Local Irradiance Sensing via Phase‐Lag Detection in Intensity‐Modulated Upconversion Luminescence

ABSTRACT Accurate determination of local optical irradiance is crucial across a wide range of photonic and biomedical applications. However, its precise measurement remains challenging in complex or confined environments, where scattering, absorption, and focusing effects significantly distort the spatial power distribution. Here, we introduce a nanoscale irradiance sensing strategy based on the phase lag between a sinusoidally intensity‐modulated near‐infrared excitation and the resulting upconversion luminescence from /‐doped nanoparticles. By monitoring the phase delay of the emission relative to the excitation modulation, we obtain a robust metric that directly reports the local excitation irradiance at the nanoparticle location, with enhanced sensitivity. The method has been experimentally implemented using a fiber‐optic reflectance/luminescence probe, enabling minimally invasive and portable irradiance sensing in complex media and hard‐to‐access geometries. Finally, a rate‐equation model identifies the average population of the intermediate energy level of as the physical origin of the observed phase response, providing a theoretical framework for optimizing sensor performance.