Low Refractive Index Material Research Articles

Design and fabrication of highly efficient antireflective coating in MWIR on germanium using ion-assisted e-beam deposition

We present the design, fabrication, and characterization of a multilayer anti-reflective (AR) coating on a Germanium (Ge) substrate for mid-wavelength infrared (MWIR) applications using the ion-assisted electron-beam evaporation (IAPVD) technique. The AR coating structure comprises five layers of thin films, alternating between low (nL) and high (nH) refractive index materials, specifically silicon dioxide (SiO2) and Ge. The total thickness of the multilayer structure is maintained below 2 μm. The design was meticulously optimized for the 3.6–4.9 μm MWIR range using OptiLayer software, achieving an impressive average transmission of 99.492 % and an average reflectance of merely 0.508 % over a broadband spectral width of 1300 nm. Following the fabrication process using the IAPVD technique, the empirical results demonstrated an average transmission of 98.56 % and a reflectance value of 0.177 % within the 3.6–4.9 μm range. A strong correlation was observed between the simulated and the fabricated results, with a deviation of only 0.331 % on the reflectance value, proving the accuracy and reliability of the design and fabrication process. This work introduces a highly efficient solution for AR coatings on Ge surfaces, enhancing the performance of electro-optical applications in the MWIR region. To the best of our knowledge, the newly designed Ge/SiO2 multilayer structure and the results achieved have not been previously documented in the literature, and offer substantial improvements in transmission efficiency and reflectance reduction for Ge-based optical systems.

Magnetron Sputter Deposition of Amorphous Silicon–SiO2 Quantized Nanolaminates

Quantization effects in nanolaminate structures of oxide materials are proposed and experimentally demonstrated only recently. Herein, the material combination of amorphous silicon and SiO2 deposited by magnetron sputtering is investigated and it is shown that the quantization effect can be observed indeed. Transmission electron microscopy characterization gives evidence of continuous layers of amorphous silicon and SiO2 with well‐defined interfaces. The deposition process is described and the tunability of the refractive index and the bandgap energy is demonstrated. By doing so, the advantages of this novel material over classical optical materials are shown and feasibility is proved. As an example, a longpass optical interference filter with edge at 720 nm is deposited using quantized nanolaminates as the high and SiO2 as the low refractive index material. This filter can be deposited successfully with close match to the design. It shows a blocking range throughout the visible spectrum whereas a comparable filter based on SiO2–TiO2 only blocks 500–700 nm.

Study on fluorine vacancy defects in yttrium fluoride coating materials

Abstract Rare-earth fluoride coating materials have attracted wide attention as a substitute for ThF4 low-refractive index materials. However, the coating materials and coated films fail to achieve the desired results due to vacancy defects caused by their fluorine-loss decomposition characteristics. To explore the rare earth fluoride coating materials’ fluorine loss influence law, the melt crystallization method is used to simulate the yttrium fluoride (YF3) pre-melting process through the electronic paramagnetic resonance (EPR) obtained at different temperatures under the fluorine vacancy defects change rule. According to the number of unpaired spintrons, the number of spintrons in each vacancy is calculated. The results show that the fluorine vacancy defects increase with the increase of temperature, the formation of YF3 fluorine vacancy defects does not affect the change of crystal structure, and the addition of the additive ammonium hydrogen fluoride (NH4HF2) can reduce the oxygen content of YF3 and effectively inhibit the formation of fluorine vacancy. The effect is gradually obvious with the amount of additives added.

Effect of YF3 Coating Process on Film Stress

YF3 rare-earth fluoride coating material has received wide attention as an alternative to ThF4 low refractive index material, but the film layer can be seriously affected due to the different coating processes. Therefore, to investigate the rules between the coating processes of YF3 rare-earth fluoride coating materials, YF3 thin films were coated with different deposition rates and substrate temperatures. The stresses of YF3 thin films under different processes were obtained by a thin film stress meter, and the microscopic morphology of the thin films was characterized by atomic force microscopy. The results show that: YF3 monolayer film is tensile stress; in the deposition rate between 0.5nm/s~1.5nm/s, with the increase of the coating deposition rate YF3 monolayer film stress tends to decrease, the roughness of the film is also gradually increased, and the overall thickness of the film surface decreases; in the substrate temperature is greater than 50 ℃, the film stress is significantly reduced.

