Role of Vibrational-Assisted Scattering and Surface-Enhanced Raman Scattering in Colloidal Plexcitonic Materials

Strong coupling between excitons and an electromagnetic mode leads to the formation of polaritonic materials. These half-light half-matter states obey Bose-Einstein statistics and have therefore promised a route toward room temperature condensates and low-threshold polariton lasers. However, our understanding of how to enhance the rate of relaxation toward the lowest energy excited state must be greatly enhanced for electrically driven organic condensates and polariton lasers to be realized. Here, the mechanism of excited-state relaxation in colloidal plexcitonic materials (CPMs) is explored. CPMs are a subgroup of polaritonic materials formed when an exciton interacts strongly with a plasmonic resonance of a nanoparticle. Based on our current understanding of relaxation in polaritonic systems, which is based on experiments done using Fabry-Pérot cavities, CPMs are expected to have high relaxation rates through the vibrationally assisted scattering (VAS) mechanism. However, so far, it has been unclear whether we can transfer the knowledge gained from Fabry-Pérot cavities to plasmonic cavities. Our results indicate that not only VAS but also surface-enhanced Raman scattering (SERS) is active in CPMs and that the predominant mechanism depends on to which state excitation occurs. Therefore, caution must be exercised when interpreting the emission from plexcitonic materials and when using theories obtained from polaritonic materials prepared with Fabry-Pérot cavities on plexcitonic materials. Additionally, we found that plexcitonic materials can provide an electromagnetic enhancement of both the excitation and emission part in SERS, increasing its enhancement factor and allowing tuning of the sensitivity to specific vibrations.