HIGH-FIELD THZ GENERATION AND BEAM CHARACTERIZATION WITH LASER BASED INTENSE THZ SOURCES
Milchberg, Howard M
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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.