Strong Optical Anisotropy Research Articles

Low temperature magneto-photoluminescence of GaAsBi /GaAs quantum well heterostructures

Strong optical anisotropy is observed in the emission from a GaAs1−xBix (x ∼ 0.04) quantum well grown by low temperature molecular beam epitaxy on (001) GaAs by means of low temperature magneto-photoluminescence (MPL) taken at 2 K in Faraday geometry for magnetic fields, B, up to 10 T. A significant diamagnetic shift (∼2.5 meV) develops for magnetic fields above ∼8 T, which is accompanied by a narrowing of the emission bandwidth and a substantial increase in the difference between the integrated intensities of the σ+ and σ− polarizations in the MPL spectra. This, along with a peculiar spectral dependence of the polarization degree which evolves with increasing magnetic field, is interpreted in terms of bound and free magneto excitons in the system where Bi-related levels become hybridized to different extents with the valence and conduction bands of the GaAs host material.

Strong orientation correlation and optical anisotropy in blend of cellulose ester and poly(ethylene 2,6‐naphthalate) oligomer

ABSTRACTMolecular orientation of two aromatic additives in cellulose acetate propionate (CAP) matrix was investigated from orientation birefringence data and an intermolecular orientation correlation represented by nematic interaction (NI) was evaluated. Poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) oligomers with low molecular weight (ca. 400 g mol−1) were used as the additives. The NI strengths of CAP‐PET and CAP‐PEN were determined to be 0.28 ± 0.05 and 0.96 ± 0.15, respectively. In particular, compared with other polymer/polymer and polymer/small additive blends, the NI value in CAP/PEN blend is much stronger and represents the perfect orientation correlation. The strong orientation correlation is possibly due to the rigid naphtalate structure in PEN. Contrary, a relation between birefringence and orientation function for CAP in bulk and blend showed the same trend, suggesting that the orientation behavior of CAP determines the orientation birefringence. As two ester groups in CAP are responsible for birefringence, the alignment of the ester groups is affected by only the main chain orientation. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40570.

Electronic structure and optical properties of Si, Ge and diamond in the lonsdaleite phase

Crystalline semiconductors may exist in different polytypic phases with significantly different electronic and optical properties. In this paper, we calculate the electronic structure and optical properties of diamond, Si and Ge in the lonsdaleite (hexagonal diamond) phase using a transferable model empirical pseudopotential method with spin–orbit interactions. We calculate their band structures and extract various relevant parameters. Differences between the cubic and hexagonal phases are highlighted by comparing their densities of states. While diamond and Si remain indirect gap semiconductors in the lonsdaleite phase, Ge transforms into a direct gap semiconductor with a much smaller bandgap. We also calculate complex dielectric functions for different optical polarizations and find strong optical anisotropy. We further provide expansion parameters for the dielectric functions in terms of Lorentz oscillators.

Optical properties of ZnO/(Zn, Mg)O quantum wells

This paper reviews the optical properties of ZnO/(Zn, Mg)O single quantum wells grown by molecular beam epitaxy. Both heteroepitaxial quantum well growth along the polar c-direction and homoepitaxial quantum well growth on the nonpolar M plane cases are considered. The optical properties of these quantum wells are investigated by using reectance, continuous wave photoluminescence, and time-resolved photoluminescence spectroscopies. The quantum-conned Stark eect dominates the properties of the excitons for polar quantum wells. The magnitude of the internal electric eld that is induced by both spontaneous and piezoelectric polarizations is determined by comparing the experimental results with a variational calculation of excitonic energies and lifetimes. For nonpolar quantum wells, the optical spectra reveal strong in-plane optical anisotropies, as predicted by the group theory. Moreover, the radiative recombination of free excitons is dominating the quantum well photoluminescence even at room temperature.

