THz transition radiation of electron bunch laser-accelerated in long-scale near-critical density plasmas

Direct laser electron acceleration in near-critical density plasma produces collimated electron beams with high charge $Q$ (up to $\mu$C). This regime could be of interest for high energy THz radiation generation, as many of the mechanisms have a scaling $\propto Q^2$. In this work we focused specifically on challenges that arise during numerical investigation of transition radiation in such interaction. Detailed analytical calculations that include both diffraction and decoherence effects of characteristics of transition radiation in the THz range were conducted with the input parameters obtained from 3D PIC and hydrodynamic simulations. The calculated characteristics of THz radiation are in good agreement with the experimentally measured ones. Therefore, this approach can be used both to optimize properties of THz radiation and distinguish the transition radiation contribution if several mechanisms of THz radiation generation are considered

Towards Terawatt-Scale Spectrally Tunable Terahertz Pulses via Relativistic Laser-Foil Interactions

An ever-increasing number of strong-field applications, such as ultrafast coherent control over matter and light, require driver light pulses that are both high power and spectrally tunable. The realization of such a source in the terahertz (THz) band has long been a formidable challenge. Here, we demonstrate, via experiment and theory, efficient production of terawatt (TW)-level THz pulses from high-intensity picosecond laser irradiation on a metal foil. It is shown that the THz spectrum can be manipulated effectively by tuning the laser pulse duration or target size. A general analytical framework for THz generation is developed, involving both the high-current electron emission and a time-varying electron sheath at the target rear, and the spectral tunability is found to stem from the change of the dominant THz generation mechanism. In addition to being an ultrabright source (brightness temperature of about 1021 K) for extreme THz science, the THz radiation presented here also enables a unique in situ laser-plasma diagnostic. Employing the THz radiation to quantify the escaping electrons and the transient sheath shows good agreement with experimental measurements

Laser‐driven strong‐field Terahertz sources

A review on the recent development of intense laser‐driven terahertz (THz) sources is provided here. The technologies discussed include various types of sources based on optical rectification (OR), spintronic emitters, and laser‐filament‐induced plasma. The emphasis is on OR using pump pulses with tilted intensity front. Illustrative examples of newly emerging applications are briefly discussed, in particular strong‐field THz control of materials and acceleration and manipulation of charged particles

Modeling terahertz emissions from energetic electrons and ions in foil targets irradiated by ultraintense femtosecond laser pulses

Terahertz (THz) emissions from fast electron and ion currents driven in relativistic, femtosecond laser-foil interactions are examined theoretically. We first consider the radiation from the energetic electrons exiting the backside of the target. Our kinetic model takes account of the coherent transition radiation due to these electrons crossing the plasma-vacuum interface as well as of the synchrotron radiation due to their deflection and deceleration in the sheath field they set up in vacuum. After showing that both mechanisms tend to largely compensate each other when all the electrons are pulled back into the target, we investigate the scaling of the net radiation with the sheath field strength. We then demonstrate the sensitivity of this radiation to a percent-level fraction of escaping electrons. We also study the influence of the target thickness and laser focusing. The same sheath field that confines most of the fast electrons around the target rapidly sets into motion the surface ions. We describe the THz emission from these accelerated ions and their accompanying hot electrons by means of a plasma expansion model that allows for finite foil size and multidimensional effects. Again, we explore the dependencies of this radiation mechanism on the laser-target parameters. Under conditions typical of current ultrashort laser-solid experiments, we find that the THz radiation from the expanding plasma is much less energetic -- by one to three orders of magnitude -- than that due to the early-time motion of the fast electrons

Terahertz generation from laser-driven ultrafast current propagation along a wire target

