A two‐tier nanostructure comprising a 20‐nm‐diameter “nanograss” on 100‐nm‐diameter nanopillars exhibits robust and near‐perfect superhydrophobicity, that is, water contact angles close to 180° and sliding angles close to 0°. Although the solid fraction was very low (∼0.02%), this surface could support a drop of water under a pressure of 234 Pa.
The unique functionalities of nanoscale structures in the natural world are an inspiration to the development of new nano-manufacturing techniques. For example, the cornea of the moth's eye features a sub-wavelength natural antireflective architecture. To date, almost all optical research into moth eye structures has been focused on their antireflective properties. No studies of inverse polarization phenomena at the Brewster angle have been reported, especially in biomimetic structures. For the first time, we discovered a unique inverse polarization phenomenon on moth eye structures that arises from TM-polarized light having a higher reflectance than TE-polarized light on moth eye structures at angles of incidence near the Brewster angle, unlike the behavior of polarized light on flat interfaces. Herein, we report a one-step colloidal lithography process that allows the fabrication of several kinds of moth eye structures. We characterized these moth eye structures experimentally and through rigorous coupled-wave analysis to understand the mechanism underlying this inverse polarization phenomenon in both visible and near infrared ray (NIR) regimes. Controlling the structural height and degree of non-close-packing of the moth eye structures had a dramatic effect on the extent of inverse polarization. This study is potentially important for various polarization-dependent devices and measurements.
An autocollimation-based method for measuring the refractive indices of solid or liquids using a Littrow prism is presented. Measurement accuracy is enhanced by use of a telescope. In solids, the refractive index is accurate to three decimal places. Similar accuracy is obtained in liquids by correcting for the wedge angle of the liquid container window. The proposed prismatic method confers high accuracy, compactness, and automation. It is suitable for index measurement applications in undergraduate laboratories.
In this paper we demonstrate a technique for improving the conversion efficiency in conventional silicon solar cells by using surface plasmon resonance (SPR) effects to harvest incident light energy over metal finger electrodes. According to three-dimensional finite-difference time-domain (3D-FDTD) analysis, incident light covering a broad bandwidth of the solar spectrum can be transmitted through metallic hole array structures. Although the light absorption region beneath the metal finger electrodes cannot generate a photocurrent, in this study, we employ the extraordinary transmission (EOT) phenomenon, due to SPR effects, to dramatically increase the degree of light harvesting below the metal electrodes and, thereby, improve the efficiency of the entire solar cell. Experimental data reveal that the excess photocurrent density was approximately 190% of the normal current density of a standard solar cell.Therefore, the negative effect of covering the absorption area with opaque metal finger electrodes can be minimized or eliminated completely by taking advantage of the SPR effect of the metal electrodes.
Broader contextThis study presents a useful technique to enhance photoelectric conversion efficiency of a conventional crystalline Si solar cell by using the surface plasma resonance (SPR) effect to harvest the incident light energy over the metal nger electrodes in a standard semiconductor process. According to the threedimensional nite-difference time-domain (3D-FDTD) simulations, we prepared samples featuring metallic hole array structures on their metal nger electrodes with specic diameters and periods to stimulate the SPR phenomenon within both the visible and NIR regimes. Experimentally, we found that the excess photocurrent density of the optimized device was 190% times the normal current density of a standard solar cell; normalized external quantum efficiency (EQE) measurements conrmed the enhanced efficiency. Therefore, the negative effect of covering the absorption area with an opaque metal nger electrode can be minimized by exploiting SPR effects on metal electrodes. In this study, using the SPR approach provided enhanced efficiency, not only breaking the tradeoff rule for metallic electrodes in conventional solar cells but also allowing additional harvesting of incident light energy over the surfaces of the metal nger electrodes.
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