Volume 104, Issue 7, 17 February 2014
- photonics and optoelectronics
- surfaces and interfaces
- structural, mechanical, optical, and thermodynamic properties of advanced materials
- magnetics and spintronics
- superconductivity and superconducting electronics
- dielectrics, ferroelectrics, and multiferroics
- nanoscale science and technology
- organic electronics and photonics
- device physics
- biophysics and bio-inspired systems
- energy conversion and storage
- interdisciplinary and general physics
Index of content:
Gradient index media, which are designed by varying local element properties in given geometry, have been utilized to manipulate acoustic waves for a variety of devices. This study presents a cylindrical, two-dimensional acoustic “black hole” design that functions as an omnidirectional absorber for underwater applications. The design features a metamaterial shell that focuses acoustic energy into the shell's core. Multiple scattering theory was used to design layers of rubber cylinders with varying filling fractions to produce a linearly graded sound speed profile through the structure. Measured pressure intensity agreed with predicted results over a range of frequencies within the homogenization limit.
- PHOTONICS AND OPTOELECTRONICS
Laser-ablative engineering of phase singularities in plasmonic metamaterial arrays for biosensing applications104(2014); http://dx.doi.org/10.1063/1.4865553View Description Hide Description
By using methods of laser-induced transfer combined with nanoparticle lithography, we design and fabricate large-area gold nanoparticle-based metamaterial arrays exhibiting extreme Heaviside-like phase jumps in reflected light due to a strong diffractive coupling of localized plasmons. When employed in sensing schemes, these phase singularities provide the sensitivity of 5 × 104 deg. of phase shift per refractive index unit change that is comparable with best values reported for plasmonic biosensors. The implementation of sensor platforms on the basis of such metamaterial arrays promises a drastic improvement of sensitivity and cost efficiency of plasmonic biosensing devices.
Electrical addressing of polymer stabilized hyper-twisted chiral nematic liquid crystals with interdigitated electrodes: Experiment and model104(2014); http://dx.doi.org/10.1063/1.4865558View Description Hide Description
Electro-optic switching in short-pitch polymer stabilized chiral nematic liquid crystals was studied and the relative contributions of flexoelectric and dielectric coupling were investigated: polymer stabilization was found to effectively suppress unwanted textural transitions of the chiral nematic liquid crystal and thereby enhance the electro-optical performance (high optical contrast for visible light, a near ideal optical hysteresis, fast electro-optic response). Test cells were studied that possessed interdigitated electrodes to electrically address the liquid crystal. Based on simulations, a well-fitted phenomenological description of the electro-optic response was derived considering both flexoelectro-optic and Kerr-effect based electro-optic response.
Atomistic tight-binding study of electronic structure and interband optical transitions in GaBi x As1− x /GaAs quantum wells104(2014); http://dx.doi.org/10.1063/1.4865827View Description Hide Description
Large-supercell tight-binding calculations are presented for GaBi x As1− x /GaAs single quantum wells (QWs) with Bi fractions x of 3.125% and 12.5%. Our results highlight significant distortion of the valence band states due to the alloy disorder. A large full-width-half-maximum (FWHM) is estimated in the ground state interband transition energy (≈33 meV) at 3.125% Bi, consistent with recent photovoltage measurements for similar Bi compositions. Additionally, the alloy disorder effects are predicted to become more pronounced as the QW width is increased. However, they are less strong at the higher Bi composition (12.5%) required for the design of temperature-stable lasers, with a calculated FWHM of ≈23.5 meV at x = 12.5%.
104(2014); http://dx.doi.org/10.1063/1.4865906View Description Hide Description
Terahertz emission by ultrafast optical excitation of semiconductor/metal interfaces strongly depends on the strength of the depletion-field. Here, we report on the strong enhancement of the emission after optical excitation of surface plasmons at these interfaces. The enhancement is caused by the plasmonic localization of the pump light near the metal surface, where the depletion-field is the strongest. Compared to the case where no surface plasmons are excited, a terahertz field enhancement of more than an order of magnitude is obtained for a particular thickness of cuprous oxide layer on gold, where localized surface plasmons are excited at the interface.