Wavelength Scale Singlet Achromatic Microlenses Based on High Refractive Index Materials

AbstractThe common methods used for correcting chromatic aberration are typically based on multi‐lens and multi‐material systems, resulting in lens thicknesses that are several orders of magnitude greater than the wavelength and complex combination designs. A method to achieve the singlet achromatic microlens of the wavelength‐scale thickness by utilizing high refractive index materials with an aspherical profile is proposed. A theoretical model based on the dispersion effect is developed to guide the selection of materials and the design of thicknesses for achieving chromatic aberration correction in singlet microlenses of a given diameter and numerical aperture. H‐ZLaF68N (68N) glass, sapphire, and fused silica with relatively high to low refractive index are selected to prepare the singlet achromatic microlenses to verify the validity of the model. The thicknesses of three microlenses are 573, 737 nm, and 1.27 µm, respectively, and all of them have achieved achromatic correction as designed. This indicates that the high refractive index material not only achieves achromatic aberration but also reduces the thickness by ≈50% compared with the conventional low refractive index material of silica glass. The presented wavelength‐scale singlet achromatic microlens hold significant promise for compact wearable devices, dynamic holography, and color projection displays.

3D-printed multilayer structures for high-numerical aperture achromatic metalenses.

Flat optics consisting of nanostructures of high-refractive index materials produce lenses with thin form factors that tend to operate only at specific wavelengths. Recent attempts to achieve achromatic lenses uncover a trade-off between the numerical aperture (NA) and bandwidth, which limits performance. Here, we propose a new approach to design high-NA, broadband, and polarization-insensitive multilayer achromatic metalenses (MAMs). We combine topology optimization and full-wave simulations to inversely design MAMs and fabricate the structures in low-refractive index materials by two-photon polymerization lithography. MAMs measuring 20 μm in diameter operating in the visible range of 400 to 800 nm with 0.5 and 0.7 NA were achieved with efficiencies of up to 42%. We demonstrate broadband imaging performance of the fabricated MAM under white light and RGB narrowband illuminations. These results highlight the potential of the 3D-printed multilayer structures for realizing broadband and multifunctional meta-devices with inverse design.

Improved Light Extraction in Organic Light‐Emitting Diodes via Semiconductor Dilution

AbstractIncreasing the internal light extraction efficiency of organic light‐emitting diodes (OLEDs) is key to improving their performance for solid‐state lighting applications; however, it is challenging to do this in a way that is compatible with high volume manufacturing. Here, it is shown that the outcoupling efficiency of OLEDs can be improved by diluting their hole transport layer (HTL) with the low refractive index material trifluoropropyl oligomeric silsesquioxane (F‐POSS). Specifically, co‐evaporating 40 vol.% F‐POSS in the HTL of single and multi‐stack phosphorescent OLEDs decreases its refractive index by Δn ≈ 0.2, which in turn yields a ≈12% increase in their outcoupling efficiency with no impact on electrical performance or operational lifetime. This result is significant because F‐POSS is a small molecule that sublimes cleanly, does not aggregate, and is compatible with state‐of‐the‐art HTL materials, making it a realistic path to increase light extraction in commercial OLEDs manufactured on existing production lines.

Ultra-high-Q and ultra-sensitive refractive index sensing in low refractive index dielectric gratings based on bound states in the continuum

The realization of a high quality factor (Q-factor) and strong local optical fields has long been of great interest in the field of nanophotonics. Unfortunately, it is still challenging to achieve high-Q and strong localized fields in nanostructures made of low refractive index materials. In this study, drawing upon the concept of bound states in the continuum (BICs), an asymmetric dielectric grating composed of low refractive index materials is demonstrated to generate an ultra-high-Q symmetry-protected quasi-BIC in the visible wavelength. Importantly, the design of BIC (quasi-BIC) mode enables the strong localized confinement of light in air. By leveraging this property, we realize ultra-sensitive refractive index sensing with a remarkable sensitivity of 669 nm/RIU and a high figure of merit (FOM) of 45,314RIU−1. This study offers an approach to achieve highly sensitive and high precision refractive index sensing with potential applications in the practical realization of strong light–matter interactions using low index materials.

