Coherent THz manipulation of ensembles of hydrogenic electrons in gallium arsenide

Electrons bound to shallow donors in GaAs have energy levels that approximate those of the hydrogen atom. The effective mass of the electron and the dielectric constant of GaAs combine to rescale the energies of the orbital state transitions to the THz frequency range. These states are of interest for quantum information schemes, and I wish to explore the dynamics of coherent manipulations of the states.; After providing a theoretical description of the hydrogenic model, and an overview of previous related experiments, I proceed to describe experimental techniques and experiments. The UCSB Free Electron Laser is used to generate the required THz radiation. I use a magnetic field to tune the states into resonance with the available THz frequencies and read out the final state of the electrons using photoconductivity.; By varying the polarization of the THz radiation, I first examine the selection rules for the hydrogenic transitions. Verification of these selection rules is important for the design of THz photonic cavities that may interact with hydrogenic transitions and for selective addressing of state transitions. I then drive Rabi oscillations between hydrogenic states by applying short intense pulses of THz radiation 10--100 ps in length. These short pulses are generated by using laser-activated semiconductor switches to "slice" short pulses from the output of the UCSB Free Electron Laser.; I model the behavior of these hydrogenic electrons using the density matrix equations of motion. Using this model to fit to experimental data, I extract information on the dephasing time of the ensemble of electrons. For the 1s to 2p+ transition at 2.52 THz, the dephasing time is of order 20 ps and appears to be limited by the presence of the excited state in the conduction band. With numerical calculations using the model, I explore the range of parameter space in which Rabi oscillations can be observed.