Silicon-on-sapphire ring resonators operate at a 5.5 μm wavelength

Researchers at the University of Washington (Seattle, WA) and Cornell University (Ithaca, NY) have created the first silicon-waveguide ring resonators for wavelengths between 5.4 and 5.6 μm, opening up a new region for ring-resonator applications such as biosensing and modulation. The resonators, as well as ridge waveguides, were fabricated on a silicon-on-sapphire (SOS) substrate.

The chip was patterned using electron-beam lithography and contained various ridge waveguides and ring resonators. Waveguides with a height of 0.6 μm and varying widths were fabricated; as predicted, a 1.8 μm wide waveguide properly channeled the fundamental mode of linearly polarized light at a 5.5 μm wavelength. The measured loss of the ridge waveguide was 4.0 ± 0.7 dB/cm at laser powers ranging from 6 to 100 mW (with an insertion loss of 25 dB), indicating minimal nonlinear loss. The ring resonators had various radii and edge-to-edge spacings; a sample ring had a 40 μm radius and a 0.25 μm edge-to-edge spacing, and exhibited a cavity Q factor of 3000, a free spectral range of 29.7 nm, and an associated group index of 3.99. Optimizing the drop port should boost the Q closer to its theoretical value of 25,000. Contact Alexander Spott at


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