Similar Refractive Index Research Articles

Optical transition properties, internal quantum efficiencies, and temperature sensing of Er3+ doped BaGd2O4 phosphor with low maximum phonon energy

AbstractThe hosts with low maximum phonon energy (MPE) are preferred since the nonradiative consumption of the luminescence centers in them are low. Among the low MPE hosts, the oxide ones are more favored owing to their excellent stability and easy synthesis. In this work, the optical and spectroscopic properties of BaGd2O4:Er3+ phosphor were studied. The MPE of BaGd2O4 host was observed from Eu3+ phonon sideband (PSB) spectrum and Raman spectrum to be 477 cm−1 which does not second to the fluoride hosts. The refractive index, which is indispensable for Judd–Ofelt calculation, was confirmed from the both approaches of the Eu3+‐probe and the band gap energy, and the similar refractive indices were confirmed, therefore the average refractive index 2.01 was used in the Judd–Ofelt calculation. The Judd–Ofelt parameters of Er3+ in BaGd2O4 host was confirmed to be = 7.91 × 10−21 cm2, = 2.36 × 10−21 cm2, and = 9.00 × 10−22 cm2. Furthermore, the internal quantum efficiencies for 4F9/2 and 4IJ (J = 9/2, 11/2, and 13/2) levels were determined. Finally, the optical temperature sensing properties were studied in detail, and the temperature calibration curve was experimentally derived, meanwhile the maximum absolute sensitivity was confirmed to be 0.0028 K−1.

Large-scale CO2 laser-based sol-gel annealing of titanium dioxide on borosilicate glass

There are several ways to create optical filters on glass, such as the low-cost sol-gel coating. However, to make this process even more effective and flexible, a laser instead of a conventional furnace process was used to anneal the borosilicate glass samples. In previous studies, it was demonstrated that it is possible to generate similar refractive indices and film thicknesses with a CO2 laser as with the furnace method. In this study, TiO2-coated borosilicate glasses are annealed with a CO2 laser. In particular, the main goal is to scan a large processing area (up to 475 × 468 mm²) and to achieve process speeds comparable to those in the furnace process. Microscopic images show a homogeneous layer and an ellipsometrically determined refractive index comparable to the reference sample from the furnace (laser sample: 2.32; furnace reference: 2.20). With a suitable process setup, it is therefore possible to process glass efficiently on an industrial scale.

A multilayer transparent wood prepared by laminating two kinds of tree species

AbstractTransparent wood (TW) is a new material with a certain light transmission performance, which can be widely used as new energy‐saving material. However, the application and development of TW are limited by material and thickness. In this work, the representative species of softwood and hardwood were selected for lamination. After chemical treatment, the lignin in natural wood was removed, and then the epoxy resin with a similar refractive index to the cell wall was impregnated in vacuum. By using the viscosity of epoxy resin itself, the single‐layer materials could be effectively laminated together to prepare a new type of multilayer transparent wood (MLTW). By testing the optical properties, mechanical properties. and surface properties of MLTW materials, we found that the optical transmittance and mechanical properties can be effectively controlled and selected through the selection of materials and assembly methods. Besides, the surface properties of MLTW are mainly related to its surface material. In short, MLTW is a new composite material prepared by simple lamination at the macro level. The properties of the material can be controlled through the selection of single‐layer materials and assembly methods. Compared with the traditional TW, MLTW has better performance regulation ability.

Refractive‐Index‐Matching‐Based Encryption of Photonic Crystal Prints with Multistage and Reconfigurable Information

AbstractEncryption of photonic prints with multiple information is highly desired due to their potential applications in high security of anticounterfeiting and information protection. Here, a new refractive‐index‐matching‐based technology is developed to construct encrypted photonic crystals (EPCs) with multistage and reconfigurable information by introducing solvents with different refractive indexes into PCs. The multilevel information of the EPCs can be hidden at normal conditions since the similar refractive index between the PC and solvent induces the diminishment of the structural color of the EPC. Programmable showing of multistage information can be realized by simply heating EPCs for different times owing to the diverse evaporation rate of the solvents. The switch of the EPCs between the encryption and decryption states is fully reversible. Based on these unique characteristics, the information of EPCs can be reconfigured repeatedly by rational design and choosing of solvents. This work paves a new way in the fabrication of EPCs with multilevel and reconfigurable information and will promote their applications in displays, printing, and anticounterfeiting.