Structural, elastic, electronic and optical properties of layered alkaline-earth halofluoride scintillators

A systematic investigation of structural properties at ambient as well as at high pressure has been carried out for layered scintillators CaClF, CaBrF, SrClF, SrBrF and SrIF based on density functional theory. Semi-empirical dispersion correction scheme has been used to account for the van der Waals interactions and the obtained results are in good agreement with experimental data. The pressure-dependent structural and elastic properties reveal that the c-axis is more compressible than the a-axis (C ) in all these materials due to weakly bonded layers stacked along the c-axis. In addition, the electronic structure and optical properties of these materials are calculated using Tran-Blaha-modified Becke-Johnson (TB-mBJ) potential. Among the five investigated compounds which are structurally anisotropic, a weak optical anisotropy is found in CaClF and SrClF and strong optical anisotropy in CaBrF, SrBrF and SrIF. The present study suggests that unlike alkaline-earth dihalides which are fast scintillators, these materials can act as storage phosphors and the possible reason is speculated from the band structure calculations.

Optical absorption and DFT calculations inL-aspartic acid anhydrous crystals: Charge carrier effective masses point to semiconducting behavior

Density functional theory (DFT) computations within the local-density approximation and generalized gradient approximation in pure form and with dispersion correction (GGA+D) were carried out to investigate the structural, electronic, and optical properties of $L$-aspartic acid anhydrous crystals. The electronic (band structure and density of states) and optical absorption properties were used to interpret the light absorption measurements we have performed in $L$-aspartic acid anhydrous crystalline powder at room temperature. We show the important role of the layered spatial disposition of $L$-aspartic acid molecules in anhydrous $L$-aspartic crystals to explain the observed electronic and optical properties. There is good agreement between the GGA+D calculated and experimental lattice parameters, with ($\ensuremath{\Delta}a$, $\ensuremath{\Delta}b$, $\ensuremath{\Delta}c$) deviations of ($0.029,\ensuremath{-}0.023,\ensuremath{-}0.024$) (units in \AA{}). Mulliken [J. Chem. Phys. 23, 1833 (1955)] and Hirshfeld [Theor. Chim. Acta 44, 129 (1977)] population analyses were also performed to assess the degree of charge polarization in the zwitterion state of the $L$-aspartic acid molecules in the DFT converged crystal. The lowest-energy optical absorption peaks related to transitions between the top of the valence band and the bottom of the conduction band involve O 2$p$ valence states and C 1$p$ and O 2$p$ conduction states, with the carboxyl and COOH lateral chain group contributing significantly to the energy band gap. Among the calculated band gaps, the lowest GGA+D (4.49-eV) gap is smaller than the experimental estimate of 5.02 eV, as obtained by optical absorption. Such a wide-band-gap energy together with the small carrier effective masses estimated from band curvatures allows us to suggest that an $L$-aspartic acid anhydrous crystal can behave as a wide-gap semiconductor. A comparison of effective masses among directions parallel and perpendicular to the $L$-aspartic molecules layers reveals that charge transport must be favored in the former case. Finally, we also show that there is a strong optical anisotropy in the dielectric function of $L$-aspartic acid anhydrous crystals.

Polarized photoreflectance and photoluminescence spectroscopy of InGaAs/GaAs quantum rods grown with As2 and As4 sources

We report photoreflectance (PR) and photoluminescence (PL) investigations of the electronic and polarization properties of different aspect ratio (height/diameter) InGaAs quantum rods (QRs) embedded in InGaAs quantum wells (QWs). These nanostructures were grown by molecular beam epitaxy using As2or As4sources. The impact of the As source on the spectral and polarization features of the QR- and QW-related interband transitions was investigated and explained in terms of the carrier confinement effects caused by variation of composition contrast between the QR material and the surrounding well. Polarized PR and PL measurements reveal that the polarization has a preferential direction along the crystal axis with a large optical anisotropy of about 60% in the (001) plane for high aspect ratio (4.1:1) InGaAs QRs. As a result, in PL spectra, the transverse magnetic mode dominated -cleaved surfaces (TM[001]>TE[110]), whereas the transverse electric mode prevailed for (110)-cleaved surfaces (). This strong optical anisotropy in the (001) plane is interpreted in terms of the hole wavefunction orientation along the direction for high aspect ratio QRs.

