Large Optical Anisotropy Research Articles

Polarization properties of excitonic qubits in single self-assembled quantum dots

We investigate polarization properties of neutral exciton emission in single self-assembled InAs/GaAs quantum dots. The in-plane shape and strain anisotropy strongly couple the heavy and light hole states and lead to large optical anisotropy with non-orthogonal linearly polarized states misaligned with respect to the crystallographic axes. Owing to a waveguiding experimental configuration, luminescence polarization along the growth axis has been observed revealing the presence of shear components of the deformation tensor out of the growth plane. Resonant luminescence experiments allowed determining the oscillator strength ratio of the two exciton eigenstates. Valence band mixing governs this ratio and can be very different from dot to dot, however the polarization anisotropy axis is quite fixed inside a scanned area of one \mum^{2} and indicates that the in-plane deformation direction to which it is related has a correlation length of the order of magnitude of one \mum^{2}.

Spectral investigation of nematic liquid crystals with high optical anisotropy at THz frequency range

Liquid crystals (LCs) with high optical anisotropy are very desirable for different applications in devices, such as filters, phase shifters, or phase gratings [T. Göbel, P. Meissner, A. Gaebler, M. Koeberle, S. Mueller, and R. Jakoby, Dual-Frequency Switching Liquid Crystal Based Tunable THz Filter, CLEO, Baltimore, MD, 2009; C.-Y. Chen, T.-R. Tsai, C.-L. Pan, and R.-P. Pan, Room temperature terahertz phase shifter based on magnetically controlled birefringence in liquid crystals, Appl. Phys. Lett. 83 (2003), pp. 4497–4499; and C.-J. Lin, C.-H. Lin, Y.-T. Li, R.-P. Pan, and C.-L. Pan, Electrically controlled liquid crystal phase grating for terahertz waves, IEEE Photon. Technol. Lett. 21 (2009), pp. 730–732]. We present spectral studies of LCs with large optical anisotropy in the range from 0.3 to 3 THz. Nematic LC mixtures which have Δn > 0.30 for visible frequency range, i.e., 1825 (Δn = 0.42 at 633 nm) were measured. Properties of LC materials like birefringence, absorption coefficients, and refractive indices for ordinary and extraordinary polarization in THz range were obtained. Orientation of LC was done by a high electric field. Measurements were performed using a TDS spectra 3000 spectrometer.

Energy Dispersions of Anisotropic Refractive Indices of Thiophene/Phenylene Co-Oligomer Crystals

We have determined the energy dispersions of the anisotropic refractive indices along the three a-, b-, and c-crystal axes of the thiophene/phenylene co-oligomer (TPCO) crystals with higher precision. The refractive indices were calculated from interference fringes observed in the vertical- and oblique-incidence transmission spectra of single crystal thin films. The optical constants of the TPCO crystals have large optical anisotropy and strong energy dispersion because of the molecular transition dipole moments oriented as H-aggregates. In particular, the refractive index of the c-axis and its energy dispersion are much larger than those of the other axes. The strong energy dispersion of the refractive indices results in the large effective refractive indices of the TPCO crystals. The determined refractive indices along the three crystal axes of BP1T crystals are very useful for optical device applications.

Synthesis and Properties of Novel Negative Dielectric Liquid Crystal Compounds with a Tetrahydropyran Ring

Novel liquid crystal compounds having a tetrahydropyran ring structure have been developed. These compounds show large optical anisotropy, large absolute negative dielectric anisotropy and higher solubility with other LC compounds compared with conventional high birefringent negative dielectric compounds. The two types of tetrahydropyran derivatives have a chair form cyclohexane ring structure with oxygen in different positions, but they show almost the same dielectric value, and their temperature dependence shows almost the same tendency. In order to clarify this the probabilities of the various conformations were determined. Practical mixtures for the VA-LCD application show improved properties suitable for LCD-TV, also three-dimensional.

