A Free-Space Interferometric Refractometer Structure With Simple Microfluidic Chips

We introduce and experimentally demonstrate an optofluidics-based refractometer structure arranged in a simple free-space Young interferometer design. Our key idea is based on the use of a very simple microfluidic chip structure that consists of one or two flow channels arranged in parallel. We then pass the optical beam through the flow channels of the microfluidic chip. Behind the flow channels, there are two small apertures where the incident optical beam is automatically divided into two optical beams. These two optical beams are propagating in free space for a desired distance before they interfere with each other at the observation plane. Key features include simplicity in design, ease of implementation, and robustness. In the experimental demonstration, we use a 655-nm wavelength laser diode; the free-space propagation distance is 57.5 cm. The two-channel microfluidic chip has a 900-μm channel spacing, a 100-μm channel width, and a 100-μm channel depth (i.e., d/L = 9). Results indicate a sensitivity of 1.34 × 10^-4 RIU in measuring the refractive index of the sucrose solution which clearly agrees with the theoretical analysis. A higher sensitivity of 6.19 × 10^-6 RIU is also accomplished when the single-channel microfluidic chip with a measured d/L ratio of 0.52 is combined with an optical mask having a 600- μm channel spacing.