Research on the Absorption Performance of Silicon-Based Pyramidal Microstructure with Ultra-Low Reflectivity

Anti-reflective structure can effectively suppress the light reflection on the surface of the object, thereby increasing the absorption of light, it has a wide range of applications in photovoltaics, such as optical sensors and photodetectors. Due to the limitation of low refractive index materials, it is difficult for existing anti-reflective coatings to achieve the expected low reflectance effect. Based on Maxwell’s electromagnetic wave theory, this paper uses Finite-Difference Time-Domain (FDTD)-solutions software to research the reflection spectrum of silicon-based pyramidal microstructure in the wavelength range of 200–1400nm, explores the influence of the height (H) and bottom side length (L) of the pyramidal on its reflection spectrum, and studies the changes of its electromagnetic field. On this basis, the absorption performance of the silicon-based pyramidal microstructure is further studied, and it is found that the absorption efficiency of silicon-based pyramidal microstructure for ultraviolet light reaches almost 100%, which provides important significance for its application in optical sensors and photodetectors.

All-dielectric terahertz wave metagrating lens based on 3D printing low refractive index material

A metagrating lens with a high numerical aperture (NA = 0.9) is designed based on the grating diffraction principle and the unit structure of the asymmetric scattering mode. The metagrating lens is composed of all-dielectric material with low refractive index and achieves good focusing at an operating frequency of 0.1 THz. In our experiments, we used 3D printing technology to fabricate the metagrating lens to further reduce the fabrication cost of the metalens and avoid the disadvantages of conventional optical devices that are large and costly.

Elevating performance of AlOxFy anti-reflection coating via controllable HF vapor fluorination

The introduction of anti-reflective coatings (ARCs) to optical devices has provided a simple and effective solution to improve their power efficiency. Among the various methods of preparing ARCs, the solution process is widely applied due to its simplicity, scalability, and naturally formable pores embedded in the film, which effectively reduce the refractive index of the material. Additionally, since low refractive index materials are always preferred for an effective ARC, strategies of fluorine substitution have been considered. Herein, we successfully introduce fluorine into Al2O3 ARCs by controlled HF vapor through abundant pores in the film. The exposure and reaction conditions were finely tunable, resulting in a precisely controlled amount of fluorine in the AlOxFy ARC. Finally, the gaseous HF vapor fluorination resulted in an effective porous ARC on a sapphire window by matching the refractive index between the surrounding media; thus, fluorinated ARC improves transparency. The maximum and the average transmittances of the ARC-applied window reach ∼99.15% and ∼97.04%, respectively. The suggested fluorination method is anticipated to have broad applicability for situations where surface chemistry modification is necessary. Highly reliable and controllable, this strategy can be used with complicated architectures without causing structural damage.

Design and fabrication of color-generating nitride based thin-film optical filters for photovoltaic applications

In this paper, we present the design and fabrication of a 11-layer notch filter for photovoltaic applications. Thickness modulation has been done on a starting quarter-wave design to obtain a 11-layer final structure. For our design parameters, we considered materials with indices of 2.09 and 1.51 as high and low refractive index materials, respectively. Since this filter aims for photovoltaic applications, we defined the substrate as silicon. The design shows a reflection peak of around 73% from 350 to 467 nm with a FWHM of 79 nm along with less than 10% reflection for longer wavelengths of the remaining high transmission region of 300–1200 nm. For fabrication, we used silicon nitride and silicon oxynitride as alternating high and low refractive index materials on a silicon substrate. Layers of silicon nitride and oxynitride were deposited with an electron cyclotron resonance plasma-enhanced chemical vapor deposition reactor and characterized with variable angle spectroscopic ellipsometry and spectrophotometry.

Silicon Oxy-Nitride for the Low Refractive Index Layers in the Mirror Coatings of the Cryogenic Laser Interferometer Gravitational Waves Detector.

