Broadband Antireflection Research Articles

Fabrication and properties of broadband antireflective coatings on inert perfluoropolymer films treated by inductively coupled oxygen plasma

Fluorinated ethylene propylene (FEP) film is an attractive candidate for mitigating target debris in high-power laser systems due to its remarkable advantages such as low cost, low absorption, and high damage threshold. However, the inert surface with bad wettability presents an enormous challenge to realize optical antireflection. In this Letter, we experimentally demonstrate that broadband antireflection of FEP film can be achieved through combining oxygen plasma treatment and solgel coating techniques. By optimizing the plasma treating time and withdrawal rate during the coating process, the treated FEP film has 4.7% enhanced transmittance compared to the untreated sample. In this case, transmittance over 99% with wide wave bands ranging from 600 to 950nm is achieved. The mechanism of broadband antireflection was revealed by investigating the fluorocarbon groups on the FEP surface. The applicable wave band of antireflective FEP film can be designed at will by changing plasma-treating conditions, which could open up a new avenue in the field of laser debris mitigation.

Performance enhancement techniques for the front and back of nanostructured “black silicon” solar cells

Nanostructured black silicon (bSi) exhibits a broadband antireflection (AR) response due to graded-index and scattering effects, unlike traditional quarter-wavelength dielectric AR coatings. We present various techniques to improve the front- and back-surface performance of nanostructured bSi solar cells. Ammonium dihydrogen phosphate (ADP) is used for proximity doping to reduce the physical impact on the bSi nanostructures during front-surface emitter formation. An optimum concentration of 2 wt. % of ADP is found to result in a typical solar cell emitter sheet resistivity of 50 Ω / sq. Potassium hydroxide is used to etch off the highly doped region of the bSi solar cell front emitter, which results in lower surface recombination and up to a 23% increase in short wavelength (400 to 600 nm) internal quantum efficiency of the bSi solar cell. To reduce the series resistance and enhance surface passivation, forming gas anneal is employed, improving bSi cell’s overall efficiency by over 31%. By optimizing the back-surface-field formed by sputtered aluminum (Al), the backside recombination rate is reduced, improving external quantum efficiency by up to 11% in the long wavelength (>900 nm) region.

Antireflective and self-cleaning glass with robust moth-eye surface nanostructures for photovoltaic utilization

Omnidirectional and broadband antireflection (AR) self-cleaning coatings are crucial for the performance enhancement of photovoltaic (PV) modules. Herein, we demonstrate biomimetic moth-eye structures on glass through inductively coupled plasma (ICP) etching process using thermal dewetted copper nanoparticles as the masks. Moreover, the etched glasses are chemically treated to strengthen the nanostructures. The moth-eye glass exhibits excellent AR effects with a reflectance below 3% over the wavelength range from 570 to 950 nm. As the incident angles of the light increasing from 0o to 60o, the relative improvement of the power conversion efficiency (PCE) of PV module with moth-eye glass produces a dramatically increases from 4.6% to 9.9%, compared with that of PV modules with flat glass. Additionally, the moth-eye glass also possesses superhydrophilic self-cleaning and antifogging abilities, which are attractive for outdoor applications.

All-Dielectric anti-reflective metasurface at visible frequencies

We analytically study the broadband antireflection of a square array of dielectric pillars with a glass substrate. The presented condition exploits the optical response of the nanoparticles to achieve complete cancellation of the reflected field by means of destructive interference.

Application of uracil for the preparation of low-index nanostructured layers

Nanostructured layers generated by plasma etching immediately after the evaporation process can exhibit an effective refractive index down to approximately 1.15. uracil, a nucleobase derived from a pyrimidine chemical structure, has been identified to form suitable bump structures in a self-organized way. It is assumed that the molecule’s ability to form aggregates plays an essential role in initiating the structure formation. A nanostructured uracil layer has been used as the final layer of an antireflection coating to demonstrate broadband and wide-angle antireflection performance.

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.

Fabrication of Broadband Antireflective Sub-Micro Structures on 4H-SiC by Mesh Patterning Etching

An approach to fabricating pseudoperiodic antireflective subwavelength structures on silicon carbide by using modified resist as etching mask is demonstrated. The etched submicron structure is a mesh-trenched pattern thus as the so called “mesh patterning etching”. The mesh patterning process is more time-efficient than the e-beam lithography or nanoimprint lithography process. The influences of the reactive-ion etching conditions and post bake temperature of the resist film to the sub-micron structure profile and its corresponding surface reflectance have been systematically investigated. Under optimal experimental conditions, the average reflectance of the silicon carbide in the range of 390-800 nm is dramatically suppressed from 40% to around 8% after introducing the mesh patterning sub-micro structures.

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.

1.6:1 bandwidth two-layer antireflection structure for silicon matched to the 190-310 GHz atmospheric window.

