Volume 101, Issue 6, 15 March 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
- applied biophysics
- interdisciplinary and general physics
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
101(2007); http://dx.doi.org/10.1063/1.2711153View Description Hide Description
We present a detailed Monte Carlo simulation of electron transport incorporating both - and -valley states in GaAs-based quantum cascade lasers(QCLs). states are calculated using the method, while states are obtained within the effective mass framework. All the relevant electron-phonon, electron-electron, and intervalley scattering mechanisms are included. We investigate the -valley leakage in two equivalent-design GaAs/AlGaAs QCLs with 33% and 45% Al-barrier compositions. We find that the dominant -valley leakage path in both laser structures is through interstage intervalley scattering, leading to a parallel leakage current. The magnitude of depends on the temperature and occupation of the subbands, which are populated primarily by the same-stage scattering from the -continuum states. At 77 K, is small up to very high fields in both QCLs. However, at room temperature the 33% QCL shows a much higher than the 45% QCL even at low fields. The reason is that in the 33% QCL the coupling between the -localized states and the next-stage states is strong, which facilitates subsequent filling of the states through efficient intrastage scattering; with high -valley population and high temperature, efficient interstage scattering yields a large . In contrast, good localization of the states in the 45% QCL ultimately leads to low -valley leakage current up to high fields. Very good agreement with experiment is obtained at both cryogenic and room temperatures.
101(2007); http://dx.doi.org/10.1063/1.2710773View Description Hide Description
Poly(methyl)methacrylate was made photoconducting by molecular doping and the photoconductivity was investigated using modulated photocurrent technique. Low-temperature current-voltage measurements showed that the transport mechanism was thermally activated hopping. An experimental investigation of the photoconductivity action spectrum along with theoretical calculation enabled an estimation of the diffusion coefficient of the material. The presence of states with a distribution of lifetimes could be understood from the frequency response of the photocurrent. The photocurrent was due to the field-assisted dissociation of these states.
The imaging property of photoacoustic Fourier imaging and tomography using an acoustic lens imaging system101(2007); http://dx.doi.org/10.1063/1.2435062View Description Hide Description
The theory of photoacoustictomographyimaging using an acoustic lens imaging system has been investigated, and the photoacoustic (PA) Fourier imagingproperty of an acoustic lens is presented. The theoretical results show that an acoustic lens is able to realize two-dimensional PA Fourier imaging and the focal depth of the acoustic lens is . The PA signals with the advantage of time resolution can be resolved in time domain by time-resolved technique. Therefore, this long focal depth characteristic of the acoustic lens can be combined with time-resolved technique to realize PA tomographyimaging. It is demonstrated by experiments that this method is able to provide PA tomographyimages of biological tissue. The images are clear and contrast sharply with their backgrounds. The depth resolution is .
Third harmonic generation in undoped and X doped ZnO films (X: Ce, F, Er, Al, Sn) deposited by spray pyrolysis101(2007); http://dx.doi.org/10.1063/1.2711143View Description Hide Description
There is a current interest in research of wide band gap semiconductor materials for the purposes of third order nonlinear optical properties in view of optoelectronics applications. Materials for nonlinear optics should present important changes of nonlinear intensity, dependence on changes of nonlinear refractive index, short response time, and weak absorption losses. We report the results of the third order nonlinear optical susceptibilities of undoped and doped(cerium, fluorine, erbium, aluminum, and tin) zinc oxide films using the third harmonic generation technique at wavelength region in picoseconds regime. Thin films were grown on glass substrate by the spray pyrolysis technique at different temperatures of substrates and characterized by using the x-ray diffraction, scanning electron microscope, transmission, and photoluminescence. A strong third harmonic signal was obtained from the studied films with a good crystallinity and roughness. We have found that at high conductivity, there is a big conversion of the third harmonic signal at different dopants and at an appropriate concentration. We might say that the morphology and the crystalline quality of the films are the main factors for this high conversion.
