@article{luo_zhang_evans_chang_2020, title={Active Periodic Magnetic Nanostructures with High Aspect Ratio and Ultrahigh Pillar Density}, volume={12}, ISSN={["1944-8252"]}, DOI={10.1021/acsami.9b18423}, abstractNote={Magnetically actuated micro/nanoscale pillars have attracted significant research interest recently due to their dynamic properties. These structures can be used for various applications, such as dry adhesion, cell manipulation, and sensors or actuators in microfluidics. Magnetically actuated structures can be fabricated by mixing magnetic particles and polymers to yield a favorable combination of magnetic permeability and mechanical compliance. However, the pillar density of demonstrated structures is relatively low, which limits the potential applications in active surface manipulation of microscale objects. Here we demonstrate active periodic nanostructures with pillar density of 0.25 pillar/μm^2, which is the highest density for magnetically actuated pillars so far. Having structure period of 2 μm, diameter of 600 nm, and high aspect ratio of up to 11, this structure can be magnetically actuated with displacement of up to 200 nm. The behaviors of the pillars under various cyclic actuation modes have been characterized, demonstrating that the actuation can be well controlled. This work can find potential applications in particle manipulation and tunable photonic elements. KEYWORDS Dynamic materials, active nanostructures, nanofabrication, magnetic nanoparticles, high aspect ratio.}, number={9}, journal={ACS APPLIED MATERIALS & INTERFACES}, author={Luo, Zhiren and Zhang, Xu A. and Evans, Benjamin Aaron and Chang, Chih-Hao}, year={2020}, month={Mar}, pages={11135–11143} }
 @article{chen_naidu_luo_chang_2019, title={Enhancing optical transmission of multilayer composites using interfacial nanostructures}, volume={126}, ISSN={["1089-7550"]}, DOI={10.1063/1.5097832}, abstractNote={We demonstrate the suppression of light reflections at solid-solid interfaces in multilayer thin and thick films using interfacial nanostructures. The embedded nanostructures have subwavelength features and function as a gradient-index medium to eliminate Fresnel losses induced by refractive index mismatch between dissimilar materials. Suppressing the interfacial reflection can reduce interference effects in thin films, and the transmittance measurement of a polymer on a silica substrate demonstrates a two-fold decrease in interference fringe contrast. A thick multilayer composite consisting of three fused silica and two polymer layers has also been fabricated and demonstrates the enhancement of optical transmission up to 30% at high incident angles. The effects of the interfacial structure geometry are examined by theoretical models based on rigorous coupled-wave analysis methods. The experimental results agree well with simulation models, which predicts that further improvements can be achieved using the optimized tapered profile. This work indicates that interfacial nanostructures can improve the broadband and wide-angle response of multilayers and can find applications in thin-film optics, optoelectronic devices, and composite windows.}, number={6}, journal={JOURNAL OF APPLIED PHYSICS}, author={Chen, Yi-An and Naidu, Sharan V and Luo, Zhiren and Chang, Chih-Hao}, year={2019}, month={Aug} }
 @article{luo_evans_chang_2019, title={Magnetically Actuated Dynamic Iridescence Inspired by the Neon Tetra}, volume={13}, ISSN={["1936-086X"]}, DOI={10.1021/acsnano.9b00822}, abstractNote={Inspired by the tropical fish neon tetra, we report a mechanism to achieve dynamic iridescence that can be magnetically tuned. This approach is based on the tilting of periodic photonic nanostructures, as opposed to the more common strain-induced color tuning. In this method, a periodic array of magnetic nanopillars serves as a template to guide the assembly of iron oxide nanoparticles when magnetized in a liquid environment. The periodic local fields induced by the magnetic template anchor the assembled particle columns, allowing the structure to tilt about the base when the angle of the applied field is changed. This effect emulates a microscopic "Venetian blind" and results in dynamic optical properties through structural coloration that is tunable in real time. The fabricated prototype demonstrates tunable reflectance spectra with peak wavelength shift from 528 to 720 nm. The magnetic actuation mechanism is reversible and has a fast response time around 0.3 s. This structure can be implemented on an arbitrary surface as dynamic camouflage, iridescent display, and tunable photonic elements, as well as in other applications such as active fluidic devices and particle manipulation.}, number={4}, journal={ACS NANO}, author={Luo, Zhiren and Evans, Benjamin Aaron and Chang, Chih-Hao}, year={2019}, month={Apr}, pages={4657–4666} }