Metasurface for broadband coherent Raman signal enhancement beyond the single-molecule detection threshold

Broadband enhancement of Raman signals is vital for molecular spectroscopy and sensing applications. However, typically, there is a compromise between the resonance width and the local field enhancement factors. Here, we merge the broadband open plasmonic cavity concepts with the geometry exploiting narrow gaps and a reflecting layer. This results in a metasurface design that maintains substantial electric field confinement in the gap volume accompanied by a broad spectral response. The confined field is enhanced through constructive interference between the plasmonically enhanced illuminating beam and the field reflected at the bottom layer. The resulting field can interact with analyte molecules and support Raman scattering. With an optimized metasurface geometry involving a pair of gold nanodisks and a thin gold film, we demonstrate numerically that our metasurface can simultaneously enhance Raman signals from various molecular species, potentially resulting in unprecedented signal intensities enhanced by up to 20 orders of magnitude, facilitating molecular detection and identification. Our design offers a practical and promising solution for improving the sensitivity of Raman techniques and has significant implications for the field of nonlinear optical spectroscopy. Moreover, our metasurface is compatible with current fabrication technologies, making it suitable for various applications in sensing and imaging.