Volume 110, Issue 10, 15 November 2011
Index of content:
- Lasers, Optics, and Optoelectronics
Microcavity effects in SiGe/Si heterogeneous nanostructures prepared by electrochemical anodization of SiGe/Si multiple quantum wells110(2011); http://dx.doi.org/10.1063/1.3653960View Description Hide Description
We present the systematic investigations of the microcavityeffects from SiGe/Si heterogeneous nanorods (HNRs) prepared by electrochemical anodization of SiGe/Si multiple quantum wells. Visible photoluminescence(PL) emission with narrow bandwidth is observed because of the wavelength selective effect of the microcavity. The resonance of the microcavity is confirmed by the temperature dependent PL measurement, which is consistent with the prediction from the thermo-optic effect. Furthermore, electroluminescence from the ITO/i-SiGe/Si HNR/n −-Si diode shows multiple peak emissions under low current density, which is in good agreement with the PL results.
Thermoelectric conversion via laser-induced voltage in highly textured polycrystalline Na x CoO2 ceramic110(2011); http://dx.doi.org/10.1063/1.3660781View Description Hide Description
We have studied and analyzed the laser-induced voltage effect in highly c-axis-oriented polycrystallineNa x CoO2. The textured and layered stacking Na x CoO2 (x ∼ 0.7) bulks were prepared by a solid-state reaction process. Under the irradiation on Na0.67CoO2 bulk surface with pulsed laser (λ = 248 nm), the induced voltage signals were observed on the inclined surface with rise time 30 ns–43 ns and peak voltage 200 mV–500 mV; the voltage peak values show a linear dependence of laser energy densities. The crystal grains orientation plays a critical role in voltage peak value whether in film or texture bulk. The transverse voltage signal brings the information of thermoelectric anisotropy. In Na x CoO2band structure, the Fermi surface is different in the ab plane and along the c axis, leading to anisotropy of Seebeck coefficient. Additionally, the artificial structure of the inclined surface for highly textured bulk enables us to obtain a transverse voltage on inclined surface. These results demonstrated the layered textured bulk has potential applications in waste-heat conversion via transverse thermoelectric effect.
110(2011); http://dx.doi.org/10.1063/1.3663527View Description Hide Description
Dynamics of the electron-hole plasma excited by the femtosecond optical pulse in wurtzite GaN/InN heterostructure is investigated by Monte Carlo simulations. The GaN/InN heterostructure for pulsed terahertz emission is suggested. The results of Monte Carlo simulations show that the power of terahertz emission from the GaN/InN heterostructure exceeds the power of terahertz emission from the surface of InN by one order of magnitude.
Mid-infrared pump-related electric-field domains in GaAs/(Al,Ga)As quantum-cascade structures for terahertz lasing without population inversion110(2011); http://dx.doi.org/10.1063/1.3660676View Description Hide Description
We investigate the effect of mid-infrared (MIR) pumping on the transport properties of GaAs/(Al,Ga)As terahertz (THz) quantum lasers (TQLs), which rely on quantum coherence effects of intersubband transitions. Aiming at THz lasing at elevated temperatures, we extend the concept of THz gain with and without population inversion of a single, MIR-pumped, electrically driven THz stage proposed by Waldmueller et al. [Phys. Rev. Lett. 99, 117401 (2007)] to an entire TQL. However, experiments using a CO2 as well as a free-electron laser and numerical simulations show that this resonant MIR pumping causes a negative differential conductivity (NDC) in addition to the NDC caused by sequential tunneling. Lasing of these TQLs is prevented by the formation of electric-field domains below the resonance field strength for gain of each single THz stage.
110(2011); http://dx.doi.org/10.1063/1.3660774View Description Hide Description
Three-dimensional finite-difference time-domain simulations are used to study the near- and far-field properties of plasmonic core–shell (CS) nanostructures of reduced symmetry. Special attention is given to silica core and gold shell nanoparticles by changing their geometry. For the simulated range of wavelengths (300–2100 nm) our calculations of the scattering and absorption efficiencies imply strong polarization sensitivity and are highly dependent on the size and geometry of the CS nanostructures. Strong enhancements of the exciting electric field associated with the excitations of nanoparticleplasmons are observed. The wavelength dependence of the scattering spectra and concentration of electromagnetic field in subwavelength volumes have a potential for biosensing and bioimaging.
