Structural Color 3D Printing By Shrinking Photonic Crystals (1905.05913v1)

Abstract: The rings, spots and stripes found on 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. Commercial nanoscale 3D printers based on two-photon polymerization are incapable of patterning photonic crystal structures with the requisite ~300 nm lattice constant to achieve photonic stopbands/ bandgaps in the visible spectrum and generate colors. Here, we introduce a means to produce 3D-printed photonic crystals with a 5x reduction in lattice constants (periodicity as small as 280 nm), achieving sub-100-nm features with a full range of colors. The reliability of this process enables us to engineer the bandstructures of woodpile photonic crystals that match experiments, showing that observed colors can be attributed to either slow light modes or stopbands. With these lattice structures as 3D color volumetric elements (voxels), we printed 3D microscopic scale objects, including the first multi-color microscopic model of the Eiffel Tower measuring only 39-microns tall with a color pixel size of 1.45 microns. 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.

• The paper introduces a heat-induced shrinkage method that reduces polymer lattice constants by up to 80% to enable visible-range photonic stopbands.
• It employs two-photon polymerization via direct laser writing to achieve lattice constants nearing 280 nm and an axial resolution of 380 nm.
• Bandstructure calculations and spectral measurements validate the approach, highlighting its potential for advanced photonic devices with precise optical control.

An Examination of Photonic Crystal Fabrication in 3D Printing through Heat-Induced Shrinkage

The paper "Structural Color 3D Printing By Shrinking Photonic Crystals" presents an innovative method for manipulating photonic crystal structures in three-dimensional (3D) printing to yield vibrant structural colors. This research addresses a significant challenge in nanoscale 3D printing: achieving the fine resolutions necessary to create photonic stopbands within the visible spectrum. Traditional 3D printing technologies have been restricted by their inability to produce sufficiently small lattice constants due to resolution limitations. By utilizing a heat-induced shrinkage process, the authors propose a solution to this limitation, potentially unlocking new avenues for vividly colored 3D printed materials without the use of synthetic dyes.

The paper focuses on utilizing two-photon polymerization via commercially available direct laser writing (DLW) systems, complemented by a subsequent shrinking process to refine the lattice constants of printed photonic crystals. This process culminates in lattice constants reduced to approximately 280 nm, enabling the manifestation of intense, angle-dependent structural colors through photonic stopbands in the visible range. This methodological innovation is characterized by its integration of DLW's capacity for 3D patterning with a heat-shrinking technique that achieves resolutions previously unattainable with conventional photonic crystal printing methodologies.

The authors demonstrate the effectiveness of their approach using woodpile photonic crystal structures, which feature repeating orthogonal grating stacks. Initial lattice constants achieved via two-photon lithography are further reduced by up to 80% through heat treatment, resulting in the transformation of polymeric structures and an increase in refractive index, as indicated by the progression from micrometer to nanometer scales. The authors provide empirical evidence of successful shrinkage, highlighted by their ability to produce a miniature, multicolored 3D model of the Eiffel Tower, marking the first instance of full-color 3D printed objects based on dielectric structural colors.

From a results-oriented perspective, the research delineates the advantageous anisotropic shrinking phenomenon where lateral resolutions surpass the vertical. This effect allows for the development of more spherical writing spots in post-shrunk structures, thereby enhancing axial resolution to 380 nm—a notable reduction from the pre-shrink limits imposed by traditional DLW setups.

Furthermore, the paper addresses how bandstructure calculations align well with experimental spectral measurements, elucidating the role of slow light modes and stopbands in producing the observable structural colors. The authors highlight that while achieving visible-range stopbands with low refractive index polymers is challenging, leveraging slow light modes enables color modulation across the entire visible spectrum.

In terms of implications, this research opens the door to a variety of practical applications in photonic devices, including those requiring precise spectral selectivity and polarization control. Such capabilities are poised to advance fields like photonic integrated circuitry and optoelectronic device design. The development of more efficient 3D color printing techniques could significantly impact the manufacturing processes of structural color filters, security labels, and novel optical components.

Looking forward, potential advancements may involve refining inorganic photoresists with heightened refractive indices or incorporating hierarchical structures for angle-independent color production. These developments could further enhance the resolution, durability, and functionality of 3D printed photonic structures across various domains, including the telecommunications industry, wearable technology, and automated optical systems.

The paper’s substantial contribution is firmly grounded in its empirical approach and methodical integration of theoretical and computational modeling, underscoring a sophisticated step forward in photonic crystal fabrication. The authors have effectively expanded the potential of nanoscale 3D printing, facilitating novel applications and sparking advancement in nanoscale optical engineering.