Longitudinal THz fields and their phase relation with respect to transverse fields

Radiation with significant longitudinal components of the electric field is of interest for excitation of systems with a dipole moment in the direction of propagation of the radiation. Another special feature of longitudinal fields occurring in the focus of radially polarized radiation is their superior focusing property as compared to transverse fields of linearly polarized beams. In contrast to previous studies in the near infrared spectral range, where the intensity distribution was recorded, our terahertz study provides direct access to the electric field. Consequently, the phase relation between longitudinal and transverse field components can be addressed. We present results for the field distribution in the focal plane for both radially and linearly polarized single cycle THz pulses generated by photoconductive emitters. In both cases a phase shift of pi/2 between transverse and longitudinal field components is found. We show that this phase shift is of universal nature as it does not depend on the type of mode, the focussing condition and the frequency [1]. The universal phase shift is a fundamental consequence Maxwell’s law div E = 0 and symmetry properties of the beams. We also show that the longitudinal field components of a radially polarized beam can be focussed to a smaller spot as compared to the transverse components of linearly polarized radiation. This is valid even for moderate focusing conditions, where the transverse fields are stronger than the longitudinal field components. Implications of the findings for imaging applications and for studies of light-matter interaction in case of vanishing Poynting vector will be discussed.
[1] S. Winnerl, R. Hubrich, M. Mittendorff, H. Schneider, and M. Helm, New J. Physics 14, 103049 (2012).