Extraordinary Refractive Indices Research Articles

Computational studies of the optical properties of 4-cyano-4-hexyl biphenyl using a combination of DFT and molecular dynamics simulation

ABSTRACT In this study, we predicted the optical properties of the liquid crystal 4-cyano-4-hexylbiphenyl (6CB) by employing a combination of Density Functional Theory (DFT) and Molecular Dynamics simulation. We obtained the polarisability and anisotropy of polarisability of the liquid crystal through DFT calculations by employing a variety of basis sets and implementing them in Gaussian 09. The density and order parameters of the liquid crystal at different temperatures were determined from Molecular Dynamics simulations and implemented in NAMD. The extraordinary and ordinary refractive indices were then predicted from the Vuks equation. The effect of long-range corrected DFT calculations on polarisability and anisotropy of polarisability was also explored with the functionals LC-BLYP and CAM-B3LYP. We found that at static frequencies, the basis set B3LYP 6–31 G (2d,p) produced excellent results in determining refractive indices with maximum errors of 4.1% and 3% in the nematic and isotropic phases, respectively. However, the B3LYP functional was unsuitable for determining refractive indices at a wavelength of 589 nm. In contrast, the long-range functional CAM-B3LYP combined with the basis set 6–31 G (2d,p) resulted in maximum errors of 3.7% in the nematic phase and 2.5% in the isotropic phase when determining refractive indices at 589 nm.

Investigation of Morphological and Optical Properties of LiNbO3 and LiNbO3:Fe 0.03 wt.% Crystals

This article is devoted to the morphological and optical properties of the photorefractive crystal LiNbO3 and LiNbO3:Fe 0.03 wt.%. According to it, the surface morphology of the samples was studied using an atomic force microscope (AFM). In addition, ordinary and extraordinary refractive indices of the LiNbO3 crystal were calculated using empirical formulas. The results of the diffraction efficiency of the LiNbO3:Fe 0.03 wt.% crystal for He-Ne and He-Cd lasers are presented.

Anisotropic tunning of MoS2-nanosheets by hydrodynamics force in microchannel: Reflection and transmission modes

In this research, a two-dimensional liquid crystal of molybdenum disulfide (LC-MoS2) was prepared using the exfoliation method in the liquid phase. A mechanical-hydrodynamic approach was proposed for anisotropy tuning in a microfluidic microchannel in two modes: transmission and reflection. By measuring the ordinary and extraordinary refractive indices at six flow rates in the reflection mode, the birefringence value of the samples was measured up to 0.006. Additionally, these measurements for the transmission mode showed a birefringence of approximately 2×10−4. Furthermore, the effect of concentration was measured in addition to the flow rate. The results indicated that increasing the flow rate of MoS2 nanosheets led to an increase in the induced birefringence. Similarly, increasing the concentration of LC-MoS2 resulted in an increase in the birefringence of the sample.

Tunable Photonic Hook Design Based on Anisotropic Cutting Liquid Crystal Microcylinder

The selective control and manipulation of nanoparticles require developing and researching new methods for designing optical tweeters, mainly based on a photonic hooks (PHs) effect. This paper first proposes a tunable PH in which a structured beam illuminates an anisotropic cutting liquid crystal microcylinder based on the Finite-DifferenceTime-Domain (FDTD) method. The PHs generated by plane wave, Gaussian, and Bessel beam are analyzed and compared. The impact of beams and LC particle parameters on the PHs are discussed. Where the influence of the extraordinary refractive index (ne) on PHs is emphasized. Our results reveal that introducing birefringence can change the bending direction of PH. Besides, the maximum intensity of the PHs increases as ne increases regardless of the beam type. The PH generated by a plane wave has a higher maximum intensity and smaller FWHM than that generated by the Gaussian and Bessel beams. The smallest FWHM and maximum intensity of the PHs generated by the Gaussian falls between that generated by the plane wave and the Bessel beam. The PH generated by a Bessel beam has the minor maximum intensity and the largest FWHM. Still, it exceeds the diffraction limit and exhibits bending twice due to its self-recovery property. This paper provides a new way to modulate PH. This work offers novel theoretical models and the degree of freedom for the design of PHs, which is beneficial for the selective manipulation of nanoparticles. It has promising applications in Mesotronics and biomedicine.

