Broadband Antireflection Research Articles

Three-layer tri-wavelength broadband antireflective coatings built from refractive indices controlled silica thin films

A three-layer tri-wavelength broadband antireflective (AR) coating with 100 % transmittance simultaneously at 1064, 532 and 355 nm was designed using TFCalc™ thin film design software. The refractive indices for the bottom, middle and top layers of the designed AR coating are 1.33, 1.21 and 1.10, respectively. To realize the designed AR coating, a template-free sol–gel process was proposed to regulate the refractive index of silica film from 1.09 to 1.44 by simple hybridization. Silica films with refractive index of 1.21–1.44 were prepared by the hybridization of acid-catalyzed silica and base-catalyzed silica, while organically modified silicates films with refractive index of 1.09–1.21 were prepared by the hybridization of tetraethoxysilane, methyltriethoxysilane and hexamethyldisilazane under base-catalyzed condition. These two hybridization methods could conveniently control the porosity, and hence the refractive index of films. Finally, the three-layer tri-wavelength broadband AR coating, which is very important in high-power laser systems, was realized with the transmittance of 99.9, 99.8 and 99.4 % at 1064, 532 and 355 nm, respectively. A three-layer tri-wavelength broadband antireflection coating with nearly 100 % transmittance simultaneously at 1064, 532 and 355 nm is designed by TFCalc™ thin film design software and prepared by a simple sol–gel process. A template-free sol–gel process was proposed to prepare silica films with controlled and ultra-low refractive index.

Nanoporosity-induced superhydrophobicity and large antireflection in InSb

A porous nanostructure evolves in InSb due to keV ion implantation which leads to superhydrophobic and large antireflective property, indicating a single-step facile fabrication to introduce both functionalities. In particular, it is observed that the contact angle of a water droplet on the nanoporous InSb surface exceeds 150°, revealing the transition to a superhydrophobic surface. Correlation between the contact angle and the porous nanostructures is qualitatively understood in light of the Cassie-Baxter model. It is found that a decrease in the fraction of solid surface wetted by the water droplet and a corresponding increase in the air-water interface fraction lead to the enhancement in the hydrophobicity. We further observe that the large broadband antireflection (in the range of 200–800 nm) is also correlated to the nanoporous structure, arising out of a large reduction in the refractive index due to its increasing porosity. Such a surface with the combination of superhydrophobicity and large antireflection can be very useful for applications of InSb nanostructures in electronic, photonic devices, or infrared detectors.

Millimeter-wave broadband antireflection coatings using laser ablation of subwavelength structures.

We report on the first use of laser ablation to make submillimeter, broadband, antireflection coatings (ARCs) based on subwavelength structures (SWSs) on alumina and sapphire. We used a 515 nm laser to produce pyramid-shaped structures with a pitch of about 320 μm and a total height of near 800 μm. Transmission measurements between 70 and 140 GHz are in agreement with simulations using electromagnetic propagation software. The simulations indicate that SWS-ARCs with the fabricated shape should have a fractional bandwidth response of Δν/νcenter=0.55 centered on 235 GHz for which reflections are below 3%. Extension of the bandwidth to both lower and higher frequencies, between a few tens of gigahertz and a few terahertz, should be straightforward with appropriate adjustment of laser ablation parameters.

SiO2–ORMOSIL double-layered broadband antireflective coating for high-power laser system

Broadband antireflective (AR) coating for high-power laser system has been realized by depositing double-layered SiO2–ORMOSIL thin films onto fused silica substrates. The refractive indices of bottom and top layers are 1.29 and 1.13, respectively. The top layer was composed of an ORMOSIL thin film which was prepared by a template-free sol–gel process by hybridization of tetraethoxysilane (TEOS) and dimethyldiethoxysilane (DDS). The hybridization of TEOS and DDS decreased gradually the refractive indices of ORMOSIL thin films from 1.21 to 1.13 as the molar ratio of DDS to TEOS increased from 0.0 to 0.8. The bottom layer was prepared by hybridization of acid-catalyzed and base-catalyzed silica sols. Finally, the double-layer broadband AR coating with transmittance of 100.0 and 99.9 % at 1064 and 532 nm, respectively, was realized. The laser-induced damage threshold (LIDT) of double-layer double-wavelength AR coating is 48.8 J cm−2. This double-layer SiO2–ORMOSIL AR coating possessed excellent transmittance at simultaneously 1064 and 532 nm, good AR stability and high LIDT, and will find important application in high-power laser system.

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.