The low refractive index layers in the mirror coatings of the room-temperature laser interferometer gravitational waves detectors are silica deposited by the ion beam sputter method. However, the silica film suffers from the cryogenic mechanical loss peak, hindering its application for the next generation detector operated at cryogenics. New low refractive index materials need to be explored. We study amorphous silicon oxy-nitride (SiON) films deposited using the method of plasma-enhanced chemical vapor deposition. By changing the N_{2}O/SiH_{4} flow rate ratio, we can tune the refractive index of the SiON smoothly from nitridelike to silicalike of ∼1.48 at 1064nm, 1550nm, and 1950nm. Thermal anneal reduced the refractive index down to ∼1.46 and effectively reduced the absorption and cryogenic mechanical loss; the reductions correlated with the N─H bond concentration decrease. Extinction coefficients of the SiONs at the three wavelengths are reduced down to 5×10^{-6}∼3×10^{-7} by annealing. Cryogenic mechanical losses at 10K and 20K (for ET and KAGRA) of the annealed SiONs are significantly lower than the annealed ion beam sputter silica. They are comparable at 120K (for LIGO-Voyager). Absorption from the vibrational modes of the N─H terminal-hydride structures dominates over the absorption from other terminal hydrides, the Urbach tail, and the silicon dangling bond states in SiON at the three wavelengths.

Self-Raman 1176 nm Laser Generation from Nd:YVO4 Crystal by Resonator Cavity Coating

Crystal coating is an important process in laser crystal applications. According to the crystal characteristics of neodymium-doped yttrium vanadate (Nd:YVO4), its intrinsic parameters, and optical film design theory, Ta2O5 and SiO2 were selected separately as high and low refractive index materials. The optical properties and surface roughness of the films were characterized by OptiLayer and Zygo interferometers, and the effects of ion source bias on refractive index and surface roughness were investigated so that the optimal ion source parameters were determined. Optical monitoring and quartz crystal control were combined to accurately control the thickness of each film layer and to reduce the monitoring error of film thickness. The prepared crystal device was successfully applied to the 1176 nm laser output system.

Optimizing Bulk and Surface Sensitivity With Fiber-Tip Leaky Mode Resonances Excited by Low Refractive Index Overlay

In this work, an all-dielectric fiber-tip sensing platform is proposed by which multiple nonpolarized leaky mode resonances (LeMRs) are generated for sensitively decoupling bulk and surface characteristics. The key point is to excite fiber-tip leaky modes with a layer of low refractive index (RI) material (here magnesium fluoride thin film) deposited on the fiber-tip end facet, but not high RI films. In this configuration, the fiber-tip thin film works as a planar film waveguide, which is confirmed by the mode equation obtained here. Both the excitation and sensing principles are obtained by analyzing the influence of bulk and surface characteristics on the mode dispersion. Similar with the case of high RI films, the results also verify the capability of decoupling bulk and surface properties with a single fiber-tip LeMR excited with low RI film. Another important advance is that both bulk and surface sensitivities can be greatly improved, respectively, up to 107.6 dB/RIU and 16.9 nm/nm (being several tenfolds higher than previous reports). The proposed fiber-tip LeMR configuration could open a new paradigm in the domain of “lab-on-optrode” technology, which is highly appealing for multifunctional plug and play biosensing.

Design and fabrication of 940 nm vertical cavity surface emitting laser single-emitter device