An investigation into the effect of Gd on the optoelectronic properties of ZnO waveguide thin films by prism coupler

GdxZn1−xO (x=0, 0.005, 0.01, 0.02, 0.04) films were elaborated via spray pyrolysis on ordinary glass to explore GdZnO's potential as a waveguide. X-ray diffraction showed the incorporation of Gd in the wurtzite structure with decreasing crystallite size and a pronounced alignment along the [002] direction. UV-Vis measurements indicated a 0.02 eV increase in the optical gap energy, starting from 3.24 eV. Hall effect measurements showed a 104 decrease in charge carriers' density, mobility increase up to 209 cm2/Vs, and a 103 increase in magnetoresistance. Photoluminescence spectra of the films revealed the presence of different kinds of point defects with a slight tendency to promote Zni and VO defects. Atomic force microscopy assessments depicted randomly oriented and elongated grains over Gd doping and overall smoother films. The waveguide properties of the films were investigated using the prism coupler techniques. A slight and general decrease was observed in ordinary refractive indices going from 1.9745 in the ZnO film to 1.9713 in the Gd0.02Zn0.98O film while the extraordinary refractive index decreased from 1.9777 to 1.9692. The birefringence was inverted for all the doped films except for Gd0.04Zn0.96O film as it experienced no guiding modes at all. The simulated results were highly aligned with the prism coupler model of distinct interfaces and step refractive index.

Optical properties and passive self Q-switching of floating zone grown Cr–Nd co-doped YVO4 crystal

Single crystals of Cr (0.7, 1.0, 1.4 and 2.0 at%) and Nd (0.7 at%) co-doped yttrium vanadate (YVO4) were grown by optical floating zone method. [100]-oriented elements were fabricated from the grown crystals for optical and lasing studies. The refractive index analysis revealed that the values of ordinary refractive indices (no) for 0.7, 1.0, 1.4 and 2.0 at% Cr doped samples were found to be almost constant except for a slight increase corresponding to 532 nm. But the extraordinary refractive indices (ne) was found to increase with Cr doping concentration and it is maximum at 1.4 at% for all the wavelengths. The thermal coefficient of ne and no increases slightly and becomes constant beynd 1.4 at% of Cr. The band gap of the crystal is polarization sensitive with the values in the range of 3.51–3.55 eV and 3.59–3.62 eV for electric field parallel and perpendicular to c axis, respectively. The band gap does not exhibit any significant change on Cr doping. The emission spectrum with excitation at 808 nm is characterized by three broad asymmetric bands centered at 896, 1064 and 1341 nm. The emission intensity for the peak around 1064 nm decreases with an increase in Cr due to a broad absorption around 1100 nm of Cr5+ ion and, therefore, this band is exploited for passive Q-switching in the crystal. Further, excitation at 400 nm of Cr band revealed that there is strong emission at 1064 nm, which may be explored for pumping the gain medium with solar radiation. Finally, a stable passive Q-switching at 1064 nm was demonstrated in [100] element fabricated from the gown crystal having 1.4 at% Cr. An output power of ∼108 mW having pulse-width ∼220 ns and repetition rate ∼239 kHz was achieved for a pumping of ∼14.6 W at 808 nm.

Enhancing Light Outcoupling Efficiency via Anisotropic Low Refractive Index Electron Transporting Materials for Efficient Perovskite Light-Emitting Diodes.

