We present a sensor design based on a Mach-Zehnder interferometer utilizing sub-wavelength gratings (SWGs) that were included in the waveguide to compensate for the short optical path length and to provide phase modulation. According to 2D finite element method simulations, it is possible to achieve 3-fold enhancement in sensitivity and 50% increase in modulation frequency with the inclusion of SWGs in the sensing arm as well as in the reference arm.
Investigating photonic components as refractive index sensors requires reliable numerical models. Performing 3D simulations of large or complex components requires powerful hardware. Alternatives to time-consuming 3D simulations are approximations such as the effective refractive index method. We propose our improved method that more closely follows 3D results, named the inverse effective index method. The accuracy of our approximation method is verified by comparing experimental results with simulations. Ring resonators with 40μm radius are simulated and fabricated. Their qualities as refractive index sensors are assessed by probing their response in DI-water and in various saline solutions. The fabricated resonators are fitted with a microfluidic channel for sample delivery. A tunable laser is used to induce resonance in the ring resonators and the resonance frequencies are measured for all solutions. The accuracy between simulation and experimental data is found to rely heavily on the approximation technique being used. The differences vary from 300% to 6% depending on the approximation method. The fabrication and characterization of the ring resonator-based refractive index sensor took approximately two days, whereas the 2D simulations took under an hour to perform. This demonstrates the usefulness of a proper simulation tool to conduct accurate performance estimations in a short amount of time.
The ability to conduct diagnostic functions on a single chip has long been of interest to the medical community. Decentralization of laboratories combined with reduced costs, increased speed and a higher throughput of potential assays are all driving forces for lab-on-a-chip technology. The small chip sizes facilitate low sample volumes, which in turn allow better control of the molecular interactions close to the sample surface. The design and quality of transducers, microfluidics and functionalization processes have all improved over recent years. Despite the growing interest for lab-on-a-chip components, several challenges remain. Combining all three disciplines into a high-quality well-functioning chip that is cheap to fabricate while providing reproducible results is challenging. A project attempting to address these challenges is presented. The main goal is to design and fabricate a labon-a-chip silicon photonic biosensor with multiple channels for detection of antigens with improved sensitivity and selectivity compared to state-of-the-art. As a proof-of-concept, the sensor is designed for simultaneous detection of three distinct antigens: C-reactive protein (CRP), lipocalin and tumor necrosis factor (TNF). The main challenge lies within their respective concentrations as well as the specificity for each analyte, where concentrations vary from the mg/ml to pg/ml regime. Multiplexing is achieved by using photonic crystal resonators, which function as drop-filters, allowing for single input/output while simultaneously probing select transducers that are functionalized for different chemistries. The individual resonator designs facilitate different limit-of-detections (LODs) and dynamic ranges for each analyte. Preliminary results from the first single channel prototype are presented, while work on the multiplexed sensor continues.