Project 2.3

This PhD thesis shall advance breadboard based PTS gas sensing solutions to more rugged and compact sensors using the technology of integrated photonic sensors. In photothermal spectroscopy, the modulated IR pump laser is partly or entirely absorbed by the medium, followed by a temperature change and consequential alteration of the medium’s refractive index. The analyte is typically placed in a Fabry-Pérot interferometer, which is monitored using a probe laser (often a Helium Neon). The change in refractive index induces a change in transmission from which the absorption is inferred. In the current systems, the Fabry-Pérot consists of dielectric coated fused silica windows. Increased sensitivity is expected upon increasing the finesse of the employed FP cavities. For stable operation, locking schemes for keeping the probe laser at the inflection point of the transmission function need to be developed. Excellent results are being achieved but the system is bulky. Integrated optics provides the means to increase both the compactness and the finesse of the resonator. ESR 2.3 will implement high Q-factor Photonic Crystal based refractive index sensors, which will consist of a fibre coupled silicon chip and PhC cavity with a strong evanescent field. Challenges due to the small size of the sensor will be overcome by using careful control of the excitation beam or the use of solid core optical (ESR 1.1). The increased finesse of the PhC resonator is expected to provide at least an order of magnitude improvement in sensitivity. A readout system based on a hybrid laser will also be developed. By building the high Q-factor PhC refractive index sensor into the laser cavity, the laser wavelength can be made sensitive to the analyte refractive index, simplifying the system. Further improvements will be made by implementing a heterodyne detection scheme where the readout laser’s output is combined with that of a reference laser, creating a beat signal that will be uniquely sensitive to the analyte refractive index, potentially giving a 100-1000 improvement in sensitivity. In the final stage of the project, the excitation laser will be provided by an integrated laser provided by ESRs 1.3, 2.1, 3.2 or 3.4, providing the ultimate in terms of compact, sensitive PTS sensors. One planned target analyte is diacetyl, of relevance in the brewing industry. At TU-WIEN this ESR will also apply his know-how in multivariate data analysis to protein analysis, taking advantage of the instruments of ESR 3.3 for liquid sensing.