Optimizing the internal electric field distribution of alternating current driven organic light-emitting devices for a reduced operating voltage104(2014); http://dx.doi.org/10.1063/1.4865928View Description Hide Description
The influence of the thickness of the insulating layer and the intrinsic organic layer on the driving voltage of p-i-n based alternating current driven organic light-emitting devices (AC-OLEDs) is investigated. A three-capacitor model is employed to predict the basic behavior of the devices, and good agreement with the experimental values is found. The proposed charge regeneration mechanism based on Zener tunneling is studied in terms of field strength across the intrinsic organic layers. A remarkable consistency between the measured field strength at the onset point of light emission (3–3.1 MV/cm) and the theoretically predicted breakdown field strength of around 3 MV/cm is obtained. The latter value represents the field required for Zener tunneling in wide band gap organic materials according to Fowler-Nordheim theory. AC-OLEDs with optimized thickness of the insulating and intrinsic layers show a reduction in the driving voltage required to reach a luminance of 1000 cd/m2 of up to 23% (8.9 V) and a corresponding 20% increase in luminous efficacy.
104(2014); http://dx.doi.org/10.1063/1.4865932View Description Hide Description
A fiber twist sensor based on the surface plasmon resonance (SPR) effect of an Au-coated tilted fiber Bragg grating (TFBG) is proposed. The SPR response to the twist effect on an Au-coated TFBG (immersing in distilled water) is studied theoretically and experimentally. The results show that the transmission power around the wavelength of SPR changes with the twist angle. For the twist ranging from 0° to 180° in clockwise or anti-clockwise directions, the proposed sensor shows sensitivities of 0.037 dBm/° (S-polarized) and 0.039 dBm/° (P-polarized), which are almost 7.5 times higher than that of the current similar existing twist sensor.
Second-harmonic generation in dry powders: A simple experimental method to determine nonlinear efficiencies under strong light scattering104(2014); http://dx.doi.org/10.1063/1.4866160View Description Hide Description
A simple experimental method for determining the efficiency of second harmonic generation (SHG) using small amounts of dry crystalline powder is described. Contrary to previous techniques, the light scattering effects are explicitly accounted for, and a procedure to eliminate their influence is proposed. The method also permits to assess whether a particular crystal is phase matchable or not. Second-order susceptibility coefficients of eight relevant materials for SHG are determined, and the agreement with the single-crystal data is, in general, fairly good. For polycrystalline ZnSe samples, a situation of the so-called random quasiphase matching is evidenced.
104(2014); http://dx.doi.org/10.1063/1.4866165View Description Hide Description
Studies of magneto-optic effects in ferrofluid-filled silica core microstructured optical fibers are reported. Amplitude, frequency, and polarization dependencies of magneto-optic responses were measured for both the transverse and longitudinal geometries of the applied AC magnetic field. Magneto-optic responses of two types, i.e., the fast and slow one, were observed. A qualitative model based on the Néel rotation for the fast response and on particle motion and agglomeration for the slow response is proposed.
High power and high temperature continuous-wave operation of distributed Bragg reflector quantum cascade lasers104(2014); http://dx.doi.org/10.1063/1.4863233View Description Hide Description
High temperature continuous-wave (CW) operation of a distributed Bragg reflector (DBR) quantum cascade laser is demonstrated up to a heat sink temperature of 80 °C. A CW output power of 2 W and a single mode operation with side mode suppression ratio of 30 dB around wavelength of 4.48 μm were achieved at 20 °C. The maximum pulsed and CW wall-plug-efficiencies reached 14.7% and 10.3% at 20 °C, respectively. A large tuning range of 5 cm− 1 between mode hopping was observed and attributed to the thermal cross-talk from the gain section to the DBR section.
