Theoretical Study On Quantum Dots And Quantum Dot Doped Fiber Gain Devices

Nanocrystal quantum dots(QDs)have discrete energy levels and adjustable band gaps due to quantum-size effects.QDs can be used as optical amplification to fabricate optical amplifiers with wider bandwidths and flatter gains.Moreover,it can also be used as gain media to form lasing resonation in a resonator due to the high photoluminescence efficiency and high gain.QDs are usually distributed in background media.The surface-polarization energy that arises from the difference in dielectric coefficients between the QDs and the background media will affect the bandgap of QDs.In this dissertation,According to the uniqueness theorem,we use an equivalent image-charges method.Because the QDs are dispersed in the background media,adding the potential energy of the interaction between electrons holes and image charges in the Hamiltonian of excitons.A general expression of the bandgap of QDs depending on the dielectric coefficient of background media is presented by solving the exciton Schr?dinger equation and using perturbation method.As examples,the size dependent bandgaps,bandgap shifts,absorption-peak wavelengths and absorption-peak-wavelength shifts of PbSe,PbS and CdSe QDs doped in different background media are determined in details.There is evidence to show that the effect of surface polarization on the bandgap and the first absorption-peak wavelength of QDs is considerable.When the dielectric coefficient of background media is less(or great)than the dielectric coefficient of QDs,surface-polarization effect blue-shifted(or red-shifted)bandgaps of QDs.The bandgap decreases with the increase of dielectric coefficient of background media,resulting into the absorption-peak wavelength to be red shifted.The absorption-peak wavelength shifts of QDs doped in different background media will reach a maximum in a certain diameter depending on the kinds of QDs.For PbSe,PbS,CdSe QDs,the diameters of QDs are 12.1,7.3,2.9nm,respectively.We establish the rate equations of QDs,power-propagation equations of signal light and pump light by using a tow-level model in regards to colloidal PbSe QD-doped fiber amplifiers.The power propagation equations is numerically solved for no signal light inputted to obtain the transmission power of the pump light in the QD doped fiber.Assuming the particle-size distribution of QDs with a Gaussian distribution,the output signal spectrum of QD-doped fiber amplifiers with different pumping powers is obtained by solving the power propagation equations for signal light inputted numerically.The calculated results are in good agreement with the experiment observation that realized previously.The variation of laser output power with pump power at different QD doping concentration is calculated.It is found that the output power of the laser varies with the pump power and there exists an obvious pumping threshold that depends on the QD doping concentration.The higher is the doping concentration of the QDs,the higher the threshold power,this may be due to the high QD doping concentration which means there are moreQDs in fiber and more power is needed to achieve population inversion.The investigation of the surface polarization effect of QDs in this dissertation reveals the fundamental effects of surface polarization on bandgap and first absorption-peak wavelength.Numerical simulations of QD-doped fiber amplifiers and QD-doped fiber lasers also provide theoretical verification of experimental results.