101(2007); http://dx.doi.org/10.1063/1.2710765View Description Hide Description
Diffractivestructures such as pillar gratings are a promising way of coupling light into or out of thin semiconductor devices, for applications in thin film solar cells and light-emitting diodes. In this paper we show that the diffuse transmittance behavior of pillar gratings can be understood using the concept of grating mode interference and that the optimum heights of the grating and an estimate of the optimum period can be predicted with the effective index method. Furthermore, the method also gives good results for structures outside the range for which it was derived, including circular pillars and quasiperiodic structures. We also show that pillar gratings offer substantially improved performance over groove gratings for thin film silicon solar cells.
101(2007); http://dx.doi.org/10.1063/1.2433711View Description Hide Description
This paper investigates fabrication of functional thick metal films using simultaneous laser sintering and patterning along with the fundamental physical phenomena that govern the laser sintering process. The effects of the processing parameters on the quality of the fabricated components are investigated through a heat transfer analysis. We show that our process has potentials for metallization of microelectronics directly onto substrates whose melting temperatures are much lower than the temperature needed for sintering, which is only possible by properly controlling the temperature field during laser sintering. Optimum properties of the fabricated components are obtained when certain thermal conditions are produced during laser heating.
Microstructural and optical properties of thin film deposited by pulsed laser deposition for low loss waveguide applications101(2007); http://dx.doi.org/10.1063/1.2711774View Description Hide Description
Barium strontium titanate ( or BST) films were grown on substrate by means of a reactive pulsed laser deposition technique. The microstructural and surface morphology of BST films were characterized by x-ray diffraction, scanning electron microscopy, and atomic force microscopy. Filmsdeposited at were found to be amorphous and present wide cracks. The crystallization of the films was induced by annealing at . The BST filmsdeposited at present a cubic perovskite structure with a dense and smooth surface. The optical constants ( and as a function of wavelength) of the films were obtained using variable angle spectroscopic ellipsometry in the UV-vis-NIR regions. Microfabricated ridged waveguides were patterned using UV photolithography and high-density plasma etching. The propagation and loss characteristics at the telecommunication wavelength of were investigated using top-view scattering and Fabry-Pérot resonance methods. For specific ridge widths, we obtained single-mode propagation with relatively low losses , thereby demonstrating the strong potential of BST films for guided-wave components for advanced optical integrated systems.
Microstructure and mechanical property characterizations of metal foil after microscale laser dynamic forming101(2007); http://dx.doi.org/10.1063/1.2710334View Description Hide Description
This article discusses the feasibility of a new microforming technique—laser dynamic forming (LDF). LDF is a new hybrid forming process, combining the advantages of laser shock peening, and metal forming, with an ultra high strain rate forming utilizing laser shock waves. Experiments are conducted on copper foils to demonstrate this forming process. After the forming process, the mechanical and microstructure of the formed work piece will be characterized. Electron backscatter diffraction will be used to investigate the grain microstructure and misorientations quantitatively. The residual stress distributions will be measured using x-ray diffraction. The key factors for the improved formability of this high strain rate microforming process will be discussed in detail. With further development, LDF may become an important microforming technology for various materials.
101(2007); http://dx.doi.org/10.1063/1.2709574View Description Hide Description
Detailed polarized spectral properties of a () cleavage crystal grown by the Czochralski method have been investigated, including the absorption cross section, the emission cross-section, and the fluorescence lifetime. The evaluation of the laser potentiality shows that the crystal is promising for tunable and ultrashort laser generations around 1 μm. The room temperature quasi-CW laser operation of an unprocessed cleavage plate has been demonstrated with diode pumping. The results indicate that the crystal is a good candidate for microchip laser gain medium by cleavage technique.
