Optical Waveguiding Nanowires And Their Sensing Applications

It is promising to develop more miniature, more function and high density nanophotonic sensing and detection devices by combining the optical fiber technique" and nanotechnique.The first part in this thesis is about the waveguiding properties of the single photonic nanowires. Firstly we compared the direct irradiation and waveguiding methods. Then we investigate the unique properties in waveguiding nanowires, such as the highly tunable evanescent wave fraction, high spatial resolution and tunable mode polarization, tailorable waveguide dispersion.The second part is about the optical sensing using waveguiding single polymer nanowires. We fabricated low-optical loss polymer nanowires by directly drawing from polymer solutions. The polymer nanowires were functionalized by doping with dyes or blending with solvated polymers before the drawing process. By waveguiding excitation light along the nanowires, we demonstrated that the interaction of light with nanowires is enhanced over3orders of magnitude compared with the irradiating excitation. Intriguing advantages such as enhanced excitation efficiency, low excitation power operation down to nW levels and high photostability are obtained. On the basis of the optical response of waveguiding polymer single nanowires when exposed to specimens, functionalized polymer nanowires were used for humidity sensing with a response time of30ms and for NO2and NH3detection down to subparts-per-million level. Selective detection of gas mixture of ammonia and humidity with ppm sensitivity were also demonstrated by using dual-wavelength analysis.The third part is about the photodetection using waveguiding single semiconductor nanowires. We demonstrated a large defect-induced sub-bandgap photoresponse in CdS nanowires over a broad spectral range. We also demonstrated infrared quenching of photoconductivity in CdSe single nanowires. Furthermore, inspired by natural ommatidium of compound eyes of insects, we also developed a source-moving thermal evaporation route and first realized the growth of a novel CdSSe alloy nanowires. Along the length of these achieved nanowires, the composition can be continuously tuned from pure CdS at one end to pure CdSe at the other end of the same wire. Via waveguiding excitation, we combined the intrinsic photoconductive effect and the infrared quenching of photoconductivity effect in the same structural schemes and demonstrated response from400nm to1.5-μm wavelength in composition-graded CdSSe alloy single nanowires.