Graduate Thesis Or Dissertation
 

Terahertz Imaging and Nonlinear Spectroscopy of Semiconductors using Plasmonic Devices

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  • In this dissertation, a series of studies in the field of terahertz (THz) science are presented, specifically using nonlinear THz spectroscopy. We exploit huge field enhancement and subwavelength confinement in plasmonic structures. There are three distinct projects which will be discussed: nonlinear THz spectroscopy using plasmonic induced transparency (PIT), THz-triggered insulator-metal transition (IMT) in nanoantenna patterned vanadium dioxide (VO2) films, and fabrication of sub-diffraction limit imaging bulls-eye structures. We used PIT structures to observe the high- field carrier dynamics in semiconductors, specifically in intrinsic, high resistivity silicon (high-rho Si) and intrinsic gallium arsenide (GaAs). The PIT structures rely on the coupling of a "bright mode" in a central half-wave dipole antenna to the "dark mode" of the adjacent split-ring resonators. We employed these structures because of their sensitivity to carrier dynamics due to the sharp resonance of the "dark mode." We observed the response of the PIT oscillation to both low and high THz fields in the presence of an optical pump. Increasing the optical pump power, and therefore the number of carriers, resulted in the damping of the oscillation. With increasing THz field strength, we observed a field induced transparency from the intervalley scattering of the excited carriers and demonstrated THz control of the PIT oscillation. By changing the delay time between the THz and optical pulses, we demonstrated pulse shaping of the PIT waveforms. We demonstrated the THz-triggered insulator-metal transition (IMT) in nanoantenna patterned vanadium dioxide (VO₂) films. Vanadium dioxide is a promising material for electronic and photonic applications due to its IMT transition lying near room temperature. We observed that the phase transition is activated on the sub-cycle time scale where strong THz fields drive the electron distribution far from equilibrium. We also observed a lowering the transition temperature of the IMT phase transition for both heating and cooling cycles in nanoslot antenna VO₂ films with increasing THz fields and also a narrowing in the width of the observed hysteresis. Using the Fresnel thin-film coefficients, Drude model, and the resistivity in semiconductors we found the activation energy in the insulating phase and show that it can be lowered with THz fields. We employed THz time domain spectroscopy to extract the frequency dependence and to observe the transiently induced IMT from the strong THz fields. We attempted to fabricate sub-diffraction-limit imaging bulls-eye structures in the Oregon State University cleanroom. During the course of the project, recipes for two different types of photoresists, SU-8 2100 and SU-8 5, were developed. We observed lack of adhesion of the metal (Al) layer for the metal-dielectric interface. Lastly the removal of metal for the apertures posed additional problems. While this project did not ultimately succeed, we present an explanation of the issues associated with their fabrication and the steps necessary to complete fabrication.
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