Xinhua Hu

Coloration strategies in peacock feathers

We report the mechanism of color production in peacock feathers. We find that the cortex in differently colored barbules, which contains a 2D photonic-crystal structure, is responsible for coloration. Simulations reveal that the photonic-crystal structure possesses a partial photonic bandgap along the direction normal to the cortex surface, for frequencies within which light is strongly reflected. Coloration strategies in peacock feathers are very ingenious and simple: controlling the lattice constant and the number of periods in the photonic-crystal structure. Varying the lattice constant produces diversified colors. The reduction of the number of periods brings additional colors, causing mixed coloration.

Enlargement of omnidirectional total reflection frequency range in one-dimensional photonic crystals by using photonic heterostructures

We show theoretically that the omnidirectional total reflection frequency range of a multilayer dielectric reflector can be substantially enlarged as desired by using photonic heterostructures. This photonic heterostructure consists of different one-dimensional (1D) photonic crystals. The constituent 1D photonic crystals have to be properly chosen such that their omnidirectional photonic band gaps of the adjacent photonic crystals overlap each other.

Stacking‐Order‐Dependent Optoelectronic Properties of Bilayer Nanofilm Photodetectors Made From Hollow ZnS and ZnO Microspheres

Innovative bilayer nanofilms composed of semiconducting ZnS and ZnO hollow microspheres are successfully fabricated by an oil-water interfacial self-assembly strategy. The photocurrent of the bilayer film-based photodetectors is dependent on the stacking orders of the building blocks. The optimal optoelectronic properties of the ZnS(up)/ZnO(down) device are much better than those of the monolayer-film based device.

Photonic crystals with silver nanowires as a near-infrared superlens

Lensing effect can be achieved in photonic crystal (PC) slabs, but in many situations, the mechanism can be traced to the self-collimation effect of a square-like constant frequency surface. We show that using a metal-in-dielectric configuration, circular constant frequency surface can be obtained, and the lensing effect then obeys fairly well the image–distance relationship characteristic of an n=−1 material. As a two-dimensional example, far-field imaging is realized in square arrays of silver nanowires in air for transverse-electric waves at 400nm. A high resolution of resolution of about 100nm obtained with a three layer slab of such PCs. By varying the matrix, the results are extended to optic and near-infrared regime.

A novel electromechanical actuation mechanism of a carbon nanotube fiber.

A spun carbon nanotube fiber functions as a torsional actuator in almost all available environmental media such as air, water, organic solvents, and electrolyte solutions. The Amperes Law among helically aligned carbon nanotubes explains the simultaneous occurrence of lengthwise contraction and rotary torsion upon applying a low current. The produced stress is over 100 times that of the strongest natural skeletal muscle with high reversibility and good stability. The use of torsional fibers for electric motors is demonstrated.

Comparison of optical absorption in Si nanowire and nanoporous Si structures for photovoltaic applications

We numerically study the optical absorption in Si nanowire and nanoporous Si structures that have potential applications in solar cells. It is found that for the same thickness and filling ratio of Si, thin nanoporous structures can have much higher absorption than thin Si nanowire arrays. Above a critical filling ratio of Si (0.25), the nanoporous structures can have higher absorption even than thin films with the same thickness. For solar cells based on thin nanoporous Si structures, the maximal ultimate efficiency occurs when the filling ratio is around 0.3.

Tubular oxide microcavity with high-index-contrast walls: Mie scattering theory and 3D confinement of resonant modes

Tubular oxide optical microcavities with thin walls (< 100 nm) have been fabricated by releasing pre-stressed Y2O3/ZrO2 bi-layered nanomembranes. Optical characterization demonstrates strong whispering gallery modes with a high quality-factor and fine structures in the visible range, which are due to their high-index-contrast property (high refractive index in thin walls). Moreover, the strong axial light confinement observed in rolled-up circular nanomembranes well agrees with our theoretical calculation by using Mie scattering theory. Novel material design and superior optical resonant properties in such self-rolled micro-tubular cavities promise many potential applications e.g. in optofluidic sensing and lasing.

Higher-order incidence transfer matrix method used in three-dimensional photonic crystal coupled-resonator array simulation

The plane-wave-based transfer matrix method with rational function interpolation and higher-order plane-wave incidence is proposed as an efficient calculation approach to simulate three-dimensional photonic crystal devices. As an example, the dispersion relations and quality factors are calculated for resonant cavity arrays embedded in a woodpile photonic crystal. An interesting ultraslow negative group velocity is observed in this structure.

Cancellation of reflection and transmission at metamaterial surfaces

Based on analytical derivations and numerical simulations, we show that both reflection and transmission can be canceled at the surface of a metamaterial (MM) with a metal-dielectric stratified structure. Strong anisotropic absorption and the surface direction are found to play important roles in this phenomenon. For the angle between the surface and the layers of MMs above a critical value, only reflection is eliminated and transmission is permitted. Since they are not related to resonance, the phenomena can occur in a broad frequency range.

Design of midinfrared photodetectors enhanced by resonant cavities with subwavelength metallic gratings

We propose a metallic Fabry–Perot cavity with a Au grating and a Au film acting as two reflectors to enhance the field and absorption in the active detector region, leading to better performance of quantum-dot-based photodetectors at a wavelength of 10 μm. One- and two-dimensional Au gratings are applied to achieve enhancement for polarized and unpolarized light, respectively. With optimizing grating parameters, the absorption can be enhanced by about 20 times in the active detector region compared to conventional photodetectors without the Au reflectors.