High-Q diamond microresonators in the long-wave infrared

High quality factor (Q) photonic devices in the room temperature thermal infrared region, corresponding to deeper long-wave infrared with wavelengths beyond 9 microns, have been demonstrated for the first time. Whispering gallery mode diamond microresonators were fabricated using single crystal diamond substrates and oxygen-based inductively coupled plasma (ICP) reactive ion etching (RIE) at high angles. The spectral characteristics of the devices were probed at room temperature using a tunable quantum cascade laser that was free space-coupled into the resonators. Light was extracted via an arsenic selenide (As₂Se₃) chalcogenide infrared fiber and directed to a cryogenically cooled mercury cadmium telluride (HgCdTe) detector. The quality factors were tested in multiple microresonators across a wide spectral range from 9 to 9.7 microns with similar performance. One example resonance (of many comparables) was found to reach 3648 at 9.601 µm. Fourier analysis of the many resonances of each device showed free spectral ranges slightly greater than 40 GHz, matching theoretical expectations for the microresonator diameter and the overlap of the whispering gallery mode with the diamond.