Intrinsic optical anisotropy of [001]-grown short-period InAs/GaSb superlattices

We theoretically investigate the intrinsic optical anisotropy or polarization induced by the microscopic interface asymmetry (MIA) in no-common-atom (NCA) InAs/GaSb superlattices (SLs) grown along the [001] direction. The eight-band $\mathbf{K}\ensuremath{\cdot}\mathbf{P}$ model is used to calculate the electronic band structures and incorporates the MIA effect. A Boltzmann equation approach is employed to calculate the optical properties. We found that in NCA InAs/GaSb SLs, the MIA effect causes a large in-plane optical anisotropy for linearly polarized light and the largest anisotropy occurs for light polarized along the [110] and $[1\overline{1}0]$ directions. The relative difference between the optical-absorption coefficient for [110]-polarized light and that for $[1\overline{1}0]$-polarized light is found to be larger than 50%. The dependence of the in-plane optical anisotropy on temperature, photoexcited carrier density, and layer width is examined in detail. This study is important for optical devices which require the polarization control and selectivity.

Refractive indices and order parameters of some tolane-based nematic liquid crystals

Four tolane-based liquid crystals, 4′-pentyl-, 4′-hexyl-, and 4′-heptyl-3-fluoro-4-isothiocyanatotolane and 4′-hexyloxy-3′-fluoro-4-isothiocyanatotolane have been investigated throughout the nematic and isotropic phase in order to determine their refractive indices and order parameters using the Vuks method. These factors characterize anisotropic behaviors of selected liquid crystals that are essential for their applications and improvement. The temperature dependence of refractive indices, birefringence and order parameters in anisotropic nematic phase, have been investigated for these four nematic liquid crystals. All the compounds have positive and large optical anisotropies that increase with decreasing temperature.

First-principles study of the electronic structure and optical properties of the pure BaY 2F 8

Ab-initio calculations based on density-functional theory have been employed to study the electronic structure and some optical properties of the pure BaY 2F 8 (BYF) crystal. The band gap is calculated to be 7.5 eV. The electronic structure calculations revealed that the top of the valence band is dominated by the fluoride p-states, while the very bottom of the conduction band has predominantly yttrium d-character. The optical response in ultra-violet region is determined by calculating the complex dielectric tensor. It was found that the BYF crystal does not exhibit large optical anisotropy. Its principal absorption occurs due to electronic transitions from populated 2p states of the F to empty d-states of neighboring Y and Ba atoms.

Enhancement of electron transport by horizontal molecular orientation of oxadiazole planar molecules in organic amorphous films

To demonstrate the effect of molecular orientation on charge transport characteristics in vacuum-deposited organic amorphous films, the electrical and optical properties of films of two oxadiazole derivatives were investigated. One of them has bulky tert-butyl terminals and forms an isotropic film, whereas the other has planar bipyridyl terminals and forms a highly anisotropic film. The very large optical anisotropy of the latter means that the planar molecules stack horizontally, leading to large overlaps of π-orbitals and more than 30 times higher electron mobility though the geometric and electronic structures of the main conformers of the two derivatives are quite similar.

Optical absorption of polarized light in InAs/GaSb quantum wells

Using an eight-band k ⋅ p model Hamiltonian with the Burt–Foreman envelope function theory, we have investigated the optical absorption of both linearly and circularly polarized light, as well as related phenomena in InAs/GaSb broken-gap quantum wells grown along the [0 0 1] direction, with emphasis on the effects of electron–hole hybridization and the various symmetry-breaking mechanisms such as structural inversion asymmetry, bulk inversion asymmetry and interface Hamiltonian. The optical matrix elements exhibit unusual angular dependence in close connection with the spin-flip transitions which are originally forbidden. The spin split of the 2e subband results in two profound absorption peaks for the 1hh–2e transition for both linearly polarized and circularly polarized light. A large lateral optical anisotropy appears in the absorption coefficient of linearly polarized light, which can reach almost 100% with a reducing thickness of the quantum well. For the absorption of circularly polarized light, we found a large enhancement of electron spin polarization in the upper 2e subband, which was generally considered as forbidden if the polarization is along the direction perpendicular to the plane-of-light incidence.

