Light Field And The Thz Field Role Download The Stream Sub-dynamics Study

Semiconductor intersubband lasers and carrier coherent dynamics in nanostruc-tures have attracted much attention in recent years. Interaction between the external fields and semiconductor nanostructures can generate a lot of novel phenomena. In this thesis, the microprocesses in the optical pumped Raman laser are numerically simulated, interaction between the external ultrafast optical field with the semiconductor subbands carriers is studied, and a new type of Green's function is introduced to simplify the description of the periodic field driven system. These results are important for the study of carrier coherent dynamics and the design of semiconductor intersubband lasers. The main results of this thesis are listed as following.1. Electron-phonon interaction in the asymmetric quantum well is studied with the dielectric continuum model. When the quantum well width is small, the interface phonon mode is the dominant relaxation mechanism. If the energy split of two subbands is approximately the same as the interface phonon mode, there will be a resonant transition. The quasi-antisymmetric phonon modes give rise to much larger transition rates than those assisted by quasi-symmetric ones. It is found that with increasing pumping intensity, the Raman gain shows saturation. This is in accordance with the experimental results.2. The transient absorption spectrum and the subband population distribution of the semiconductor quantum well illuminated by ultrafast pulses are studied by using the semiconductor Bloch equations. By adjusting the phase shift and time delay of two identical optical pulses, selective transitions between different sub-bands in a quantum well is theoretically investigated. Meanwhile, the electron coherent oscillation in the subbands is expected to generate THz radiations. Our calculations show that the absorption spectrum and the subband population can be influenced by the intensity and phase of the external THz fields.3. If an electron system is under the influence of an external periodic field, we can expand the Hilbert space defined by the Hamiltonian to include the external field exactly. Expectation values of an operator defined on this enlarged Hilbert space are given and a new type of Green's function is defined on the enlarged Hilbert space. Using this newly-defined Green's function, we calculate the density of states and thermal properties of a two dimensional electron gas. This method is physically clear and can be generalized to the case where there are several periodic fields exist at the same time.