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Numerical Simulation, Tunability and Experimental Investigation of Random Lasers and Using ZnO Nanosheets in these Lasers and UV Photodetectors

Ghasempour Ardakani, Abbas | 2012

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 43244 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Mahdavi, Mohammad; Bahrampour, Ali Reza; Taghavinia, Nima
  7. Abstract:
  8. In this thesis, first, random lasing in a three dimensional disordered medium has been simulated using FDTD method and some properties of random lasers which were observed by others in the previous experiments have theoretically been shown. Our simulation results show that when the density of particles increases to an upper limit, the intensity of lasing modes is enhanced. Also, the effects of pumping rate and particle size on the number of lasing modes and their intensity are studied. Lasing threshold of laser modes in different disordered systems is calculated and it is shown that by an appropriate selection of the central frequency of gain line-shape, the output power intensity of random lasers increases. Then, a simple and fast method is introduced based on transfer matrix method for calculation of lasing frequency, threshold pumping rate of different modes of fiber Bragg grating random lasers. In this model cross and self saturation effects are considered. Numerical calculations show that by increasing the number of fiber Bragg grating, the number of lasing modes increases, while the threshold pumping rate of the first mode decreases. Then, it is proposed a one-dimensional tunable random laser based on superconducting layers and it is theoretically demonstrated that it is tunable with ambient temperature in near infrared and terahertz domain. Random laser characteristics have been calculated using the one dimensional FDTD method. Here, it is observed that transition from metal phase to pure superconducting phase leads to the enhancement of the laser emission and number of lasing modes. It is also shown that the tunability of random lasers with external magnetic field is possible using magneto-optic layers. It is numerically shown that due to magneto-optical Voigt effect, the emission spectrum of a semiconductor-based random laser can be made tunable by adjusting the external magnetic field.
    Here, a time dependent model is also presented for investigation of random lasers in the presence of an inhomogeneous broadened gain medium such as PbSe quantum dots with a size distribution. By employing finite difference time domain, the governing equations are numerically solved and lasing spectra and spatial distribution of the electric field are calculated. The results show that the number of lasing modes and their intensities increase with pumping rate. It is also demonstrated that increasing the standard deviation leads to decrease of the laser intensity. Then, we fabricate two random lasers based on WO3 powder and ZnO sheets as dielectric elements. In both lasing systems, an organic dye plays the role of active medium.
    Because ZnO is a suitable material for fabrication of random lasers and UV photodetectors, in this thesis, a UV photodetector is fabricated based on electrodeposited ZnO nano-sheets thin films. Before electro-deposition, a thin buffer layer of ZnO is deposited on the glass by pulsed laser deposition method. Our results show that photoresponse properties of the photodetectors strongly depend on the sheet sizes. The smaller sheets exhibited enhanced photosensitivity, shortened fall times and decreased gain compared to larger ones. We showed that photodetectors based on ZnO sheets have a faster response than ones based on polycrystalline films. It was also shown that even less response time could be obtained by using comb-like electrodes instead of two-electrode
  9. Keywords:
  10. Anderson Localization ; Heteroscedasticity ; Finite Difference Time Domain (FDTD) ; Inhomogeneity ; Random Lasers ; Tunability ; Ultraviolet Photo Detector ; Zinc Oxide Sheets

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