We investigate thermal tuning of an ultra-high quality (Q) terahertz (THz) whispering-gallery mode resonator (WGMR) made of low loss silicon. The Si THz WGMR can be continuously tuned with a rate of about 29 MHz/K in the frequency range from 650 GHz to 666 GHz. Furthermore, we utilize the Si WGMR to extract the thermo-optic coefficient of Si at 459 GHz and 659 GHz in the temperature range from 295 K to 363 K.

The terahertz spectrum provides tremendous opportunities for broadband gas-phase spectroscopy, as numerous molecules exhibit strong fundamental resonances in the THz frequency range. However, cutting-edge THz gas-phase spectrometer require cumbersome multi-pass gas cells to reach sufficient sensitivity for trace level gas detection. Here, we report on the first demonstration of a THz gas-phase spectrometer using a sub-wavelength thick ultrahigh-Q THz disc microresonator. Leveraging the microresonator’s ultrahigh quality factor in excess of 120,000 as well as the intrinsically large evanescent field, allows for the implementation of a very compact spectrometer without the need for complex multi-pass gas cells. Water vapour concentrations as low as 4 parts per million at atmospheric conditions have been readily detected in proof-of-concept experiments.

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