Volume 101, Issue 10, 03 September 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:
We report a very simple, robust, and reliable on-chip fabrication method of a chemoresistive sensor based on siliconnanowires (NWs). Our method permits the use of nanowires without the need of their removal and transfer to a support different from the growth substrate. Our method, completely based on the silicon technology platform, exploits nanowires directly grown onto a selected area, over and between pre-patterned, interdigitated electrodes defined on oxidizedsilicon. The fabricated sensor is capable to detect NO2 down to a few ppb levels operating at room temperature. The sensor characteristics benefit of the presence of self-welded nanowires.
- PHOTONICS AND OPTOELECTRONICS
Controlling spatial distribution of thermal poling induced second-order optical nonlinearity with multilayered structures101(2012); http://dx.doi.org/10.1063/1.4749814View Description Hide Description
Fused silica plates with boron-doped silicate multilayered thin films are thermally poled to create second-order optical nonlinearity for nonlinear optical applications. Measurement results from second harmonic microscopy show that the spatial distribution of the induced nonlinearity peaks at the interfaces between different layers where there is an abrupt change in boron concentration. A simple model is suggested to simulate the nonlinearity-trapping effect at the interfaces. This nonlinearity-trapping effect is not restricted to interfaces with abrupt refractive index changes; results from silica plates with deposited multilayered pure silicathin films show that the interface between the silica substrate and the silicathin film also has the nonlinearity-trapping effect.
Impact of resonator geometry and its coupling with ground plane on ultrathin metamaterial perfect absorbers101(2012); http://dx.doi.org/10.1063/1.4749823View Description Hide Description
We investigate the impact of resonator geometry and its coupling with ground plane on the performance of metamaterial perfect absorbers. Using a cross-resonator as an example structure, we find that the absorber thickness can be further reduced through modifying the geometric dimensions of the resonators. Numerical simulations and theoretical calculations reveal that destructive interference of multiple reflections is responsible for the near-unity absorption. The near-field coupling between the resonator array and ground plane can be significant. When this coupling is taken into account, the theoretical results calculated using the interference model are in excellent agreement with experiments and numerical simulations.
101(2012); http://dx.doi.org/10.1063/1.4750062View Description Hide Description
We report a buried heterostructure vertical-cavity surface-emitting laser fabricated by epitaxial regrowth over an InGaAsquantum well gain medium. The regrowth technique enables microscale lateral confinement that preserves a high cavity quality factor (loaded ) and eliminates parasitic charging effects found in existing approaches. Under optimal spectral overlap between gain medium and cavity mode (achieved here at T = 40 K), lasing was obtained with an incident optical power as low as . The laser linewidth was found to be at .
101(2012); http://dx.doi.org/10.1063/1.4750071View Description Hide Description
We demonstrate an apodized focusing subwavelength grating (SWG) for suspended membrane waveguides on silicon-on-insulator. Finite-difference time-domain simulation predicts −1.7 dB coupling efficiency and a 3 dB bandwidth of ∼50 nm for the transverse-magnetic mode apodized SWG, which has 98% field overlap with propagation mode in the single mode fiber. A modified phase matching formula is proposed to design the focusing apodized SWG. Better than −3.0 dB coupling efficiency and a 3 dB optical bandwidth of ∼50 nm is demonstrated experimentally.
101(2012); http://dx.doi.org/10.1063/1.4750244View Description Hide Description
We report on high power terahertz (THz) emission from ErAs-enhanced superlattices for operation at 1550 nm. ErAs clusters act as efficient recombination centers. The optical power is distributed among a large, microstructured area in order to reduce the local optical intensity. A THz field strength of 0.7 V/cm (1 V/cm peak-to-peak) at 100 mW average optical power has been obtained, with emission up to about 4 THz in air, limited by the detection crystal used in the system.
101(2012); http://dx.doi.org/10.1063/1.4751336View Description Hide Description
High-quality-factor one-dimensional photonic crystal nanobeam cavities embedding GaN/AlN quantum dots are fabricated by an epilayer transfer method. The GaN/AlN quantum dots are first grown on SiC before being transferred to a Si substrate using a hydrogen silsesquioxane bonding layer and highly selective back-etching of the SiC. Nanobeam cavities are then fabricated by electron-beam lithography, dry etching, and HF underetching of the bonding layer. The resulting nanocavity exhibits quality factors larger than 6.3 × 103, the highest quality factor reported to date for an optically active group-III nitride photonic crystal nanocavity.
