Unique Polarization Research Articles

Ultrafast laser assisted fabrication of ZnO nanorod arrays for photon detection applications

Orderly aligned ZnO nanorod arrays were grown by the ultrafast laser assisted ablation deposition method. These nanorod arrays were further used to make efficient p–n heterojunction photodetector arrays, which have the potential to have nanoscale spatial resolution for imaging, unique incident polarization discrimination capability, and much improved quantum efficiency as well as detection sensitivity. Both front- and back-illumination photodetection schemes were demonstrated by growing those ZnO nanorod arrays on p-type silicon and p-type Zn 0.9Mg 0.1O-coated Al 2O 3 (0 0 0 1) substrates, respectively. Typical diode rectification behavior and photosensitivity were observed in both designs through I– V and photocurrent measurements.

Diffraction Properties of Nematic Phase Gratings with Photoregulated Liquid Crystal Cells

A detailed analysis of the recently proposed [Appl. Phys. Lett. 87 (2005) 161112] nematic phase gratings with photoregulated liquid crystal cells is presented. Nematic phase gratings with photoregulated photo-cross-linkable polymer liquid crystals were prepared and characterized. The phase grating consists of homogeneous and 90°-twisted nematic (TN) areas and shows unique polarization conversion in the diffraction process. The diffraction properties, including higher-ordered diffractions, were well explained by means of the elastic continuum theory, Jones calculus, and diffraction theory.

DC and RF Characteristics of AlGaN/GaN/InGaN/GaN Double-Heterojunction HEMTs

We present the detailed dc and radio-frequency characteristics of an Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> N/GaN/In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9 </sub> N/GaN double-heterojunction HEMT (DH-HEMT) structure. This structure incorporates a thin (3 nm) In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> N notch layer inserted at a location that is 6-nm away from the AlGaN/GaN heterointerface. The In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> N layer provides a unique piezoelectric polarization field which results in a higher potential barrier at the backside of the two-dimensional electron gas channel, effectively improving the carrier confinement and then reducing the buffer leakage. Both depletion-mode (D-mode) and enhancement-mode (E-mode) devices were fabricated on this new structure. Compared with the baseline AlGaN/GaN HEMTs, the DH-HEMT shows lower drain leakage current. The gate leakage current is also found to be reduced, owing to an improved surface morphology in InGaN-incorporated epitaxial structures. DC and small- and large-signal microwave characteristics, together with the linearity performances, have been investigated. The channel transit delay time analysis also revealed that there was a minor channel in the InGaN layer in which the electrons exhibited a mobility slightly lower than the GaN channel. The E-mode DH-HEMTs were also fabricated using our recently developed CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -based plasma treatment technique. The large-signal operation of the E-mode GaN-based HEMTs was reported for the first time. At 2 GHz, a 1times100 mum E-mode device demonstrated a maximum output power of 3.12 W/mm and a power-added efficiency of 49% with single-polarity biases (a gate bias of +0.5 V and a drain bias of 35 V). An output third-order interception point of 34.7 dBm was obtained in the E-mode HEMTs

Atmosphere models of magnetized neutron stars: QED effects, radiation spectra and polarization signals