Thanks to the extensive efforts toward optimizing perovskite crystallization properties, high-quality perovskite films with near-unity photoluminescence quantum yield are successfully achieved. However, the light outcoupling efficiency of perovskite light-emitting diodes (PeLEDs) is impeded by insufficient light extraction, which poses a challenge to the further advancement of PeLEDs. Here, an anisotropic multifunctional electron transporting material, 9,10-bis(4-(2-phenyl-1H-benzo[d]imidazole-1-yl)phenyl) anthracene (BPBiPA), with a low extraordinary refractive index (ne) and high electron mobility is developed for fabricating high-efficiency PeLEDs. The anisotropic molecular orientations of BPBiPA can result in a low ne of 1.59 along the z-axis direction. Optical simulations show that the low ne of BPBiPA can effectively mitigate the surface plasmon polariton loss and enhance the photon extraction efficiency in waveguide mode, thereby improving the light outcoupling efficiency of PeLEDs. In addition, the high electron mobility of BPBiPA can facilitate balanced carrier injection in PeLEDs. As a result, high-efficiency green PeLEDs with a record external quantum efficiency of 32.1% and a current efficiency of 111.7cd A-1 are obtained, which provides new inspirations for the design of electron transporting materials for high-performance PeLEDs.

Controlling the Optical Properties of Transparent Auxetic Liquid Crystal Elastomers.

Determining the tunability of the optical coefficients, order parameter, and transition temperatures in optically transparent auxetic liquid crystal elastomers (LCEs) is vital for applications, including impact-resistant glass laminates. Here, we report measurements of the refractive indices, order parameters, and transition temperatures in a family of acrylate-based LCEs in which the mesogenic content varies from ∼50 to ∼85%. Modifications in the precursor mixture allow the order parameter, ⟨P2⟩, of the LCE to be adjusted from 0.46 to 0.73. The extraordinary refractive index changes most significantly with composition, from ∼1.66 to ∼1.69, in moving from a low to high mesogenic content. We demonstrate that all LCE refractive indices decrease with increasing temperature, with temperature coefficients of ∼10-4 K-1, comparable to optical plastics. In these LCEs, the average refractive index and the refractive index anisotropy are tunable via both chemical composition and order parameter control; we report design rules for both.

Propagation of Lorentz-Gaussian elliptical multi-Gaussian correlated Schell-Model beam in uniaxial crystal

The evolution behavior of the intensity profile, beam spreading, and spectral degree of coherence of the Lorentz-Gaussian elliptical multi-Gaussian correlated Schell-Model beam propagating in uniaxial crystal are studied. The expression of the cross spectral density of the beam in uniaxial crystal is obtained based on Huygens-Fresnel integration, and the expressions of spectral coherence and root mean square (rms) beam width are derived. The beam spot is circular or elliptical while the ordinary and extraordinary refractive indices of the crystal are equal or unequal, respectively. The ellipticity of the light spot is affected by the Lorentz parameters. The larger the ratio of ordinary to extraordinary refractive index in uniaxial crystal is, the larger the spectral coherence of beam and rms beam waist width is. The spectral coherence increases with the increase of the Gaussian beam waist and the decrease of the Lorentz parameters. The rms beam width of the beam increases with the increase of the Gaussian beam width and Lorentz partial parameters. With the increase of the transmission distance, the influence of the light source parameters on the rms beam width gradually weakens. The total number of terms of multi-Gaussian correlated Schell-Model (MGSM) source and the spatial coherence lengths add additional degree of freedom for adjusting the spot pattern inside crystal.

Birefringent optical responses of single-chirality carbon nanotube membranes

Membranes composed of single-walled carbon nanotubes (SWCNTs) with a specific chiral structure (chirality) possess unique optical properties dominated by thermally robust quasi-one-dimensional excitons. Therefore they have emerged as promising materials for photonic and/or thermo-optic applications. As SWCNTs in the membranes are usually aligned two-dimensionally in a plane, and the optical responses of individual SWCNTs are highly anisotropic, the macroscopic responses of membranes are expected to depend on the light-propagation direction. However, the complex refractive-index spectra of some membrane axes are unknown, hindering the optical design of SWCNT-based devices that respond to arbitrarily propagating light. Here, we report the anisotropic complex refractive index spectra of a single-chirality SWCNT membrane fabricated via vacuum filtration. The polarization- and angle-resolved reflection spectra were obtained in the near-infrared-to-visible region. All spectral features in different polarizations and incident angle configurations were consistent with a uniaxial birefringent membrane, reflecting the random in-plane orientations of the SWCNTs. The ordinary (in-plane) refractive index spectra presented a series of sharp resonances of parallel-polarized excitons. The extraordinary (out-of-plane) refractive index in the 0.8–2.4 eV range was determined as ∼1.9. The extraordinary spectra included small but indispensable contributions from the cross-polarized exciton resonance, which are necessary for properly predicting the angle-dependent optical responses of the membrane. By completely unveiling the birefringent optical responses of single-chirality SWCNT membranes, this work paves the way for integrating SWNCT membranes into precise and diverse photonic and/or thermo-optic devices.

