Whispering-gallery mode Raman spectroscopy in liquids with metal nanostructure-enhanced microresonators

Whispering-gallery mode (WGM) microresonators are amongst the most promising optical sensors for detection of biochemical targets. A number of laser interrogation methods have been proposed and demonstrated over the last decade, based on scattering and absorption losses or resonance splitting and shift, harnessing the high quality factor and light confinement in the ultra-small volume of WGMs. Regardless of the sensitivity gain, in chemical sensing applications where the chemical targets are present only in rare traces, WGM sensors operation may be augmented by plasmonic interaction, e.g. metallic nanostructures or substrates that provide impressive local field amplification via the so-called Surface-Enhanced Raman Scattering (SERS). Here, we use a silica microsphere and efficiently excite WGMs collecting the relevant information from the transmitted and back-scattered light, which is then analyzed to extract the Raman spectral signature a chemical analyte. We show that a near-infrared diode laser locked on a WGM resonance provides real-time, fast sensing of dielectric nanoparticles approaching the surface with no need for signal post-processing. Also, the Raman spectrum of light scattered from the microsphere surface clearly shows the enhancement of Purcell effect due to the oscillating WGMs. The implementation of a SERS-modified spectroscopic scheme is underway.