Volume 101, Issue 6, 06 August 2012
- 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 dynamics of a magnetic infinity-shaped nanostructure has been experimentally studied by two different techniques such as the sinusoidal resonance excitation and the damped short pulse excitation to measure the eigenfrequency of domain walls. Direct observation of the magnetic domain wall nucleation has been measured in the frequency domain. Electrical measurements of the domain wall dynamics in the frequency domain reveal the existence of multi-eigenmodes for large excitation amplitudes. The time-resolved measurements show that the frequency of the damped gyration is similar to that of the frequency domain and indicate coexistence of spin wave excitations.
- PHOTONICS AND OPTOELECTRONICS
101(2012); http://dx.doi.org/10.1063/1.4742889View Description Hide Description
Adaptive optical devices based on electric field induced deformation of dielectricelastomers require transparent and highly compliant electrodes to conform to large shape changes. Electrical, optical, and actuation properties of acrylic elastomerelectrodes fabricated with single-walled carbon nanotubes(SWCNTs) and silver nanowires (AgNWs) have been evaluated. Based on these properties, a figure of merit is introduced for evaluating the overall performance of deformable transparent electrodes. This clearly indicates that SWCNTs outperform AgNWs. Under optimal conditions, optical transparency as high as 91% at 190% maximum actuation strain is readily achievable using SWCNTelectrodes.
Partially sandwiched graphene as transparent conductive layer for InGaN-based vertical light emitting diodes101(2012); http://dx.doi.org/10.1063/1.4742892View Description Hide Description
InGaN-based vertical structure light emitting diodes (VLEDs) with multi-layer graphene transparent electrodes with higher optical output have been fabricated and tested. High temperature annealing introduced inter-diffusion of metal atoms and Ga atoms and generated the partially sandwiched graphene structure, which contributed to the performance improvement of VLEDs.
Polarized light emission from GaInN light-emitting diodes embedded with subwavelength aluminum wire-grid polarizers101(2012); http://dx.doi.org/10.1063/1.4744422View Description Hide Description
We demonstrate a back-emitting (sapphire-substrate emitting) linearly polarized GaInN light-emitting diode(LED) embedded with a subwavelength-sized aluminum wire-grid polarizer (WGP). Rigorous coupled wave analysis is implemented to study the polarization characteristics of such a WGP LED. The aluminum nanowire grating with a period of 150 nm is located on the sapphire backside of a GaInN LED structure and is fabricated by electron-beam lithography and inductively coupled plasma reactive-ion etching. A polarization ratio of 0.96 is demonstrated for a WGP GaInN LED in good agreement with simulation results.
Mid-infrared distributed-feedback interband cascade lasers with continuous-wave single-mode emission to 80 °C101(2012); http://dx.doi.org/10.1063/1.4744445View Description Hide Description
We report continuous-wave (cw) distributed-feedback interband cascade lasers operating in a single spectral mode (λ = 3.7–3.8 μm) at temperatures between 20 and 80 °C. The first-order gratings were realized by patterning high-index germanium layers on top of narrow ridges with relatively thin top claddings. One device generated over 27 mW of cw single-mode output at 40 °C, with a side-mode-suppression ratio >30 dB, while at 80 °C it still emitted >1 mW. At 20 °C, a second device lased in a single spectral mode with <100 mW of drive power. The tuning range was 21.5 nm with temperature and 10 nm with current.
Scattering performance of plasmonic nanorod antennas in randomly tilted disordered and Fibonacci configurations101(2012); http://dx.doi.org/10.1063/1.4738984View Description Hide Description
In this letter, we formulate and investigate the optical performance of finite arrays of plasmonic nanorod antennas in randomly tilted disordered and Fibonacci configurations. To efficiently model the scattering performance of a nanorod, we take advantage of characteristic basis function method in conjunction with macro basis functions. We study how random rotations of the nanorods, as a disorder, may affect their electromagnetic response. Also, the impact of quasi-periodic order on the performance of the array in the form of two-dimensional Fibonacci quasi-lattice comprising of plasmonic nanorods with two different lengths is studied.
101(2012); http://dx.doi.org/10.1063/1.4740258View Description Hide Description
A prism-coupled metal-dielectric-metal waveguidestructure is presented to detect slight variations in wavelength. Strong spatial dispersion ability is exhibited at the guided-mode resonance, which leads to a rapid changing of the reflected beam shift with respect to the light wavelength. It is demonstrated to be an effective way to enlarge the lateral shift of the reflected beam by increasing the guiding layer thickness, which helps to improve the sensitivity of the wavelength-shift monitoring. A wavelength resolution as high as 0.027 pm is achieved in the experiment.