Artificial Chameleon Skin with Super-Sensitive Thermal and Mechanochromic Response.

Both the nonclose-packed structure and the large refractive index contrast of guanine nanocrystals and cytosols in iridophores play a vital role in the dynamic camouflage of chameleons, including the bright skin color and color tuning sensitivity to external stimulus. Here, the nonclose-packed photonic crystals consisting of ZnS nanospheres and polymers, which have similar refractive indices with guanine nanocrystals and cytosols, respectively, are constructed by a two-step filling strategy. ZnS@SiO2 nanospheres are self-assembled to build intermediate close-packed photonic crystals followed by filling polymers in their interstices. The nonclose-packed photonic crystal is successfully achieved when the silica portion is etched by HF solution and refilled by polymers. Excitingly, the stimulus response of the designed photonic crystal is as sensitive as the skin of chameleons due to the similar contrast of refractive indices and nonclose-packed structure. The reflection peak of the structure can blue-shift more than 200 nm as the temperature increases from 30 to 55 °C or under 20% compressional strain. This work not only builds the nonclose-packed photonic crystals by introducing a two-step filling strategy but also proves that high refractive contrast in photonic crystals is an effective strategy to achieve ultrasensitivity, which is highly desirable for various applications.

Analysis of Effects of Surface Roughness on Sensing Performance of Surface Plasmon Resonance Detection for Refractive Index Sensing Application.

This paper provides a theoretical framework to analyze and quantify roughness effects on sensing performance parameters of surface plasmon resonance measurements. Rigorous coupled-wave analysis and the Monte Carlo method were applied to compute plasmonic reflectance spectra for different surface roughness profiles. The rough surfaces were generated using the low pass frequency filtering method. Different coating and surface treatments and their reported root-mean-square roughness in the literature were extracted and investigated in this study to calculate the refractive index sensing performance parameters, including sensitivity, full width at half maximum, plasmonic dip intensity, plasmonic dip position, and figure of merit. Here, we propose a figure-of-merit equation considering optical intensity contrast and signal-to-noise ratio. The proposed figure-of-merit equation could predict a similar refractive index sensing performance compared to experimental results reported in the literature. The surface roughness height strongly affected all the performance parameters, resulting in a degraded figure of merit for surface plasmon resonance measurement.

Optical characterisation of alumina–mullite materials for solar particle receiver applications

Alumina–mullite particles are used in high-temperature solar thermal applications such as solar particle receivers. In this study, optical properties of alumina–mullite materials with variable content of alumina and mullite are determined in the spectral range of 0.193–1.69 μm. Variable angle spectroscopic ellipsometry is performed for alumina–mullite thin films, which are fabricated by magnetron sputtering. The thin films are characterised by scanning electron microscopy, atomic force microscopy, and energy dispersive spectroscopy methods. The B-spline model is employed to generate ellipsometric parameters to fit the measured data and to obtain the optical properties. The investigated materials of variable content of alumina and mullite have a similar refractive index in the considered spectral range. The absorptive index of the alumina–mullite materials in the spectral range of 0.193–0.4 μm is higher than in the range 0.4–1.69 μm. The absorptive index decreases with increasing content of alumina in the spectral range of 0.193–0.4 μm. The material composed of similar proportions of alumina and mullite yields the highest absorptive index in the spectral range of 0.4–1.1 μm. The optical properties determined for the alumina–mullite materials are applied to obtain the radiative properties of spherical homogeneous particles. Mie theory is used to calculate absorption and scattering efficiency factors, as well as the scattering phase function. In addition, the scattering phase functions are obtained using the Henyey–Greenstein approximation and the transport approximation. The Monte Carlo ray-tracing method is employed to study the radiative transfer in a model one-dimensional particle curtain containing polydisperse particles exposed to high-flux solar irradiation. It is found that the overall reflectance, absorptance and transmittance of the particles only weakly depend on the optical properties of the materials investigated.

