Silicon-based photonic integrated circuit for label-free biosensing

Silicon-based Photonic Integrated Circuit (PIC) is a device that integrates several optical components using the mature semiconductor technology platform, developed through years for the needs of electronic integrated circuits. In recent years, silicon PICs have been demonstrated as a powerful platform for biosensing systems - devices which play an omnipresent role in such essential life aspects as health care, environmental monitoring, food safety, etc. The growing importance of silicon photonics technologies for biosensor development consists in the possibility of realization compact and simple-to-use instruments, socalled Lab-On-a-Chip (LOC), enabling rapid point-of-care analysis. This dissertation was carried out within the European project Symphony (grant number 610580), dedicated to the development of a portable sensor instrument for timely detection of highly carcinogenic compound Aflatoxin M1 in milk. The main objective of this Thesis is a development of one of the key element of such system - a photonic chip for label-free biosensing. The PIC operates at the short-wave near-infrared wavelength range (at 850 nm) and integrates an array of silicon oxynitride microring resonators for multiplexed analysis of the samples in aqueous ambient. An innovative technology is developed for monolithic on-chip integration of a low-cost silicon photodetector, used to monitor the sensor response. The theoretical background includes the basics of microring resonators, the optical label-free biosensing principles and the fundamentals of photodetectors. The design of photonic circuit elements and different numerical models, used for the device simulations, are discussed in details. A great part of the work is dedicated to the optimization of the fabrication process, which is comprehensively explained and illustrated in the thesis. A complete characterization of the realized optical components is presented, including the study of typical performance parameters of waveguides, splitters and microring resonators. To evaluate the sensors functionality, the volumetric sensing experiments are performed using the home-made microfluidic module. The results reveal a high sensor performance, comparable with the state-of-the-art: the devices are able to sense the changes in the refractive index of solution with the limit of detection down to 2*10^􀀀-6 refractive index units. Finally, the characterization of the on-chip integrated PIN photodiodes is presented. The detectors demonstrate the very encouraging results, showing a good responsivity at 850 nm and a capability to efficiently monitor the response of the biosensor array. The successful integration of silicon photodetectors on the highly-sensitive sensor chip can permit the realization of tiny, robust and cost-effective optical biosensors, which gives an important contribution towards the simplification of development of LOC devices.