About detecting steam condensation by means of polymer racetrack micro-resonators: highlighting the dynamics of such a soft-matter process

We have investigated the effect of brutal steam 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 steam condensation lab-on-chip sensor, with the possibility of data treatment with an embedded system. We have designed and fabricated integrated photonic micro-resonators (MRs) devices in polymer UV210 by means of Deep-UV photolithography. The UV210 polymer is a Chemically Amplified (CA) positive resin containing a Photo Acid Generator (PAG), featuring an absorption band in the Deep-UV domain. Such a property allows us to take advantage of a smaller insolation wavelength than that of a traditional i-line photolithography (here, λ insolation =248nm), so as to make smaller and more precise structures. Due to exposure, the PAG entails a cascade of chemical reactions that leads to a change of polarity in the resin, going from a lipophilic state to a hydrophilic one. This changing of state makes possible the development by means of the basic solvent tetra-methyl ammonium hydroxide (TMAH). Thanks to this technique, we have achieved racetrack shaped micro-resonators coupled to an access waveguide. We have made such MRs with different geometrical characteristics while changing respectively; the coupling length (L C), the radius of curvature (R) and the width (w) of the guides. The chosen values for the set of parameters L C-R-w (in µm) are: 5-5-3, 5-5-4, 10-10-3 and 10-10-4. The laser source used with the injection bench is a Gaussian broadband laser (λ central =790nm, FWHM=40nm) 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 on which Labview and Matlab software record and process the data in real time. The relevant characteristics to be tracked are the Free Spectral Range (FSR), the Full Width at Half Maximum (FWHM), the extinction ratio, the transmitted energy and the shift of the different resonances. These quantities can be linked to the physical characteristics of the structure such as effective refractive index, coupling coefficient and absorption coefficient. The experimental setup also includes various movies, a top-view imaging camera of the chip (MRs) during such a soft matter process, so as to correlate the changes in the transduced spectrum and the behavior of the condensed steam mechanisms (condensation, coalescence and evaporation). Then, the chip is linked to a temperature controller, so as to carry out measurements at different temperatures: 22°C, 25°C, 28°C and 31°C.