Volume 101, Issue 10, 15 May 2007
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic structure and transport
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- interdisciplinary and general physics
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
101(2007); http://dx.doi.org/10.1063/1.2732412View Description Hide Description
Wave propagationcharacteristics of apertures were analyzed to explain the light transmission of metallic nanoapertures. Based on Maxwell’sequations, the wavedispersion relations of wave propagation modes in nanoapertures were derived. The resonance frequency shift of the aperture and the variation of the spot size are explained with the dispersion relations. The relationship between near-field and far-field light transmission power throughput and spot size is also shown with the wave mode change predicted by the dispersion relations.
Characterization of excitonic features in self-assembled InAs/GaAs quantum dot superlattice structures via surface photovoltage spectroscopy101(2007); http://dx.doi.org/10.1063/1.2733992View Description Hide Description
This work systematically investigates the influence of InAsgrowth conditions and superlattice parameters on the optical properties of InAs/GaAs quantum dot(QD)superlatticestructuresgrown by molecular beam epitaxy. Using surface photovoltage spectroscopy, one directly obtains the absorption spectra up to the highest confined QD levels at room temperature. Based on photoluminescence measurements at different excitation wavelengths, a feature below the fundamental transition is attributed to the transition from uncoupled dots in the bottom layers. The QD transition energy shift was found to be correlated with material intermixing, driven by enhanced strain strength with the increase of layer number and the decrease of spacer thickness, and the growth rate of InAs. A blueshift observed in low growth rate samples is indicative of greatly enhanced intermixing. For QDsuperlatticesgrown at a relatively high deposition rate of InAs, the material intermixing effect is gradually enhanced with increasing layer numbers, which will compensate the electronically coupled effect and will prevent further energy shift toward the lower energy side. The absorption wavelengths in 10- and 30-period InAs/GaAs QDsuperlattices with higher growth rates are near . The results suggest that these QDsuperlatticestructures can be used as promising active media for long-wavelength QD lasers operating at room temperature.
101(2007); http://dx.doi.org/10.1063/1.2732691View Description Hide Description
An approach named the defect-induced extension of photonic band gaps(PBGs) is proposed, in which a stratified periodic structure is combined with another stratified structure containing defect layers. For comparison, three structures composed of and alternating layers, perfect, five-defect and combined-photonic crystals (PCs), were fabricated by using an electron-beam evaporation system. The measured PBG width of combined-PCs extended to 370 nm at the normal incidence, nearly three times more than that of perfect ones, 130 nm, but a few ripples are observed within the PBG. In order to elucidate the origin of the ripples, the influence of the refractive-index contrast on the reflectance was analyzed theoretically. When the refractive-index contrast , the ripples are eliminated. More importantly, an overdoubled omnidirectional PBG is realized at , covering the visible region completely. This approach is considered to be based on the light localization and a heterostructure resulting from introducing the impurity band and compensating for it using PCs with a strictly periodic structure, in which the thickness of every layer remains unchanged. These characteristics distinguish it from other methods proposed before, with the fabrication much easier.
Single-photon source characterization with twin infrared-sensitive superconducting single-photon detectors101(2007); http://dx.doi.org/10.1063/1.2717582View Description Hide Description
We report on the high fidelity characterization, via spontaneous emission lifetime and measurements, of a cavity-coupled quantum dot single-photon source at using a pair of nanowire-based superconducting single-photon detectors (SSPDs). We analyze the suitability of the twin SSPD scheme reported here for the characterization of single-photon sources at telecommunications wavelengths (1310 and ).
Enhancement of upconverted luminescence in glass on increasing the temperature and concentration of beyond the critical limit101(2007); http://dx.doi.org/10.1063/1.2727451View Description Hide Description
Enhancement of upconverted luminescence in has been observed with increase in the concentration of on excitation with laser radiation. It is further noted that the upconversion is increased even above the quenching limit on increasing the temperature of the glass. The incident radiation excites indirectly via resulting in a strong blue emission from and green and red emissions from . The UV emission from at involves three near infrared photons when there is no in the glass but when is also present, this behaves as a two photon process. The lifetime of the level of in glasses containing only and also those containing both and has also been measured in different conditions.