Although high-resistivity, low-loss silicon is an excellent material for terahertz transmission optics, its high refractive index necessitates an antireflection treatment. We fabricated a wide-bandwidth, two-layer antireflection treatment by cutting subwavelength structures into the silicon surface using multi-depth deep reactive-ion etching (DRIE). A wafer with this treatment on both sides has <-20 dB (<1%) reflectance over 187-317GHz at a 15° angle of incidence in TE polarization. We also demonstrated that bonding wafers introduce no reflection features above the -20 dB level (also in TE at 15°), reproducing previous work. Together these developments immediately enable construction of wide-bandwidth silicon vacuum windows and represent two important steps toward gradient-index silicon optics with integral broadband antireflection treatment.

Origin and compensation of deposition errors in a broadband antireflection coating prepared using quartz crystal monitoring

To improve the performance of TiO2/SiO2 broadband antireflection (BBAR) coating prepared using ion assisted deposition, a hybrid approach was proposed to probe, deduce, and compensate for the thickness errors resulting from quartz crystal monitoring. Using transmission electron microscopy (TEM), a thickness-dependent relative error was found in the prepared BBAR coating. Thinner layer had larger relative error and this was the main reason for the degradation of the performance of the BBAR coating. By fitting the thickness errors via a linear model, the origin of the thickness-dependent relative error is determined to be a constant thickness offset of the quartz crystal monitor. The thickness offsets are 1.5 nm and 2.7 nm for TiO2 and SiO2, respectively. Finally, an approach through reverse engineering of a 4-layer coating was proposed to obtain the offsets for both materials. The results achieved are comparable to that obtained using TEM, but this approach is much more convenient. After compensating these offsets, a BBAR coating was prepared with transmittance higher than 99% from 450 to 900 nm.

Pushing the Limits of Broadband and High-Frequency Metamaterial Silicon Antireflection Coatings

Broadband refractive optics realized from high index materials provide compelling design solutions for the next generation of observatories for the Cosmic Microwave Background (CMB), and for sub-millimeter astronomy. In this paper, work is presented which extends the state of the art in silicon lenses with metamaterial antireflection (AR) coatings towards larger bandwidth and higher frequency operation. Examples presented include octave bandwidth coatings with less than $0.5\%$ reflection, a prototype 4:1 bandwidth coating, and a coating optimized for 1.4 THz. For these coatings the detailed design, fabrication and testing processes are described as well as the inherent performance trade offs.

Design and fabrication of PbTe/BaF2 hydrophobic high-efficiency broad-band antireflection coating on Ge substrate in long-wave infrared region

Hydrophobic high efficiency broad-band antireflection (BBAR) coating in long-wave infrared (LWIR) region (7.5–11 µm) was designed and fabricated on germanium substrate by PVD technique. The multi-layer coating materials used in the design were Lead Telluride (PbTe) as high index material and Barium Fluoride (BaF2) as low index material. Hydrophobic high-efficiency BBAR coating with an average water contact angle of 102° was obtained only by the intrinsic surface roughness of BaF2 layer and without any surface treatment. AFM and FTIR spectrophotometer were used to investigate the surface topography and average reflection of the multi-layer coating, respectively. The average roughness obtained about 13 nm. The average reflection of the hydrophobic high-efficiency BBAR coating was 0.72% in the wavelength range from 7.5 to 11 µm that leads to eliminate Narcissus effect. Infrared optical windows and domes, having hydrophobic BBAR coatings, can effectively be applicable to humid or marine environments.

Direct nanoimprinting of moth-eye structures in chalcogenide glass for broadband antireflection in the mid-infrared

Fresnel reflection at the boundary between two media of differing refractive indices is a major contributing factor to the overall loss in mid-infrared optical systems based on high-index materials such as chalcogenide glasses. In this paper, we present a study of broadband antireflective moth-eye structures directly nanoimprinted on the surfaces of arsenic triselenide (As2Se3)-based optical windows. Using rigorous coupled-wave analysis, we identify a relief design optimized for high transmittance ( 12%) in the 5.9–7.3 μm spectral range as well as improved omnidirectional properties. Finally, we demonstrate the adaptability of nanoimprinted surface reliefs by tailoring the nanostructure pitch and height, achieving both extremely broadband antireflective and highly efficient antireflective surface reliefs. The results and methods presented herein provide an efficient and scalable solution for improving the transmission of bulk optics, waveguides, and photonic devices in the mid-infrared.

