Volume 105, Issue 7, 18 August 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:
Most lipid formulations need cholesterol for efficient transfection, but the precise motivation remains unclear. Here, we have investigated the effect of cholesterol on the transfection efficiency (TE) of cationic liposomes made of 1,2-dioleoyl-3-trimethylammonium-propane and dioleoylphosphocholine in Chinese hamster ovary cells. The transfection mechanisms of cholesterol-containing lipoplexes have been investigated by TE, synchrotron small angle X-ray scattering, and laser scanning confocal microscopy experiments. We prove that cholesterol-containing lipoplexes enter the cells using different endocytosis pathways. Formulations with high cholesterol content efficiently escape from endosomes and exhibit a lamellar-nonlamellar phase transition in mixture with biomembrane mimicking lipid formulations. This might explain both the DNA release ability and the high transfection efficiency. These studies highlight the enrichment in cholesterol as a decisive factor for transfection and will contribute to the rational design of lipid nanocarriers with superior TE.
- PHOTONICS AND OPTOELECTRONICS
105(2014); http://dx.doi.org/10.1063/1.4893576View Description Hide Description
Long-wavelength InP-based diode lasers emitting at 1.53 μm have been optimized for maximum continuous-wave (CW) electrical-to-optical power conversion efficiency, so-called wallplug efficiency (WPE). Efficient electron and hole capture into a single-quantum-well (SQW) active region as well as suppression of electron and hole leakage out of the SQW result in high values for the internal differential efficiency: ∼97% for long-cavity (≥2 mm) uncoated-facet devices and ∼85%–89% for short-cavity (1.5 mm) optimized facet-coated devices. The characteristic temperature of the slope efficiency, T 1, reaches a high value of 323 K. Doping-level optimization of the p-cladding layer and the use of the SQW result in low values for the internal loss coefficient: ∼1.1 cm−1 for long-cavity (≥2 mm) uncoated-facet devices and ∼1.5–2.0 cm−1 for short-cavity (1.5 mm) optimized facet-coated devices. In turn, a maximum CW WPE value of 50% is achieved at room temperature and ∼1 W output power from conductively-cooled 100 μm-wide-aperture devices. The maximum CW power is 2.5 W. One beneficial byproduct of the CW-WPE maximization process is a large transverse spot size which, in turn, provides a very narrow transverse beamwidth: 26° full width half maximum. Reliability tests show no degradation when devices are run CW at high currents (4–5 A) and high temperatures (40–50 °C) for over 4000 h, at ∼2 W output power.
105(2014); http://dx.doi.org/10.1063/1.4893594View Description Hide Description
We report the photonic bandgap engineering of Bragg fibers by controlling the thickness profile of the fiber during the thermal drawing. Conical hollow core Bragg fibers were produced by thermal drawing under a rapidly alternating load, which was applied by introducing steep changes to the fiber drawing speed. In conventional cylindrical Bragg fibers, light is guided by omnidirectional reflections from interior dielectric mirrors with a single quarter wave stack period. In conical fibers, the diameter reduction introduced a gradient of the quarter wave stack period along the length of the fiber. Therefore, the light guided within the fiber encountered slightly smaller dielectric layer thicknesses at each reflection, resulting in a progressive blueshift of the reflectance spectrum. As the reflectance spectrum shifts, longer wavelengths of the initial bandgap cease to be omnidirectionally reflected and exit through the cladding, which narrows the photonic bandgap. A narrow transmission bandwidth is particularly desirable in hollow waveguide mid-infrared sensing schemes, where broadband light is coupled to the fiber and the analyte vapor is introduced into the hollow core to measure infrared absorption. We carried out sensing simulations using the absorption spectrum of isopropyl alcohol vapor to demonstrate the importance of narrow bandgap fibers in chemical sensing applications.