Nanorod-Depolarized Dynamic Light Scattering in a Gelling Liquid

We exploit the strong optical anisotropy of metal nanorods to measure their mobility in a complex fluid. Gold rods in hot agarose solutions cause dynamic, depolarized scattering with an exponentially decaying autocorrelation. As the solution cools down, its decay constant increases. At a certain temperature, the autocorrelation drastically changes its shape and the dynamic contrast drops. We show that, at this temperature, the gelling liquid confines the rods and dampens their motion almost entirely. Depolarized scattering proves to be extraordinarily sensitive to the transition from Brownian to confined motion. We calculate true mobilities for the particles using Pusey and van Megen’s correction of the Siegert relation for nonergodic systems. Multipoint measurements show that the rods are immobilized throughout the gel.

Surface optical phonon modes in GaN nanowire arrays: Dependence on nanowire density and diameter

We present a systematic study, by means of Raman scattering, of the surface optical modes of GaN nanowires (NW) as a function of the fill factor, defined as the relative concentration of GaN NWs and the surrounding air. We show that changes in the fill factor from 0.8 to 0.3 result in a shift of the surface optical mode of more than 60 cm${}^{\ensuremath{-}1}$, which is explained by theoretical calculations of surface vibrations for a cylindrical nanowire based on the dielectric continuum model. Two cases are considered: the effective dielectric function approximation (Maxwell-Garnett approximation) and a model for isolated NWs with various diameters. We conclude, in agreement with the Maxwell Garnett approximation, that the dependence of the surface optical (SO) frequency with fill factor is dominated by dipolar interaction between neighboring NWs. Finally, a splitting of the SO mode is observed and ascribed to the anticrossing between surface modes with ${A}_{1}$ and ${E}_{1}$ symmetry, due to a deviation of the exciting light from the NW axis. This splitting is a consequence of the strong optical anisotropy and the polar nature of GaN, and should be present in NWs based on materials with similar characteristics.

Giant optical anisotropy of oblique-aligned ZnO nanowire arrays

A combined method of modified oblique-angle deposition and hydrothermal growth was adopted to grow an optically anisotropic nanomaterial based on single crystalline ZnO nanowire arrays (NWAs) with highly oblique angles (75°-85°), exhibiting giant in-plane birefringence and optical polarization degree in emission. The in-plane birefringence of oblique-aligned ZnO NWAs is almost one order of magnitude higher than that of natural quartz. The strong optical anisotropy in emission due to the optical confinement was observed. The oblique-aligned NWAs not only allow important technological applications in passive photonic components but also benefit the development of the optoelectronic devices in polarized light sensing and emission.

Anisotropic Organization and Microscopic Manipulation of Self-Assembling Synthetic Porphyrin Microrods That Mimic Chlorosomes: Bacterial Light-Harvesting Systems

Being able to control in time and space the positioning, orientation, movement, and sense of rotation of nano- to microscale objects is currently an active research area in nanoscience, having diverse nanotechnological applications. In this paper, we demonstrate unprecedented control and maneuvering of rod-shaped or tubular nanostructures with high aspect ratios which are formed by self-assembling synthetic porphyrins. The self-assembly algorithm, encoded by appended chemical-recognition groups on the periphery of these porphyrins, is the same as the one operating for chlorosomal bacteriochlorophylls (BChl's). Chlorosomes, rod-shaped organelles with relatively long-range molecular order, are the most efficient naturally occurring light-harvesting systems. They are used by green photosynthetic bacteria to trap visible and infrared light of minute intensities even at great depths, e.g., 100 m below water surface or in volcanic vents in the absence of solar radiation. In contrast to most other natural light-harvesting systems, the chlorosomal antennae are devoid of a protein scaffold to orient the BChl's; thus, they are an attractive goal for mimicry by synthetic chemists, who are able to engineer more robust chromophores to self-assemble. Functional devices with environmentally friendly chromophores-which should be able to act as photosensitizers within hybrid solar cells, leading to high photon-to-current conversion efficiencies even under low illumination conditions-have yet to be fabricated. The orderly manner in which the BChl's and their synthetic counterparts self-assemble imparts strong diamagnetic and optical anisotropies and flow/shear characteristics to their nanostructured assemblies, allowing them to be manipulated by electrical, magnetic, or tribomechanical forces.

