Broadband Antireflection Properties Research Articles

Quantitative analysis of gradient effective refractive index in silicon nanowires for broadband light trapping and anti-reflective properties

Controlling the light reflection using vertically-aligned nanowires has great importance in fundamental research with interesting applications in photonic devices. Here, we discuss the spatial- and polarization-dependent reflectivity measurements from the cross-sectional as well as from the top surface of vertically-aligned disordered silicon nanowires. The gradient variation in an effective refractive index along the nanowire length is estimated using the cross-sectional reflectivity measurements. We have studied the gradient variation of an effective refractive index profile and its tunability with the nanowire length. The reflectivity is measured to be as low as 5% irrespective of spatial directions and the polarization of incident light in a broad wavelength range. This constitutes the signature of a broadband omnidirectional anti-reflector that is scalable with the nanowire length. The reflectivity measurements are in good agreement with theoretical calculations. Such omnidirectional anti-reflection in a broad wavelength range is useful for applications such as photon management in photovoltaic devices and disorder-induced light scattering.

Optical properties and fabrication of dielectric metasurfaces based on amorphous silicon nanodisk arrays.

Dielectric metasurfaces based on amorphous silicon (a-Si) nanodisks are interesting for nanophotonic applications due to the high refractive index and mature/low temperature fabrication of a-Si. The investigated metasurfaces consist of a-Si nanodisk arrays embedded in a transparent film. The diameter-dependent optical properties of the nanodisk Mie resonators have been investigated by finite-difference time-domain (FDTD) simulations and spectrally-resolved reflectivity and transmission measurements. Well-ordered substrate-free a-Si nanodisk arrays were fabricated and characterized with regard to their broadband anti-reflection properties when placed on a crystalline silicon (c-Si) surface, and reflectivity/ transmission properties when embedded in a polydimethylsiloxane (PDMS) film. Our results confirm broadband anti-reflection when placed on silicon, while the optical characteristics of the nanodisks embedded in PDMS are shown to be potentially useful for color/NIR filter applications as well as for coloring on the micro/nanoscale.

Design and Fabrication of Moth-Eye Subwavelength Structure with a Waist on Silicon for Broadband and Wide-Angle Anti-Reflection Property

Reflection loss on the optical component surface is detrimental to performance. Several researchers have discovered that the eyes of moths are covered with micro- and nanostructured films that reduce broadband and wide-angle light reflection. This research proposes a new type of moth-eye subwavelength structure with a waist, which is equivalent to a gradient refractive index film layer with high–low–high hyperbolic-type fill factor distribution. The diffraction order characteristics of a moth-eye subwavelength structure are first analyzed using a rigorous coupled wave analysis. The moth-eye structural parameters are optimized within the spectral range of 2–5 μm using the finite-difference time-domain method. The experimental fabrication of the moth-eye structure with a waist array upon a silicon substrate is demonstrated by using three-beam laser interferometric lithography and an inductively coupled plasma process. The experimental and simulation results show good agreement. The experimental results show that the reflectivity of the moth-eye structure with a waist is less than 1.3% when the incidence angle is less than 30°, and less than 4% when the incidence angle is less than 60°. This research can guide the development of AR broadband optical components and wide-angle applications.

Self-Erasable Nanocone Antireflection Films Based on the Shape Memory Effect of Polyvinyl Alcohol (PVA) Polymers.

Arrays of nanostructure that are capable of broadband antireflection and light trapping properties are implemented in photovoltaic and photonic devices. However, most of the existing antireflection films have been hindered by a complicated fabricated method and structurally rigid. Here, we report a simple preparation method for self-erasable nanocone antireflection films using the surface replication method. Arrays of nanocone with sub-100 nm surface features could be replicated easily on the shape memory polyvinyl alcohol (PVA) film, and are erased by thermal stimulation. The reflectivity of self-erasable antireflection film can be switched from the 4.5% to 0.6% averaged over the visible spectral range by controlling the temperature below or above 80 °C. Theoretical simulations have been demonstrated. The unique smart film is expected to be used to further extend the application of smart optical windows and digital screens.

