A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Silicon-based terahertz waveguides(2015) Li, Shanshan; Murphy, Thomas E.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we present the design, fabrication and application of two types of silicon-based terahertz waveguides. The first is anisotropically etched highly doped silicon surface for supporting terahertz plasmonic guided wave. We demonstrate propagation of terahertz waves confined to a semiconductor surface that is periodically corrugated with subwavelength structures. We observe that the grating structure creates resonant modes that are confined near the surface. The degree of confinement and frequency of the resonant mode is found to be related to the pitch and depth of structures. The second is silicon dielectric ridge waveguide used to confine terahertz pulses and study silicon's terahertz intensity-dependent absorption. We observe that the absorption saturates under strong terahertz fields. By comparing the response between lightly-doped and intrinsic silicon waveguides, we confirm the role of hot carriers in this saturable absorption. We introduce a nonlinear dynamical model of Drude conductivity that, when incorporated into a wave propagation equation, predicts a comparable field-induced transparency and elucidates the physical mechanisms underlying this nonlinear effect. The results are numerically confirmed by Monte Carlo simulations of the Boltzmann transport equation, coupled with split-step nonlinear wave propagation.Item THE DESIGN AND STUDY OF THE SUB-MILLIMETER WAVE LENGTH GYROTRON AND FUNDAMENTAL AND SECOND CYCLOTRON HARMONICS(2015) Pu, Ruifeng; Granatstein, Victor L.; Nusinovich, Gregory S.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation documents research activities directed toward designing high power, high efficiency gyrotrons to operate in the sub-millimeter wavelength region. The gyrotron is to produce pulsed RF power at 670 GHz, with possible application to a novel scheme for detecting concealed radioactive materials. High efficiency is to be achieved by designing a cavity resonator in which the electrons interact with the high-order TE31,8 mode. The choice of resonating mode helps to alleviate Ohmic losses in the cavity walls, and simulation results show that the output efficiency could be more than 30%. The design study takes into account a variety of known effects that could affect efficiency, such as orbital velocity spread, voltage depression and after-cavity interaction. The 670GHz gyrotron was built using the resonator design; operation confirmed that record high efficiency was achieved at an output power level of about 200 kilowatts. In addition, the issue of radial spread in electron guiding centers, which is related to the design of the magnetron injection gun used in the 670GHz gyrotron, was also examined. This spread degrades the interaction between the electrons and the RF field. This often overlooked issue is important for future electron gun designs; this thesis presents analytical methods for estimating how much the degradation affects gyrotron efficiency. The analytical method was verified with numerical simulation, showing that the efficiency's sensitivity to spread in guiding centers is highly dependent on the location of an annular electron beam: when the beam is injected in the inner peak of the desired mode, the radial spread should be kept to less than 1/3 of the RF wavelength. Finally, the dissertation investigates the possibility of further extending the operating parameters of the gyrotron by using the second harmonic of the electron cyclotron resonance. An average output power could be increased by operating the gyrotron continuously rather than in pulses. Using the second cyclotron harmonic allows the magnetic field requirement for resonance condition to be reduced by a factor of two, so that in 670 GHz gyrotrons the pulsed solenoid can be replaced with a cryo-magnet. The investigation shows that for the TE31,8 mode at the second cyclotron harmonic, the operating mode has only one competing mode at the fundamental cyclotron harmonic that could present a mode stability issue. Numerical simulation shows that this mode is TE11,6, the operating mode can suppress this mode, while achieving 20% interaction efficiency. Results also reveal that the resonator for the operating mode at second cyclotron harmonic must be modified to increase the Q-factor. Continuously operating gyrotrons using cryo-magnets have been used for plasma heating in controlled thermonuclear fusion research, albeit at lower frequency than the 670 GHz of the current study.Item INTENSE TERAHERTZ GENERATION VIA TWO-COLOR LASER FILAMENTATION(2013) Oh, Taek il; Milchberg, Howard M; Kim, Kiyong; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The main focus of this dissertation is intense terahertz (THz) generation using two-color laser mixing in air plasma. In this scheme, the fundamental and second harmonic of an ultrashort pulsed laser are combined and focused into air to create a gaseous plasma, which produces an intense THz pulse in the far field. To understand the generation process, we have developed a two-dimensional (2-D) plasma current model. Using this model, we have simulated the conditions for optimal THz generation and verified them experimentally. A full control of THz output is demonstrated by varying the phases and polarization states of the input laser pulses. We have studied how the generated THz energy scales with various focal lengths and input laser energies up to 60 mJ. For high enough energy inputs, the resulting THz saturates. This arises from inefficient laser energy coupling into the plasma, which results from plasma-induced laser defocusing in filamentation. We have overcome the saturation effect by elongating the filaments and achieved 7 µJ of THz energy with 60 mJ laser energy. This provides a conversion efficiency of 10-4 from optical to THz energy. In addition, we have investigated high-power THz generation in two-color laser filamentation with terawatt (TW) lasers including a 0.5 TW, 1 kHz repetition (rep) rate system, as well as, 2 TW and 30 TW systems, both operating at 10 Hz rep rate. In particular, our 1 kHz rep rate THz source can provide high-energy (>1 µJ), high-average power (>1 mW), intense (>1 MV/cm), and broadband (0.01~60 THz) THz radiation via two-color filamentation in air. Based on our observed scaling law, a ~30 TW laser can produce >0.1 mJ of THz radiation with multi-gigawatt (GW) peak power in ~1.5 m long filamentation. We have also studied various THz detection methods covering a broad range of THz frequency bands. We observe our THz source produces extremely broad electromagnetic (EM) radiation ranging from radio-micro waves to infrared frequencies, confirmed by our complementary THz detection methods. This source could be a useful tool for broadband linear and nonlinear THz spectroscopy.