Volume 100, Issue 23, 04 June 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:
Using grapheneresonator, we perform electromechanical measurements in quantum Hall regime to probe the coupling between a quantum Hall (QH) system and its mechanical motion. Mechanically perturbing the QH state through resonance modifies the DC resistance of the system and results in a Fano-lineshape due to electronic interference. Magnetization of the system modifies the resonator’s equilibrium position and effective stiffness leading to changes in resonant frequency. Our experiments show that there is an intimate coupling between the quantum Hall state and mechanics—electron transport is affected by physical motion and in turn the magnetization modifies the electromechanical response.
- PHOTONICS AND OPTOELECTRONICS
100(2012); http://dx.doi.org/10.1063/1.4725486View Description Hide Description
Electrically pumped lasing actions from the metastable CdZnO films have hardly been achieved as yet. Herein, we have demonstrated electrically pumped wavelength-tunable blue random lasing from the hexagonal CdZnO films with different Cd contents, with central wavelength changing from ∼490 to 425 nm. The devices based on the metal-insulator-semiconductor structures of Au/SiO2/CdZnO on silicon substrates are constructed for electrical pumping of the CdZnO films. The insulator layers of SiO2 onto the CdZnO films in the devices should be annealed at sufficiently low temperature such as 400 °C so that the CdZnO films can be kept their integrity in terms of near-band-edge emissions.
100(2012); http://dx.doi.org/10.1063/1.4725490View Description Hide Description
We demonstrate optical near field coupling of small quantum dot(QD) ensembles and surface plasmons propagating along a silvernanowire. The nanowire fabrication and the aligned QD deposition close to one nanowire end rely on a two-step electron beam lithography procedure. We observe both the addressing of QDs by plasmons and the excitation of plasmonicnanowire modes by QDs. We use the fluorescence signals to quantify the QD/plasmon coupling and show that part of the plasmon-induced QDfluorescence couples back to plasmonic modes.
100(2012); http://dx.doi.org/10.1063/1.4726043View Description Hide Description
InGaP microdisk optical resonators are embedded in indiumtin oxide (ITO) to investigate how the optical absorption in electrodes impacts the resonance properties. The quality factor is slightly reduced in comparison with air-embedding microdisks, and the measured quality factor is 960 for a 1.2 μm diameter, 170 nm thick disk embedded in ITO media. The experiments show that the absorption loss in ITO media has smaller contribution to the reduction of total quality factor than the scattering and radiation losses. The analysis indicates that the suppression of non-absorption loss would improve the quality factor to 104.
A high-resolution spectrometer based on a compact planar two dimensional photonic crystal cavity array100(2012); http://dx.doi.org/10.1063/1.4724177View Description Hide Description
We demonstrate a compact spectrometer based on an array of high-quality-factor photonic crystal nanocavities, coupled via a planar two-dimensional waveguide. This architecture enables spectralanalysis of incident light with resolution as high as the bandwidth of the cavity mode–0.3 nm at 840 nm for our device. The design is easily extended to the visible and deep-infrared spectral ranges. The two-dimensional cavity array can be mated to commercial two-dimensional optical detector arrays, creating a compact and high-resolution spectrometer suitable for a range of applications including materials and chemical analysis.
100(2012); http://dx.doi.org/10.1063/1.4726038View Description Hide Description
We demonstrate a flexible, electrowetting-driven, variable-focus liquid microlens. The microlens is fabricated using a soft polymerpolydimethylsiloxane. The lens can be smoothly wrapped onto a curved surface. A low-temperature fabrication process was developed to reduce the stress on and to avoid any damage to the polymer. The focal length of the microlens varies between −15.0 mm to +28.0 mm, depending on the applied voltage. The resolving power of the microlens is 25.39 line pairs per mm using a 1951 United States Air Force resolution chart. The typical response time of the lens is around 50 ms.
100(2012); http://dx.doi.org/10.1063/1.4722938View Description Hide Description
We realized a potential energy gradient—a ramp—for indirect excitons using a shaped electrode at constant voltage. We studied transport of indirect excitons along the ramp and observed that the exciton transport distance increases with increasing density and temperature.
100(2012); http://dx.doi.org/10.1063/1.4719065View Description Hide Description
We demonstrate a method for tuning a semiconductor quantum dot(QD) onto resonance with a cavity mode all-optically using a system comprised of two evanescently coupled cavities containing a single QD. One resonance of the coupled cavity system is utilized to generate a cavity enhanced optical Stark shift, enabling the QD to be resonantly tuned to the other cavity mode. A twenty-seven fold increase in photon emission from the QD is measured when the off-resonant QD is Stark shifted onto the cavity mode resonance, which is attributed to radiative enhancement of the QD. A maximum tuning of 0.06 nm is achieved for the QD at an incident power of 88 μW.