Non-linear absorption of 1.3-μm wavelength femtosecond laser pulses focused inside semiconductors: Finite difference time domain-two temperature model combined computational study110(2011); http://dx.doi.org/10.1063/1.3662192View Description Hide Description
We present a theoretical model, which describes local energy deposition inside IR-transparent silicon and gallium arsenide with focused 1.3-μm wavelength femtosecond laser pulses. Our work relies on the ionization rate equation and two temperature model (TTM), as we simulate the non-linear propagation of focused femtosecond light pulses by using a 3D finite difference time domain method. We find a strong absorption dependence on the initial free electron density (doping concentration) that evidences the role of avalancheionization. Despite an influence of Kerr-type self-focusing at intensity required for non-linear absorption, we show the laser energy deposition remains confined when the focus position is moved down to 1-mm below the surface. Our simulation results are in agreement with the degree of control observed in a simple model experiment.
110(2011); http://dx.doi.org/10.1063/1.3663276View Description Hide Description
To clarify the degradation of confocality in laser Raman microscopy depth profiling (optical sectioning) and the influence of pinhole filtering on it, we investigate the confocal volume in detail based on Gaussian beam optics and scalar wave optics. Theoretical depth profiles of a homogeneous transparent sample for four different pinhole sizes, which are computed using the measured incident beam waist radius w 0 and only a few optical system specific parameters such as a numerical aperture (NA) and a focal length, show a good agreement with the corresponding measured depth profiles. The computed confocal volume demonstrates that the pinhole size affects the actual probe depth as well as the axial resolution and the total intensity loss.
110(2011); http://dx.doi.org/10.1063/1.3662956View Description Hide Description
We present a new method of optical trapping based on the intensity gradient that is created by boundary diffraction of light at a metalthin-film edge. The structure consists of an optically thick metal-film step formed on a semi-transparent thin-film-metal-coated glass substrate. While the underlying thin layer of metal serves the purpose of suppressing the thermophoretic effect, the metalfilm step is found to induce a highly localized intensity distribution of light around the edge via self-interference of an incident wave and its boundary diffraction wave. Two-dimensional (2D) optical trapping of micron-sized dielectric particles is experimentally demonstrated with a 100-nm-thick Aufilm edge formed on a 10-nm-thick-Cr-coated glass slide. For a 2-µm polystyrene sphere, ∼2-pN trapping force is measured at 30-mW incident power of a 1064-nm laser beam. Not involving surface plasmon fields, this thin-film edge trapping is polarization independent and can be easily incorporated into an on-chip microfluidic configuration.
- Plasmas and Electrical Discharges
110(2011); http://dx.doi.org/10.1063/1.3660690View Description Hide Description
Measurements and analysis of air breakdown processes and plasma production by focusing 193 nm, 300 mJ, 15 MW high power laser radiation inside a 6 cm diameter helical radio frequency (RF) coil are presented. Quantum resonant multi-photon ionization (REMPI) and collisional cascade laser ionization processes are exploited that have been shown to produce high-density (ne ∼ 7 × 1016/cm3) cylindrical seed plasmas at 760 Torr. Air breakdown in lower pressures (from 7–22 Torr), where REMPI is the dominant laser ionization process, is investigated using an UV 18 cm focal length lens, resulting in a laser flux of 5.5 GW/cm2 at the focal spot. The focused laser power absorption and associated shock wave produce seed plasmas for sustainment by the RF (5 kW incident power, 1.5 s) pulse. Measurements of the helical RF antenna load impedance in the inductive and capacitive coupling regimes are obtained by measuring the loaded antennareflection coefficient. A 105 GHz interferometer is used to measure the plasma electron density and collision frequency. Spectroscopic measurements of the plasma and comparison with the SPECAIR code are made to determine translational, rotational, and vibrational neutral temperatures and the associated neutral gas temperature. From this and the associated measurement of the gas pressure the electron temperature is obtained. Experiments show that the laser-formed seed plasma allows RF sustainment at higher initial air pressures (up to 22 Torr) than that obtained via RF-only initiation (<18 Torr) by means of a 0.3 J UV laser pulse.
110(2011); http://dx.doi.org/10.1063/1.3658249View Description Hide Description
Experimental and theoretical investigations for growth of silicon nanoparticles (4 to 14 nm) in radio frequency discharge were carried out. Growth processes were performed with gas mixtures of SiH4 and Ar in a plasma chemical reactor at low pressure. A distinctive feature of presented kinetic model of generation and growth of nanoparticles (compared to our earlier model) is its ability to investigate small “critical” dimensions of clusters, determining the rate of particle production and taking into account the influence of SiH2 and Si2Hm dimer radicals. The experiments in the present study were extended to high pressure (≥20 Pa) and discharge power (≥40 W). Model calculations were compared to experimental measurements, investigating the dimension of silicon nanoparticles as a function of time, discharge power, gas mixture, total pressure, and gas flow.
