Postdoctoral researcher "Micro mechanical resonator for photo-acoustic gas detection"

Applications are invited for a 18 months postdoctoral position at the University of Montpellier (France), in the NANOMIR group. The group is located at the Electronics and Systems Institute (IES), on the University premises. IES (Electronics and Systems Institute) is a laboratory specialized in reliable electronic systems and components for observation and measurement applications.

Gas sensors find more and more various applications such as human health, with breath analysis and diagnostic, agri-food monitoring, or air quality qualification. To be efficient, they must exhibit three important qualities: sensitivity, selectivity and stability. A considerable amount of commercial gas sensors are available based on different techniques, among the most developed: electrochemical sensors, mass sensors and optical sensors. Despite the great diversity of these gas sensors, only sensors based on optical absorption like TDLS (tunable diode laser spectroscopy), can perfectly discriminate the spectral signature of a specie among others, thus give a perfect selectivity, combined with a very high sensitivity [Namjou 1998]. However, in most optical technic the detection sensitivity is strongly related to the used photodetector performances and spectral range. In photo-acoustic spectroscopy, the optical detection is replaced by an acoustic detection [Castledan 1981]. A laser, emitting at a wavelength corresponding to an absorption line of a targeted gas specie, is focused into a gas chamber and the measurement is performed by detecting the acoustic pressure generated by the local warming caused by optical absorption. In 2002, Rice University proposed to use a piezoelectric mechanical resonator (a commercial quartz tuning fork QTF) instead of a microphone to amplify the response of the photo-acoustic effect. This new technique was called QEPAS (Quartz-Enhanced Photo Acoustic Spectroscopy).

This method avoids using expensive mid-infrared optical detectors with restricted spectral ranges while providing sensitivities down to a few particles per billion in volume, equivalent to those of the best photo-acoustic systems. QEPAS has been exhaustively studied in our group [Rousseau 2019] and is potentially the best method to achieve selective, sensitive, compact and stable sensors. However, the main limitation of QEPAS systems comes from the use of quartz tuning forks (QTF). These components are cheap and easy to use, but they are not specifically designed for this application. It is for this reason that our group is developing mechanical micro-resonators specifically designed and optimized for the photo-acoustic gas detection. Theses mechanical resonators specifically designed for the photo-acoustic will allow creating very compact, robust, portable systems [Chamassi 2019].

Never has a micro-mechanical resonator been entirely thought and designed for photo-acoustic detection. This postdoctoral position aims at studying all physical concepts to optimize the micro-resonator design and improve the sensor sensitivity. The micro-resonators will be developed in the silicon technology, in order to benefit from its maturity. More specifically, a SOI (Silicon On Insulator) based fabrication technique is chosen to fabricate the first micro-resonators. The technological process is then simpler and opens the way to optimized geometries that can improve the sensibility of the sensor. This sensibility improvement will allow placing the exciting laser next to the micro-resonator, eliminating the need for any optical element used to focalize laser light. Without optical elements, we will be able to realize very compact and robust gas sensors.

The postdoctoral research proposed will evolve in the context of the NOMADE project of the national French research agency and the COMPACT project of the Occitanie region, whose goal is to demonstrate opto-mechanical gas sensor operating with a silicon micro-resonator.

The successful applicant will be an energetic individual with strong academic record and experience in semiconductor physics, optical physics. She/he will have completed a PhD program in Physics, Optics or Engineering. Numerical simulation capabilities, characterization tools and fabrication facilities are available at the host institution.

The position is available on the first of January 2020.

[Castleden 1981] Castleden, S. L., Kirkbright, G. F., & Spillane, D. E. M. (1981). Wavelength Modulation in Photo-Acoustic Spectroscopy. Anal. Chem., 53(14), 2228-2231.

[Chamassi 2019] Chamassi, K., Vicet, A., Rousseau, R., Oussalah, D., & Bahriz, M. (2019). New silicon micro-electromechanical resonator for enhanced photoacoustic spectroscopy applications. Applied Physics Letters, 83(24), 9320. https://doi.org/10.1063/1.5098140

[Namjou 1998] Namjou, K., Cai, S., Whittaker, E. A., Faist, J., Gmachl, C., Capasso, F., ... Cho, A. Y. (1998). Sensitive absorption spectroscopy with a room-temperature distributed-feedback quantum-cascade laser. Optics Letters, 23(3), 219. https://doi.org/10.1364/OL.23.000219

[Rousseau 2019] Rousseau, R., Loghmari, Z., Bahriz, M., Chamassi, K., Teissier, R., Baranov, A. N., & Vicet, A. (2019). Off-beam QEPAS sensor using an 11-μm DFB-QCL with an optimized acoustic resonator. Optics Express, 27(5), 7435. https://doi.org/10.1364/OE.27.007435