Image scanning microscopy based on multifocal metalens for sub-diffraction-limited imaging of brain organoids.

Image scanning microscopy (ISM) is a promising imaging technique that offers sub-diffraction-limited resolution and optical sectioning. Theoretically, ISM can improve the optical resolution by a factor of two through pixel reassignment and deconvolution. Multifocal array illumination and scanning have been widely adopted to implement ISM because of their simplicity. Conventionally, digital micromirror devices (DMDs)(1) and microlens arrays (MLAs)(2,3) have been used to generate dense and uniform multifocal arrays for ISM, which are critical for achieving fast imaging and high-quality ISM reconstruction. However, these approaches have limitations in terms of cost, numerical aperture (NA), pitch, and uniformity, making it challenging to create dense and high-quality multifocal arrays at high NA. To overcome these limitations, we introduced a novel multifocal metalens design strategy called the hybrid multiplexing method, which combines two conventional multiplexing approaches: phase addition and random multiplexing. Through numerical simulations, we demonstrate that the proposed method generates more uniform and denser multifocal arrays than conventional methods, even at small pitches. As a proof of concept, we fabricated a multifocal metalens generating 40 × 40 array of foci with a 3 μm pitch and NA of 0.7 operating at a wavelength of 488 nm and then constructed the multifocal metalens-based ISM (MMISM). We demonstrated that MMISM successfully resolved sub-diffraction-limited features in imaging of microbead samples and forebrain organoid sections. The results showed that MMISM imaging achieved twice the diffraction-limited resolution and revealed clearer structural features of neurons compared to wide-field images. We anticipate that our novel design strategy can be widely applied to produce multifunctional optical elements and replace conventional optical elements in specialized applications.