Versatile titanium dioxide inverse opal composite photonic hydrogel films towards multi-solvents chip sensors

Photonic crystals with orderly periodic structure and manipulated structural colors have garnered widespread attention in sensors field. However, the unsatisfactory detection range, low sensitivity and inconvenient integration highly limit their further practical applications. In this paper, by combining TiO2 inverse opal photonic crystal with polyacrylamide hydrogel, a novel organic-inorganic composite solvents responsive photonic hydrogel (SRPH) was developed. By encapsulating TiO2 inverse opal photonic crystal with high refractive index in this hydrogel, the photonic hydrogels, therefore, possess bright structural colors. Due to the discrepancy in swelling rate of SRPH film in various solvents (ethanol, dimethyl formamide, dimethyl sulfoxide, etc.), it can identify these organic solvents through the rapid change of structural colors and detect the concentrations of the solvent in a wide range. Even for solvents with similar refractive index, it can be clearly distinguished, along with the excellent repeatability. Benefiting from the wide detection range and high sensitivity, the chip sensors were then developed by integrating multiple hydrogel films into the microfluidic system, which have high efficiency in solvent naked-eye multi-identification and concentration multi-detection. The new composite photonic hydrogels provide a reference for the design of functional photonic crystals and next-generation application of sensors.

Single-step etched grating couplers for silicon nitride loaded lithium niobate on insulator platform

Dielectrically loaded thin-film lithium niobate (LiNbO3) on insulator (LNOI) platforms have enabled a range of photonic integrated circuit components, such as high-speed optical modulators, switches, and nonlinear devices, while avoiding the direct etching of the LiNbO3 thin film. Silicon nitride (Si3N4) is one of the most attractive dielectric loading materials as it has a similar refractive index and transparency window to LiNbO3 and can be deposited and patterned by mature fabrication processes. The patterning of Si3N4 opens the opportunity to fabricate grating couplers in the same fabrication step, providing efficient optical interfaces for wafer-scale testing. In this paper, we investigate and demonstrate single-step etched grating couplers on a Si3N4-LNOI (X-cut) platform. The grating couplers (straight and curved) are designed and fabricated for TE-polarized modes along the Y and Z crystallographic directions, considering the LiNbO3 crystal’s birefringence. The experimentally demonstrated coupling losses are as low as 4.02 and 4.24 dB along the crystallographic Y and Z directions, respectively. The corresponding peak wavelengths are 1609 and 1615 nm, respectively. The measured 3-dB bandwidths are wider than 70 nm for both crystallographic directions. We also numerically investigated the influence of fabrication variations and the fiber angle on the transmission. To the best of our knowledge, this work is the first demonstration of grating couplers with different light propagation directions on the Si3N4 loaded LNOI platform.

Nanoparticle Determination in Water by LED-Excited Surface Plasmon Resonance Imaging

The increasing popularity of nanoparticles in many applications has led to the fact that these persistent materials pollute our environment and threaten our health. An online sensor system for monitoring the presence of nanoparticles in fresh water would be highly desired. We propose a label-free sensor based on SPR imaging. The sensitivity was enhanced by a factor of about 100 by improving the detector by using a high-resolution camera. This revealed that the light source also needed to be improved by using LED excitation instead of a laser light source. As a receptor, different self-assembled monolayers have been screened. It can be seen that the nanoparticle receptor interaction is of a complex nature. The best system when taking sensitivity as well as reversibility into account is given by a dodecanethiol monolayer on the gold sensor surface. Lanthanide-doped nanoparticles, 29 nm in diameter and with a similar refractive index to the most common silica nanoparticles were detected in water down to 1.5 µg mL−1. The sensor can be fully regenerated within one hour without the need for any washing buffer. This sensing concept is expected to be easily adapted for the detection of nanoparticles of different size, shape, and composition, and upon miniaturization, suitable for long-term applications to monitor the quality of water.

Dual‐Modal Invisible Photonic Crystal Prints from Photo/Water Responsive Photonic Crystals

Invisible photonic crystal prints (IPCPs) are appealing for anti‐counterfeiting and information protection due to their capability in hiding and showing the encrypted information under specific conditions. Herein, the dual‐modal IPCPs based on responsive photonic crystals (PCs) with tunable photoluminescent (PL) and structural colors are reported. The responsive PCs are prepared by the self‐assembly of dyes doped silica particles into non/swellable polymers. The independent design and control over the optical response of the PL of the particles and structural colors of PCs enable the successful encryption of dual‐modal patterns. Under normal conditions, the as‐fabricated IPCP shows a fake pattern with uniform structural colors and thus hides the encrypted pattern due to the similar particles size, lattice constant, and refractive index of each region. In striking contrast, PL and structural colors‐based new patterns can be instantly and reversibly revealed when the IPCP is exposed to UV illumination or soaked in water. This work provides a new concept in designing and fabricating dual‐modal IPCPs, and facilitates their applications in the fields of color display, antifake package, and multilevel anti‐counterfeiting.

