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Design and fabrication of a compact gas sensor integrating a polymer micro resonator and a 850nm VCSEL source

Qingyue Li 1
1 LAAS-MICA - Équipe MICrosystèmes d'Analyse
LAAS - Laboratoire d'analyse et d'architecture des systèmes
Abstract : There is a growing demand for portable gas sensors, from both environmental and health sciences as well as the industry. Resonant optical sensors, in particular planar micro resonators, combine high sensitivity and small dimensions, which makes them good candidates for these applications. The sensing principle of these evanescent field sensors is based on a variation in their optical response linked to a modification of the effective index in the presence of the target molecules. While many studies show the effectiveness of this approach, its implementation on sub-micron dimensions requires the use of complex optical benches, bulky, and difficult to miniaturize. We propose here a global approach based on the complete measurement system, by exploiting multidisciplinary skills on (i) vertical cavity surface emitting diodes (VCSEL) and the shaping of their beam; (ii) the design and manufacture of polymer optical micro resonators and coupling networks; (iii) gas sensors and surface functionalization. The use of a VCSEL source tunable in current over 5nm, combined with beam shaping and the use of grating couplers, provides better tolerance to misalignment between the source and the waveguide input, and thus simplify the injection of light into micrometric or submicronic structures in a portable system. Vertical coupling of the micro-resonator is favored to facilitate the deposition of the sensitive layer and to isolate the coupling networks and the waveguides. Since the 850nm wavelength does not allow direct detection of the absorption of the gas molecules of interest, a polyaniline (PANI) sensitive layer is added in order to provide selectivity and sensitivity to ammonia (NH_3). Its exposure to NH_3 causes dedoping, which is reversible at room temperature, and results in changes in its optical index and a variation in the resonance spectrum at the output of the micro-resonator, which can be measured using a simple photodiode thanks to the tunability. The compactness and integrability of micro-resonators, VCSELs sources and photodetectors makes it possible to envision arrays of functionalized micro-resonators for multi-target detection. The manuscript begins with an overall presentation of the microsystem and its positioning in relation to the state of the art. The interest of polymer materials and technologies is discussed to achieve a low-cost lab-on-chip system, based on our previous work and our technological constraints. The integration of polymeric micro-optical elements on VCSEL is theoretically and experimentally studied to obtain a collimation and a spot size appropriate for the working distance considered, in order to reduce its natural divergence and to maximize coupling efficiency. Two post-production methods are used, discussed and compared: a method based on laser lithography and inkjet printing, and a 3D method using 2-photon stereolithography. Analytical calculation and FDTD modeling including technological constraints is presented to design coupling networks, waveguides and micro-resonators. The integration of a silicon photodetector is discussed to achieve complete integration. Next, several technological approaches are presented to create the structure designed in the technology and manufacturing section. The optical characterization platforms allowing the performance evaluation of each of the elements of the microsystem are detailed. Preliminary results are presented and novel ideas to improve the test method are proposed. Finally, the integration of a chemically sensitive material at the surface of the waveguide material for NH_3 sensing is developed and tested. The prospects for future research are also discussed.
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Submitted on : Monday, October 4, 2021 - 4:54:20 PM
Last modification on : Monday, April 4, 2022 - 3:24:36 PM

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  • HAL Id : tel-03299372, version 2

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Qingyue Li. Design and fabrication of a compact gas sensor integrating a polymer micro resonator and a 850nm VCSEL source. Micro and nanotechnologies/Microelectronics. Université Paul Sabatier - Toulouse III, 2021. English. ⟨NNT : 2021TOU30048⟩. ⟨tel-03299372v2⟩

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