Volume 101, Issue 13, 24 September 2012
- photonics and optoelectronics
- surfaces and interfaces
- structural, mechanical, optical, and thermodynamic properties of advanced materials
- magnetics and spintronics
- 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:
A light scattering peak of ZnO rods is presented at 543.2 nm. The radiation peaks of ZnO nanospines correspond to 496.6 nm and 630.6 nm due to the breaking of the symmetry of the ZnO rods. The radiation peaks of sea-urchin shaped ZnO was observed and confirmed by utilizing the dipole approximation. Sea-urchin shaped ZnO can tune and then filter different frequencies of light by utilizing incident light to illuminate at the different positions of sea-urchin shaped ZnO which works like a nanolight filter device and has potential applications in photonic computers, bio-light emission device, and solar cells.
- PHOTONICS AND OPTOELECTRONICS
101(2012); http://dx.doi.org/10.1063/1.4754533View Description Hide Description
Self-assembledInGaNquantum dots(QDs) were fabricated by metal-organic chemical vapor deposition. Abnormal temperature dependence of photoluminescence(PL) was observed. The integrated PL intensity of QDs sample shows a dramatic increase in a temperature range from 160 K to 215 K and reaches the maximum value at 215 K instead of 10 K as usual. To interpret this phenomenon, a theoretic model of temperature induced carrier redistribution mechanism is designed using rate equation, which fits closely with the experimental result. It is concluded that carriers’ redistribution from shallow QDs or wetting layer to deep QDs gives rise to the unique behavior for InGaNQDs structure.
101(2012); http://dx.doi.org/10.1063/1.4754534View Description Hide Description
Plasmonic hotspots located in the nanogaps of infrared optical antennas are mapped in the near-field. The enhanced evanescent field resonance is shown to depend strongly on excitation wavelength, the excitation and detection laser polarization, and gap size. In addition, we demonstrate that in nanogap hotspot imaging using scattering probes, the probe tip can be considered as a load in the gap of the antenna, and the impedance of the load can then be tuned from inductive to capacitive or vice versa by changing the dielectric value of the tip load. Experimental results are in agreement with finite-difference time-domain simulations.
GaN-based light emitting diodes with micro- and nano-patterned structures by femtosecond laser nonlinear decomposition101(2012); http://dx.doi.org/10.1063/1.4754569View Description Hide Description
The multiple micro/nano-patterning on p-GaN surface by femtosecond (fs) laser irradiation through a micro-ball lens array (MBLA) mask has been established to improve the light extraction efficiency of GaN-based light-emitting diodes(LEDs). This technique was meant for the high-speed surface micro/nano-patterning of a p-GaN surface on a large area using fs laser irradiation. Compared with conventional LEDs with flat surfaces, dual-scale roughened structures on p-GaN surfaceLEDs used in the present experiment scatter and increase the effective critical angle, increasing the escape probability of photons. The relationship of the hole size on p-GaN surface and the refractive index of the MBLA material would be also discussed. With an injection current of 20 mA, the output power of the experimental LEDs markedly improved by a magnitude of 48% compared with conventional GaN-based LEDs.
101(2012); http://dx.doi.org/10.1063/1.4754575View Description Hide Description
We present a method for all-optical switching using a microcavity exciton-polariton system. Unlike many other switching methods, we use a single wavelength for both the signal and gate, which means that this method could be used for cascading optical logic gates and amplification within an all-optical circuit. We resonantly pump the sample at an angle with a laser beam and probe the sample with another beam at normal incidence. Upon saturation of the exciton-polariton states, the normal-incidence resonance increases in energy to permit the probe beam to be transmitted through the sample. Experimental results demonstrate switching using a GaAs/AlAs microcavity. Switching times on the order of ten picoseconds and on/off ratios on the order of 10:1 have been observed, and we propose design options to improve upon these.
101(2012); http://dx.doi.org/10.1063/1.4754615View Description Hide Description
A fluorescent dielectric multilayer is exploited for label-free sensing in aqueous micro-environment. Fluorescence is laser-excited and collected through prism-coupling to a surface electromagnetic mode, also known as Bloch surface waves (BSW) localized at the interface between the multilayer and the outer aqueous medium. By detecting the spectral changes of the BSW-coupled light emission due to an external perturbation of the refractive index (Δn), a sensitivity of ∼2500 nm/RIU and a limit of detection down to Δn ∼ 3 × 10−6 are obtained.
101(2012); http://dx.doi.org/10.1063/1.4754616View Description Hide Description
We develop the fluctuational electrodynamics of metamaterials with hyperbolic dispersion and show the existence of broadband thermal emission beyond the black body limit in the near field. This arises due to the thermal excitation of unique bulk metamaterial modes, which do not occur in conventional media. We consider a practical realization of the hyperbolic metamaterial and estimate that the effect will be observable using the characteristic dispersion (topological transitions) of the metamaterial states. Our work paves the way for engineering the near-field thermal emission using metamaterials.