PIV-Measurements of Centrifugal Instabilities in a Rectangular Curved Duct with a Small Aspect Ratio

In this study, experimental measurements were undertaken using non-intrusive particle image velocimetry (PIV) to investigate fluid flow within a 180° rectangular, curved duct geometry of a height-to-width aspect ratio of 0.167 and a curvature of 0.54. The duct was constructed from Plexiglas to permit optical access to flow pattern observations and flow velocity field measurements. Silicone oil was used as working fluid because it has a similar refractive index to Plexiglas. The measured velocity fields within the Reynolds number ranged from 116 to 203 and were presented at the curved channel section inlet and outlet, as well as at the mid-channel height over the complete duct length. It was observed from spanwise measurements that the transition to unsteady secondary flows generated the creation of wavy structures linked with the formation of Dean vortices close to the outer channel wall. This flow structure became unsteady with increasing Reynolds number. Simultaneously, the presence of Dean vortices in the spanwise direction influenced the velocity distribution in the streamwise direction. Two distinct regions defined by a higher velocity distribution were observed. Fluid particles were accelerated near the inner wall of the channel bend and subsequently downstream near the outer channel wall.

Sodium nanoparticles in alkali halide minerals: Why is villiaumite red and halite blue?

Abstract The presence of metal Na nanoparticles causes the bright, thermally unstable colors of villiaumite (NaF) and halite (NaCl). These nanoparticles have been suspected for a long time to be caused by external irradiation. Metal nanoparticles, often referred to as metal colloids, cause surface plasmon resonance effects, characterized by a single Lorentzian-shaped absorption band. The color of these minerals is due to metal Na nanoparticles of 2.5–3 nm. A key point is that the resonance wavelength, which corresponds to the maximum of the absorption band, is inversely related to the value of the refractive index of the embedding mineral. This causes the position of the main absorption band to be offset downward by 140 nm in halite relative to villiaumite. As a consequence, the optical transmission window is shifted from the long to the short wavelength domain, explaining the color of blue halite and red villiaumite, respectively. Similar refractive index dependence may explain the purple color of fluorite caused by metallic Ca nanoparticles. Finally, the origin of the villiaumite irradiation may be the presence of Th-rich (about 8.8 wt% ThO2) nano-inclusions, about 500 nm large, illustrating the specific geochemistry of peralkaline rocks where villiaumite is found.

Boosting Long-Range Surface-Enhanced Raman Scattering on Plasmonic Nanohole Arrays for Ultrasensitive Detection of MiRNA

A fundamental challenge, particularly, in surface-enhanced Raman scattering (SERS) analysis is the detection of analytes that are distant from the sensing surface. To tackle this challenge, we herein report a long-range SERS (LR-SERS) substrate supporting an extension of electric field afforded by long-range surface plasmon resonance (LRSPR) excited in symmetrical dielectric environments. The LR-SERS substrate has a sandwich configuration with a triangle-shaped gold nanohole array embedded between two dielectrics with similar refractive indices (i.e., MgF2 and water). The finite-difference time-domain simulation was applied to guide the design of the LR-SERS substrate, which was engineered to have a wavelength-matched LRSPR with 785 nm excitation. The simulations predict that the LR-SERS substrate exhibits great SERS enhancement at distances of more than 10 nm beyond its top surface, and the enhancement factor (EF) has been improved by three orders of magnitude on LR-SERS substrates compared to that on conventional substrates. The experimental results show good agreement with the simulations, an EF of 4.1 × 105 remains available at 22 nm above the LR-SERS substrate surface. The LR-SERS substrate was further applied as a sensing platform to detect microRNA (miRNA) let-7a coupled with a hybridization chain reaction (HCR) strategy. The developed sensor displays a wide linear range from 10 aM to 1 nM and an ultralow detection limit of 8.5 aM, making it the most sensitive among the current detection strategies for miRNAs based on the SERS-HCR combination to the best of our knowledge.