100(2012); http://dx.doi.org/10.1063/1.4724204View Description Hide Description
We demonstrate a terahertz multichannel microfluidicsensor based on a parallel-plate waveguide geometry with two independent integrated resonant cavities. The resonant frequency of each cavity exhibits an approximately linear dependence on the index of refraction of the material inside the cavity and each cavity is demonstrated to respond independently with no measurable crosstalk. The sensitivities of the two cavities in terms of the change in resonant wavelength per refractive index unit (RIU) are measured to be 1.21 × 106 nm/RIU and 6.77 × 105 nm/RIU.
Quantitative analysis and near-field observation of strong coupling between plasmonic nanogap and silicon waveguides100(2012); http://dx.doi.org/10.1063/1.4725511View Description Hide Description
We present a near field optical study of a plasmonic gap waveguide vertically integrated on silicon. The experimental study is based on a near field scanning optical microscope configured in perturbation mode. This operation mode is described and modeled to give a physical insight into the measured signal. A high spatial resolution allows for the characteristics of the plasmonic gap modes, such as near field distributions, effective indices, direction of propagation, and coupling between perpendicularly polarized modes, to be imaged and analyzed with accuracy. This experimental work is supported by numerical simulations based on finite elementoptical mode solvers and by the application of the strongly coupled-mode theory to the device.
Influence of polarity on carrier transport in semipolar () and () multiple-quantum-well light-emitting diodes100(2012); http://dx.doi.org/10.1063/1.4726106View Description Hide Description
We investigate the influence of polarity on carrier transport in single-quantum-well and multiple-quantum-well(MQW)light-emitting diodes(LEDs) grown on the semipolar () and () orientations of free-standing GaN. For semipolar MQWLEDs with the opposite polarity to conventional Ga-polar c-plane LEDs, the polarization-related electric field in the QWs results in an additional energy barrier for carriers to escape the QWs. We show that semipolar () MQWLEDs with the same polarity to Ga-polar c-plane LEDs have a more uniform carrier distribution and lower forward voltage than () MQWLEDs.
100(2012); http://dx.doi.org/10.1063/1.4726122View Description Hide Description
We present an optical-electronic approach to generating microwave signals with high spectral purity. By circumventing shot noise and operating near fundamental thermal limits, we demonstrate 10 GHz signals with an absolute timing jitter for a single hybrid oscillator of 420 attoseconds (1 Hz–5 GHz).
100(2012); http://dx.doi.org/10.1063/1.4726189View Description Hide Description
We report on triggered single photon emission from GaAsquantum dots, grown on Si substrates and obtained by means of fabrication protocols compatible with the monolithic integration on Si based microelectronics. Very bright and sharp individual exciton lines are resolved in the spectra and can be followed up to 150 K. The nature of quantum emitters of single photon pulses can be measured up to liquid nitrogen temperature by Hanbury Brown and Twiss interferometric correlations.
100(2012); http://dx.doi.org/10.1063/1.4726123View Description Hide Description
GaN-based light-emitting diodes(LEDs) were transferred to paper substrates after a laser lift-off (LLO) process with an ArF excimer laser system (λ = 193 nm) to remove the sapphire substrate and produce freestanding blue LED templates. The threshold voltage (∼2.7 V), current-voltage characteristics, and peak emission wavelength (442 nm) were not changed after the paper substrate was subsequently wrinkled. We were able to demonstrate transfers to both planar and folded (origami) paper structures, showing the promise of the LLO process for transferring LEDs to arbitrary surfaces.
100(2012); http://dx.doi.org/10.1063/1.4727898View Description Hide Description
A detailed rate-equation-based model is developed to study carrier transport effects on optical and electrical characteristics of the multiple quantum wellheterojunction bipolar transistor laser in time-domain. Simulation results extracted using numerical techniques in small-signal regime predict significant enhancement in device optical bandwidth when multiple quantum wells are used. Cavity length and base width are also modified to optimize the optoelectronic performances of the device. An optical bandwidth of ≈60 GHz is achieved in the case of 5 quantum wells each of 70 Å widths and a cavity length of 200 μm.
