  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.