Angle-independent responsive organogel retroreflective structural color film for colorimetric sensing of humidity and organic vapors

The angle dependence of photonic crystals (PCs) dramatically limits their practical applications in the colorimetrical sensing of humidity and volatile organic compound (VOC) vapors. In addition, it is challenging for inverse opal PCs to colorimetrically distinguish between vapors with similar refractive indices. Different from the mechanism of PC-based sensors, here, we report an angle-independent polyacrylamide (PAAm) organogel structural color film based on the mechanisms of retroreflection, total internal reflection (TIR) and interference with a shape similar to a single-sided “egg waffle”. During the process of responding to humidity and VOC vapors, the color of the film remains angle-independent in the normal angle range of 0° to 45° under coaxial illumination and observation conditions. At the same time, the film can colorimetrically distinguish between vapors with similar refractive indices, such as methanol and ethanol, which is mainly due to the differences in their polarity and solubility parameters. The film shows good stability, reversibility and selectivity when exposed to vapors. A colorimetric sensor with a new response mechanism is proposed and has the potential to effectively distinguish between vapors with similar refractive indices. Furthermore, this responsive retroreflective structural color film (RRSCF) provides a universal strategy to develop targeted angle-independent structural color sensors by selecting optimized materials.

Profile control of femtosecond laser-fabricated moth-eye structures on Si substrate

The present study focuses on controlling the antireflective (AR) characteristics of one-dimensional moth eye structures on a high-resistivity silicon substrate by adjusting the profile (i.e., refractive index distribution) of its micro-tapers using femtosecond laser processing. Moth eye structures that comprise periodically arranged tapers with varying sizes and profiles (triangle, parabola, stair, and triangle–stair) were fabricated using femtosecond laser ablation via a fine adjustment of the processing pattern. The AR characteristics of these samples were experimentally evaluated by a standard terahertz (THz) time-domain spectroscopy and modeled by employing the method of Finite-difference time-domain (FDTD). The measurement data agreed well with the simulation results, and the power reflectance of these structures was induced from approximately 30% (bare silicon substrate) to a very low value (less than 5%) for a broad THz band. Furthermore, the profiled structures showed a varying power reflectance distribution with respect to the frequency in the THz region, especially for the low reflection one (<5%) approximately from 0.5 to 1.0 THz. Three samples with different aspect ratios (approximately 1–3) but having the similar refractive index distribution were also fabricated by using an industrial high-power femtosecond laser (40 W) to demonstrate the relationship between the AR characteristics and the aspect ratio of the moth-eye structures, as well as the processing efficiency. The structures displayed a very good AR performance for a broadband THz region. Besides the broad AR band and low reflectance, this adjustability for the profile of laser-fabricated tapers could improve the flexibility of anti-reflection for THz devices, such as THz wave emitters, receivers, and filters, and the antireflective frequencies less than 1 THz suggest that this technology can be used in the next generation of mobile communication.

Sequential in situ Raman spectroscopy for observing dissociation behavior of filled‐ice Ih of methane hydrate at high pressure

AbstractIn situ Raman spectroscopy was performed to investigate the stability and dissociation behavior of filled‐ice Ih of methane hydrate at high‐pressure (up to 5.1 GPa) and high‐temperature (up to 408 K) using an externally heated diamond anvil cell. The results revealed that filled‐ice Ih was stable up to 383 K at 4.5 GPa, and the dissociation conditions intersected with the melting curve of ice VII at approximately 2.5 GPa and with that of solid methane at approximately 2.2 GPa. Consequently, the dissociation into water and methane components occurred via two different mechanisms depending on pressure: (1) melting into liquid water and fluid methane below approximately 2.5 GPa and (2) solid–solid decomposition into ice VII and solid methane above approximately 2.5 GPa. Visual observations with an optical microscope synchronized with in situ Raman spectroscopy detected considerable changes in the sample in the case of melting, whereas they could not detect any changes in the decomposition case because both decomposition products were solid phases with similar refractive indices. This result demonstrated that the synchronized and sequential Raman spectroscopy with optical observations is indispensable for evaluating the dissociation behavior; otherwise, the solid–solid decomposition is overlooked when the evaluation is performed using a microscope only. The stability field of filled‐ice Ih obtained in the experiment can help infer the states of methane hydrates in the interior of icy bodies such as Saturn's largest moon, Titan.

