Broadband Absorbance in a Liquid-Core Optical Ring Resonator

We have demonstrated the application of broadband absorption spectroscopy in a liquid-core optical ring resonator. An initial proof of concept of the broadband liquid-core optical ring resonator (BLCORR) was constructed using a thinned-wall, 250-μm-inner-diameter fused silica capillary, tapered multimode optical fibers for input and output coupling, and a light-emitting diode (LED) source. When compared with standard cuvette measurements, an apparent path length as high as 5 cm was observed for methylene blue (MB). MB is a cationic dye that exhibits strong surface interaction with bare silica. Bromothymol blue (BTB), on the other hand, has a similar absorbance spectrum but does not share this same surface activity. On comparing these two dyes, the apparent path length for MB was found to reach more than 50 times that of BTB, confirming the expectation that the sensing region being probed is largely within the evanescent field at the inner surface of the capillary. The BLCORR may also inherit, from attenuated total reflection (ATR) spectroscopy, the ability to analyze highly concentrated chromophores. Concentrations of BTB as high as 10⁻² and 10⁻³ M were easily distinguished from each other at the λ_max in the BLCORR, whereas this was not the case in a 4-mm cuvette cell. Our presented device employs commercially available materials and could incorporate well into micro-fluidic systems. These benefits, along with the demonstrated ability to take enhanced surface absorbance measurements in a capillary, give the BLCORR potential in a variety of applications.