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This research investigates the optical anisotropy and structure-induced birefringence in low-index nanolattices. By designing the unit-cell geometry using 3-dimentional (3D) colloidal lithography, nanolattices can exhibit different refractive indices along orthogonal directions due to the structure geometry. The out-of-plane and in-plane indices are characterized using spectroscopic ellipsometry and agree well with the anisotropic Cauchy material model. Exhibit positive-uniaxial birefringence, the nanolattices can have up to Δn = 0.003 for nanolattices with low indices that range from 1.04 to 1.12. The birefringence is modeled using the finite-difference-time-domain (FDTD) method, where the reflectance of an anisotropic film is calculated to iteratively solve for the indices. The theoretical model and experimental data indicate that the birefringence can be controlled by the unit-cell geometry based on the relative length scale of the particle diameter to the exposure wavelength. This work demonstrates that it is possible to precisely design optical birefringence in 3D nanolattices, which can find applications in polarizing optics, nanophotonics, and wearable electronics. 

Although there has been significant interest in the novel material properties of bio-inspired nanostructures, engineering them to become mechanically durable remains a significant challenge. This work demonstrates the fabrication of sapphire nanostructures with anti-glare, anti-fogging, anti-dust and scratch-resistant properties. The fabricated nanostructures demonstrated a period of 330 nm and an aspect ratio of 2.1, the highest reported for sapphire thus far. The nanostructured sapphire sample exhibited broadband and omnidirectional antireflection properties, with an enhanced transmission of up to 95.8% at a wavelength of 1360 nm. The sapphire nanostructures also exhibited enhanced wetting performance and could mitigate fogging from water condensation or repel water droplets. Furthermore, owing to their sharp features, the fabricated structures could prevent particulate adhesion and maintain a 98.7% dust-free surface area solely using gravity. Furthermore, nanoindentation and scratch tests indicated that the sapphire nanostructures have an indentation modulus and hardness of 182 GPa and 3.7 GPa, respectively, which are similar to those of bulk glass and scratch-resistant metals such as tungsten. These sapphire nanostructures can be fabricated using high-throughput nanomanufacturing techniques and can find applications in scratch-resistant optics for photonics, electronic displays, and protective windows. 

Nanostructured materials and nanolattices with high porosity can have novel optical and mechanical properties that are attractive for nanophotonic devices. One existing challenge is the integration of microstructures that can be used as waveguides or electrodes on such nanostructures without filling in the pores. This study investigates the fabrication of TiO2 microstructures on nanolattices using a stencil mask. In this approach, the nanostructures are planarized with a polymer film while the microstructures are patterned in a sequential shadow deposition step. Our results demonstrate the successful fabrication of a “dog-bone” microstructure with 400 μm length, 100 μm width, and 30–560 nm thicknesses on nanostructure with 390 and 500 nm period. The experimental results show that cracks can form in the microstructures, which can be attributed to residual stress and the thermal annealing cycle. A key finding is that the film cracks decrease as the TiO2 layer becomes thinner, highlighting an important relationship between grain size distribution and the film thickness. The mechanical stability of the underlying nanolattices also plays a key role, where interconnected architecture mitigated the crack formation when compared with isolated structures. The demonstrated fabrication process can lead to integrated waveguides and microelectrodes on nanolattices, which can find applications for next-generation photonic and electronic devices. 

Dielectric mirrors based on Bragg reflection and photonic crystals have broad application in controlling light reflection with low optical losses. One key parameter in the design of these optical multilayers is the refractive index contrast, which controls the reflector performance. This work reports the demonstration of a high-reflectivity multilayer photonic reflector that consists of alternating layers of TiO₂films and nanolattices with low refractive index. The use of nanolattices enables high-index contrast between the high- and low-index layers, allowing high reflectivity with fewer layers. The broadband reflectance of the nanolattice reflectors with one to three layers has been characterized with peak reflectance of 91.9% at 527 nm and agrees well with theoretical optical models. The high-index contrast induced by the nanolattice layer enables a normalize reflectance band of Δλ/λ_oof 43.6%, the broadest demonstrated to date. The proposed nanolattice reflectors can find applications in nanophotonics, radiative cooling, and thermal insulation.