Highly Efficient Detection of Homologues and Isomers by the Dynamic Swelling Reflection Spectrum.

Precise and efficient detection of solvents with similar refractive index is highly desired but remains a big challenge for the conventional opal because the shift of its reflection wavelength only depends on the refractive index of the solvent to be detected. Here, homologues (alcohols, acids, alkalis, esters, and aromatic hydrocarbons), isomers, and other solvents with similar refractive index and structures were precisely distinguished through the dynamic swelling reflection spectrum (DSRS) pattern based on the different swelling behavior of swellable photonic paper in solvents. The one reflection signal of photonic paper will split into two reflection peaks, which then tend to merge together during the swelling process. The variation of the reflection signals and merging time are highly sensitive to the polarity and refractive index of the solvent, and the differences can be significantly amplified in DSRS, resulting in the distinction of the solvent from its unique geometric pattern. Moreover, the variation tendency of the reflectance provides an additional parameter in recognition of the solvent, which can be explained by calculation and comparison of the practical volume ratio of the solvent swelled into the photonic paper and the corresponding critical volume ratio of the solvent determined by its refractive index.

Nanometer-Scale Heterogeneous Interfacial Sapphire Wafer Bonding for Enabling Plasmonic-Enhanced Nanofluidic Mid-Infrared Spectroscopy.

As one of the most effective surface-enhanced infrared absorption (SEIRA) techniques, metal-insulator-metal structured metamaterial perfect absorbers possess an ultrahigh sensitivity and selectivity in molecular infrared fingerprint detection. However, most of the localized electromagnetic fields (i.e., hotspots) are confined in the dielectric layer, hindering the interaction between analytes and hotspots. By replacing the dielectric layer with the nanofluidic channel, we develop a sapphire (Al2O3)-based mid-infrared (MIR) hybrid nanofluidic-SEIRA (HN-SEIRA) platform for liquid sensors with the aid of a low-temperature interfacial heterogeneous sapphire wafer direct bonding technique. The robust atomic bonding interface is confirmed by transmission electron microscope observation. We also establish a design methodology for the HN-SEIRA sensor using coupled-mode theory to carry out the loss engineering and experimentally validate its feasibility through the accurate nanogap control. Thanks to the capillary force, liquid analytes can be driven into sensing hotspots without external actuation systems. Besides, we demonstrate an in situ real-time dynamic monitoring process for the acetone molecular diffusion in deionized water. A small concentration change of 0.29% is distinguished and an ultrahigh sensitivity (0.8364 pmol-1 %) is achieved. With the aid of IR fingerprint absorption, our HN-SEIRA platform brings the selectivity of liquid molecules with similar refractive indexes. It also resolves water absorption issues in traditional IR liquid sensors thanks to the sub-nm long light path. Considering the wide transparency window of Al2O3 in MIR (up to 5.2 μm), the HN-SEIRA platform covers more IR absorption range for liquid sensing compared to fused glass commonly used in micro/nanofluidics. Leveraging the aforementioned advantages, our work provides insights into developing a MIR real-time liquid sensing platform with intrinsic IR fingerprint selectivity, label-free ultrahigh sensitivity, and ultralow analyte volume, demonstrating a way toward quantitative molecule identification and dynamic analysis for the chemical and biological reaction processes.

Additive Combination of Spectra Reflected from Porous Silicon and Carbon/Porous Silicon Rugate Filters to Improve Vapor Selectivity.

Selectivity remains a challenge for rapid optical vapor sensing via light reflected from porous silicon photonic crystals. This work highlights a method to increase optical vapor selectivity of porous silicon rugate filters by analyzing additive spectra from two rugate filter substrates with different functionalities, an oxidized and carbonized surface. Individually, both porous silicon rugate filters demonstrated sensitivity but not selectivity toward the vapor analytes. However, differences in peak shift trends between the two substrates suggested differences in vapor affinities for the surfaces. By adding the two spectra, improvements to selectivity relative to the individual surfaces were observed even at low vapor pressures and for analytes of similar polarity, refractive index, and concentration. These results are expected to contribute toward optical vapor selectivity improvements in one-dimensional porous silicon photonic crystals.