(Invited paper) Efficient THz generation by femtosecond laser pulses and using THz radiation for plasma diagnostics

Andrey N. Stepanov ^1*, Sergey B. Bodrov ^{1, 3}, Igor E. Ilyakov ¹, Aleksey I. Korytin ¹, Nickolay L. Aleksandrov ², and Nickolay V. Vvedenskii ^{1, 3
1} Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod 603950, Russia
² Moscow Institute of Physics and Technology, Dolgoprudny 141700, Russia
³ University of Nizhny Novgorod, Nizhny Novgorod 603950, Russia
^*1 Email: step@ufp.appl.sci-nnov.ru

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Abstract: Optical-to-terahertz conversion of femtosecond laser radiation by optical rectification in the LiNbO₃ crystal and two-color gas ionization in air is discussed. It is shown that a high energy conversion efficiency (~0.25%) is achieved by ?erenkov radiation of line-focused femtosecond pulses propagated in a sandwich structure with a thin LiNbO₃ core covered with a Si prism. The effect of different substrates of the sandwich structure as well as laser pulse duration on the generation efficiency and terahertz spectrum is explored. The tilted pulse front technique for phase-matched THz generation using room-temperature and cryogenically cooled LiNbO₃ is studied under conditions of weak and strong laser pump for different crystal lengths and pump pulse durations. It is shown that the optical-to-terahertz conversion efficiency saturates as a result of the Kerr self-phase modulation of the optical pump. Low-frequency THz generation by two-color femtosecond laser pulse ionization of the ambient air is studied both experimentally and theoretically in the case of the main field at the fundamental and a weak additional field tunable near the half-harmonic frequency. The dependences of the THz yield and polarization on the parameters of the two-color pulse are determined. The analytical formulas obtained using the model of the free-electron residual current density give an excellent agreement with the experimental results. For demonstration of the THz radiation application, we use scattering of terahertz radiation to measure the air plasma density decay in a filament produced by an intense femtosecond laser pulse in air. It is shown that the plasma density decreases by about two orders of magnitude within 2 ns after the ionization and the rate of plasma decay can be decreased by applying an additional high (several kV/cm) external electric field. Numerical simulation of the electron density decay and electron temperature evolution is performed taking into consideration dissociative and three-body electron-ion recombinations as well as the formation of complex positive ions, in good agreement with the experimental data.

Keywords: Efficient terahertz generation, Optical-to-terahertz conversion, Terahertz scattering, Plasma density decay

Received: 2014-1-11

Published: 2014-9-30

Acknowledgments: This work was supported by the RFBR grant Nos. 12-02-33087, 12-02-31598, and 14-02-00847, the Program of the Presidium of RAS "Extreme Light Fields and Their Application" and the Ministry of Education and Science of the Russian Federation through Agreement Nos. 11.G34.31.0011, 14.B25.31.0008, and 14.B37.21.0770.

Cite this article:
Andrey N. Stepanov, Sergey B. Bodrov, Igor E. Ilyakov, Aleksey I. Korytin, Nickolay L. Aleksandrov, and Nickolay V. Vvedenskii. Efficient THz generation by femtosecond laser pulses and using THz radiation for plasma diagnostics[J]. International Journal of Terahertz Science and Technology, 2014, Vol.7, No.3: 108-127.  DOI:10.11906/TST.108-127.2014.09.09

URL: http://www.tstnetwork.org/10.11906/TST.108-127.2014.09.09