Enhancing Spectral Coverage, Efficiency, and Photometric Accuracy in Ultrafast Fluorescence Upconversion Spectroscopy

Abstract Time‐resolved fluorescence techniques are essential for studying the excited‐state dynamics of molecules and materials, as well as their interactions with the surrounding media. Fluorescence upconversion provides femtosecond time resolution, but its limited spectral coverage often restricts its application. Broadband variants of fluorescence upconversion spectroscopy (FLUPS) offer improved spectral coverage but require balancing spectral bandwidth against signal strength. In this paper, we introduce a three‐angle method (3‐AM) for FLUPS designed to overcome these limitations. This method enhances spectral coverage, photometric accuracy, and signal‐to‐noise ratio by recording and averaging spectrally resolved upconversion signals at three distinct crystal angles. Unlike other techniques that rely on spectral reconstruction or continuous crystal rotation during acquisition, the 3‐AM relies on fixed crystal positions, allowing for robust post‐acquisition corrections. We validate the performance and reproducibility of the 3‐AM by comparing it with the conventional fixed‐angle approach, as well as through blind testing in two independent laboratories. Despite variations in experimental configuration, the 3‐AM consistently produces reproducible spectral line shapes across the two setups. The enhanced performance and reliability demonstrate its practicality for laboratories with broadband FLUPS capabilities. Thus, the 3‐AM is a valuable tool for investigating ultrafast radiative processes in previously inaccessible molecular and material systems.