Phase characterization in broadband THz wave detection through field-induced second harmonic generation.

We present a theoretical and experimental investigation of the THz pulse phase measured by a broadband heterodyne detection method via field-induced second-harmonic generation in ambient air. The dependence of the detected THz phase spectra on the positions of the wire shaped electrodes scanning along the detection plasma is discussed. An additional phase shift around the beam focus is observed. Theoretical deductions reveal that it is caused by the Gouy shift of the optical probe beam and THz beam during the heterodyne detection process.