Heterochromatic Nonlinear Optical Responses in Upconversion Nanoparticles for Super-Resolution Nanoscopy

Point spread function (PSF) engineering by an emitter's response can code higher-spatial-frequency information of an image for microscopy to achieve super-resolution. However, complexed excitation optics or repetitive scans are needed, which explains the issues of low speed, poor stability, and operational complexity associated with the current laser scanning microscopy approaches. Here, the diverse emission responses of upconversion nanoparticles (UCNPs) are reported for super-resolution nanoscopy to improve the imaging quality and speed. The method only needs a doughnut-shaped scanning excitation beam at an appropriate power density. By collecting the four-photon emission of single UCNPs, the high-frequency information of a super-resolution image can be resolved through the doughnut-emission PSF. Meanwhile, the two-photon state of the same nanoparticle is oversaturated, so that the complementary lower-frequency information of the super-resolution image can be simultaneously collected by the Gaussian-like emission PSF. This leads to a method of Fourier-domain heterochromatic fusion, which allows the extended capability of the engineered PSFs to cover both low- and high-frequency information to yield optimized image quality. This approach achieves a spatial resolution of 40 nm, 1/24th of the excitation wavelength. This work suggests a new scope for developing nonlinear multi-color emitting probes in super-resolution nanoscopy.