Photonic micro-resonators for steam evaporation dynamic sensing

Abstract

We have investigated the effect of sudden water condensation processes and the behavior of its condensed water prior evaporation, with an integrated resonant photonic structure and dynamic tracking of its transduced signal. The aim of this analysis is to develop a water condensation lab-on-chip sensor, with the possibility of data treatment with an embedded system. Integrated photonic micro-resonator (MR) devices have been designed and fabricated with polymer UV210 by means of Deep-UV photolithography. Thanks to this technique, we have achieved racetrack shaped micro-resonators coupled to suited access waveguides. We have assessed such MRs with different geometrical characteristics while changing, respectively, the coupling length (LC), the radius of curvature (R) and the width (w) of the guides. The chosen values for the set of parameters LC-R-w (in μm) are 5-5-3 and 10-10-3. The laser source used with the injection bench is a Gaussian broadband laser (λcentral = 790 nm, FWHM = 40 nm) allowing us to visualize several resonances at the same time in order to multiplex the relevant measurements. The transduced spectrum is then acquired with an optical spectrum analyzer (OSA) linked to a computer with Labview and MATLAB software recording and processing data in real time. Then, relevant characteristics to be tracked are the Free Spectral Range (FSR) and the transmitted energy; these quantities can be linked to the physical characteristics of the structure considering both the effective refractive index and the absorption coefficient. The experimental setup also includes various movies with a top-view imaging camera of the chip (MRs) recording the soft matter process steps, so as to correlate the changes in the transduced spectrum and the behavior of the condensed water mechanisms (condensation, coalescence and evaporation). Then, the chip is fitted with a temperature controller, so as to carry out measurements at different temperatures: 20, 24 and 28 °C.