Broadband antireflective coating stack based on mesoporous silica by acid-catalyzed sol-gel method for concentrated photovoltaic application

Silica multi-layer stacks have been designed with the aim to provide broadband antireflective (AR) properties for glass components in concentrated photovoltaic (CPV) application. Silica porous coatings were grown by combining acid-catalyzed sol-gel route and evaporation induced self-assembly (EISA) method with four types of organic/inorganic systems. Sols were prepared using tetraethylorthosilicate (TEOS) as inorganic precursor assembled with two di-block copolymers, one tri-block copolymer and one cationic surfactant as organic templates. Optical properties were characterized by ellipsometry and spectrophotometry while the material structure was analyzed by environmental ellipsometric porosimetry (EEP) and atomic force microscopy (AFM). The concentration of inorganic and organic phases was optimized and a broadband AR bi-layer stack was obtained providing a 7.2% (under the reference AM1.5 solar spectral irradiance) increase in transmittance over bare glass in the wavelength range 300–2000 nm when coated on both sides.

Broadband antireflection property of conformally grown zinc tin oxide thin films on nanorippled- and nanofaceted-Si substrates

In this paper, we show the broadband antireflection property of radio frequency sputter-grown zinc tin oxide (ZTO) thin films grown on ion-beam fabricated self-organized nanoscale rippled- (R-Si) and faceted-Si (F-Si) substrates. For the sake of comparison, ZTO film has been deposited simultaneously on pristine-Si (P-Si) substrate as well. X-ray diffraction studies show the amorphous nature of the films. Atomic force microscopic studies reveal the conformal growth of ZTO thin films (granular in nature) on both R- and F-Si substrates (up to 60 nm). Surface reflectance measurements show a large reduction in the average broadband (300–800 nm) surface reflectance (at 550 nm) from 43.9% for P-Si substrate to 42% and 32% for R-Si and F-Si substrate, respectively. In addition, we show a further reduction in the surface reflectance following the growth of ZTO overlayers (in the range of 15–60 nm) on P-Si, R-Si, and F-Si substrates. As a matter of fact, there is a continuous reduction in surface reflectance up to a thickness of 60 nm, beyond which the reflectance minimum starts increasing. The large reduction in the surface reflectance of ZTO coated P-, R-, and F-Si substrates are attributed to the graded refractive index of the heterostructures driven by the surface morphology of the substrates and the different average surface heights of the films. This study will be very useful for efficient ZTO-based optoelectronic devices and heterojunction solar cell applications.

Surface-Nanostructured Al–AlN Composite Thin Films with Excellent Broad-Band Antireflection Properties Fabricated by Limited Reactive Sputtering

Al–AlN composite thin films with surface nanostructures possessing excellent broad-band antireflection properties can be simply fabricated by using a simple sputtering-based method, namely, limited reactive sputtering at elevated temperature. A mixture of Ar and N2 gases with limited gas flow of N2 is used. During the process, the limited amount of the N ions and radicals can only consume some of the sputtered Al atoms for the formation of AlN, so that the Al–AlN composite thin film can finally be deposited on the substrate. Compared to the traditional cosputtering for composite deposition, which works necessarily with two or more targets simultaneously, here only a single Al target is required. When the substrate temperature is higher than 100 °C, surface nanostructures can be evolved because of the different diffusion kinetics of Al and AlN. The total reflection was reduced drastically (<10%) in the spectral range 300–2000 nm. The Al–AlN composite thin films exhibit also a good electric conductivity.

Growth mechanism of one-step self-masking reactive-ion-etching (RIE) broadband antireflective and superhydrophilic structures induced by metal nanodots on fused silica.

This work presents a low-cost, simple, convenient, advanced technology to prepare large-area defect-free subwavelength structures (SWSs). SWSs were obtained by a metal-induced one-step self-masking RIE process on a fused-silica surface, in which metal-fluoride (mainly ferrous-fluoride) nanodots were used to induce and gather stable fluorocarbon polymer etching inhibitors in the RIE polymers as masks. The SWS growth processes are visible with an increase in etching time and some exhibit prominent broadband antireflective properties from the visible to the near-infrared wavelength range. Transmission in the 600-900-nm range increased from approximately 93% for the polished fused silica to above 99% for the double-side SWSs on fused silica. A theoretical simulation by a finite-difference time-domain method agreed well with the experiments. Moreover, the surface of the SWSs exhibits excellent superhydrophilic properties.

