Longitudinal Super-Resolution Spherical Multi-Focus Array Based on Column Vector Light Modulation

: Featured by the capability of multi degree-of-freedom light-field manipulations while reserving high spatial resolution, multifocal laser arrays have been widely applied in femtosecond laser micro/nanofabrication, optical trapping, and so forth. Yet, due to the relatively lower axial resolution of single focuses within the array in comparison with the lateral resolution of their own, multifocal laser array has been refrained from isotropic 3D nanofabrication. Herein, we propose a feasible method for generation of axially super-resolved multifocal array with quasi-spherical focal spots. In particular, quasi-spherical multifocal array is optically synthesized via precise modulation on the coherent superposition of the orthogonal radially polarized beam (RPB) and azimuthally polarized beam (APB) states in the focal region based on annular amplitude modulation. We show theoretically the generation of quasi-spherical multifocal array with a high uniformity up to 99 % . The average axial and lateral full-width-half maximum (FWHM) of the focal array are measured to be 0.76 λ with the standard deviations in the axial and lateral directions being 0.005 λ and 0.019 λ , respectively. The presented strategy for synthesis of quasi-spherical multifocal array with high uniformity paves the way for high-precision laser fabrication of 3D micro/nano devices. Overview: Featured by the capability of multi degree-of-freedom light-field manipulations while reserving high spatial resolution, multifocal laser arrays have been widely applied in femtosecond laser micro/nanofabrication, optical trapping, etc. However, for lens diffraction, the smaller momentum spread along the optical axis with respect to that in the transverse direction could introduce a larger position spread in real space, which in turn leads to lower axial resolution than the transverse resolution. The anisotropy of the focused laser beam, inherent regardless of paraxial or tight-focusing cases, has been a great hurdle for laser printing of functional microdevices with precise control on feature size and improved mechanical performances. To this end, in this research, a feasible method for generation of isotropic focused laser beam with quasi-spherical 3D point spread function (PSF) is developed based on vectorial light field modulation. We demonstrate that through simultaneous implementation of phase modulation and amplitude modulation, homogeneous multifocal array with quasi-spherical focal spots can be generated. Particularly, with the use of a well-designed annular mask, the suppression on the axial spread of field is accomplished via accurate control on the coherent superposition of the orthogonal radially polarized beam (RPB) and azimuthally polarized beam (APB) in the focal region since the depolarized axial component of the AP beam vanishes in vicinity of the gaussian focus even under tight focusing condition. Using the proposed method, isotropic 3D PSF with identical axial and transverse FWHM of 0.71 λ is achieved. Meanwhile, based on iterative phase retrieval algorithm, phase-only holograms are designed and employed transforming the incident wavelet as the summation of sub-wavelets, yielding multiple converging sites in 3D space, thereby generating the multifocal array. We further present the synthesis of quasi-spherical multifocal array. A high uniformity up to 99 % for a 10-by-10 multifocal array, in which the single focus elements share near-identical axial and transverse FWHM, being 0.76