Silicon mid-infrared waveguide-based bio-chemical sensors

Abstract

The MIR spectral region contains strong absorption bands for many molecules and substances such as gases, liquids, proteins or drugs. Photonic devices operating in the MIR can be used, for example, in the treatment of aspirin and paracetamol overdose patients to identify and measure the concentrations of drugs in the body. The main aim of the project is to develop the MIR silicon photonic circuits for absorption spectroscopy, to be able to detect the quality and quantity of samples. By analysing the absorption spectra, identifiable absorption peaks and troughs at certain wavelengths can give knowledge of the composition of the sample present and the level of absorption can give information about the samples concentration. In the first part of this work, a preliminary demonstration of a silicon-on-insulator (SOI) device with a microfluidic channel is carried out in which the absorption spectra of different concentrations of water-IPA solutions are measured at wavelengths between 3.725 µm and 3.888 µm. The devices were used to detect an expected IPA absorption peak at 3.77 µm, and a concentration as low as 1.5% IPA in water (by volume) was detected. The second part of this work aims to reduce the noise floor. A new on-chip liquid sensor in the mid-infrared wavelength region was demonstrated, which uses Si waveguide switches and a microfluidic channel to allow for the circuit transmission to be switched between a reference waveguide and a sensing waveguide faster than fluctuations due to the noise. The results of this study show that the switch sensor can reduce the noise floor by a factor of 11 compared to a simple waveguide absorption sensor, which would ultimately allow the switch sensor to measure lower concentrations of a target analyte, and to reach a lower limit of detection. In future work, different methods of achieving longer wavelength (7-10 µm) sensing are discussed, with the current progress outlined.

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