Experimental Gouy phase shift compensation in Terahertz time-domain spectroscopy

Terahertz time-domain spectroscopy experiments are usually carried out with focused beams. In this case the Gouy phase shift modifies the retrieved dielectric spectra of thick samples [Opt. Express 18, 15338 (2010)]. We show that a significant influence of the Gouy shift occurs also in commercial THz spectrometers where the focusing conditions may be a priori unknown. A method of experimental compensation of the Gouy shift influence is described and demonstrated.