101(2007); http://dx.doi.org/10.1063/1.2713942View Description Hide Description
In this study, the growth and properties of material and a nonpolar GaN-based light-emitting-diode (LED)structure on have been investigated. The material is grown by the Czochralski pulling technique and is used as a substrate for nonpolar nitride growth. An improved surface roughness can be obtained by a four-step polishing process. With subsequent nitridation treatment, a pure -plane GaN can be obtained. An electron microscope shows an abundance of cracks that are oriented parallel to the (001) and (100) planes of the substrate on the rear surface of GaN. The absence of the polarization-induced electric field of a GaN-based LEDstructure on was shown by using photoluminescence measurements. Therefore, this approach is promising to further increase the luminescence performance of GaN-based LEDs.
101(2007); http://dx.doi.org/10.1063/1.2730570View Description Hide Description
We show the realization of quantum cascade lasers(QCL)grown on -oriented InP substrates instead of the usual -oriented surface.Growth along orientation allows the excitation of the nonlinear susceptibility for the transverse magnetic wave produced by the QCL. Two devices were realized, at and wavelengths. They are compared with the same structuresgrown on the usual -oriented surface.Second harmonic generation is demonstrated in one of the devices.
101(2007); http://dx.doi.org/10.1063/1.2736342View Description Hide Description
Dynamics and state filling of the resonantly excited ground state in a strongly confined CdSequantum dot were investigated by degenerate pump-probe measurements. With increasing the electron-hole (e-h) pairs per dot the state filling and bleaching were observed with e-h pairs. While radiative recombination was dominant with a small number of the e-h pairs , the Auger-type recombination became significant near the bleaching number of e-h pairs . This result suggests that, for resonant excitation, either cold electron or hole of the ground state is scattered into the higher-energy states of a ZnS shell via Auger recombination process .
101(2007); http://dx.doi.org/10.1063/1.2435960View Description Hide Description
Semianalytical formulas have been used to model the pulse propagation dynamics of low gain free electron laseroscillators. Most of the scaling relations, including pulse propagation effects, are limited to the low gain regime. We discuss the problem of oscillator devices operating with small signal gain coefficients larger than 1. We comment on the relevant physical aspects and show that gain and saturated power can be reproduced by a set of simple formulas, analogous to those employed for the low gain case.
101(2007); http://dx.doi.org/10.1063/1.2734870View Description Hide Description
The anchoring energy and cell gap effects on liquid crystal response time is analyzed theoretically and validated experimentally. Analytical expressions are derived using two different approaches: effective cell gap and surface dynamic equation methods. Consistent results are deduced from these two approaches. A simplified equation also fits the experimental data well, where is the liquid crystal cell gap and is the exponent. Under two extreme (strong and weak) anchoring limits, the exponent approaches 2 and 1, respectively. This information is helpful for optimizing liquid crystal devices for display applications.
101(2007); http://dx.doi.org/10.1063/1.2736943View Description Hide Description
Interdiffusion in multiple quantum wells with well widths of 2 and , respectively, was investigated both experimentally and theoretically. Maximum blueshifts of 206 and in the photoluminescence spectra were observed. Secondary ion mass spectrometry showed that both In–Ga and N–As interdiffusions played key roles for the large blueshifts. The significant In–Ga interdiffusion occurred at while the N diffusion occurred at a temperature above . The theoretical results are in good agreement with the experimental observations.
101(2007); http://dx.doi.org/10.1063/1.2720095View Description Hide Description
Siliconnanoparticles synthesized in the gas phase are studied. From time-resolved photoluminescence measurements we determine, quantitatively, the size-dependence of the oscillator strength of the nanoparticles. We investigate experimentally the absorption and photoluminescence emission of nanoparticle ensembles with a broad size distribution. Using a model which accounts for size-effects in both oscillator strength and quantum-confinement, we are able to calculate absorption and emission spectra of ensemble samples. From these results we have determined, whether siliconnanoparticles should be regarded as indirect or direct semiconductors. Moreover, we systematically study the influence of the particle size-distribution on the optical spectra.
