QUPERS: Quantum Enhanced Raman Spectroscopy
We present experimental demonstration of a recently predicted path interference phenomenon [Nanophotonics 7, 1687 (2018)]. A SERS process becomes resonant when both incident and the converted frequencies match with two plasmon resonances -- a condition which is hard to satisfy experimentally. Yet, presence of a quantum dot~(QD) coupled to the lower-energy plasmon mode has been predicted to introduce interference effect in the conversion paths and to bring the SERS process into resonance. Here, we experimentally demonstrate a 7-fold additional enhancement factor~(EF) {\it multiplying} the conventional SERS enhancement owing to the presence of QDs in addition to metal nanoparticles~(MNPs). We eliminate influence of alternative enhancement mechanisms and show that observed additional EF of 7 is solely because of the path interference~(Fano) effect taking place in the presence of the QDs. As the most significant evidence: we observe that when QD/MNP ratio exceeds $1$, EF starts to decrease. Two or more QDs sitting at different~(random) positions on a nanostar MNP surface starts to degrade the path interference effect. Even though the theoretically predicted maximum EFs of $\sim$100--1000 when QD is in the MNP hotspot are not always experimentally feasible in stochastic nature of large area SERS substrate, still, observation of a 7-fold overall increase in such a setting is significant for fabricating Raman-based sensor applications. More importantly, our scheme allows for designing selective SERS substrates. As the QD resonances can actively be shifted by external factors, such as by applying an electric field (Stark effect), it provides a previously-not-existing tool: active selective enhancement of Raman bands via an applied voltage.