Refractive Behavior Research Articles

Optical properties of Fe-doped BaTiO3 films deposited on quartz substrates by sol-gel method

Fe-doped BaTiO3 films were prepared on quartz substrates by sol-gel route. Linearly optical constants of the films were obtained by fitting transmittance spectra with Adachi’s model. With increasing Fe composition, refractive index n at 532 nm increases, single oscillator energy Ec and dispersion energy Ed increase, but optical band gap Eg decreases from 3.78 to 3.63 eV. In addition, nonlinear optical properties of the Fe-doped BaTiO3 films were analyzed by Z-scan method. Nonlinear optical responses of the BaTiO3 films were enhanced by Fe doping. For BaTi0.96Fe0.04O3 film, the third-order nonlinear refractive index coefficient γ, the nonlinear refractive index coefficient n2, the nonlinear absorption coefficient β and the third-order nonlinear susceptibility χ(3) are 1.98 × 10−13 m2/W, 1.03 × 10−6 esu, 1.33 × 10−7 m/W, 2.38 × 10−7 esu respectively. The nonlinearity is predominated by the nonlinear refractive behavior. These results suggest that Fe doping may tune the linear and nonlinear optical properties of BaTiO3 films, which indicates that Fe-doped BaTiO3 films are promising materials for applications in optical devices.

The role of two-particle effects in the behavior of refraction of single-component liquids and two-component solutions

The role of irreducible two-particle contributions to the refraction properties of liquids and solutions has been studied. Two-particle contributions to the polarization vector of the system are calculated by electrostatics methods, which can be explained by weak deformations and overlaps of the electron shells of atoms and molecules. It is shown that, when the two-particle effects are taken into account, the constant of molecular refraction for binary solutions becomes a weakly nonadditive function of the constant of refraction of the components. It is shown by the example of aqueous solutions of ethanol and methanol that our values of the refractive indices and constants of refraction of the solutions are in good agreement with the experimental data.

Negative refraction and imaging of acoustic waves in a two-dimensional square chiral lattice structure

The negative refraction behavior and imaging effect for acoustic waves in a kind of two-dimensional square chiral lattice structure are studied in this paper. The unit cell of the proposed structure consists of four zigzag arms connected through a thin circular ring at the central part. The relation of the symmetry of the unit cell and the negative refraction phenomenon is investigated. Using the finite element method, we calculate the band structures and the equi-frequency surfaces of the system, and confirm the frequency range where the negative refraction is present. Due to the rotational symmetry of the unit cell, a phase difference is induced to the waves propagating from a point source through the structure to the other side. The phase difference is related to the width of the structure and the frequency of the source, so we can get a tunable deviated imaging. This kind of phenomenon is also demonstrated by the numerical simulation of two Gaussian beams that are symmetrical about the interface normal with the same incident angle, and the different negative refractive indexes are presented. Based on this special performance, a double-functional mirror-symmetrical slab is proposed for realizing acoustic focusing and beam separation. La réfraction négative et l'imagerie des ondes acoustiques dans une structure périodique chirale bidimensionnelle sont étudiées dans cet article. La cellule élémentaire de la structure considérée comporte quatre bras en zigzag, connectés dans la partie centrale par un fin anneau circulaire. La relation entre la symétrie de la cellule élémentaire et le phénomène de réfraction négative est explorée. À l'aide de la méthode des éléments finis, nous calculons la structure de bande et les surfaces équifréquence du système, ce qui nous permet de confirmer la gamme de fréquence pour laquelle la réfraction négative est obtenue. En raison de la symétrie par rotation de la cellule élémentaire, une différence de phase est introduite pour les ondes émises par un point source et traversant la structure. Cette différence de phase est liée à la largeur de la structure ainsi qu'à la fréquence de la source, ce qui nous permet d'obtenir une image défléchie avec un angle ajustable. Le même phénomène est également démontré par simulation numérique pour deux faisceaux gaussiens incidents, avec des angles symétriques par rapport à la normale à l'interface. Dans ce cas, différents indices de réfraction négatifs sont obtenus. Sur la base de ce comportement spécifique, une dalle présentant une symétrie miroir est proposée pour réaliser la double fonction de focalisation et de séparation de faisceaux acoustiques.

Investigation on fluorescence, third order non-linear optical and mechanical performance of glycine doped ADP crystal for applications of photonic devices

Optically transparent glycine doped ammonium dihydrogen phosphate crystals have been grown from aqueous solution by slow solution evaporation method. Crystallographic data of the grown crystal has been determined using the single crystal X-ray diffraction technique. The photoexcited colour centred fluorescence emission spectrum of the grown crystal has been analysed at an emission wavelength of 254 nm in the range of 300 to 700 nm. Z-scan studies have been performed at 632·8 nm to confirm the negative third order non-linear refraction behaviour of the grown crystal. The non-linear refractive index (n2), non-linear absorption coefficient (β) and third order non-linearity susceptibility (χ3) of the grown crystal were calculated using the Z-scan data to explore the material suitability for photonic applications. Vicker's microhardness test was carried out to analyse the mechanical stability and determine the hardness parameters of the grown crystal.

