Study of laser-plasma THz using the high-power laser system: THz-plasma interaction and application in plasma diagnostics

Abstract

This Ph.D. thesis work focuses on mainly two topics that I had been involved in during my Ph.D. period. The first topic is the generation of terahertz (THz) radiation through laser-plasma, and the second topic is about the THz-based plasma diagnosis, which uses the THz radiation generated from the laser-induced plasma. Both works were performed using laser pulses from the regenerative amplifier system (central wavelength: 800 nm, pulse duration: 35 fs), which is a part of the 20 TW/35 fs laser system in the Laser Plasma Acceleration Laboratory (LPAL) at Gwangju Institute of Science and Technology (GIST). Throughout this work, the THz generation is mainly achieved by the two-color photoionization method. The generated THz radiation had strong field amplitude (~kV/cm), ultra-broadband (~70 THz) bandwidth, and the conical shape of spatial distribution. Detection of the THz wave has been performed by two different methods: THz time-domain spectroscopy (THz-TDS) and spectral encoding (SE). For the plasma diagnostics work using the laser-plasma THz, THz-plasma interaction studies have been done. The homemade inductively-coupled plasma (ICP) driven by a 13.56 MHz RF power source was used as the plasma source. When the THz radiation traverses through the plasma, the THz wave could contain the information of the target plasma. The THz waves have been detected by both the THz-TDS method and the SE method, and the detection results were the proof-of-principle data that show the feasibility of the SE method as a tool for fast plasma diagnosis. In addition to the experimental studies, a more profound calculation study on the THz-plasma interaction was also carried out by using the plasma refractive index from the Drude model. The calculation was used to predict how the THz wave would react against the plasma under different conditions: plasma density, plasma length, electron-neutral collision, etc. The THz waveform expected from the calculation through the model was compared to the THz waveform experimentally obtained. The diagnosis is conducted by finding the plasma parameters that make the calculated THz waveform similar to that from the experiment. This specific method is named as `waveform comparison' method, and the results from this method showed its possibility as a new or auxiliary plasma diagnostic method.