- SURFACES AND INTERFACES
104(2014); http://dx.doi.org/10.1063/1.4866283View Description Hide Description
The adsorption of up to one monolayer (ML) of copper phthalocyanine (CuPc) molecules on a room temperature Cu(111) surface has been studied using scanning tunneling microscopy (STM). Below 1 ML the molecules are in a fluid state and are highly mobile on the surface. At 1 ML coverage the molecules coalesce into a highly ordered 2D crystal phase. At sub-ML coverages, chemisorption of individual CuPc molecules can be induced through exposure to tunneling electrons at a tunneling bias voltage exceeding a threshold value. This tunneling electron induced effect has been exploited to perform molecular STM lithography.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
104(2014); http://dx.doi.org/10.1063/1.4865902View Description Hide Description
We performed density functional theory study on the electronic and magnetic properties of armchair MoS2 nanoribbons (AMoS2NR) with different edge hydrogenation. Although bare and fully passivated AMoS2NRs are nonmagnetic semiconductors, it was found that hydrogenation in certain patterns can induce localized ferromagnetic edge state in AMoS2NRs and make AMoS2NRs become antiferromagnetic semiconductors or ferromagnetic semiconductors. Electric field effects on the bandgap and magnetic moment of AMoS2NRs were investigated. Partial edge hydrogenation can change a small-sized AMoS2NR from semiconductor to metal or semimetal under a moderate transverse electric field. Since the rate of edge hydrogenation can be controlled experimentally via the temperature, pressure and concentration of H2, our results suggest edge hydrogenation is a useful method to engineer the band structure of AMoS2NRs.
104(2014); http://dx.doi.org/10.1063/1.4866004View Description Hide Description
We report the strain state and transport properties of V2O3 layers and V2O3/Cr2O3 bilayers deposited by molecular beam epitaxy on (0001)-Al2O3. By changing the layer on top of which V2O3 is grown, we change the lattice parameters of ultrathin V2O3 films significantly. We find that the metal-insulator transition is strongly attenuated in ultrathin V2O3 layers grown coherently on Al2O3. This is in contrast with ultrathin V2O3 layers grown on Cr2O3 buffer layers, where the metal-insulator transition is preserved. Our results provide evidence that the existence of the transition in ultrathin films is closely linked with the lattice deformation.
104(2014); http://dx.doi.org/10.1063/1.4863408View Description Hide Description
We investigated the terahertz (THz) transmission characteristics of semiconductor VO2 film and its THz suppression behavior after the phase transition. The VO2 films were deposited by the sol-gel method, and an in situ growth with surface nanocrystallization occurring in the films with increasing thickness was presented. Morphology-induced percolation leads to high THz transparency in the semiconductor VO2 film, and the more compact nanostructure could account for the enhanced THz switching ratio in the metallic film. These results may offer insights into the artificial design of VO2 films for THz device applications.
104(2014); http://dx.doi.org/10.1063/1.4865259View Description Hide Description
The origin of a blue luminescence band at 2.7 eV and a luminescence excitation band at 5.2 eV of hafnia has been studied in stoichiometric and non-stoichiometric hafnium oxide films. Experimental and calculated results from the first principles valence band spectra showed that the stoichiometry violation leads to the formation of the peak density of states in the band gap caused by oxygen vacancies. Cathodoluminescence in the non-stoichiometric film exhibits a band at 2.65 eV that is excited at the energy of 5.2 eV. The optical absorption spectrum calculated for the cubic phase of HfO2 with oxygen vacancies shows a peak at 5.3 eV. Thus, it could be concluded that the blue luminescence band at 2.7 eV and HfOx excitation peak at 5.2 eV are due to oxygen vacancies. The thermal trap energy in hafnia was estimated.
104(2014); http://dx.doi.org/10.1063/1.4865930View Description Hide Description
Tensile ductility of the Ti-based composites, which consist of a β-Ti phase surrounded by ultrafine structured intermetallics, is tunable through the control of intermetallics. The two Ti-based alloys studied exhibit similar compressive yield strength (about 1000 MPa) and strain (about 35%–40%) but show a distinct difference in their tensile plasticity. The alloy Ti71.8 Nb 14.1 Ni 7.4 Al 6.7 fractures at the yield stress while the alloy Ti71.8 Nb 14.1Co7.4 Al 6.7 exhibits about 4.5% of tensile plastic deformation. To clarify the effect of microstructure on the deformation behavior of these alloys, tensile tests were carried out in the scanning electron microscope. It is shown that the distribution as well as the type of intermetallics affects the tensile ductility of the alloys.