110(2011); http://dx.doi.org/10.1063/1.3656446View Description Hide Description
We present a non-stationary model proposed for high power impulse magnetron sputtering discharges, which is based on a global description of the plasma processes. The model takes into account a typical structure of magnetron discharges by dividing the plasma volume into two zones, the magnetically confined high-density zone above the target racetrack and the bulk plasma zone, where the transport of particles onto the substrate and the chamber walls dominates. The comparisons of the calculated data with measured results for distinct experimental conditions in two different high power impulse magnetron sputtering systems show a good agreement, suggesting that all relevant plasma processes were correctly incorporated into the model equations. The model can be used to gain a more detailed insight into the complicated processes in such types of discharges and to predict the influence of various process parameters on the deposition characteristics.
Multi-component non-stationary exponential distributions of the breakdown voltages and time delays in neon ramp breakdown experiments110(2011); http://dx.doi.org/10.1063/1.3660687View Description Hide Description
The concept of physically based distributions used in studies concerning gas electrical breakdowns is introduced in this paper. The non-stationary exponential distribution of the breakdown voltages and time delays with time dependent distribution parameter is theoretically derived based on physical grounds starting from a binomial distribution for electron occurrence in the interelectrode gap. The experimental distributions of breakdown voltages and time delays are obtained by applying linearly rising (ramp) voltage pulses to the discharge tube with a hard galvanic layer of gold on the cathode and modeled by multi-component non-stationary exponential distribution, as well as by a Weibull distribution for the sake of comparison. In order to fit the experimental data, the multi-component voltage/time dependent distribution parameter is introduced, where is electron yield (number of generated electrons in the interelectrode gap per second), and is breakdown probability (the probability of one electron to cause a breakdown). It is shown that multi-component non-stationary exponential distribution is suitable for modeling of the experimental data when time varying voltage pulses are applied to the discharge tube.
Field-emission-assisted approach to dry micro-electro-discharge machining of carbon-nanotube forests110(2011); http://dx.doi.org/10.1063/1.3663438View Description Hide Description
This work investigates dry micro-electro-discharge machining (μEDM) of vertically aligned carbon nanotube(CNT) forests that are used as cathodes in the process, as opposed to conventional μEDM where the material to be machined forms the anode, toward achieving higher precision in the patterned microstructures. The new configuration with the reversed polarity is observed to generate higher discharge currents in the process, presumably due to effective field-emission from CNTs. This effect allows the process to be performed at very low discharge energies, approximately 80× smaller than in the conventional normal-polarity case, with the machining voltage and tolerance down to 10 V and 2.5 μm, respectively, enabling high-precision high-aspect-ratio micropatterning in the forests. The new approach is also demonstrated to make the process faster, cleaner, and more stable than conventional processing. Spectroscopic analyses of the forests processed by reverse μEDM show no evidence of significant crystalline deterioration or contamination in the CNTs.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
Guided propagation of surface acoustic waves and piezoelectric field enhancement in ZnO/GaAs systems110(2011); http://dx.doi.org/10.1063/1.3660215View Description Hide Description
The characteristics and dispersion of the distinct surface acoustic waves(SAWs) propagating in ZnO/GaAs heterostructures have been studied experimentally and theoretically. Besides the Rayleigh mode, strong Sezawa modes, which propagate confined in the overlayer, arise due to the smaller sound velocity in ZnO than in the substrate. The design parameters of the structure providing the strongest piezoelectric field at a given depth within the layered system for the different modes have been determined. The piezoelectric field of the Rayleigh mode is shown to be more than 10 times stronger at the interface region of the tailored ZnO/GaAs structure than at the surface region of the bulk GaAs, whereas the same comparison for the first Sezawa mode yields a factor of 2. This enhancement, together with the capacity of selecting waves with different piezoelectric and strain field depth profiles, will facilitate the development of SAW-modulated optoelectronic applications in GaAs-based systems.
Localization effects on recombination dynamics in InAs/InP self-assembled quantum wires emitting at 1.5 μm110(2011); http://dx.doi.org/10.1063/1.3660260View Description Hide Description
We have studied the temperature dependence of the photoluminescence of a single layer of InAs/InP(001) self-assembledquantum wires emitting at 1.5 μm. The non-radiative mechanisms responsible for the quenching of the emission band have been identified. The exciton dynamics has been investigated using time resolved photoluminescence measurements. The results have been explained through the interplay between free excitons and localized states (arising from size fluctuations in the quantum wires).
110(2011); http://dx.doi.org/10.1063/1.3660782View Description Hide Description
Cuprous oxide (Cu2O) thin films were grown via radio frequency sputteringdeposition at various temperatures. The dielectric functions and luminescence properties of the Cu2O thin films were measured using spectroscopic ellipsometry and photoluminescence, respectively. High-energy peaks were observed in the photoluminescence spectra. Several critical points (CPs) were found using second derivative spectra of the dielectric functions and the standard critical point model. The electronic band structure and the dielectric functions were calculated using density functional theory, and the CP energies were estimated to compare with the experimental data. We identified the high-energy photoluminescence peaks to quasi-direct transitions which arose from the granular structures of the Cu2O thin films.
