Abstract-Subwavelength thick ultrahigh-Q terahertz disc microresonators

Dominik Walter Vogt, Angus Harvey Jones, Thomas Alan Haase, and Rainer Leonhardt

 (a) Schematic illustration of a THz disc resonator with subwavelength thickness. The insert depicts 2 orders of magnitude of the normalized electric field distribution of the fundamental TM mode of a disc resonator with 12 mm diameter and 66.5 μm thickness at 0.6 THz on a logarithmic scale. The HRFZ-Si disc is indicated with the grey solid line. (b) Simulated intrinsic 𝑄$Q$ factor 𝑄0$Q_0$ for two discs with 6 mm diameter and 72 μm thickness (blue dots) and 12 mm diameter and 66.5 μm thickness (orange dots). The green-shaded area indicates the 𝑄0$Q_0$ of a solid sphere with 6 mm diameter. For simplicity, a constant permittivity corresponding to a material absorption of 𝛼=0.006 cm−1$\alpha =0.006{\mathrm{cm}}^{-1}$ is assumed. (c) Optimal disc thickness (black) and maximal intrinsic 𝑄$Q$ factor (brown) for diameters from 6 to 60 mm at a design frequency of about 560 GHz. The green-shaded area shows the intrinsic 𝑄$Q$ factors for solid sphere resonators. (d) FSRs of the disc resonators for diameters from 6 to 60 mm with optimal thicknesses (blue) and solid spheres (green). The solid lines are interpolations of the simulated data points to guide the eye.

https://www.osapublishing.org/prj/abstract.cfm?uri=prj-8-7-1183

Artificial structures that exhibit narrow resonance features are key to a myriad of scientific advances and technologies. In particular, exploration of the terahertz (THz) spectrum—the final frontier of the electromagnetic spectrum—would greatly benefit from high-quality resonant structures. Here we present a new paradigm of terahertz silicon disc microresonators with subwavelength thickness. Experimental results utilizing continuous-wave THz spectroscopy establish quality factors in excess of 120,000 at 0.6 THz. Reduction of the disc thickness to a fraction of the wavelength reduces the losses from the silicon substrate and paves the way to unparalleled possibilities for light–matter interaction in the THz frequency range.

© 2020 Chinese Laser Press

Abstract-Free-space coupling to symmetric high-Q terahertz whispering-gallery mode resonators

Dominik Walter Vogt, Angus Harvey Jones, and Rainer Leonhardt


https://www.osapublishing.org/ol/abstract.cfm?uri=ol-44-9-2220

We report on the coupling of a free-space Gaussian beam to symmetric high-quality (Q) whispering-gallery mode resonators (WGMRs) for terahertz (THz) radiation. We achieve very high excitation efficiencies up to 50% to THz WGMs with a Q-factor of 1.5×104$1.5\times {10}^4$ at 0.7 THz. The high coupling efficiencies have been realized by leveraging a Gaussian beam with a nearly diffraction-limited focal spot, as well as readily available low-loss, high-index silicon spheres with diameters comparable to the wavelength. The results convincingly underline the viability of free-space coupling in the THz frequency range.

© 2019 Optical Society of America

Abstract-Anomalous blue-shift of terahertz whispering-gallery modes via dielectric and metallic tuning

Dominik Walter Vogt, Angus Harvey Jones, Harald G. L. Schwefel, and Rainer Leonhardt

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-44-6-1319

The vast majority of resonant systems show a red-shift for the resonance frequency when a perturbation, e.g., losses, is introduced to the system. In contrast, here we report for the first time, to the best of our knowledge, the experimental demonstration of both red- and anomalous blue-shifting of whispering-gallery modes (WGMs) using dielectric and metallic substrates. The maximum blue-shift is more than three times as large as the expected red-shift, proving that the anomalous blue-shift is more than a peculiar curiosity. The experiments are performed in the terahertz frequency range with coherent continuous-wave spectroscopy. The results establish dielectric and metallic tuning as a novel and viable approach to tune high-quality WGMs and provide valuable insights into the anomalous blue-shift of WGM cavity systems. The tuning capabilities for these compact monolithic resonators are of significant interest for fundamental science and technological applications alike.

© 2019 Optical Society of America

Abstract-Prism coupling of high-Q terahertz whispering-gallery-modes over two octaves from 0.2 THz to 1.1 THz

Dominik Walter Vogt, Angus Harvey Jones, Harald G. L. Schwefel, and Rainer Leonhardt

Fig. 1 (a) Schematic of the CW THz spectroscopy system with fiber coupled photo-conductive antennas (PCAs) using two-inch diameter s-p THz lenses to focus the THz radiation onto the base of the HRFZ-Si prism. The length of the base of the prism is about 13 mm. The spherical HRFZ-Si WGMR is mounted on a 3D manual translation stage, and the position is observed using two microscope cameras. (b) and (c) show the corresponding top and side view, respectively, of the 8 mm HRFZ-Si sphere next to the HRFZ-Si prism. Please note that (c) is focused on the prism surface.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-24-31190

We report on prism coupling of high-quality (high-Q) terahertz (THz) whispering-gallery modes (WGMs) in spherical high resistivity float zone grown silicon (HRFZ-Si) resonators over two octaves from 0.2 THz to 1.1 THz. The WGMs are excited using a HRFZ-Si prism and show unprecedented quality factors of up to 2.2 × 104. A detailed discussion of the phase-and mode-matching criteria of the prism coupling scheme implemented in the continuous wave THz spectroscopy system is presented. The results provide numerous op

portunities for passive ultra-broadband high-Q devices operating in the THz frequency range.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement