Multi-reflection polarimetry with MEMS-machining and microfluidics

Polarimetry provides useful insights into the molecular structure of optically active mediums such as chiral molecules in solution, enabling the measurement of the rotation of the polarization of a transverse wave, while also determining the concentration of the analyte present. The field of microfluidics promises new portable, low-cost, sensing systems, as well as the capabilities to measure the properties of precious samples, for which only small volumes are available. However, when optical sensing methods are involved, using microfluidic channels in millimeter to micrometer range results in a short path length over which the signal can be measured. Polarimetry is an optical sensing modality which would greatly benefit from using small volumes, as it can provide relevant information on the structure of chiral biomarkers in life sciences. However, the small interrogation volumes associated with the minute samples decreases the numbers of molecules in the light path that cause an optical rotation, and, as such, reduces the sensitivity. This thesis is aimed at the development of new methodologies to increase the sensitivity of polarimetry measurements for microfluidics systems. The work focuses on a new geometry whereby an optical beam, passing through a chiral sample, undergoes multiple reflections from suitably aligned and configured external micromirrors. The usual cancelling out of the optical rotation that occurs when the rotated polarized beam is passed back through a solution following reflection at a single mirror can be then negated. This enables the chirality of molecular species present in a microfluidic sample to be measured with increased sensitivity. This approach was validated experimentally using multiple passes of a linearly polarized He-Ne laser and a 403 nm diode laser through solutions of D-(+)-glucose as a chiral molecule, leading to a detection limit enhanced by ~27 times after only 11 passes. The configuration was also extended in a microscale format by creating mirrored silicon structures for micro-optics. New methods are introduced and characterised to fabricate both 45° and 90° etch-planes from monocrystalline silicon for use as retro-reflective sidewalls in a microfluidic device. Both techniques (to achieve the respective angles) use the same photolithographic pattern orientation, but with two different etchants. Etching on direction in Si(100) with potassium hydroxide gives vertical surfaces, whilst tetramethylammonium hydroxide gives 45° sidewalls. Optical rotations of glucose were measured using an integrated multi-reflections zigzag structure, enabling the use of 20 times smaller sample volumes. In general, the new method for polarimetry demonstrated in this work, based on multiple reflections across the microchannel, enables to significantly increase the optical path length in optofluidic devices, opening new applications in drug discovery and diagnostics where the sample structure of active molecules is of interest