One of the crucial aspects in healthcare is medical diagnostics by offering earlier detection and diagnosis of a disease. Currently, the analytic methods are suffering from time-consuming, high cost, and the need for professional people to achieve this purpose. Thus, the progressive demand for fast and cheap analytic methods and its utilized possibility by non-skilled people became an essential trend in recent years. For that reason, enormous efforts are continuously invested to optimize modern strategies to realize this aim.
The outstanding characteristics of silicon photonics offering an ideal solution for the implementation of a biosensor. Its compatibility with the mature (CMOS) technology leads to a reduction in fabrication costs. The high refractive index of silicon allowed to minimize its size and increasing the sensitivity. In addition, the possibility of novel concepts to incorporate all the functions such as optical, microfluidics, electronics and chemical in a single chip, open prospects to use this technology in inducing lab-on-a-chip (LOC) device. The term LOC indicates to a platform that capable to realize all the assay requirements on the same chip, starting from sample preparation and ending with the readout of the sensor. The improvement of such devices is achieved depending on enhancements of photonic integration technologies.
Despite the silicon material shows great performance as a biosensor platform, the crystalline silicon (c-Si) platform needs costly silicon-on-insulator (SOI) wafers. Furthermore, it lacks flexibility in regards to process integration.
Hydrogenated amorphous silicon (a-Si: H) that is deposited by plasma-enhanced chemical vapor deposition (PECVD) reveals itself as a substitute material for the biosensor platform. It offers the same optical properties as c-Si as low-loss material for near-infrared photonics and high refractive (n≈3.5). Moreover, the capability to deposit it at relatively low temperatures (≤ 300 °C), gained it more flexibility compared with c-Si and open the door to use glass and plastic material as a substrate.
The majority of photonic biosensors rely on the basic idea of evanescent field detection. It is based on the fraction of light interacting with the next interface parallel to the core, which reflection causing changes in the overall propagation. Such an altered optical signal can be observed by measuring the spectral properties. The wavelength shift and the intensity variation are related to the kind and concentration of the target analyte.
The generation of this field is attributed to a fraction of light signal that escapes outside the core waveguide during the propagation process. The interaction between this field and analyte molecules, which in turn lead to change the effective refractive index at the sensor surface or the surrounding area, will alter the optical signal properties (wavelength and intensity). This change is related to the concentration of the target analyte. The transduction signal can be measured by monitoring the wavelength shift or the intensity variation. Depending on the position of the analyte molecules, the sensing mechanism is classified into homogenous sensing and surface sensing.
Among integrated optical biosensors, several configurations were proposed during recent years: Photonic crystals, diffraction gratings, interferometers, and microring resonators, which will be the main focus within this research.
The objective of this project is to explore, design, and implement a photonic biosensor based on photonic integration with high performance detection capabilities and smart readout functionality fulfilling the following characteristics:
- High sensitivity and reliability with low detection limits
- High integration density due to multiplexing capability
- Low fabrication cost
- Flexible application options
- Real-time measurement capabilities
- Increased level of portability compared to state of the art
