Refractive Index Curves Research Articles

THz-TDS characterization of stress evolution of EB-PVD TBCs under thermal cycling

The failure of thermal barrier coatings (TBCs) during service is a critical issue affecting aeroengine efficiency. In this study, electron beam physical vapor deposition (EB-PVD) TBCs were subjected to thermal cycling at 1100 °C. Terahertz time-domain spectroscopy (THz-TDS) was employed to characterize the relationship between the evolution of characteristic frequency and the number of thermal cycles, along with the development of stress in the ceramic layer. In situ tensile test revealed a linear correlation between the characteristic frequency and micro strain derived from THz-TDS data, with a proportional coefficient of −9.94 between the characteristic frequency and the slope of the refractive index curve. The results show that the stress evolution can be effectively monitored by recording the trend of characteristic frequency. During the non-failure stage, the characteristic frequency followed a parabolic trend as thermal cycles increased, influenced by the growth of thermally grown oxide (TGO), ceramic layer sintering, and thermal mismatch. Approaching the failure stage, stress evolution was dominated by the expansion of vertical cracks in the ceramic layer and the widening of transverse cracks at the interface, which corresponded to a sudden change in the characteristic frequency.

Tailoring refractive index dispersion in ionic liquids: The influence of charge delocalization in cations

In this work, we study the contributions that different molecular blocks have in the wavelength-dependence of the refractive index in ionic liquids. The ionic liquids chosen for this work are combinations of the bis(trifluoromethylsulfonyl)imide anion with cations based on four different heterocycles with different extents of charge delocalization. The analysis is performed in terms of the experimental electronic polarizability, which is obtained by combining measurements of refractive index curves and densities via the Lorentz-Lorenz equation. Exploiting the additivity of electronic polarizability in ionic liquids, the contribution of the anion and the heterocycles of the cations is separated from that of the alkyl chains. Our results show important differences in these contributions, revealing a key influence of the charge delocalization in the cationic rings on the behavior of the refractive index dispersion. The understanding of how different parts of ionic liquids affect their refractive index dependence on wavelength would allow to gain precise control of this magnitude, enabling the development of customized optical materials for diverse applications in photonics and sensing technologies.

Improving the light transmission of silica glass using silicone as an anti-reflection layer for solar panel applications

The anti-reflection (AR) technology currently used in photovoltaic (PV) glass has reached its operational limit as the refractive index of existing materials cannot be lowered further. To overcome this, in this study, we selected formed methylsiloxane as an AR layer for PV glass. Its low refractive index (∼1.37) originates from the high content of methyl groups and the formation of voids during crosslinking. We acquired and compared the refractive index curves, conducted structural analyses, characterizations (optical, thermal, and surface), and performance evaluations to confirm the advantages of silicone as an AR layer for PV panels. The spreading and subsequent room-temperature self-curing characteristics of silicone considerably enhanced its applicability for upscaling and repair. The hydrophobic nature of the silicone AR layer imparted a new self-cleaning function to the solar panels; further, the methyl-silicone coating enhanced light transmission, resulting in electrical gain. Furthermore, the addition of the methyl-silicone layer created a refractive index gradient on the glass surface, making it fully compatible for use with the mainstream AR process, namely, the sol-gel method employed for PV glass.

Method for bandgap interpolation of perovskite's spectral complex refractive index.

Lead halide perovskites are a promising class of materials for solar cell applications. The perovskite bandgap depends on the material composition and is highly tunable. Opto-electrical device modelling is commonly used to find the optimum perovskite bandgap that maximizes device efficiency or energy yield, either in single junction or multi-junction configuration. The first step in this calculation is the optical modelling of the spectral absorptance. This requires as input the perovskite's complex refractive index N as a function of wavelength λ. The complex refractive index consists of real part n(λ) and imaginary part k(λ). For the most commonly used perovskites, n and k curves are available from spectroscopic ellipsometry measurements, but usually only for a few discrete bandgap energies. For solar cell optimization, these curves are required for a continuous range of bandgap energies. We introduce new methods for generating the n and k curves for an arbitrary bandgap, based on interpolating measured complex refractive index data. First, different dispersion models (Cody-Lorentz, Ullrich-Lorentz and Forouhi-Bloomer) are used to fit the measured data. Then, a linear regression is applied to the fit parameters with respect to the bandgap energy. From the interpolated parameters, the refractive index curve of perovskite with any desired bandgap energy is finally reconstructed. To validate our method, we compare our results with methods from literature and then use it to simulate the absorptance of a single junction perovskite and a perovskite/silicon tandem cell. This shows that our method based on the Forouhi-Bloomer model is more accurate than existing methods in predicting the complex refractive index of perovskite for arbitrary bandgaps.

