UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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Item HIGH-FIELD THZ GENERATION AND BEAM CHARACTERIZATION WITH LASER BASED INTENSE THZ SOURCES(2018) Yoo, Yungjun; Milchberg, Howard M; Kim, Ki-Yong; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The main topic of this dissertation is about the generation of intense terahertz (THz) pulses with field strengths up to tens of MV/cm and their characterization with energy, pulse duration, and spot size measurements. As a strong THz source, we used two-color laser mixing in air, which can produce coherent, high energy (> µJ), broadband (0.01~100 THz) THz radiation. In this scheme, 800-nm, 1-kHz, 30-fs laser pulses are weakly focused onto a BBO (Beta Barium Borate) crystal to generate the 2nd harmonic (400 nm) pulses. The original (800 nm) and second harmonic (400 nm) pulses are focused together to generate plasma filaments in air, and this works as a broadband source of THz radiation. In particular, we have studied THz energy scaling with various focal length conditions and input laser energies up to 10 mJ. With high laser input energy, the THz output energy does not simply increase but rather saturates or even decreases. We find that this occurs due to plasma-induced laser defocusing, which prohibits efficient laser energy coupling into the plasma. We have overcome this saturation effect by increasing the plasma volume in the longitudinal or transverse direction. At a high repetition rate (1 kHz), we have achieved 2.6 µJ of THz energy with 10 mJ laser energy by elongating the plasma length (~7 cm). This provides a conversion efficiency of 2.6 10-4 from optical to THz energy. Also, at a low repetition rate (10 Hz) with high laser input energy (~50 mJ), we increased the plasma volume in the transverse direction by generating a 2-dimensional plasma sheet and obtained 31 µJ of THz energy. We have also investigated THz generation from two-color laser filamentation in different types of gases (room air, nitrogen, oxygen, carbon dioxide, helium, argon, krypton, and xenon) at various gas pressures. By elongating the plasma length in a long gas cell, we have achieved laser-to-THz conversion efficiency of ~0.1%, one order of magnitude higher than a typical value (0.01%) obtained in two-color laser focusing in air. To obtain strong THz fields, we have performed tight refocusing of the emitted THz radiation. Previously, it was speculated that a large plasma volume could produce more THz energy but not necessarily assure strong THz field strengths because of its ineffective refocusing of the emitted THz radiation. Contrary to the concern, we have achieved a small THz spot size near its diffraction limit (~40 µm) even with long filamentation. This gives THz field strengths up to ~30 MV/cm in our gas cell experiment. We have also studied various THz detection methods to cover a broad frequency range of THz radiation. To measure THz energy, we used broadband thermopile and pyro-electric detectors. We have also developed a real-time lock-in imaging technique to characterize frequency-dependent THz radiation profiles by using an uncooled microbolometer along with THz bandpass filters. We have characterized THz waveforms and spectra with electro-optic (EO) sampling and/or Fourier Transform Infrared Spectroscopy (FTIR). We find that our THz source produces extremely broad electromagnetic (EM) radiation ranging from radio-micro waves to infrared frequencies. This source can be a useful tool to study broadband linear and nonlinear THz spectroscopy.Item The Response of Molecular Gases and Modulated Plasmas to Short Intense Laser Pulses(2011) Pearson, Andrew; Antonsen, Thomas M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we study the response of two systems to short, intense laser pulses. The first system is a gas of diatomic molecules whose ensemble-averaged alignment features rotational revivals. We analyze the effect of a background plasma on the revival peaks. Both the revivals and the plasma are the result of a laser pulse passing through the gas. The second system is a density-modulated plasma channel. We study the generation of electromagnetic radiation by a laser pulse passing through this structure. The molecules in the gas are modeled as rigid rotors that interact first with the cycle-averaged electric field of the laser pulse, and second with the fluctuating electric field of the background plasma. The laser pulse generates a broad superposition of angular momentum eigenstates, resulting in the transient alignment of the molecules. Because of the time evolution properties of the angular momentum states, the alignment re-occurs periodically in field-free conditions. The alignment is calculated using a density matrix, and the background plasma is modeled using dressed particles. The result is decoherence between the phases of the basis states of the wavefunction, which causes decay of subsequent alignment peaks. We find that field-induced decoherence is competitive with collisional decoherence for small ionization fractions. The corrugated plasma channel is modeled using linear plasma theory, and the laser pulse is non-evolving. Corrugated channels support EM modes that have a Floquet dispersion relation, and thus consist of many spatial harmonics with subluminal phase velocities. This allows phase matching between the pulse and the EM modes. Since the pulse bandwidth includes THz frequencies, significant THz generation is possible. Here we consider realistic density profiles to obtain predictions of the THz power output and mode structure. We then estimate pulse depletion effects. The fraction of laser energy converted to THz is independent of laser pulse energy in the linear regime, and we find it to be around one percent. Extrapolating to a pulse energy of 0.5 J gives a THz power output of 6 mJ, with a pulse depletion length of less than 20 cm.