Radial photocurrents for single-cycle terahertz pulse excitation and propagation in hollow coaxial waveguides

Low dispersion of single-cycle terahertz (THz) pulses can be achieved in very few hollow waveguides such as dielectric-lined and coaxial waveguides. The former has been widely investigated [1] because, among other reasons, the in-coupling is very efficient with typical THz emitters. Conversely, the fundamental mode of a coaxial waveguide, the transverse electromagnetic (TEM) mode, is radially-polarized and it exhibits poor mode matching with standard linearly-polarized THz emitters, making the in-coupling not efficient for this, otherwise attractive, waveguide [2].
We solve this problem by exploiting the radially-polarized THz pulses generated from photoexcited charge density gradients in narrowband semiconductors [3]. This leads to a simple coupling scheme for THz coaxial waveguides using standard optical (IR) beams with a Gaussian profile [4] (Fig 1(a)). In our experiments, a 1 μm thick indium arsenide layer is placed at the input of a 100 mm long copper coaxial waveguide with external diameter of 2.8 mm (or 2 mm) and inner rod diameter of 1.6 mm (or 1 mm). At the waveguide output, the field is detected by an integrated sub-wavelength aperture (10×10 μm²) THz near-field probe. We demonstrate that, for maximum efficiency, the spot size of the incident optical beam needs to be slightly larger than the inner rod diameter (Fig. 1(b)). The TEM mode (see the xy-map in Fig. 1(a)) has unnoticeable dispersion (Fig. 1(c)). The estimated attenuation of the TEM mode ranges from 15 (~0.035 cm^-1) to 40 dB/m (~0.104 cm^-1) at the low and high end of the spectral window investigated.
When comparing the results with those obtained for a standard linearly-polarized THz emitter optimized for maximum coupling to the TEM mode, we observe higher discrimination against higher-order modes for the here proposed scheme. This suggests that quasi-single TEM propagation can be achieved for this simple coupling scheme. This work may trigger further studies of the widely used (at microwaves) coaxial waveguides for THz applications and suggests that a spatial THz modulator can be realized with a spatial light modulator that tailors the optical photoexcitation.
[1] O. Mitrofanov, et al., IEEE Trans. THz Sci. Tech. 1, 124-132 (2011).[2] X. Wang, et al., Opt. Express 20, 7706-7715 (2012).[3] S.C. Corzo-Garcia, et al., Phys. Rev. B 94, 045301 (2016).[4] M. Navarro-Cía, et al., Sci. Rep. to be published