105(2014); http://dx.doi.org/10.1063/1.4893664View Description Hide Description
A class of photonic crystal resonant reflectors known as guided mode resonant filters are optical structures that are widely used in the field of refractive index sensing, particularly in biosensing. For the purposes of understanding and design, their behavior has traditionally been modeled numerically with methods such as rigorous coupled wave analysis. Here it is demonstrated how the absolute resonance wavelengths of such structures can be predicted by analytically modeling them as slab waveguides in which the propagation constant is determined by a phase matching condition. The model is experimentally verified to be capable of predicting the absolute resonance wavelengths to an accuracy of within 0.75 nm, as well as resonance wavelength shifts due to changes in cladding index within an accuracy of 0.45 nm across the visible wavelength regime in the case where material dispersion is taken into account. Furthermore, it is demonstrated that the model is valid beyond the limit of low grating modulation, for periodically discontinuous waveguide layers, high refractive index contrasts, and highly dispersive media.
Observation of enhanced visible and infrared emissions in photonic crystal thin-film light-emitting diodes105(2014); http://dx.doi.org/10.1063/1.4893739View Description Hide Description
Photonic crystals, in the form of closed-packed nano-pillar arrays patterned by nanosphere lithography, have been formed on the n-faces of InGaN thin-film vertical light-emitting diodes (LEDs). Through laser lift-off of the sapphire substrate, the thin-film LEDs conduct vertically with reduced dynamic resistances, as well as reduced thermal resistances. The photonic crystal plays a role in enhancing light extraction, not only at visible wavelengths but also at infrared wavelengths boosting heat radiation at high currents, so that heat-induced effects on internal quantum efficiencies are minimized. The observations are consistent with predictions from finite-difference time-domain simulations.
105(2014); http://dx.doi.org/10.1063/1.4893762View Description Hide Description
We have devised a simple means for determining the size of a nanoparticle in one binding event (i.e., real time) by utilizing two polar modes of a slightly eccentric Whispering Gallery Mode (WGM) spheroidal resonator. The ratio of shifts of these modes locates the absolute latitude angle at which a nano-particle binds. From this location, the size of the nanoparticle is calculated using the reactive sensing principle. Although our latitude-only micro-global positioning scheme is applied to nanoparticle sizing using slightly eccentric spheroids in aqueous solution, this approach can be applied to WGM micro-resonators having a variety of shapes.
High power operation of λ ∼ 5.2–11 μm strain balanced quantum cascade lasers based on the same material composition105(2014); http://dx.doi.org/10.1063/1.4893746View Description Hide Description
A technique based on composite quantum wells for design and growth of strain balanced Al0.63In0.37As/Ga0.35In0.65As/Ga0.47In0.53As quantum cascade lasers (QCLs) by molecular beam epitaxy (MBE), emitting in 5.2–11 μm wavelength range, is reported. The strained Al0.63In0.37As provides good electron confinement at all wavelengths, and strain balancing can be achieved through composite wells of Ga0.35In0.65As/Ga0.47In0.53As for different wavelength. The use of these fixed composition materials can avoid the need for frequent calibration of a MBE reactor to grow active regions with different strain levels for different wavelengths. Experimental results for QCLs emitting at 5.2, 6.7, 8.2, 9.1, and 11 μm exhibit good wall plug efficiencies and power across the whole wavelength range. It is shown that the emission wavelength can be predictably changed using the same design template. These lasers are also compatible with a heterogeneous broadband active region, consisting of multiple QCL cores, which can be produced in a single growth run.
105(2014); http://dx.doi.org/10.1063/1.4893570View Description Hide Description
We investigate the optical and thermal hysteresis of single-domain vanadium dioxide nanocrystals fabricated by ion beam synthesis in a fused silica matrix. The nanocrystals exhibit a giant hysteresis, which permits to optically generate a long-time stable supercooled metallic phase persistent down to practically room temperature. Spatial patterns of supercooled and insulating nanocrystals feature a large dielectric contrast, in particular, for telecom wavelengths. We utilize this contrast to optically imprint reconfigurable photonic elements comprising diffraction gratings as well as on- and off-axis zone plates. The structures allow for highly repetitive (>104) cycling through the phase transition without structural damage.
