title: The effect of the Gouy phase in optical-pump-THz-probe spectroscopy.
abstract: We show theoretically as well as experimentally that the Gouy-phase shift, which depends on the exact positioning of a sample in relation to the focus of a probe beam in a pump-probe experiment, may have a pronounced effect on the shape of the pump-probe signal. The effect occurs only when single-cycle probe pulses are used, i.e. when the slowly varying envelope approximation breaks down, while it disappears for multi-cycle pulses. The effect is thus most relevant in THz time-resolved spectroscopy, where such single cycle pulses are most commonly used, but it should not be overlooked also in other spectral regimes when correspondingly short pulses are involved.
authors.forename: Saima
authors.initials: S
authors.lastname: Ahmed
authors.forename: Janne
authors.initials: J
authors.lastname: Savolainen
authors.forename: Peter
authors.initials: P
authors.lastname: Hamm