Refractive Index Research Articles

Bio-material-based deformable out-coupling film with UV-blocking properties for stabilized organic light-emitting diodes

In multilayer organic light-emitting diodes (OLEDs), 70% of the generated light is lost due to differences in the refractive indices, resulting in total internal reflection. Therefore, external light-extraction films that can easily enhance efficiency are being researched. However, most of these films use petroleum-based polymers, raising waste disposal concerns. Additionally, ultraviolet (UV) light degrades organic materials. Hence, UV protection is essential for mitigating this damage. This study fabricated an eco-friendly external light-extraction film with UV-blocking capabilities using carboxymethyl cellulose (CMC) and sodium lignosulfonate (LS) extracted from plants. The phenolic groups in LS absorb UV light, while the sulfonate groups enhance its hydrophilicity, rendering it soluble in water. Therefore, a simple process was employed by casting a CMC-LS solution onto a microlens-array (MLA) template to fabricate a CMC-LS MLA film. The CMC-LS MLA film achieved 90% transmittance in the visible light range and exhibited excellent UV-blocking capabilities, simultaneously enhancing the lifespan of the OLED and improving the light-extraction efficiency by 30%.

Tunning the omnidirectional bandgap of nanoporous silicon using a semi-sinusoidal refractive index profile

We report the theoretical comparison of the omnidirectional bandgap in a 1-D photonic crystal using sinusoidal and semi-sinusoidal refractive index profiles. It is found that the corresponding omnidirectional bandgap of the semi-sinusoidal widens and shifts to a higher wavelength range as a function of the asymmetric ratio of semi-sinusoidal profile. The asymmetric ratio plays an essential role in tunning the width of the omnidirectional bandgap due to the changed average refractive index and optical thickness. The semi-sinusoidal refractive index is experimentally achieved by changing the current waveform. Novel omnidirectional nanoporous silicon mirrors with an omnidirectional bandgap covering optical communication wavelength (1550 nm) were fabricated using a semi-sinusoidal current waveform. The experimental analogy was carried out by building up the multilayered dielectric structures of omnidirectional mirrors by anodic etching using a semi-sinusoidal current waveform. The experimental results were compared with the theoretical results investigated by the Transfer matrix method. It was shown that the distorted current profile impacts the quality of the omnidirectional bandgap although it does not affect the porous size range.

Characterising Optical Properties of Doped Metal Complex Nanocomposite Films with PVA/PVAC for Optoelectronics

This study investigates the optical properties of nanocomposite films made from polyvinyl alcohol (PVA), polyvinyl acetate (PVAC), and a 50/50 polyblend of PVA and PVAC, doped with varying concentrations 0, 3, 6, and 9 wt.% of the metal complex Fluoro pentaamine cobalt (III) fluoride [Co(NH₃)₅F]F₂ (FACF). The films were prepared using a solvent casting method and analysed with a computerised UVـــVISـــIR spectrophotometer over the wavelength range of 250–850 nm. Key optical parameters, including the refractive index, extinction coefficient, and optical conductivity, were determined through absorption studies. The optical absorbance increased with higher FACF concentrations. UV-Vis spectra were used to estimate the band gap energies and investigate the electronic transitions from the valence band to the conduction band. A decrease in the band gap for allowed direct transitions was observed: from 4.0 eV to 2.55 eV for PVA, 4.1 eV to 2.6 eV for PVAC, and 4.35 eV to 2.6 eV for the polyblend as the FACF content increased. Additionally, both the extinction coefficient and refractive index increased with higher FACF concentrations. These nanocomposite films show potential for use in photovoltaic cells, optical sensors, LEDs, photodetectors and other luminescent devices, though further research is needed. KEYWORDS absorbance spectrum, casting method, energy gap, polyblend, polyvinyl acetate, polyvinyl alcohol

Advanced spectroscopic approach for exploring the structural, optical, and electronic properties in dye-functionalized chitosan biopolymers