As a key part of vertical cavity surface emitting laser (VCSEL), active region will seriously affect the threshold and efficiency of the device. To obtain the appropriate laser wavelength and material gain, the design of In<sub>0.18</sub>Ga<sub>0.82</sub>As strain compensated quantum well is optimized. The relationship between the lasing wavelength of multiple quantum wells (MQWs) and the thickness is calculated. Considering the influence between the active region temperature and the lasing wavelength, the thickness of the quantum well is chosen as 6 nm, and the quantum barrier thickness is chosen as 8 nm, corresponding to the lasing wavelength of 929 nm. The material gain characteristics of the MQWs at different temperatures are simulated by Rsoft. The material gain exceeds 3300/cm at 300 K, and the temperature drift coefficient of the peak wavelength is 0.3 nm/K. In this work, Al<sub>0.09</sub>Ga<sub>0.91</sub>As and A<sub>l0.89</sub>Ga<sub>0.11</sub>As are chosen as the high- and the low-refractive index material of distributed Bragg reflector (DBR), and 20 nm graded layer is inserted between two types of materials. The influence of the graded layer thickness of DBR on the valence band barrier and reflection spectrum are calculated and analyzed. The increase of graded layer thickness can lead the band barrier peak and the reflection spectrum bandwidth to decrease. The reflection spectrum and phase spectrum of P-DBR and N-DBR are calculated by the transmission matrix mode (TMM): the reflectance of DBR is over 99% and the phase shift is zero at 940 nm. The optical field distribution of the whole VCSEL structure is simulated, in which the standing wave peak overlaps with the active region, and the maximum gain can be obtained. Using the finite element method (FEM), the effect of oxidation confined layer on the injection current is simulated. The current in the active region is effectively limited to the position corresponding to the oxidation confined hole, and its current density is stronger and more uniform. The optical field distributions in different modes of photonic crystal-vertical cavity surface emitting laser (PC-VCSEL) are simulated, and different modes have different resonant wavelengths. The values of quality factor Q in different modes of VCSEL and PC-VCSEL are calculated, Q of the fundamental mode is higher than that of higher transverse mode. It is demonstrated that the photonic crystal air hole structure can realize the output of basic transverse mode by increasing the loss of high order transverse mode. The VCSEL and PC-VCSEL with oxidation hole size of 22 μm are successfully fabricated, in which the photonic crystal period is 5 μm, the air pore diameter is 2.5 μm, and the etching depth is 2 μm. Under continuous current test, the maximum slope efficiency of VCSEL is 0.66 mW/mA, the output power is 9.3 mW at 22 mA, and the lasing wavelength is 948.64 nm at 20 mA injection current. Multiple wavelengths and large spectrum width are observed in the spectrum of VSCEL, which is an obvious multi-transverse mode. The maximum fundamental transverse mode output of PC-VCSEL reaches 2.55 mW, the side mode suppression ratio (SMSR) is more than 25 dB, and the spectrum width is less than 0.2 nm, indicating that the photonic crystal air hole has a strong control effect on the transverse mode, and the laser wavelength is 946.4 nm at 17 mA.

SPR based refractive index modulation of nanostructured SiO2 films grown using GLAD assisted RF sputtering technique

Low refractive index materials have always gained attention for avoiding optical losses in numerous devices like Light emitting diodes (LEDs), Distributed Bragg Reflectors (DBRs) etc. Glancing angle deposition (GLAD) has proven to be an effective method for depositing porous nanostructures with low refractive index. SiO2 has the lowest refractive index in comparison to most of the materials present in nature and is therefore utilized extensively in the fabrication of optical devices with minimum optical losses. The present work focuses on the SiO2 nanostructured thin films grown using glancing angle (GLAD) configuration in RF Sputtering technique under varying deposition conditions (glancing angle and pressure) at fixed thickness. The deposited porous SiO2 nanostructured layers have well-defined nanorods of feature size smaller than 100 nm. The refractive index corresponding to all the deposited SiO2 nanostructured films came out to be smaller than the corresponding value (1.49) mentioned in the literature for bulk SiO2. The refractive index has been observed to vary in a wide range (1.05 to 1.40). Characterization of the deposited SiO2 nanostructured films was carried out using Fourier Transform Infrared (FTIR) spectroscopy, Energy Dispersive X-Ray (EDS) analysis, and Atomic Force Microscopy (AFM) to assess the qualitative properties of the deposited nanostructured SiO2 films. Surface Plasmon Resonance (SPR) and Ellipsometry studies are performed to determine the refractive index value of the deposited nanostructured SiO2 thin films. A refractive index value as low as ∼ 1.05 at a wavelength of 633 nm is achieved for SiO2 nanostructured thin film deposited at 30 mTorr deposition pressure and 80° glancing angle.

