Integration of optically pumped magnetometers and microfluidics for the determination of the magnetic moments of single micro-particles (Micromagnetic Moments)
Final Report Abstract
Within the project a measurement systems should be developed, which enables the detection of single particles or objects by their micromagnetic moments. In particular the detection of single magnetotactic bacteria within an ensemble should become possible, measured via the magnetic field of the nagnetosomes within the bacteria. This measurement demands for a high magnetic-field resolution in very small distance from the magnetic particle or bacterium. Furthermore, a possibility had to be created which forces them to flow definitely along the magnetometer. In order to solve these tasks, various technologies and expertise available at the Leibniz-IPHT Jena have been combined on a detector chip using microsystems technologies. An optically pumped magnetometer serves for the magnetic field detection. Its central part is a cavity in a silicon wafer, closed by anodically bonded glass plates. Optically pumped atoms of cesium vapor within the cavity serve for the magnetic-field detection. This common configuration had to be changed and complemented in various parts. For the achievement of the smallest possible distance between magnetic particle and magnetometer, in one glass plate a microfluidic canal was integrated where the particles or bacteria are pumped through. It is separated from the cesium vapor by a 1 µm thick silicon nitride membrane, which originates from the development of highly sensitive infrared detectors. The optical magnetometer must measure only in the vicinity of the microfluidic canal. All other parts of the cesium vapor may not be sensitive to magnetic fields. This task was solved by the use of two separate beams for pump and probe, whereby the magnetic-field sensitive range is limited to a tiny beam crossing point near the canal. For the sake of simplicity of the sensor a new operational mode of the optically pumped magnetometer was developed. The pump beam is amplitudemodulated and the probe beam is read out by circular dichroism. This mode allows the use of one single laser for pump und probe. No additional B1-field is needed. The checkout with a common cesium cell having low buffer-gas pressure showed good magnetic-field resolution, comparable to established operational modes. In the small magnetometer with integrated microfluidics much more cesium azide than usual had to be dissolved to get enough cesium atoms. This originates increased nitrogen buffer-gas pressure and demands for higher cell temperature. Both together ends up in a two orders of magnitude worse magnetic-field resolution. Hence the real detection of magnetotactic bacteria was not possible within the project. The interplay of microfluidic pumping of nanoparticles and magnetic-field measurement could be shown successfully, however. For the improvement of the magnetic-field resolution of the integrated optical magnetometer further technological developments are planned after the end of the project.
Publications
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An optically pumped magnetometer for counting magnetotactic bacteria, Workshop on Optically-Pumped Magnetometers (WOPM), Mainz 2019
T. Fremberg, V. Schultze, F. Wittkämper, M. Kielpinski, T. Henkel, R. Stolz
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An optically pumped magnetometer to count magnetotactic bacteria, Heraeus Seminar - Quantum Sensing & Magnetometry, Bad Honnef 2019
T. Fremberg, V. Schultze, F. Wittkämper, M. Kielpinski, T. Henkel, and R. Stolz
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Challenges and solutions of fabricating fully integrated alkali vapour cells, Workshop on Optically-Pumped Magnetometers (WOPM), Mainz 2019
F. Wittkämper, C. B. Schmidt, G. Oelsner, R. IJsselsteijn, V. Schultze, R. Stolz
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An optically pumped magnetometer based on a pump-probe scheme with amplitude modulated light, Hot Vapor Workshop, Stuttgart 2021
T. Fremberg, V. Schultze, F. Wittkämper, M. Kielpinski, T. Scholtes, R. Stolz
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Towards detection of individual magnetotactic bacteria using optically pumped magnetometers, Workshop on Optically-Pumped Magnetometers (WOPM), Strathclyde 2021
T. Fremberg, V. Schultze, F. Wittkämper, M. Kielpinski, A. Ihring, G. Mayer, T. Scholtes, R. Stolz