105(2014); http://dx.doi.org/10.1063/1.4892830View Description Hide Description
We studied the effect of multiple interruptions during the quantum well growth on emission-efficiency enhancement of InGaN-based yellow-green light emitting diodes on c-plane sapphire substrate. The output power and dominant wavelength at 20 mA are 0.24 mW and 556.3 nm. High resolution x-ray diffraction, photoluminescence, and electroluminescence measurements demonstrate that efficiency enhancement could be partially attributed to crystal quality improvement of the active region resulted from reduced In clusters and relevant defects on the surface of InGaN layer by introducing interruptions. The less tilted energy band in the quantum well is also caused by the decrease of In-content gradient along c-axis resulted from In segregation during the interruptions, which increases spatial overlap of electron-hole wavefunction and thus the internal quantum efficiency. The latter also leads to smaller blueshift of dominant wavelength with current increasing.
Extracting the anisotropic optical parameters of chiral plasmonic nanostructured thin films using generalized ellipsometry105(2014); http://dx.doi.org/10.1063/1.4893785View Description Hide Description
Using a commercial ellipsometer and analytical inversion, we show that both linear and circular birefringence-dichroism pairs can be extracted from a single generalized ellipsometry measurement, providing a complete description of the polarization properties of anisotropic chiral films, which is a distinct advantage over typical circular dichroism measurements. This is demonstrated by measuring the anisotropic optical parameters of post-like and helical composite Ti/Ag thin films fabricated by dynamic shadowing growth. These films are both chiral and highly aligned, and the measured linear and circular birefringence-dichroism pairs scale with the shape anisotropy and chirality. Furthermore, because the total polarization anisotropy is measured through generalized ellipsometry, we are able to determine that the polarization eigenstates can be effectively tuned from purely circular to approximately linear by changing the pitch number, N, of plasmonic helices for N ≤ 1.
105(2014); http://dx.doi.org/10.1063/1.4893786View Description Hide Description
Helmholtz resonators are widely used acoustic components able to select a single frequency. Here, based on an analogy between acoustics and electromagnetism wave equations, we present an electromagnetic 2D Helmholtz resonator made of a metallic slit-box structure. At the resonance, the light is funneled in the λ/800 apertures, and is subsequently absorbed in the cavity. As in acoustics, there is no higher order of resonance, which is an appealing feature for applications such as photodetection or thermal emission. Eventually, we demonstrate that the slit is of capacitive nature while the box behaves inductively. We derive an analytical formula for the resonance wavelength, which does not rely on wave propagation and therefore does not depend on the permittivity of the material filling the box. Besides, in contrast with half-wavelength resonators, the resonance wavelength can be engineered by both the slit aspect ratio and the box area.
105(2014); http://dx.doi.org/10.1063/1.4893788View Description Hide Description
Single mode distributed feedback (DFB) interband cascade lasers were realized by placing metal gratings laterally to dry etched ridges. A discrete tuning range of 104 nm could be realized on the same gain material by a variation of the grating period. At room temperature, a 2.4 mm long and 9.8 μm wide ridge with as-cleaved facets emitted more than 6 mW of single mode output power in continuous-wave (cw) mode at a wavelength around 3.8 μm. With typical temperature- and current-tuning rates of 0.31 nm/ °C and 0.065 nm/mA, respectively, a total tuning bandwidth of more than 10 nm could be covered with a single device.
