UC Riverside

In the past century, electric current dominated the field of information generation, transportation, and process. In the last decades, the photon has been replacing the electric current due to its extraordinarily large bandwidth and faster transmission speed. 1D nanostructures possess an exceptional advantage over the other nanostructures for the guidance of light because it has a smaller small footprint and high mechanical stability and flexibility and requires low materials. Metal nanowires that are synthesized with a polyol methods exhibit atomically smooth surface, high crystallinity, and low impurities, has been intensively studied recently due to its ability of deep-subwavelength confinement. Recently, localized surface plasmon resonance (LSPS) using a plasmonic hotspot formed in the gap (<5nm) remotely excited by metallic nanowire waveguide have stirred-up great research interests in chemical imaging and sensing. In this work, I seek to improve the performance and reliability of chemical imaging and analysis at the nanoscale by developing an effective method for the confinement and guidance of the light. The enabling technology is a novel adiabatic nanofocusing to concentrate light to a nano-confined Raman sensing volume with high efficiency and energy throughput. Here, the adiabatic means no energy loss in the process of focusing light at the nanoscale. The critical component of the adiabatic nanofocusing is selective excitation of fundamental surface plasmon polariton (SPP) modes in AgNW using a tapered optical fiber with a minimal scattering and propagation losses. The SPP propagates along the NW and eventually creates a localized light at the sharp NW tip, with a hotspot size (<5nm). This optical path can be used not only for concentrating of light to but also extracting the vibrational spectra from the hot spot. Compared to the objective lens based conventional light focusing method, this NW waveguide works as a tunnel for light to propagates without a significant propagation loss due to a high absorption or diffraction in or reflection from the liquid sample. Therefore, imaging or sensing in the liquid phase is also available such as living cell analysis in culture media. The versatility and performance of our adiabatic nanofocusing were successfully demonstrated by measuring Raman spectra of specific molecules on the solid surface and in a single living cell. The transformative advances in light focusing and extracting are instrumental in increasing the prevalence of chemical imaging and analysis at the nanoscale as a practical, reliable and powerful tool for researchers in materials science, catalysis, energy conversion, electronics, and biology. This novel light focusing technique will be the pillar of the chemical imaging and analysis at the nanoscale and contribute to innovative findings in diverse scientific fields in the future.