Unidirectional launching of surface plasmons with subwavelength metallic gratings around the plasmonic critical angle101(2012); http://dx.doi.org/10.1063/1.4745919View Description Hide Description
We find and verify that unidirectional surface plasmon polaritons (SPPs) are highly excited by subwavelength metallic gratings with back-side illumination around the plasmonic critical angle (PCA). The conditions and physical origins of the SPP excitation around the PCA are clarified. Moreover, the experimental results support our numerical calculations very well. Potential applications are also suggested.
101(2012); http://dx.doi.org/10.1063/1.4745182View Description Hide Description
Recent years have seen significant advances in material diagnostics and analysis using terahertz (THz) time domain spectroscopy (TDS) and imaging. Despite its widespread application, the interaction between THz radiation and materials with random structure is not yet fully studied. Separation of absorption and scattering is required to extract the true absorption spectra, thus enabling direct comparison with pure samples in a spectral data base for automated recognition. Here, we present a discrete wavelet transform based iterative reconstruction technique that reduces the scattering contribution in THz-TDSmeasurements, in composites with absorbing constituents that exhibit sharp absorption features.
101(2012); http://dx.doi.org/10.1063/1.4744539View Description Hide Description
We investigate the acousto-optic coupling, based on both photo-elastic and opto-mechanical mechanisms, in periodic structures with simultaneous photonic and phononic band gaps. The investigations are focused on a cavity defect in which the strong confinement of acoustic and optic waves enhances the interaction. We calculate the modulation of each photonic mode frequency by each phononic mode confined in the cavity. We compare the strength for the photo-elastic and opto-mechanical effects in the different cases. Both mechanisms can be in phase or out of phase and produce additive or subtractive effects in the total acousto-optic coupling.
Nonthermal carrier distributions in the subbands of 2-phonon resonance mid-infrared quantum cascade laser101(2012); http://dx.doi.org/10.1063/1.4745013View Description Hide Description
Nonequilibrium Green’s functionsmodel of mid-infrared quantum cascade laser that utilizes double-phonon resonance scheme is analyzed. Good agreement with experimental data is found. For quantities not accessible experimentally, it is shown that electronic distributions in lower active region subbands are nonthermal and have local maximum at higher in-plane energy. In upper subband, carrier distribution is thermal-like. Upper state lifetime is strongly reduced from optical phonon value by interface roughness and alloy scatterings. Net gain originates from population inversion at subbands bottom which is not suppressed by high-k absorption because of nonparabolicity which shifts absorption peak to lower frequencies.
- SURFACES AND INTERFACES
Electromechanical properties of freestanding graphene functionalized with tin oxide (SnO2) nanoparticles101(2012); http://dx.doi.org/10.1063/1.4745780View Description Hide Description
Freestanding graphene membranes were functionalized with SnO2nanoparticles. A detailed procedure providing uniform coverage and chemical synthesis is presented. Elemental composition was determined using scanning electron microscopy combined with energy dispersive x-ray analysis. A technique called electrostatic-manipulation scanning tunneling microscopy was used to probe the electromechanical properties of functionalized freestanding graphene samples. We found ten times larger movement perpendicular to the plane compared to pristine freestanding graphene and propose a nanoparticle encapsulation model.
Hard x-ray photoelectron spectroscopy study of As and Ga out-diffusion in In0.53Ga0.47As/Al2O3 film systems101(2012); http://dx.doi.org/10.1063/1.4745207View Description Hide Description
Hard x-ray photoelectron spectroscopy (HAXPES) was performed on In0.53Ga0.47As/Al2O3 gate stacks as deposited and annealed at 400 °C, 500 °C, and 700 °C to test for out-diffusion of substrate elements. Ga and As core-level intensities increase with increasing anneal temperature, while the In intensity decreases. HAXPES was performed at two different beam energies to vary the surface sensitivity; results demonstrate Ga and As out-diffuse into the Al2O3film.Analysis suggests the presence of an interlayer containing Ga and As oxides, which thickens with increasing anneal temperature. Further diffusion, especially of Ga, into the Al2O3film is also observed with increasing anneal temperature.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
101(2012); http://dx.doi.org/10.1063/1.4742859View Description Hide Description
We achieve the resonant actuation of a microcantilever by using a pulse wave of one-nth the microcantilever resonant frequency. Our studies on the relationship between pulse duration and microcantilever resonant amplitude reveal that the most effective actuation can be acquired when the pulse duration is odd multiples of the half period of microcantilever resonant oscillation. This actuation method is generally applicable and may find applications in ultrahigh frequency actuation of microcantilever or similar resonant systems for high sensitivity detection.
101(2012); http://dx.doi.org/10.1063/1.4742896View Description Hide Description
We present our recent achievement of a transparent ceramic able to produce white light when directly combined with commercially available blue light emitting diodes. The photoluminescence properties of ceramicphosphor (Y1-xCex)3Al5O12 are studied as a function of doping fraction (x = 0.0005–0.0020). The emission color is tunable by variations of Ce3+ concentration and ceramicphosphor thickness. A maximum luminous efficacy exceeding 93 lm/W at a low correlated color temperature of ∼4600 K is obtained, which is superior to samples made from commercial phosphorpowders. Hence, the present transparent ceramicphosphor is expected to be an ideal candidate for generating white light.