Horizontal molecular orientation in vacuum-deposited organic amorphous films of hole and electron transport materials

Using wide-range variable angle spectroscopic ellipsometry, the authors demonstrate large optical uniaxial anisotropy of vacuum-deposited organic amorphous films of hole and electron transport materials having long or planar molecular structures. The ordinary refractive indices and extinction coefficients were higher than the extraordinary ones, revealing that the molecules in the amorphous films are horizontally oriented. The horizontal orientation requires significant modifications in the understanding of both the electrical and optical characteristics of amorphous films when we use materials having long or planar molecular structures.

Electronic structure and optical properties of freestanding [0001] oriented GaN nanowires and nanotubes

We report on a theoretical study of the electronic and optical properties of freestanding, [0001] oriented wurtzite GaN nanowires and nanotubes based on an atomistic tight binding approach. The developments of band structure, optical properties, and effective mass are studied as functions of nanowire size. It is shown that the valence band structure of the nanowire depends on the lateral size of the nanowire and that the order between the first two valence bands is reversed above a critical size. The fundamental optical transition is found to be strong for nanowire sizes below, and weak for nanowire sizes above, this critical size. The first strong optical transition is found to have a very large optical polarization anisotropy with the dominant component parallel to the nanowire axis. It is also shown that there is a simple functional relationship between the conduction band effective mass and the subband energy, while no such general relation can be found for the valence bands. For the nanotubes the change in energy compared to the solid nanowire is found to be strongly related to the distribution of the original nanowire state wave function. The incorporation of a hole in the nanowire will force a change in the ordering between the first two valence band states compared to a below critical size nanowire.

Fabrication of laser induced periodic surface structure for geometrical engineering

The paper presents the highly ordered geometrical structures of laser induced periodic surface structure (LIPSS) in azobenzene urethane polymer (DR19 polymer) from 4-( N, N-dihydroxyethylamino)-4′-nitroazobenzene (Disperse red 19) with tolylene-2,4-diisocyanate (TDI). One or two regulated striped LIPSS was formed in confined spaces between surface relief gratings (SRG) induced by the s-polarized interfered beams. The pitch of LIPSS was one-half or one-third of SRG pitch. Standing wave with some selected mode between SRG in the surface waveguide is responsible for the formation of the regulated striped LIPSS. The crossed illumination of the interfered beams showed the waffle-like structure for s-polarization beam and the egg crate-like (ECL) structure for p-polarized beam. Photoinduced microscopic molecular ordering was also investigated. The linear polarized beam gave the large optical anisotropy in the polymer and the circularly polarized beam produced the chiral structure. The circular dichroism spectra showed the sharp peak due to the circular Bragg reflection from which the chiral pitch was evaluated.

Infrared resonances of self-assembled Pb nanorods

The formation of self-assembled Pb nanorods on a vicinal Si(335)/Au surface at 305K was monitored in situ with polarization dependent infrared spectroscopy. The rods have formed from Pb evaporated on a single domain Au-stabilized Si(335) surface under ultrahigh vacuum conditions and reached lengths of about 1μm. As the rods are aligned almost parallel, a large optical anisotropy of the transmitted infrared light was detected. A plasmonic antennalike resonance appeared in the spectra. Since such resonance frequencies are mainly determined by the length of the rods, the growth process was directly monitored via the shift of the resonance frequency. The estimated extinction cross section at resonance frequency indicates field enhancement similar to that observed for gold nanorods.

