Multi‐Photon Super‐Linear Image Scanning Microscopy Using Upconversion Nanoparticles

Abstract Super‐resolution fluorescence microscopy is of great interest in life science studies for visualizing subcellular structures at the nanometer scale. Among various kinds of super‐resolution approaches, image scanning microscopy (ISM) offers a doubled resolution enhancement in a simple and straightforward manner, based on the commonly used confocal microscopes. ISM is also suitable to be integrated with multi‐photon microscopy techniques, such as two‐photon excitation and second‐harmonic generation imaging, for deep tissue imaging, but it remains the twofold limited resolution enhancement and requires expensive femtosecond lasers. Here, the super‐linear ISM (SL‐ISM) pushes the resolution enhancement beyond the factor of two is presented and experimentally demonstrated, with a single low‐power, continuous‐wave, and near‐infrared laser, by harnessing the emission nonlinearity within the multiphoton excitation process of lanthanide‐doped upconversion nanoparticles (UCNPs). Based on a modified confocal microscope, a resolution of ≈120 nm, 1/8th of the excitation wavelength is achieved. Furthermore, a parallel detection strategy of SL‐ISM with the multifocal structured excitation pattern is demonstrated, to speed up the acquisition frame rate. This method suggests a new perspective for super‐resolution imaging or sensing, multi‐photon imaging, and deep‐tissue imaging with simple, low‐cost, and straightforward implementations.