Optical Fireworks Based on Multifocal Three-Dimensional Color Prints.

Colorful three-dimensional (3D) prints are promising as practical anticounterfeiting labels with easily recognizable and striking visual effects. However, existing colorful 3D displays either require specific illumination conditions with multiple coherent lasers, hence suffer from speckles, or are unsuitable as passive labels. Here, we report a concept of a virtual 3D color object consisting of colorful focal spots in free space. The colors and corresponding "floating heights" of these spots are independently controlled via the design of 3D printed microlens profiles and heights of nanopillars that act as structural-color filters. Despite the unremarkable appearance of the printed substrate under both optical and electron microscopy, illumination with incoherent white light reveals information in the form of bright colorful spots appearing at designated heights above the plane of the substrate. The term "optical fireworks" refers to the way these spots appear and disappear under an optical microscope as one continuously shifts the focal plane. Our 3D printed optical fireworks security labels introduce applications for optical elements integrated with nanostructures in 3D colorful displays and anticounterfeiting labels.