Microfluidics and Integrated Optics Glass Sensor for In-Line Microprobing of Nuclear Samples

We study the miniaturization of Thermal Lens Spectrometry (TLS) towards Lab-on-chip integration in order to reduce the volume of fluid assays in nuclear process control. TLS is of great interest in this context since it combines the advantages of optical detection methods with an inherent suitability for small-scale samples. After validating the experimental principle in a classical thermal lens crossed-beam setup, we show the integration of a Young-interferometer with a microcapillary on a glass substrate, reducing the necessary sample size to 400 nl. The interferometer translates the photothermally induced refractive index change in the fluid to a phase shift of the fringe pattern, which can then be detected by a camera. Measurements of Co(II) in ethanol yield a detection limit of $c = 5 \times 10^{-4}$ M for the crossed-beam setup and $c = 6 \times 10^{-3}$ M for the integrated sensor. At an interaction length of 10 $\mu$ m, it detects a minimum absorbance of $K = 1.2 \times 10^{-4}$ in a probed volume of 14 pl.