101(2012); http://dx.doi.org/10.1063/1.4742919View Description Hide Description
EpitaxialGe2Sb2Te5filmsgrown on Si(111) by molecular beam epitaxy were reversibly switched between crystalline and amorphous states over a large area using femtosecond laser pulses. The structural and spatial homogeneity of the as-grown epitaxial and laser-switched areas on the sample were investigated by synchrotron nanofocus high resolution x-ray diffraction. The investigation clearly demonstrated that the single crystalline metastable cubic phase of Ge2Sb2Te5 is restored after switching. No polycrystalline features, not only on the average but even on the nanometer scale of the x-ray beam, were observed.
Blue-white tunable luminescence for white light-emitting diodes and wideband near-infrared luminescence from Sm3+-doped borophosphate glass101(2012); http://dx.doi.org/10.1063/1.4743008View Description Hide Description
Highly transparent samarium (Sm3+) doped borophosphate glasses were prepared using the melt-quenching technique. The tunable light emission and wideband near-infrared luminescence properties of Sm3+-doped glasses were investigated systemically. Tuning the Sm3+ concentration and excitation wavelength can generate hues that vary from blue to white. Two wide luminescence bands in the 850 nm to 1070 nm range and in the 1100 nm to 1250 nm range, respectively, were also achieved. The results suggest that Sm3+-doped borophosphate glasses can be used as conversion materials for blue light-emitting diode chips to generate white light-emitting diodes and for optical amplification.
101(2012); http://dx.doi.org/10.1063/1.4745211View Description Hide Description
The enhanced internal quantum efficiency of InGaN/GaN multiple-quantum-wells(MQWs) structure is demonstrated by paving the graphene layers on the MQWs surface. Compared to the conventional MQWs, the internal quantum efficiency of the graphene/InGaN MQWs hybrid structure exhibits a remarkable 2-fold increase. The high charge carrier density in graphene layer is accounted for the enhanced internal quantum efficiency. Moreover, the negligible photoluminescence emission peak shift with increasing the excitation power as well as the decrease of radiative recombination lifetime are attributed to the reduced quantum-confined Stark effect, which correlates to the screening of the polarization field in the c-plane nitride-based quantum well structure.
101(2012); http://dx.doi.org/10.1063/1.4745654View Description Hide Description
In sodium nanoring dimers, plasmon resonances and the plasmon-induced field enhancement are investigated by time-dependent density functional theory. The optical absorption, the induced charge response, and the frequency dependent current demonstrate that the main plasmon resonance modes are the charge transferplasmon mode and the bonding dimer plasmon mode (BDP). Moreover, there are also two small hybridized plasmon modes. The induced field enhancement of each spatial region depends on the gap distance and the plasmon mode. For the narrow gap, the field enhancement at different positions of the straight line segments between two nanorings is almost uniformly distributed. However, for large separations, along the axial direction, the field enhancement gradually decreases in the region within the radius of the sodium atom. Then, the change of the field enhancement is nonlinear. For different plasmon modes, there is a different number of field enhancement extrema. The largest extreme value is located in the middle region. These findings are expected to play an important role in designing plasmonic nanostructures for practical applications that require coupled metallic nanoparticles with enhanced electric fields.
Kinetic study of GeO disproportionation into a GeO2/Ge system using x-ray photoelectron spectroscopy101(2012); http://dx.doi.org/10.1063/1.4738892View Description Hide Description
GeO disproportionation into GeO2 and Ge is studied through x-ray photoelectron spectroscopy. Direct evidence for the reaction 2GeO → GeO2 + Ge after annealing in ultra-high vacuum is presented. Activation energy for GeO disproportionation is found to be about 0.7 ± 0.2 eV through kinetic and thermodynamic calculations. A kinetic model of GeO disproportionation is established by considering oxygen transfer in the GeO network. The relationship between GeO disproportionation and GeO desorption induced by GeO2/Ge interfacial reaction is discussed, and the apparent contradiction between GeO desorption via interfacial redox reaction and GeO disproportionation into Ge and GeO2 is explained by considering the oxygen vacancy.
101(2012); http://dx.doi.org/10.1063/1.4744951View Description Hide Description
Noise measurements were performed to determine the quality factor Q and the resonating frequency shift as a function of gas pressure P for microcantilevers with modified surfaces and geometries. In the molecular and continuum regimes, energy loss is dominated by the surrounding fluid leading to reduction of the Q factor and shift of the resonance frequency by Δf, which becomes significant in the continuum regime showing sensitivity on surface changes. This is shown via three methods: frequency shift Δf vs. P, Q factor vs. P, and direct calculation using surface roughness details acquired via atomic force microscopy.