Observations of surface emission from isolated neutron stars (NSs) provide unique challenges to theoretical modelling of radiative transfer in magnetized NS atmospheres. Recent work has demonstrated the critical role of vacuum polarization effects in determining NS spectra and polarization signals, in particular the conversion of photon modes (due to the 'vacuum resonance' between plasma and vacuum polarization) propagating in the density gradient of the NS atmosphere. Previous NS atmosphere models incorporated the mode conversion effect approximately, relying on transfer equations for the photon modes. Such treatments are inadequate near the vacuum resonance, particularly for magnetic field strengths around B ∼ B l ≃ 7 × 10 13 G, where the vacuum resonance occurs near the photosphere. In this paper, we provide an accurate treatment of the mode conversion effect in magnetized NS atmosphere models, employing both the modal radiative transfer equations coupled with an accurate mode conversion probability at the vacuum resonance, and the full evolution equations for the photon Stokes parameters. In doing so, we are able to quantitatively calculate the effects of vacuum polarization on atmosphere structure, emission spectra and beam patterns, and polarizations for the entire range of magnetic field strengths, B = 10 12 -10 15 G. In agreement with previous works, we find that for NSs with magnetic field strength B ≥ 2B l , vacuum polarization reduces the widths of spectral features, and softens the hard spectral tail typical of magnetized atmosphere models. For B ≤ B l /2, vacuum polarization does not change the emission spectra, but can significantly affect the polarization signals. Our new, accurate treatment of vacuum polarization is particularly important for quantitative modelling of NS atmospheres with 'intermediate' magnetic fields, B ≃ (0.5-2) B l . We provide fitting formulae for the temperature profiles for a suite of NS atmosphere models with different field strengths, effective temperatures and chemical compositions (ionized H or He). These analytical profiles are useful for direct modelling of various observed properties of NS surface emission. As an example, we calculate the observed intensity and polarization light curves from a rotating NS hotspot, taking into account the evolution of photon polarization in the magnetosphere. We show that vacuum polarization induces a unique energy-dependent linear polarization signature, and that circular polarization can be generated in the magnetosphere of rapidly rotating NSs. We discuss the implications of our results to recent observations of thermally emitting isolated NSs and magnetars, as well as the prospects of future spectral and polarization observations.

Enhanced Electroweak Penguin Amplitude inB→VVDecays

We discuss a novel electromagnetic penguin contribution to the transverse helicity amplitudes in B decays to two vector mesons, which is enhanced by two powers of mB/Lambda relative to the standard penguin amplitudes. This leads to unique polarization signatures in penguin-dominated decay modes such as B-->rhoK* similar to polarization effects in the radiative decay B-->K*gamma and offers new opportunities to probe the magnitude and chirality of flavor-changing neutral current couplings to photons.

Nano-interface Surface Dielectric Polarization Phenomena: Detection and Application

Nano-interfacial dielectric polarization phenomena were discussed from the viewpoint of electronics, paying attention to the alignment of dipoles and dipolar energy. Firstly, we summarized dielectric polarization phenomena of monolayer comprised of polar molecules, and showed that monolayer is quite different from bulk materials, and the unique dielectric properties of monolayers can be discussed in terms of orientational order parameters. Maxwell-displacement-current (MDC) measurement coupled with optical second harmonic generation (SHG) measurement is shown as an effective way for detecting unique dielectric polarization phenomena of surface monolayers. Then, the importance of dielectric energy is discussed from the viewpoint of monolayer pattern formation and pointed out that the control of dipolar energy can be new pattem formation technology.

Experimental observation of the elliptically polarized fundamental vector soliton of isotropic Kerr media

We report the experimental observation of the elliptically polarized fundamental vector soliton of isotropic Kerr media and its unique polarization evolution. This was achieved in the spatial domain in a nonbirefringent CS2 planar waveguide.

Liquid crystal phase gratings using photoregulated photocrosslinkable polymer liquid crystals

We have presented liquid crystal phase gratings using photoregulated photocrosslinkable polymer liquid crystals. The phase grating consists of homogeneous and 90°-twisted nematic area and shows unique polarization conversion in the diffraction process. The diffraction properties are well explained by the theory based on Jones calculus.