Broadband and wide-angle hybrid antireflection coatings prepared by combining interference multilayers with subwavelength structures

To reduce the intensity of the Fresnel reflections of optical components, subwavelength structures prepared by reactive ion etching of SiO2 thin films were combined as the outermost layer with a multilayer system made of conventional thin-film materials. A hybrid coating was thus realized, with the nanoscaled structured outermost layer expected to further improve the antireflection properties of common interference stacks. The microscopic and optical spectroscopic analysis of the subwavelength structures revealed that pillar-shaped nanostructures formed during etching exhibit low-refractive-index properties and have a depth-dependent refractive index. To take into account the refractive-index gradient in the coating design, the optical properties of the nanostructures were modeled using the effective-medium approximation. The calculated average effective refractive index turned out to be 1.11 at 500-nm wavelength. A hybrid coating was designed to minimize the residual reflectance in the 400-nm to 900-nm spectral range for BK7 glass substrate. Experimental results demonstrated that the hybrid-coating approach yields a low residual reflectance with very good omnidirectional properties, owing to the properties of the nanostructured surface.

In Situ Surface Assembly Derived Ultralow Refractive Index MgF2–SiO2 Hybrid Film for Tri‐Layer Broadband Antireflective Coating

For the broadband antireflective (AR) coating working in specific wavelengths, the precise control over refractive index and film thickness of each layer is critical to guarantee the peak position locating at the targeted wavelengths. In this paper, a simple sol–gel procedure without post‐treatment is used to prepare MgF2–SiO2 hybrid coating with tunable refractive index for the fabrication of tri‐layer tri‐wavelength broadband AR coating. The MgF2–SiO2 coating with ultralow refractive index of 1.12 is realized through in situ surface assembly of negatively charged silanol groups on vesicle‐like MgF2 particles to obtain porous crosslinking structure. The electrostatic attraction between the negatively charged silanol groups and positively charged MgF2 colloidal particles plays a key role. Furthermore, the MgF2–SiO2 coating with a wide range of refractive index between 1.20 and 1.44 is achieved by adjusting the addition amount of acid‐catalyzed silica sol into the vesicle‐like MgF2 sol. According to the theoretical design, a tri‐layer broadband AR coating with refractive index model of 1.12/1.26/1.36 is prepared, which can enhance the transmittance of quartz substrate to near 100% simultaneously at 351, 527, and 1053 nm.

Broadband Antireflection Subwavelength Structures on Fused Silica Using Lower Temperatures Normal Atmosphere Thermal Dewetted Au Nanopatterns

The antireflection (AR) performance and far-field transmittance, depending on the morphology and parameters of subwavelength structures (SWSs), have been investigated by a 3-D finite-difference time-domain (FDTD) method. It is found that far-field transmission characteristics are dominated mainly by period of SWSs. If the period is increased above 220 nm, the far-field transmittance rapidly decreases for a short-wavelength region. An effective and inexpensive method for fabricating disordered SWSs on fused silica has been demonstrated in this paper. An Au nanoscale island mask was formed by lower temperatures thermal dewetted at normal atmosphere. Then, a disordered SWS was fabricated by reactive ion etching (RIE). The average diameter of the Au nanoscale island was easily controlled by the thickness of Au thin films and annealing temperature. The etched depth and shape of fused silica SWSs depend additionally on RIE duration, thus achieving efficient AR characteristics. The measured data and calculated results obtained by the FDTD method exhibit reasonably similar tendencies. The optimized single-side fused silica SWS that is tapered on the tips leads to a significantly high transmissivity value of 96.2%. In addition, it exhibits a broadband AR property at a wavelength range of 400–1100 nm.

Broadband antireflection for a high-index substrate using SiNx/SiO2 by inductively coupled plasma chemical vapour deposition

This paper presents the development of broadband antireflection coating for a high-index substrate such as Si using SiNx/SiO2 by inductively coupled plasma chemical vapour deposition (ICP-CVD). The thin-film design employs a simulated annealing method for a minimal average reflectance over the wavelength range and incidence angles involved, which gives the optimized refractive index and thickness of each layer of the thin-film stack under different layer numbers. Using ICP-CVD, the SiNx material system is optimized by tuning the SiH4/N2 gas ratio. The corresponding thin-film characterization shows the precise refractive index/film thickness control in deposition, and the deposited film also has a low absorption coefficient and smooth surface. The double-layer SiNx/SiO2 coating with the optimized refractive index and thickness for broadband antireflection is demonstrated experimentally. The average reflectance of the Si surface is reduced from ~32% to ~3.17% at normal incidence for a wavelength range from 400 to 1100 nm.

Perforating domed plasmonic films for broadband and omnidirectional antireflection.

Domed Ag nano-hole/disk array films exhibit a reflectivity of less than 0.7% over a wide spectral range (400-1000 nm) and even lower values down to 0.05% with an oblique incidence angle; this unique optical response is attributed to three key factors: diffractive scattering loss on nanostructures, localized plasmonic absorption and curved surface (domed units).

