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Tracing the Gouy phase shift of focused, radially polarized THz pulses

THz microscopic imaging is used to extract the Gouy phase shift of the transverse and longitudinal field components of a tightly focused, radially polarized beam. We demonstrate that the applied THz time domain approach allows to observe directly the evolution of the geometric phase as the wave propagates through the focus. Our method yields a Gouy phase shift of 2π for the transverse component and of π for the longitudinal component. In addition, we apply our method to the well-known case of a focused, linearly polarized beam and pinpoint a fundamental connection between the field components of tightly focused, radially and linearly polarized light. The applied procedure is universal and may even allow to determine the geometric phase of arbitrary shaped and polarized propagating waves.