Effect Of Cylindrical Defects Introduced In Random Gain Media On Lasing Mode

As a new type of light source,random lasers have potential applications in imaging,biological medicine,and so on.The design and control of random laser is one of the research focuses for the development and application of random lasers.In this thesis,the influence of introducing cylindrical defects into dye-doped nematic liquid crystals on the random laser threshold and mode characteristics is observed and analyzed experimentally.Then,two-dimensional random scattering systems with and without the gain medium are constructed,and FDTD method is used to simulate the effect of cylindrical defects on quasimodes and random lasing modes.A 100-?m-thick dye-doped liquid crystal cell containing a cylindrical defect is prepared.Then the threshold and spectral differences of random lasers radiation from the defect with disordered nematic director orientations and the area with uniform orientation of nematic director are studied and compared,by using a experimental setup which can accurately control the displacement of the pump position relative to the central position of the defect.And the random lasing modes in the weak scattering medium can be controlled by moving the pump position.The two-dimensional random scattering model is constructed,and the quasimodes in the random scattering system are simulated by FDTD method.By controlling the parameters(such as filling rate,size,arrangement and distribution)of scattering particles of the random scattering system,the relationship between quasimodes and scattering structure is given.Then,the cylindrical defect region without filling scattering particles is introduced into the system,and the influence of such defect on quasimodes is discussed.On the basis of the above two-dimensional random scattering system,a uniform gain medium is added into the random scattering system to simulate the random lasing behavior using FDTD method.Firstly,by changing the arrangement of scattering particles and relative refractive index of scattering particles relative to background,the influence of scattering structure on the light field distribution and random lasing modes in the system is studied,and the effect of scattering intensity on light localization and spectrum is considered.Then,a single cylindrical defect region without filling scattering particles is introduced into the two-dimensional random gain system.To control the size and position of the defect,competition and coupling between random lasing modes changes,resulting in the variation of mode behavior of the random laser and the formation of new lasing modes.Changing the defect into a cylindrical defect region with high refractive index,the formation of Whispering gallery mode in such cylindrical defect and its control effect on the random lasing modes are explored.