Study on the Performance of Sunflower Quasi-Periodic Photonic Crystal Superlens

A study for the performance of the Sunflower (circular symmetric structure) quasi-periodic photonic crystal superlens is presented. A series of parameters, including thickness, refractive index and duty cycle, of the Sunflower structure are discussed. The analysis of this study illustrates that the Sunflower structure could provide excellent focusing imaging due to its good circular symmetry and also have negative refraction in low refractive index materials. It provides an important way to choose an alternative to positive refraction lenses in the future.

Meta-Heuristics Optimization of Mirrors for Gravitational Wave Detectors: Cryogenic Case

In this paper, we explore the behavior of several optimization methods for reducing coating Brownian noise in the mirrors of gravitational wave detectors. We will refer to cryogenic operating temperatures, where the low refractive index material has mechanical losses higher than those of the high refractive index material. This situation is the exact opposite of that which occurs at room temperature, which is already widely known. The optimal design of the dielectric mirror (without a priori assumptions on thicknesses) can be obtained through the combined multi-objective optimization of transmittance and thermal noise. In the following, we apply several multi-objective meta-heuristics to compute the Pareto front related to the optimization problem of dielectric mirror thicknesses made of two materials (binary coatings). This approach gives us more certainty about the structure of the final result. We find strong evidence that all meta-heuristics converge to the same solution. The final result can be interpreted with simple physical considerations, providing useful rules to simplify the thicknesses of the optimization algorithm.

Design and Fabrication of Multiple-Color-Generating Thin-Film Optical Filters for Photovoltaic Applications

The use of electric vehicles (EVs) can reduce greenhouse gas emissions, air pollution, dependency on fossil fuels, and their adverse health effects on humans. But, we can only utilize the full environmental benefits of EVs when they are charged with renewable energy sources with zero or low carbon emissions. As a solution, Mobarak et al. [1] suggested integrating low-cost, flexible, and thin-film copper indium gallium selenide (CIGS) solar cells directly onto the steel of all the upward-facing body parts of the vehicles. But, this integration of solar cells comes with an aesthetic drawback. Previously, colorful photovoltaics (PVs) have been designed with one-dimensional (1D) photonic crystals or various 1D and 2D metallic nanostructures for aesthetic building-integrated photovoltaics (BIPVs) [2, 3]. However, the functionality of our application differs from that of BIPV as we need maximum absorption of the solar spectrum to obtain maximum conversion efficiency. Thus, we propose replacing the anti-reflective coating (ARC) present in the solar cells with a notch filter (a narrow high-reflection region in the visible range along with high transmission for the rest of the solar spectrum) to obtain colors.High-performance notch filters with a narrow and ultra-steep notch are well known in literature [4, 5]. Generally, high-performance notch filters are designed with a minimum of 45 layers. It is challenging to use filters with many layers on solar cells due to fabrication and thickness complexities. Thus, we created designs with a maximum of 27 layers for possible integration with photovoltaics. We used OptiLayer [6] to simulate our designs and the gradual evolution technique was used to optimize the designs. We performed our simulations with a multilayer structure of alternating high and low refractive indices of 2.09 and 1.45, respectively, on top of a silicon substrate. We optimized this multilayer structure for three reference wavelengths (400 nm, 550 nm, and 700 nm) resembling three colors. Our designs have notch widths of less than 100 nm for all the reference wavelengths with an average of 70% reflection in the high-reflection region and less than 20% reflection in the high-transmission area.To fabricate our designs, we need materials that are transparent to the solar spectrum targeted by the active material of the solar cells. The materials also need to have refractive indices closer to our simulation. Thus, we chose the combination of silicon nitride and silicon dioxide as our high and low refractive index material, respectively [7, 8]. To better understand our designs’ optical characteristics, we fabricated a scaled-down version of our structure with 5-10 layers. We used electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) to deposit the multilayer structure on silicon wafers. To obtain the silicon nitride and silicon dioxide layers, we used a SiH4/N2/O2/Ar precursor mixture. By tuning the gas flow rate in the reactor chamber, we tuned the stoichiometry and obtained the required refractive index for each layer. To characterize the refractive index and thickness for each layer, we used variable angle spectroscopic ellipsometry (VASE). We made a detailed comparison of our simulation and fabrication results.