Since the role of quantum mechanics in molecular structure was identified by researchers such as Linus Pauling in the early 20th century, chemists have increasingly been using optical analytical tools to understand the structure and dynamics of molecules. In molecular science, fluorescence spectroscopy is an extremely important tool, which has found its use in a wide range of applications from chemi-luminescence and monitoring of atmospheric gases to single molecule microscopy. Since many important chemicals have vibrational states in the mid-infrared part of the electromagnetic spectrum, the development of new spectroscopic instrumentation in the mid-infrared range is a powerful enabler of future scientific advances.
Li Chen and co-authors have demonstrated laser-induced infrared fluorescence (LIIF) spectroscopy in the mid-infrared using a superconducting nanowire single-photon detector (SNSPD) integrated into a custom-made cryogenic spectroscopy setup. Made of a tungsten silicide (WSi) nanowire and operated at a temperature of 0.3 K, their SNSPD is highly sensitive to mid-infrared photons with a wavelength up to 7 µm. Employing the SNSPD, the authors were able to obtain a strong LIIF signal from a sub-monolayer of CO on a NaCl crystal surface. It is worth noting that measurements with such sensitivity are not possible with a conventional InSb detector. Another distinctive advantage of SNSPD is its sub-nanosecond temporal response, which is crucial for time resolved spectroscopic studies. Although optical coupling and read-out electronics set limits on the present levels of sensitivity and timing resolution of mid-infrared SNSPD, there is a significant potential for engineering further improvement. This work demonstrates the potential of SNSPD technology for advanced single-molecule fluorescence microscopy, extending the usage of this technique from the visible to mid-infrared. Moreover, rapid advancements in cryogenic engineering make this technique ready for wider adoption as an analytics tool in physical chemistry.
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