26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

MHz‑Resolution Terahertz Near‑Field Mapping for High‑Quality Resonators

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral ANZOS | Photonics and Optics (ANZCOP)

Description

High-quality terahertz resonators are key enablers of enhanced light–matter interaction in the terahertz domain. This enhancement underpins improved sensitivity and reduced detection limits in sensing and spectroscopy, as well as more efficient nonlinear mixing between optical and terahertz signals for telecommunications. However, the design of high‑Q terahertz resonators remains largely dependent on numerical optimisation, often limited by uncertainties in material parameters and fabrication-induced deviations that are difficult to quantify.

Here, we introduce a near-field imaging technique that enables direct experimental characterisation of high‑Q terahertz resonators, providing access to their unperturbed spatial field distribution and effective refractive index. The approach combines a fibre‑coupled photoconductive antenna with a continuous‑wave terahertz spectrometer offering MHz spectral resolution, together with Hilbert transform-based data analysis to reconstruct complex field profiles. Subsequently, a 2D Fourier transform is performed to map the data into momentum space, allowing direct extraction of the effective refractive index of the structure. To the best of our knowledge, no existing near-field imaging technique offers this combination of capabilities.

We validate the method using two systems: a single-mode step-index waveguide for proof-of-concept demonstration, which we then extend to a high‑Q silicon terahertz disc microresonator. In both cases, excellent agreement with numerical simulations is achieved. Notably, the results demonstrate that the near‑field probe does not measurably perturb the resonator mode, confirming the technique’s non-invasive nature. Furthermore, this work highlights a novel application of Hilbert transform analysis in terahertz near-field imaging.

This work establishes a robust and quantitative experimental framework for validating and refining terahertz resonator designs, reducing reliance on purely numerical approaches and accelerating the development of high‑performance terahertz devices for sensing, spectroscopy, and communication technologies.