Volume 103, Issue 22, 25 November 2013
- 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:
The influence of a transverse spin–polarized current on long ferromagnetic nanostripes is studied numerically. The magnetization behavior is analyzed for all range of the applied currents, up to the saturation. It is shown that the saturation current is a nonmonotonic function of the stripe width. A number of stable periodic magnetization structures are observed below the saturation. Type of the periodical structure depends on the stripe width. Besides the one–dimensional domain structure, typical for narrow wires, and the two–dimensional vortex–antivortex lattice, typical for wide films, a number of intermediate structures are observed, e.g., cross–tie and diamond state.
- PHOTONICS AND OPTOELECTRONICS
103(2013); http://dx.doi.org/10.1063/1.4832824View Description Hide Description
We report on current injection induced terahertz electroluminescence from 4H-SiC p-n junctions with operating temperature up to 270 K. The emission is assigned to intracenter optical transitions in donor centers, initiated by the injection of non-equilibrium carriers into the n-doped region of a SiC p-n junction. At a pumping current of 300 mA at 100 K, the integrated output power was 58 μW from the device surface with an area of 3 mm2. These results suggest that THz emitting devices can be fabricated with simple structures of SiC p-n junctions, with relatively high operating temperatures and reasonable output powers.
103(2013); http://dx.doi.org/10.1063/1.4832825View Description Hide Description
We have investigated the terahertz-induced third-order (Kerr) nonlinear optical properties of the amorphous chalcogenide glasses As2S3 and As2Se3. Chalcogenide glasses are known for their high optical Kerr nonlinearities which can be several hundred times greater than those of fused silica. We use high-intensity, single-cycle terahertz pulses with a maximum electrical field strength exceeding 400 kV/cm and frequency content from 0.2 to 3.0 THz. By optical Kerr-gate sampling, we measured the terahertz-induced nonlinear refractive indices at 800 nm to be for As2S3 and for As2Se3.
103(2013); http://dx.doi.org/10.1063/1.4836195View Description Hide Description
Impact of sub-wavelength-size dielectric particles on Zenneck surface waves on planar metallic antennas is investigated at terahertz (THz) frequencies with THz near-field probe microscopy. Perturbations of the surface waves show the particle presence, despite its sub-wavelength size. The experimental configuration, which utilizes excitation of surface waves at metallic edges, is suitable for THz imaging of dielectric sub-wavelength size objects. As a proof of concept, the effects of a small strontium titanate rectangular particle and a titanium dioxide sphere on the surface field of a bow-tie antenna are experimentally detected and verified using full-wave simulations.
103(2013); http://dx.doi.org/10.1063/1.4832478View Description Hide Description
In this work, we study experimentally the interaction of spatial optical soliton in chiral nematic liquid crystals with disclination line created in a wedge shaped cell. We show that in most cases the self-confined beam preserves this interaction. We demonstrate that this interaction can be employed for efficient bending of the soliton trajectory, as a result of reflection and refraction.
103(2013); http://dx.doi.org/10.1063/1.4836477View Description Hide Description
Brillouin microscopy is an emerging imaging modality that provides fundamental information about mechanical properties of media in a non-contact manner. To date, low numerical aperture (NA) optics have been used, due to noticeable angular broadening of the Brillouin spectrum at higher NAs. In this work, we investigate theoretically and experimentally the dependence of spectral broadening effects in Brillouin imaging on system NA, for both and scattering geometries. Lineshape deformations and broadening are found to be minimised in a backscattering geometry, hence paving the way for high resolution in-vivo mechanical imaging.
103(2013); http://dx.doi.org/10.1063/1.4836995View Description Hide Description
Ghost imaging has become increasingly popular in quantum and optical application fields. Here, we report three-dimensional (3D) optical security using ghost imaging. The series of random phase-only masks are sparsified, which are further converted into particle-like distributions placed in 3D space. We show that either an optical or digital approach can be employed for the encoding. The results illustrate that a larger key space can be generated due to the application of 3D space compared with previous works.
103(2013); http://dx.doi.org/10.1063/1.4837015View Description Hide Description
The freely-suspended glass membranes in a dual-nanoweb fiber, driven at resonance by intensity-modulated light, exhibit a giant optomechanical nonlinearity. We experimentally investigate the effect of squeezed-film damping by exploring the pressure dependence of resonant frequency and mechanical quality factor. As a consequence of the unusually narrow slot between the nanowebs (22 μm by 550 nm), the gas-spring effect causes a pressure-dependent frequency shift that is ∼15 times greater than typically measured in micro-electro-mechanical devices. When evacuated, the dual-nanoweb fiber yields a quality factor of ∼3 600 and a resonant optomechanical nonlinear coefficient that is ∼60 000 times larger than the Kerr effect.
