What a Cathedral Taught Us About Light: The Science Behind Whispering Gallery Resonators

A recent study published in the Journal of the Royal Society of New Zealand presents an experimental investigation into thermo-optic tuning in silicon-based whispering gallery mode resonators (WGMRs). Using a 500 µm radius silicon sphere, the researchers demonstrated all-optical tuning of near-infrared WGMs by introducing a 405 nm pump laser to induce local heating.

The experiment yielded a red-shift of up to 0.63 nm at 6.32 mW pump power. These shifts were attributed to temperature-dependent changes in the silicon’s refractive index and slight thermal expansion of the resonator. The measured Q-factors reached ~10⁵, consistent with the expected linewidths based on Lorentzian fits.

The term "whispering gallery mode" originates from acoustics, specifically the phenomenon observed in circular galleries like the dome of St. Paul’s Cathedral in London, where whispers can travel along curved surfaces to distant listeners. This same principle applies to light in WGMRs, where electromagnetic waves are confined by total internal reflection along the curved boundary of a resonator, creating discrete, high-Q resonant modes. The analogy underscores the role of geometry in sustaining wave propagation without significant loss.

The study also examined the transient response of WGMs under modulated pump conditions, confirming that dynamic tuning could be achieved. Although the coupling method—optical fibre half-coupling (OFHC)—limited coupling efficiency and spectral control, the results show clear potential for developing tunable silicon-based photonic devices.

Silicon’s high refractive index and compatibility with integrated photonics make it a strong candidate for WGMR-based applications such as temperature sensing, optical filtering, and nonlinear optics. While traditional tuning methods like strain or electro-optic modulation require additional materials or mechanical setups, this all-optical approach offers a compact and CMOS-compatible alternative.

This work contributes to ongoing efforts in photonic device miniaturization and control, particularly within the context of silicon photonics and integrated resonator design.

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