Volume 105, Issue 24, 15 December 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:
Electrical Properties Tomography (EPT) technique utilizes measurable radio frequency (RF) coil induced magnetic fields (B1 fields) in a Magnetic Resonance Imaging (MRI) system to quantitatively reconstruct the local electrical properties (EP) of biological tissues. Information derived from the same data set, e.g., complex numbers of B1 distribution towards electric field calculation, can be used to estimate, on a subject-specific basis, local Specific Absorption Rate (SAR). SAR plays a significant role in RF pulse design for high-field MRI applications, where maximum local tissue heating remains one of the most constraining limits. The purpose of the present work is to investigate the feasibility of such B1-based local SAR estimation, expanding on previously proposed EPT approaches. To this end, B1 calibration was obtained in a gelatin phantom at 7 T with a multi-channel transmit coil, under a particular multi-channel B1-shim setting (B1-shim I). Using this unique set of B1 calibration, local SAR distribution was subsequently predicted for B1-shim I, as well as for another B1-shim setting (B1-shim II), considering a specific set of parameter for a heating MRI protocol consisting of RF pulses plaid at 1% duty cycle. Local SAR results, which could not be directly measured with MRI, were subsequently converted into temperature change which in turn were validated against temperature changes measured by MRI Thermometry based on the proton chemical shift.
- PHOTONICS AND OPTOELECTRONICS
105(2014); http://dx.doi.org/10.1063/1.4904222View Description Hide Description
We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.
105(2014); http://dx.doi.org/10.1063/1.4904264View Description Hide Description
Generation of THz radiation based on the Smith-Purcell effect in graphene is investigated numerically. The specific device geometry considered involves an electrically biased single-layer sheet of graphene deposited on a periodic array of holes in a solid substrate. Rigorous electrodynamic simulations combined with a basic model of charge transport are presented, showing that technologically significant output power levels can be obtained at geometrically tunable THz frequencies. These results suggest that graphene is a uniquely suited materials platform for the demonstration of THz electron-beam radiation mechanisms in compact solid-state systems.
Low temperature p-type doping of (Al)GaN layers using ammonia molecular beam epitaxy for InGaN laser diodes105(2014); http://dx.doi.org/10.1063/1.4904272View Description Hide Description
We demonstrate state-of-the-art p-type (Al)GaN layers deposited at low temperature (740 °C) by ammonia molecular beam epitaxy (NH3-MBE) to be used as top cladding of laser diodes (LDs) with the aim of further reducing the thermal budget on the InGaN quantum well active region. Typical p-type GaN resistivities and contact resistances are 0.4 Ω cm and 5 × 10−4 Ω cm2, respectively. As a test bed, we fabricated a hybrid laser structure emitting at 400 nm combining n-type AlGaN cladding and InGaN active region grown by metal-organic vapor phase epitaxy, with the p- doped waveguide and cladding layers grown by NH3-MBE. Single-mode ridge-waveguide LD exhibits a threshold voltage as low as 4.3 V for an 800 × 2 μm2 ridge dimension and a threshold current density of ∼5 kA cm−2 in continuous wave operation. The series resistance of the device is 6 Ω and the resistivity is 1.5 Ω cm, confirming thereby the excellent electrical properties of p-type Al0.06Ga0.94N:Mg despite the low growth temperature.
Terbium gallium garnet ceramic-based Faraday isolator with compensation of thermally induced depolarization for high-energy pulsed lasers with kilowatt average power105(2014); http://dx.doi.org/10.1063/1.4904461View Description Hide Description
A scalable aperture Faraday isolator for high-energy pulsed lasers with kW-level average power was demonstrated using terbium gallium garnet ceramics with water cooling and compensation of thermally induced depolarization in a magnetic field. An isolation ratio of 35 dB (depolarization ratio γ of 3.4 × 10−4) was experimentally observed at a maximum laser power of 740 W. By using this result, we estimated that this isolator maintains an isolation ratio of 30 dB for laser powers of up to 2.7 kW. Our results provide the solution for achieving optical isolation in high-energy (100 J to kJ) laser systems with a repetition rate greater than 10 Hz.