This study employed advanced spectroscopic techniques to investigate the structural and optical properties of chitosan (CS) biopolymer films modified with natural dyes from Cosmos Sulphureus Cav. (CSC) flowers. FTIR results indicated that the inclusion of CSC dyes led to broader absorbance and decreased transmittance. Distinct absorption regions were identified, and the optical energy band gap (OEBG), transport gap, and exciton binding energy were calculated using Tauc’s method. The OEBG was found to be 5.44 eV for CS while for CS-CSC dye samples, it dropped to 2.24 eV and The Urbach energy increased from 0.44 eV to 0.60 eV, indicating the presence of high tail states in the band gap region. The electron–phonon interaction was found to increase from 11.35 to 15.58. The oscillator energy values (4.13 eV-2.33 eV) at low energies obtained using Wemple-DiDomenico model are found to be close to OEBGs using Tauc’s model. Additionally, from the Drude-Lorentz model the N/m* was found to increase from 1.69 × 1052 to 1.27 × 1054. The third order non-linear polarizability parameter, the linear optical susceptibility and the non-linear index of refractions were all found to increase upon increasing the CSC dye concentrations.

Beyond two-octave coherent OAM supercontinuum generation in air-core Ge-doped ring fiber

Orbital angular momentum (OAM) has emerged as a revolutionary technology for communication networks due to its ability to significantly increase the channel capacity. However, traditional optical fibers present significant hurdles to harnessing OAM’s full potential, including dispersion and limited bandwidth, which facilitates investigations on supercontinuum (SC) generation for OAM beams. In this paper, an air-core Ge-doped ring fiber is proposed and designed to support high-order OAM mode up to |l| = 24. To achieve this, the fiber has a high refractive index difference between the ring core and the cladding, enabling stable transmission of high-order OAM modes. A key feature of this design is the OAM24,1 mode, which exhibits near-zero and flat dispersion. This characteristic translates into high coherence and a remarkably broad SC generation. The generated SC spans over two octaves (2336 nm) within the infrared wavelength range (764 nm to 3100 nm) at a power level of −40 dB. Furthermore, by optimizing the structural parameters, we ensure near-zero and flat dispersion characteristics for the other OAM modes (|l| < 24), along with broad SC generation exceeding two octaves. This fiber design holds significant promise for future advancements in OAM beam transmission within the infrared spectrum.

Photothermally‐Engineered Crystallization of GAP‐Se Bulk Chalcogenide Nanocomposites toward the Realization of 3D Gradient Refractive Index Profiles

AbstractTailorability of a medium's optical properties, specifically refractive index and dispersion, is key to enabling compact optical designs. Chalcogenide glasses (ChGs) are widely used for infrared (IR) imaging applications, and the development of gradient refractive index (GRIN) optics. This work extends efforts to create and characterize 3D GRIN profiles in bulk multi‐component Ge‐As‐Pb‐Se (GAP‐Se) ChGs through spatially selective conversion of commercial glass to glass ceramic. This work extends prior efforts on bulk and film lab‐scale glass media, to that of a commercially produced material with improved optical homogeneity. Laser‐induced crystallization upon heat treatment results in the formation of high index Pb‐containing crystals that contribute to an increase in the nanocomposite's resulting effective refractive index, neff. The material's induced crystallinity imparted via laser exposure and heat treatment using metrology tools such as refractometry, X‐ray diffraction, FTIR, and TEM are studied. The resulting material response is quantified which is shown to be modulated via laser dose in both lateral and for the first time, axial directions enabling the first demonstration of a true, 3D GRIN profile. By comparing these outcomes to prior radial GRIN structures, the promise of these media as candidate materials for infrared systems.

