Design and Applications of a Soft X-Ray Detector Using Gallium Arsenide Multiple Quantum Wells.

This thesis focuses on two separate, but related problems. These are: (1) the design and applications of a novel quantum well based soft x-ray detector; (2) the study of absorption lineshapes in quantum confined systems. Described in this thesis is a detector in the 70-500 eV range with spatial resolution of 1mu m-5mu m, temporal resolution of 20 ps, energy resolution of 85 eV and a sensitivity of 25 photons/mu m^2. The detector design is based on observing a change in the optical susceptibility in semiconductor multiple quantum well structures induced by the absorption of x-ray photons. Such a detector can play a potentially important role in plasma diagnostics and x-ray microscopy. The latter application is explored in depth. The high resolution and sensitivity is achieved by adjusting the optical impedance of the detector to the optical probe beam so that the transmission in the absence of x-rays is negligible. The presence of free-carriers from the absorbed x-rays modifies the optical susceptibility in the vicinity of the exciton absorption lines causing the transmission to be significantly modified (over 10%), which is readily detectable. The detector described in this thesis is optimized with respect to temperature and quantum well parameters using a semi-empirical lineshape model. The lineshape model is based on a conventional line-broadening analysis similar in spirit to that used in atomic and plasma physics in which line positions, oscillator strengths, energy and continuum shifts and broadening are described using a combination of theory and experiments. The lineshape model accurately describes absorption and optical nonlinearity as a function of temperature, free carrier density and quantum well quality. Complementing this investigation of lineshapes using semi-empirical models, this thesis also investigates lineshapes from a theoretical viewpoint as well. Presented here are: (1) new analytical results and asymptotic scaling for the renormalization of the band gap in the presence of free carriers, (2) new analytical expressions for the exciton-phonon matrix element and (3) a new lineshape function that accurately describes both the Lorentzian and non-Lorentzian features in the absorption lineshape such as the Urbach's rule. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.).