Universal phase relation between longitudinal and transverse fields observed in focused terahertz beams

TL;DR

This paper experimentally demonstrates a universal phase relation between longitudinal and transverse fields in focused terahertz beams, confirming theoretical predictions and revealing differences in focus size based on polarization.

Contribution

It provides direct experimental evidence of the pi/2 phase shift between longitudinal and transverse fields in focused terahertz beams, and compares focus sizes for different polarizations.

Findings

Longitudinal fields are pi/2 phase shifted from transverse fields.

Longitudinal fields are observed in all frequency components.

Radially polarized beams have smaller focus spots than linearly polarized beams.

Abstract

We directly observe longitudinal electromagnetic fields in focused freely propagating terahertz beams of radial and linear polarization. In accordance with theory, the longitudinal fields are phase shifted by a value of pi/2 with respect to the transverse field components. This behavior is found for all frequency components of single cycle THz radiation pulses. Additionally we show that the longitudinal field of a radially polarized THz beam has a smaller spot size as compared to the transverse field of a linearly polarized beam, that is focused under the same conditions.