Systematical study on the role of laser second harmonic generation in the terahertz air coherent detection

•The role of laser SHG in the THz air coherent detection is studied.•Two parameters are used to characterize the conversion efficiency of laser SHG.•A new scheme with a biased DC field to detect broad THz radiation is proposed. Terahertz field-induced laser second harmonic has been used to detect the broadband terahertz radiation. After this method is developed to air coherent detection technique with a biased modulated field, it has been widely used in the broadband terahertz science and technology. In this complicated process, both the laser plasma interaction and the four-wave mixing generate laser second harmonic factually. In this paper, the role of laser second harmonic generation in such detection method is investigated systematically based on the theory and the calculations. Two parameters are given to express the efficiency of second harmonic generated from the interaction between the laser plasma. Then, several different schemes to implement terahertz detection are discussed with the calculations as illustrations. This work will help to understand the process of the air coherent detection technique when implementing it.