Ultralow Refractive Index Research Articles

Synthesis of hydrophobic silica without traditional aging process to construct the anti-reflective film with ultralow refractive index for improving power conversion efficiency

The fabrication of anti-reflective films to optimize sunlight utilization encounters various challenges. Herein, we have synthesized modified silica with hydrophobic properties, eliminating the conventional aging process to create an anti-reflective film with an ultralow refractive index of 1.05, marking the lowest value reported to date. This innovative methodology reduces tasks that typically require days or weeks to just a few hours, thereby significantly accelerating production. The power conversion efficiency of the photovoltaic device can increase by 6.2 % when coated with this film. This research not only advances the synthesis of silica but also develops anti-reflective films with record-low refractive indices. This breakthrough represents a substantial contribution, highlighting its significant impact on the photovoltaic field.

Multilayer anti-reflective coating with ultra-low refractive index SiO2 nanopillars for high efficiency multi-junction GaAs solar cells

The anti-reflective coating (ARC) is an essential component in inverted metamorphic triple-junction gallium arsenide (GaAs) solar cells (IMM-SCs) to improve the light absorption and efficiency. The common approach to realize AR function is to construct a multilayer thin-film with a gradient refractive index (n) change from air to the solar cell. In this study, SiO2 nanopillars (SiO2-NPs) are implemented as the top layer in multilayer thin-films to enhance their broadband anti-reflection capability. The transfer matrix method and electron beam (oblique angle) deposition process are applied for the design and fabrication of the multilayer thin-films, respectively. By optimizing the thickness and n of SiO2-NPs thin-films, an IMM-SC with a SiO2-NPs/Al2O3/TiO2 ARC is achieved with an average reflectivity as low as 5.506% in the 300–1300 nm range. The proposed ARCs have significant implications for the window layer designs of high-performance multi-junction solar cells.

Influence of polyethylene content on preparation and properties of lead zirconate titanate piezoelectric ceramics via vat photopolymerization

Vat photopolymerization has garnered significant research interest due to its capabilities in achieving complicated designed structures and high resolution. However, there are still significant technical difficulties in manufacturing some piezoceramic parts because of the high refractive index and ultraviolet light absorptivity of these ceramic powders. Herein, we overcome this challenge for lead zirconate titanate (PZT) ceramics by introducing polyethylene (PE) additions with ultralow refractive index and UV light absorptivity into PZT slurries. As the PE content increases, the curing depth shows a clear upward trend. When the PE content is 30 vol%, the curing depth of PZT slurry could ensure the adequate interlayer combination. Besides, PZT ceramics with different volumes of porosity could be obtained by modulating PE content. The relative density of sintered PZT ceramics decreased from 83.12 % to 68.06 %. Furthermore, the samples with 30 vol% PE exhibited satisfactory dielectric and piezoelectric properties (ϵr =1911, tanδ=0.027, d33 = 308 pC/N, g33 = 1.82 × 10−2 Vm/N and d33g33= 5.61 × 10−12 m2/N). These results indicate that the addition of PE additives is an effective and universal approach to improve the curing depth of ceramic slurries and expand the range of processable materials for vat photopolymerization technologies.

Design of a novel D-shaped SPR sensor with high sensitivity based on PCF for ultra-low refractive index detection

In this paper, a novel D-shaped plasmon resonance sensor based on photonic crystal fiber (PCF) is proposed to detect analytical materials with low refractive index. There are three arc-shaped small holes on both upper and lower edges of the fiber core, which can enhance the surface plasmon effect. The operation wavelength of the sensor is from 2243 nm to 2651 nm when its measurable refractive index range is between 1.17 and 1.27. The maximum spectral sensitivity of the sensor can be reached to 20 300 nm RIU−1, and the optimal resolution is 4.93 × 10−6 RIU. In our knowledge, the current literature indicates that when the refractive index is less than 1.3, this is the sensor with the highest spectral sensitivity among the reported low refractive index PCF-surface plasmon resonance sensors.

