Antireflection effects at nanostructured material interfaces and the suppression of thin-film interference

Antireflection Structures for VIS and NIR on Arbitrarily Shaped Fused Silica Substrates with Colloidal Polystyrene Nanosphere Lithography

Antireflective (AR) nanostructures offer an effective, broadband alternative to conventional AR coatings that could be used even under extreme conditions. In this publication, a possible fabrication process based on colloidal polystyrene (PS) nanosphere lithography for the fabrication of such AR structures on arbitrarily shaped fused silica substrates is presented and evaluated. Special emphasis is placed on the involved manufacturing steps in order to be able to produce tailored and effective structures. An improved Langmuir-Blodgett self-assembly lithography technique enabled the deposition of 200 nm PS spheres on curved surfaces, independent of shape or material-specific characteristics such as hydrophobicity. The AR structures were fabricated on planar fused silica wafers and aspherical planoconvex lenses. Broadband AR structures with losses (reflection + transmissive scattering) of <1% per surface in the spectral range of 750-2000 nm were produced. At the best performance level, losses were less than 0.5%, which corresponds to an improvement factor of 6.7 compared to unstructured reference substrates.

Measuring compound eye optics with microscope and microCT images

With a great variety of shapes and sizes, compound eye morphologies give insight into visual ecology, development, and evolution, and inspire novel engineering. In contrast to our own camera-type eyes, compound eyes reveal their resolution, sensitivity, and field of view externally, provided they have spherical curvature and orthogonal ommatidia. Non-spherical compound eyes with skewed ommatidia require measuring internal structures, such as with MicroCT (µCT). Thus far, there is no efficient tool to characterize compound eye optics, from either 2D or 3D data, automatically. Here we present two open-source programs: (1) the ommatidia detecting algorithm (ODA), which measures ommatidia count and diameter in 2D images, and (2) a µCT pipeline (ODA-3D), which calculates anatomical acuity, sensitivity, and field of view across the eye by applying the ODA to 3D data. We validate these algorithms on images, images of replicas, and µCT eye scans from ants, fruit flies, moths, and a bee.

Transparent Glass Surfaces with Silica Nanopillars for Radiative Cooling

The increasing global use of cooling systems and the need of reducing greenhouse effect are pushing the emergence of more efficient cooling methods. In particular, passive radiative cooling technology extracts heat from objects by tailoring their optical emissivity using surface micro- and nanostructuring. Being capable of increasing thermal emissivity is especially relevant for widespread glass structures and devices, e.g., displays, car and building windows, and solar cells. In this paper, we propose a scalable lithography-free nanostructuring method to increase the infrared (IR) emissivity of glass by reducing the high reflection associated with the SiO₂ Reststrahlen band around 9 μm wavelength. Furthermore, we show that with an additional thin polymer coating the scattering (haze) in the visible due to the deep nanostructures can be dramatically reduced while maintaining the large IR emissivity. We experimentally prove that our nanostructured surface can extract more heat via radiation emission than the bare glass substrate, while keeping full transparency.

Effect of wrinkles on extreme ultraviolet pellicle reflectivity and local critical dimension

Extreme ultraviolet (EUV) pellicles must have an EUV reflectance (EUVR) below 0.04% to prevent the reduction of critical dimension (CD). However, pellicle wrinkles cause localized CD variation by locally amplifying the EUVR. This study demonstrates that wrinkles can increase the pellicle's EUVR by approximately four times, and the CD drop depends on the relative position of the reflected light from the wrinkle to the 0th- or 1st-order diffracted light. The CD decreases by 6 nm. Therefore, even if the pellicle satisfies the requirement for the EUVR, we need to tightly control the generation of wrinkles to suppress CD variation during the entire exposure process.

Omnidirectional and broadband photon harvesting in self-organized Ge columnar nanovoids

Highly porous Germanium surfaces with uniformly distributed columnar nanovoid structures are fabricated over a large area (wafer scale) by large fluence Sn⁺irradiation through a thin silicon nitride layer. The latter represents a one-step highly reproducible approach with no material loss to strongly increase photon harvesting into a semiconductor active layer by exploiting the moth-eye antireflection effect. The ion implantation through the nitride cap layer allows fabricating porous nanostructures with high aspect ratio, which can be tailored by varying ion fluence. By comparing the reflectivity of nanoporous Ge films with a flat reference we demonstrate a strong and omnidirectional reduction in the optical reflectivity by a factor of 96% in the selected spectral regions around 960 nm and by a factor of 67.1% averaged over the broad spectral range from 350 to 1800 nm. Such highly anti-reflective nanostructured Ge films prepared over large-areas with a self-organized maskless approach have the potential to impact real world applications aiming at energy harvesting.