Influencing modulation properties of quantum-dot semiconductor lasers by carrier lifetime engineering101(2012); http://dx.doi.org/10.1063/1.4754588View Description Hide Description
The relaxation oscillation (RO) parameters and modulation properties of quantum-dot lasers are investigated depending on effective charge carrierscattering lifetimes of the confined quantum-dot states. We find three dynamical regimes of the laser, characterized by the level of synchronization between carrier dynamics in quantum-dots and quantum-well. For scattering rates similar to the RO frequency, a strong damping is found. On either side of this regime, simulations show low RO damping and improved dynamical response. Depending on the regime, the modulation response differs from conventional analytical predictions. Our results suggest the possibility of tailoring quantum-dot laser dynamical behavior via bandstructure engineering.
Record-low propagation losses of 154 dB/cm for substrate-type W1 photonic crystal waveguides by means of hole shape engineering101(2012); http://dx.doi.org/10.1063/1.4753808View Description Hide Description
Large propagation losses in the order 600-1000 dB/cm were reported in the past for planar line-defect photonic crystal waveguides with a weak vertical refractive index contrast (substrate-type). Loss-relevant factors are a well-suited design of the photonic crystal pattern and a high-quality fabrication technology. Here, the focus is on the latter. Two methods—a thermally driven mass transport process and an ultra-slow, selective wet-etching process—are assessed with respect to their capability of enabling low-loss waveguides. With the resulting hole shape, we experimentally demonstrate record-low propagation losses of 154 dB/cm for W1 photonic crystal waveguides in the InP/InGaAsP system.
Direct observation of the terahertz optical free induction decay of molecular rotation absorption lines in the sub-nanosecond time scale101(2012); http://dx.doi.org/10.1063/1.4754826View Description Hide Description
Optical free induction decay (FID) in the region of 60–75 cm−1 was detected using 120 ps pulses of free electron laser. Signals were detected in real time using ultra-fast Schottky diode detectors. The oscillations corresponding to the splitting of absorption lines in deuterated water vapor (Δf = 0.15 cm−1) and hydrogen bromide (Δf = 0.02 cm−1) were detected. At high optical density, we observed the oscillations arising from “top-hat” shape of absorption lines. Free induction decay signals could be detected in a single shot. This observation allowed obtaining a spectrum in one laser pulse, which facilitates studies of very fast processes.
Degenerate parallel conducting layer and conductivity type conversion observed from p-Ge1−ySny (y = 0.06%) grown on n-Si substrate101(2012); http://dx.doi.org/10.1063/1.4754625View Description Hide Description
Electrical properties of p-Ge1−ySny (y = 0.06%) grown on n-Si substrate were investigated through temperature-dependent Hall-effect measurements. It was found that there exists a degenerate parallel conducting layer in Ge1−ySny/Si and a second, deeper acceptor in addition to a shallow acceptor. This parallel conducting layer dominates the electrical properties of the Ge1−ySny layer below 50 K and also significantly affects those properties at higher temperatures. Additionally, a conductivity type conversion from p to n was observed around 370 K for this sample. A two-layer conducting model was used to extract the carrier concentration and mobility of the Ge1−ySny layer alone.
Influence of indium content and temperature on Auger-like recombination in InGaN quantum wells grown on (111) silicon substrates101(2012); http://dx.doi.org/10.1063/1.4754688View Description Hide Description
High-efficiency InGaN-based light-emitting diodes have been grown on (111) silicon substrates and investigated with regard to efficiency and carrier lifetime as a function of current density. Using a single quantum well active layer ensures a well-defined active volume which enables the precise determination of the recombination coefficients in the ABC rate model for different emission wavelengths and junction temperatures. Good agreement of the resulting C values with calculated Auger coefficients is found both with respect to absolute value as well as their dependence on bandgap energy and temperature.
101(2012); http://dx.doi.org/10.1063/1.4755777View Description Hide Description
The role of polariton-electron scattering on the performance characteristics of an electrically injected GaAs-based quantum well(QW)microcavity diode in the strong coupling regime has been investigated. An electron gas is introduced in the quantum wells by modulation doping with silicon dopants. It is observed that polariton-electron scattering suppresses the relaxation bottleneck in the lower polariton branch. However, it is not adequate to produce a degenerate coherent condensate at k∥ ∼ 0 and coherent emission.
The reduction of efficiency droop by Al0.82In0.18N/GaN superlattice electron blocking layer in (0001) oriented GaN-based light emitting diodes101(2012); http://dx.doi.org/10.1063/1.4756791View Description Hide Description
To investigate the effect of Al0.82In0.18N electron blocking layer (EBL) on the efficiency droop, (0001) oriented InGaNlight emitting diodes(LEDs) were grown with two different types of EBLs—single Al0.82In0.18N:Mg layer and Al0.82In0.18N:Mg (2 nm)/GaN:Mg (2 nm) superlattice(SL) structure with 7 periods. It was found that the output power and operating voltage of single Al0.82In0.18N EBL LED were sensitive to EBL thickness due to the difficulty in growing high quality MgdopedAl0.82In0.18N. On the other hand, LED with SL EBL showed no deterioration of optical power and operating voltage while its efficiency droop (17% at 300 A/cm2) reduced by more than a half compared to a conventional Al0.2Ga0.8N (20 nm) EBL LED (36% at 300 A/cm2).