Strain effect on structural, electronic and optical properties of Gra/MoS2 heterobilayer

Abstract Heterobilayers formed by stacking graphene and MoS2 monolayers have been successfully synthesized in the laboratory, presenting a highly promising material with versatile applications due to its flexibility and enhanced optical properties. In this study, we employ density functional theory to investigate the impact of biaxial and uniaxial strain on the structural, electronic and optical properties of Graphene/MoS2 (Gra/MoS2) heterobilayer. The unstrained Gra/MoS2 is shown to be a thermodynamically stable structure with binding energy -0.736 eV and a band gap of 1.899 meV, which opens at the Dirac point of graphene. Gra/MoS2 is also shown to have a better refractive index and absorption coefficient when compared to MoS2 monolayer in the visible region. Under biaxial strain, heterobilayer became unstable as the structure of both monolayers changed. The variation of biaxial strain from -4% to 4% enables tuning of the band gap within a range of 1.07 meV to 6.623 meV. The Fermi level shifted towards the conduction band under compressive strain and towards the valence band under expansive strain. Manipulation of biaxial strain also facilitates tunability in energy peaks for optical properties such as the real and imaginary parts of the dielectric function, refractive index and absorption coefficient in the visible spectrum. In contrast, uniaxial strain induces minimal changes in the overall structure and stability of the heterobilayer, resulting in a limited band gap range of 1.118 meV to 2.4 meV. Furthermore, uniaxial strain fails to introduce significant tunability in the optical properties of the heterobilayer.

An investigation on structural, morphological, thermal, optical, and antibacterial properties of chitosan-oleic acid (CH-OA) composite thin films

Recent researches show an increasing interest in developing polymer composite films to improve their properties for use in biodegradable food packaging and optoelectronic applications. For this purpose, this study aims to fabricate and characterize chitosan-oleic acid (CH-OA) composite films for different optical applications. Solution casting process was used to prepare CH-OA composite films. XRD patterns of the films showed that the addition of OA up to 3.0 % (v/w) reduced their crystalline structure. The morphology of the composites was studied using SAED and SEM. SAED patterns indicated good mixing ability between OA and CH, while SEM analysis revealed the heterogeneous surface of the films and proved the incorporation of OA into the CH matrix. The thermal stability of the films was studied using DSC and TGA analyses of the composites, and their thermograms indicated the interactions between CH and OA which improved their thermal stability. The optical properties of the films were performed in the spectral region of 250–2500 nm. The near-infrared results showed that increasing the OA concentration can lead to strong local interactions between OA and other groups in the CH matrix. The transmittance spectra of CH-OA composites at UV wavelength ≤ 400 nm showed values ​​close to 0 %, indicating that all the films have UV-blocking features. Significant changes in color and opacity parameters were detected. In the visible range (400–700 nm), the absorption coefficients, absorption indices and refractive indices of the films were evaluated. The absorption edges, Urbach energies, direct and indirect energy band gaps were in the ranges of 2.491–1.849 eV, 0.591–2.008 eV, 2.698–2.578 eV and 2.167–1.422 eV, respectively, as well as the refractive index decreased from 1.837 to 1.735 to 1.149–1.119 by increasing the OA concentration to 3.0 % (v/w). The produced composites have high optical conductivities up to 1010S−1 indicating good photoresponse and enhanced electronic excitation due to increased absorption coefficient of the film. The Wemple-DiDomenico model was used to evaluate the dispersion energies and related parameters. The values ​​of dispersion energy and oscillator energy decreased from 12.388 to 1.327 eV and 6.7268 to 5.7525 eV, respectively, with the OA content increasing to 3.0 % (v/w). The swelling degree and degradation rate were also determined and the results showed an improvement in the swelling and degradation ability of the films. The composites showed the appearance of inhibition zones indicating antibacterial activity against Bacillus cereus, Staphylococcus aureus, and Escherichia coli. These obtained results indicate that CH-OA composite films exhibited better physico-chemical properties with significant improvement and thus can be used in biodegradable food packaging and optoelectronic devices.