105(2014); http://dx.doi.org/10.1063/1.4893984View Description Hide Description
We report the experimental observation of nonlinear switching dynamics in an InP photonic crystal nanocavity. Usually, the regime of relatively small cavity perturbations is explored, where the signal transmitted through the cavity follows the temporal variation of the cavity resonance. When the cavity is perturbed by strong pulses, we observe several nonlinear effects, i.e., saturation of the switching contrast, broadening of the switching window, and even initial reduction of the transmission. The effects are analyzed by comparison with nonlinear coupled mode theory and explained in terms of large dynamical variations of the cavity resonance in combination with nonlinear losses. The results provide insight into the nonlinear optical processes that govern the dynamics of nanocavities and are important for applications in optical signal processing, where one wants to optimize the switching contrast.
- SURFACES AND INTERFACES
Improvement of magnetic and structural stabilities in high-quality Co2FeSi1− x Al x /Si heterointerfaces105(2014); http://dx.doi.org/10.1063/1.4893608View Description Hide Description
We study high-quality Co2FeSi1− x Al x Heusler compound/Si (0 ≤ x ≤ 1) heterointerfaces for silicon (Si)-based spintronic applications. In thermal treatment conditions, the magnetic and structural stabilities of the Co2FeSi1− x Al x /Si heterointerfaces are improved with increasing x in Co2FeSi1− x Al x . Compared with L21-ordered Co2FeSi/Si, B2-ordered Co2FeAl/Si can suppress the diffusion of Si atoms into the Heusler-compound structure. This experimental study will provide an important knowledge for applications in Si-based spin transistors with metallic source/drain contacts.
105(2014); http://dx.doi.org/10.1063/1.4893700View Description Hide Description
The magnesium (Mg) phase characterized within Mg/Nb multilayers can adopt either a body-centered cubic (bcc-Mg) or hexagonal close packed (hcp-Mg) structure, depending on the Mg layer thickness. Using first-principles density functional theory, we find that bcc-Mg has a similar weight density of hcp-Mg, lower Young's modulus, and higher shear modulus than hcp-Mg, and the same conventional slip systems as the bcc structure. A simple theoretical model is developed to predict the structural stability of both the bcc-Mg/Nb and hcp-Mg/Nb multilayers. It shows that the bcc-Mg/Nb multilayer is energetically favorable when the bcc-Mg layer is less than 4.2 nm.
Formation mechanism of threading-dislocation array in AlN layers grown on 6H-SiC (0001) substrates with 3-bilayer-high surface steps105(2014); http://dx.doi.org/10.1063/1.4892807View Description Hide Description
We grew AlN layers on 6H-SiC (0001) substrates with three Si-C bilayer high (0.75 nm) steps. In the AlN layers, most of the threading dislocations (TDs) were arranged in rows. The TD row consisted of arrays of a half-loop dislocation, which was formed by an AlN/SiC interfacial dislocation along the step edges of the SiC substrate surfaces and a TD pair at both ends. The configuration of the interfacial dislocation was highly relevant with two-dimensional AlN nuclei at the initial stage of growth. We concluded that the half-loop dislocation arrays were generated in the AlN nucleus coalescence over the SiC step edges.
105(2014); http://dx.doi.org/10.1063/1.4893768View Description Hide Description
Wide-bandgap (WBG) semiconductor technologies are maturing and may provide increased device performance in many fields of applications, such as high-temperature electronics. However, there are still issues regarding the stability and reliability of WBG devices. Of particular importance is the high-temperature stability of interconnects for electronic systems based on WBG-semiconductors. For metallization without proper encapsulation, morphological degradation can occur at elevated temperatures. Sandwiching Ag films between Ta and/or TaN layers in this study is found to be electrically and morphologically stabilize the Ag metallization up to 800 °C, compared to 600 °C for uncapped films. However, the barrier layer plays a key role and TaN is found to be superior to Ta, resulting in the best achieved stability, whereas the difference between Ta and TaN caps is negligible. The β-to-α phase transition in the underlying Ta barrier layer is identified as the major cause responsible for the morphological instability observed above 600 °C. It is shown that this phase transition can be avoided using a stacked Ta/TaN barrier.
