Effects Of Spin Polarization And Spatial Confinement On Optical Properties Of Bulk Semiconductors And Doped Quantum Wells
We correlated experimental results with theoretical estimations of the dielectric function ε(ω) in two contexts: the effect of an electric ﬁeld in quantum wells and that of the spin polarization of an interacting electron-hole plasma in bulk semiconductors. In the ﬁrst part, we recorded photoreﬂectance spectra from Ge/GeSi quantum wells of different widths but having comparable builtin electric ﬁelds caused by doping. The reason why the spectra differed in overall shape was difﬁcult to understand by conventional methods, for example, by calculating the allowed transition energies or by ﬁtting the data with lineshape functions at each transition energy. Instead, we computed the photoreﬂectance spectra from ﬁrst-principles by using the conﬁned electron and hole wavefunctions. This method showed that the spectra differ in overall shape because of the experimentally hitherto unobserved trend in quantum well electro-optical properties, from the quantum conﬁned Franz-Keldysh effect to the bulk Franz-Keldysh effect, as the well width is increased. The second part develops a threeband microscopic theory for the optical properties due to spin-polarized carriers in quasiequilibrium. We show that calculations based on this theory reproduce all the trends observed in a recent circularly polarized pump-probe experiment reported in the literature. To make the computation less intensive, we proposed a simpliﬁed, two-band version of this theory which captured the main experimental features. Besides, we constructed a cw diode laser-based pump-probe setup for our own optical studies of spin-polarized carriers by Kerr rotation. We achieved a sensitivity of detection of Kerr rotation of 3 x 10¯ 8 rad, corresponding to an order of magnitude improvement over the best reports in the literature. The efﬁcacy of our setup allowed for the demonstration of a pumpinduced spin polarization in bulk GaAs, under the unfavorable conditions of steady-state and room temperature.
- Physics (PHY)