Tailoring Optical Performance of Polyvinyl Alcohol/Crystal Violet Band-Pass Filters via Solvent Features

Optical filters are essential components for a variety of applicative fields, such as communications, chemical analysis and optical signal processing. This article describes the preparation and characterization of a new optical filter made of polyvinyl alcohol and incremental amounts of crystal violet. By using distinct solvents (H2O, dimethyl sulfoxide (DMSO) and H2O2) to obtain the dyed polymer films, new insights were gained into the pathway that underlies the possibility of tailoring the material’s optical performance. The effect of the dye content on the sample’s main properties was inspected via UV–VIS spectroscopy analysis combined with colorimetry, refractometry and atomic force microscopy experiments. The results revealed that the colorimetric parameters are affected by the dye amount and are dramatically changed when the solvent used for film preparation is different. The rise in the refractive index upon polymer dyeing was due to the synergistic effect of the larger polarizability of the dye and the occurrence of hydrogen bonds among the system components. Spectral data evidenced that samples prepared in H2O and DMSO preserve the absorption characteristics of the added dye, whereas H2O2 acts as an oxidizing agent and enhances transparency. Also, for the first two solvents, multiple absorption edges were noted as a result of dye incorporation, which was responsible for the occurrence of new exciton-like states, hence the band gap reduction. The films processed in H2O were able to block radiations in the 506–633 nm range while allowing other wavelengths to pass with a transmittance above 90%. The samples attained in DMSO presented similar properties, with the difference that the domain of light attenuation was shifted towards higher wavelengths. Atomic force microscopy showed the dye’s effect on the level of surface roughness uniformity and morphology isotropy. The dyed polymer foils in non-oxidizing agents have suitable features for use as band-pass filters.

Investigating the physical properties of the binary compound CdTe under different pressures using FP-LMTO method

The structural, electronic and optical properties of Cadmium Telluride (CdTe) are investigated at different pressures using the full potential linear muffin-tin orbital FP-LMTO method based on density functional theory (DFT), within the local density approximation LDA and the generalized gradient approximation GGA approximations were used to calculate the exchange-correlation potential. The ground state properties of CdTe compound were determined for diverse phases, and the band structure and optical parameters were calculated at pressures ranging from 0 GPa to 50 GPa. Optical parameters, including real part, imaginary part of the dielectric function, reflectivity, refractive index, extinction coefficient and energy loss function, were calculated for over an energy range from 0 eV to 13.6058 eV. The results were compared with available experimental and theoretical data, showing good agreement with earlier reported result. These particularities reveal that CdTe as favorable material for optoelectronic applications, as it exhibits together important electronic and optical responses under different pressure.

Dynamics of lead selenide films modification by laser annealing

The paper considers the modification of the structure and optical properties of chalcogenide films of lead selenide (PbSe) as a result of exposure to a continuous wave (CW) laser with 405 nm wavelength. The mechanisms of oxide layer formation and the accompanying changes in the optical properties of films during laser exposure were considered. Based on the measured reflection and transmission spectra of film regions modified by laser radiation with different exposure times (1–12 s), the extinction coefficient and refractive index were calculated. The study reveals significant modifications in the films' optical constants during the early stages of laser irradiation, leading to oxidation and melting, which are further stabilized by phase transitions. Additionally, temperature field analysis showed that the film's surface temperature in the laser spot's center is strongly influenced by the time-dependent absorption capacity, especially from the 3rd second of exposure onward.

Whispery gallery mode plasmonic biosensor based on intensifying graphene layer

This study presents a novel whispery gallery mode (WGM) biosensor based on a plasmonic nanoring structure. The biosensor's design incorporates a unique combination of gold-coated plasmonic nanoring and graphene in which graphene serves as both an intensifying and receptor layer, allowing binding and detection. According to the classification of essential biomolecules, the proposed biosensor is capable of different biomolecules detection including DNA, RNA, adenine, cytosine, guanine, thymine, and uracil. The biosensor's performance was optimized using the FEM solution method, simulating the optimal sizes of the plasmonic layer and the dimensions of the silicon nanoring. The obtained results confirm high sensitivity within the studied range of refractive index, around 105 nm/RIU. Additionally, the highest Q-factor, figure of merit and minimum detection limit are around 4179.078, 276.315 RIU−1and 3.619 ×10−3RIU, respectively. This biosensor demonstrates great potential in identifying biomolecular structures as presented in this paper, serving as a valuable tool for detecting and analyzing complex biological materials.