On the role of chemical reactions in initiating ultraviolet laser ablation in poly(methyl methacrylate)101(2007); http://dx.doi.org/10.1063/1.2740340View Description Hide Description
The role of chemical reactions is investigated versus the thermal and mechanical processes occurring in a polymer substrate during irradiation by a laser pulse and subsequent ablation. Molecular dynamics simulations with an embedded Monte Carlo based reaction scheme were used to study ultraviolet ablation of poly(methyl methacrylate) at . We discuss the onset of ablation, the mechanisms leading to ablation, and the role of stress relaxation of the polymer matrix during ablation. Laser induced heating and chemical decomposition of the polymer substrate are considered as ablation pathways. It is shown that heating the substrate can set off ablation via mechanical failure of the material only for very short laser pulses. For longer pulses, the mechanism of ejection is thermally driven limited by the critical number of bonds broken in the substrate. Alternatively, if the photon energy goes towards direct bond breaking, it initiates chemical reactions,polymer unzipping, and formation of gaseous products, leading to a nearly complete decomposition of the top layers of substrates. The ejection of small molecules has a hollowing out effect on the weakly connected substrates which can lead to lift-off of larger chunks. Excessive pressure buildup upon the creation of gaseous molecules does not lead to enhanced yield. The larger clusters are thermally ejected, and an entrainment of larger polymer fragments in gaseous molecules is not observed.
- PLASMAS AND ELECTRICAL DISCHARGES
Role of ambient gas and laser fluence in governing the dynamics of the plasma plumes produced by laser blow off of LiF–C thin film101(2007); http://dx.doi.org/10.1063/1.2732446View Description Hide Description
The time- and space-resolved emission profiles of and emission lines from the laser-blow-off plumes of a multilayered LiF–C thin film have been studied using spectroscopic technique. The evolution features were analyzed in different ambient environments ranging from high vacuum to of argon pressures and at various fluences of the ablating laser. During the evolution of the plume, a transition region was found to exist between 4 and . Here, the plume dynamics changed from free expansion to collisional regime, where the plume experienced viscous force of the medium. The enhancement observed in neutral lines, in comparison with ionic lines, is explained in terms of the yield difference in electron impact excitation and ionization processes. Substantial difference in the arrival time distribution of the plume species was observed for and lines at high ambient pressures. Three expansion models are invoked to explain the evolution of the plume in different ambient conditions. The laser fluence was found to control the ratio of ions and neutrals.
Density effect on proton acceleration from carbon-containing high-density thin foils irradiated by high-intensity laser pulses101(2007); http://dx.doi.org/10.1063/1.2730565View Description Hide Description
The acceleration of protons in dense plastic foils irradiated by ultrahigh intensity laser pulses is simulated using a two-dimensional hybridparticle-in-cell scheme. For the chosen parameters of the overdense foils of densities , 1, and and of an ultrahigh intensity laser pulse, our simulations illustrate that a high-density target is favorable to high collimation of the target-normal-sheath acceleration protons but less energy for a short acceleration time . In particular, the difference of strong local heating of the carbon ion for different plasma densities is clearly observed at both the front and rear surfaces of thin solid targets, suggesting that the effect of the density and composition of the targets are also important for correctly simulating energetic ion generation in ultraintense laser-solid interactions.
Two-dimensional electromagnetic model of a microwave plasma reactor operated by an axial injection torch101(2007); http://dx.doi.org/10.1063/1.2732508View Description Hide Description
This paper presents a two-dimensional electromagnetic model for a microwave (2.45 GHz) plasma reactor operated by an axial injection torch. The model solves Maxwell’sequations, adopting a harmonic time description and considering the collision dispersion features of the plasma. Perfect-conductor boundary conditions are satisfied at the reactor walls, and absorbing boundary conditions are used at the open end of the coaxial waveguide powering the system. Simulations yield the distribution of the electromagnetic fields and the average power absorbed by the system for a given spatial profile of the plasma density (tailored from previous experimental measurements), with maximum values in the range . Model results reveal that the system exhibits features similar to those of an air-filled, one-end-shorted circular metal waveguide, supporting evanescent or oscillatory solutions for radial dimensions below or above a critical radius, respectively. Results also show that the fractional average power absorbed by the plasma is strongly influenced by the system dimensions, which play a major role in defining the geometry pattern of the electromagnetic field distribution. Simulations are used to provide general guidelines for device optimization.