Partial strain relaxation effects on polarization anisotropy of semipolar InGaN/GaN quantum well structures103(2013); http://dx.doi.org/10.1063/1.4833277View Description Hide Description
Partial strain relaxation effects on polarization anisotropy of semipolar (11 2) InGaN/GaN quantum well (QW) structures were investigated using the multiband effective-mass theory. In the case of strain relaxation of along -axis, the polarization ratio gradually decreases with increasing strain relaxation. Also, with the strain relaxation by the same amount, the variation of the polarization ratio along -axis is shown to be much larger than that along -axis. However, the polarization switching is not observed even at a high In composition of 0.4 due to a small strain component ( ) with no strain relaxation. On the other hand, in the case of strain relaxation of along -axis, the polarization switching is observed, and the optical anisotropy is found to change from positive to negative with increasing strain relaxation. Also, the absolute value of the polarization ratio gradually decreases with increasing carrier density. However, the polarization switching due to the carrier density is not observed. Thus, the polarization switching observed at high carrier density may be attributed to inhomogeneous strain distribution in the InGaN layer.
See-through amorphous silicon solar cells with selectively transparent and conducting photonic crystal back reflectors for building integrated photovoltaics103(2013); http://dx.doi.org/10.1063/1.4833542View Description Hide Description
Thin semi-transparent hydrogenated amorphous silicon (a-Si:H) solar cells with selectively transparent and conducting photonic crystal (STCPC) back-reflectors are demonstrated. Short circuit current density of a 135 nm thick a-Si:H cell with a given STCPC back-reflector is enhanced by as much as 23% in comparison to a reference cell with an ITO film functioning as its rear contact. Concurrently, solar irradiance of 295 W/m2 and illuminance of 3480 lux are transmitted through the cell with a given STCPC back reflector under AM1.5 Global tilt illumination, indicating its utility as a source of space heating and lighting, respectively, in building integrated photovoltaic applications.
Investigations on correlation between I–V characteristic and internal quantum efficiency of blue (AlGaIn)N light-emitting diodes103(2013); http://dx.doi.org/10.1063/1.4833895View Description Hide Description
We have studied the electrical and optical characteristics of (AlGaIn)N multiple quantum well light-emitting diodes. Minimizing contact effects by utilizing platinum as p-contact metal, ideality factors as low as 1.1 have been achieved. In agreement with basic semiconductor theory, a correlation between ideality factor and small-current efficiency was found. We were able to emulate the experimental current-voltage characteristic over seven orders of magnitude utilizing a two diode model. This model enables a very good prediction of internal quantum efficiency at moderate current densities out of purely electrically derived parameters.
The role of polarization fields in Auger-induced efficiency droop in nitride-based light-emitting diodes103(2013); http://dx.doi.org/10.1063/1.4833915View Description Hide Description
The rates of non-radiative Auger recombination (AR) and radiative recombination (RR) in polar GaN/AlN quantum wells (QWs) are calculated. It is shown that in these QWs the polarization field not only suppresses the RR but also strongly enhances the rate of AR. As a result, the polarization field triggers the Auger-induced efficiency droop, which, according to the calculations, does not exist in non-polar GaN/AlN QWs. We demonstrate that in polar QWs the droop can be overcome by suppression of AR using a gradual variation of the QW layer composition, which compensates the effect of the electric field acting on holes.
103(2013); http://dx.doi.org/10.1063/1.4835216View Description Hide Description
We experimentally demonstrate efficient lasing from a Rhodamine-nanoscatterer random laser when pumped with unconventional wavelengths, at which the absorption of Rhodamine is negligible. Förster-type energy transfer was realized by using Coumarin molecules as donors. Explicit time-resolved spectroscopy provided direct evidence for the nonradiative transfer with ∼48% efficiency. We obtained lasing at reduced thresholds by a factor of over 3 and increased amplification rates by a factor of ∼4 in the Förster regime, even in samples with sub-diffusive disorder strength. We characterize the efficacy of the Förster transfer induced lasing over a range of unconventional wavelengths for the Rh-based system.
103(2013); http://dx.doi.org/10.1063/1.4837035View Description Hide Description
We experimentally demonstrated that multiphoton-ionization-induced plasma grating in air could be precisely manipulated by impulsive molecular alignment. In the linear region, the impulsively aligned molecules modulated the diffraction efficiency of the plasma grating for a time-delayed femtosecond laser pulse. In the nonlinear region, the third harmonic generation from the plasma grating was either enhanced or suppressed by following the alignment of the molecules.
