Visualization and selective manipulation of sub-terahertz whispering gallery modes
26 November 2025·,
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0 min read
Anna R. Petersen
Pablo Paulsen
Florian Sedlmeir
Nicholas J. Lambert
Harald G. L. Schwefel
Mallika Suresh
Abstract
The development of technologies based on electromagnetic resonant structures usually relies on precise understanding of the spatial form of the supported modes. In millimeter-sized resonators that support millimeter-wave modes, this is often done numerically. However, simulations often fail to accurately predict the resonance frequencies and mode numbers. Such discrepancies can arise from uncertainties in the dielectric properties of the materials or inaccuracies in the geometric characterization of the structures. Here, we experimentally map the spatial distribution of terahertz modes in a silicon disk resonator by perturbing the mode with a metal needle scanned across the disk. This allows us to identify the modes, and hence to design a perturbative periodic structure to selectively manipulate specific modes. This technique could be useful for targeted tuning of such modes for spectral engineering.
Type
Publication
Applied Physics Letters, 127(21), 211102
Status
Peer-reviewed
Authors
Expert — Resonator Physics
Florian Sedlmeir is a Research Fellow and Marie Skłodowska-Curie Fellow at the University of Otago. His research focuses on high-Q whispering-gallery-mode resonators, nonlinear optics, and the conversion of microwave and terahertz signals to the optical domain. He completed his PhD at Friedrich-Alexander University Erlangen-Nürnberg, carrying out his research at the Max Planck Institute for the Science of Light, and subsequently worked in both academic research and industry before returning to the University of Otago in 2022. His current work explores electro-optic frequency conversion for low-noise millimetre-wave and terahertz detection, as well as microwave-to-optical interfaces for quantum technologies.
Authors
Programme Leader / Principal Investigator
Harald Schwefel is a Professor in the Physics Department of the University of Otago and Science Lead of Tuwhiri. He is also the Deputy Director Science in Te Whai Ao – Dodd-Walls Centre for Photonic and Quantum Technologies and Principal Investigator in Quantum Technologies Aotearoa (QTA).
His research interests are in the use of optical resonators. Such high-quality resonators are great to generate very high field intensities. With high fields the interaction of light with matter can show significant nonlinearities and light can interact with itself and new colours can be created. A side effect of these high-quality resonators is that they are extremely sensitive to changes in the environment making them an ideal candidate to measure such changes.
Authors
Lead, Photonics
Mallika Suresh (Mika) is a Research Fellow at the Physics Department of the University of Otago and the Photonics Lead of Tuwhiri. For the last 5 years, her research has been focused on electro-optic nonlinear mixing of terahertz signals with optical modes in whispering gallery mode resonators. Before that, during her PhD, Mika worked with femtosecond pulses in gas-filled hollow-core fibres under the supervision of Prof. Philip Russell and Dr. Francesco Tani at the Max Planck Institute for the Science of Light. Soon after joining Harald’s team, she supervised the setting up of a femtosecond laser ablation system at the Physics Department for (among other applications) the fabrication of dielectric whispering gallery mode resonators for nonlinear optical experiments. She was the Agnes-Blackie Research Fellow of Te Whai Ao – Dodd-Walls Centre for Photonic and Quantum Technologies in 2024 – 2026 and now is an Associate Investigator in Te Whai Ao. Mika is also leading collaborations with international experts in the field of terahertz photonics such as Prof. Withawat Withayachumnankul at the University of Adelaide and Prof. Ileana-Cristina Benea-Chelmus at École Polytechnique Fédérale de Lausanne as part of the Tuwhiri programme.