Volume 100, Issue 8, 20 February 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 fluid-borne superparamagnetic bead transport by field-driven domain walls in submicrometer ferromagnetic tracks is studied experimentally together with numerical and analytical modeling. Experiments show that nanotrack-guided domain walls can propel individual trapped beads through an aqueous medium at speeds approaching 1000 μm/s, 10 to 100 times faster than through any previously demonstrated mechanism.
- PHOTONICS AND OPTOELECTRONICS
100(2012); http://dx.doi.org/10.1063/1.3687702View Description Hide Description
Metal–insulator–metal (MIM) capacitors with Al2O3/TiO2/Al2O3 (ATA) dielectrics were fabricated and investigated. At 0 V and frequencies of 100 kHz and 1 MHz, the MIM capacitors with ATA (3/20/3 nm) and ATA (6/20/6 nm) thin films had low leakage current densities of approximately 5.2 × 10−13 and 1.5 × 10−13 A/cm2, respectively, and high capacitance densities of ∼19.48 and ∼20.13 fF/μm2, respectively. The frequency dispersion effect for these MIM capacitors was very small. The electrical transport mechanism, which is the device conduction mechanism, was determined for the varying structures of MIM capacitors.
100(2012); http://dx.doi.org/10.1063/1.3688032View Description Hide Description
We report on a method to stabilize a hot electronbolometer (HEB) mixer at 2.5 THz. The technique utilizes feedback control of the local oscillator (LO) laser power by means of a swing-arm actuator placed in the optical beam path. We demonstrate that this technique yields a factor of 50 improvement in the spectroscopic Allan variance time which is shown to be over 30 s in a 12 MHz noise fluctuation bandwidth. Furthermore, broadband signal direct detection effects may be minimized by this technique. The technique is versatile and can be applied to practically any local oscillator at any frequency.
85 °C error-free operation at 38 Gb/s of oxide-confined 980-nm vertical-cavity surface-emitting lasers100(2012); http://dx.doi.org/10.1063/1.3688040View Description Hide Description
Extremely temperature stable oxide-confined high-speed 980-nm vertical-cavity surface-emitting lasers(VCSELs) for optical interconnects are presented. Error-free performance at 38 Gb/s and 40 Gb/s is demonstrated at temperatures as high as 85 °C and 75 °C, respectively. No adjustment of driving conditions was found to be necessary from room temperature up to 85 °C. In addition, energy-efficient 35 Gb/s operation at a very low pump current of only 4 mA is demonstrated with a low dissipated heat-to-bit rate ratio of 233 mW/Tbps. These are by far the highest bit rates reported for VCSELs at such temperatures.
Highly modified spontaneous emissions in YVO4:Eu3+ inverse opal and refractive index sensing application100(2012); http://dx.doi.org/10.1063/1.3688167View Description Hide Description
Here, we present strong modification of photonic crystals on the spontaneous emissions of Eu3+ions in YVO4:Eu3+ inverse opal. It is interesting that due to the effect of local field, the 5D0-7FJ radiative lifetimes of Eu3+ions in the PCs are all prolonged ∼2.5 times in contrast to the grinded reference (REF) in air, consistent with the empty cavity model. The photonic stop band and the radiative lifetime are both highly sensitive to the refractive index of the infiltrated solutions, in accordance with the real cavity model.
100(2012); http://dx.doi.org/10.1063/1.3687910View Description Hide Description
We compare the dielectric functions of silver interfaces obtained via thermal evaporation with those obtained with template stripping. Ellipsometry measurements show that the smoother template-stripped surfaces exhibit effective dielectric functions with a more negative real component and a smaller imaginary component, implying higher conductivity and less energy loss, respectively. These results agree with the relation between dielectric function and surface roughness derived from combining the effective-medium model and the Drude-Lorentz model. The improvement in the effective dielectric properties shows that metallic films prepared via template stripping can be favorable for applications in electronics, nanophotonics, and plasmonics.
Temperature dependent efficiency droop in GaInN light-emitting diodes with different current densities100(2012); http://dx.doi.org/10.1063/1.3688041View Description Hide Description
The effect of chip area on the temperature-dependent light-output power (LOP) in GaInN-based light-emitting diodes(LEDs) is investigated. The larger the chip size, the faster the reduction in LOP with increasing temperature becomes, indicating that increasing the size of LED chips, a technology trend for reducing the efficiency droop at high currents, is detrimental for high temperature-tolerant LEDs. In addition, it is found that regardless of chip size, the temperature-dependent LOP is identical for the LEDs operating at the same current density.