Enhanced power conversion efficiency of an n-Si/PEDOT:PSS hybrid solar cell using nanostructured silicon and gold nanoparticles

Herein, the effect of nanostructured silicon and gold nanoparticles (AuNPs) on the power conversion efficiency (PCE) of an n-type silicon/poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (n-Si/PEDOT:PSS) hybrid solar cell was investigated. The Si surface modified with different nanostructures including Si nanopyramids (SiNPs), Si nanoholes (SiNHs) and Si nanowires (SiNWs) was utilized to improve light trapping and photo-carrier collection. The highest power conversion efficiency (PCE) of 8.15% was obtained with the hybrid solar cell employing SiNWs, which is about 8%, 20% and 40% higher compared to the devices using SiNHs, SiNPs and planar Si, respectively. The enhancement is attributed to the low reflectance of the SiNW structures and large PEDOT:PSS/Si interfacial area. In addition, the influence of AuNPs on the hybrid solar cell's performance was examined. The PCE of the SiNW/PEDOT:PSS hybrid solar cell with 0.5 wt% AuNP is 8.89%, which is ca. 9% higher than that of the device without AuNPs (8.15%). This is attributed to the increase in the electrical conductivity and localized surface plasmon resonance of the AuNP-incorporated PEDOT:PSS coating layer.

Parallel Aligned Nickel Nanocone Arrays for Multiband Microwave Absorption

Magnetic nanostructures with conical shape are highly desired for pursuing extraordinary magnetic properties and microwave absorption. However, the fabrication of such nanostructures with controlled shape and size uniformities and alignment is not yet realized. Accordingly, the magnetic properties and their application as microwave absorber are not well understood. Here, we report on the first demonstration of controlled fabrication of soft magnetic nickel nanocone arrays with sharp geometry, large aspect ratio, uniform size, and parallel alignment. The imaginary part of the relative complex permeability shows multiband absorption in the 2-17 GHz range. Such an exceptional microwave absorption results from the uniform conical shape and size and the parallel alignment. The absorption mechanisms are discussed under the framework of natural resonance and exchange resonance. The natural resonance is dependent on the shape anisotropy and facilitated by the conical geometry. The exchange resonance is well explained by the observation of the bulk spin waves with exchange coupling at the tip of nanocones using the inelastic light scattering and is consistent with exchange theory predictions for the quantization of bulk spin waves. We expect that our work will shed light on the physical insights into the magnetic properties of nanocones and find great potential in applications of microwave absorption.

Enhanced Surface Properties of Light-Trapping Si Nanowires Using Synergetic Effects of Metal-Assisted and Anisotropic Chemical Etchings

Metal-assisted chemical etching (MACE) has been widely explored for developing silicon (Si)-based energy and optical devices with its benefits for low-cost and large-area fabrication of Si nanostructures of high aspect ratios. Surface structures and properties of Si nanostructures fabricated through MACE are significantly affected by experimental and environmental conditions of etchings. Herein, we showed that surfaces and interfacial energy states of fabricated Si nanowires can be critically affected by oxidants of MACE etching solutions. Surfaces of fabricated Si nanowires are porous and their tips are fully covered with lots of Si nano-sized grains. Strongly increased photoluminescence (PL) intensities, compared to that of the crystalline Si substrate, are observed for MACE-fabricated Si nanowires due to interfacial energy states of Si and SiO_x of Si nano-sized grains. These Si grains can be completely removed from the nanowires by an additional etching process of the anisotropic chemical etching (ACE) of Si to taper the nanowires and enhance light trapping of the nanowires. Compared with the MACE-fabricated Si nanowires, ACE-fabricated tapered Si nanowires have similar Raman and PL spectra to those of the crystalline Si substrate, indicating the successful removal of Si grains from the nanowire surfaces by the ACE process.

Mechanisms of 2π phase control in dielectric metasurface and transmission enhancement effect

Metasurfaces, two-dimensional structures composed of nanoantennas in an array configuration, can be used to fully control electromagnetic waves, which requires a 2π phase shift. Herein, we apply the silicon metasurface as an example to interpret the mechanisms of full 2π phase coverage. It is found that the mechanism varies from Fabry-Pérot resonance to Mie resonance as the period increases for a metasurface with certain height. Particularly, there is a transition region between these two types of resonance. We present the corresponding periods and wavelength regions of the different mechanisms when considering the phase-gradient metasurface with at most three diffraction orders. Moreover, the transmission enhancement of metasurface is investigated. The transmission efficiency can be effectively improved when the nanoantenna is changed from a uniform structure to a gradient-index one. We expect that the results can simplify the design process and provide a reference for the future design of all-dielectric metasurface with 2π phase control.

