Ultra-high-Q microresonators in the mid-infrared:
toward utilizing strong interactions between light and molecules on a chip

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Hansuek Lee

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Department of Physics, KAIST

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The mid-infrared wavelength region, where fundamental absorption bands of various molecules exist, is of great importance for many applications to monitor and manipulate molecules such as molecular sensing, biochemical imaging, time-resolved spectroscopy, and photochemical processing. As an ideal material platform in the mid-IR, chalcogenide glasses (ChGs) have attracted much attention due to their high transparency and large optical nonlinearity in contrast to conventional on-chip optical materials such as silica and silicon nitride which suffer from significant losses due to multiphonon absorption. However, the loss of on-chip ChG components has remained significantly higher than that of the optical fiber form, preventing the full exploitation of their inherent large optical nonlinearities on a chip in the mid-IR.

In this talk, we present on-chip ChG resonators with Q-factors larger than 30 million, exceeding previous records in the mid-IR by more than 30 times. By exploiting this ultra-high Q-factor together with the controllability of free spectral range provided by microfabrication, we have successfully demonstrated Brillouin lasing in this wavelength region for the first time. Our results exhibit a threshold power of 0.1 mW and a Schawlow-Townes linewidth of 85 Hz, significantly surpassing that of commercialized quantum cascade lasers. We will also discuss future research opportunities enabled by cavity-enhanced strong interactions between light and molecules, such as molecule-induced nonlinear optics and vacuum field catalysis.