Recovery of image distorted by turbulent atmosphere using phase-conjugate image generated by difference frequency generation105(2014); http://dx.doi.org/10.1063/1.4904474View Description Hide Description
We demonstrate dynamic recovery of blurred images caused by atmospheric turbulence. In particular, using a phase-conjugate wave generated by a second-order nonlinear crystal or composite, we restore the original quality of the image after the optical radiation forming the image propagates through the turbulent atmosphere. One of the key elements for our experiment is a rotating phase plate being placed in the beam path for simulating turbulent atmosphere. Using the nonlinear composite, we demonstrate that the image recovery is insensitive to the polarization of the optical radiation forming the image.
105(2014); http://dx.doi.org/10.1063/1.4904005View Description Hide Description
We report on terahertz (THz) generation via optical rectification in a room temperature gallium arsenide (GaAs) crystal pumped at a wavelength of 1.8 μm using a modified tilted-pulse-front scheme, leading to a 0.05% energy conversion efficiency. The spectral content of the measured THz pulses, ranging from 0.1 to 3 THz, confirmed a good broadband phase matching between the pump and the THz pulses over several millimeters (>20 mm) in a semi-insulating 〈110〉 cut bulk GaAs crystal. Our findings also suggest that the pump-to-THz conversion efficiency can be further increased by using a pump source with a narrower bandwidth.
105(2014); http://dx.doi.org/10.1063/1.4904883View Description Hide Description
Linear and nonlinear electron dynamics of polycrystalline gold (Au) ultrathin films with thicknesses ranging from 1.4 to 5.8 nm were investigated via transmittance terahertz (THz) spectroscopy with intense electric field transients. We prepared ultrathin films with low surface roughness formed on a Si–(7 × 7) reconstructed surface, leading to the observation of monotonic decrease in THz transmittance with respect to film thickness. Furthermore, at all tested thicknesses, the transmittance decreased nonlinearly by 10%–30% with the application if high-intensity THz electric fields. Based on a Drude-model analysis, we found a significant decrease in the damping constant induced by the THz electric field, indicating that electrons are driven beyond the polycrystalline grain boundaries in Au thin films, and consequently leading to the suppression of the electron–boundary scattering rate.
High spectral uniformity of AlGaN with a high Al content evidenced by scanning near-field photoluminescence spectroscopy105(2014); http://dx.doi.org/10.1063/1.4904710View Description Hide Description
Scanning near-field photoluminescence (PL) spectroscopy was applied to study spatial variations of emission spectra of Al xGa1−xN epilayers with . PL spectra were found to be spatially uniform with peak wavelength standard deviations of only ∼2 meV and ratios between peak intensity standard deviations and average peak intensity values of 0.06. The observed absence of correlation between the PL peak wavelength and intensity shows that spatial distribution of nonradiative recombination centers is not related to band potential fluctuations. Our results demonstrate that the homogeneous broadening and the random cation distribution primarily determine PL linewidths for layers grown under optimized conditions.
An orbital angular momentum radio communication system optimized by intensity controlled masks effectively: Theoretical design and experimental verification105(2014); http://dx.doi.org/10.1063/1.4904090View Description Hide Description
A system of generating and receiving orbital angular momentum (OAM) radio beams, which are collectively formed by two circular array antennas (CAAs) and effectively optimized by two intensity controlled masks, is proposed and experimentally investigated. The scheme is effective in blocking of the unwanted OAM modes and enhancing the power of received radio signals, which results in the capacity gain of system and extended transmission distance of the OAM radio beams. The operation principle of the intensity controlled masks, which can be regarded as both collimator and filter, is feasible and simple to realize. Numerical simulations of intensity and phase distributions at each key cross-sectional plane of the radio beams demonstrate the collimated results. The experimental results match well with the theoretical analysis and the receive distance of the OAM radio beam at radio frequency (RF) 20 GHz is extended up to 200 times of the wavelength of the RF signals, the measured distance is 5 times of the original measured distance. The presented proof-of-concept experiment demonstrates the feasibility of the system.
