<p> Time-domain terahertz spectroscopy is now a well-established technique. Of the many methods available for a terahertz source for terahertz spectroscopy, the most widely used may be the GaAs-based photoconductive antenna, as it provides relatively high power at terahertz frequencies, commercially available up to 150 &micro;W, and a wide-bandwidth, approximately 70 GHz to 3.5 THz. One of the limitations for developing more accurate and sensitive terahertz interrogation techniques is the lack of higher power sources. Because of our research interests in terahertz spectroscopy, we investigated detailed design and fabrication parameters involved in the photoconductive antenna, which exploits the surface plasma oscillation to produce a wideband pulse. The investigation enabled us to develop a new photoconductive antenna that is capable of generating a high power terahertz beam, at least twenty times stronger than those currently available. Throughout this research, it was discovered that antenna electrodes with particular geometries could produce superradiance, also known as the Dicke effect. Chaotic electrodes with a predisposition to lead charge-carriers into chaotic trajectories, e.g. rippled geometry, were exploited to reduce undesirable heat effects by driving thermal-electrons away from the terahertz generation site, i.e. the location of the surface plasma, while concentrating the removed charge-carriers in separate locations slightly away from the surface plasma. Then, spontaneous emission of coherent terahertz radiation may occur when the terahertz pulse generated by the surface plasma stimulates the concentrated carriers. This spontaneous emission enhanced the total coherent terahertz beam strength, as it occurs almost simultaneously with the primary terahertz beam. In principle, the spontaneous emission power increases as N², with the number N of dipole moments resulted from the concentrated charge carriers. Hence, if the design parameters are optimized, it may be possible to increase the strength of coherent terahertz beam by more than one order of magnitude with a photoconductive antenna containing rippled electrodes. However, as the parameters are yet to be optimized, we have only demonstrated 10-20 % enhancement with our current photoconductive antennas. </p><p> Photoconductive antennas were fabricated via photolithography and characterized by time-domain terahertz spectroscopy and pyroelectric detection. In addition to chaotic electrodes, a variety of other parameters were characterized, including GaAs substrate thickness, GaAs crystal lattice orientation, trench depth for electrodes, metal-semiconductor contact, and bias voltage across electrodes. Nearly all parameters were found to play a crucial role influencing terahertz beam emission and carrier dynamics. By exploiting wave chaos and other antenna parameters, we developed a new photoconductive antenna capable of continuous operation with terahertz power twenty times larger than that of the conventional photoconductive antennas, improving from 150 &micro;W to 3 mW. With further optimizations of the parameters, we expect more dramatic improvement of the photoconductive antenna in the near future.</p>

<p> The purpose of this study is to design, fabricate and test a CMOS compatible 3-axis Hall effect sensor capable of detecting the earth&rsquo;s magnetic field, with strength&rsquo;s of &sim;50 &mu;T. Preliminary testing of N-well Van Der Pauw structures using strong neodymium magnets showed proof of concept for hall voltage sensing, however, poor geometry of the structures led to a high offset voltage. A 1-axis Hall effect sensor was designed, fabricated and tested with a sensitivity of 1.12x10^-3 mV/Gauss using the RIT metal gate PMOS process. Poor geometry and insufficient design produced an offset voltage of 0.1238 volts in the 1-axis design; prevented sensing of the earth&rsquo;s magnetic field. The new design features improved geometry for sensing application, improved sensitivity and use the RIT sub-CMOS process. The completed 2-axis device showed an average sensitivity to large magnetic fields of 0.0258 &mu;V/Gauss at 10 mA supply current.</p>

This thesis discusses the full scope of a project exploring the physics of hierarchical clusters of interacting nanomagnets. These clusters may be relevant for novel applications such as multilevel data storage devices. The work can be grouped into three main activities: micromagnetic simulation, fabrication and characterization of proof-of-concept prototype devices, and efforts to scale down the structures by creating the hierarchical structures with the aid of diblock copolymer self assembly. Theoretical micromagnetic studies and simulations based on Landau-Lifshitz-Gilbert (LLG) equation were conducted on nanoscale single domain magnetic entities. For the simulated nanomagnet clusters with perpendicular uniaxial anisotropy, the simulation showed the switching field distributions, the stability of the magnetostatic states with distinctive total cluster perpendicular moments, and the stepwise magnetic switching curves. For simulated nanomagnet clusters with in-plane shape anisotropy, the simulation showed the stepwise switching behaviors governed by thermal agitation and cluster configurations. Proof-of-concept cluster devices with three interacting Co nanomagnets were fabricated by e-beam lithography (EBL) and pulse-reverse electrochemical deposition (PRECD). EBL patterning on a suspended 100 nm SiN membrane showed improved lateral lithography resolution to 30 nm. The Co nanomagnets deposited using the PRECD method showed perpendicular anisotropy. The switching experiments with external applied fields were able to switch the Co nanomagnets through the four magnetostatic states with distinctive total perpendicular cluster magnetization, and proved the feasibility of multilevel data storage devices based on the cluster concept. Shrinking the structures size was experimented by the aid of diblock copolymer. Thick poly(styrene)-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer templates aligned with external electrical field were used to fabricate long Ni/Fe magnetic nanowire array, dominant shape anisotropy was observed and compared to the result from previously reported Co nanowire array with strong crystalline anisotropy. Guided diblock copolymer poly(styrene)-b-poly(4-vinyl pyridine) (PS- b-P4VP) self assembly was performed to generate clustered microdomains. Direct e-beam patterning on PS-b-P4VP thin film showed precise and arbitrary patterning on the lateral ordering of the self assembly. Graphoepitaxy of self-assembled PS-b-P4VP copolymers on isolated SiN triangular plateaus successfully resulted in the exact clusters of three microdomains. Theoretical consideration and system modeling based on the micellar configuration of the microdomains were done, and the distribution of the cluster’s size and number of elements were explained qualitatively.

The synthesis and physical characterization of a series of poly(p-phenylene vinylene-co-2,5-thienylene vinylene) (PPV-co-PTV) were reported. The synthetic route used was a variation on the Wessling route to poly(p-phenylene vinylene) which involved a precursor polymer. The copolymeric nature of the materials produced were investigated by IR analyses of the trans-vinylene absorption of these systems. The results of the attempts to produce other polyarylene vinylene (PAV) copolymers were also reported. The conductivity and orientational properties of PPV-co-PTV in comparison with those of the blends of poly(p-phenylene vinylene) (PPV) and poly(2,5-thienylene vinylene) (PTV) were investigated. The interrelation of the PTV contents, sequence distributions, conductivities, draw ratios, morphology and order parameters for the copolymers and blends were discussed. The effects due to the random copolymeric nature of PPV-co-PTV were emphasized.