Искусственно созданная дисперсия в полностью диэлектрических градиентных наноструктурах: частотно-избирательные поверхности раздела и туннельные широкополосные просветляющие покрытия

Искусственно созданная дисперсия в полностью диэлектрических градиентных наноструктурах: частотно-избирательные поверхности раздела и туннельные широкополосные просветляющие покрытия

Facile synthesis of a superhydrophobic and colossal broadband antireflective nanoporous GaSb surface

Ion implantation creates a superhydrophobic and colossal antireflective nanoporous GaSb.

Optimization of the spectral performance of an antireflection coating on a micro-spherical substrate

To improve the optical performance of an antireflection (AR) coating on a micro-spherical substrate, the ray angle of the incidence distribution and the thickness profile are taken into consideration during the optical coating design. For a convex spherical substrate with a radius of curvature of 10 mm and a clear aperture of 10 mm, three strategies are used for the optimization of the spectral performance of a broadband AR coating in the spectral region from 480 to 720 nm. By comparing the calculated residual reflectance and spectral uniformity, the developed method demonstrates its superiority in spectral performance optimization of an AR coating on a micro-spherical substrate.

Growth kinetics and surface properties of single-crystalline aluminum-doped zinc oxide nanowires on silicon substrates

We present here the results from a systematic investigation on the growth kinetics and surface properties of Al-doped ZnO (AZO) nanowires synthesized on (001)Si substrates under different hydrothermal conditions. The as-synthesized vertical AZO nanowires exhibited a hydrophilic characteristic and their crystal structures were determined to be perfectly single crystalline with the axis of the wire parallel to the [0001] direction. TEM and EDS results revealed that the as-synthesized AZO nanowires have tapered tips, and the Al-doped concentration in the AZO nanowires was about 1.6at%. After a series of SEM examinations, the average length of AZO nanowires synthesized at each temperature studied was found to follow a linear relationship with the reaction time, indicating that the hydrothermal growth of AZO nanowires was a reaction-controlled process. The activation energy for linear growth of AZO nanowires on Si substrate, as obtained from an Arrhenius plot, was found to be about 46kJ/mol. From UV–vis spectroscopic measurements, it was found that the Si substrate coated with vertically-aligned AZO nanowire arrays exhibited remarkably reduced reflectance (10–12%) over a wide range of visible wavelengths (400–800nm) and angles of light incidence (8–60°). The good broadband and omnidirectional antireflection characteristics can be attributed to the light trapping effect and the graded refractive index resulting from the tapered AZO nanowire structures.

Pulsed laser deposition of refractive-index-graded broadband antireflection coatings for silicon solar cells

This paper reports the design, fabrication and characterization of broadband antireflection coatings for silicon-based solar cells, using a graded-index technique and a novel dual-beam pulsed laser deposition method. A graded refractive index was realized by mixing two different optical materials, silicon and soda-lime glass, in a way that the composition of the mixture changes gradually from silicon (that of the solar cell) to glass (that of the outside protective glass) along the coating thickness direction following pre-designed refractive-index profiles. In this study, two different refractive-index profiles were fabricated, linear and Southwell profiles, on silicon substrates. In order to emulate a protective cover glass for solar-cells, coatings with an additional 400nm-thick outer glass layer were also fabricated. The antireflection coatings showed reflectance of 2.2–5.5% (mostly around 4%) for a wavelength range of 400–1000nm, and for the Southwell profile with a top glass layer, the reflectance was found to be between 2.2% and 4%, which is believed to be close to the theoretically best antireflection performance of a solar cell with a cover glass. Finite-difference time-domain simulations were also performed and the numerical results were in line with the measurement results. Because these coatings are based on silicon and glass and can be fabricated by the one-step pulsed laser deposition procedure, it can simplify the solar-cell structure considerably. The fabricated antireflection coatings may be also used for many other optical applications, such as thin-film optical attenuator and some optical sensors, and the presented pulsed laser deposition method is capable of fabricating a variety of high-performance optical coatings, which cannot be fabricated by other methods.

The Broadband Anti-reflection Coated Extended Hemispherical Silicon Lenses for Polarbear-2 Experiment

Polarbear-2 (PB-2) is a next-generation receiver that is part of the Simons Array cosmic microwave background (CMB) polarization experiment which is located in the Atacama desert in Northern Chile. The primary scientific goals of the Simons Array are a deep search for the CMB B-mode signature of gravitational waves from inflation and the characterization of large-scale structure using its effect on CMB polarization. The PB-2 receiver will deploy with 1897 dual-polarization sinuous antenna-coupled pixels, each with a directly contacting extended hemispherical silicon lens. Every pixel has dual polarization sensitivity in two spectral bands centered at 95 and 150 GHz, for a total of 7588 transition edge sensor bolometers operating at 270 mK. To achieve the PB-2 detector requirements, we developed a broadband anti-reflection (AR) coating for the extended hemispherical lenses that uses two molds to apply two layers of epoxy, Stycast 1090 and Stycast 2850FT. Our measurements of the absorption loss from the AR coating on a flat surface at cryogenic temperatures show less than 1 % absorption, and the coating has survived multiple thermal cycles. We can control the diameter of the coating within 25 \({\upmu }\)m and translation errors are within 25 \({\upmu }\)m in all directions, which results in less than 1 % decrease in transmittance. We also find the performance of the AR-coated lens matches very well with simulations.

