Theoretical Study On The Polarization Dependence And Threshold Property Of Random Lasers

Random laser, which is a new-type laser, is based on the lasing phenomenon in the amplifying disordered media. It has been a hot subject in laser physics and it presents an important value in application. Since random laser phenomenon was explained by experiments about 10 years before, researchers have done lots of experimental and theoretical work on it.The main contents of this thesis are summarized as follows:(1) A summary of the concept and characteristic of random lasers is given, and some theoretical models and experimental results at present are expatiated.(2) The principle and thoughtway of Finite Difference Time Domain (FDTD) method is introduced , and then, we analyze the conditions of stability,Perfectly Matched Layer (PML) boundary conditions and the setting of excitation sources.(3) The output intensity of a lasing mode varying with the pumping rate is simulated for a one-dimensional random active medium, and the mode's localization extent is also discussed. Subsequently, the influence of the localization extent on the pumping threshold of quasi-state modes is also analyzed.(4) The quasi-state'threshold in one-dimensional random media which has different number of particles for scattering is investigated, and we also analyzed the reason why the value of the pumping threshold of random lasers'quasi-state modes will decrease when the number of particles is increased.(5) Based on the time dependent theory of random lasers, the characteristic of the two-dimensional random laser for both transverse electric and transverse magnetic polarization fashions is analyzed, and we investigate the difference between the two polarization fashions in the same random pattern , such as the modes'frequency,the intensity of radiation,the width of the spectrum,the value of the pumping threshold and so on. The phenomenon that TM modes have a lower value of the pumping threshold than TE modes is found. Finally, we indicated that, as the increase of the orientational disorder of particles'optical axes in the anisotropic media, the laser threshold would decrease.