101(2007); http://dx.doi.org/10.1063/1.2490382View Description Hide Description
Continuous wave and femtosecond mode-locked laser operation of in the tetragonal crystal host is demonstrated by pumping with a Ti:sapphire laser. Pumping with at , a maximum output power of was achieved at . The slope efficiency was in excess of 60%. The laser performance was similar for the two polarizations. By inserting a birefringent filter the output wavelength was tunable from . Pulses as short as with an average power of were generated by passive mode locking at a repetition rate of . These attractive laser properties of are related to the inhomogeneous broadening of the spectral features resulting from the local disorder of the host crystal. We report the spectroscopic properties of in the temperature range and the optical properties of the host at room temperature.
101(2007); http://dx.doi.org/10.1063/1.2709870View Description Hide Description
We consider hybrid free electron laser devices consisting of Cerenkov and undulator sections. We will show that they can in principle be used as segmented devices and also show the possibility of exploiting Cerenkov devices for the generation of nonlinear harmonic coherent power. We discuss both oscillator and amplifier schemes.
101(2007); http://dx.doi.org/10.1063/1.2713936View Description Hide Description
We report a single-beam heterodyne-scan method capable of measuring slow response nonlinear refraction of absorbers with excellent sensitivity. By imposing a small sinusoidal modulation on the incoming continuous wave laser beam and recording both the magnitude and phase of the intensity transmitted through a small aperture, we are able to eliminate parasitic linear effects and achieve a sensitivity good enough to determine the absorption coefficient of a 2 mm-thick water sample at 532 nm with moderate power.
- PLASMAS AND ELECTRICAL DISCHARGES
101(2007); http://dx.doi.org/10.1063/1.2710763View Description Hide Description
Measurements, simplified analyses, and two-dimensional numerical simulations with a fluid plasma model show that classical resistivity cannot account for the elevated electron temperatures and steep plasma potential gradients measured in a electric propulsion hollow cathode. The cathode consisted of a hollow tube with an diameter orifice and was operated with (SCCM denotes cubic centimeter per minute at STP) of xenon flow using two different anode geometries: a segmented cone and a circular flat plate. The numerical simulations show that classical resistivity yields as much as four times colder electron temperatures compared to the measured values in the orifice and near-plume regions of the cathode. Classical transport and Ohm’s law also predict exceedingly high electron-ion relative drift speeds compared to the electron thermal speed . It is found that the addition of anomalous resistivity based on existing growth rate formulas for electron-ion streaming instabilities improves qualitatively the comparison between the numerical results and the time-averaged measurements. Simplified analyses that have been based largely on the axial measurements support the conclusion that additional resistivity is required in Ohm’s law to explain the measurements. The combined results from the two-dimensional simulations and the analyses bound the range of enhanced resistivity to be 3–100 times the classical value.
101(2007); http://dx.doi.org/10.1063/1.2711811View Description Hide Description
A fluid model to prescribe microwaveplasma is proposed. The conservation equations for electron are reduced to only one differential equation. Therefore, the computational cost is also reduced as small as possible. One may extend functions of existing program for vacuum field analysis to microwaveplasma simulator by coupling this differential equation with Maxwell equations. While the model is quite simple, this model can prescribe distributions of electron temperature as well as the number density, whereas effects of inelastic collisions with ions and neutrals are taken into account. Test calculation is carried out in three dimensional system and well reproduces experimental observation and prediction obtained by other model.
101(2007); http://dx.doi.org/10.1063/1.2709524View Description Hide Description
A cylindrical Langmuir probe in a low-density, collisionless plasma (density, electron temperature 0.2 eV) has been scanned radially through the presheath region to determine the effect of distance from the axis on the current-voltage characteristic. In the ion part of the probe characteristic, the collected ion current decreases with distance from the axis as a consequence of ion acceleration by the presheath. The part of the ion current from charge-exchange collisions remains relatively constant. In the electron part of the probe characteristic, the collected current decreases with distance from the axis, consistent with the existence of a small potential barrier from the presheath between the axis and the probe. The electron temperature from the slope of the probe characteristic is nearly constant across the presheath region. The plasma potential from the Langmuir probecharacteristic is also nearly constant, indicating that the probe analysis finds the plasma potential on the axis, even when the probe is not on the axis. The plasma potential from an emissive probe shows an approximately parabolic profile. The plasma potential from the emissive probe and the Boltzmann relation give nearly the same density profile in the presheath that is obtained from the Langmuir probe data.
