On-chip nanophotonic structures for biosensing devices

Abstract

Label-free optical biosensors which are capable of detecting small changes in the refractive index of analyte are widely used in viral disease detection, environmental monitoring, and biochemical application. Light can be confined in a high refractive index or low refractive index waveguide or by creating deformity states in a periodic structure. One of the finest examples of guiding light in low refractive index media is the slot waveguide which has been utilized to realize the number of photonic functionalities for a variety of applications. The grating waveguide supports a low-loss optical mode that propagates in the wave-guiding region and enhances the field overlaps with analytes and suppresses reflection. The optical detection method is based on a change in effective refractive index, a shift in narrow resonance peak and a change in photocurrent. This dissertation focuses on the design, fabrication and characterization of engineered nanophotonic slot waveguides for applications in optical biosensors. The optical biosensor depends on the wavelength shift of the cavity and the sharpness of the resonant peak. The slot waveguide configuration explored includes one dimensional silicon–air grating, a combination of dielectric and plasmonic waveguiding and nanocomposites. The introduction of silicon grating in the slot-based waveguide can enhance the sensitivity of the label-free biosensors. The waveguide grating usually composed of silicon blocks or in disk shape are arranged in a periodic manner in the direction of propagation having a period smaller than the operating wavelength. By optimizing the pitch, width and duty cycle of the grating, the effective index of the medium can be engineered. The grating loaded nanophotonic waveguide is based on the introduction of grating with TiO2 overlay on slot waveguide. The effect of TiO2 as an overlay on the grating of the device is to enhance the sensitivity. The proposed device is numerically showing to detect principal marker of hepatitis-B viral infection, with a high sensitivity of 1200 nm/RIU with a figure of merit of 300. To improve the sensing performance the combination of nano-disk with a grating in engineered optical slot waveguide is designed for the detection of viral infections using Fano resonance. APTMS layer is synthesized as a functionalized layer to support the immobilization of antibodies to bind antigens. The proposed sensor device manifests bulk sensitivity of 1463 nm/RIU for 0.02 refractive index change of analyte and limit of detection of 1.02 x10-3 RIU-1 . The proposed device can detect the virus analytes with the smallest refractive index change of 0.0008 in serum medium. To enhance the sensing performance, nanophotonic slot waveguides with subwavelength grating structures are designed on SOI technology. Due to the diffraction limit, the footprints of SOI waveguide devices cannot be decreased. To design more compact devices for subwavelength optical confinement hybrid plasmonic waveguide is designed. Hybrid plasmonic devices support the combination of photonic and plasmonic modes with confinement in the subwavelength region and relatively low losses. To confine optical mode in horizontal as well as vertical slot regions, a hybrid plasmonic waveguide was designed for polarization-independent bio chemical sensing. The bulk sensitivity in TM mode is increased by 1.5 times with gold cover compare to TE mode. The bulk sensitivity for TE and TM modes are reported to be 1.024 nm-1 and 0.95 nm-1 respectively. Surface sensitivity is investigated numerically for DNA hybridization and Hepatitis surface antigen detection. Food borne bacterial pathogens are an important cause of a variety of human diseases. A biosensing platform with a high-throughput mechanism, free from complex fabrication processes and highly selective to bacterial contamination is required in medicine, food and water safety. Nanocomposite materials are synthesized for the development of optical biosensors because of their biocompatibility, tunable conduction characteristics and large surface area for adsorption of biomolecules. Cu ZnO/TiO2 nanocomposite based photocatalytic biosensor is used in the detection of food borne pathogens due to their high stability and excellent charge transfer characteristics. The label free photocurrent biosensing platform based on ITO/CZT nanocomposite thin film for the detection of E. coli in buffer was investigated. The biosensor is based on the interaction of E. coli with the functionalized surface of copper doped ZnO /TiO2 (CZT) film over ITO thin film. A change of 20 µA is detected with a change in concentration of E. coli on the application of 4 volts. To improve the sensor performance a slot based nanophotonic structure with electric control based on photocatalytic nanocomposite is proposed to realize. The fabricated nanocomposite slot waveguide exhibits a measured photocurrent difference of 9 μA with E. coli in LB media solution. The limit of detection of E. coli bacteria concentrations is 5000 CFU/ml. The slot waveguide provides 10 times higher current density compared to thin film for the detection of food pathogens. The proposed methods carry a great potential for on-chip and off-chip nanoscale optical devices and sensors.