Ag nanowire percolating network embedded in indium tin oxide nanoparticles for printable transparent conducting electrodes104(2014); http://dx.doi.org/10.1063/1.4866007View Description Hide Description
Solution-based printable transparent conducting electrodes consisting of Ag nanowire (NW) and indium tin oxide (ITO) nanoparticles (NPs) were fabricated by simple brush painting at room temperature under atmospheric ambient conditions. Effectively embedding the Ag NW percolating network into the ITO NPs provided a conduction path, led to a metallic conduction behavior of the ITO NPs/Ag NW/ITO NPs multilayer and supplied electrons into the ITO NPs. The optimized ITO NPs/Ag NW/ITO NPs multilayer showed a sheet resistance of 16.57 Ω/sq and an optical transparency of 79.50% without post annealing. Based on high resolution transmission electron microscope analysis, we investigated the microstructure and interface structure of the ITO NPs/Ag NW/ITO NPs multilayer electrodes and suggested a possible mechanism to explain the low resistivity of the multilayers.
104(2014); http://dx.doi.org/10.1063/1.4865586View Description Hide Description
The nanoscale crystal nuclei in an amorphous Ge2Sb2Te5 bit in a phase change memory device were evaluated by fluctuation transmission electron microscopy. The quench time in the device (∼10 ns) afforded more and larger nuclei in the melt-quenched state than in the as-deposited state. However, nuclei were even more numerous and larger in a test structure with a longer quench time (∼100 ns), verifying the prediction of nucleation theory that slower cooling produces more nuclei. It also demonstrates that the thermal design of devices will strongly influence the population of nuclei, and thus the speed and data retention characteristics.
Direct imaging of light elements by annular dark-field aberration-corrected scanning transmission electron microscopy104(2014); http://dx.doi.org/10.1063/1.4866185View Description Hide Description
In this report, we show that an annular dark-field detector in an aberration-corrected scanning transmission electron microscope allows the direct observation of light element columns in crystalline lattices. At specific imaging conditions, an enhancement of the intensities of light element columns in the presence of heavy element columns is observed. Experimental results are presented for imaging the nitrogen and carbon atomic columns at the GaN-SiC interface and within the GaN and SiC compounds. The crystal polarity of GaN at the interface is identified. The obtained findings are discussed and are well supported by image simulations.
Textured metastable VO2 (B) thin films on SrTiO3 substrates with significantly enhanced conductivity104(2014); http://dx.doi.org/10.1063/1.4865898View Description Hide Description
Textured metastable VO2 (B) thin films with a layered structure were grown on SrTiO3 (001) by pulsed laser deposition. The X-ray diffraction and transmission electron microscopy results indicate that VO2 (B) films exhibit c-axis out-of-plane, while the films have 4 possible in-plane matching relations. In addition, a small amount of VO2 (M) phase can co-grow in the VO2 (B) phase when the film thickness exceeds a threshold. The thick VO2 films on STO exhibit a sharp metal-insulator transition with an increase of electrical conductivity in two orders of magnitude. This study may provide an alternative approach to enhance the performance of insulating VO2 (B) based batteries with increased electrical conductivity by incorporating VO2 (M) phase in the VO2 (B) phase layered network.
Behavior of a hollow core photonic crystal fiber under high radial pressure for downhole application104(2014); http://dx.doi.org/10.1063/1.4866334View Description Hide Description
Pressure fiber sensors play an important role in downhole high pressure measurements to withstand long term operation. The purpose of this paper is to present an application of hollow core photonic crystal fiber (HC-PCF) as a high pressure sensor head for downhole application based on dispersion variation. We used a high pressure stainless steel unit to exert pressure on the sensor. The experimental results show that different wavelengths based on sagnac loop interferometer have additive sensitivities from 5 × 10−5 nm/psi at 1480 nm to 1.3 × 10−3 nm/psi at 1680 nm. We developed a simulation to understand the reason for difference in sensitivity of wavelengths and also the relationship between deformation of HC-PCF and dispersion variation under pressure. For this purpose, by using the finite element method, we investigated the effect of structural variation of HC-PCF on spectral transformation of two linear polarizations under 1000 psi pressure. The simulation and experimental results show exponential decay behavior of dispersion variation from −3.4 × 10−6 1/psi to −1.3 × 10−6 1/psi and from −5 × 10−6 1/psi to −1.8 × 10−6 1/psi, respectively, which were in a good accordance with each other.