Ultraviolet true zero-order wave plate made of birefringent porous silica

Porous Si made from (110) Si wafers exhibits strong in-plane optical anisotropy (birefringence) in the visible and near-IR ranges. Oxidation of the birefringent porous Si results in the formation of birefringent porous silica. We demonstrate that the degree of the birefringence of porous silica can be controlled by the oxidation condition, and very small birefringence can be achieved. The smallest anisotropy of the refractive index (Δn) is 0.001, which is about ten times smaller than that of quartz. The small birefringence allows us to produce true zero-order wave plates operating in the UV range.

Polarized light microscopy and spectroscopy of individual single-walled carbon nanotubes

Polarized light microscopy (PLM) is used to image individual single-walled carbon nanotubes (SWNTs) suspended in air across a slit opening. The imaging contrast relies on the strong optical anisotropy typical of SWNTs. We combine PLM with a tunable light source to enable hyperspectral excitation spectroscopy and nanotube chirality assignment. Comparison with fluorescence microscopy and spectroscopy confirms the assignment made with PLM. This represents a versatile new approach to imaging SWNTs and related structures. Open image in new window

In situcharacterization of one-dimensional plasmonic Ag nanocluster arrays

One-dimensional Ag nanoparticle arrays have been grown on step-bunched vicinal Al${}_{2}$O${}_{3}$ in ultrahigh vacuum using deposition at a glancing angle. The structures grown showed a strong optical anisotropy in the visible region of the spectrum. The optical anisotropy was measured in situ using reflection anisotropy spectroscopy. Relevant optical properties were determined as a function of deposition angle and Ag thickness. A simple phenomenological model was developed to reproduce the features seen in the spectra. With this model it was possible to use the inhomogeneous broadening as a guide to the nanoparticle dispersion.

Low temperature reflectivity study of nonpolar ZnO/(Zn,Mg)O quantum wells grown on M-plane ZnO substrates

We report growth of high quality ZnO/Zn0.8Mg0.2O quantum well on M-plane oriented ZnO substrates. The optical properties of these quantum wells are studied by using reflectance spectroscopy. The optical spectra reveal strong in-plane optical anisotropies, as predicted by group theory, and marked reflectance structures, as an evidence of good interface morphologies. Signatures ofc onfined excitons built from the spin-orbit split-off valence band, the analog of exciton C in bulk ZnO are detected in normal incidence reflectivity experiments using a photon polarized along the c axis of the wurtzite lattice. Experiments performed in the context of an orthogonal photon polarization, at 90^{\circ}; of this axis, reveal confined states analogs of A and B bulk excitons. Envelope function calculations which include excitonic interaction nicely account for the experimental report.

Structure−Property Correlations in Hybrid Polymer−Nanoparticle Electrospun Fibers and Plasmonic Control over their Dichroic Behavior

Electrospinning constitutes a simple and versatile approach of fabricating polymer heterostructures composed of nanofibers. A preferred alignment of polymer crystallites stems from complex shear elongational forces and generates a strong intrinsic optical anisotropy in typical electrospun fibers of semicrystalline polymers. While it can prove useful for certain devices, this intrinsic anisotropy can be extremely detrimental for other key applications such as high-performance polymer-based lighting and solar-energy harvesting platforms. We report a dramatic reduction in the intrinsic dichroism of electrospun poly(ethylene oxide) fibers resulting from the incorporation of inorganic nanoparticles in the polymer matrix. This effect is shown to originate from a controllable randomization of the orientational ordering of the crystalline domains in the hybrid nanofibers and not merely from a reduction in crystallinity. This improved understanding of the crystalline structure-optical property correlation then leads to a better control over the intrinsic anisotropy of electrospun fibers using localized surface-plasmon enhancement effects around metallic nanoparticles.

