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Abstract Sapphire is an attractive material that stands to benefit from surface functionalization effects stemming from micro/nanostructures. Here we investigate the use of ultrafast lasers for fabricating sapphire nanostructures by exploring the relationship between irradiation parameters, morphology change, and selective etching. In this approach a femtosecond laser pulse is focused on the substrate to change the crystalline morphology to amorphous or polycrystalline, which is characterized by examining different vibrational modes using Raman spectroscopy. The irradiated regions are removed using a subsequent hydrofluoric acid etch. Laser confocal measurements quantify the degree of selective etching. The results indicate a threshold laser pulse intensity required for selective etching. This process was used to fabricate hierarchical sapphire nanostructures over large areas with enhanced hydrophobicity, with an apparent contact angle of 140 degrees, and a high roll-off angle, characteristic of the rose petal effect. Additionally, the structures have high broadband diffuse transmittance of up to 81.8% with low loss, with applications in optical diffusers. Our findings provide new insights into the interplay between the light-matter interactions, where Raman shifts associated with different vibrational modes can predict selective etching. These results advance sapphire nanostructure fabrication, with applications in infrared optics, protective windows, and consumer electronics. 

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.