Developing core-shell silica nanoparticles to create multifunctional coating with excellent antireflection, enhanced antifogging and anticorrosion performances

How to fabricate a multifunctional coating with controllable ultralow refractive index (RI), high antifogging and anticorrosion performances by a facile method is still one of the great challenges. In this work, a simple and time-saving approach involving a microwave-assisted sol-sel process was developed to fabricate a novel core-shell structured silica nanoparticle sol by co-condensation of perfluorodecyltriethoxysilane (F-DTOS) and tetraethoxysilane (TEOS) and self-condensation of F-DTOS. Results suggest that both the co-condensations and self-condensations not only give a layer of low surface energy –C10H4F17 groups on the silica particles but also enlarge the hybrid silica particle size, providing more and larger voids to form the high porosity. After spinning the silica nanoparticles on the substrate, the obtained optical coating exhibits the controllable ultralow refractive index (RI) and outstanding transparency. The minimum RI and maximum transmittance of our developed coating is up to ~1.04 and ~100 %, respectively. Moreover, because of the high porosity and the presence of low surface energy –C10H4F17 groups in the coating, the coating has the high hydrophobicity (water contact angle ~108.8°), which not only makes it excellent antifogging performance but also affords it high-efficient anticorrosion property. After immersion in a 3.5 wt% NaCl solution for 7 days, the Rct value of the coating could be higher than 7.3 × 104 Ω cm2. It is believed that our developed method used to fabricate such multifunctional coating does not need any costly equipments, complex steps and time-consuming, making it well suited for industrial application.

Nanoporous Silicon Dioxide Films for Large Area and Low-Cost Fabrication of Ultra-Low Refractive Index Coatings

Nanoporous Silicon Dioxide Films for Large Area and Low-Cost Fabrication of Ultra-Low Refractive Index Coatings

Low polarization-sensitive ultra-broadband anti-reflection coatings with improved reliability.

Broader spectra, lower reflectivity and higher reliability are the performance requirements for broadband antireflective (BBAR) films. In this work, a BBAR film structure was proposed, which maintains extremely low reflectivity, ultra-wide spectra, low polarization sensitivity and practical reliability. The BBAR film consists of a dense multilayer interference stack on the bottom and a nano-grass-like alumina (NGLA) layer with a gradient low refractive index distribution on the top. The film was deposited by atomic layer deposition, while the NGLA layer was formed by means of a hot water bath on Al2O3 layer. The top NGLA layer has extremely high porosity and ultra-low refractive index, along with extremely fragile structure. To surmount the fragility of NGLA layer, a sub-nano layer of SiO2 was grown by atomic layer deposition to solidify its structure and also to adjust the refractive index with different thicknesses of SiO2. Finally, in the wide wavelength range of 400-1100 nm, the average transmittance of the double-sided coated fused quartz reaches 99.2%. The absorption, light scattering, reliability and polarization characteristics of BBAR films were investigated. An optimized BBAR film with low polarization-sensitivity and improved reliability was realized, which should be potentially promising for application in optical systems.

3D printing of lead zirconate titanate piezoelectric ceramics via digital light processing (DLP)

Research of piezoelectric ceramics via DLP technology has drawn considerable interest due to its high geometrical flexibility. However, the poor curing properties limit the use of DLP technology to fabricate piezoelectric ceramics. Herein, we overcome this challenge in prepared lead zirconate titanate (PZT) piezoelectric ceramics via DLP technology by incorporating ultralow refractive index and ultraviolet light absorptivity of soluble starch in PZT ceramic slurry. After the post-process, the relative density of printed PZT ceramic with 20 vol% soluble starch could reach about 89.51% of theoretical density. The printed PZT ceramic exhibits remarkable dielectric permittivity, remnant polarization, piezoelectric strain constant, piezoelectric voltage constant, figure of merit and electromechanical coupling factor of 2196, 35.56 μC/cm2, 373 pC/N, 1.92 × 10−2 Vm/N, 7.16 × 10−12 m2/N and 42.28%, respectively. The results achieved in this research indicate that our work provides a promising route for fabrication of advanced ceramics with high performance and designed complex structures by digital light processing.

Fabrication of a porous SiO2 thin film with an ultralow refractive index for anti-reflective coatings

Anti-reflective (AR) coatings are used in various optical products, such as lenses, filters, and transparent conductive films. High-performance multilayer AR coatings can be achieved by reducing the refractive index of the top layer. Preventing light scattering by forming finely structured film is also important for a high-performance antireflection film. In this study, porous SiO2 thin films with ultralow refractive indices were prepared using a simple sol–gel method. A high-quality porous SiO2 thin film with an ultralow refractive index of 1.17 bearing a fine porous structure was fabricated by carefully selecting the solvent and base species. Observation and simulation of the film structure revealed the factors responsible for the low scattering of light and the ultralow refractive index exhibited by the film. Hydrophobic treatment of the film was conducted to reduce the fluctuation of optical performance caused by changes in the refractive index owing to the water adsorbed. Therefore, porous SiO2 thin films can be used in the lenses and filters of various optical instruments.Graphical abstractA transparent film with low refractive index was obtained. The film has porous structure, consisting of silica particles of a few nm in size. The pores size was found to be a few nm by image analysis. Thin films with low refractive index and low scattering for optics were successfully prepared by a simple sol–gel method.