Differential derivative method for highly accurate calculation of fiber refractive index curves

The refractive index of an optical fiber is an important parameter, which is closely related to the characteristics of dispersion, birefringence, and attenuation. It is vital to accurately calculate the refractive index in designing special optical fibers. However, the refractive index curves for different transmission modes of an optical fiber may be crossed or closed at some specific wavelengths, where the calculator may easily be tricked. A differential derivation method (DDM) is demonstrated to accurately calculate the refractive index curves by enhancing the mode tracking ability. The effectiveness is verified in the calculations of the refractive index curves for the HE11, HE12, TE01, and EH11 modes of a custom-designed fiber.

Elimination of the fundamental mode hybridization on an x-cut lithium-niobate-on-insulator by using a densely packed bent waveguide array.

Lithium niobate (L i N b O 3, LN) is a promising material for integrated photonics due to its natural advantages. The commercialization of thin-film LN technology has revitalized this platform, enabling low-loss waveguides, micro-rings, and compact electro-optical modulators. However, the anisotropic birefringent nature of X-cut LN leads to mode hybridization of TE and TM modes, which is detrimental to most polarization-sensitive integrated optical waveguide devices. A novel structure, to the best of our knowldege, utilizing a densely packed bent waveguide array is presented in this paper to eliminate mode hybridization. The refractive index is modulated in a manner that eliminates the avoided crossing of the refractive index curves of the TE and TM fundamental modes; thus, mode hybridization is prevented. The structures are readily accessible in the full range of commercially available LN film thicknesses from 400 to 720nm and in any etching depth. The proposed structures give a polarization extinction ratio of -30d B across all bend radii, while simultaneously maintaining low excess loss of less than -1d B after reaching a 100µm bend radius.

Growth, Thermal, and Optical Properties of a Novel Mid-Infrared Ho:LuYAP Laser Crystal

A novel mid-infrared Ho:Lu0.1Y0.9AlO3 laser crystal with a size of Φ26 mm × 80 mm was grown successfully by the Czochralski method. The crystal structure was characterized in detail by powder and single-crystal X-ray diffractions: Pnma (62) space group, a = 5.3234 Å, b = 7.3517 Å, c = 5.1670 Å, V = 202.22 Å3, and Z = 1. The X-ray rocking curves show that the crystal possesses high crystalline quality. It was observed by a transmission electron microscope that all atoms are neatly arranged and all elements are uniformly distributed. Meanwhile, the refractive index curves from 400 to 3000 nm at three crystalline axes and Mohs hardness were also achieved. The thermal expansion coefficient is 4.382 × 10–6 K–1, and thermal conductivity is 5.872 W·m–1·K–1. Particularly, the maximum phonon energy is only 552 cm–1, as exhibited by the Raman spectrum. Finally, the transmission, fluorescence spectra, and level lifetimes were measured and compared, which provided a basis for subsequent spectral theoretical analysis and laser experiment.

Fiber Residual Stress Effects on Modal Gain Equalization of Few-Mode Fiber Amplifier.

The modal gain equalization (MGE) of few-mode fiber amplifiers (FMFAs) ensures the stability of signal transmission. MGE mainly relies on the multi-step refractive index (RI) and doping profile of few-mode erbium-doped fibers (FM-EDFs). However, complex RI and doping profiles lead to uncontrollable residual stress variations in fiber fabrication. Variable residual stress apparently affects MGE due to its impacts on the RI. So, this paper focuses on the residual stress effects on MGE. The residual stress distributions of passive and active FMFs were measured using a self-constructed residual stress test configuration. As the erbium doping concentration increased, the residual stress of the fiber core decreased, and the residual stress of the active fibers was two orders of magnitude lower than that of the passive fiber. Compared with the passive FMF and the FM-EDFs, the residual stress of the fiber core completely transformed from tensile stress to compressive stress. This transformation led to an obvious smooth RI curve variation. The measurement values were analyzed with FMFA theory, and the results show that the differential modal gain of the FMFA increased from 0.96 to 1.67 dB as the residual stress decreased from 4.86 to 0.01 MPa.