Isolation and Characterization of Aspergillus oryzae Vacuolar Protein Sorting Mutants

The vacuolar protein sorting (vps) system in the filamentous fungus Aspergillus oryzae, which has unique cell polarity and the ability to secrete large amounts of proteins, was evaluated by using mutants that missort vacuolar proteins into the medium. Vacuolar carboxypeptidase Y (CPY) fused with enhanced green fluorescent protein (EGFP) was used as a vacuolar marker. Twenty dfc (dim EGFP fluorescence in conidia) mutants with reduced intracellular EGFP fluorescence in conidia were isolated by fluorescence-activated cell sorting from approximately 20,000 UV-treated conidia. Similarly, 22 hfm (hyper-EGFP fluorescence released into the medium) mutants with increased extracellular EGFP fluorescence were isolated by using a fluorescence microplate reader from approximately 20,000 UV-treated conidia. The dfc and hfm mutant phenotypes were pH dependent, and missorting of CPY-EGFP could vary by 10- to 40-fold depending on the ambient pH. At pH 5.5, the dfc-14 and hfm-4 mutants had an abnormal hyphal morphology that is consistent with fragmentation of vacuoles and defects in cell polarity. In contrast, the hyphal and vacuolar morphology of the dfc-14 and hfm-4 mutants was normal at pH 8.0, although CPY-EGFP accumulated in perivacuolar dot-like structures similar to the class E compartments in Saccharomyces cerevisiae vps mutants. In hfm-21, CPY-EGFP localized at the Spitzenkörper when the mutant was grown at pH 8.0 but not in vacuoles, suggesting that hfm-21 may transport CPY-EGFP via a novel pathway that involves the Spitzenkörper. Correlations between vacuolar protein sorting, pH response, and cell polarity are reported for the first time for filamentous fungi.

High extinction ratio in-fiber polarizers based on 45° tilted fiber Bragg gratings

We report a near-ideal in-fiber polarizer implemented by use of 45 degrees tilted fiber Bragg grating structures that are UV inscribed in hydrogenated Ge-doped fiber. We demonstrate a polarization-extinction ratio of 33 dB over a 100-nm operation range near 1550 nm. We further show an achievement of 99.5% degree of polarization for unpolarized light with these gratings. We also theoretically investigate tilted grating structures based on the Green's function calculation, therein revealing the unique polarization characteristics, which are in excellent agreement with experimental data.

Polarization of III-nitride blue and ultraviolet light-emitting diodes

Polarization-resolved electroluminescence studies of III-nitride blue and ultraviolet (UV) light-emitting diodes (LEDs) were performed. The LEDs were fabricated on nitride materials grown by metalorganic chemical vapor deposition on sapphire substrates (0001). Transverse electric (TE) polarization dominates in the InGaN∕GaN quantum-well (QW) blue LEDs (λ′=458nm), whereas transverse magnetic (TM) polarization is dominant in the AlInGaN QW UV LEDs (λ=333nm). For the case of edge emission in blue LEDs, a ratio (r=I⊥∕I‖) of about 1.8:1 was observed between the EL intensities with polarization E⊥c (TE mode) and E‖c (TM mode), which corresponds to a degree of polarization ∼0.29. The UV LEDs exhibit a ratio r of about 1:2.3, corresponding to a degree of polarization ∼0.4. This is due to the fact that the degree of polarization of the bandedge emission of the AlxInyGa1−x−yN active layer changes with Al concentration. The low emission efficiency of nitride UV LEDs is partly related to this polarization property. Possible consequences and ways to enhance UV emitter performances related to this unique polarization property are discussed.

A mechanistic model for eukaryotic gradient sensing: Spontaneous and induced phosphoinositide polarization

The crawling movement of cells in response to a chemoattractant gradient is a complex process requiring coordination of various subcellular activities. Although a complete description of the mechanisms underlying cell movement remains elusive, the very first step of gradient sensing, enabling the cell to perceive the imposed gradient, is becoming more transparent. The increased understanding of this step has been driven by the discovery that within 5–10 s of applying a weak chemoattractant gradient, membrane phosphoinositides such as PIP 3 localize at the front end of the cell. It is currently believed that the gradient sensing mechanism is precisely the mechanism leading to this localization. We have formulated a reaction–diffusion model based on the phosphoinositide cycle which predicts various responses of motile cells in addition to the phosphoinositide polarization induced by chemoattractant gradients. The responses include: (a) Polarized sensitivity wherein a polarized cell responds to a change in the direction of the gradient by turning its existing front. (b) Spontaneous polarization wherein cells polarize in a random direction even if the surrounding chemoattractant concentration is uniform. (c) Unique localization which refers to the formation of a unique polarity even in the face of multiple chemoattractant sources. The above responses preclude the hypothesis that the cell merely amplifies the external signal. Our model indicates that the cell must be viewed as a system that nonlinearly processes chemoattractant inputs. We show in particular that these seemingly complex dynamics can be explained very simply in terms of the instabilities and wavefront dynamics that are characteristic of the activator–inhibitor class of models.