Effect of water on sulfur dioxide and nitrogen oxides removal from flue gas in a direct current corona discharge reactor101(2007); http://dx.doi.org/10.1063/1.2733762View Description Hide Description
A direct current(dc)corona discharge reactor composed of needle-plate electrodes in a glass container filled with flue gas was designed. To clarify the influence of water on discharge characteristics, water was introduced in the plasma reactor as electrode where plate electrode is immersed, under the application of dc voltage. Experiment results show that (1) corona wind forming between high-voltage needle electrode and water by corona discharge enhances the cleaning efficiency of flue gas due to the existence of water and the cleaning efficiency will increase with the increase of applied dc voltage within definite range and (2) both removal efficiencies of and increased in the presence of water, which reach up to 98% for , and about 85% for under suitable conditions. These results play an important role in flue gas cleanup research.
Optical emission spectroscopy characterization of oxygen plasma during degradation of Escherichia coli101(2007); http://dx.doi.org/10.1063/1.2732693View Description Hide Description
Optical emission spectroscopy was applied for plasma characterization during sterilization of substrates contaminated with bacteria. The amount of cells of Escherichia coli was carefully applied to glass substrates and exposed to oxygen plasmaglow discharge at different pressures between 30 and . Plasma was created in a glass discharge tube by an inductively coupled rf generator at the frequency of and output power of about . The electron temperature and plasma density were estimated with a double Langmuir probe. They were between 3 and and 2 and . Density of neutral oxygen atoms was measured with a catalytic probe, and was between 2 and . Optical emission spectroscopy was performed with a low resolution spectrometer. The emission from carbon monoxide and nitrogen molecules was used to monitor the evolution of bacteria degradation. Both signals expressed a well defined maximum corresponding to peak erosion of bacteria by plasma radicals. As the sterilization was accomplished, both CO and lines fell below the detection limit of the spectrometer. The bacteria degradation was also monitored by scanning electron microscope(SEM) and culturing. The SEM images corresponded well with the evolution of CO and lines so the optical emission spectroscopy found a reliable tool for monitoring the sterilization process.
Electrowetting on plasma-deposited fluorocarbon hydrophobic films for biofluid transport in microfluidics101(2007); http://dx.doi.org/10.1063/1.2735682View Description Hide Description
The present work focuses on the plasma deposition of fluorocarbon (FC) films on surfaces and the electrostatic control of their wettability (electrowetting). Such films can be employed for actuation of fluid transport in microfluidic devices, when deposited over patterned electrodes. Here, the deposition was performed using and the plasma parameters that permit the creation of films with optimized properties desirable for electrowetting were established. The wettability of the plasma-deposited surfaces was characterized by means of contact angle measurements (in the static and dynamic mode). The thickness of the depositedfilms was probed in situ by means of spectroscopic ellipsometry, while the surface roughness was provided by atomic force microscopy. These plasma-deposited FC films in combination with silicon nitride, a material of high dielectric constant, were used to create a dielectric structure that requires reduced voltages for successful electrowetting.Electrowetting experiments using protein solutions were conducted on such optimized dielectric structures and were compared with similar structures bearing commercial spin-coated Teflon® amorphous fluoropolymer (AF) film as the hydrophobic top layer. Our results show that plasma-deposited FC films have desirable electrowetting behavior and minimal proteinadsorption, a requirement for successful transport of biological solutions in “digital” microfluidics.
101(2007); http://dx.doi.org/10.1063/1.2714646View Description Hide Description
A rf microplasma jet working at atmospheric pressure has been characterized for Ar, He, and and mixtures. The microdischarge has a coaxial configuration, with a gap between the inner and outer electrodes of . The main flow runs through the gap of the coaxial structure, while the reactive gases are inserted through a capillary as inner electrode. The discharge is excited using a rf of , and rms voltages around and rms currents of are obtained. Electron densities around and gas temperatures lower than have been measured using optical emission spectroscopy for main flows of and inner capillary flows of . By adjusting the flows, the flow pattern prevents the mixing of the reactive species with the ambient air in the discharge region, so that no traces of air are found even when the microplasma is operated in an open atmosphere. This is shown in and plasmas, where no CO and CN species are present and the optical emission spectroscopy spectra are mainly dominated by CH and bands. The ratio of these two species follows different trends with the amount of precursor for and mixtures, showing the presence of distinct chemistries in each of them. In plasmas, species are produced mainly by electron impact dissociation of molecules, and the ratio is independent of the precursor amount. In mixtures, species are formed mainly by recombination of species through three-body reactions, so that the ratio depends on the amount of present in the mixture. All these properties make our microplasma design of great interest for applications such as thin film growth or surface treatment.