Propagation Properties of Vortex Beams in a Helically Twisted Photonic Bandgap Fiber

We propose a helically twisted hollow-core photonic bandgap fiber (PBGF) for orbital angular momentum (OAM) generation. Results demonstrate that this fiber can effectively separate and transmit higher-order OAM modes while reducing crosstalk. The mode effective refractive index curves as a function of wavelength and the transmission characteristics of OAM are investigated in this letter. We show that this fiber has the potential to be used in OAM generators, light-controlling devices, and all-fiber optical communication applications.

Solubility, density and refractive index of formamide/N-methylformamide/N, N-dimethylformamide + Rb2SO4 + H2O ternary systems at 283.2, 298.2 and 313.2 K

In this work, the solid–liquid equilibrium (SLE) of the ternary systems formamide (FA)/N-methylformamide (NMF)/N, N-dimethylformamide (DMF) + Rb2SO4 + H2O was measured at temperatures of 283.2, 298.2, and 313.2 K. The isothermal solubility of Rb2SO4 in mixed solvents, the density and refractive index of the saturated solutions were determined. At a given temperature, with the increase of the amide content in the mixed solvent, the saturated solubility of Rb2SO4 decreased significantly. The density trend was consistent with the solubility. However, the refractive index increased with increasing the content of amide. The solubility increased with increasing the temperature, whereas, the density curves and refractive index curves at the three temperatures showed a point of intersection. In addition, the influence of the polarity and structure of amide on the solubility of the ternary system was also discussed. Finally, the experimental data were fitted with empirical equations.

A wavelength calibration method for photoelastic-modulated Fourier transform spectrometers

PurposeAn interferogram is produced by modulating the difference between the extraordinary refractive index and the ordinary refractive index for photoelastic crystals in photoelastic-modulated Fourier transform spectrometers (PEM-FTs). Due to the influence of the refractive index dispersion characteristics on the maximum optical path difference of the interferogram, it is necessary to study wavelength calibration methods.Design/methodology/approachA wavelength calibration method for PEM-FTs was proposed based on the modulation principle of the photoelastic-modulated interferometer and the relationship between the maximum optical path difference and the refractive index difference. A 632.8 nm narrow-pulse laser was used as a reference source to measure the maximum optical path difference () of the interferogram, and the parameter was used to calculate the discrete frequency points in the frequency domain. To account for the influence of refractive index dispersion on the maximum optical path difference, the refractive index curve for the photoelastic crystal was used to adjust the discrete frequency coordinates.FindingsThe error in the reconstructed spectral frequency coordinates clearly decreased. The maximum relative error was 2.5%. A good solar absorption spectrum was obtained with a PEM-FT experimental platform and the wavelength calibration method.Originality/valueThe interferogram is produced by adjusting the difference between extraordinary refractive index and ordinary refractive index for the photoelastic crystal in the PEM-FTs. Given the wavelength dependence on the refractive indices, in view of the modulation principle of the photoelastic modulated interferometer, the relationship between the maximum optical path difference and the refractive index difference, the variation law of the refractive index of the photoelastic crystal and the process of spectral reconstruction is presented in this paper.

Compact Ga2O3 Thin Films Deposited by Plasma Enhanced Atomic Layer Deposition at Low Temperature.