Magneto-optical effects in the Landau level manifold of 2D lattices with spin-orbit interaction

Silicene is a competitive and promising 2D material, possessing interesting topological, electronic and optical properties. The presence of strong spin orbit interaction in silicene and its analogues, germanene and tinene, leads to the opening of a gapin the energy spectrum and spin-splitting of the bands in each valley. Building upon prior work we discuss a general method to determine the magneto-optic response of silicene when a Gaussian beam is incident on silicene grown on a dielectric substrate in the presence of a static magnetic field. We use a semiclassical treatment to describe the Faraday rotation (FR) and Magneto-optical Kerr effect (MOKE). The response can be modulated both electrically and magnetically. We derive analytic expressions for valley and spin polarized FR and MOKE for arbitrary polarization of incident light in the terahertz regime. We demonstrate that large FR and MOKE can be achieved by tuning the electric field, magnetic fields and chemical potential in these fascinating 2D materials.

Biomimetic Omnidirectional Antireflective Glass via Direct Ultrafast Laser Nanostructuring

Here, a single-step, biomimetic approach for the realization of omnidirectional transparent antireflective glass is reported. In particular, it is shown that circularly polarized ultrashort laser pulses produce self-organized nanopillar structures on fused silica (SiO₂ ). The laser-induced nanostructures are selectively textured on the glass surface in order to mimic the spatial randomness, pillar-like morphology, as well as the remarkable antireflection properties found on the wings of the glasswing butterfly, Greta oto, and various Cicada species. The artificial structures exhibit impressive antireflective properties, both in the visible and infrared frequency ranges, which are remarkably stable over time. Accordingly, the laser-processed glass surfaces show reflectivity smaller than 1% for various angles of incidence in the visible spectrum for s-p linearly polarized configurations. However, in the near-infrared spectrum, the laser-textured glass shows higher transmittance compared to the pristine. It is envisaged that the current results will revolutionize the technology of antireflective transparent surfaces and impact numerous applications from glass displays to optoelectronic devices.

Realization of perfect selective absorber based on multipole modes in all-dielectric moth-eye structure

Perfect absorbers play crucial roles in optical functional devices. Among various types of absorbers, moth-eye structures are known for their excellent absorbing efficiency. In this paper, we apply an electromagnetic multipole expansion method to treat an isolated all-dielectric moth-eye structure as a large particle and calculate various electric and magnetic multipole modes within the moth-eye structure. In periodical array, the multipole modes within each particle interact with each other. These constructive or destructive interactions cause shifts in the multipole resonant peaks. The multipole modes inside the particle array introduce reflecting peaks for loss-less materials. The absorption enhancement inside moth-eye structures can be attributed to the electric field enhancement resulting from these electric and magnetic multipole modes. Based on our theoretical study, we propose a near-ideal selective absorber based on moth-eye array, which achieves near 100% absorption within wavelength range from 400 nm to 1500 nm while achieving near 0% absorption over about 1700 nm.

Ultra-broadband, polarization-independent, wide-angle absorption in impedance-matched metamaterials with anti-reflective moth-eye surfaces

We computationally study periodic impedance-matched metal-dielectric metamaterials and the advantage of imprinting moth-eye surfaces on them. Impedance-matched metamaterials are known to act as strong, polarization-independent, broadband absorbers. However, in the infrared region far from the metal's plasma frequency, the reflection from metal layers dominates over the absorption. Using anti-reflective moth-eye surfaces we show that it is possible to obtain absorption independent of polarization or incidence angle, over an exceptionally broad frequency range from 400 nm to 6 μm.

Diffraction efficiency performance of random anti-reflecting subwavelength surface structures on prefabricated fused silica binary gratings

Random anti-reflecting subwavelength surface structures have been reported to enhance transmission of optical windows and lenses. Specifically, for fused silica substrates, 99.9% specular transmission has been verified by various groups. Diffractive optical elements, such as gratings, also experience net Fresnel losses on both their planar and structured surfaces. We investigated the performance of prefabricated 50% duty-cycle, binary, fused silica linear gratings, with a period of 1.6 μm, before and after application of random anti-reflecting subwavelength surface structures, in order to reduce their initial Fresnel reflectivity. We compared the diffraction order directions and their efficiencies at three test wavelengths: 594, 612, and 633 nm, for both TE(s) and TM(p) incident light polarization states, under three different mountings: normal, first Bragg, and second Bragg incidence. We report transmission enhancement of the sum of all propagating grating orders for all cases tested by factors between 2% and 10%, with reduction of the respective reflected orders by similar ratios. Transmission enhancement of the -2 diffraction order at Bragg incidence suggests that the random etch has different rates between the raised and lowered linear grating topography.