Low driving-voltage four-armed pentaerythritol and coumarin based compounds as potential materials for LCDs

The electro-optical properties of some four-armed liquid crystalline materials with pentaerytrithol core based on coumarin as mesogenic moiety is investigated. The mesogenic moiety is connected to the core via spacers with various chain lengths differing. The compounds are investigated via differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) equiped with a temperature controlled hot-stage. Field-Emission Scanning Electron Microscope (FESEM) and “High-Resolution Transmission Electron Microscope (HRTEM) measurements are exhibited a relatively spherical morphology with particle sizes of 145–174 nm. The electro-optical performance of the materials as thin films coated on indium-tin-oxide (ITO) glass substrates are investigated via applied voltage versus transmittance plots, exhibiting relatively low driving voltage values as low as 2.5–3 V. The ordinary (no) and extraordinary (ne) refractive indexes of the compounds at different temperatures are measured showing a good correspondence with the POM results. In conclusion, the studied materials have exhibited proper physicochemical and electro-optical properties as materials for liquid crystal displays (LCD).

Modeling of the high-resolution optical-coherence diagnostics of bi-refringent biological tissues

We present a computer model of the polarization-sensitive interference diagnostics of the bi-refringent biological media, with a particular example of the lamella of eye cornea. The diagnostic procedure employs the modified Mach–Zehnder interferometer with controllable phase retardation of the reference wave, separate observation of the orthogonal linearly-polarized interference signals, and evaluation of the phases and amplitudes of their variable (AC) components. The data obtained permit to determine the mean refractive index as well as the difference between the extraordinary and ordinary refractive indices, which, in turn, indicates the optical axis and the collagen fibers’ orientation in the lamella. The modelled procedure enables the sample structure diagnostics with the longitudinal and lateral resolution ∼100 nm and ∼1.8 μm, correspondingly. In particular, it permits a reliable detection and quantitative characterization of a thin (<100 nm) near-surface layer where the mean refractive index differs by less than 1% from that in the main volume (due to the different orientation of the collagen fibers). The diagnostic approach, developed in the paper, can be useful in various problems of structure characterization of optically-anisotropic biological tissues.

Linear and nonlinear optical properties of the langasite crystal Ca3TaAl3Si2O14

We have shown that the optical sign of the uniaxial langasite crystal Ca3TaAl3Si2O14 is positive. We also measured the phase-matching conditions of second-harmonic generation and sum-frequency generation up to 3.5 µm. Simultaneous fitting of these experimental data allowed us to report the Sellmeier equations of the ordinary and extraordinary principal refractive indices, and the magnitude of the nonlinear coefficient.

Investigation of the effect of Cr co-doping on the refractive index, spectroscopic parameters and lasing of Nd:GdVO4 crystals