A method to measure fracture toughness using indentation in REBa2Cu3O7-δ superconductor single crystals110(2011); http://dx.doi.org/10.1063/1.3662121View Description Hide Description
A method is proposed to estimate indentation fracture toughness based on the critical load for cracknucleation calculated by the derivative of the loading curve. This method is applied to REBa2Cu3O7-δsuperconductorsingle crystals based on xenotime, where is difficult to observe well defined radial crack lengths due to its lamellar structure. The indentation fracture toughness agrees well with that obtained from the traditional method based on radial crack propagation. The proposed procedure is an alternative technique to estimate indentation fracture toughness without the necessity of measuring radial cracks lengths while keeping the advantages of the traditional method.
110(2011); http://dx.doi.org/10.1063/1.3660523View Description Hide Description
Classical molecular dynamics methods have been used to investigate the atomic-scale dynamics of collisions between two Al and Ni crystals with rough surfaces. The crystals were approached along the direction perpendicular to the surfaces and simultaneously displaced along the direction parallel to them at relative velocities in the range between 1 and 10 nm ns−1. The mechanical stresses operating at collision determine a local deformation of Al and Ni lattices, accompanied by a significant temperature rise. As the Al melting point is reached, the Al crystal partially melts and Ni atoms start dissolving into the molten phase. The significant heat of mixing liberated further promotes the Al melting and the Nidissolution processes. In the absence of neighboring Al-Ni interfaces, the heat dissipation processes and the limited rate of Nidissolution gradually lead to the extinction of the reactive behavior. Conversely, the presence of Al-Ni interfaces in the vicinity of the Al-Ni one formed by collision permits the propagation of the high-temperature chemical reaction. It is shown that the ignition and propagation of the self-sustaining reaction is sensitive to the distance between Al-Ni interfaces and to their degree of chemical mixing.
High carrier concentration induced effects on the bowing parameter and the temperature dependence of the band gap of GaxIn1−xN110(2011); http://dx.doi.org/10.1063/1.3660692View Description Hide Description
The influence of intrinsic carrier concentration on the compositional and temperature dependence of the bandgap of GaxIn1−xN is investigated in nominally undoped samples with Ga fractions of x = 0.019, 0.062, 0.324, 0.52, and 0.56. Hall Effect results show that the free carrier density has a very weak temperature dependence and increases about a factor of 4, when the Ga composition increases from x = 0.019 to 0.56. The photoluminescence(PL) peak energy has also weak temperature dependence shifting to higher energies and the PL line shape becomes increasingly asymmetrical and broadens with increasing Ga composition. The observed characteristics of the PL spectra are explained in terms of the transitions from free electron to localized tail states and the high electron density induced many-body effects. The bowing parameter of GaxIn1−xN is obtained from the raw PL data as 2.5 eV. However, when the high carrier density induced effects are taken into account, it increases by about 14% to 2.9 eV. Furthermore, the temperature dependence of the PL peak becomes more pronounced and follows the expected temperature dependence of the bandgap variation.
First-principles investigations of elastic stability and electronic structure of cubic platinum carbide under pressure110(2011); http://dx.doi.org/10.1063/1.3662143View Description Hide Description
The authors have presented a detailed investigation on the phase stabilities and electronic properties of ideal stoichiometric platinum carbide (PtC) in the rock-salt (RS) and zinc-blende (ZB) structures under high pressure.Theoretical calculations are performed using the first-principles pseudopotential density functional method, in which we employ the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof form and local density approximation (LDA) of Ceperly and Adler parameterized by Perdew and Zunger together with plane-wave basis sets for expanding the periodic electron density. Through a series of tests, such as the total energy as a function of volume, the Gibbs free energy as a function of pressure, the P-Vequation of states, the elastic stabilities, and the electronic band structures of PtC with ZB and RS phases, we have confirmed that the recently synthesized compound PtC is crystallized in the ZB structure at zero pressure and that the RS structure is a high-pressure phase; the phase transition studied from the usual condition of equal enthalpies occurs at the pressures of 46.6 and 46.5 GPa for GGA and LDA calculations, respectively. Our conclusions are consistent with the theoretical prediction obtained from the full-potential linearized augmented plane-wave method, but are reversed with the DAC experimental results and other pseudopotential plane-wave theoretical results. Therefore, the experimental observation of the RS structure in PtC remains a puzzle, and our study indicates that further experimental and theoretical investigations need to be carried out to find the cause of the stability of the PtC.