Sunday, January 4, 2015

Abstract-Theoretical and Experimental Studies on Strong Terahertz Emission from Ultra-Short Two-Color Laser Interaction with Gases


Terahertz(1THz=1012Hz) radiation refers to electromagnetic waves propagating at frequencies in the terahertz range,0.1THz-10THz,which is between infrared and microwave. Because of its wide range of potential applications in fundamental science research and practical applications, such as Medical imaging, public security, communications etc, THz waves have attracted significant interest. However, the technologies of THz wave sources still lag behind the demand for the applications mentioned above. A few new schemes for strong THz sources have been proposed. One of them is by use of short pulse laser interaction with gas targets. Because it has the potential to produce high peak power THz emission, it has drawn a great deal of attention. This thesis has focused on the theoretical and experimental studies on strong THz emission from the interaction of ultra-short two-color lasers with air plasma. The main results are listed as follows.Firstly, based upon a theoretical model of field ionization currents for terahertz emission in laser-gas interaction, the parameter optimization for strong THz emission via the two-color laser scheme has been explored. It is found from numerical calculation that the ionization currents are mainly due to the first-order ionization process and the contribution from high-order ionization is ignorable. In order to produce stronger THz emission, the ionization currents can be enhanced by adjusting the amplitude ratio between the fundamental and its second harmonic laser pulses as well as their relative phase difference. The ionization currents can also be increased by use of laser pulses at mid-infrared and by changing the gas species. Under the same laser pulse and gas density conditions, the ionization current from the Helium gas is almost twice that from the Neon gas. Besides, it is shown that a dc bias field can be converted efficiently into terahertz radiation.Secondly, on the experimental side, we successfully built a system to generate terahertz radiation based upon the Electric-optical sampling method and detected terahertz radiation by focusing two-color lasers in air. This has laid the foundation for the follow-up experimental studies.

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