Plasmonic Bragg Grating for Optical Feedback Raman Detection

Optical feedback for either the excitation or the signal is an effective approach to enhance the detection sensitivity of Raman scattering spectroscopy. A combination of plasmonic and optical feedback schemes may supply multifold enhancement mechanisms with further improved efficiency. Herein, a plasmonic Bragg grating (PBG) consisting of periodically distributed silver nanoslabs coated onto the sidewalls of the grating lines is reported. Localized surface plasmons excited in the silver nanoslabs with a large width/thickness ratio facilitate enhanced local field and Raman interaction. Asymmetric Bragg diffraction not only supplies optical feedback along the grating plane to extend largely the laser–molecule interaction distance but also enables efficient collection of the Raman signal through back‐diffraction into the detection system. Such a design accomplishes “volume” surface‐enhanced Raman scattering with a modulation depth of more than 500 nm and strong optical feedback, extending largely the limited interaction distance of surface plasmons. With respect to the scheme using Ag films for enhancement, a further enhancement factor larger than 2.8 × 10 3 has been achieved by the PBG device in the direct detection of the low‐concentration water solution of R6G molecules. The discovered photophysics is potentially important for practical applications in in situ water pollutant detection.