Direct measurement of laser-induced frequency shift rate of ultracold cesium molecules by analyzing losses of trapped atoms101(2012); http://dx.doi.org/10.1063/1.4756801View Description Hide Description
We report on a quantitative experimental determination of the laser-induced frequency shift rate of the ultracold cesium molecules formed via photoassociation (PA) by means of the trap loss measurement of the losses of trapped atoms in a standard magneto-optical trap. The experiment was directly performed by varying the photoassociation laser intensity without any additional frequency monitor technologies. Our experimental method utilized dependences of the losses on the laser-induced frequency shift rate based on the conditions of the identified photoassociation spectral shape. We demonstrated that the method is sensitive enough to determine small frequency shifts of rovibrational levels of ultracold cesium molecules.
101(2012); http://dx.doi.org/10.1063/1.4755761View Description Hide Description
We present the light-induced manipulation of absorbing liquid droplets in air. Ink droplets from a printer cartridge are used to demonstrate that absorbing liquids—just like their solid counterparts—can interact with regions of high light intensity due to the photophoretic force. It is shown that droplets follow a quasi-ballistic trajectory after bouncing off a high intensity light sheet. We estimate the intensities necessary for this rebound of airborne droplets and change the droplet trajectories through a variation of the manipulating light field.
- SURFACES AND INTERFACES
Pulsed laser ablation of complex oxides: The role of congruent ablation and preferential scattering for the film stoichiometry101(2012); http://dx.doi.org/10.1063/1.4754112View Description Hide Description
By combining structural and chemical thin film analysis with detailed plume diagnostics and modeling of the laser plume dynamics, we are able to elucidate the different physical mechanisms determining the stoichiometry of the complex oxides model material SrTiO3 during pulsed laser deposition. Deviations between thin film and target stoichiometry are basically a result of two effects, namely, incongruent ablation and preferential scattering of lighter ablated species during their motion towards the substrate in the O2 background gas. On the one hand, a progressive preferential ablation of the Ti species with increasing laser fluence leads to a regime of Ti-rich thin film growth at larger fluences. On the other hand, in the low laser fluence regime, a more effective scattering of the lighter Ti plume species results in Sr rich films.
101(2012); http://dx.doi.org/10.1063/1.4754689View Description Hide Description
The thermal characteristics in the interaction process between waterdroplets on a hydrophobicinsulator surface under high direct current voltages have been investigated with the infrared thermography. Discharge inception under a sufficiently high electric field resulted in the temperature rise between droplets and the formation of a liquid channel. The channel was transient for low conductivity droplets followed by their coalescence while stable for highly conductive ones. Asymmetric temperature distributions in the associated droplets appeared, and localized drying in the low conductivity droplet near the higher potential electrode initiated intermittent discharge. Mechanisms of these experimental phenomena have been discussed.
101(2012); http://dx.doi.org/10.1063/1.4754707View Description Hide Description
We report measurements of the thermoelectric power and electrical resistivity of superlattices composed of the high-temperature superconductor YBa2Cu3O7−δ (YBCO) and the metallic ferromagnet La2/3Ca1/3MnO3 (LCMO) with individual layer thicknesses between 5 and 50 nm. Whereas YBCO and LCMO reference films prepared under the same conditions exhibit negative Seebeck coefficients, in excellent agreement with data on bulk compounds of identical composition, those of all superlattices are positive, regardless of the individual layer thickness. Having ruled out lattice strain and incomplete oxygenation, we attribute the observed sign reversal of the Seebeck coefficient to a long-range electronic reconstruction nucleated at the YBCO-LCMO interfaces.
101(2012); http://dx.doi.org/10.1063/1.4755839View Description Hide Description
We investigated the Schottky barrier height and S-parameter at nonpolar (11-20) a-plane p-GaN surfaces by using Schottky diodes fabricated with various metals, including Ti, Cu, Ni, and Pt. A barrier inhomogeneity model was used to explain anomalous carrier transport behavior at the nonpolar p-GaN surfaces, yielding the mean barrier heights of 2.01, 1.73, 1.82, and 1.92 eV for the Ti, Cu, Ni, and Pt contacts, respectively. The extracted S-parameter was as low as 0.02, indicating perfect pinning of the surface Fermi level at around 1.9 eV above the valence band.
101(2012); http://dx.doi.org/10.1063/1.4751986View Description Hide Description
We present here the spontaneous formation of catalyst-free, self-aligned crystalline (wurtzite) nanorods on Si(111) surfaces modified by surface nitridation. Nanorodsgrown by molecular beam epitaxy on bare Si(111) and non-stoichiometric silicon nitride interface are found to be single crystalline but disoriented. Those grown on single crystalline Si3N4 intermediate layer are highly dense c-oriented hexagonal shaped nanorods. The morphology and the self-assembly of the nanorods shows an ordered epitaxial hexagonal superstructure, suggesting that they are nucleated at screw dislocations at the interface and grow spirally in the c-direction. The aligned nanorod assembly shows high-quality structural and optical emission properties.