Broadband antireflection property of silicon nanocone arrays with porous sidewalls fabricated by Ag-catalyzed etching

Siliconnanocone (SiNC) arrays with porous sidewallswere successfully fabricated through a simple, low-cost Ag-catalyzed etching method. By electron spin resonance technique and TEM analysis, it has been verified that the formation of porous SiNC arrays is due to the gradual dissolution of the pristine Ag nanoislands as they sank into the Si and the lateral etching of the regenerated Ag nanoparticles to the SiNC sidewalls. Theoretical calculation results suggest that the reflectance of the porous SiNC arrays is much lower than that of the smooth SiNC arrays over wide wavelengths ranging from 300nm to 1700nm. The long-wavelength reflectance can be further suppressed by increasing surface porosity of the SiNCs and their length. Experimental test results show the lowest average reflectance of 1.7% in the wavelength range of 300-1000nm while approximately 30% in the wavelength ranges of 1200-1700nm, which is generally consistent with the theoretical results. This shows that the porous SiNC arrays had excellent broadband antireflection properties, making them attractive for a wide range of potential applications in Si-based optoelectronic devices.

Rational Design and Construction of Well-Organized Macro-Mesoporous SiO2/TiO2 Nanostructure toward Robust High-Performance Self-Cleaning Antireflective Thin Films

Antireflection (AR) thin films on optical substrates are of great significance in high-performance optoelectronic devices. Here, we present a rational design and construction of well-organized macro-mesoporous nanostructure toward robust high-performance self-cleaning antireflective thin films on the basis of effective medium theory and finite difference time domain (FDTD) simulations that combine the optical design principle. A hierarchical macro-mesoporous SiO2 thin film with very high porosity and gradient refractive indexes works as a λ/4-wavelength AR layer and significantly suppresses the reflection in the range from 350 to 1200 nm. Even after dip-coating a layer of high refractive index TiO2 nanocrystals, the nanostructured thin film still exhibits broadband AR properties which are much superior to conventional flat SiO2/TiO2 thin films, especially in the range of 350-500 nm. In addition, the obtained thin film exhibits photocatalytic self-cleaning and durable superhydrophilicity. The advantages brought by the well-organized macro-mesoporous structure are also testified through comparing to the solely mesoporous SiO2/TiO2 film counterpart. Moreover, the pencil hardness test and sandpaper abrasion test show favorable robustness and functional durability of the thin film, which make it extremely attractive for practical applications in optical devices, display devices, and photovoltaic cells.

Sol-gel derived single-layer zeolite-based coatings on glass for broadband antireflection properties

In this study, a simple and effective sol-gel method was used to generate antireflection coatings on borosilicate glass substrates. Commercially available zeolite 4Å powder and tetraethyl orthosilicate were used as the starting materials to synthesize the sol. Single layer antireflection coatings were generated on borosilicate glass substrates using dip coating technique and densified in air at 450°C. Optical properties were analyzed using UV–Vis-NIR transmittance/reflectance spectrophotometer and variable angle spectroscopic ellipsometer. Mechanical properties were analyzed using adhesion, pencil scratch hardness testing as well as using crockmeter tests. The antireflection coatings were found to exhibit excellent broad band transmittance over the visible wavelength range. An average transmittance of 96.8±1% could be obtained for the wavelength range 400–800nm. The reflectance of the bare borosilicate glass substrate was found to decrease from 8.6±0.1% to 2.5±0.05% after deposition of the antireflection coating. The scratch hardness of the coatings was determined to be 6H and adhesion tests showed the adhesion to be of rank 5B. The transmittance loss was 0.9±0.1% after 1000cycles of crockmeter test using a cotton fabric as the abrading medium. The single layer zeolite-based antireflection coatings were found to exhibit good mechanical properties and could be used for a variety of applications.

Optimization and continuous fabrication of moth-eye nanostructure array on flexible polyethylene terephthalate substrate towards broadband antireflection.

Reflection loss can cause harmful effects on the performance of optoelectronic devices, such as cell phones, notebooks, displays, solar cells, and light-emitting diode (LED) devices. In order to obtain broadband antireflection (AR) properties, many researchers have utilized surface texture techniques to produce AR subwavelength structures on the interfaces. Among the AR subwavelength structures, the moth-eye nanostructure is one of the most promising structures, with the potential for commercialization in the near future. In this research, to obtain broadband AR performance, the optimization of moth-eye nanostructures was first carried out using the finite difference time domain method within the spectral ranges of 400-800nm, including the optimization of shape, height, pitch, and residual layer thickness. In addition, the continuous production of moth-eye nanostructure array upon a flexible polyethylene terephthalate substrate was demonstrated by using the roll-to-roll ultraviolet nanoimprint lithography (R2R UV-NIL) process and anodic aluminum oxide mold, which provided a solution for the cost-effective fabrication of moth-eye nanostructure array. The AR performance of moth-eye nanostructure array obtained by the R2R UV-NIL process was also investigated experimentally, and good consistence was shown with the simulated results. This research can provide a beneficial direction for the optimization and cost-effective production of the moth-eye nanostructure array.

