Sensitivity enhancement of time-gated Raman spectroscopy by reducing the systematic noise of residual fluorescence background.

Abstract

Fluorescence interference is a pervasive challenge in Raman spectroscopy, often limiting its broader application. Time-gated Raman spectroscopy offers a more universal solution by temporally separating Raman signals from fluorescence; however, it faces significant challenges when dealing with samples that exhibit short fluorescence lifetimes. Achieving high time resolution to effectively distinguish these signals typically requires advanced detectors that are not only costly but also difficult to source commercially, often resulting in substantial residual fluorescence that diminishes overall signal quality. In this work, we identified that the dominant noise in time-gated Raman spectroscopy is wavelength-to-wavelength fluctuation noise, which cannot be reduced by simply extending the collection time. Through our analysis, we discovered that this noise is linearly proportional to the fluorescence background and remains consistent across different time windows when collected using the time-correlated single-photon counting (TCSPC) technology. Recognizing this consistent pattern, we developed a novel, to the best of our knowledge, method to effectively remove this noise by leveraging the time-resolved fluorescence spectrum. For example, in the case of sesame oil excited with a 532 nm laser, it is typically difficult to obtain a recognizable Raman spectrum when the gate width exceeds 300 ps. However, using our method, we were able to achieve a decent signal even with a gate width of 4 ns. By correcting the Raman spectrum using the captured pure fluorescence spectrum, we achieve up to a 23-fold improvement in the signal-to-noise ratio (SNR). This innovation significantly reduces the dependence on high-cost, high-time-resolution detectors, potentially expanding the adoption and applicability of time-gated Raman spectroscopy across various fields.