Refractive Index Sensing in Microfluidic Channels With Integrated Reflectors by Measuring Light Spot Displacement

Abstract

As several optical properties of a fluid under test depend on its refractive index (RI), there is the need of new sensing approaches for its contactless detection on small sample volumes. To tackle this request, we report a micro-optofluidic sensing platform for measuring the bulk RI of liquid samples inside rectangular section glass microcapillaries with integrated reflectors. In the instrumental setup, a laser beam travels obliquely toward the fluidic channel and it is zigzag guided between the mirrors, crossing the fluid multiple times. When samples with different refractive indices are tested, the light beam is deflected at different angles inside the channel; as a consequence, the output beam position shifts along the microcapillary surface. The position change of the output light spot is then measured by means of a 1-D position-sensitive detector (PSD). Hence, by proper calibration, it is possible to retrieve volumetric RI variations with respect to a reference fluid. Experimental measurements showed a beam displacement per RI unit up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2669~\mu \text{m}$ </tex-math></inline-formula> /RIU, in agreement with the prediction of the theoretical model developed to study light traveling through the microcapillary. The proposed readout technique is contactless and noninvasive, being thus highly suitable for application in several fields, ranging from biology to chemistry to food and beverage industry.