Colossal Broad Band Antireflection from Conformally Grown Cu2O on Pyramidally Textured Si and its Photovoltaic Applications

In this paper, we show the efficacy of a conformally grown, self‐organized Cu2O overlayer on chemically synthesized, pyramidally textured silicon substrate for a colossal suppression of optical reflection loss. In particular, we demonstrate that p‐Cu2O film deposited on a chemically textured n‐Si substrate shows an ultra‐low average surface reflectance (≈0.25%) over the entire spectral range (300–2400 nm), whereas the same turns out to be ≈25% for the film deposited on an as‐obtained n‐type Si substrate. It is observed that in the former case, Cu2O grows in the form of leaf‐like nanostructures, which is attributed to be responsible for such a low reflectance. In addition, under both configurations, the p‐Cu2O/n‐Si heterostructure shows a non‐linear current–voltage characteristics, clearly suggesting the formation of barrier junctions. Further, it is found that a solar cell fabricated with p‐Cu2O overlayer grown on a pyramidally textured n‐Si substrate shows an eight times higher efficiency compared to the one grown on an as‐obtained n‐Si substrate. This study will pave the way to design cost‐effective and efficient copper oxide‐based solar cells.

Fabrication of robust, self-cleaning, broadband TiO2[sbnd]SiO2 double-layer antireflective coatings with closed-pore structure through a surface sol-gel process

Multifunctional antireflective (AR) coatings are widely used in fields of optical devices, flat-panel displays, windows and solar cell devices. In this paper, silica (SiO2) and titanium dioxide (TiO2) nanomaterials were used as building blocks to construct multifunctional AR coatings. The surface of the SiO2 coating derived from SiO2 nanoparticles consists of a large number of nanopores, and its refractive index is tuned conveniently from 1.19 to 1.45 by controlled incorporation of TiO2 into these nanopores through a surface sol-gel process. By tuning the coating structure, substrates with double-layer TiO2SiO2 coatings presented a maxium transmittance of 98.4% and average transmittance as high as 97.7% over the visible and near infrared region. The nanocomposite TiO2SiO2 coatings demonstrated a good hydrophilicity, favorable robustness and high photocatalytic activity. In addition, the double-layer TiO2SiO2 coatings had a long-term durability after accelerated-aging test, while the porous SiO2 coatings with a great of open nanopores showed a severe decrease. This double-layer TiO2SiO2 coatings have the benefit of designing, fabricating, and developing multifunctional AR coatings with broadband transmittance, high photocatalytic activity and high environmental stability, which makes them extremely appealing for applications in lenses, solar photovoltaic cells, etc.. Moreover, this work not only provides an alternative way to tune the refractive indices of AR coatings, but also opens up a new avenue towards broadband AR coatings.

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.

Broadband Antireflection Coatings Based on Low Surface Energy/Refractive Index Silica/Fluorinated Polymer Nanocomposites

We demonstrated the fabrication of broadband antireflection coatings (ARCs) comprising low-surface energy/refractive index (RI) silica/polymer nanocomposites by silica mineralization of layer-by-layer (LbL) assembled poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2,2,3,3-tetrafluoropropyl methacrylate)/poly(L-glutamic acid) (PDMA-b-PTFP/PGA) multilayer films without any post-treatments. The introduction of the fluorinated polymer (PTFP segments) effectively lowered not only the RI of the as-fabricated coatings but also the surface energy of the constituted pore surface, which rendered the ARCs with high transmittance and durable AR performance by preventing the absorption and capillary condensation of moisture at ambient conditions. Moreover, the formation of nanosized PDMA-b-PTFP vesicles can render the ARCs exhibiting small pore size, which can improve their light transmittance. The coated substrate with an average transmittance over 97.0% was obtained at the visible wavelength region. The combina...

Characterization of sol-gel ORMOSIL antireflective coatings from phenyltriethoxysilane and tetraethoxysilane: Microstructure control and application

Chemical structure, microstructure and properties of organically modified silicate (ORMOSIL) antireflective (AR) coatings from phenyltriethoxysilane (PTES) and tetraethoxysilane (TEOS) were characterized. The reaction between PTES and TEOS was verified by means of FTIR, XPS, and 29Si NMR. N2 adsorption-desorption analysis and TEM were utilized to investigate the pore structure and microstructure control of ORMOSIL AR coatings. It was revealed that, by increasing PTES/TEOS molar ratio, the porosity of ORMOSIL AR coating decreased and then the refractive index increased gradually from 1.21 to 1.27. The hydrophobicity of ORMOSIL AR coatings was also improved significantly by increasing the water contact angle from 21.2° to 112°. Most importantly, the OIH mechanism between PTES and TEOS and hence the microstructure control of ORMOSIL AR coatings were induced in detail based on a deep discussion of results from 29Si MAS NMR, N2 adsorption-desorption analysis and TEM characterization. Finally, the feasibility of application of PTES/TEOS ORMOSIL thin films in preparation of double-layer broadband AR coatings was verified.

In situ formation of artificial moth-eye structure by spontaneous nano-phase separation

Unprecedented in situ formation of artificial moth-eye structure is demonstrated by spontaneous nano-phase separation of a silica-based system on substrate. The moth-eye thin film with a homogenously distributed nipples array shows broadband antireflection functionalities. The mechanism of nano-phase separation is unveiled as spinodal decomposition by chemical freezing method and thermodynamic analysis. The current method may provide a new avenue to ready fabrication of patterned nanostructures toward a variety of applications.