Wide angle light collection with ultralow reflection and super scattering by silicon micro-nanostructures for thin crystalline silicon solar cell applications

Conventional c-Si solar cells employ micron-sized pyramids for achieving reduced reflection (∼10%) and enhanced light trapping by multiple bounces (maximum 3) of the incident light. Alternatively, bio-mimetic, moth-eye sub-wavelength nanostructures offer broadband antireflection properties (∼3%) suitable for solar cell applications in the optical regime. However, such structures do not provide any advantage in the charge carrier extraction process as radial junctions cannot be formed in such sub-wavelength dimensions and they have high surface area causing increased charged carrier recombination. The choice of the geometry for achieving optimum photon–electron harvesting for solar applications is therefore very critical. Cross-fertilization of the conventional solar cell light-trapping techniques and the sub-wavelength nanostructures results in unique micro-nanostructures (structures having sub-wavelength dimensions as well as dimensions of the order of few microns) which provide advanced light management capabilities along with the ability of realizing radial junctions. It is seen that an ultralow reflection along with wide angle light collection is obtained which enables such structures to overcome the morning, evening and winter light losses in solar cells. Further, super-scattering in the structures offer enhanced light trapping not only in the structure itself but also in the substrate housing the structure. Ray and wave optics have been used to understand the optical benefits of the structures. It is seen that the aspect ratio of the structures plays the most significant role for achieving such light management capabilities, and efficiencies as high as 12% can be attained. Experiments have been carried out to fabricate a unique micro-nanomaze-like structure instead of a periodic array of micro-nanostructures with the help of nanosphere lithography and the MacEtch technique. It is seen that randomized micro-nanomaze geometry offers very good antireflection properties (∼1%) which are close to the expected optical behaviour of the periodic array at a much lower surface area.

Terahertz filter integrated with a subwavelength structured antireflection coating

Micro-pyramid shaped subwavelength structures (SWSs) were integrated on both sides of a terahertz (THz) filter by means of stamping methods. Two silicon-based stamping molds fabricated via crystallographic wet etching were utilized to replicate SWSs onto cyclo-olefin copolymer (COC) films coated onto both sides of a THz filter at the same time. The SWSs act as an broadband antireflection coating to reduce the surface reflection loss in a frequency range of 0.2 THz to 1.4 THz. Compared to a THz filter without SWSs, the filter integrated with double-sided SWSs exhibits a low standing wave ratio inside the substrate and THz signal transmission enhancement of up to 10.8%.

Low temperature and large-scale growth of ZnO nanoneedle arrays with enhanced optical and surface-enhanced Raman scattering properties

Large-scale ZnO nanoneedle arrays have been grown on four-inch silicon wafers with ZnO seed film by an aqueous chemical growth method at a low growth temperature and short reaction time. The volumes of 1,3 diaminopropane play an important role in controlling the dimension and optical emission properties of ZnO nanoneedle arrays, which exhibit a very prominent green emission and weak UV emission from defect and band gaps in the cathodoluminescence spectrum, respectively. The ZnO nanoneedle arrays with large alignment variations display broadband and omnidirectional antireflection properties from the gradual index profile, and can provide a higher surface-to-volume ratio and stronger defect emission, which results in a peak photocatalytic performance at a light irradiation of 10W UV. The appropriate Ag sputtering durations on the ZnO nanoneedle arrays have been optimized to yield the greatest surface-enhanced Raman scattering effect in the rhodamine 6G molecule. The ZnO/Ag composite arrays provide a facile, high enhancement, low detection limit and low cost fabrication, which shall be of significant value for practical applications of other SERS sensing systems.

Broadband antireflection coating for optimized terahertz beam splitters

We investigate the potential of anti-ferromagnetic nanofilms as broadband antireflection coatings in the terahertz frequency range. The anti-ferromagnetic layer is modeled by an analytic wave-impedance matching approach. The experimental results of the transmission and reflection measurements demonstrate the effectiveness of our antireflection coatings. Furthermore, we use anti-ferromagnetic nanofilms as antireflection coating for a terahertz beam splitter. Compared with conventional terahertz beam splitters consisting of an uncoated thick silicon wafer, the coated silicon beam splitter has two advantages: elimination of multiple reflections and improvement of the signal-to-noise ratio for terahertz time-domain spectroscopy in reflection geometry.