Demonstration of composite signal enhancement from surface enhanced Raman spectroscopy in a liquid core optical ring resonator

摘要

Surface enhanced Raman spectroscopy (SERS) utilizing silver colloids for localized plasmonic enhancement has been heavily researched due to its tremendous increase in the Raman signal of bio/chemical molecules. We demonstrate further enhancement by multiplying the SERS effect by the resonant enhancement of a ring resonator microcavity. The liquid core optical ring resonator (LCORR) offers a high-performance and practical design to obtain this composite enhancement for bio/chemical molecule detection. The LCORR integrates an array of optical ring resonators into a capillary-based microfluidic channel to form a novel bio/chemical sensing platform. The circular cross-section of the glass capillary acts as an optical ring resonator, with the evanescent field of the resonant light interacting with the sample passing through the capillary. The LCORR has already been well-studied for applications in label free biomolecule sensing. In this work, we utilize a silver colloid solution inside the capillary to perform SERS-based detection. In contrast to a typical SERS system where the incident light interacts with the colloid and target molecules only once, in the LCORR system, the tightly confined light resonates around the capillary wall, repeatedly interacting with the SERS system. Our experimental results show the increased enhancement due to the composite effect of the cavity resonance and the localized plasmonic effect of the nanoparticles inside the cavity. We have achieved detection of 3.3 nM R6G inside the LCORR. In addition to the excellent sensitivity, this detection system represents an advancement in the development of practical SERS bio/chemical sensors due to the arrayed nature of the sensors combined with the integrated microfluidics of the LCORR.