In-plane optical anisotropy in GaAsN/GaAs single-quantum well investigated by reflectance-difference spectroscopy

The interface properties of GaNxAs1−x/GaAs single-quantum well is investigated at 80 K by reflectance difference spectroscopy. Strong in-plane optical anisotropies (IPOA) are observed. Numerical calculations based on a 4 band K⋅P Hamiltonian are performed to analyze the origin of the optical anisotropy. It is found that the IPOA can be mainly attributed to anisotropic strain effect, which increases with the concentration of nitrogen. The origin of the strain component ϵxy is also discussed.

Internal stresses in hypercrosslinked polystyrene: Polarized optical studies

Polarized optical microscopy is used for studying the hypercrosslinked PS network prepared by the crosslinking of DC swollen spherical beads of styrene copolymer containing 0.5% of divinylbenzene by monochlorodimethyl ether up to 100% crosslinking. The removal of the solvent decreases the volume of the sample and leads to the development of internal stresses, which manifest themselves as strong optical anisotropy of dry beads. On the contrary, upon swelling in toluene, the network is expanded and shows anisotropy. When the network swells in any thermodynamically good solvent, existing stresses invert their sign and this network appears to be free of any internal stresses. This state of the network should be treated as the ϑ state, in which beads are optically isotropic. This state can also be achieved by swelling of a hypercrosslinked polymer in methanol, which eliminates compressive stresses in the hypercrosslinked network via increase in volume but cannot extend it like toluene. Conventional gel copolymers of styrene with divinylbenzene prepared without any toluene or in the presence of minor amounts of toluene are fully isotropic only in their dry state. These copolymers are inert to methanol but swell in hexane and cyclohexane, and their swelling is accompanied by the development of internal stresses.

Optical polarizer made of uniaxially aligned short single-wall carbon nanotubes embedded in a polymer film

An intrinsic characteristic of carbon nanotubes (CNTs), which are allotropes of carbon, is one-dimensional anisotropy. It is derived from the peculiar geometrical shape of the molecules and is manifested in their electrical, thermal, mechanical, and optical properties. Here, we report an optical polarizer that makes effective use of such anisotropy of CNTs. The polarizer we fabricated is made of uniaxially aligned CNTs embedded in a polymer film. Thanks to the $\ensuremath{\pi}$ plasmon-originated broad absorption spectrum and strong optical anisotropy of single-wall CNTs (SWCNTs), the film exhibits a degree of polarization of $\ensuremath{\sim}90%$ with keeping flat transmittance through the spectral region from 350 to 800 nm. In order to efficiently obtain well-aligned CNTs, we used SWCNTs shortened into a length of less than 200 nm. We also observed enhancement of the degree of polarization at the wavelengths of Van Hove singularities.

New perspectives on van der Waals–London interactions of materials. From planar interfaces to carbon nanotubes

The drive towards nanoscale assembly necessitates an accurate understanding of all the fundamental forces present in a given system to ensure the greatest chance of success. The van der Waals - London dispersion (vdW-Ld) interaction is the universal, long range, interaction that is present in all materials systems. However, scientists and engineers often either ignore or crudely approximate the vdW-Ld interactions because the calculations often appear impractical due to the 1) lack of the required full spectra optical properties and 2) lack of the proper geometrical formulation to give meaningful results. These two barriers are being actively eliminated by the introduction of robust ab initiocodes that can calculate anisotropic full spectral optical properties and by proper extensions to the Lifshitz vdW-Ld formulation that take into account anisotropic spectral optical properties as well as novel geometries. These new capabilities are of broad utility, especially in the biological community, because of the difficulty in experimental determination of full spectral optical properties of nanoscale, liquid phase biomolecules. Here we compare 3 levels of complexity of vdW-Ld interactions (optically isotropic planar, optically anisotropic planar, and optically anisotropic solid cylinder) as well as calculate and compare a variety of Hamaker coefficients relevant to these systems. For the latter two, more complex, cases, we use the ab initiooptical properties of single wall carbon nanotubes (SWCNTs). Our results show the effects of strong optical anisotropy upon the overall vdW-Ld interaction strength as well as the presence of strong dispersion-driven torques in both anisotropic cases, which can play a role in CNT alignment with other CNTs and also preferred CNT alignment directions with optically anisotropic substrates[1, 2].