Generation of intense coherent THz radiation by obliquely incidenting an intense laser pulse on a wire target is studied using particle-in-cell simulation. The laser-accelerated fast electrons are confined and guided along the surface of the wire, which then acts like a current-carrying line antenna and under appropriate conditions can emit electromagnetic radiation in the THz regime. For a driving laser intensity ∼3×1018W/cm2 and pulse duration ∼10 fs, a transient current above 10 KA is produced on the wire surface. The emission-cone angle of the resulting ∼0.15 mJ (∼58 GV/m peak electric field) THz radiation is ∼30°. The conversion efficiency of laser-to-THz energy is ∼0.75%. A simple analytical model that well reproduces the simulated result is presented

Observation of energetic terahertz pulses from relativistic solid density plasmas

We report the first experimental observation of terahertz (THz) radiation from the rear surface of a solid target while interacting with an intense laser pulse. Experimental and two-dimensional particle-in-cell simulations show that the observed THz radiation is mostly emitted at large angles to the target normal. Numerical results point out that a large part of the emission originates from a micron-scale plasma sheath at the rear surface of the target, which is also responsible for the ion acceleration. This opens a perspective for the application of THz radiation detection for on-site diagnostics of particle acceleration in laser-produced plasmas

High Power Laser - Plasma Interactions for Homeland Security Applications

Advance in laser technology over the last few decades have allowed progress in intense laser-plasma interaction research. The relativistic plasma generated by intense laser pulses can generate many different forms of radiation. This radiation, including X-rays, has been studied intensively due to the numerous potential applications of these sources. For example, for Homeland Security, radiation sources are already utilized to detect dangerous materials and hidden items that threaten civil safety. Neutrons and THz radiation have been studied as candidates for next generation screening, which may complement typical X-ray techniques. This thesis contains three experimental studies of high-power laser-plasma interactions as sources of radiation for Homeland Security applications, especially at kilohertz repetition-rates using few- millijoule pulses. First, a neutron generation experiment was conducted using a high repetition-rate laser system (1⁄2 kHz) at the University of Michigan. A heavy water (D2O) stream was irradiated by 40 fs pulses, each containing a few millijoules of energy. Acceleration of deuterons (to E < 1 MeV) was achieved through plasma sheath acceleration. Ensuing DD nuclear fusion reactions, in turn, generated neutron fluxes of up to 10^5 s^−1 into 4π steradians. In order to understand the neutron source characteristics, deuteron spectra were measured with CR39 detectors and compared to particle-in-cell (PIC) relativistic plasma dynamics simulations. The neutron source characteristics were analyzed using various neutron detection techniques, including Time-of-Flight measurements, bubble detectors, and neutron-capture gamma-ray measurements. Second, THz generation from laser filamentation in air was investigated. For security applications, THz can complement X-ray scanning, because THz can detect non-metallic materials and dangerous chemicals while not ionizing the sample. Even though there have been extensive studies on THz generation from laser filamentation processes, the exact generation mechanisms are yet to be determined. In this thesis, optimization of THz radiation using an adaptive optic with active feedback was demonstrated. Using a genetic algorithm, the THz radiation was improved six-fold without the need for detailed knowledge of the mechanisms. In particular, the use of a high repetition-rate laser system accelerated the optimization of the THz signal. Another strength of this optimization system is that it can enhance certain THz generation mechanisms depending on the experimental circumstances. Lastly, using a nanosecond pulsed high-power laser system (10 Hz), a long-range detection technique was developed for detection of special nuclear materials. Although direct detection of radiation from nuclear materials can be defeated by radiation shielding, leakage of radiation-ionized gases can provide an alternative indicator of the existence of nuclear materials. For instance, in the presence of ionizing radiation, the ratio of ionized nitrogen to neutral nitrogen would be higher than in no-source air-plasma conditions. By inducing optical breakdown (plasma) near a sample’s position, the ionization levels of the surrounding air were analyzed. To enhance the detection efficiency, an adaptive-optic feedback system was introduced with this ratio as a figure-of-merit. This resulted in a 50 % enhancement in the spectral ratio of the nitrogen lines. In addition, aerosol-initiated plasma spectra were distinguished from the original air-breakdown plasma, as a step toward practical deployment.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/145909/1/jmhah_1.pd