The coloration of some butterflies, Pachyrhynchus weevils, and many chameleons are notable examples of natural organisms employing photonic crystals to produce colorful patterns. Despite advances in nanotechnology, we still lack the ability to print arbitrary colors and shapes in all three dimensions at this microscopic length scale. Here, we introduce a heat-shrinking method to produce 3D-printed photonic crystals with a 5x reduction in lattice constants, achieving sub-100-nm features with a full range of colors. With these lattice structures as 3D color volumetric elements, we printed 3D microscopic scale objects, including the first multi-color microscopic model of the Eiffel Tower measuring only 39 µm tall with a color pixel size of 1.45 µm. The technology to print 3D structures in color at the microscopic scale promises the direct patterning and integration of spectrally selective devices, such as photonic crystal-based color filters, onto free-form optical elements and curved surfaces.

Conventional optical security devices provide authentication by manipulating a specific property of light to produce a distinctive optical signature. For instance, microscopic colour prints modulate the amplitude, whereas holograms typically modulate the phase of light. However, their relatively simple structure and behaviour is easily imitated. We designed a pixel that overlays a structural colour element onto a phase plate to control both the phase and amplitude of light, and arrayed these pixels into monolithic prints that exhibit complex behaviour. Our fabricated prints appear as colour images under white light, while projecting up to three different holograms under red, green, or blue laser illumination. These holographic colour prints are readily verified but challenging to emulate, and can provide enhanced security in anti-counterfeiting applications. As the prints encode information only in the surface relief of a single polymeric material, nanoscale 3D printing of customised masters may enable their mass-manufacture by nanoimprint lithography.

Lanthanide‐doped nanophosphors are promising in anti‐counterfeiting and security printing applications. These nanophosphors can be incorporated as transparent inks that fluoresce by upconverting near‐infrared illumination into visible light to allow easy verification of documents. However, these inks typically exhibit a single luminescent color, low emission efficiency, and low print resolutions. Tunable resonator‐upconverted emission (TRUE) is achieved by placing upconversion nanoparticles (UCNPs) within plasmonic nanoresonators. A range of TRUE colors are obtained from a single‐UCNP species self‐assembled within size‐tuned gap‐plasmon resonances in Al nanodisk arrays. The luminescence intensities are enhanced by two orders of magnitude through emission and absorption enhancements. The enhanced emissive and plasmonic colors are simultaneously employed to generate TRUE color prints that exhibit one appearance under ambient white light, and a multicolored luminescence appearance that is revealed under near‐infrared excitation. The printed color and luminescent images are of ultrahigh resolutions (≈50 000 dpi), and enable multiple colors from a single excitation source for increased level of security.

hologram (CGH) algorithms, such as the algorithms of Gerchberg-Saxton (GS), [14] Yang-Gu, [15] Fienup, [16] and Simulated Annealing. [17][18][19] Diffractive optical elements (DOEs) are a common application of CGH. In particular, DOEs consisting of square phase elements (i.e., pixels) are the easiest to design and fabricate. Each of these elements is a structure with a discrete height or refractive index designed to locally modulate the phase of the transmitted light. When combined, DOEs with diffraction efficiencies as high as 80% can be routinely achieved. [20] DOEs inevitably suffer from the zero order spot that appears as a bright spot in the middle of the projected image. The projected image corresponds to the intensity distribution in the Fourier plane, which is obtained by Fourier transform (FT) of the modified wavefront immediately after the hologram plane. Hence, the zero order spot is also referred to as the DC noise. This bright spot is exacerbated by fabrication imperfections and illumination beyond the extent of the DOE. [20][21][22] For instance, a commercialized liquid crystal spatial light modulator (SLM) with a filling fact or of ≈90% would result in a zero order spot due to light that passes through the dead regions. [23] Transparent dielectric structures fabricated by etching and polymer structure made by lithography would possess defects in the surface topography [11,24] that lead to the zero order spot. Furthermore, the inhomogeneity and dispersive optical property of the phase element materials can impede the performance of DOEs. [25] Ineluctably, a bright zero order spot appears in the center of the Fourier plane, [22,25] Diffractive optical elements (DOEs) provide a compact and energy-efficient solution to project arbitrary grayscale images onto a distant screen. Unfortunately, they invariably suffer from the zero order spot, which is caused mainly by undiffracted light that travels along the optical axis of an illuminating laser beam. To produce projected images without the bright spot, one can either shift the intended projection off-axis or block the zero order. However, images projected by these methods are occluded by the dark fringes of the diffraction pattern ("shadowing" effect) or increase the complexity of the optical setup. Here, a new type of DOE is introduced with blazed facets to shift the laser power into the off-axis direction. By adding blazed facets onto each phase element of a computer-generated hologram, far-field projections that are free of both the zero order spot and shadowing effects are produced while maintaining a diffraction efficiency as high as 86%. The blazed facets are fabricated by 3D direct laser writing, which enables continuous phase modulation within a single pixel. This concept of sub-pixel level modification of diffractive optical elements can be extended to other applications requiring precise wavefront shaping or detection, such as 3D displays, mixed-reality technology, and optical analog computing.

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