Alex F. Kaplan

Structural Colors: From Plasmonic to Carbon Nanostructures

In addition to colorant-based pigmentation, structure is a major contributor to a materials color. In nature, structural color is often caused by the interaction of light with dielectric structures whose dimensions are on the order of visible-light wavelengths. Different optical interactions including multilayer interference, light scattering, the photonic crystal effect, and combinations thereof give rise to selective transmission or reflection of particular light wavelengths, which leads to the generation of structural color. Recent developments in nanofabrication of plasmonic and carbon nanostructures have opened another efficient way to control light properties at the subwavelength scale, including visible-light wavelength selection, which can produce structural color. In this Concept, the most relevant and representative achievements demonstrated over the last several years are presented and analyzed. These plasmonic and carbon nanostructures are believed to offer great potential for high-resolution color displays and spectral filtering applications.

High efficiency resonance-based spectrum filters with tunable transmission bandwidth fabricated using nanoimprint lithography

We propose a nanostructured color filter based on a metallic resonant waveguide structure capable of extremely high transmission efficiency. As an experimental demonstration, a blue and a red device were fabricated over a large area using nanoimprint lithography. Achieving transmission as high as 90% with a variable transmission bandwidth, these devices exhibit desirable features for numerous color filter applications.

Single-mode chirally-coupled-core fibers with larger than 50µm diameter cores

In this paper, we report an advance in increasing core size of effective single-mode chirally-coupled-core (CCC) Ge-doped and Yb-doped double-clad fibers into 55 µm to 60 µm range, and experimentally demonstrate their robust single-mode performance. Theoretical and numerical description of CCC fibers structures with multiple side cores and polygon-shaped central core is consistent with experimental results. Detailed experimental characterization of 55 µm-core CCC fibers based on spatially and spectrally resolved broadband measurements (S(2) technique) shows that modal performance of these large core fibers well exceeds that of standard 20 μm core step-index large mode area fibers.

Two dimensional silicon nanowalls for lithium ion batteries

One-dimensional (1-D) nanostructures such as nanowires and nanotubes have been widely explored for anodes with high specific capacity in Li-ion batteries, which effectively release the mechanical stress to avoid structure pulverization. However, 1-D nanostructures typically have a high surface area, which leads to a large irreversible capacity in the first cycle due to a solid electrolyte interface (SEI) formation. Two dimensional (2-D) nanowalls can address the same challenges as 1-D nanostructures, with a much lower surface area. For the first time, we demonstrated a 2-D nanowall structure with silicon for Li-ion batteries. Excellent performance for the first Coulombic efficiency (CE) has been achieved. Such a 2-D nanowall structure can also be applied in other devices with improved performance where nanostructures are needed but a high surface area is problematic.

Multilayer pattern transfer for plasmonic color filter applications

Contact printing involves transferring a material deposited on a prepatterned mold directly to a substrate with the application of uniform pressure and temperature. This process has traditionally been used to transfer metal layers to act as electrodes or masks for subsequent etch steps. In this work, the authors propose that devices with multiple layers, such as metal-insulator-metal (MIM) structures, can be transferred over using similar processing techniques. Using a SiO2 grating mold, the authors demonstrate the transfer of a MIM pattern to a flexible polycarbonate substrate in order to create a thin film, reflective color filter. This method could be optimized for roll-to-roll nanoimprint lithography and could be used to efficiently fabricate large-area structures on various substrates for display applications.

Finite element simulation of a perturbed axial-symmetric whispering-gallery mode and its use for intensity enhancement with a nanoparticle coupled to a microtoroid.

We present an optical mode solver for a whispering gallery resonator coupled to an adjacent arbitrary shaped nano-particle that breaks the axial symmetry of the resonator. Such a hybrid resonator-nanoparticle is similar to what was recently used for bio-detection and for field enhancement. We demonstrate our solver by parametrically studying a toroid-nanoplasmonic device and get the optimal nano-plasmonic size for maximal enhancement. We investigate cases near a plasmonic resonance as well as far from a plasmonic resonance. Unlike common plasmons that typically benefit from working near their resonance, here working far from plasmonic resonance provides comparable performance. This is because the plasmonic resonance enhancement is accompanied by cavity quality degradation through plasmonic absorption.

Plasma etch fabrication of 60:1 aspect ratio silicon nanogratings with 200 nm pitch

The authors present a breakthrough multistage dry-etch process to create 100 nm half-pitch gratings in silicon with depths up to 6 μm. Interference lithography was used to pattern gratings in an optically matched stack of materials to form a 400-nm-thick silicon oxide hard-mask. The oxide was then used to mask the subsequent deep reactive-ion etching of silicon. In this article, the authors describe their grating patterning, pattern transfer, and deep etch processes, and present progress toward combining this technique with coarser scale lithography steps designed to form an integrated mechanical support structure to produce freestanding x-ray diffraction gratings.

Subwavelength grating structures with magnetic resonances at visible frequencies fabricated by nanoimprint lithography for large area applications

Transmission of TM-polarized light through a subwavelength metal-dielectric grating structure exhibits strong resonance in the visible range. Simulations by finite-difference time-domain and finite-element methods show that the resonance can be attributed to the magnetic response. Further simulation shows that the grating structure can be optimized by adding an index matching dielectric layer to produce negative refractive index response in the visible band. Easily fabricated using nanoimprint lithography and conveniently able to be excited by incident light normal to the fabrication plane, such metal-dielectric grating structure could find potential use in large area negative refractive index applications.

Critical-angle transmission grating spectrometer for high-resolution soft x-ray spectroscopy on the International X-ray Observatory

High-resolution spectroscopy at energies below 1 keV covers the lines of C, N, O, Ne and Fe ions, and is central to studies of the Interstellar Medium, the Warm Hot Intergalactic Medium, warm absorption and outflows in Active Galactic Nuclei, coronal emission from stars, etc. The large collecting area, long focal length, and 5 arcsecond half power diameter telescope point-spread function of the International X-ray Observatory will present unprecedented opportunity for a grating spectrometer to address these areas at the forefront of astronomy and astrophysics. We present the current status of a transmission grating spectrometer based on recently developed high-efficiency critical-angle transmission (CAT) gratings that combine the traditional advantages of blazed reflection and transmission gratings. The optical design places light-weight grating arrays close to the telescope mirrors, which maximizes dispersion distance and thus spectral resolution and minimizes demands on mirror performance. It merges features from the Chandra High Energy Transmission Grating Spectrometer and the XMM-Newton Reflection Grating Spectrometer, and provides resolving power R = E/ΔE = 3000 - 5000 (full width half max) and effective area >1000 cm2 in the soft x-ray band. We discuss recent results on ray-tracing and optimization of the optical design, instrument configuration studies, and grating fabrication.

Characterization of single-mode performance of Chirally-Coupled-Core fibers with cores larger than 50μm

We demonstrate robust single-spatial mode performance in fabricated Ge-doped 50μm -60μm core Chirally-Coupled-Core fibers using spatially and spectrally resolved (S²) measurements.