100(2012); http://dx.doi.org/10.1063/1.3688301View Description Hide Description
A photonic crystal three missing holes nanocavity, having only a few modes, is coupled to a 60 missing holes long multimode cavity, both fabricated in the same InGaAsP membrane. The coupling was studied in detail by the photothermal tuning of the small cavity over about three free spectral ranges of the large cavity. Strong coupling effects, involving at least three large cavity modes simultaneously, were observed from level anticrossing data. The observations are excellently reproduced by a model of coupled Fabry Perot resonators.
100(2012); http://dx.doi.org/10.1063/1.3688601View Description Hide Description
We demonstrated a method to excite selected whispering gallery modes in optical bottle microresonators (BMR) by inscribing microgroove scars on their surface by focused ion beammilling. Substantial spectral clean-up is obtained in appropriately scarred BMRs, providing the potential for high performance sensors and other optical devices.
100(2012); http://dx.doi.org/10.1063/1.3688604View Description Hide Description
A turn-on of a quantum dotsemiconductor laser is analyzed in detail both theoretically and experimentally. We show that quantum dot lasers have a nonlinear damping rate which strongly affects laser turn-on dynamics due to the non-instantaneous capture of carriers to a dot. It results in nonvanishing turn-on delay even at very high pumping in good agreement with experiment.
100(2012); http://dx.doi.org/10.1063/1.3689765View Description Hide Description
In this paper, improved electrical and optical properties of aluminum gallium nitride (AlGaN)-based ultraviolet light-emitting diodes using fluorine-doped indium tin oxide (F-ITO) electrodes are reported. F-doping was found to increase the work function as well as the energy bandgap of the ITO and, thereby, reduce the Shottky barrier height in contact with p-(Al)GaN. As a result, the optical transmittance increased from 79.7% to 86.9% at 380 nm, while the specific contact resistance decreased from 1.04 × 10−3 Ω·cm2 to 9.12 × 10−4 Ω·cm2 after F-doping, which led to an increase in the output power from 2.41 mW to 5.99 mW.
100(2012); http://dx.doi.org/10.1063/1.3689775View Description Hide Description
We introduce the concept of controlling the nonlinear response of the metamaterial by altering its internal structure. We experimentally demonstrate tuning of the nonlinear response of two coupled split-ring resonators by changing their mutual position. This effect is achieved through modification of the structure of the coupled resonant modes and their interaction with the incident field. By offsetting the resonators we control the maximum currents through the nonlinear elements, which affect the nonlinear response of the system.
Controlling extraordinary transmission characteristics of metal hole arrays with spoof surface plasmons100(2012); http://dx.doi.org/10.1063/1.3689784View Description Hide Description
We experimentally controlled the spectral characteristics of the extraordinary transmissions observed in metal hole arrays by analogy with spoof surface plasmon-polaritons (spoof SPPs). We constructed doubly periodic metal hole arrays. The larger and smaller periodic holes are for inducing extraordinary transmissions and modifying the spoof SPPs’ characteristics, respectively. Introducing the smaller holes that surround the larger hole arrays can modify the dispersion curve of the surface waves excited on a structuredmetal surface analogous to the spoof SPP model, and consequently, the extraordinary transmission peak will show a redshift.
- SURFACES AND INTERFACES
100(2012); http://dx.doi.org/10.1063/1.3687706View Description Hide Description
We have studied conductive LaAlO3/SrTiO3heterostructures deposited at different oxygen pressures.Photoluminescence spectra confirm the presence of a significant amount of oxygen vacancies in samples deposited at low oxygen pressures. Power law fitting of resistance versus temperature measurements reveals fundamental characteristics of the conduction mechanism at the interface. A distinct non-Fermi-liquid behavior is observed for samples grown in higher oxygen pressure, which give two-dimensionally confined conducting interfaces, whereas characteristic electron-electron scattering is observed for samples grown in lower oxygen pressures, as seen in bulk doped SrTiO3 (i.e., oxygen deficient SrTiO3). Transitions between different conduction modes occur throughout the studied temperature range (10–270 K) as a result of structural transformations in the near-surface region of the SrTiO3.
100(2012); http://dx.doi.org/10.1063/1.3685713View Description Hide Description
The windmill-like chiral nature of individual ZnPc molecules adsorbed on Cu(100) surface at room temperature has been revealed by scanning tunneling microscopy(STM) and the origin of such chirality is attributed to asymmetrical charge transfer between the molecules and the coppersurface. Such chiral enantiomers do recognize each other in molecular level and spontaneously form second-level chiral supramolecular structures with the same chirality during thermally driven movements. The interactions between the ZnPc molecules during such chiral recognition process have been discussed based on the analysis of the sub-molecule-resolution STM images.
