All-metal nanostructured substrates as subtractive color reflectors with near-perfect absorptance.

All-metal structures consisting of nanoprotrusions on a bulk silver layer are theoretically investigated and shown to have narrow near-perfect absorption peaks (>95%). Within the constraints of constant nanostructure height (50 nm) and pitch (250 nm), these peaks are tunable across the visible spectrum by adjusting the width and shape of the protrusion. The peaks are caused by localized surface plasmon resonances leading to dissipation on the surface of the protrusions. As the peaks occur in the visible range, they produce subtractive colors with high saturation, in accordance with Schrödinger's rule for maximum pigment purity.

Enhancing LED spectral output with perylene dye-based remote phosphor.

LEDs offer a wide range of spectral output with high efficiencies. However, the efficiencies of solid-state LEDs with green and yellow wavelengths are rather low due to the lack of suitable direct bandgap materials. Here, we introduce and develop perylene-enhanced green LEDs that produce a higher wall-plug efficiency of 48% compared to 38% for a solid-state green LED. While the wall-plug efficiency of the perylene-enhanced red LED is still lower than that of a solid-state red LED, we demonstrate that remote phosphor colour converters are effective solutions for targeted spectral tuning across the visible spectrum for horticultural lighting. In this work, we retrofit existing white LEDs and augment photosynthesis via spectral output tuning to achieve a higher red-to-blue ratio. Our results show a significant improvement in plant growth by up to 39%, after a 4-month growth cycle. We observe no visible degradation of the colour converter even under continuous illumination with a current of 400 mA. This opens up new opportunities for using perylene-based colour converters for tuneable illumination with high brightness.

Structural color three-dimensional printing by shrinking photonic crystals.

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.

Stacking of colors in exfoliable plasmonic superlattices.

Color printing with plasmonic resonators can overcome limitations in pigment-based printing approaches. While layering in pigment-based prints results in familiar color mixing effects, the color effects of stacking plasmonic resonator structures have not been investigated. Here, we demonstrate an experimental strategy to fabricate a 3-tiered complex superlattice of nanostructures with multiple sets of building blocks. Laser interference lithography was used to fabricate the nanostructures and a thin-layer of aluminum was deposited to introduce plasmonic colors. Interestingly, the structures exhibited drastic color changes when the layers of structures were sequentially exfoliated. Our theoretical analysis shows that the colors of the superlattice nanostructure were predominantly determined by the plasmonic properties of the two topmost layers. These results suggest the feasibility of the sub-wavelength vertical stacking of multiple plasmonic colors for applications in sensitive tamper-evident seals, dense 3D barcoding, and substrates for plasmonic color laser printing.