Fluorescence enabled Raman amplification using photonic crystal resonant modes

Abstract

Although plasmonic and photonic crystal substrates represent fertile ground for plasmon-enhanced fluorescence, Raman scattering, and surface-enhanced Raman scattering based diagnostic tool development, extracting quantifiable Raman information from strongly fluorescent analytes without photobleaching, signal gating, or multi-step sample preparation has remained a long-standing challenge. In this work, we introduce Fluorescence Enabled Raman Amplification (FERA) as a mechanism that triggers the resonances of a photonic crystal surface and plasmonic nanoparticles via the molecular emission of a fluorescence-emitting radiating dipole, which, in turn, feeds back into molecular Raman scattering of the same molecules. This self-reinforcing feedback mechanism of FERA is experimentally demonstrated using multiple lasers and objectives and validated through COMSOL Multiphysics simulations. While the mesoscopic engineering presented valuable insights toward the generation of intense photonic-plasmonic hotspots, the microscopic engineering demonstrates the functionality of the radiating dipole as a dynamic entity with tailorable electronic and vibrational energy levels. By offering a simple, scalable, and label-compatible approach to photonic crystal-enhanced fluorescence in the transmittance mode and FERA in the reflectance mode, our study represents a pathway in the design of multifunctional plasmonic-photonic substrates and invites further exploration into light-matter interactions at the nanoscale.