Fabry-Pérot sensors: microfluidic channels and transparent membranes

Abstract

We present two realizations of a highly sensitive platform useful in environmental sensing and diagnostics -a Fabry-Perot (FP) interferometer -(i) a pair of semi-transparent mirrors integrated into a microfluidic channel and (ii) a silicon membrane of sub-micrometer thickness. Simple way to make microfluidic channels by (i) hot-embossing into a sheet of technical grade PMMA and (ii) double-sided tape fixed glass with Au-coated mirrors are presented. By changing the thickness of the Au coating, the roughness and porosity of mirror surface is controlled. In turn, this provides a method to tune finesse of the FP cavity to monitor solutions flowwing between the FP-mirrors. In case of silicon, the FP cavity is formed by coating two sides of a Si-membrane. These two different approaches to harness a high sensitivity of the FP interferometry are proposed: changes of FP cavity caused by materials in the channel can be monitored, while the coated membrane is used to monitor the effects which are induced by membrane's ambiance. The finesse of the FP cavity is optimized for the maximum spectral sensitivity at the cost of transmitted light intensity in case of microfluidic channel and silicon membrane. Via optimization of the finesse (in the range 2-5) and overall transmission of a FP-pair (20-60%) practical solutions are proposed for spectral sensing of (i) refractive index and mechanical channel width's changes in a microfluidic channel as well as (ii) temperature changes of membrane's environment. Asymmetric thickness of the FP mirrors can be used to optimize sensitivity.

Measuring concentration fields in microfluidic channels in situ with a Fabry–Perot interferometer

“…Various interferometric techniques have been adapted to microfluidic systems to precisely measure the average refractive index of a single (as low as~μm 3 ) point in space, down to 10 −4 to 10 −7 RIU, including a Mach-Zehnder interferometer, 23 a back scattering interferometer, 24 a Young interferometer, 25 and Fabry-Perot interferometers. [26][27][28] To our knowledge, only one refractometry method, consisting of a Fabry-Perot interferometer with a monochromator, is able to resolve spatial refractive index profiles in a fluidic system. 29 This technique is optimized for measurements in nanofluidic channels and not immediately applicable to measuring evolving concentration profiles in microfluidic channels.…”