Amorphous Gallium oxide (Ga2O3) thin films were grown by plasma-enhanced atomic layer deposition using O2 plasma as reactant and trimethylgallium as a gallium source. The growth rate of the Ga2O3 films was about 0.6 Å/cycle and was acquired at a temperature ranging from 80 to 250 °C. The investigation of transmittance and the adsorption edge of Ga2O3 films prepared on sapphire substrates showed that the band gap energy gradually decreases from 5.04 to 4.76 eV with the increasing temperature. X-ray photoelectron spectroscopy (XPS) analysis indicated that all the Ga2O3 thin films showed a good stoichiometric ratio, and the atomic ratio of Ga/O was close to 0.7. According to XPS analysis, the proportion of Ga3+ and lattice oxygen increases with the increase in temperature resulting in denser films. By analyzing the film density from X-ray reflectivity and by a refractive index curve, it was found that the higher temperature, the denser the film. Atomic force microscopic analysis showed that the surface roughness values increased from 0.091 to 0.187 nm with the increasing substrate temperature. X-ray diffraction and transmission electron microscopy investigation showed that Ga2O3 films grown at temperatures from 80 to 200 °C were amorphous, and the Ga2O3 film grown at 250 °C was slightly crystalline with some nanocrystalline structures.

Determination of the atomic cohesive energy and Debye temperature from thermal stiffening of the dielectric constant of semiconductors

From the perspective of bond-order-length-strength correlation, we have formulated the thermally stiffened dielectric constant and refractive index for semiconductors. It is clarified that: (i) The dielectric constant and refractive index of semiconductors turn from non-linear to linear thermal stiffening at one third of its Debye temperature; (ⅱ) The Debye temperature determines the width of the non-linear part and the reciprocal atomic cohesive energy determines the slope of linear part at high temperatures of the dielectric constant and refractive index curves.

Growth, physicochemical and optical properties of LuYSGG garnet single crystal

We investigate the physical and chemical properties of a Lu1Y2Sc2Ga3O12 (LuYSGG) single crystal with high quality grown by the Czochralski (Cz) method. The XRC result shows the crystal has high crystalline quality and the lattice constant is calculated to be 12.4253 Å, and the lattice constant value could be adjusted by changing the ratio of Lu and Y. The dislocation density on the (111)-crystalline face is achieved to be about 200 cm−2. The Mohs hardness is obtained to be 6.86. The surface roughness (Ra) of the polished LuYSGG wafer is only 0.23 nm. The thermal expansion coefficient and the thermal conductivity are measured to be 8.5 × 10−6 K−1 and 4.79 W m−1 K−1, respectively, indicating the LuYSGG has better thermal properties. In addition, the Sellmeier equation is fitted by the measured refractive indices at a few distinct wavelengths using the method of minimum deviation angle, and the refractive index curve in the range of 400–3000 nm is displayed. The maximum phonon energy of LuYSGG crystal is determined to be 880.7 cm−1. All the obtained results can provide useful reference parameters for the further investigation of LuYSGG crystal as epitaxial magneto-optical thin film substrate and laser host.

Investigation of defect, mechanical, thermal properties and refractive index on an Er:LuYSGG mixed laser crystal

We investigate the defect, mechanical, thermal properties, and refractive index of Er:LuYSGG mixed crystal grown by the Czochralski method. The chemical etching pit patterns present a regular tetrahedron, which consists well with the corresponding atomic arrangement schematics. The Mohs hardness is obtained to be 6.60. The thermal expansion coefficient and the thermal conductivity along <111> crystallographic orientation are achieved to be 7.91 × 10−6 K−1 and 3.423 W m−1 K−1, respectively, indicating the Er:LuYSGG has a relatively high damage threshold. Based on the measured values of refractive index, the Sellmeier equation is fitted and the refractive index curve in the range of 400–3000 nm is displayed. All these results are of great significance for the further research and laser application of Er:LuYSGG crystal.

Variation of structure and optical material dispersion in lead borate glass containing multi valence chromium and germanium cations

In pursuit of manufacturing glass having a high refractive index and low dispersion, a series of lead and borate glasses containing germanium and chromium oxide by composition 25B2O3-73.8PbO-xGeO2-(1.2 − x)Cr2O3, with 0 ≤ x ≤ 1.2 mol.%, are prepared. IR measurements are carried out to explore the structure of as-prepared samples. The deconvoluted IR peaks revealed presence of bridging oxygen up to x = 0.6 mol%. At x > 0.6 mol% none bridging oxygen is the dominant. The dependence of the measured hydrostatic density and corresponding molar volume for the studied compositions are analyzed and discussed. The dispersion curves of refractive index (n) and absorption index (k) are calculated in the spectral range 300–2500 nm. The effect of increasing the ratio GeO2/[GeO2 + Cr2O3] on the calculated dispersion energy, lattice energy, oscillator energy and material dispersion are deduced. For photonic applications, the wavelength at zero material dispersion for the investigated samples is calculated and compared with other systems based on Borates, silicates and Germinate fiber. The results recommend that the present glass could be used in different modern optoelectronics devices.