Multilayer mirror with enhanced spectral selectivity for the next generation extreme ultraviolet lithography103(2013); http://dx.doi.org/10.1063/1.4837335View Description Hide Description
We have demonstrated a hybrid extreme ultraviolet (EUV) multilayer mirror for 6.x nm radiation that provides selective suppression for infrared (IR) radiation. The mirror consists of an IR-transparent LaN∕B multilayer stack which is used as EUV-reflective coating and antireflective (AR) coating to suppress IR. The AR coating can be optimized to suppress CO2 laser radiation at the wavelength of 10.6 μm, which is of interest for application in next-generation EUV lithography systems.
103(2013); http://dx.doi.org/10.1063/1.4837355View Description Hide Description
The emission of millions of fluorescence photons from a chromophore is controlled by the absorption of a few tens of photons in a photochromic molecule. The parameters that determine the efficiency of this process are investigated, providing insights for the development of an all-optical gate.
Reconfigurable liquid metal based terahertz metamaterials via selective erasure and refilling to the unit cell level103(2013); http://dx.doi.org/10.1063/1.4837675View Description Hide Description
We demonstrate a technique for selectively erasing and refilling unit cells of terahertz (THz) metamaterials. The structures are formed by injecting eutectic gallium indium (EGaIn), a liquid metal at room temperature, into microchannels within a polydimethylsiloxane (PDMS) mold fabricated using conventional soft lithography techniques. The thin oxide layer that forms on the surface of EGaIn can be locally dissolved via exposure to hydrochloric acid (HCl) introduced at the surface of the gas permeable PDMS mold. In the absence of the oxide skin, the liquid metal retracts to a position where a stable new oxide layer can be formed, effectively erasing the liquid metal structure in the presence of HCl. After erasing selected structures, EGaIn can be re-injected into microchannels to yield the initial structure. In the case of small unit cells, we show that mechanical pressure can be used to effectively erase individual elements. We use THz time-domain spectroscopy to characterize the distinct transmission properties for each of these different structures.
103(2013); http://dx.doi.org/10.1063/1.4834360View Description Hide Description
We present an evanescent-field device based on a right-angled waveguide. This consists of orthogonal waveguides, with their points of intersection lying along an angled facet of the chip. Light guided along one waveguide is incident at the angled dielectric-air facet at an angle exceeding the critical angle, so that the totally internally reflected light is coupled into the second waveguide. By depositing a nanotube film on the angled surface, the chip is then used to mode-lock an Erbium doped fiber ring laser with a repetition rate of 26 MHz, and pulse duration of 800 fs.
- SURFACES AND INTERFACES
103(2013); http://dx.doi.org/10.1063/1.4832996View Description Hide Description
Physical mechanisms of time-averaged structuring of adatoms induced by a standing surface acoustic wave (SAW) on a solid substrate are studied. Despite some similarity with conventional mechanisms based on averaging of fast oscillation-type motion or radiation-pressure effects, we demonstrate that, for diffusional (i.e., strongly damped) adatom motion, the origin of time-averaged structuring is essentially different. The proposed analytical model and kinetic Monte–Carlo (kMC) simulations reveal several distinct structuring regimes and directly relate them to the transient modification of diffusion barriers and adiabatic temperature variations induced by SAW strains.
103(2013); http://dx.doi.org/10.1063/1.4833543View Description Hide Description
Critical heat flux (CHF) enhancement by surface modifications has been an extensively researched area in pool boiling heat transfer. Here we report a fundamental mechanism of CHF enhancement where nano/micro ridges are fabricated on surfaces to fragment and evaporate the metastable non-evaporating/adsorbed film present at the base of a bubble in the contact line region. CHF increase of ∼125% is obtained with only ∼40% increase in surface area. An analytical model is extended to explain the CHF enhancement and to determine the average non-evaporating film thickness, which serves as the critical height for nano/micro structures for pool boiling heat transfer enhancement.
103(2013); http://dx.doi.org/10.1063/1.4830378View Description Hide Description
A theoretical study of ultrashort laser-induced electron emission from a negatively biased metallic cathode has been performed. Classical as well as tunneling electron emission mechanisms are considered. It was found that electron emission is governed by an interplay of processes inside as well as above the cathode. A hybrid model is proposed, where the electron distribution within the target is retrieved from Boltzmann scattering integrals, while the charge distribution above it is studied by a Particle-In-Cell simulation. The results indicate that non-equilibrium effects determine the initial emission process, whereas the space charge above the target suppresses the effectively emitted charge.