Cr co-doped (0.5, 1.0, 1.2 and 1.5 at.%) Nd (0.5 at.%):GdVO4 single crystals, used as laser gain medium, were grown by the optical floating zone technique to investigate the effect of Cr on the refractive index and spectroscopic properties. The ordinary (no) and extra-ordinary (ne) refractive index of 0.5 at.% Cr co-doped Nd:GdVO4 crystals was found to be lower than the 1.0 at.% Cr co-doped crystals, and thereafter it becomes almost constant up to 1.5 at.% of Cr co-doping. Further, the thermo-optic coefficient (dn/dT) of Cr co-doped Nd:GdVO4 crystals is positive with values ∼10−5/°C and the value is higher for ne compared to no for all the Cr doping concentrations. Further, the measured fluorescence lifetime of Nd ion decreases from 98 to 85 μs with the increase in Cr concentration. The Judd-Ofelt (JO) analysis revealed that the JO parameters for all the samples are Ω2 ∼9.6–10.6 × 10−20 cm2, Ω4 ∼6.5–7.0 × 10−20 cm2 and Ω6 ∼6.5–7.0 × 10−20 cm2. The radiative lifetime (∼100 μs) and emission cross section (∼3.2–4.1 × 10−19 cm2) are not affected significantly by the Cr co-doping. The investigation suggests that the inclusion of Cr ion in the crystal lattice does not deteriorate the overall spectroscopic properties of the Nd ions, thus Cr may act as an excellent activator for Q-switching in the Nd:GdVO4 laser gain medium. Further, the self Q-switching in the Cr co-doped Nd:GdVO4 laser element was demonstrated. A stable pulsed laser output at 1066 nm with power ∼105 mW, pulse width ∼477 ns and repetition rate ∼550 kHz was obtained in the 1.0 at.% Cr co-doped crystal.

Optogeometric and waveguiding properties of multimode ZnO planar waveguide sprayed thin films

ZnO thin films with different thicknesses (T1=230.0, T2=450.0, T3=634.0, T4=926.9 and T5=1153.4nm) were deposited by spray pyrolysis on ordinary glass. All of the films had wurtzite crystal structure. The texture coefficient of the (002) plan decreased from 2.74 to 1.64 while that of the (103) increased from 0.65 to 1.23 over the thickness. Flaky shaped grains were observed in the SEM images with a tendency to get larger and hexagonal in shape. The surface evolution was verified by AFM microscopy. The ordinary and extraordinary refractive indices increased with thickness from 1.9344 and 1.9365 to 1.9768 and 1.9830 respectively. The mode order had a significant impact on the confinement of the electric field in the TE mode, therefore the power confinement factor increased by 16.58% with respect to thickness, reaching over 99% whilst it was reduced by 16.39% when the mode order increased from 0 to 4 in T5.

The Evolution Characteristics of Twisted Hermite–Gaussian Schell-Model Beams Propagating in a Uniaxial Crystal

In this paper, we study the propagation properties of twisted Hermite–Gaussian Schell- model (THGSM) beams propagating in a uniaxial crystal orthogonal to the optical axis. We derive the concrete analytical expression of the cross-spectral density (CSD) function in the crystal and simulate the evolution characteristics of such beams, including normalized spectral intensity, the spectral degree of coherence (DOC), and effective beam width. We find that the spectral intensity distribution exhibits a non-circular symmetric self-splitting while rotating, and the distribution of the spectral DOC is non-circular symmetric rotationally distorted, which is quite different from that in an isotropic medium. The initial beam parameters and crystal parameters both affect the distribution of spectral intensity and DOC. Furthermore, increasing the twist factor and adjusting the ratio of the extraordinary light refractive index and the ordinary light refractive index ne/no of the uniaxial crystal can suppress the beam expansion as propagating in the crystal. Our results show that the uniaxial crystal can be used to determine whether light beams carry a twist phase or not, and to modulate the characteristics of light beams.

Development of a Mercury Bromide Birefringence Measurement System Based on Brewster's Angle.

