Fabry-Perot sensors: microfluidic channels and transparent membranes

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.