Tunable fiber-optic imaging bundle SERS substrates

Abstract

Chemical imaging not only provides structural and spatial information about a sample but also chemical information about the sample. Raman spectroscopy can be a powerful transduction mechanism for chemical imaging due to the narrow vibrational bandwidths and unique spectral fingerprints. Unfortunately, Raman cross-sections are extremely weak (~10⁻³⁰cm⁻²), often necessitating long exposure times, making dynamic chemical imaging impractical, particularly for high-resolution images. By utilizing a Raman enhancement technique such as surface enhanced Raman spectroscopy (SERS), the effective scattering cross-sections are increased making practical imaging times feasible. This paper will discuss the fabrication, characterization, and demonstration of a novel SERS substrate and instrumental system for non-scanning SERS chemical imaging with sub-diffraction limited spatial resolution. These substrates are fabricated by chemically etching a polished fiber optic imaging bundle consisting of 30,000, hexagonally packed, 4- micron diameter elements. The chemical etching process creates uniform array of cladding spikes onto which a SERS active metal is vacuum deposited, forming the SERS active surface. By varying the size of the silver islands deposited on the cladding peaks active surface plasmons can be tuned to various excitation frequencies. SERS signals measured both on and off the plasmon absorption band demonstrate that these SERS fiber bundles can be tuned for various excitation frequencies.