Near-field Raman imaging with the use of dielectric microspheres.

• Main Author: Seow, Cheryl Kai Ling.
• Other Authors: Shen Zexiang
• Format: Final Year Project
• Language: English
• Published: 2010
• Subjects: DRNTU::Science::Physics::Optics and light
• Online Access: http://hdl.handle.net/10356/40825
• Institution: Nanyang Technological University

Raman spectroscopy is a powerful research tool used to provide the chemical information of samples under study. It has wide applications in popular research areas such as nanotechnology and semiconductor technology and has recently been used as an imaging technique, though it still faces limitations of poor spatial resolution and weak signal. These issues need to be dealt with to keep it relevant for current research as we further develop nanoscale techniques. A novel near-field method developed by our group involves the use of a dielectric microsphere to improve spatial resolutions of images obtained by Raman spectroscopy into sub 100nm ranges. The microsphere focused the light into a photonic nanojet: a sub-diffraction limited light beam produced on the shadow-side of the microsphere. The microsphere was held into position by the same laser incident on the surface through the well-known optical tweezers mechanism. The purpose of this study is to further improve this technique and explore its potential applications in the field of plasmonics. We investigated the light emission properties of gold nanopatterns generated by Nano-Sphere Lithography. Using the dielectric microsphere increased the resolution and allowed for observation of plasmon coupling on the nanopatterns for the first time. This was previously impossible in the far-field regime due to the small feature size. The results obtained using a 1 μm sphere showed excellent spatial resolution, going beyond the diffraction limited resolution of conventional Raman spectroscopy. Our study also revealed the polarization dependence of surface plasmon coupling from the gold nanopattern, which is very helpful for research in plasmonics and has great potential for application in Surface Enhanced Raman Spectroscopy (SERS). The other project undertaken was to study the feasibility of using a cantilever-held microsphere. Optically trapped microspheres face certain challenges such as Brownian motion which greatly affects the spatial resolution. Furthermore, such optical trapping must be performed in liquid. Such setup does not allow for water-soluble samples, making in-situ studies impossible. Hence the cantilever-held microsphere was proposed as a solution. Our studies determined that smaller spot sizes were achievable, allowing this technique to become a standard microscopy technique. This thesis concludes that dielectric microspheres as near-field lenses have much potential and could bring Raman spectroscopy to new heights.