Fabrication of PEDOT Nanocone Arrays with Electrochemically Modulated Broadband Antireflective Properties.

Ordered nanocone arrays of the electroactive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) were fabricated by the simultaneous oxygen plasma etching of an electrodeposited PEDOT thin film coated with a hexagonally closed packed polystyrene bead monolayer. PEDOT nanocone arrays with an intercone spacing of 200 nm and an average nanocone height of 350 nm exhibited a low broadband reflectivity of <1.5% from 550 to 800 nm. Electrochemical modulation of the oxidation state of the PEDOT nanocone array film was used to change both its ex situ absorption spectrum (electrochromism) and reflection spectrum (electroreflectivity). The sign of the PEDOT nanocone array electroreflectivity was opposite to that observed from unmodified PEDOT thin films; this significant difference is attributed to the unique optical behavior of nanostructured surfaces with an interfacial layer that contains a graded mix of air and highly absorptive nanocones. The combined electrochromic and electroreflective behavior of the antireflective PEDOT nanocone array films should find promising applications in solar energy cells, sensors and other optical devices.

Self-Templated Fabrication of Robust Moth-Eye-Like Nanostructures with Broadband and Quasi-Omnidirectional Antireflection Properties

Antireflection coatings (ARCs) have been realized by “bottom-up” depositing self-templated moth-eye patterns on glass substrates. The substrates were dip-coated by a layer of silica nanoparticles and further textured into patterns of nanodomes by precursor-derived one-step assembly (POA) of a mesoporous silica layer. Typically, the compact stacked nanodomes were about 260 nm in width and about 90 nm in height, under which was a layer comprising the lower halves of silica nanoparticles and mesoporous silica layer. The protuberant nanodomes with quarter-wavelength thickness and smoothly varying refraction index make significant contributions to the realization of broadband and quasi-omnidirectional antireflection. The mesoporous silica layer grown on the surface of silica nanoparticles and substrate through chemical bonding endows the coating with excellent structural integrity, thus, guaranteeing the high mechanical durability of the coating. The optimized coating reduces the double-sided reflection of gla...

Fabrication of high aspect ratio broadband antireflection porous nano-network on glass using candle soot as a sacrificial layer for solar energy utilization

In this paper, we report the fabrication of broadband antireflection porous nano-network on the glass substrate using the combination of candle soot and HF-based vapor phase etching method. Candle soot layer plays a key role to control the pore size during the etching process. Field emission electron microscopy results showed that the pores have the tapering profile, and the pores size was restricted to the sub-wavelength dimension. Therefore, an excellent broadband antireflection with an enhancement of ∼7% in the maximum total transmittance as compared to plain glass has been achieved. Moreover, reflectance from the etched surface remains quite low (&amp;lt;4%) over a broad range of incident angles up to 58°. The broadband antireflection property was further examined by finite difference time domain simulation. Further, we demonstrate a relative improvement of ∼6% in the Jsc (ΔJsc/Jsc) of solar cell covered with the etched glass.

Lithography-free wide-angle antireflective self-cleaning silicon nanocones.

Low-cost, wideband, and wide-angle antireflective layers are of prime importance to photovoltaic and other optoelectronic applications. We report a novel fabrication methodology of random textured silicon nanocones (SiNCs) array through metal-assisted chemical etching combined with oxidation. The optical properties of the fabricated structure are studied theoretically and experimentally. The random textured SiNCs array showed very promising broadband antireflective properties through the entire visible wavelength range at different incident angles up to ±60°. In addition, the nanostructures inherently could become self-cleaning due to the high contact angle. This random cheap textured SiNCs array increases the absorption efficiency of photodetectors and reduces its cost.