Hollow silica nanospheres synthesized by one-step etching method to construct optical coatings with controllable ultra-low refractive index

HypothesisIt is generally believed that silica nanospheres deposited on substrates by sol-gel method to form coatings with low refractive index, if hollow structures are introduced into silica nanospheres, the obtained coatings can easily have an ultra-low refractive index (<1.15). However, with the currently reported techniques, facile and large-scale preparation of hollow silica nanospheres (HSNs) with diameters suitable for transparent optical coatings in a one-step method is hard to realize. Therefore, it is crucial to propose a one-step strategy to prepare HSNs colloids, and also to achieve controllable morphologies. ExperimentalStöber silica nanospheres were used as initial nanospheres, and diameters of nanospheres were controlled by the concentration of ammonia. Nanospheres of different diameters were etched using different concentrations of HF, and the structural transformation of nanospheres before and after etching was recorded. Etched nanospheres were deposited into coatings, refractive index was measured and appropriate coatings were selected to design and prepare a double-layer coating with broadband antireflection performance. FindingsExperiments indicated that nanospheres of different sizes exhibited different structural transformations after being etched by HF, and smaller nanospheres were aggregated and larger ones were hollowed out. It was the first time that HSNs have been prepared by one-step etching method. The refractive index of etched nanospheres was 1.387–1.057. The double-layer broadband antireflective coating designed and prepared using coatings with refractive index of 1.057 and 1.339 as upper and bottom layers respectively had an average transmittance of 99.5% in the visible region and exhibited a good antifogging property and weather durability.

Highly Sensitive Plasmonic Biosensors with Precise Phase Singularity Coupling on the Metastructures.

In this paper, we demonstrated the ability of a plasmonic metasensor to detect ultra-low refractive index changes (in the order of ∆n = 10−10 RIU), using an innovative phase-change material, vanadium dioxide (VO2), as the sensing layer. Different from current cumbersome plasmonic biosensing setups based on optical-phase-singularity measurement, our phase signal detection is based on the direct measurement of the phase-related lateral position shift (Goos–Hänchen) at the sensing interface. The high sensitivity (1.393 × 108 μm/RIU for ∆n = 10−10 RIU), based on the Goos–Hänchen lateral shift of the reflected wave, becomes significant when the sensor is excited at resonance, due to the near-zero reflectivity dip, which corresponds to the absolute dark point (lower than 10−6). GH shifts in the order of 2.997 × 103 μm were obtained using the optimal metasurface configuration. The surface plasmon resonance (SPR) curves (reflectivity, phase, GH) and electromagnetic simulations were derived using the MATLAB programming algorithm (by the transfer matrix method) and Comsol modeling (by finite element analysis), respectively. These results will provide a feasible way for the detection of cancer biomarkers.

A novel ultra-low refractive index photonic crystal fiber sensor based on surface plasmon resonance

Since many biopharmaceuticals have low refractive indexes, the demand for corresponding low refractive index photonic crystal fiber surface plasmon resonance (PCF-SPR) sensors increasing rapidly. In this paper, a novel ultra-low refractive indexes PCF sensor based on SPR is proposed. The sensor structure is simple photonic crystal fiber without grinding. The gold pillars are filled in the y-direction pores. The effect of gold pillars of different shapes, positions and sizes on sensitivity is discussed. The effect of two pores size and pitch are also investigated. The sensitivity increases with the dimension of two pores. However, the sensitivity increases first and then decreases with cycle. As a result, the sensor can monitor the analyte refractive index in 1.21–1.49. The maximum x-polarization sensitivity is 5200 nm/RIU, and the maximum y-polarization sensitivity is 4700 nm/RIU in near-infrared wavelength region. Moreover, the corresponding y-polarization and x-polarization linear fits are 0.9999 and 0.9998, respectively. The proposed ultra-low RI PCF-SPR sensor also works well with wide range refractive indexes.

