Volume 87, Issue 17, 24 October 2005
- lasers, optics, and optoelectronics
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
We demonstrate the determination of the vortex chirality using a nonlocal spin-valve measurement technique in a lateral spin valve consisting of a Permalloy (Py) disk in diameter and a Py wire in width. The vortex chirality is determined under two probe configurations; the disk as the current probe and the disk as the voltage probe. The vortex chirality is found to depend on the probe configuration because of the current-induced Oersted field.
- LASERS, OPTICS, AND OPTOELECTRONICS
87(2005); http://dx.doi.org/10.1063/1.2112196View Description Hide Description
This paper examines the thermodynamic limits of photovoltaicsolar energy conversion by fluorescent collectors. The maximum efficiency of a fluorescent collector corresponds to the Shockley–Queisser limit for a nonconcentrating solar cell with a single bandgapenergy. To achieve this efficiency, the collector requires a photonicstructure at its surface that acts as an omnidirectional spectral band stop filter. The large potential of photonicstructures for the efficiency enhancement of idealized as well as real fluorescent collectors is highlighted.
87(2005); http://dx.doi.org/10.1063/1.2112197View Description Hide Description
We report time-domain measurements of ultraslow light propagation in a semiconductor quantum-well structure using coherent population oscillation. Delays greater than 1 ns are achieved for an amplitude-modulated optical beam propagating through a 195-nm-long active region, corresponding to group velocities less than . Delays can be easily varied by adjusting the intensity of the control laser. The bandwidth is suitable to delay sub-GHz modulated optical signals.
87(2005); http://dx.doi.org/10.1063/1.2115097View Description Hide Description
We demonstrate self-fabrication of a submicrometer-sized void array in fused silica using a Ti:Sapphire femtosecond laser and a high 0.9 numerical aperture (NA) objective lens. The effect of the focusing conditions of NA, laser energy, and pulse number on the shape of the fabricatedvoid was investigated. The void has a linearly drawn shape in the direction of the laser irradiation when a single pulse is irradiated and an increasing number of incident pulses resulted in the break up of the long void into multiple spherical ones, leading to a periodically aligned void array. The void shape also varied with the depth of the focus point beneath the fused silica surface, because the amount of self-focusing has a significant effect on the generation of the voids. The void shape was narrower and longer when the laser pulse was focused with the higher NA (up to 0.9) objective lens in the deeper position (up to ) in the fused silica.
87(2005); http://dx.doi.org/10.1063/1.2115081View Description Hide Description
In this letter, we present a matching stage for reflection reduction based on the principle of gradual change to efficiently couple light to propagating modes of photonic crystals (PCs). Basic physical considerations in designing these matching stages are investigated and a systematic yet simple design procedure is suggested. We show that matching stages obtained using this method are wideband in frequency, have a wide acceptance angle, and are robust against fabrication imperfections. Therefore, they are the preferred choice in general-purpose matching stages to be used along with dispersion-based PC devices.
87(2005); http://dx.doi.org/10.1063/1.2117610View Description Hide Description
The development of a high-performance photorefractive polymercomposite operating at 1550 nm is reported. We show 40% internal diffraction efficiency with response time of 35 ms and a net gain of in four-wave mixing and two-beam coupling experiments, respectively. This is more than an order of magnitude improvement in the diffraction efficiency and net two beam coupling gain and two orders of magnitude in the response time than the previously reported photorefractive polymer operating at this technologically important wavelength. The improvement in photorefractive characteristics is accomplished by an enhanced orientation of the nonlinear optical chromophore in the present composite.
Experimental demonstration of Fano-type resonance in photoluminescence of one-dimensional photonic crystals87(2005); http://dx.doi.org/10.1063/1.2117611View Description Hide Description
We investigated the photoluminescence(PL) properties of one-dimensional photonic-crystal structures. PLspectra were measured from two opposite directions perpendicular to the sample. The cavity mode emission measured from the sample surface showed asymmetric spectral shape; measurements from the back side of the sample showed a symmetric spectral shape. The experimental spectra were analyzed by a simple model calculation based on the transfer matrix method. From the model calculation, it was found that the asymmetric shape observed in cavity mode emission is caused by Fano-type resonance which is the coupling effect between discrete emission from the ZnS:Mn cavity layer and continuous background emission from all ZnS:Mn layers except the cavity layer.
