs a minimum value at some value of disorder strength.