Mercury bromide (Hg2Br2) has been used to develop acousto-optic tunable filters (AOTFs) because it has several advantages, including a high refractive index, a broad optical bandwidth, and a relatively high figure of merit. Therefore, the measurement of its birefringence is a highly important factor for ensuring AOTF quality. However, for single crystals, it is difficult (at the millimeter scale) to quantify the birefringence using an ellipsometer, as this equipment is only designed to conduct measurements on thin films. In this study, a simple birefringence measurement system for Hg2Br2 was developed based on Brewster's angle at the millimeter scale. The planar distributions of the Hg2Br2 crystal along the (100), (010), and (001) planes were used in the experiments. The developed measurement system can measure the reflected light intensity of the Hg2Br2 crystal depending on the incidence angles (rotations at 0.01125° steps) and can calculate the ordinary and extraordinary refractive indices and birefringence. The calculated birefringence of the Hg2Br2 crystal was 0.8548; this value exhibits an error of 0.64% compared with a value of 0.86 reported in the literature. The developed measurement system demonstrates the ability to be used to evaluate the quality of birefringent single crystals.

Optoelectronic properties of sprayed MnxZn1−xO optical waveguide thin films: Refractive index and birefringence tailoring

This study investigated the properties of MnxZn1−xO thin films with varying Mn concentrations (x = 0, 0.03, 0.05, 0.07, and 0.09) fabricated via spray pyrolysis on glass substrates. The films exhibited a wurtzite structure with (002) orientation and decreasing crystal size. By utilizing the first derivative technique, it was found that the optical gap energy increased linearly by 0.06 eV, starting at 3.24 eV. Photoluminescence spectra deconvolution revealed the presence of multiple point defects, particularly VO, which correlated with the increase in optical gap energy. The Hall effect-based electric measurements indicated a decline in charge carriers and mobility accompanied by an increase in magnetoresistance and resistivity. The SEM observations revealed a morphological transition from flaky to dome-shaped grains with a reduction in size. The waveguiding properties of the films were studied using the m-lines technique, which showed that all films were dual-mode waveguides. The ordinary and extraordinary refractive indices increased to 2.0019 and 2.0005, respectively with Mn doping, while the birefringence inversion began at Mn0.05Zn0.95O and remained constant at Mn content of 7 and 9 at.%. Overall, these findings provide valuable insights into the structural, optical, electrical, and waveguiding properties of MnxZn1−xO thin films, which could be useful for the development of new optoelectronic devices.

The orientational order parameter of some ferroelectric liquid crystals by refractivity and image analysis methods

This manuscript describes the determination of the orientational order parameter of the ferroelectric liquid crystals having chemical formulas C36H47NO5, C37H49NO5 and C39H53NO5. The ordinary and extraordinary refractive indices of these compounds in the nematic phase are measured by using a small angle prism and modified spectrometer. By well-known Vuk’s method the orientational order parameter is estimated. The optical textures of these compounds are apprehended using an optical polarizing microscope. The microscopic textures are analysed using MATLAB software.and the orientational order parameter is estimated. The limitations and merits of the image analysis technique is discussed.

Argon flow rate effects on the optical waveguide properties of DC sputtered TiO2 thin films

TiO2 thin films were successfully prepared by DC sputtering using pure titanium target in mixed gases (Argon, Oxygen) plasma on glass substrates. The films were deposited at a constant substrate temperature (350 °C) with different Argon flow rate values (15, 30, 45, and 60 sccm). Raman spectroscopy analysis shows that all films crystallized in the Anatase phase. The crystallinity was found to improve with increasing Argon flow rate up to 45 sccm and then deteriorate sharply at 60 sccm. The crystallite size varied between 9.1 and 9.7 nm. Atomic force microscopy (AFM) revealed that the roughness fluctuated between 1.30 and 6.01 nm with an overall increase. The grain shape went from sharp needles like shape to dome like shape with an enlargement in their size by up to 60 nm. The UV–Vis spectrophotometer displayed that the films were highly transparent. The optical band gap ranged from 3.65 to 3.49 eV. Prism coupler analysis exhibited single guided modes in both transverse electric and transverse magnetic polarizations for the sample prepared at 15 sccm Argon flow rate and bi-guided modes in both polarizations for the rest of the samples. Both ordinary and extraordinary refractive indices decreased over Argon flow by 9.912% and 6.441% respectively. The thickness, porosity as well as the birefringence where found to increase by 155 nm, 16.16% and 0.0832 respectively as Argon flow rate went from 15 to 60 sccm.