Whispering resonances to the terahertz regime

Aside from their rather evocative name, whispering-gallery waves able to travel along concave surfaces feature the useful quality of making extremely high quality (Q) resonators possible. Whispering-gallery mode (WGM) devices already find wide application in optical technologies such as lasers and sensing devices. Experimental work published in APL Photonics demonstrates that silicon-based WGM resonators promise similar advantages in the burgeoning field of terahertz (THz) electronics and photonics.

While THz resonators do not require the ultraprecise machining of optical devices because of the long THz wavelengths, their strong energy-absorbing characteristics make the choice of resonator material vital in order to achieve a high-Q factor. For their WGM resonator, the authors chose high resistivity flat zone silicon (HRFZ-Si), which exhibits very low absorption over a wide range of frequencies. The spherical device, which was slightly less than 4 millimeters, was connected to a silica optical fiber with its cladding removed, serving as a waveguide through which THz radiation was transmitted and detected.

Considering the refractive index of the HRFZ-Si material and the radius of the resonator, the experimenters were able to derive resonance frequencies and Q factors of the observed WGMs using Mie-Debye-Aden-Kerker (MDAK) theory. This allowed them to achieve critical coupling between the silica waveguide and HRFZ-Si material despite their disparate refractive indices. Phase matching could be altered by changing the waveguide-resonator distance.

The work demonstrates WGMs with Q factors as high as 1.5 × 10⁴ and thus provides the first experimental evidence of ultrahigh-Q THz WGMs, showing how the resonator-waveguide combination can be designed to create specific WGM modes for various possible applications.

Source: “Ultra-high Q terahertz whispering-gallery modes in a silicon resonator,” by Dominik Walter Vogt and Rainer Leonhardt, APL Photonics (2018). The article can be accessed at https://doi.org/10.1063/1.5010364 .