101(2012); http://dx.doi.org/10.1063/1.4750060View Description Hide Description
In this paper, we present the results of doping nematic liquid crystals (nLCs) with semiconductorquantum dots(QDs) where we discuss the CdS and CdSeQDs influence on the optical properties of investigated liquid crystal structures, i.e., diffraction efficiency enhancement. We also present the mathematical model describing the interaction between QDs and nLC molecules in the liquid crystal volume. The aim of this study is to improve the comprehension of the mechanisms of photorefractive effect observed in functionalized liquid crystal structures, which can lead to the development of more efficient holographic materials for dynamic data processing applications.
101(2012); http://dx.doi.org/10.1063/1.4748875View Description Hide Description
We propose two approaches for optical encryption based on computational ghost imaging. These methods have the capability of encoding ghost imagesreconstructed from gray-scale images and colored objects. We experimentally demonstrate our approaches under eavesdropping in two different setups, thereby proving the robustness and simplicity thereof for encryption compared with previous algorithms.
101(2012); http://dx.doi.org/10.1063/1.4748979View Description Hide Description
We present experimental observation of self-accelerating parabolic beams in quadratic nonlinear media. We show that the intensity peaks of the first and second harmonics are asynchronous with respect to one another in the two transverse coordinates. In addition, the two coupled harmonics have the same acceleration within and after the nonlinear medium. We also study the evolution of second harmonic accelerating beams inside the quadratic media and their correlation with theoretical beams.
101(2012); http://dx.doi.org/10.1063/1.4751850View Description Hide Description
We present a Raman fiber laser based on a chalcogenide glass microwire. The microwire is pumped in the C-band, and the resulting laser oscillates in the L-band. The power conversion efficiency of the device is ∼17%, and the laser threshold is 23.2 dBm (pulse energy = 4.65 pJ) in peak pump power. By extension, the device can be used for all-optical wavelength conversion of existing laser sources in the useful transmission window of chalcogenide, from the C-band up to a wavelength of ∼10 μm in the mid-infrared.
101(2012); http://dx.doi.org/10.1063/1.4751247View Description Hide Description
We demonstrate an experimental scheme to simultaneously stabilize the frequency and amplitude of a 3.5 THz third-order distributed feedback quantum cascade laser as a local oscillator. The frequency stabilization has been realized using a methanol absorption line, a power detector, and a proportional-integral-derivative (PID) loop. The amplitude stabilization of the incident power has been achieved using a swing-arm voice coil actuator as a fast optical attenuator, using the direct detection output of a superconducting mixer in combination with a 2nd PID loop. Improved Allan variance times of the entire receiver, as well as the heterodyne molecular spectra, are demonstrated.
- SURFACES AND INTERFACES
Size effect on the electron wind force for electromigration at the top metal-dielectric interface in nanoscale interconnects101(2012); http://dx.doi.org/10.1063/1.4750067View Description Hide Description
We report a classical model on the size effect of the electron wind force on a metal atom at the metal-dielectric interface in nanoscale interconnects. The effect is expressed as a size factor S for the effective charge Z*e. It is found that the size factor decreases with scaling due to reduced electron drift momentum as a result of scattering at interfaces and grain boundaries. The electron wind force on the metal atoms at the top metal-dielectric interface is enhanced by the interface scattering. This force enhancement is partially mitigated by the grain boundaryscattering.
101(2012); http://dx.doi.org/10.1063/1.4751109View Description Hide Description
For some spherical micro-particles on a vibrating flat substrate, in addition to their predicted rocking resonance frequencies, other resonance peaks at their doubles are observed. Here, a mathematical model and experimental results are presented for explaining this observation. It is determined that the frequency doubling is caused by nonlinear coupling between the out-of-plane and rocking modes of motion, and whirling-like motion that must be present for the presence of doubling effect. It is also found that the work-of-adhesion values extracted from the experimental resonance frequencies of a set of particles using the developed model are in good agreement with those reported in the literature.
- STRUCTURAL, MECHANICAL, OPTICAL, AND THERMODYNAMIC PROPERTIES OF ADVANCED MATERIALS
101(2012); http://dx.doi.org/10.1063/1.4750252View Description Hide Description
Electroplating bath additives become problematic when incorporated into electrochemically depositedcopper as impurities. The surface segregation of these impurities during spontaneous microstructural transformation was monitored by secondary ion mass spectrometry and found to be no different than the surface chemistry of the non-transforming plated samples or even sputtered copper. The results show that microstructural transformation of electroplated copper progresses despite the presence of impurities, suggesting foreign constituents are not in fact responsible for grain boundary pinning in these films.