Synthesis, growth and optical studies of novel organometallic NLO crystal: Calcium bis-thiourea chloride

A novel organometallic crystal calcium bis-thiourea chloride (CBTC) has been synthesized and confirmed by means of EDX and FT-IR analysis. The (25×9×10)mm3 CBTC crystal has been grown from aqueous solution by slow solution evaporation technique. The UV–visible spectral study has been carried in the range of 200–900nm to assess the optical transparency of CBTC crystal. The optical constants such as band gap, optical conductivity, refractive index and reflectance of grown crystal have been evaluated using the transmittance data. The color centered fluorescence behavior of grown crystal has been explored in visible region. The Z-scan technique has been employed to determine the negative third order nonlinear refraction behavior of CBTC crystal. The magnitude of third order nonlinear refraction (n2), nonlinear susceptibility (χ3), nonlinear absorption (β) and FOM of grown crystal has been calculated using the Z-scan data. The χ3 of magnitude 3.56×10−5esu confirms the strong polarizing nature of CBTC crystal. The Kurtz-perry test confirmed the SHG efficiency of CBTC crystal is 1.15 times higher than KDP crystal material.

Study of various cladding modes effective refractive indices behavior in long period fiber grating based refractive index sensor: Employing two layer and three layer geometry approach

In this work, we have calculated various cladding modes effective refractive indices (ERI) by using two layer and three layer geometry in long period fiber grating (LPFG) and studied the variation of various cladding ERI due to variation in surrounding refractive index (SRI). Employing both two layer and three layer geometry approach, it is observed that in LPFG based refractive index sensor, sensitivity to SRI can be improved by using higher order cladding modes. Both two layer and three layer geometry results in nearly same values of ERIs for lower order cladding modes for any value of SRI but there is drastically increase in difference of results obtained by using two layer from three layer geometry as cladding mode order increases. Actually investigation of LPFG as refractive index sensor using two layer geometry is very simple and can be used for lower order cladding modes. But to achieve accurate results with enhanced sensitivity for higher order cladding modes, three layer geometry is best suited for studying LPFG as refractive index sensor. The response has been studied for surrounding refractive index variation from 1 to 1.45.

Observation of negative refraction of Dirac fermions in graphene

Half a century ago, Veselago proposed left-handed materials with negative permittivity and permeability, in which waves propagate with phase and group velocities in opposite directions. Significant work has been undertaken to attain this left-handed response, such as establishing a negative refractive index in so-called metamaterials, which consist of periodic sub-wavelength structures. However, an electronic counterpart has not been demonstrated owing to difficulties in creating repeated structures smaller than the electronic Fermi wavelength (\lambda_F) of the order ~ 10 nm. Here, without needing to engineer sub-wavelength structures, we demonstrate negative refractive behaviour of Dirac fermions in graphene, exploiting its unique relativistic band structure. Analysis of both electron focusing through a n-p-n flat lens and negative refraction across n-p junctions confirms left-handed behaviour in the electronic system. This new approach to electronic optics is of particular relevance to the on-going efforts to develop novel quantum devices with emerging layered materials.

Theoretical predictions and experimental suggestions for refraction behaviors occurring at lossy interfaces

According to the energy streamline method, refraction of energy flow of the transverse magnetic (electric) polarized electromagnetic waves occurring at the lossy/lossless interface may be either positive or negative, depending on the sign of the real part of permittivity (permeability) of the lossy medium. On the other hand, based on the reflection and refraction formulas developed by us, it is demonstrated that refraction behaviors of the electromagnetic wave occurring at the lossy/lossless interface relate to the difference between the electric damping angle and the magnetic angle of the lossy medium. Furthermore, the two-dimensional transmission lines with adjustable effective permittivity and permeability are proposed to be applied to experimentally test these predictions. This work systematically provides the basic refraction properties of electromagnetic waves occurring at lossy/lossless interfaces; it may be useful to further understand the observed refraction behaviors associated with the lossy interfaces and fully test the validity of reflection and refraction theories.

Design of fiber metamaterials with negative refractive index in the infrared.

Metamaterials possess intricate, sub-wavelength microstructures, making scalability a salient concern in regard to their practicality. Fiber-drawing offers a route to producing large quantities of material at relatively low cost, though to our knowledge, a fiber-based design capable of negative refractive index behaviour has not yet been proposed. We submit that the electric and magnetic dipole resonance modes of the fiber can be enhanced by including in the fiber aligned metallic inclusions. Addition of a solid metallic core can effect a synchronisation of these modes, allowing a collection of the fibers to possess negative refractive index.