105(2014); http://dx.doi.org/10.1063/1.4893731View Description Hide Description
Surfaces and interfaces between materials are of paramount importance for various phenomena, such as painting a house, catalyst driven chemical reactions, intricate life processes, corrosion of materials, and fabrication of various semiconductor devices. Interface of silicon or other such substrates with any of the oxides has profound effect on the performance of metal oxide field effect transistors and other similar devices. Since a surface is an abrupt termination of a periodic crystal, surface atoms will have some unsaturated valence electrons and these unsaturated bonds at the semiconductor surface make it chemically highly reactive. Other than annealing, there is not much that can be done to manage these unsaturated bonds. This study was initiated to explore the possibility of repairing these unsaturated dangling bonds that are formed at the silicon and oxide interface during the deposition of oxide layer above silicon, by the use of proton irradiation. In order to improve the interface characteristics, we present a method to modify the interface of silicon and hafnium dioxide after its fabrication, through proton irradiation. Results of the study are promising and probably this method might be used along with other methods such as annealing to modify the interface, after its fabrication.
Influence of graphene-substrate interactions on configurations of organic molecules on graphene: Pentacene/epitaxial graphene/SiC105(2014); http://dx.doi.org/10.1063/1.4893880View Description Hide Description
We demonstrate that molecular ordering of pentacene (Pn) on graphene depends on the interaction between graphene and its underlying SiC substrate. The adsorption of Pn molecules on zero-layer (ZL) and single-layer (SL) graphene, which were grown on a Si-faced 6H-SiC(0001) wafer, was studied using scanning tunneling microscopy (STM). Pn molecules form a quasi-amorphous layer on ZL graphene, which interacts strongly with the underlying SiC substrate. In contrast, they form a uniformly ordered layer on the SL graphene having a weak graphene-SiC interaction. Furthermore, we could change the configuration of Pn molecules on the SL graphene by using STM tips. The results suggest that the molecular ordering of Pn on graphene and the Pn/graphene interface structure can be controlled by a graphene-substrate interaction.
Enhanced surface flashover strength in vacuum of polymethylmethacrylate by surface modification using atmospheric-pressure dielectric barrier discharge105(2014); http://dx.doi.org/10.1063/1.4893884View Description Hide Description
Polymer materials, such as polymethylmethacrylate (PMMA), are widely used as insulators in vacuum. The insulating performance of a high-voltage vacuum system is mainly limited by surface flashover of the insulators rather than bulk breakdown. Non-thermal plasmas are an efficient method to modify the chemical and physical properties of polymer material surfaces, and enhance the surface insulating performance. In this letter, an atmospheric-pressure dielectric barrier discharge is used to treat the PMMA surface to improve the surface flashover strength in vacuum. Experimental results indicate that the plasma treatment method using Ar and CF4 (10:1) as the working gas can etch the PMMA surface, introduce fluoride groups to the surface, and then alter the surface characteristics of the PMMA. The increase in the surface roughness can introduce physical traps that can capture free electrons, and the fluorination can enhance the charge capturing ability. The increase in the surface roughness and the introduction of the fluoride groups can enhance the PMMA hydrophobic ability, improve the charge capturing ability, decrease the secondary electron emission yield, increase the surface resistance, and improve the surface flashover voltage in vacuum.
105(2014); http://dx.doi.org/10.1063/1.4893945View Description Hide Description
This Letter demonstrates a simple method to achieve underwater anisotropic oil-wetting using silicon surfaces with a microgroove array produced by femtosecond laser ablation. The oil contact angles along the direction perpendicular to the grooves are consistently larger than those parallel to the microgroove arrays in water because the oil droplet is restricted by the energy barrier that exists between the non-irradiated domain and the trapped water in the laser-ablated microgrooves. This underwater anisotropic oil-wetting is able to be controlled, and the anisotropy can be tuned from 0° to ∼20° by adjusting the period of the microgroove arrays.