Label free detection techniques such as surface plasmon resonance, carbon nanotubes, nanowires, and interferometry have been progressing rapidly for biosensing applications. Surface plasmon resonance is considered one of the most promising label free optical techniques. The use of nanohole arrays in a metal film allows for extraordinary transmission and has been motivated by their application as biosensors. Nanohole arrays present several advantages like smaller foot print, dense integration, lower limits of detection, and collinear optical detection. This thesis presents the design parameters for the optimization of sensitivity and resolution of nanohole arrays for refractive index sensing. A systematic study is provided of the influence of the nanohole array periodicity, diameter, and gold thickness. Focused ion beam was used to fabricate the nanohole arrays. A microfluidic device with a set of embedded nanohole arrays was developed and used to measure the sensing characteristics. The results are encouraging for potential future biosensing tests. / Graduate

Optical fibers are getting significant considerations in the field of the sensors and sensing beyond its applications in optical communications. Because of several advantages, e.g., low profile of the sensors, immunity to electromagnetic noises, the ability of multiplexing, etc., the use of the fiber optic sensor is increasing in the field of physical, chemical, and biomedical sensing. In this study, we have developed two new fiber optic sensors based on fiber loop ringdown technique (FLRD) and have demonstrated their applications in the field of sensing. In the first part of this study, we report on the development of a high-sensitivity FLRD strain sensor. For the design of the strain sensor, the fiber loop was cut at the middle, and then the two fiber ends from broken fiber loop were cleaved and aligned carefully to couple the light from one end to another end. Any strain during the measurement changes the alignment of the fiber ends, consequently, the ringdown time changes. With this scheme, the FLRD strain sensor has shown the strain detection limit of 65 nanostrain, which is five times better than any FLRD strain sensors reported in the literature. Furthermore, The FLRD strain sensors were successfully embedded into prestressed concrete-beams.The FLRD strain sensor was able to monitor stress on a post-tensioned rod, as well as the load applied on the concrete-beam during the three-point loading test, thus exhibiting immense potential in structural health monitoring. For the chemical sensor, a new scheme of interrogation for a fiber optic surface plasmon sensor was developed with the use of the FLRD technique. A gold nanolayer was deposited on an uncladded fiber section, and the fiber section was integrated into the FLRD system as a sensor head. The gold layer facilitates for increased interaction of sample of interest, with the light pulse confined in the fiber waveguide. Moreover, with the affinity of the gold with specific biomolecules, the sensor has the potential for applications in biochemical sensing. In the experiment, the SP-FLRD sensor was used for refractive index sensing, and index detection limit of 4.6×10-5 RIU was achieved.