Optically tunable microfiber-knot resonator.

This paper demonstrates light-induced tuning of the optical spectrum by a microfiber-knot resonator overlaid with a photoresponsive liquid crystal (LC) mixture containing photosensitive diluents (non-mesogenic azobenzene molecules), a chiral dopant and a nematic LC. The high-quality resonator is made by drawing a single mode fiber to a micro-size diameter and causing the microfiber to self-twist into a knot. A thin layer of a photosensitive mixture was placed on the overlap (knot) area and gentle heating was used to obtain a uniform thin film which coated the fiber's surface. Upon irradiation with UV light, noticeable changes to the peak resonance wavelengths were observed which we associate with a local change in the refractive index (RI) in the fiber's tapering area. Repeatable and reversible spectral shifting (0.15 nm) of the resonance wavelength is demonstrated by irradiation with 50 mW/cm² UV light.