87(2005); http://dx.doi.org/10.1063/1.2084340View Description Hide Description
Integration of III-nitride electrical devices on the ferroelectric materiallithium niobate has been demonstrated. As a ferroelectric material,lithium niobate has a polarization which may provide excellent control of the polarity of III-nitrides. However, while high temperature, , thermal treatments produce atomically smooth surfaces, improving adhesion of GaN epitaxial layers on lithium niobate, repolarization of the substrate in local domains occurs. These effects result in multi domains of mixed polarization in , producing inversion domains in subsequent GaN epilayers. However, it is found that AlN buffer layers suppress inversion domains of III-nitrides. Therefore, two-dimensional electron gases in heterojunction structures are obtained. Herein, the demonstration of the monolithic integration of high power devices with ferroelectric materials presents possibilities to control modulators on compact optoelectronic/electronic chips.
87(2005); http://dx.doi.org/10.1063/1.2112199View Description Hide Description
We report on the nonlinear optical performance of sub-ZnOfilmsgrown by metal organic aerosol deposition on -plane sapphire substrates. These films display scale-like nanocrystalline structures. Layers of different crystallite sizes and shapes are studied. Both, x-ray diffractometry and the characteristic angular and polarization dependence of the second harmonic generation, indicate a strongly uniform -axis orientation of the crystallites. Using 35-fs laser pulses, we demonstrate much higher conversion efficiencies for ZnO layers than previously reported. The robust performance at normal incidence makes this device suitable for advanced pulse characterization techniques.
- STRUCTURAL, MECHANICAL, THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER
87(2005); http://dx.doi.org/10.1063/1.2108108View Description Hide Description
We report on the investigation of the local element distribution in as-grown (Ga,In)(N,As) quantum wells with high In and N contents by using low-loss electron energy-loss spectroscopy combined with dark-field transmission electron microscopy. The (Ga,In)(N,As) quantum wells were grown on GaAs(001) substrates at different growth temperatures by molecular-beam epitaxy. Lateral modulations on the nanometer scale were detected with reversal In and N distributions pointing to the existence of regions with a more favorable Ga–N and In–As bond configurations, respectively. These composition fluctuations are the driving force for the morphological instabilities at the interfaces. Lowering the growth temperature of the quantum well results in a more homogeneous element distribution of the quaternary compound. This result is discussed with regard to the influence of the epitaxial strain and cohesive bond energy on the alloy formation during epitaxialgrowth.
87(2005); http://dx.doi.org/10.1063/1.2112198View Description Hide Description
A process is reported for creating arbitrary patterns of sub-10-nm Ge islands on a Si(100) substrate by directed self-assembly. Carbon-based templates are created on Si substrates by electron-beam-induced deposition using high-resolution electron beam lithography. Ozone etching, followed by annealing in ultra-high vacuum, yields small SiC nucleation sites for subsequently depositedGe. Quantitative analysis of atomic force microscope images reveals templated Ge islands with mean diameter , averaging atoms per island, with controlled spacings as small as 35 nm, and 2 nm absolute positional accuracy. The nanostructures reported here may find use in end-of-scaling classical computing and single-electron devices and spin-based quantum computing architectures.
87(2005); http://dx.doi.org/10.1063/1.2056590View Description Hide Description
Exceptionally high thermoelectric figure of merit , has been reported for , which may involve the nanoscale microstructure. However, conflicting reports on the same materials claim only of 1 or less. Here we show that materials are multiphase on the scale of millimeters despite appearing homogeneous by x-ray diffraction and routine electron microscopy. Using a scanning Seebeck microprobe, we find significant variation of Seebeck coefficient (including both -type and -type behavior in the same sample) that can explain the discrepancy in reported . More homogeneous samples can be prepared with faster cooling rates.
87(2005); http://dx.doi.org/10.1063/1.2112188View Description Hide Description
In this work, we demonstrate that the image charges effect induced by the dielectric mismatch strongly modifies the electronic structure of quantum wells(QWs) in such a way that the ideal QW confinement model is no longer suitable for the description of these systems. Particularly, two different confinement regimes were observed for narrow and wide QWs. In the former, electrons, light and heavy holes are spatially localized in the same region. In wide QWs, heavy holes are confined in the interfacial regions due to the strong attraction of the image charges, which does not occur for electrons and light holes. As a consequence, optical transitions involving electrons and heavy holes become less efficient in wide QWs.