Optical polarization anisotropies in GaN films for different nonpolar orientations

AbstractWe have used polarized photoreflectance spectroscopy to study the electronic‐band‐structure modification of GaN films grown along different nonpolar orientations due to biaxial, anisotropic in‐plane strain. For nonpolar oriented films, the c‐axis of GaN lies in the film plane. In four different GaN films with a nonpolar orientation (M ‐ and A ‐plane), we observe a large in‐plane optical polarization anisotropy, which is much larger for the M ‐plane films due to the much larger strain in at least one of the in‐plane directions. The experimental results for the transition energies and oscillator strengths are compared with theoretical results obtained using the k·p perturbation approach. The large optical polarization anisotropy in M ‐plane GaN films can be used for polarization filtering and polarization‐sensitive photodetectors. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electro-optical modulation for a boron nitride nanotube probed by first-principles calculations

The electro-optical modulation effect of a (10,0) boron nitride nanotube is investigated by first-principles, density functional theory calculations. The optical properties of boron nitride nanotubes can be tuned through perturbation by a transverse electric field. Large optical anisotropy in variations of the absorption spectra and refractive indices in response to external electric field is found. In addition, a method called the band-resolved absorption density analysis technique is developed to investigate the detailed optical absorption transition processes. The result is consistent with the recent experimental observation [M. Ishigami et al., Phys. Rev. Lett. 94, 56804 (2005)] and suggests that the boron nitride nanotubes can be a promising material for electro-optical modulation device applications.

Transparent Image Sensors Using an Organic Multilayer Photodiode

The most fascinating advantages of organic photodiodes in the context of information-conversion applications are their ability for large-area surface light detection, the potentially wide selection of materials that allows color tuning, and the possibility for low-cost fabrication of flexible devices. The photosensitivity of organic photodiodes, either multilayer-heterojunction type or bulk-heterojunction type, is quite high, and internal photon-to-electron conversion quantum efficiency in some cases approaches unity. Thus, we have a good opportunity to utilize organic photodiodes for high-performance photosensors, even though power-conversion efficiency of organic photovoltaic solar cells under solar light irradiation remains less than 5 %. Fabrications of organic photodiodes with good photoresponse, such as fast-response photodetectors, full-color image sensors, and position-sensitive photodetectors, have been reported. The use of anisotropy of optical absorption results from molecular orientation is one of the most fascinating advantages of organic photodiodes. The combination of transparent electrodes and polarized light detection that results from optical anisotropy of molecules provides unique image-sensing devices that capture images under light illumination behind them. Figure 1 demonstrates the basic concept for reading a black and white picture using our transparent photodiodes with polarization-detection capability. Let us assume that white light or monochromatic light is transmitted through a linear polarizer to the polarization-sensitive photodetector. At present, for simplicity, we assume that the photodetector is composed of oriented molecules with large optical anisotropy. When polarized light perpendicular to the transition moments of the oriented molecules irradiates the photodetector, light passes through the detector with little optical absorption. Thus, a very small photocurrent is detected by the photodiode when the incident light is perpendicularly polarized. The transmitted polarized light is reflected back according to the reflectivity of the object. When white paper is used, a large fraction of the light is reflected back and depolarized by scattering. Thus, the light component with parallel polarization reaches the photodetector and a large photocurrent signal is detected. On the other hand, when the transmitted polarized light reaches a black object, all the light is absorbed and no photocurrent is detected. Thus, the transparent and polarization-sensitive photodetectors are useful for the detection of reflectivity of objects. We note that this principle is applicable to grayscale sensing as well as simple black and white detection. When addressable crossed-array photodiodes are used or 2D scanning of incident light is available, 2D picture images are obtainable. We have successfully developed polarization-sensitive photodiodes using a pn-heterojunction-type structure composed of 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI) for the n-type layer and titanyl phthalocyanine (TiOPc) for the p-type layer. In this report, we demonstrate the feasibility of our transparent photodetector composed of a well-aligned PTCBI and a non-aligned TiOPc layer for image sensing. As shown in Figure 2a, the device configuration was indium tin oxide (ITO)/indium/aligned PTCBI/TiOPc/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/ Au. A well-aligned PTCBI layer with the optical axis parallel to the rubbing direction was grown. A large dichroic ratio of optical absorption was obtained at the peak wavelength of 540 nm. Figure 2b shows absorption spectra of both aligned PTCBI and non-aligned TiOPc films. An aligned PTCBI showed a dichroic ratio of 4.7:1 at the peak wavelength of 540 nm, while non-aligned TiOPc showed isotropic absorpC O M M U N IC A TI O N S

