UMD Theses and Dissertations

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    PHYSICS AND APPLICATIONS OF EXTENDED AIR HEATING BY FEMTOSECOND LASER FILAMENTATION
    (2022) Larkin, Ilia; Milchberg, Howard M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Femtosecond laser pulses of sufficient energy can propagate as filaments in air due to a dynamic interplay between nonlinear self-focusing and ionization-induced defocusing. A filament in air is characterized by a narrow, 100 m diameter core propagating at high intensity for many Rayleigh ranges corresponding to the core diameter, surrounded by a lower intensity reservoir that exchanges optical energy with the core. The high intensity core ionizes the air and excites molecular rotational wavepackets in N2 and O2. Thermal relaxation of these excitations leads to air heating over very long and narrow volumes, launching acoustic waves and imprinting density profiles in air. These features enable longitudinal mapping of energy absorption, interaction with aerosols in air, guiding of high voltage discharges, and the generation of long air waveguides for subsequent laser pulses. All of these topics are detailed in this dissertation.In particular, we present: (1) Single shot axially resolved energy deposition measurements, using a synchronized array of microphones, to see on a shot-by-shot basis the effect of air turbulence on nonlinear pulse propagation. (2) Measurements of the pre-breakdown evolution of a laser triggered high voltage spark gap, induced by a density channel imprinted by femtosecond laser pulses. By interferometrically measuring air heating and current leakage through the spark gap we clarify the role of laser plasma vs laser air heating in triggering breakdowns. (3) Air waveguiding experiments extended to ranges up to 50 m from the original ~1 m experiments. (4) Fog droplet clearing experiments showing that in natural filamentation of a collimated beam, direct optical interactions are the dominant clearing mechanism rather than acoustic effects.
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    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.
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    HIGH FIELD OPTICAL NONLINEARITIES IN GASES
    (2013) Cheng, Yu-Hsiang; MILCHBERG, HOWARD M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Optical femtosecond self-channeling in gases, also called femtosecond filamentation, has become an important area of research in high field nonlinear optics. Filamentation occurs when laser light self-focuses in a gas owing to self-induced nonlinearity, and then defocuses in the plasma generated by the self-focused beam. The result of this process repeating itself multiple times is an extended region of plasma formation. Filamentation studies have been motivated by the extremely broad range of applications, especially in air, including pulse compression, supercontinuum generation, broadband high power terahertz pulse generation, discharge triggering and guiding, and remote sensing. Despite the worldwide work in filamentation, the fundamental gas nonlinearities governing self-focusing had never been directly measured in the range of laser intensity up to and including the ionization threshold. This dissertation presents the first such measurements. We absolutely measured the temporal refractive index change of O2, N2, Ar, H2, D2 and N2O caused by highfield ultrashort optical pulses with single-shot supercontinuum spectral interferometry, cleanly separating for the first time the instantaneous electronic and delayed rotational nonlinear response in diatomic gases. We conclusively showed that a recent claim by several European groups that the optical bound electron nonlinearity saturates and goes negative is not correct. Such a phenomenon would preclude the need for plasma to provide the defocusing contribution for filamentation. Our results show that the `standard model of filamentation', where the defocusing is provided by plasma, is correct. Finally, we demonstrated that high repetition rate femtosecond laser pulses filamenting in gases can generate long-lived gas density `holes' which persist on millisecond timescales, long after the plasma has recombined. Gas density decrements up to ~20% have been measured. The density hole refilling is dominated by thermal diffusion. These density holes will affect all other experiments involving nonlinear high repetition-rate laser pulse energy absorption by gases.
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    The ultrafast nonlinear response of air molecules and its effect on femtosecond laser plasma filaments in atmosphere
    (2011) Chen, Yu-hsin; Milchberg, Howard M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The nonlinear propagation of an intense ultrafast laser pulse in atmosphere or other gas media leads to filamentation, a phenomenon useful for applications such as remote sensing, spectral broadening and shaping of ultrashort laser pulses, terahertz generation, and guiding of electrical discharges. Axially extended optical filaments result from the dynamic balance between nonlinear self-focusing in the gas and refraction from the free electron distribution generated by laser ionization. In the air, self-focusing is caused by two nonlinear optical processes: (1) the nearly-instantaneous, electronic response owing to the distortion of electron orbitals, and (2) the delayed, orientational effect due to the torque applied by the laser field on the molecules with anisotropic polarizability. To study their roles in filamentary propagation as well as influences on plasma generation in atmosphere, these effects were experimentally examined by a sensitive, space- and time-resolved technique based on single-shot supercontinuum spectral interferometry (SSSI), which is capable of measuring ultrafast refractive index shift in the optical medium. A proof-of-principle experiment was first performed in optical glass and argon gas, showing good agreement between the laser pulse shape and the refractive index temporal evolution owing to pure instantaneous n2 effect. Then the delayed occurrence of the molecular alignment in the temporal vicinity of the femtosecond laser pulse, as well as the subsequent periodic “alignment revivals” due to the coherently excited rotational wavepacket were measured in various linear gas molecules, and the results agreed well with quantum perturbation theory. It was found that the magnitude of orientational response is much higher than the electronic response in N2 and O2, which implies that the molecular alignment is the dominant nonlinear effect in atmospheric propagation when the pulse duration is longer than ∼40 fs, the rotational response timescale of air molecules. Realizing the possibility of manipulating plasma generation by aligning air molecules, the molecular orientational effect was further investigated by a technique developed to directly measure, for the first time, the radial and axial plasma density in a meter-long filament. The experiment was performed using both ∼40 fs and ∼120 fs laser pulse durations while keeping the peak power fixed under various focusing conditions, and the alignment-assisted filamenation with ∼2–3 times plasma density and much longer axial length was consistently observed with the longer pulse, which experienced larger refractive index shift and thus stronger self-focusing. Simulations reproduced the axial electron density measurements well for both long and short pulse durations, when using a peak magnitude of instantaneous response as <15% of the rotational response.