Time-stretched quantum spectroscopy of midinfrared plasmonic nanostructures

Plasmonic nanostructures offer a prominent foundation for plasmon sensing owing to their capacity to confine light within nanoscale regions and enhance the intensity of the local electromagnetic field. Nevertheless, the development of plasmonic sensors that operate in the midinfrared (MIR) region still presents a notable obstacle, primarily due to the limited detection sensitivity of commercial MIR detectors. To break this bottleneck, we demonstrate the characterization of the plasmonic nanostructures' MIR spectral response using a time-stretched quantum spectroscopy with frequency upconversion. Quantum spectroscopy of correlated photon pairs offers a novel approach to overcome the limitations associated with the detection sensitivity of localized surface plasmon resonances (LSPRs), particularly under conditions of excessive noise. This scheme not only extends the application of quantum spectroscopy for the plasmonic nanostructures to the MIR regions, but also circumvents the necessity of employing various types of MIR detectors. In addition, by wavelength-to-time mapping and using a single-pixel detector to obtain the MIR spectral response of a plasmonic sample, nonlocal time stretching significantly increases system stability and efficiently reduces the measurement time under a low-light-level illumination. With an integration time of 600 s, time-stretched quantum spectroscopy demonstrates a detection sensitivity of 0.22 photons per pulse. The outcomes of our research pave the way for the realization of enhanced detection sensitivity in the investigation of plasmonic nanostructures in the MIR range.