87(2005); http://dx.doi.org/10.1063/1.2112193View Description Hide Description
Anisotropic spin dynamics of two-dimensional electrons in strained (110) quantum wells(QWs) is investigated by a time-resolved Faraday rotation technique. Strong anisotropy of the relaxation time for the electron spins in parallel and perpendicular to the QWs is observed at 150 K as a result of the enhanced D'yakonov–Perel' (DP) spin relaxation mechanism. At 5 K, an anisotropic feature of the spin relaxation time is also observed in the presence of in-plane magnetic field, suggesting that the DP mechanism is effective for low-temperature spin relaxation.
87(2005); http://dx.doi.org/10.1063/1.2112200View Description Hide Description
The chemical stability of Si(111), terminated with bilayer AlSe and GaSe, upon exposure to atmosphere, and , was investigated with core-level and valence band photoelectron spectroscopy. Si(111):GaSe and Si(111):AlSe both form stable, unreconstructed surfaces with no states in the silicon energy gap; their atomic structures are nearly identical. However, similarities in surface electronic and atomic structure do not imply similar chemical passivity. While Si(111):GaSe is largely unaffected by the exposures, Si(111):AlSe reacts irreversibly with both pure and atmosphere, removing over of the Se and permanently destroying long-range order.
87(2005); http://dx.doi.org/10.1063/1.2112205View Description Hide Description
This letter presents the observation of second-harmonic generation in pure silica and germanium-doped glass samples after heat treatment at elevated temperatures followed by rapid quenching to room temperature. No applied voltage has been used and the samples do not contain crystals. The induced nonlinearity is located near the surfaces of the sample. The possible origin of the second-order nonlinearity is discussed.
87(2005); http://dx.doi.org/10.1063/1.2115072View Description Hide Description
Co-sputtered nickel silicide films were evaluated on thin layers of gate dielectrics.Work function values ranging from for Ni rich films to were observed at and were found to be a strong function of the Ni and Si ratio in the films. Phase analysis indicated the presence of different phases of for varying concentrations of Ni and Si. High-temperature characteristics, leakage, and change in equivalent oxide thickness values were also evaluated for selected conditions. Rutherford backscattering, x-ray diffraction,Auger electron spectroscopy and high-resolution transmission electron microscopy were used for material analyses.
Atomically flat aluminum-oxide barrier layers constituting magnetic tunnel junctions observed by in situ scanning tunneling microscopy87(2005); http://dx.doi.org/10.1063/1.2108121View Description Hide Description
Observation using in situscanning tunneling microscopy of the layers constituting a magnetic tunnel junction with a naturally oxidizedaluminum barrier layer revealed an extremely flat aluminum-oxide surface. It was clarified from line-scan images that the aluminum-oxide barrier layer has atomic steps. This flatness, which is surprising given that the aluminum-oxide film is amorphous, reduced electron scattering within the barrier, leading to momentum-dependent tunneling, which should enable the fabrication of advanced devices, such as spin-polarized resonant tunneling transistors.
87(2005); http://dx.doi.org/10.1063/1.2115079View Description Hide Description
Electroluminescence and photoluminescence of two fluorinated dyes emitting in the blue and in the green, blended with polyvinylcarbazole and an oxadiazole compound, are reported. Organic light-emitting diodes realized with about dye concentration show interesting performances. Excited state complexes reduce the effectiveness of energy transfer in the blue-emitting device while, for the green-emitting device, the main mechanism of exciton generation is based on charge trapping at the emissive dye.
87(2005); http://dx.doi.org/10.1063/1.2115082View Description Hide Description
Intense blue-upconversion in -codoped gallate–bismuth–lead glasses has been achieved under an excitation from a commercially available laser diode. Energy transfer processes and excited-state absorption account for the population of the emitting level of the . Although the addition of has enhanced the glass thermal stability, the phonon mode associated with vibration of has almost no influence on the blue-upconversion intensity and the radiative lifetime of level. The dependence of the phonon energy of the host on contributions from multiphonon decay on the fluorescence has been discussed. Significant enhancement of the blue-upconversion has also been observed in gallate–bismuth–lead glasses with the incorporation of content.
87(2005); http://dx.doi.org/10.1063/1.2117615View Description Hide Description
-axis-oriented thin films have been grown directly on glass (fused silica) substrate by pulsed laser deposition. Detailed microstructure analysis showed stacking faults abundant throughout the films. However, the Seebeck coefficient and resistivity of these films on glass substrate at room temperature were found comparable to those of the single-crystal samples. The presence of these structural defects could reduce thermal conductivity, and thus enhance the overall performance of cobaltate films to be potentially used in the thermoelectric devices.