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.

Additive manufacturing at small scales enables advances in micro-and nanoelectromechanical systems, micro-optics, and medical devices. Materials that lend themselves to AM at the nano-scale, especially for optical applications, are limited. State-of-the-art AM processes for high refractive index materials typically suffer from high porosity, poor repeatability, and require complex experimental procedures. We developed an AM process to fabricate complex 3D architectures out of fully dense titanium dioxide (TiO 2) with a refractive index of 2.3 and nano-sized critical dimensions. Transmission Electron Microscopy (TEM) analysis proves this material to be rutile phase of nanocrystalline TiO 2 , with an average grain size of 110 nm and <1% porosity. Proof-ofconcept woodpile architectures with 300-600 nm beam dimensions exhibit a full photonic bandgap centered at 1.8-2.9 μm, revealed by Fourier-transform Infrared Spectroscopy (FTIR) and supported by Plane Wave Expansion simulations. The developed AM process enables advances in 3D MEMS, micro-optics, and prototyping of 3D dielectric PhCs.

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We report the optical properties obtained through experiments, simulation, and theory of ultrathin (<0.1λ), amorphous Si nanopillar arrays embedded in a thin film of SiO 2 designed for narrowband filtering for multi-and hyperspectral imaging in the near-infrared. The fabricated nanopillar arrays are square-packed with subwavelength periodicity, heights of ∼100 nm, and a radius-to-spacing ratio, r/a, of ∼0.2. Specular reflection measurements at normal incidence demonstrate that these arrays behave as narrow stopband filters in the near-infrared (λ = 1300−1700 nm) and attain ∼90% reflectivity in band and a full width at halfmaximum as low as 20 nm. Using a combination of full-wave simulations and theory, we demonstrate that these narrowband filtering properties arise from efficient grating coupling of light into guided modes of the array because the nanopillar arrays serve as photonic crystal slabs. This phenomenon is known as a guided mode resonance. We discover that the spectral location of these resonances is passively tunable by modifying array geometry and is most sensitive to nanopillar spacing. Theoretical photonic crystal slab band diagrams accurately predict the spectral locations of the observed resonance and provide physical insights into and support the guided mode resonance formulation. This work demonstrates that these ultrathin all-dielectric nanopillar arrays have advantages over existing hyperspectral filter designs because they are polarization independent, do not suffer from material absorption loss, and have significant implications for minimizing imaging device size.

Engineering of the dispersion properties of a photonic crystal (PhC) opens a new paradigm for the design and function of PhC devices. Exploiting the dispersion properties of PhCs allows control over wave propagation within a PhC. We describe the design, fabrication, and experimental observation of photonic bands for 3D PhCs capable of negative refraction in the mid-infrared. Band structure and equifrequency contours were calculated to inform the design of 3D polymer−germanium core− shell PhCs, which were fabricated using two-photon lithography direct laser writing and sputtering. We successfully characterized a polymer−Ge core−shell lattice and mapped its band structure, which we then used to calculate the PhC refraction behavior. An analysis of wave propagation revealed that this 3D core−shell PhC refracts light negatively and possesses an effective negative index of refraction in the experimentally observed region. These results suggest that architected nanolattices have the potential to serve as new optical components and devices across infrared frequencies.