Chromatic Aberration in Wavefront Coding Imaging with Trefoil Phase Mask

The refractive index of the lenses used in optical designs varies with wavelength, causing light rays to fail when focusing on a single plane. This phenomenon is known as chromatic aberration (CA), chromatic distortion, or color fringing, among other terms. Images affected by CA display colored halos and experience a loss of resolution. Fully achromatic systems can be achieved through complex and costly lens designs and/or computationally when digital sensors capture the image. In this work, we propose using the wavefront coding (WFC) technique with a trefoil-shaped phase modulation plate in the optical system to effectively increase the resolution of images affected by longitudinal chromatic aberration (LCA), significantly simplifying the optical design and reducing costs. Experimental results with three LEDs simulating RGB images verify that WFC with trefoil phase plates effectively corrects longitudinal chromatic aberration. Transverse chromatic aberration (TCA) is corrected computationally. Furthermore, we demonstrate that the optical system maintains depth of focus (DoF) for color images.

Polycrystalline magneto-optical transparent Pr2Zr2O7 pyrochlore ceramic for Faraday rotation.

To design an innovative magneto-optical material aimed at a large Verdet constant coincides with the development trend of state-of-the-art modern optical devices. In this work, a magneto-optical transparent Pr2Zr2O7 ceramic with pyrochlore structure was successfully fabricated by vacuum sintering plus hydrogen reduction for the first time to our knowledge. The two- and three-dimensional images observed on the laser scanning confocal microscopy reveal that the grain-boundary dent depth of the polished Pr2Zr2O7 ceramic is only ∼1.6 µm upon thermal etching at a high temperature of 1675°C for 2 h, showing excellent heat-resistance ability. The crystal-clear Pr2Zr2O7 ceramic has a high in-line transmittance of ∼71.6% at 780 nm (∼92.3% of the theoretical value), a high refractive index of ∼2.1, and an effective magnetic moment of ∼5.28 µ B. In the visible region, the O2- polarizability and optical basicity of the Pr2Zr2O7 compound generally retain constants around 2.37 ± 0.07 Å3 and 0.965 ± 0.015, respectively. The magneto-optical transparent Pr2Zr2O7 ceramic exhibits high Verdet constants of -336 ± 4, -208 ± 3, -108 ± 1, and -48 ± 2 rad·T-1·m-1 at 515, 635, 780, and 1064 nm, respectively, which were ∼1.55-fold higher than the commercial Tb3Ga5O12 single crystal. Our developed new-type ceramic material has excellent potential application for Faraday rotation.

Fano Resonance-Associated Plasmonic Circular Dichroism in a Multiple-Dipole Interaction Born–Kuhn Model

Plasmon chirality has garnered significant interest in sensing application due to its strong electromagnetic field localization and highly tunable optical properties. Understanding the effects of mode coupling in chiral structures on chiral optical activity is particularly important for advancing this field. In this work, we numerically investigate the circular dichroism (CD) of elliptical nanodisk dimers arranged in an up-and-down configuration with a specific rotation angle. By adjusting the inter-particle distance and geometric parameters, we introduce the coupling between dipole and electric hexapole modes, forming an extended Born–Kuhn model that achieves strong CD. Our findings show that the coupling of dipole modes with electric hexapole modes in elliptical nanodisks can also show obvious Fano resonance and a strong CD effect, and the structure with the largest Fano asymmetry factor shows the highest CD. In addition, CD spectroscopy is highly sensitive to changes in the refractive index of the surrounding medium, especially in the visible and near-infrared regions, highlighting its potential for application in high-sensitivity refractive index sensors.