Refractive Index and Temperature Sensing Performance of Microfiber Modified by UV Glue Distributed Nanoparticles.

Dielectric materials with high refractive index have been widely studied to develop novel photonic devices for modulating optical signals. In this paper, the microfibers were modified by silicon nanoparticles (NPs) and silver NPs mixed in UV glue with ultra-low refractive index, respectively, whose corresponding optical and sensing properties have been studied and compared. The influence from either the morphological parameters of microfiber or the concentration of NPs on the refractive index sensing performance of microfiber has been investigated. The refractive index sensitivities for the microfiber tapers elaborated with silver NPs and silicon NPs were experimentally demonstrated to be 1382.3 nm/RIU and 1769.7 nm/RIU, respectively. Furthermore, the proposed microfiber was encapsulated in one cut of capillary to develop a miniature temperature probe, whose sensitivity was determined as 2.08 nm/°C, ranging from 28 °C to 43 °C.

Iridescent Colloidal Crystals Composed of SiO2 Porous Hollow Sphere for SERS Application.

In recent years, the application of low-refractive-index materials in the optical field has attracted considerable attention due to it high transmittance and high optical sensitivity. In this study, we synthesized SiO2 porous hollow spheres (SPHS) with an ultralow refractive index (n = 1.05) by using a templating method. Their refractive indices could be easily controlled from 1.05 to 1.08 by tuning the thickness of shell. In addition, a droplet coatings method is proposed for SPHS colloidal crystal (CC) by controlling the temperature and humidity. The SPHS CCs displayed distinct structural colors when the incident angle was adjusted and demonstrated high angular resolution. Moreover, the iridescent color changes could be observed with the naked eye. For surface-enhanced Raman scattering application, more analyte could be absorbed by the porous shells, and metal nanoparticles were coated on the SPHSs surface to increase the hot spot density for improving the SERS intensity.

Фокусирующие системы на основе двумерных фотонных кристаллов

Methods of focusing electromagnetic waves based on the use of two-dimensional photonic crystals formed by metal and dielectric elements are considered. In the first case the physical basis for the creation of radiation focusing systems consists in the manifestation of the properties of a homogeneous medium with ultra-low refractive index by metal photonic crystals at the frequencies of the first allowed zone. In the second case the possibility of constructive interference of direct transmission waves and edge waves diffracting at the edges of spatially bounded dielectric structures is used for focusing.

Spherical-chain silica with super-hydrophobic surface and ultra-low refractive index for multi-functional broadband antireflective coatings

We present a new type of ultra-low refractive index antireflective (AR) coating based on spherical-chain silica particle with super-hydrophobic surface. This AR coating is fabricated using methyltriethoxysilane (MTES) and ethylsilicate (TEOS) as co-precursors to direct silica particles to form a spherical-chain structure. Then hexamethyldisilazane (HMDS) was further used to modify the surface of spherical-chain silica particle, which greatly decreases the refractive index of the silica thin film from 1.139 to 1.098. The formation mechanism of the ultralow refractive index thin film is proposed. The spherical-chain silica particle surface modification with HMDS significantly improves the hydrophobicity of the coated film; the water contact angle of the film increases from 42° to 168°. Ultimately, using this hydrophobic ultralow-index coating as a top-layer, a high-performance double-layer antireflective coating was fabricated with self-cleaning, environmental resistance and a high average transmittance of 99.64% in the wavelength range of 400–800 nm. This multi-functional broadband antireflective coating has potential value in the field of solar cells.

Aggregation of Silica Nanoparticles in Sol-Gel Processes to Create Optical Coatings with Controllable Ultralow Refractive Indices.