Electrokinetic micropump and micromixer design based on ac faradaic polarization

A microfluidic pump and mixer design based on ac faradaic polarization is proposed. Unlike ac electrokinetic devices based on capacitive charging of the electrodes, the design yields a net electro-osmotic flow for high-conductivity electrolytes at high voltages and frequencies without producing gas bubbles or generating pH gradients. The average velocity, which can be more than an order of magnitude higher than that generated by the capacitive mechanism, has an exponential dependence on the voltage and increases monotonically at low frequencies. Vortices and net flows with linear velocities in excess of 1mm∕s are generated with orthogonal microfabricated planar electrodes based on the unique flow and polarization features of this new ac charging mechanism.

Unique optical properties of AlGaN alloys and related ultraviolet emitters

Deep UV photoluminescence spectroscopy has been employed to study the optical properties of AlxGa1−xN alloys (0⩽x⩽1). The emission intensity with polarization of E⊥c and the degree of polarization were found to decrease with increasing x. This is a consequence of the fact that the dominant band edge emission in GaN (AlN) is with polarization of E⊥c(E∥c). Our experimental results suggest that the decreased emission efficiency in AlxGa1−xN alloys and related UV emitters could also be related with their unique polarization property, i.e., the intensity of light emission with polarization of E⊥c decreases with x. It is thus concluded that UV emitters with AlGaN alloys as active layers have very different properties from InGaN and other semiconductor emitters.

Identification of singlet and triplet states of negatively charged excitons in CdTe‐based quantum wells

We present comprehensive study of negatively charged exciton in high magnetic fields for filling factors ν < 1. In magneto-optical spectra the fine structure was found to be contributed by neutral exciton and different a set of bound states of charged exciton. These states were identified due to their unique polarization properties charecteristics in emission and absorption spectra. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced classification of prairie landscapes using simulated RADARSAT-2 imagery

For the past decade, we have been using Landsat TM imagery to monitor changes in the area and distribution of ecosystems in a relatively undisturbed natural region in southwestern Manitoba, Canada. The 40 000 ha of Canadian Forces Base Shilo are protected from urbanization, commercialization, and agriculture, and are disturbed only by modest and controlled amounts of military training. A significant problem with the use of TM imagery for this study is that some of the more sensitive indicators of environmental change, such as the infestation of disturbed grassland by leafy spurge (Euphorbia esula), are very difficult to identify and delineate in the visible and near-IR bands. A fully polarized synthetic aperture radar that is sensitive to vegetation morphology may not have the same difficulty. C-band radars, such as that being constructed for RADARSAT-2, are known to have difficulty differentiating vegetation types in regions of dense foliage, but, in a prairie ecosystem such as the one under study here, even the densest foliage should be sparse enough to yield a unique polarization signature. To test this hypothesis, we have simulated RADARSAT-2 HH, HV, and VV images, using TM images of the area, polarization ratios extracted from the literature for vegetation types similar to those found in the area, and a RADARSAT-1 HH image of the area collected almost simultaneously with one of the TM images. Estimation of class separation by Bhattacharyya distance, first using only HH information and then using the full HH, HV, VV imagery, shows an increase of statistically separable classes and class groups when the fully polarized class characterization is used. Visual comparison of synthesized imagery from different years shows sufficient differences to imply that RADARSAT-2 imagery will likely be a useful tool to augment our continuing monitoring and assessment of this prairie ecosystem.

