Standoff Time-Resolved SECARS on Ag-Decorated Silicon Nanowires

Surface-enhanced coherent anti-Stokes Raman scattering (SECARS) has been widely explored for ultrasensitive vibrational spectroscopy, primarily in microscopy configurations employing high-numerical-aperture optics. Here, we demonstrate standoff detection of molecular vibrational fingerprints using plasmon-enhanced time-resolved SECARS at a working distance of 20 cm. We employ Ag-decorated silicon nanowires as a plasmonic substrate supporting the analyte molecules. Broadband pump (575 nm) and Stokes (625 nm) femtosecond pulses together with a narrowband delayed probe (515 nm) are used to generate the SECARS signal, with excitation and anti-Stokes wavelengths overlapping the plasmonic response of the substrate. System performance is first calibrated through Raman and CARS measurements of liquid pyridine and compressed Na₂DPA powder. SECARS spectra of Rhodamine B and Na₂DPA deposited on Ag-SiNWs are then recorded in backscattering at 20 cm distance, revealing clear vibrational fingerprints down to 10^-7 M for Rhodamine B and 10^-6 M for Na₂DPA. The Na₂DPA measurements provide an estimate of the overall SECARS/Raman signal amplification, yielding an effective enhancement of the order of 10¹³ under the present experimental conditions. These results establish a proof-of-concept for plasmon-enhanced standoff vibrational spectroscopy from analyte-loaded nanostructured substrates, including dipicolinate targets relevant to bacterial-spore detection.