Simultaneous determination of the constituent azimuthal and radial mode indices for light fields possessing orbital angular momentum100(2012); http://dx.doi.org/10.1063/1.4728111View Description Hide Description
A wide array of diffractive structures such as arrays of pinholes, triangular apertures, slits, and holograms have all recently been used to measure the azimuthal index of individual Laguerre-Gaussian beams. Here, we demonstrate a powerful approach to simultaneously measure both the radial and azimuthal indices of pure Laguerre-Gaussian light fields using the method of principal component analysis. We find that the shape of the diffracting element used to measure the mode indices is in fact of little importance and the crucial step is training any diffractingoptical system and transforming the observed pattern into uncorrelated variables. The method is generic and may be extended to other families of light fields such as Bessel or Hermite-Gaussian beams.
- SURFACES AND INTERFACES
100(2012); http://dx.doi.org/10.1063/1.4722785View Description Hide Description
Binary Ni-C thin-film alloys, which have been shown to be passive against corrosion in hot sulphuric acid solution whilst also being electrocatalytically active, were investigated by XPS to determine the oxidation state of the metal and carbon components. The Ni component produces a Ni 2p spectrum similar to that of metallic nickel (i.e., no oxidation occurs) but with a 0.3 eV shift to higher binding energy (BE) due to electron donation to the carbon matrix. The C 1s peak shows a shift to lower BE by accepting electrons from the Ni nanocrystals. A cluster-model analysis of the observed Ni 2p spectrum is consistent with the electron transfer from the nickel to the carbon.
100(2012); http://dx.doi.org/10.1063/1.4726054View Description Hide Description
We investigate the room-temperature oxidation of In nanoparticles at the tips of Si nanowires. This geometry allows a direct comparison of oxidation at the gas-indium interface and the interface to the Si nanowire. While the In2O3 at the free surface is polycrystalline with small limiting thickness, the oxidation at the nanoscale interface to Si gives rise to single crystalline In2O3 with a tenfold-enhanced thickness. Our results demonstrate interfacial modifications of oxidation at the nanoscale, which need to be considered in scaling electronic devices, and which can become a route for forming high-quality semiconductor-oxide interfaces in nanostructured materials, such as nanowires.
Oxygen migration process in the interfaces during bipolar resistance switching behavior of WO 3−x -based nanoionics devices100(2012); http://dx.doi.org/10.1063/1.4726084View Description Hide Description
Bipolar resistance switching (BRS) behavior and the effects of atmosphere (air, vacuum, O2 gas, or N2 gas) on BRS behavior occurred in the top and bottom interfaces in the M (top electrode)/WO 3−x /Pt(bottom electrode) (M = Pt, Au) devices were investigated. Stable BRS only can be obtained in the interface with Pt electrode. And, the top Pt/WO 3−x interface exhibited stable BRS only in an oxygen-rich atmosphere (air and O2 gas). In contrast, the bottom WO 3−x /Pt interface showed stable BRS under any atmosphere. Based on the x-ray photoelectron spectroscopy measurement on Pt, Au/WO 3−x interfaces, it is identified that the oxygen migration process during resistance switching mainly occurs between the Pt/WO 3−x interface and Pt electrode.
100(2012); http://dx.doi.org/10.1063/1.4726114View Description Hide Description
Compared to preparation on metal substrates, graphene synthesis on non-metalsurfaces is highly desirable to avoid the deleterious metallic effects in fabrication of electronic devices. However, study of graphene growth mechanism on non-metalsurfaces is rare and little understood. Here, we report that few-layers graphene films can be grown directly on silicon-on-insulator surface. Furthermore, the graphene growth mechanism on non-metalsurfaces is proposed as a surface reaction, adsorption, decomposition, and accumulation process.
Plasma plume effects on the conductivity of amorphous-LaAlO3/SrTiO3 interfaces grown by pulsed laser deposition in O2 and Ar100(2012); http://dx.doi.org/10.1063/1.4727905View Description Hide Description
Amorphous-LaAlO3/SrTiO3interfaces exhibit metallic conductivity similar to those found for the extensively studied crystalline-LaAlO3/SrTiO3interfaces. Here, we investigate the conductivity of the amorphous-LaAlO3/SrTiO3interfacesgrown in different pressures of O2 and Ar background gases. During the deposition, the LaAlO3 ablation plume is also studied, in situ, by fast photography and space-resolved optical emission spectroscopy. An interesting correlation between interfacial conductivity and kinetic energy of the Al atoms in the plume is observed: to assure conducting interfaces of amorphous-LaAlO3/SrTiO3, the kinetic energy of Al should be higher than 1 eV. Our findings add further insights on mechanisms leading to interfacial conductivity in SrTiO3-based oxide heterostructures.