Enhanced photoelectrochemical performance of multi-leg TiO2 nanotubes through efficient light harvesting

Multi-leg TiO2 nanotubes have been synthesised by electrochemical anodisation of Ti foils with suitable optimisation of the electrolyte (containing NH4F and HF) composition and concentration. As a result, multi-leg nanotube morphology shows a 4-fold enhancement in photocurrent density compared with that of compact tubes of comparable length under UV-VIS (320–500 nm) light illumination. We attribute this enhancement in photocurrent to efficient light scattering within the multi-leg morphology and the availability of a large contact area with the electrolyte compared to that of compact nanotubes. A model has been presented for light trapping and scattering ability on the basis of the graded refractive index behaviour associated with multi-leg nanotubes. Such graded refractive index behaviour is found to reduce specular reflectance, leading to a greater scattering of light into the nanotube structure. The diffused reflectance spectra confirm higher scattering in the multi-leg nanotube arrays, which indicates greater light harvesting by the multi-leg nanotube arrays.

Nonlocal effective medium approximation for metallic nanorod metamaterials

We present an analytical nonlocal effective medium approximation to describe the optical nonlocal effects in metallic nanorod metamaterials based on Mie scattering theory. It is shown that the developed nonlocal effective medium theory can predict a coexistence state of two modes around the epsilon-near-zero region, where strong optical nonlocal effects lead to the behavior of both positive refraction and negative refraction in the nanorod metamaterials. Outside of the coexistence region, only one mode can be excited and its behavior can be well described using the local effective medium theory.

Structural characterization and optical properties of pulsed laser deposition of Se75Te25 and Se75Te17Ge8 amorphous thin films

Thin films of Se75Te25 and Se75Te17Ge8 were deposited onto glass substrates using a pulsed laser deposition technique. The target materials used for the films deposition were prepared by conventional solid state reaction in a vacuum sealed silica tube to avoid oxide formation. Chemical composition and structural characterization were investigated using energy-dispersive X-ray spectrometry (EDX) and X-ray diffraction (XRD). The optical constants were determined from the spectrophotometric measurements of the transmittance and reflectance spectra carried out on the prepared films. The refractive index behavior is adequately described by the single oscillator model proposed by Wemple and DiDomenico whereby the oscillator parameters values were determined. The optical absorption coefficient was analyzed to identify the type of the optical transition and for determination of the corresponding energy values.

Theoretical study of visible light refraction phenomena occurring at noble metal–air interfaces

Based on the generalized reflection and refraction theory, the visible light wavelength-dependent refraction behaviors occurring at both Ag–air and Au–air interfaces are qualitatively attributed to the non-synchronized variations of electric and magnetic parameters of the light. Numerical results show that distribution of time-averaged Poynting vector of the transmitted light traveling through a wedge shaped noble metal film may be affected by width and divergence of the incident light beam, energy losses, and so on. Careful consideration of effects of multi-factors on the refraction phenomena are necessary to establish reasonable relationship among measured results by using different experimental methods. Our work may provide a new possible angle of view to understand the refraction phenomena occurring at noble metal–air interface.

Low-threshold Optical limiting and Nonlinear refraction in Nanocomposite films of Light Green dye-polymer system

Poly(vinyl alcohol) (PVA) /Light Green (LG) dye nanocomposites were fabricated and their structure, microstructure, linear and nonlinear optical properties were investigated. The samples were characterized as nanocomposites with dye molecules encapsulated between the larger molecules (molecular chains) of the polymer host PVA. The nonlinear refraction behaviour was investigated employing the Z-scan technique, using a continuous wave (cw) He- Ne laser operating at 632.8 nm, as the excitation source. The samples displayed negative nonlinear refraction (self-defocusing) under the experimental conditions. Low-threshold optical limiting behaviour exhibited by the PVA based nanocomposites was demonstrated using an aperture limited geometry. The low limiting thresholds (~4 mW and ~2.4 mW) observed for the samples indicate that these low cost and durable nanocomposite films are potential media for optical limiting application under cw laser light excitation.

Multiple refraction switches realized by stretching elastomeric scatterers in sonic crystals

The effect of out-of-plane pre-stretch on the 2D sonic crystal with stretchable elastomeric scatterers is explored. The hyperelastic scatterers are characterized by a compressible neo-Hookean model. The Dirichlet-to-Neumann (DtN) map is adopted to obtain the band structure and equi-frequency contours. We focus on the first passband and find that a variety of switching functionalities for refraction behaviors can be realized in selected frequencies under a specific pre-stretch range. These refraction switches enable an active control of wave propagation and are applicable in advanced technologies where switchable and multifunctional sonic crystals are required.

