| Abstract [eng] |
Currently, due to its unique properties for applications in medicine, telecomunications, homeland security, etc. terahertz (THz) radiation has become an object of great interest. Usually THz radiation is generated in photoconductive antennas by illuminating the surface of semiconductor substrate with a femtosecond laser, or by optical rectification process in nonlinear crystals. However, these materials, are vulnerable to material damage when high energy laser pulses are used, which fundamentally limits THz energy scaling. In contrast, plasma is an ideal medium for scalable THz generation because it is already broken down and there is no concern about material demage. This motivates us to investigate properties of THz radiation generated in laser-induced air plasma. In this work THz radiation was generated using two different setups: i) by focusing fundamental and its second harmonic pulses into ambient air, and ii) by focusing three different frequency laser pulses into ambient air. It was experimentally found that in second setup generation efficiency was more than 5 times greater than obtained in the first one. In both cases measured spatial profiles of generated THz beam indicate that this radiation is conical because of phase mismatch between the pump and generated radiation. The far field divergence of THz cone was found to be about 2.5◦. Finally, it was shown that spectrum of THz radiation generated by mixing three different frequency optical pulses is more than 4 times wider than obtained by mixing bichromatic laser pulses. |