Plasmonic enhancement in hybrid organic/Si heterojunction solar cells enabled by embedded gold nanoparticles105(2014); http://dx.doi.org/10.1063/1.4904955View Description Hide Description
We investigate the incorporation of gold nanoparticles (Au NPs) into poly(3,4-ethylenedioxythiophene)/polystyrenesulfonate for organic/Si heterojunction hybrid solar cells. The incorporation of Au NPs can enhance the short-circuit current density (Jsc), consequently, leading to a higher power conversion efficiency (PCE). The hybrid devices incorporating Au NPs with optimized size achieve a PCE of over 12.9% with about 10% enhancement of the Jsc compared with that of reference ones. Au NPs with different sizes are incorporated to explore the mechanism for the enhanced device performance. According to the detailed experimental characterization and numerical simulation results, the increased PCE is attributed to the light scattering and the local electromagnetic field enhancement, which is originated from the excitation of the localized surface plasmon resonance of Au NPs.
105(2014); http://dx.doi.org/10.1063/1.4904477View Description Hide Description
Metamaterial waveguides based on a concept to generate highly controllable negative-permittivity or permeability bands in the transmission-line (TL) metamaterials are proposed. In this concept, periodic alternation of input impedance of a TL segment (such as with open- or short-circuited end), which can be used to generate multiple negative-permittivity or permeability bands periodically in a frequency domain, is changed by an additional admittance attached at a specific point of the segment. Such segment with the admittance gives a possibility to effectively control separation between two negative-permittivity or permeability bands and the bandwidth, which were originally determined by the periodic alternation of input impedance of the segment. This approach leads to much broader applications of metamaterials such as in multiband wireless communication systems. Enlargement or contraction of band separation and enhancement of bandwidth on negative permittivity bands are experimentally demonstrated in two-dimensional isotropic waveguides at microwave frequencies.
105(2014); http://dx.doi.org/10.1063/1.4904809View Description Hide Description
Cathodoluminescence (CL) analysis enables characterization of optoelectronic materials and devices with high spatial resolution. However, data interpretation is complicated by the competitive nature of the CL generation process. Specifically, spatially resolved CL profiles are affected by both CL center distributions, and by the unknown distributions of recombination centers that do not generate peaks in measured CL spectra. Here, we use depth-resolved CL to show that the contribution of the latter can be deduced and removed from spatially resolved CL data. The utility of this technique is demonstrated using CL depth profiles of color centers in diamond.
105(2014); http://dx.doi.org/10.1063/1.4904827View Description Hide Description
Light reflection at the boundary of two different media is one of the fundamental phenomena in optics, and reduction of reflection is highly desirable in many optical systems. Traditionally, optical antireflection has been accomplished using single- or multiple-layer dielectric films and graded index surface structures in various wavelength ranges. However, these approaches either impose strict requirements on the refractive index matching and film thickness, or involve complicated fabrication processes and non-planar surfaces that are challenging for device integration. Here, we demonstrate an antireflection coating strategy, both experimentally and numerically, by using metasurfaces with designer optical properties in the mid-wave infrared. Our results show that the metasurface antireflection is capable of eliminating reflection and enhancing transmission over a broad spectral band and a wide incidence angle range. The demonstrated antireflection technique has no requirement on the choice of materials and is scalable to other wavelengths.
- SURFACES AND INTERFACES
Visible light illumination-induced phase transition to the intermediate states between the metallic and insulating states for the LaAlO3/SrTiO3 interfaces105(2014); http://dx.doi.org/10.1063/1.4904460View Description Hide Description
Photoexcitation usually drives the LaAlO3/SrTiO3 interface from the insulating state into a totally metallic state, without experiencing any intermediate states. Here, we reported on an illumination-induced transition of the insulating LaAlO3(3uc)/SrTiO3 interface to a series of state between a totally insulating state and a totally metallic state. We found that appropriate light illumination can cause an insulator-to-semiconductor transition in the temperature range above ∼150 K and an insulator-to-metal transition below ∼60 K, while the original state recovers immediately after the removal of the illumination, without persistent photoconductivity as previously reported. Moreover, a remarkable resistive anomaly corresponding to the structural transition of SrTiO3 at ∼105 K appears, indicating a phase-transition-induced carrier density change.
