Theoretical foundations of detection of terahertz radiation in laser-plasma interactions
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Theoretical Foundations of Detection of Terahertz Radiation in Laser–Plasma Interactions A. A. Frolov Joint Institute for High Temperatures, Russian Academy of Sciences, Izhorskaya ul. 132, Moscow, 125412 Russia email: [email protected] Received March 24, 2012; in final form, June 20, 2012
Abstract—A theory is developed enabling one to calculate the temporal profile and spectrum of a terahertz wave packet from the energy of the second harmonic of optical radiation generated during the nonlinear interaction between terahertz and circularly polarized laser pulses in the skin layer of an overdense plasma. It is shown that the spectral and temporal characteristics of the envelope of the second harmonic of optical radi ation coincide with those of the terahertz pulse only at small durations of the detecting laser radiation. For long laser pulses, the temporal profile and spectrum of the second harmonic are mainly determined by the characteristics of optical radiation at the carrier frequency. DOI: 10.1134/S1063780X12120021
1. INTRODUCTION In recent years, the theory and practice of genera tion of radiation in the terahertz (THz) spectral band (1 THz = 1012 Hz) have attracted significant interest due to the possibility of wide application of such radi ation in science and engineering. At present, THz pulses with high average and peak powers are mainly generated by means of optical lasers radiating pulses of femtosecond (1 fs = 10–15 s) duration. For the first time, generation of THz radiation in gaseous and solidstate targets under the action of intense femto second laser pulses was observed experimentally in [1]. Later, generation of THz radiation under the laser action on matter was detected in many experiments (see, e.g., [2–5]). In the literature, various physical mechanisms of generation of THz radiation under the action of intense laser pulses on matter, when plasma is produced due to photoionization of the medium, were considered [4–17]. Along with the problem of creating THz radiation sources, the problem of detect ing THz pulses is also very important. Various devices, such as bolometers, Golay cells, Schottky diodes, and photoconducting antennas, are employed to detect THz pulses. Since recently, optical lasers are also used to detect THz radiation. The advantages of lasers gen erating femtosecond pulses are related to the possibil ity of detecting THz radiation in a wider frequency range and at room temperature. The method of elec trooptical detection based on the transformation of the THzband frequency into an optical frequency in a nonlinear crystal is also used for these purposes [18, 19]. There is also a method for determining the tem poral profile of the THz pulse by measuring the energy and intensity of the second harmonic of optical radia tion generated in the nonlinear interaction between
THz and laser pulses in air in the presence of a con stant external electric field [20–22]. When using low intensity optical pulses, when no ionization of air and plasma formation occur, t
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