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

## TL;DR

This paper demonstrates for the first time that whispering-gallery modes in the terahertz range can exhibit an anomalous blue-shift when tuned with dielectric and metallic substrates, challenging the typical red-shift behavior.

## Contribution

It introduces dielectric and metallic tuning as a novel method to achieve and study anomalous blue-shifts in high-Q whispering-gallery modes in the terahertz frequency range.

## Key findings

- Blue-shift exceeds three times the expected red-shift
- Experimental verification of anomalous blue-shift in THz WGMs
- Tuning method applicable to high-Q monolithic resonators

## Abstract

The vast majority of resonant systems show a red-shift for the resonance frequency when a perturbation, e.g. losses, are introduced to the system. In contrast, here we report for the first time 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 (THz) frequency range with coherent continuous-wave spectroscopy. The results establish dielectric and metallic tuning as a novel, and viable approach to tune high quality (high-Q) WGMs, and provide valuable insights into the anomalous blue-shift of WGM cavity systems. The tuning capabilities for these compact monolithic resonators is of significant interest for fundamental science and technological applications alike.