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This thesis presents the fabrication and measurement of LT-GaAs based terahertz (THz) photo conductive antennas (PCAs) and arrays. The LT-GaAs THz PCAs are fabricated to serve as reference devices to new 2D material black phosphorous (BP) based THz PCAs. The LT-GaAs and BP devices have identical metallic electrodes, allowing for a comparison of emitted THz intensity and bandwidth. All PCAs have been measured using an open bench pulsed time-domain spectroscopy (TDS) system with a usable bandwidth from 0.1-4 THz, pumped with a 780nm Ti:Sapphire femtosecond laser. The results have shown LT-GaAs devices outperforming BP devices in signal amplitude and bandwidth at identical DC bias voltages and pump powers. Three other electrode shape designs were achieved: circular, slotted, and fractal, fabricated on LT-GaAs. The effect of electrode shape on the amplitude and bandwidth of the THz pulse has been experimentally characterized. A comparison of all four shapes has shown that the bowtie electrodes provide a nominal increase in pulse amplitude under identical biasing conditions. Further, a polarization study using an x-cut quartz rotator was conducted, validating that all four electrode shapes are highly linearly polarized. Results show that the co-polarized THz pulse is two orders of magnitude greater than the cross-polarized THz pulse. In addition, two element THz PCAs with electrode spacings of 75µm, 150µm, and 300µm have been investigated. A novel feed network using two beam splitters has been designed, and implemented into the existing open bench TDS system. This feed network gives individual control over the position and path length of the beams feeding each elements. Further, a DC bias splitting PCB and switchboard were designed to allow each element to be turned on and off, aiding in laser alignment validation. The measurement of all three devices have shown the array THz pulse having a higher amplitude than either individual element, however, an insignificant effect on the array bandwidth has been observed.

Recent developments in nanotechnology have led to renewed interest and breakthroughs in structural biopolymers, specifically silk protein, as functional materials. The exceptional mechanical properties and the bio-compatibility of silk has enabled wide range of applications from biomedical devices, optics, electronics, to transient implants. Understanding the mechanisms that underpin the β-sheet formation and deformation as well as the formulation of strategies to control inter- and intramolecular bonds within silk protein matrices is paramount for the control of protein structures and the improvement of material properties. However, conventional imaging techniques that are used to characterize and recapitulate silk structure–function relationships present challenges at the nanoscale given their limitations in chemical sensitivity (for example, electron microscopy and atomic force microscopy (AFM)) or limited spatial resolution (for example, ‘far-field’ infrared (IR) spectroscopy). In this context, my research focuses on the understanding of the conformational transitions of silk fibroin and recombinant spider silk, and the interaction between the protein and energy or other biomolecules at nanoscale using near-field optics. In particular, the complete conformational transition of the silk protein under the electron bombardment have been visualized, guiding the creation of novel 3D nanostructures using Electron Beam Lithography (EBL). Meanwhile, the dual-tone structural formation of silk structures under ion beam irradiation have been thoroughly investigated, resulting in the “Protein Lego” manufacturing paradigm. The UV enabled silk protein cross-linking has also been utilized for scalable manufacturing of bio-structures. The interaction between the silk protein and other types of biological materials (such as cells, bacteria, and virus) has been studied to explore the stabilization capability of the silk matrix. The comprehensive investigation of the interplay between the protein material, energy input, as well as other chemical/biological species will pave the way for the bio-compatible, bio-degradable, and multi-functional platforms, serving as the building blocks of the green bio-manufacturing paradigm

Investigation of photoelectric effect in Transition Metal Dichalcogenides Field Effect TransistorsBy: I-Hsi Daniel LuAbstract:One atomically thin two-dimensional materials like graphene was brought into the spotlight as an ideal high mobility material for electronics once it was synthesized by Dr. Andre Geim and Dr. Kostya Novoselov. The two professors found an ingenious way to extract graphene from graphite through a method later known as exfoliation. This opened a gateway for people to learn more about 2-D materials. However, graphene lacks a direct bandgap due to its single carbon sheet structure. Thus, transition metal dichalcogenide (TMDs) has been studied for its direct bandgap and decent mobility properties. Two-dimensional materials have then been synthesized through different methods such as exfoliation and chemical vapor deposition (CVD). TMDs are materials with the form of MX2 (M= Transition metal; X= Chalcogen). Each different combinations of such materials can provide different properties. Due to its unique crystal structure, it forms a direct bandgap in between the transition metal and chalcogen in single layer form. This property allows for functions of optoelectronic devices. In my work, I demonstrate photocurrent emission of monolayer MoS2 films using a home-built scanning photocurrent microscope. The purpose of the work is to investigate the Schottky barrier difference among different metal composition.

Very high energy (VHE; E>100 GeV) blazars are a type of active galaxy detected above 100 GeV with a jet pointed toward the observer. This work investigates VHE blazars through broadband observations, starting with a description of the VHE-discovery and time-independent modeling of the non-thermal emission from RX J0648.7+1516. Additionally, synchrotron self-Compton models are applied to six non-VHE blazars, with the VHE flux of each blazar being constrained by non-detection during observation by VERITAS. The general lack of physical measurements of model parameters is highlighted and a scheme of supplementary observations involving millimeter carbon monoxide (CO) luminosity and soft X-ray absorption measurements is explored for three VHE blazars. The limited sample supports a possible connection between the existence of CO in the vicinity of the blazar and additional soft X-ray absorption beyond what can be attributed to the Milky Way. RGB J0710+590 and W Comae both lack a significant level of CO and do not require additional absorption for the description of the soft X-ray emission as observed by Swift XRT. 1ES 1959+650, on the other hand, shows a significant level of CO in the vicinity of the blazar and requires additional absorption to describe the soft X-ray emission. The positive detection of CO in the vicinity of 1ES 1959+650 is used as motivation to apply a mirrored emission scenario to broadband variability data. Limits on the redshifts of the two VHE blazars 3C 66A and PKS 1424+240 are derived from HST/COS observations of intervening Lyman absorption. These observations show 3C 66A to reside at a redshift below the tentative z=0.444 at 99.9% confidence and reveal PKS 1424+240 to be the most distant VHE-detected blazar thus far. The redshift information is paired with VERITAS and Fermi Large Area Telescope gamma-ray observations to probe the density of the extragalactic background light and correct the observed gamma-ray spectra to the intrinsically emitted spectra, allowing insight into the acceleration mechanisms at work within the blazar jets and the propagation of gamma-ray photons through intergalactic space.

Abstract: Electron Beam Lithography (EBL) is a powerful tool for structuring materials at the deep nanoscale. Modeling and simulation of electron-beam interactions at this length scale is vital to understanding and optimizing nanofabrication using EBL. The low to high voltage (5 keV – 100 keV) regimes of EBL have been studied for decades. However, the ultra-low regime (< 5 keV) provides an opportunity to further rationalize the understanding of this powerful technique and explore its applications. The ultra-low voltage regime was studied using (poly) methyl- methacrylate (PMMA) as the resist, and important metrics at this regime such as dose windows, exposure sensitivity and dose variation with exposure energy are explored. An application of low voltage EBL is presented to develop a process for pattern density multiplication in a single step. Density multiplication was demonstrated for arrays of nanostructures using a combination of experiments and simulation. This approach increased the areal density of the lithographically patterned lines by a factor of approximately 2 for dots and lines in a single exposure and development step. This application is not only interesting from a technological perspective, but also demonstrates the power of a combined experimental and simulation optimization strategy.