Ultra Uniform Pb0.865La0.09(Zr0.65Ti0.35)O₃ Thin Films with Tunable Optical Properties Fabricated via Pulsed Laser Deposition

Ferroelectric thin films have been utilized in a wide range of electronic and optical applications, in which their morphologies and properties can be inherently tuned by a qualitative control during growth. In this work, we demonstrate the evolution of the Pb_0.865La_0.09(Zr_0.65Ti_0.35)O₃ (PLZT) thin films on MgO (200) with high uniformity and optimized optical property via the controls of the deposition temperatures and oxygen pressures. The perovskite phase can only be obtained at the deposition temperature above 700 °C and oxygen pressure over 50 Pa due to the improved crystallinity. Meanwhile, the surface morphologies gradually become smooth and compact owing to spontaneously increased nucleation sites with the elevated temperatures, and the crystallization of PLZT thin films also sensitively respond to the oxygen vacancies with the variation of oxygen pressures. Correspondingly, the refractive indices gradually develop with variations of the deposition temperatures and oxygen pressures resulted from the various slight loss, and the extinction coefficient for each sample is similarly near to zero due to the relatively smooth morphology. The resulting PLZT thin films exhibit the ferroelectricity, and the dielectric constant sensitively varies as a function of electric filed, which can be potentially applied in the electronic and optical applications.

Mechanical Strength and Broadband Transparency Improvement of Glass Wafers via Surface Nanostructures

In this study, we mechanically strengthened a borosilicate glass wafer by doubling its bending strength and simultaneously enhancing its transparency using surface nanostructures for different applications including sensors, displays and panels. A fabrication method that combines dry and wet etching is used for surface nanostructure fabrication. Specifically, we improved the bending strength of plain borosilicate glass by 96% using these surface nanostructures on both sides. Besides bending strength improvement, a limited optical transmittance enhancement of 3% was also observed in the visible light wavelength region (400–800 nm). Both strength and transparency were improved by using surface nanostructures of 500 nm depth on both sides of the borosilicate glass without affecting its bulk properties or the glass manufacturing process. Moreover, we observed comparatively smaller fragments during the breaking of the nanostructured glass, which is indicative of strengthening. The range for the nanostructure depth is defined for different applications with which improvements of the strength and transparency of borosilicate glass substrate are obtained.

Fine-Tunable Absorption of Uniformly Aligned Polyurea Thin Films for Optical Filters Using Sequentially Self-Limited Molecular Layer Deposition

Development of methods enabling the preparation of uniformly aligned polymer thin films at the molecular level is a prerequisite for realizing their optoelectronic characteristics as innovative materials; however, these methods often involve a compromise between scalability and accuracy. In this study, we have grown uniformly aligned polyurea thin films on a SiO2 substrate using molecular layer deposition (MLD) based on sequential and self-limiting surface reactions. By integrating plane-polarized Fourier-transform infrared, Raman spectroscopic tools, and density functional theory calculations, we demonstrated the uniform alignment of polyurea MLD films. Furthermore, the selective-wavelength absorption characteristics of thickness-controlled MLD films were investigated by integrating optical measurements and finite-difference time-domain simulations of reflection spectra, resulting from their thickness-dependent fine resonance with photons, which could be used as color filters in optoelectronics.

High-efficiency nanostructured silicon solar cells on a large scale realized through the suppression of recombination channels

Nanostructured silicon solar cells show great potential for new-generation photovoltaics due to their ability to approach ideal light-trapping. However, the nanofeatured morphology that brings about the optical benefits also introduces new recombination channels, and severe deterioration in the electrical performance even outweighs the gain in optics in most attempts. This Research News article aims to review the recent progress in the suppression of carrier recombination in silicon nanostructures, with the emphasis on the optimization of surface morphology and controllable nanostructure height and emitter doping concentration, as well as application of dielectric passivation coatings, providing design rules to realize high-efficiency nanostructured silicon solar cells on a large scale.

Antireflection-enhanced color by a natural graded refractive index (GRIN) structure

Nanostructured materials like graded refractive index (GRIN) structures in moth eyes have inspired the design of novel antireflective coatings. Such structures are more flexible than uniform coatings, but applications have been mainly limited to broadband antireflection in solar cells and LEDs. Here we show that cylindrical pigment granules in two bird species (Polyplectron bicalcaratum and Patagioenas fasciata) form a GRIN that suppresses interference and expands the range of colors produced by a multilayer. These results demonstrate that a GRIN structure can function like a pigment (i.e. through selective, independent wavelength blocking) to generate unique colors and may inspire the design of novel antireflective and structurally colored coatings.