105(2014); http://dx.doi.org/10.1063/1.4903824View Description Hide Description
The influence of sodium on the band structure of MoS2(0001) and the comparison of the experimental band dispersion with density functional theory show excellent agreement for the occupied states (angle-resolved photoemission) and qualitative agreement for the unoccupied states (inverse photoemission spectroscopy). Na-adsorption leads to charge transfer to the MoS2 surface causing an effect similar to n-type doping of a semiconductor. The MoS2 occupied valence band structure shifts rigidly to greater binding with little change in the occupied state dispersion. Likewise, the unoccupied states shift downward, approaching the Fermi level, yet the amount of the shift for the unoccupied states is greater than that of the occupied states, effectively causing a narrowing of the MoS2 bandgap.
105(2014); http://dx.doi.org/10.1063/1.4904459View Description Hide Description
Chemically sharp interface was obtained on single phase single oriented Fe3O4 (001) thin film (7 nm) grown on NiO (001) substrate using oxygen assisted molecular beam epitaxy. Refinement of the atomic structure, stoichiometry, and oxygen vacancies were determined by soft and hard x-ray photoelectron spectroscopy, low energy electron diffraction and synchrotron based X-ray reflectivity, and X-ray diffraction. Our results demonstrate an epitaxial growth of the magnetite layer, perfect iron stoichiometry, absence of oxygen vacancies, and the existence of an intermixing free interface. Consistent magnetic and electrical characterizations are also shown.
105(2014); http://dx.doi.org/10.1063/1.4904471View Description Hide Description
We report exchange bias effect in Fe3O4 films epitaxially grown on SrTiO3 substrates. This effect is related to the formation of Ti3+-vacancy complexes at the surface of SrTiO3 in ultrahigh vacuum that in turn triggers the growth of a thin antiferromagnetic (AFM) FeO layer (∼5 nm) at the interface. The picture of antiferromagnetic FeO interacting with native ferrimagnetic Fe3O4 matrix reasonably accounts for this anomalous magnetic behavior. With increasing film thickness from 17 to 43 nm, the exchange bias effect and the magnetization anomaly associated with the AFM phase transition of the FeO layer are progressively weakened due to the increase in the volume fraction of the Fe3O4 phase, indicating the interfacial nature of the exchange coupling. Our results highlight the important role of interface engineering in controlling the magnetic properties of iron oxide thin films.
105(2014); http://dx.doi.org/10.1063/1.4904803View Description Hide Description
We report on the controlled removal of an amorphous Se capping layer from Bi2Te3 and Bi2Se3 topological insulators. We show that the Se coalesces into micron-sized islands before desorbing from the surface at a temperature of ∼150 °C. In situ Auger Electron Spectroscopy reveals that Se replaces a significant fraction of the Te near the top surface of the Bi2Te3. Rutherford Backscattering Spectrometry and Transmission Electron Microscopy show that after heating, Se has been incorporated in the Bi2Te3 lattice down to ∼7 nm from its top surface while remaining iso-structural.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
Large entropy change associated with the elastocaloric effect in polycrystalline Ni-Mn-Sb-Co magnetic shape memory alloys105(2014); http://dx.doi.org/10.1063/1.4904419View Description Hide Description
We report on compressive strain measurements in polycrystalline magnetic shape memory alloys aimed at determining the entropy change associated with their elastocaloric effect. It is shown that for a maximum applied stress of 100 MPa, the stress-induced entropy change amounts to J/kg K. This value compares well to the values reported for nonmagnetic shape memory alloys, and it is of the same order as those reported for the best giant magnetocaloric materials at moderate magnetic fields.
105(2014); http://dx.doi.org/10.1063/1.4904462View Description Hide Description
Nanoscale Bi particles embedded in a Zn matrix were obtained by casting and melt-spinning, resulting in quenching rate-dependent sizes and shapes. With decreasing Bi particle size, an increasing aspect ratio was observed. Due to high resolution transmission electron microscopy performed for different orientations of the nanoparticles and the matrix, the three-dimensional shape and the respective crystallographic orientations of the Bi nanoparticles as well as the orientation relationship with the matrix have been evaluated. It is suggested that the size-dependence of the nanoparticle morphologies has a strong impact on their thermal stabilities thus affecting the size dependence of the melting temperature.