The use of femtosecond (fs) laser-induced plasma in gases to generate and detect terahertz (THz) radiation has been a subject of considerable interest in recent years. When used for THz detection, it was demonstrated that broadband THz wave detection using gas plasma as the sensor was valid, which shown superior bandwidth covering the entire THz gap and well beyond, as well as large dynamic range. However, the present approaches for THz coherent detection with gaseous mediums usually require very strong laser probe or high biased voltage on the plasma, which greatly limit their applications in practical THz system or in the field such as remote THz sensing.
Prof. Jinsong Liu and his team members from Nonlinear Terahertz Laboratory of WNLO, using few cycle laser pulses with fixed carrier envelope phase (CEP) as probes, developed a new THz wave coherent detection method via gaseous mediums. A well-developed photo-current model was performed to simulate the nonlinear process of the interaction of laser probe pulse and THz pulse. The results show that THz pulse can be coherent detected by the few cycle laser pulses with CEP fixed at certain points and the probe energy needed is quite low. Since biased field is not required, this THz coherent 
detection method is very promising in the field of THz remote sensing. It’s published in Optics Letters, Vol. 38, No. 7, P. 1104-1106 (2013).

This research is supported by the National Natural Science Foundation of China, Grant No. 61177095; the Hubei Natural Science Foundation, Grant No. 2012FFA074; and the Ph.D. Programs Foundation of the Ministry of Education of China, Grant No. 20100142110042.

Fig. 1.  (a)-(c) The calculated I2w0  as the functions of the time interval Δt and the laser pulse  ΦCEPwhen w=5, 50 and 80uJ, respectively. (d)-(i)  I2ab signal waveforms obtained at some special CEP. The TFWHM of the laser pulse is 6fs.