Felipe successfully defended his PhD Proposal on August 25, 2011.

TITLE:
Modeling and design of waveguide structures for generation of terahertz pulses through optical rectification

ABSTRACT:
In this work I propose to develop theoretical models to study the generation of broadband terahertz (THz) pulses through optical rectification (OR) of an input infrared (IR) femtosecond (fs) pulse in waveguide structures composed of a layer of poled electro-optic (EO) polymers embedded within dielectric cladding layers and possibly metal capping layers. The main goal is to be able to predict the generated THz bandwidth for given input pulse characteristics (pulsed duration, pulse shape, transverse mode profile, etc) and waveguide structure composition (geometrical dimensions and the optical properties of the waveguide composites). The models should also allow to optimize the output bandwidth through variation of the waveguide dimensions and its predictions should agree and somehow correlate to the experimental data of already built waveguide structures.

Malachi successfully defended his PhD Proposal on August 26, 2011.

TITLE:
Modeling of Outflows in Active Galactic Nuclei using X-Ray Data

ABSTRACT:
X-ray spectra of Active Galactic Nuclei (AGN) show absorption and emission features that are thought to arise from highly-ionized, fast accretion disk winds. These outflows carry a significant fraction of the accretion energy and connect the supermassive black holes of AGN to their host galaxies. However, the physical mechanism driving them, the conditions in the gas, and the radial location of the outflow have yet to be understood. In the research that will comprise my thesis, I propose using an existing Compton-thick wind model and archived X-ray observational data to study and better understand the wind components of AGN. The sample of AGN used in this study are selected because of their bright X-ray uxes and long exposure times on the current X-ray telescopes (Chandra, XMM-Newton, and Suzaku). Our goal is to test the model to determine if it constrains the wind parameters. This test may offer insight into how dominant the disk wind model is in the X-ray band.

Sheng successfully defended his PhD dissertation on August 30, 2011.

TITLE:
Investigation of Carrier Dynamics and Nonlinear Effects in Quantum Cascade Lasers using Femtosecond Mid-Infrared Pulses

ABSTRACT:
Quantum cascade lasers (QCLs) are semiconductor lasers based on intersubband transitions and resonant tunneling, emitting mid- to far-infrared light. This dissertation used femtosecond (fs) mid-IR pulses generated by difference frequency generation (DFG) to investigate the ultrafast carrier dynamics and nonlinear effects in QCLs.

In this work, the carrier dynamics of high wall-plug efficiency designed QCLs were investigated utilizing fs mid-IR degenerate pump-probe technique. We observed ultrafast gain recovery within the first 200 fs due to the thin injector barrier inducing fast electron resonant tunneling, fast depletion from lower lasing level by two-phonon resonance design, and relaxation from the continuum region. The observed gain recovery oscillation is interpreted as electrons excited by incoming photons up to higher subbands or continuum region then falling into the next periods of active region so as to dramatically increase the gain on a short time scale. Later, slower gain recovery (2-3 picoseconds) is observed by electron transport through the injector region of the superlattice structure and tunneling into the next period of active region. Another ultraslow recovery component (hundreds of picoseconds) is likely caused by the electron heating and cooling effects as well as the collection of electrons into the lower subbands in real space and compensated by the external bias supply. The latter contribution is support by the observed positive photoconductivity with increased current and decreased voltage across the QCL.

We studied the SHG signal generated by focusing different polarizations (TE and TM modes) fs mid-IR pulses into the QCLs active core. SHG due to intersubband transitions is observed when the pump is TM polarized (SHG_TM). The measured SHG_TM spectrum narrows when the bias across the QCL increases due to the electron population re-distribution and subband realignment. The expected quadratic dependence of the SHG_TM with mid-IR pump power is observed, but saturates at higher pump powers. The observed SHG pumped by TE polarized fs mid-IR pulses (SHG_TE) is most likely generated by the bulk nonlinearity of the InP substrate. The second order nonlinearity for SHG_TE is also confirmed, without observing the saturation. We estimated linear-to-nonlinear power conversion efficiency for both SHG_TM and SHG_TE.