SY9-1. Predicting and preventing long-term cardiovascular disease after preeclampsia

We investigate the defect, mechanical, thermal properties, and refractive index of Er:LuYSGG mixed crystal grown by the Czochralski method. The chemical etching pit patterns present a regular tetrahedron, which consists well with the corresponding atomic arrangement schematics. The Mohs hardness is obtained to be 6.60. The thermal expansion coefficient and the thermal conductivity along <111> crystallographic orientation are achieved to be 7.91 × 10−6 K−1 and 3.423 W m−1 K−1, respectively, indicating the Er:LuYSGG has a relatively high damage threshold. Based on the measured values of refractive index, the Sellmeier equation is fitted and the refractive index curve in the range of 400–3000 nm is displayed. All these results are of great significance for the further research and laser application of Er:LuYSGG crystal.

Effect of film growth thickness on the refractive index and crystallization of HfO2 film

A series of Hafnium dioxide (HfO2) thin films with nominal thickness of 5–350 nm were reactively deposited on silicon（100）substrates at 200 °C using electron-beam evaporation. Based on the measurement and characterization techniques of spectroscopic ellipsometry, X ray diffraction, scanning electron microscopy and atomic force microscopy, the effect of growth thickness on the refractive index and crystallization of HfO2 film grown under the same conditions was studied. The results indicate that the change of refractive index of HfO2 film is closely related to the change of microstructure i.e., grain size and crystallization which change obviously with the increasing growth thickness. The average refractive index at 633 nm decreases about from 1.97 to 1.84 with the increase of the average size about from 0.5 nm to 7~8 nm and the crystallinity from zero to 74% when the thickness of HfO2 film increases from 5 nm to 350 nm. Meanwhile, a critical growth thickness of HfO2 film, somewhere on order of 130 nm, is confirmed at which the film transforms from an amorphous to a polycrystalline monoclinic structure. The sharp change of microstructure near the critical thickness value of HfO2 film leads to the abrupt change of optical properties of HfO2 film, such as the turning behaviors of the refractive index curve. The correlation between phase transformation, size and orientation of crystallite, packing density and hence the refractive index is established. The possible interpretations are proposed to well understand the underlying mechanism of the evolution of optical properties in HfO2 film-forming process.

Design of a Hyperbolic Metamaterial as a Waveguide for Low-Loss Propagation of Plasmonic Wave

A stratiform hyperbolic metamaterial comprises multiple units of symmetrical metal-dielectric film, stacked to have a precisely equivalent refractive index, admittance, and iso-frequency curve. A metamaterial that is composed of stacks of symmetrical films as a waveguide to couple a diffracted wave into a horizontally propagating plasmonic wave is designed herein. By tuning the parameters of the constituent thin films within a hyperbolic metamaterial, both the loss of the plasmonic wave and admittance matching are minimized and optimized, respectively.

Improving thin film solar cells performance via designing moth-eye-like nanostructure arrays

Herein, bioinspired moth-eye-like nanostructures are presented as a possible solution for overcoming the limitation of weak absorption by thin film silicon solar cells. The interaction mechanism between the incident wave and such an artificial nanostructure is revealed by using the finite-difference time-domain method. Optical simulations show that the addition of biomimetic nanostructured surface reduces the refractive indices difference from external media environment to cell devices, thereby leads more incident light propagated into the active material. The optical design yield power conversion efficiencies of 14.6% and 17.5% for 2 and 5 μm thick absorbing layers, respectively. Further analysis depended on the equivalent medium theory demonstrates that a smooth, non– abrupt effective refractive index curve plays a key role for the light harvesting. The proposed method of addressing the weak absorption problem by thin film solar cells shows great promise.