  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, we present and experimentally demonstrate the
super-linear ISM (SL-ISM) to push the resolution enhancement beyond the factor
of two, 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, we achieve a resolution of about 120 nm, 1/8th of the
excitation wavelength. Furthermore, we demonstrate a parallel detection
strategy of SL-ISM with the multifocal structured excitation pattern, 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.
