Breakthrough in Luminescence Thermometry─Supersensitive Emission Line Shift of Whispering Gallery Modes in Rhodamine B-Doped Cellulose Fiber Microresonators.

The development of optically active materials enabling rapid, precise, and accurate remote detection of physical parameters is crucial for advancing science and modern technology. In this work, we investigate resonant effects and light propagation in cellulose fibers doped with Rhodamine B for optical thermometry applications. These fibers were successfully produced by using the spinning method with N-methylmorpholine N-oxide. Their optical properties were investigated through absorption and emission spectroscopy, confirming the integration of Rhodamine B into the cellulose matrix. Notably, the cylindrical shape of the modified fibers significantly affects the emission spectra when excited at the fiber edge, revealing sharp and superimposed whispering gallery modes (WGMs). A confocal system with a 532 nm laser was used to analyze for the first time the WGM emission from the optically active cellulose microfibers. The WGMs displayed high susceptibility to the negative thermo-optical coefficient of the resonating cavity, leading to a giant spectral shift. This unprecedented temperature-induced blue shift of the WGMs provides the highest reported sensitivity-27 times higher than other microresonators─demonstrating a spectral shift of ∼0.47 nm K^-1. With excellent temperature resolution (≈0.17 K), our findings highlight the great potential of this method and material as a supersensitive optical thermometer.