Direct determination of multiphoton absorption cross-sections by transient absorption spectroscopy.

Single- and multi-photon absorption cross-sections quantify the likelihood that a material will absorb one or more photons at a given wavelength. This critical parameter is fundamental to understanding light-matter interactions that underpin key applications in spectroscopy, photochemistry and advanced imaging techniques like multi-photon microscopy and deep tissue imaging. Conventional methods for measuring absorption cross-sections are often limited by sensitivity to sample morphology, type, concentration, and high excitation intensities - factors that can compromise reliability, increase experimental complexity, and risk sample damage. Herein, we present a direct, robust, and versatile method for quantifying absorption cross-sections across single- to multi-photon regimes, based on the saturation behaviour of transient absorption signals. Using this approach, we report for the first time the three-photon and four-photon absorption cross-sections of CsPbI perovskite nanocrystals and CdSe/ZnS quantum dots under 1700 nm and 2100 nm excitation. These values exceed those of incumbent materials used for mouse deep-brain imaging by at least an order of magnitude. Our method does not rely on photoluminescence signals, making it suitable for weakly or non-emissive materials. Importantly, our work provides a powerful generalizable tool to accelerate the discovery and optimization of next generation photon-harvesting materials.