We have developed a system for tapered fiber measurements of optomechanical resonators inside a dilution refrigerator, which is compatible with both on- and off-chip devices. Our apparatus features full three-dimensional control of the taper-resonator coupling conditions enabling critical coupling, with an overall fiber transmission efficiency of up to 70%. Notably, our design incorporates an optical microscope system consisting of a coherent bundle of 37,000 optical fibers for real-time imaging of the experiment at a resolution of ∼1 μm. We present cryogenic optical and optomechanical measurements of resonators coupled to tapered fibers at temperatures as low as 9 mK.

We demonstrate aqueous refractive index sensing with 15 to 30 µm diameter silicon nitride microdisk resonators to detect small concentrations of Li salt. A dimpled-tapered fiber is used to couple 780 nm visible light to the microdisks, in order to perform spectroscopy their optical resonances. The dimpled fiber probe allows testing of multiple devices on a chip in a single experiment. This sensing system is versatile and easy to use, while remaining competitive with other refractometric sensors. For example, from a 20 µm diameter device we measure a sensitivity of 200 ± 30 nm/RIU with a loaded quality factor of 1.5 × 10 4 , and a limit of detection down to (1.3 ± 0.1) × 10 −6 RIU.Optical whispering-gallery mode (WGM) resonators are an area under avid research as they promise fast, sensitive and label-free detection of chemical and biological samples.