High-gain composite microstrip patch antenna with the near-zero-refractive-index metamaterial

A high-gain rectangular microstrip patch antenna which is covered by a single layer metamaterial (MTM) superstrate with the near zero refractive index is proposed. The refraction of the metamaterial at frequency 3.51GHz–3.57GHz is very close to zero. The metamaterial with the near zero refractive index is placed 42mm above an ordinary rectangular microstrip patch antenna. The effectively zero refractive index behavior of metamaterial superstrate can gather the wave emitted from the microstrip patch antenna and collimate it toward the normal direction of the antenna. The finite element method (FEM) and the finite difference time domain (FDTD) method are used to study the characteristics of this antenna. The results of the two methods indicate that the realized gain of the proposed antenna is increased by more than 6dB, and the antenna has a flatness high gain in the predicted frequency band, where the proposed MTM is designed to have a near zero index of refraction. Therefore, the high-gain antenna is effectively enhanced based on the near-zero-refractive-index metamaterial.

Artificial isotropic magnetism and negative refraction of metamaterial consisting of dielectric spherical shells

We present a detailed study on the artificial magnetism of high-permittivity dielectric spherical shell array and the negative refraction behavior of the composite consisting of dielectric spherical shells and metal wire lattice. The spherical shell array in cubic lattice arrangement can exhibit an isotropic negative magnetism when excited into magnetic dipole resonance. When the dielectric spherical shell and metal wire lattice are combined in a body-centered cubic structure, full-wave simulations show that the bulk metamaterial with this structure can exhibit a negative refraction behavior. It is shown that artificial magnetic response can originate from differently structured dielectric elements, similarly to the conventional metamaterial constituents based on metal resonance structures.

Investigation of Carbon ion-implanted waveguides in tungsten bronze (Ca0.28Ba0.72)0.25(Sr0.6Ba0.4)0.75Nb2O6 single crystal

Planar optical waveguides were fabricated in (Ca0.28Ba0.72)0.25(Sr0.6Ba0.4)0.75Nb2O6 (CSBN25) crystal by 6.0-MeV C+ ion implantation with fluences of 2, 4 and 6×1014ions/cm2 at room temperature. The mode parameters, refractive indices profiles are measured and the refractive indices behavior in the waveguide region is discussed. The shape of nuclear energy loss distribution of the C+ implantation was similar to those of the waveguide refractive index profiles, which means an inherent relationship between the waveguide formation and the energetic energy deposition. The extraordinary refractive index has a small positive change in the surface region after the implantation.

Nonlinearity management of photonic composites and observation of spatial-modulation instability due to quintic nonlinearity

We present a procedure for nonlinearity management of metal-dielectric nanocomposites (MDNCs). Varying the volume fraction occupied by silver nanoparticles (NPs) suspended in acetone we could investigate the nonlinear (NL) response of the MDNC. In particular, we could cancel the NL refractive index related to the effective third-order susceptibility, ${\ensuremath{\chi}}_{\mathrm{eff}}^{(3)}$, and thus the NL refractive behavior of the MDNC was due to the effective fifth-order susceptibility, ${\ensuremath{\chi}}_{\mathrm{eff}}^{(5)}$. Hence, in a cross-phase modulation experiment, we demonstrated the effect of spatial-modulation instability due to ${\ensuremath{\chi}}_{\mathrm{eff}}^{(5)}$. The experimental results are corroborated with numerical calculations based on a generalized Maxwell-Garnet model that includes the direct contributions of the NPs and the host medium as well as cascade contributions due to the third- and fifth-order susceptibilities of the NPs.

Effects of the ZnSe concentration on the structural and optical properties of ZnSe/PVA nanocomposite thin film

This study investigated the effects of ZnSe nanoparticles (NPs) on the structural and (linear and nonlinear) optical properties of polyvinyl alcohol (PVA) thin film. Three samples of ZnSe NP-doped PVA thin films with different concentrations of ZnSe were produced on a glass substrate. The ZnSe NPs were synthesized by pulsed laser ablation of the ZnSe bulk target immersed in distilled water using a 1064nm wavelength and a high frequency pulsed Nd:YAG laser. The optical bandgap energies of the films were extracted from their UV-Vis-NIR absorption spectra. The corresponding energy bandgaps of the nanocomposite films declined as the ZnSe NPs doping concentration increased. X-ray diffraction analysis was used to characterize the crystalline phases of the ZnSe/PVA nanocomposite films. The concentration-dependent nonlinear optical absorption and nonlinear refraction behaviors of the films after exposure to 532-nm nanosecond laser pulses were investigated using the Z-scan technique. The nonlinear absorption response of the films was positive when measured using an open aperture scheme, which was attributed to the two-photon absorption mechanism. In addition, the nonlinear refraction indices had a negative value and they increased as the concentration of ZnSe NPs in the films increased.