Optical characterization of tristructural isotropic fuel particle cross-sections using generalized ellipsometry

One particularly important measure of quality of tristructural isotropic (TRISO) fuel particles is the degree of preferred orientation of crystallites in the polycrystalline pyrolytic carbon coatings. Excessive crystallographic anisotropy leads to unwanted anisotropic dimensional changes during irradiation that can cause the TRISO coatings to fail. Early optical methods were developed in the 60s and 70s to measure this anisotropy by taking advantage of the large optical anisotropy of graphite. Since that time, there have been significant improvements in both the theoretical understanding and experimental techniques in the understanding of optical anisotropy. Here we discuss a new method, based on the two-modulator generalized ellipsometer (2-MGE) to measure the optical anisotropy. This technique has been demonstrated to measure the optical diattenuation to an accuracy from ±0.001 to ±0.005 and the preferred direction of the crystallites to an accuracy of better than ±2° with a spatial resolution of better than 5μm. Diattenuation ‘pictures’ of the nuclear fuel cross-sections reveal that the inner pyrocarbon layer (IPyC) is far from uniform both in the degree of diattenuation and in the direction of the principal axis. The 2-MGE technique is faster, more accurate, and collects considerably more data than previous optical anisotropy measurements of TRISO fuel particles.

Photoinduced Reorientation and Multiple Optical Data Storage in Photo-Cross-Linkable Liquid Crystalline Copolymer Films Using 405 nm Light

New polymethacrylate liquid crystalline copolymers, which are comprised of photo-cross-linkable 4-(4-methoxycinnamoyloxy)biphenyl (MCB) and photosensitizing 4-nitrobiphenyl (NB) or 4-nitrophenyl (NP) side groups, were synthesized. The irradiation with linearly polarized (LP) 405 nm light was performed to investigate the photoinduced reorientation behavior of the thin films. On the basis of the polarization-preserved excited energy transfer from the photosensitizing groups to the MCB groups, a photoinduced optical anisotropy due to the axis-selective photoreaction of the MCB groups accompanied the photoreaction of the film. In all cases, annealing the film reversely amplified the photoinduced optical anisotropy and an in-plane order parameter greater than 0.7 was achieved for the NB-containing copolymer films. The axis-selectivity of the photoabsorption of the photosensitizing groups and the effective energy transfer played important roles in generating the large photoinduced optical anisotropy and the eff...

Optical detection of deoxyribonucleic acid hybridization using an anchoring transition of liquid crystal alignment

We demonstrate an optical method for detecting specific binding events in an oligo deoxyribonucleic acid (DNA) chip using an anchoring transition of a nematic liquid crystal (NLC) as a result of DNA hybridization. A homeotropic orientation of the NLC supported by oligoDNA changes to a random planar orientation after hybridization. Such DNA hybridization and subsequent NLC reorientation are easily observable with a simple detection system via long-range orientational order and large optical anisotropy of the NLC.

Dynamic birefringence of cyclic olefin copolymers

Dynamic birefringence and viscoelasticity of amorphous polymers containing cyclic olefins were simultaneously measured over a wide frequency region from the rubbery to the glassy zone. The obtained results are discussed in terms of optical anisotropy and main chain flexibility. For copolymers of ethyelene and cyclododecene, the stress optical coefficient in the rubbery region, C R, decreased with increasing of cyclododecene contents. This could be attributed to the reduction in the nematic interaction and flexible main chain structure of cyclic olefin unit. On the other hand,in copolymers of ethylene and norbornene, systematic trend of composition dependence of C R was unclear, suggesting differences in the sequence distribution of monomers in the main chain, probably due to usage of different catalyses. Main chain flexibility decreased with increasing of cyclic olefin units when its content exceeds the 50 mol%. On the other hand, polymers produced by ring-opening polymerization of cyclic olefin showed larger C R values among the studied polymers, which can be attributed to large optical anisotropy of resulting cyclic olefin units and less-flexible main chain structure.