  Multicolor super-resolution imaging remains an intractable challenge for both
far-field and near-field based super-resolution techniques. Planar
super-oscillatory lens (SOL), a far-field subwavelength-focusing diffractive
lens device, holds great potential for achieving sub-diffraction-limit imaging
at multiple wavelengths. However, conventional SOL devices suffer from a
numerical aperture (NA) related intrinsic tradeoff among the depth of focus
(DoF), chromatic dispersion and focus spot size, being an essential
characteristics of common diffractive optical elements. Typically, the limited
DoF and significant chromatism associated with high NA can lead to unfavorable
degradation of image quality although increasing NA imporves the resolution.
Here, we apply a multi-objective genetic algorithm (GA) optimization approach
to design an apochromatic binary-phase SOL that generates axially jointed
multifoci concurrently having prolonged DoF, customized working distance (WD)
and suppressed side-lobes yet minimized main-lobe size, optimizing the
aforementioned NA-dependent tradeoff. Experimental implementation of this
GA-optimized SOL demonstrates simultaneous focusing of blue, green and red
light beams into an optical needle half of the incident wavelength in diameter
at 428 um WD, resulting in an ultimate resolution better than one third of the
incident wavelength in the lateral dimension. By integrating this apochromatic
SOL device with a commercial fluorescence microscope, we employ the optical
needle to perform, for the first time, three-dimensional super-resolution
multicolor fluorescence imaging of the unseen fine structure of neurons at one
go. The present study provides not only a practical route to far-field
multicolor super-resolution imaging but also a viable approach for constructing
imaging systems avoiding complex sample positioning and unfavorable
photobleaching.
