The Gouy phase shift in terahertz time-domain spectroscopy and its experimental estimation, modelling and compensation

Spatial properties of pulsed beams become more important in the terahertz range, which causes that reference and sample measurements can differ in the spectral dependent complex phase called the Gouy phase. It originates from propagation of the beam wavefront close to the focus. In such a case, the plane wave approximation has not to be fulfilled and measured data gives incorrect absorptions and dispersions. We present techniques to compensate these phenomena as well as the procedure to achieve the Gouy phase from measured data. Moreover, the model to simulate the Gouy phase detection will be shown in this paper. It gives powerful tool to avoid experimental errors caused by the Gouy phase in terahertz time-domain spectroscopy.