Magnetic and electronic properties of the interface between half metallic Fe3O4 and semiconducting ZnO100(2012); http://dx.doi.org/10.1063/1.3687731View Description Hide Description
We have investigated the magnetic depth profile of an epitaxial Fe3O4thin filmgrown directly on a semiconducting ZnO substrate by soft x-ray resonant magnetic reflectometry (XRMR) and electron energy loss spectroscopy(EELS). Consistent chemical profiles at the interface between ZnO and Fe3O4 are found from both methods. Valence selective EELS and XRMR reveal independently that the first monolayer of Fe at the interface between ZnO and Fe3O4 contains only Fe3+ ions. Besides this narrow 2.5 Å interface layer, Fe3O4 shows magnetic bulk properties throughout the whole film making highly efficient spin injection in this system feasible.
Droplet jumping by electrowetting and its application to the three-dimensional digital microfluidics100(2012); http://dx.doi.org/10.1063/1.3688487View Description Hide Description
We introduce droplet jumping by electrowetting (DJE), which stretches droplets to store energy for jumping by electrowetting. The capillarity-driven droplet jumping is effective to overcome the energy barrier, where the threshold for jumping is less than 100 V. We studied the detailed jumping mechanisms with regard to the jumping height and the energy conversion and demonstrated the transport of sessile droplets to upper surfaces under diverse electrode configurations. While the droplet jumping on the superhydrophobicsurface is the primary focus of our research, DJE is also found to be possible on conventional Teflon surfaces, envisioning the three-dimensional droplet-based digital microfluidics.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
100(2012); http://dx.doi.org/10.1063/1.3687695View Description Hide Description
We report the nonlinear optical absorption studies in two differently sized water-soluble cadmium telluride quantum dot(QD) samples, exhibiting first excitonic absorption peaks at 493 nm and 551 nm, respectively. An optical limiting behavior is observed for near-resonant excitation at 532 nm using nanosecond laser pulses, originating from the effective two-photon absorption(TPA) mechanism. The effective TPA coefficient (βeff) is measured to be in the range of 10−12 m/W. This is one order of magnitude higher than the TPA coefficient (β) reported for off-resonant excitation. At this excitation wavelength, the smaller QD shows a relatively weaker photoluminescence and stronger nonlinear absorption.
100(2012); http://dx.doi.org/10.1063/1.3688051View Description Hide Description
Stimulated emission (SE) is studied in AlGaN/AlGaN multiple quantum wells(MQWs) with different Al content grown on sapphire substrate. The spectra of spontaneous and stimulated emission and their transformations with increasing temperature as well as stimulated emission thresholds were measured in the temperature range from 8 to 300 K. Phonon-assisted band broadening in low-Al-content MQWs and double-scaled potential profile in high-Al-content MQWs were observed in the samples and linked with carrier localization conditions. The temperature dependence of the stimulated emission threshold was similar in the samples where the stimulated transitions occur between extended states and in the samples where the transitions occur in localized states. The stimulated emission threshold depends predominantly on the density of nonradiative recombination centers.
Micro-fabricated channel with ultra-thin yet ultra-strong windows enables electron microscopy under 4-bar pressure100(2012); http://dx.doi.org/10.1063/1.3688490View Description Hide Description
Transmission electron microscopy(TEM) of (de-)hydrogenation reactions is crucial to characterize efficiency of hydrogen storagematerials. The nanoreactor, a micromachined channel with 15-nm-thick windows, effectively confines the gas flow to an electron-transparent chamber during TEM of reactions. Realistic experiments require very high pressures to be sustained by the device. Nanomechanical bulge tests and simulations show that due to a very strong size effect, ultra-thin device components can reliably withstand tensile stresses as high as 19.5 GPa enabling high pressure operation. We use the device to characterize Pd particles under a 4-bar H2pressure within the ultra-high-vacuum of the TEM.
100(2012); http://dx.doi.org/10.1063/1.3689776View Description Hide Description
The optical control of the molecular motions in chloroform CHCl3 at room temperature through the non-resonant excitation was enhanced by means of the double-pulse pump-probe technique. When the separation time of the pump pulses and their relative intensity were varied, the amplification or the cancellation of the coherent vibrations of the molecules was achieved. The molecular responses were detected by the time-resolved optically heterodyne-detected optical-Kerr-effect technique.