Demonstration of composite microsphere cavity and surface enhanced raman spectroscopy for improved sensitivity

Abstract

Surface enhanced Raman spectroscopy (SERS) has widely been used for material composition analysis because it can provide good selectivity and sensitivity without the labeling process required by fluorescence detection. SERS enhancements on the order of 10 14 have been demonstrated, which can enable the detection of a single molecule. However, further enhancement is necessary to increase the sensitivity of SERS systems, and to make single molecule detection and analysis more practical. In this work, we demonstrate a composite system of silver nanoparticles and an optical microsphere resonator to create an even higher average Raman enhancement than SERS alone. The Raman pump is coupled into the microsphere resonator, where it repeatedly circulates around the surface via total internal reflection in the form of whispering gallery modes (WGMs). Microsphere resonators can have Q-factor values higher than 10 6 , which results in a tremendous local field enhancement. The evanescent field of the WGM interacts with the silver nanoparticles and target analytes, which are adsorbed onto the surface of the microsphere. With this composite system, we demonstrate an increase in Raman enhancement of approximately 300. Engineering improvements to this experimental prototype system may increase the enhancement by an order of magnitude. Further improvements that can leverage the microsphere resonator system to promote stimulated Raman scattering (SRS) may result in a dramatic increase in sensitivity. Ultimately, this composite system will increase the sensitivity of SERS sensor systems, and will bring single molecule detection and analysis systems closer to practical implementation.