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N. Teigell Beneitez, B. Baumgartner, J. Missinne, B. Lendl, G. Roelkens:
"Broadband Integrated Waveguide Sensor for trace analysis in the fingerprint Mid-IR range";
Vortrag: Online Conference SCIX 2020, Sparx (eingeladen); 12.10.2020 - 15.10.2020; in: "SCIX2020", (2020), Paper-Nr. MOLEC-OD1.3, 1 S.

Advances in mid-IR sources, detectors and integrated photonic circuits (PIC´s) allow the design and fabrication of novel, highly integrated sensor systems. In this contribution we present a sensing platform for mid-IR laser spectrometers based on novel Germanium-on-Silicon waveguides incorporating dedicated optics for coupling the laser beam in and out of the PIC. Opposed to classical ATR systems, the light interaction in the PIC takes place over the whole length of the waveguide in contact with the sample, leading to higher effective path lengths and thus to potentially higher sensitivities. Furthermore, to enrich the analyte in the evanescent region, the Germanium-on-Silicon waveguides were coated with a well defined and chemically modified mesoporous silica. By means of micro-lenses etched on the Si substrate and broadband grating couplers fabricated on the PIC, the developed sensor chip can easily be interfaced to a collimated broadband laser beam and a mid-IR detector. A dedicated micro-flow cell was used as to provide passive mechanical alignment to the chip with respect to the optical ports of the laser spectrometer and to deliver the samples to the sensing region. The mesoporous coating covering the Germanium-on-Silicon waveguides serves two purposes. First, it reversibly adsorbs contaminants on its surface and thereby concentrates them in the region probed by the evanescent wave, thus increasing analyte absorption for a given concentration. On the other hand, it isolates the evanescent field from the water stream significantly reducing the water background absorption. In this work, we targeted the 6.5 μm-7. 5 μm spectral region using a MEMS based EC-QCL source and we used aqueous BTX´s solutions to evaluate the sensing performance for trace detection of apolar analytes. Different concentrations of toluene and benzene in water were measured showing almost instantaneous sensor response due to fast analyte diffusion into the mesoporous structure of the coating. When flushing the flow-cell with distilled water a rapid decrease in analyte specific absorption was observed, confirming full and complete sensor regeneration. The LODs were found to be 7ppm for toluene and 1ppm for benzene despite still rather high for noise floors of 0.02 and 0.05 AU, respectively.