Cross-polarized angular emission patterns from lens-coupled terahertz antennas

Abstract

We report detailed measurements of the cross-polarized radiation from lens-coupled terahertz dipole antennas, of the sort commonly used in terahertz time-domain spectroscopy. We compare two different antenna geometries and measure both polarization components as a function of emission angle. The qualitative features of these patterns are not specific to one particular emitter geometry but appear to be generally characteristic of these lens-coupled systems. These measurements are used to determine the angle-dependent ellipticity of the emitted terahertz beam. We conclude that a low f-number collection system (Շf/5) unavoidably results in a beam with a small but measurable ellipticity. This has important implications for any measurements in which a purely linearly polarized wave is required.

Terahertz beam propagation measured through three-dimensional amplitude profile determination

“…The specific silicon-lens configuration has been shown to strongly affect the propagation of THz radiation. 20,21 The plano-convex lens used for both the THz source and detector were hemispherical, designed for collimation of THz radiation. The lens heights were 6.56 mm and 6.42 mm for the source and detector respectively, with radii of curvature of 5 mm.…”

“…16 The temporal pulse reshaping depends strongly on the spatial profile of the THz beam. Measurements of the spatial profile of THz beams has been performed along a line orthogonal to the propagation vector [17][18][19][20][21] ; the THz radiation pattern was found to be adequately represented by a Gaussian with a waist determined by the aperture of the silicon lens used to collimate the THz radiation. This is an important result since, by approximating the THz beam as a Gaussian, the ABCD formalism 22,23 can be applied to determine temporal beam reshaping.…”

Recent Advances in Birefringence Studies at THz Frequencies

“…With regard to PCAs, THz waves emitted by two-contact electrode structures, such as dipole-, stripline-or bow-tie geometries, exhibit a preferential polarization direction, but were shown also to have a slight cross-polarization component. For PCA dipole and bow-tie structures with attached substrate lens, an ellipticity of the emitted THz radiation was found in the range of a few percent of the main polarization component [77,78]. This property appeared to be a general characteristic of these lens-coupled systems [78].…”

“…For PCA dipole and bow-tie structures with attached substrate lens, an ellipticity of the emitted THz radiation was found in the range of a few percent of the main polarization component [77,78]. This property appeared to be a general characteristic of these lens-coupled systems [78]. When two-contact PCAs are used for PS detection, they are subject to the same restrictions as for THz generation.…”

Generation of terahertz pulses with arbitrary elliptical polarization

“…3 In spite of the potential importance, only a few efforts to control the polarization of THz pulses have been reported so far. 4,5 The lack of established methods for generating elliptically polarized THz waves is partly because the most common and powerful sources such as photoconductive dipole antennas or optical rectification of ultrafast laser pulses in ͑2͒ media naturally generate linearly polarized THz radiation; additionally, the majority of THz sources using femtosecond lasers produce single-cycle pulses and the ellipticity is not well defined for the wave form. On the other hand, ellipticity is well defined for multicycle pulses.…”