Broadband and wide angle anti-reflective nanoporous surface on poly (ethylene terephthalate) substrate using a single step plasma etching for applications in flexible electronics

Poly (ethylene terephthalate) (PET) substrate is a widely used polymer in the optoelectronics industry, its optical properties, such as light transmittance, haze, etc., are of significant importance in organic light emitting diodes and photovoltaics. In this paper, we present a broadband anti-reflective nanostructure on PET substrate prepared by means of plasma etching in a mixture of argon and oxygen gas. Etching time was varied for optimizing the optical transmittance. Field emission scanning electron microscopy images revealed formation of nanostructures of sub-wavelength dimensions resembling pores at the surface. Atomic force microscopy confirmed that the graded porosity was generated from the randomly distributed pores of different sizes and depths. Due to the formation of a gradient refractive index, nanostructured surface exhibited broadband and quasi-omnidirectional anti-reflection properties. A maximum total transmittance of ~99% at 835nm was achieved for both sides treated PET without significant increase in light scattering or haze. Moreover, both sides treated PET showed polarization insensitivity upto 48°, which was not exhibited by untreated PET. X-ray photoelectron spectroscopy revealed generation of degradation products, which consisted of carbon and oxygen containing functional groups. Durability of the anti-reflection layer was tested and proved to be satisfactory.

Fabrication of broadband antireflective black metal surfaces with ultra-light-trapping structures by picosecond laser texturing and chemical fluorination

A hybrid method consisting of ultrafast laser-assisted texturing and chemical fluorination treatment was applied for efficiently enhancing the surface broadband antireflection to fabricate black titanium alloy surface with ultra-light-trapping micro-nanostructure. Based on the theoretical analysis of surface antireflective principle of micro-nanostructures and fluoride film, the ultra-light-trapping micro-nanostructures have been processed using a picosecond pulsed ultrafast laser on titanium alloy surfaces. Then fluorination treatment has been performed by using fluoroalkyl silane solution. According to X-ray diffraction phase analysis of the surface compositions and measurement of the surface reflectance using spectrophotometer, the broadband antireflective properties of titanium alloy surface with micro-nano structural characteristics were investigated before and after fluorination treatment. The results show that the surface morphology of micro-nanostructures processed by picosecond laser has significant effects on the antireflection of light waves to reduce the surface reflectance, which can be further reduced using chemical fluorination treatment. The high antireflection of over 98 % in a broad spectral range from ultraviolet to infrared on the surface of metal material has been achieved for the surface structures, and the broadband antireflective black metal surfaces with an extremely low reflectance of ultra-light-trapping structures have been obtained in the wavelength range from ultraviolet–visible to near-infrared, middle-wave infrared. The average reflectance of microgroove groups structured surface reaches as low as 2.43 % over a broad wavelength range from 200 to 2600 nm. It indicates that the hybrid method comprising of picosecond laser texturing and chemical fluorination can effectively induce the broadband antireflective black metal surface. This method has a potential application for fabricating antireflective surface used to improve the photothermal or photoelectric conversion efficiency of solar installations and enhancing the surface wave-absorbing capacity of devices.

Fabrication of hierarchical anti-reflective structures using polystyrene sphere lithography on an as-cut p-Si substrate

The broadband anti-reflective (AR) properties of hierarchical structures (HSs) have attracted considerable attention in recent years as a means to reduce Fresnel reflection in photovoltaic solar cell materials. This study employed polystyrene sphere lithography in conjunction with high density plasma dry etching in the fabrication of pure sub-wavelength structures and HSs on an as-cut p-Si substrate. Etching parameters, such as RF power, O2, and etching time, were adjusted to alter the surface morphology. Experiment results demonstrate that the resulting hierarchical paraboloidal structures suppress average reflectance to below 0.5% across a spectral range of 500–1000nm.

Quasiperiodic air hole arrays for broadband and omnidirectional suppression of reflection

Surfaces patterned with quasi-periodic array of sub-wavelength air holes have been studied for their effectiveness in suppressing air-substrate reflection in the wavelength range of 450–1350 nm. Superlattice structures formed by superposing two different quasiperiodic arrays with 450 nm deep holes showed reflectance of ∼2% (compared to 6% for unpatterned substrate) for all measured incidence angles up to 50° and also show very weak polarization dependence. Dense k-space of quasiperiodic array along with the graded index offered by tapered holes provide broadband, polarization independent, and omnidirectional antireflection property.