Dynamic Evolution‐Controlled Parabolic‐Shaped Microstructures for Ultra‐Black Surface via Self‐Assembled Microsphere Mask Etching

Suppressing broadband stray light remains a persistent obstacle in advanced optical instrumentation, where conventional blackening treatments often fall short in spectral range, surface conformity, or substrate compatibility. Here we report a fabrication approach that directly forms parabolic-shaped microstructures on a blackout-ink coating through self-assembled microsphere mask etching. The process is governed by a time-dependent shadowing evolution around the microspheres, which enables continuous tuning of the sidewall curvature-a structural parameter that is difficult to access using conventional micro/nanofabrication and emerges as a key determinant of optical attenuation. The method requires only spray-coating of the ink followed by dry etching and applies to the surfaces of glass, metal, polymer, and other engineering materials. Uniform microstructure formation is maintained across planar and curved surfaces without lithography or substrate-dependent optimization. The parabolic-shaped structures yield an average reflectance 0.89% over 300-1700 nm, enabled by continuous refractive-index grading and efficient photon trapping associated with the controlled sidewall profile. This work establishes a practical route for producing broadband ultra-black surfaces on real optical components while revealing a previously unrecognized mechanism linking microsphere-mediated morphological evolution to macroscopic optical suppression. The approach offers a straightforward and broadly applicable pathway for improving stray-light management in precision metrology, imaging, and spaceborne systems.