Studies On Terahertz Emission From Laser Plasmas

A terahertz (THz) wave is an electromagnetic wave, which usually has a frequencylocated between0.1THz-10THz (1THz=1012Hz). Since it is between the microwave andthe optical wave in the whole electromagnetic wave spectrum, which is used to be difficultto generate either in electronic ways or optical ways, sometimes it is called as a “THzGap”. In this thesis, THz wave generation from laser plasma interaction is studied withparticle-in-cell (PIC) numerical simulations and laser interaction with air-plasmaexperiments. The thesis is composed of three parts.Firstly, THz wave detection methods and three kind of THz optical sources areintroduced. The detection methods include free space electro-optic (EO) sampling method,semiconductor antenna, air-biased-coherent-detection (ABCD), Fourier-transform-infrared(FTIR) and single-shot methods, in which EO and ABCD are described in detailed sincethey are mainly used in our experiment. The THz wave sources as well as their mainfeatures from semiconductor emission, optical rectification and laser plasma interactionare introduced.Secondly, ionization currents and the corresponding THz emission from laserinteraction with gas targets are investigated theoretically and numerically. Here the theoryis semi-classical and it is confirmed by one-dimensional particle-in-cell (PIC). Accordingto the model the ionization current can emit an electromagnetic wave with its frequencyclose to the electron plasma frequency. A large net ionization current can generate a strongTHz wave when the electron plasma density is in a proper range. The schemes with two-color laser pulses (fundamental pulses and its second harmonic) and few-cycle laser pulsesare two efficient methods to produce large ionization currents. In the former, the THzwave amplitude depends upon the amplitudes of the two color lasers as well as theiramplitude ratio of the fundamental laser pulse to the second harmonic pulse. When theratio is about the1.5, the THz wave generation efficiency is relatively high. The THz waveamplitude changes periodically with the phase difference of the two pulses. For the few-cycle laser pulses, the THz generation is also a function of the laser pulse phase. For bothcases, the THz emission also depends upon the density profiles at the vacuum-gasboundaries. When the density gradient length is longer, the produced THz wave is weaker and the frequency spectrum becomes much smoother. With the laser intensity~1016w/cm2,one is able to produce strong THz emission with the field strength up to several MV/cm.In the last part of the thesis, we have investigated experimentally the THz wavegeneration from the laser interaction with air-plasma. A THz TDS system is set up andboth EO and ABCD methods are used to detect THz pulses. THz wave generation fromeither a single color laser pulse or two-color laser pulses in air-plasma is studied. It isfound that the two-color laser method can generate much stronger THz pulses than thatwith single-color lasers, consistent with previous theoretical and numerical investigation.Even though the laser pulses have duration about100femtosecond in our experiments, thebandwidth of THz pulses measured is up to10THz. With a bias dc field, the THzamplitude increases linearly with the bias field in the single color laser scheme, which is,however, not the case with the two-color laser scheme.