Plasmon-enhanced nonlinear optics for applications in sensing and biology

In this thesis, we present the results of three experiments that combine techniques from the fields of ultrafast nonlinear optics and plasmonics, with the aim of developing tools for improved surface-enhanced Raman spectroscopy and biological cell transfection. We first describe the use of femtosecond laser pulses to generate large areas of a nanostructured silicon surface which is used as a new type of substrate for surface-enhanced Raman scattering (SERS). We perform spectroscopic characterization of this substrate and find its Raman cross-section enhancement factor to be on the order of 10 7 . This large, spatially-uniform, and reproducible enhancement factor is nearly constant across the near-infrared spectral region. In a second experiment, we develop a technique to spatially isolate the "hot spots" on SERS substrates. This technique leverages the plasmonic near field enhancement of metallic nanostructures to preferentially expose a commercial photoresist using femtosecond laser pulses. By isolating the hot spots, analyte molecules adsorb only to the regions of largest electromagnetic enhancement. Compared to an unprocessed substrate covered with a sub-monolayer of benzenethiol molecules, a processed substrate shows a 27-fold improvement in its average Raman cross-section enhancement factor. Finally, we present a proof-of-principle experiment which demonstrates high-throughput ultrafast laser transfection of biological cells using large-area plasmonic substrates. Utilizing the field localization properties of a substrate fabricated using photolithography, wet etching, and template stripping, we demonstrate the introduction of silence RNA (siRNA) molecules into cells with an efficiency of approximately 50% after exposure to femtosecond laser pulses.