Polarized x-ray emission from magnetized neutron stars: signature of strong-field vacuum polarization.

In the atmospheric plasma of a strongly magnetized neutron star, vacuum polarization can induce a Mikheyev-Smirnov-Wolfenstein type resonance across which an x-ray photon may (depending on its energy) convert from one mode into the other, with significant changes in opacities and polarizations. We show that this vacuum resonance effect gives rise to a unique energy-dependent polarization signature in the surface emission from neutron stars. The detection of polarized x rays from neutron stars can provide a direct probe of strong-field quantum electrodynamics and constrain the neutron star magnetic field and geometry.

The HIγS Facility: A Free-Electron Laser Generated Gamma-Ray Beam for Research in Nuclear Physics

The High Intensity Gamma-ray Source (HIγS) is a joint project between Triangle Universities Nuclear Laboratory (TUNL) and the Duke Free Electron Laser Laboratory (DFELL). This facility utilizes intra-cavity back-scattering of the FEL photons in order to produce a γ-flux enhancement of approximately 103 over the existing sources. At present, gamma-ray beams with energies ranging from 2 to 50 MeV are available with intensities of 105–107 γ/ s , energy spreads of 1% or better (with lower intensities), and 100% linear polarization. An upgrade is presently underway which will allow for the production of γ rays up to an energy of about 225 MeV having intensities in excess of 108 γ/ sec . The primary component of the upgrade is a 1.2 GeV booster-injector which will provide for efficient injection at any chosen operating energy of the storage ring from 300 MeV to 1.2 GeV. In addition, an upgrade of the present OK-4 FEL to a helical undulator system (OK-5) is underway. This new system has many advantages over the present one, including making switchable linear and circularly polarized beams available, an increase in power and a decrease in mirror-damaging radiation. The full system, including the booster injector, is expected to be ready for use by 2006. TUNL researchers, in collaboration with outside theoretical and experimental colleagues, have proposed a broad based research program in nuclear physics which is designed to exploit the unique flux, energy resolution and polarization of the HIγS beams. A description of the presently available facility and the anticipated facility following the present upgrades will be given in this review, along with a description of recent and planned experiments.

Challenges and potential payoff for crystalline oxides in wide bandgap semiconductor technology

While growth of wide bandgap semiconductor materials on crystalline oxides (sapphire, lithium gallate, lithium aluminate, zinc oxide and others) has become routine, growth of crystalline oxides on wide bandgap materials remains challenging and minimally explored. The potential payoff in terms of enhanced device performance, increased functionality and reliability warrants examining this option. This presentation aims at targeting key areas, where crystalline oxides could improve wide bandgap semiconductor device performance. Some of these include the use of ferroelectric oxides for power switching applications, oxides with anisotropic dielectric constants for high voltage termination and oxides with large electric flux density near breakdown. Unique polarization engineered structures are described that are enabled by using lithographically defined poled regions in a ferroelectric substrate. The desired crystalline oxide properties, potential implementation challenges and potential pitfalls will be discussed.

Behavior of bulk melt-textured YBCO single domains subjected to crossed magnetic fields

We have experimentally investigated the crossed magnetic field effects on bulk melt-processed YBCO single domains. The samples were first permanently magnetized along their c-axis and then subjected to several cycles of a transverse magnetic field parallel to the ab planes. The magnetic properties along the c and ab directions were simultaneously measured using a couple of orthogonal pick-up coils as well as a Hall probe placed against the sample surface. The effects of both sweep amplitude and polarity were investigated. Field sweeps of alternate polarities are shown to affect the decay of the c-axis magnetization much more strongly than field sweeps of unique polarity do. However, the c-axis magnetization does not show any saturation even after a large number of field sweeps. Next, a micro-Hall probe scanning system was used to measure the distribution of magnetic induction over the top surface of the single domain subjected to the same combination of magnetic fields. The results are shown to be consistent with those determined with the sensing coils and bring out the role played by geometric effects.