Praseodymium valence determination in Lu2SiO5, Y2SiO5, and Lu3Al5O12 scintillators by x-ray absorption spectroscopy101(2012); http://dx.doi.org/10.1063/1.4748168View Description Hide Description
Until now, determination of both Pr3+ and Pr4+ at the low concentration levels commonly used in single crystal scintillators has proven to be difficult. We have found that it is possible to use synchrotron radiation and superconducting tunnel junction detectors to measure the X-ray absorption on the M4 and M5 edges of Pr to directly determine Pr3+ and Pr4+ in Lu2SiO5, Y2SiO5, and Lu3Al5O12. The spectra were measured at room temperature and compared to model samples of trivalent and tetravalent praseodymium, which provided clear signatures of the two charge states. The results show predominant Pr(III) in most samples.
101(2012); http://dx.doi.org/10.1063/1.4750140View Description Hide Description
The coupling of plasmonicresonances with the multiple phonon modes of a plasmonic supracrystal is studied. Ultrafast optical pump-probe spectroscopy with variable wavelength allows the selective detection of the breathing mode, the interparticle vibrations, and the vibration of the whole structure. Thanks to this selectivity, the characterization of the bonding strength between nanoparticles in different directions of the supracrystal is possible. The observation of these vibrations could be useful for the realization of future phonon modulated photonic and plasmonicdevices.
101(2012); http://dx.doi.org/10.1063/1.4751107View Description Hide Description
Laser driven dynamic compression experiments may, in materials with picosecond equilibration times, be possible with orders of magnitude less drive energy than currently used. As we show, the compression energy for geometrically similar experiments varies as the third power of the time scale of compression. For materials which equilibrate and can be characterized on picosecond time scales, the compression energy can be orders of magnitude smaller than the 1–100 ns scale time scale of many current experiments. The use of substantially lower compression energy is a great practical advantage in such experiments, potentially enabling the observation of extreme states of matter with table top scale laser systems.
Defect microstructural evolution in ion irradiated metallic nanofoils: Kinetic Monte Carlo simulation versus cluster dynamics modeling and in situ transmission electron microscopy experiments101(2012); http://dx.doi.org/10.1063/1.4748980View Description Hide Description
Understanding materials degradation under intense irradiation is important for the development of next generation nuclear power plants. Here we demonstrate that defect microstructural evolution in molybdenum nanofoils in situirradiated and observed on a transmission electron microscope can be reproduced with high fidelity using an object kinetic Monte Carlo (OKMC) simulation technique. Main characteristics of defect evolution predicted by OKMC, namely, defect density and size distribution as functions of foil thickness, ion fluence and flux, are in excellent agreement with those obtained from the in situexperiments and from previous continuum-based cluster dynamics modeling. The combination of advanced in situexperiments and high performance computer simulation/modeling is a unique tool to validate physical assumptions/mechanisms regarding materials response to irradiation, and to achieve the predictive power for materials stability and safety in nuclear facilities.
101(2012); http://dx.doi.org/10.1063/1.4751266View Description Hide Description
Single- and few-layer has recently gained attention as an interesting material system for opto-electronics. Here, we report on scanning Raman measurements on few-layer flakes prepared by exfoliation. We observe a Raman mode corresponding to a rigid shearing oscillation of adjacent layers. This mode appears at very low Raman shifts between 20 and . Its position strongly depends on the number of layers, which we independently determine using atomic force microscopy and investigation of the other characteristic Raman modes. Raman spectroscopy of the shear mode, therefore, is a useful tool to determine the number of layers for few-layer flakes.
101(2012); http://dx.doi.org/10.1063/1.4751338View Description Hide Description
The chalcopyrite semiconductors show large tolerances to deviations from stoichiometry by keeping the crystal structure. Such deviations always cause structural inhomogeneities and charge mismatches which influence the material properties. We studied the type and concentration of cationic point defects on Cu-poor CuGaSe2powders by the complementary use of neutrons and photons. It is demonstrated that the main existing defects in this Cu-poor wide gap semiconductor are copper-vacancies (VCu) and gallium on interstitial sites (Gai). The latter may explain why tailoring a highly efficient CuGaSe2solar cell is an even more challenging task than previously expected.