Dynamical structures of optical solitons for highly dispersive perturbed NLSE with $$\beta $$-fractional derivatives and a sextic power-law refractive index using a novel approach

Dynamical structures of optical solitons for highly dispersive perturbed NLSE with $$\beta $$-fractional derivatives and a sextic power-law refractive index using a novel approach

Compensation of temperature effects on imaging quality of thermal imaging objectives using deep learning techniques

Thermal imaging objectives are made from infrared materials with large thermal expansion coefficients, such as Ge, Si, and ZnSe. When the temperature changes, it leads to variations in the refractive index, curvature radius, and thickness of the lens, causing defocus shifts that degrade the image quality of the thermal imaging system. In this paper, we propose a novel method to compensate for the effects of temperature variations on the quality of thermal imaging objectives by using deep learning techniques. The temperature variations are measured using a thermal sensor. Subsequently, a U-Net network is employed to mitigate the impact of temperature on the imaging quality of the thermal imaging objectives without requiring any optical displacement or replacement of the lens. Simulation results show that the proposed method performs the effectively compensation for the influence of temperature changes on thermal imaging objective over a wide temperature range from -5 °C to 50 °C.

Measurement of plasma characteristic parameters of copper foil explosion using interferometry

The accurate testing of plasma temperature and electron density and shock wave pressure during an electroburst in a copper foil transducer is critical for the characterization of the detonation performance of its elements. In this paper, the sequence of interferograms during the detonation of a copper foil transducer is captured at a frame rate of 3×106fps in conjunction with Mach–Zehnder interferometry and high-speed photography, and the results clearly demonstrate the propagation of the shock wave wavefront and plasma. The phase differences disturbed by plasma are extracted using the Fourier transform method, and the refractive index distributions are reconstructed with the Abel algorithm. Subsequently, based on the refractive index models of the shock wave and plasma, the shock wave pressure and plasma temperature and electron density are partitioned and reconstructed. Results show that the maximum shock wave pressure in the detonation of the copper foil transducer element is 1.297 atm, the maximum plasma temperature is 16,280 K, and the maximum plasma electron density is 2.134×1017cm−3. This study provides a theoretical and technical foundation for the detonation performance testing of pyrotechnic energy-conversion components.

Influence of Bi2O3 on structural and optical characteristics of PbO⋅B2O3⋅Bi2O3⋅SiO2 glasses

In this study, the influence of Bi2O3 on the structural and optical characteristics of the glass samples with the molar composition of 17PbO⋅23B2O3⋅xBi2O3⋅(60-x)SiO2, prepared using the conventional melt quenching approach, is described. X-ray diffraction spectra reveal the amorphous nature of the prepared series. Density (D), molar volume (Vm), glass transition temperature (Tg), and other physical parameters have been evaluated. FTIR spectra show the presence of bismuth as [BiO3] and [BiO6] structural units and of borate as [BO3] and [BO4] structural units. It is found that the increase in non-bridging oxygens can be attributed to the increase in cut-off wavelength and decrease in bandgap energy with the rise in bismuth concentration. Urbach's energy values demonstrate that the presence of Bi2O3 in the existing glass system might affect the defect concentration. All glass samples possess a high refractive index suggesting that the current glasses may be useful for developing non-linear optical systems.

Spatial and reconfigurable control of photoluminescence from single-layer MoS2 using a strained VO2-based Fabry–Pérot cavity

We investigated the photoluminescence (PL) from single-layer MoS2 on VO2 platelets grown on SiO2, where the insulating and metallic phases can coexist above a bulk transition temperature of 340 K, due to the inhomogeneous strain. We found that the intensity of PL from MoS2 on metallic VO2 is higher than that on the insulating counterpart, resulting in spatially varying PL even at the sub-micrometer scale. In contrast to the intensity, the PL peak energies were observed to be nearly identical on insulating and metallic VO2, indicating that the influences of charge transfer, strain, and dielectric screening on MoS2 are comparable, regardless of the phase state. Thus, the observed difference in PL intensity is due to the difference in refractive indices of insulating and metallic VO2, leading to the phase-dependent Fabry–Pérot interference effect. We performed numerical simulations for the emission from MoS2 supported on the VO2-based Fabry–Pérot interferometer. The calculated emission intensity ratio on insulating and metallic VO2 well reproduces the experimental observations. These results suggest a strategy for controlling PL from two-dimensional semiconductors in a spatial and reconfigurable manner.