Generation of radially-polarized terahertz pulses for coupling into coaxial waveguides.

Miguel Navarro-Cía,Jiang Wu,Oleg Mitrofanov,Huiyun Liu

doi:10.1038/srep38926

Miguel Navarro-Cía, Jiang Wu + Show 2 more

Open Access

https://doi.org/10.1038/srep38926

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Abstract

Coaxial waveguides exhibit no dispersion and therefore can serve as an ideal channel for transmission of broadband THz pulses. Implementation of THz coaxial waveguide systems however requires THz beams with radially-polarized distribution. We demonstrate the launching of THz pulses into coaxial waveguides using the effect of THz pulse generation at semiconductor surfaces. We find that the radial transient photo-currents produced upon optical excitation of the surface at normal incidence radiate a THz pulse with the field distribution matching the mode of the coaxial waveguide. In this simple scheme, the optical excitation beam diameter controls the spatial profile of the generated radially-polarized THz pulse and allows us to achieve efficient coupling into the TEM waveguide mode in a hollow coaxial THz waveguide. The TEM quasi-single mode THz waveguide excitation and non-dispersive propagation of a short THz pulse is verified experimentally by time-resolved near-field mapping of the THz field at the waveguide output.

Highlights

Coaxial waveguides exhibit no dispersion and can serve as an ideal channel for transmission of broadband THz pulses
The radial symmetry of this mode eliminates the need for maintaining the polarization direction required for most waveguides, and the wave confinement within the outer conductor ensures robust wave guiding compared to the surface Sommerfeld wave of metal wires[4]
Potential applications of THz coaxial waveguides and radially-polarized THz beams have led to the development of THz antennas with special radial electrodes[2,5,6,7], and segmented nonlinear crystals with threefold rotational symmetry[8,9]

Summary

Introduction

Coaxial waveguides exhibit no dispersion and can serve as an ideal channel for transmission of broadband THz pulses. We find that the radial transient photo-currents produced upon optical excitation of the surface at normal incidence radiate a THz pulse with the field distribution matching the mode of the coaxial waveguide In this simple scheme, the optical excitation beam diameter controls the spatial profile of the generated radiallypolarized THz pulse and allows us to achieve efficient coupling into the TEM waveguide mode in a hollow coaxial THz waveguide. It was demonstrated that transient photo-currents excited on semiconductor surfaces by short (100 fs) optical pulses can have a large radial component due to the photo-generated charge density gradients[20,21,22,23] and anisotropic conductivity[24] This mechanism can provide efficiency, simplicity and tunability (i.e., dynamic control) in generation of THz beams with a tailored polarization state (e.g., radially-polarized THz beam) since the polarization state is governed by the shape of the optical beam and does not require bias. For a Gaussian beam incident on the semiconductor surface, the transient dipole moments are arranged radially and the radiated THz pulse is expected to be radially-polarized with the intensity profile in a doughnut shape[26] similar to the TEM mode of the coaxial metallic waveguide

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