Development and Characterization of Waveguides and Whispering-Gallery Mode Resonators for Terahertz Radiation

Abstract

We report on the development of terahertz (THz) waveguides and investigations on the vastly unexplored field of THz whispering-gallery mode (WGM) resonators. The waveguides are experimentally characterized with broadband coherent THz time-domain spectroscopy, while the measurements of the THz WGMs require the high-frequency resolution of coherent continuous wave (CW) THz spectroscopy. The CW THz spectroscopy measurements are analyzed using Hilbert transformation, clearly outperforming the traditional data analysis. All experimental results are supported by finite-difference timedomain and finite element method simulations, as well as analytical models. In particular, specialized Mie theory is applied to analytically analyze the investigated THz WGM resonators. The presented 3D-printed helical waveguide and tube with an anti-reflection structure facilitate single-mode, broadband, low loss and low dispersion guidance in the frequency range from 0.2 THz to 1 THz. In accordance with the simulations, the broadband low loss guidance of both waveguides approach the theoretical limit of the leaky waveguide design. We, furthermore, numerically investigate a spoof surface plasmon polariton THz metamaterial waveguide to support integrated highly confined guidance of THz radiation. A sophisticated technique to fabricate the metamaterial consisting of a ridge of metal micro pillars in a hexagonal lattice is proposed. Advancing to THz WGM resonators, we demonstrate the first high quality (Q) silica THz WGM bubble resonator with a sub-wavelength wall. Critical coupling is achieved using a sub-wavelength air-silica step-index waveguide. The design minimizes the impact of the material loss and supports a large evanescent field favorable for sensing applications. Moreover, we investigate the Fano resonances observed in a very low loss high resistivity float zone grown silicon (HRFZ-Si) sphere coupled to a multi-mode air-silicon step-index waveguide. Intriguingly, we also observe critical coupling of the THz WGMs in the HRFZ-Si resonator coupled to an air-silica step-index waveguide. The ultra-high Q factor of 1.5×104 at 0.62 THz of the reported higher order radial THz WGMs exceeds by far any other reported resonant structure in the THz frequency range. Studying the amplitude and phase profile of the observed THz WGMs for the first time, we provide unprecedented insights into the intricacies of THz WGMs. In particular, the phase profile allows establishing critical coupling with very high precision and enables us to develop a novel technique to unambiguously identify the observed THz WGMs.