Optical coatings with controllable ultralow refractive indices are of profound significance in optical areas. However, it remains a challenge to fabricate such coatings using a simple method. Here we develop an effective and simple approach to create ultra-low-index coatings. This approach was based on a modified sol-gel process, with a key process that involved the aggregation of silica nanoparticles via the addition of a polymer surfactant (e.g., polyvinylpyrrolydone) in sols before coating. The approach involves three steps: the synthesis of silica sols under ammonia catalysis in ethanol (Stöber method), the addition of polyvinylpyrrolydone in the silica sols to induce the aggregation of the silica nanoparticles, and the formation of ultra-low-index coatings by depositing the aggregated silica sols on substrates. Through varying the aggregation extent, this approach produced coatings with controllable refractive indices ranging from 1.17 to 1.07. To the best of our knowledge, the minimum index value of 1.07 from our coating is among the lowest refractive indices ever reported. The ultra-low-index coatings demonstrated excellent optical properties, with which perfect quarter-wavelength antireflection coatings (maximum transmittance ∼100%) and broadband antireflection coatings (transmittance >98% from 400 to 1100 nm) can be prepared. One advantage of the antireflection coatings is that their transmission is less dependent on the refractive index and the thickness of the stacking layer, which make it promising in large-scale production. Moreover, the coatings can be made hydrophobic (water contact angle 136°) by exposing the coatings to a hexamethyldisilazane atmosphere, exhibiting high environmental stability in a humid environment. The aggregation of silica nanoparticles in sol-gel processes provides a scalable alternative to the current approaches for creating ultra-low-index coatings.

Thermochromic VO2-based smart radiator devices with ultralow refractive index cavities for increased performance

VO2 thermochromic coatings show great potential as the main component in smart radiator devices (SRD) for spacecraft, most notably for micro- and nano-satellites. Indeed, the inherent metal to insulator transition (MIT) of VO2 allows such a coating to act as a lightweight thermal regulator, eliminating the need for heavy and failure-prone mechanical louvers. However, spacecraft industry standards require an emissivity variation Δε of at least 60%, a value which, to our knowledge, has yet to be demonstrated. To reach and surpass this value, we first apply a modeling approach to optimize the optical properties and thicknesses of the individual constituent films of a typical SRD with the following architecture: mirror | dielectric resonant cavity | VO2. This study then highlights various possible avenues to enhance the performance of the devices, one of these being the use of an infrared transparent ultralow refractive index dielectric material for the resonant cavity. This theoretical prediction is then confirmed by the deposition of various prototype devices implementing a CaF2 cavity layer with n @ 10 μm = 1.17 and with measured values of Δε in excess of 60% for the full 3 to 25 μm wavelength range. In fact, we demonstrate a prototype device with a maximum Δε value of 66%, thus bringing this technology one step closer to implementation.

Preparation of superhydrophilic silicate coating by sol–gel for double-wavelength broadband antireflective coatings

In the process of realizing double-wavelength broadband antireflection (AR) coatings, it is very important to control the refractive index in a wide range, but at present, many methods of adjusting the refractive index are too complex or expensive. Here, a simple and inexpensive sol–gel route is proposed to prepare the superhydrophilic silica (SiO2) thin films having ultra-low refractive indices by the SiO2 particles surface corrosion with hydrofluoric acid (HF) in sol; this treatment effectively reduces the refractive index of the coating from 1.22 to 1.11. The formation mechanism of the coatings with ultra-low refractive index was discussed in detail. The corrosion of HF on the particles surface significantly improves the hydrophilicity of the coating, and reduces the water contact angle of the coating from 27° to 2°. The bottom layer is prepared by mixing the acid–base complex silica sol. When the refractive indices of the bottom and top layers were set as 1.29 and 1.13, the double-layer double-wavelength AR coating with excellent optical properties has the transmittance of 99.95% and 100.00% at 532 and 1064 nm, respectively.

Graded-Index Fluoropolymer Antireflection Coatings for Invisible Plastic Optics.

Plastic optics are used in an ever-expanding range of applications and yet a durable, high performance antireflection (AR) coating remains elusive for this material class. Here, we introduce a sacrificial porogen approach to produce ultralow refractive index nanoporous fluoropolymer AR coatings via thermal coevaporation of Teflon AF and the small molecule N, N'-bis(3-methylphenyl)- N, N'-diphenylbenzidine (NPD). Using this approach, we demonstrate a five-layer, step-graded AR coating that reduces the solar spectrum-averaged (400 < λ < 2000 nm) reflectance of acrylic plastic to <0.5% for incidence angles up to 40° and withstands over 3 months of outdoor rooftop exposure with minimal degradation. A trilayer coating optimized for the visible range yields luminous reflectivity down to ∼0.1%, effectively rendering double-side coated acrylic plastic invisible under room lighting conditions. Strong adhesion to most optical plastics, an outstanding combination of mechanical, chemical, and environmental durability, and compatibility with commercial vacuum coating systems should enable this AR technology to find widespread practical use.