101(2007); http://dx.doi.org/10.1063/1.2697259View Description Hide Description
A medium-energy synchrotron capable of accelerating all-ion species is proposed. The accelerator employs a strong focusing lattice for ion-beam guiding and induction acceleration for acceleration and longitudinal capture, which is driven by a switching power supply. All ions, including cluster ions in their possible and arbitrary charge state, are accelerated in a single accelerator. Since the switching power supply employing solid-state switching elements is energized by a trigger signal, which is generated from a bunch monitor signal produced by a circulating ion bunch, the induction acceleration always synchronizes with the bunch circulation. This feature enables the realization of an almost injector-free synchrotron.
101(2007); http://dx.doi.org/10.1063/1.2433746View Description Hide Description
Plasmadoping of semiconductors is being investigated for low energy ion implantation to form ultrashallow junctions. In plasmadoping, ions are extracted from a quasicontinuous plasma using a pulsed bias on the substrate. Plasma-based implantation techniques have the potential for higher throughput than those attainable with conventional accelerator beamlines due to the higher current densities possible with plasma sources. In this work, results from a computational investigation of plasma sources for doping of semiconductors will be discussed. An inductively coupled plasma(ICP) was used to generate ions at pressures of a few to tens of millitorr. A pulsed bias up to having lengths of tens of microseconds was applied to the substrate to accelerate the ions. Results are presented for gas mixtures which serve as surrogates for the mixtures that would provide boron doping. The consequences of bias voltage waveform, ICP power, operating pressure, and aspect ratio of the reactor on discharge characteristics and ion energy and angular distributions (IEADs) to the substrate will be discussed. The shape of the bias waveform has important consequences on the IEADs not only because of the transit times of the ions but also due to the instabilities that may be launched into the plasma. The aspect ratio of the reactor influences the angular uniformity of the IEADs, particularly when using large biases.
101(2007); http://dx.doi.org/10.1063/1.2496368View Description Hide Description
Emission spectroscopy was used for the diagnostic of a large-scale, slot antenna excited microwaveplasma source operating in pure hydrogen and in helium-hydrogen and argon-hydrogen mixtures at low pressures and microwave power ranging from . No evidence was found for excessive broadening of the line under the present operating conditions, even though this line was found to be broader than the helium singlet line at . The Dopplertemperatures corresponding to this helium line are close to the rotational temperatures determined from the branch of the Fulcher- band under the same conditions. The kinetic temperature of H atoms corresponding to the Doppler broadening of the line varies spatially between 3100 and (95% He–5% , , and ) and between 3350 and (95% Ar–5% , , and ) keeping constant the mixture composition, pressure, and microwave power. The rotational temperature in argon-hydrogen mixtures varies from under the same conditions. Therefore, the results presented here indicate that the kinetic temperature of H atoms is higher than the background gas temperature.
Influence of the filling gas on plasma focus assisted diamondlike carbon coating at room temperature101(2007); http://dx.doi.org/10.1063/1.2713086View Description Hide Description
Amorphous diamondlike carbonfilms (up to thick) are deposited by dense plasma focus system using nitrogen, methane, and neon gases. The peak intensity ratio of the -band to -band and the -peak position in the Raman spectra are used to characterize the filmsdeposited on silicon substrate placed at different axial and angular positions with respect to focus axis. Stress and cluster size present in the films are discussed with shift in -peak position, since higher content and residual compressive growth stress shifts the -peak position to higher frequencies. The peak intensity ratio is related to ratio to estimate the fourfold carbon networks. Thickness values obtained by cross-sectional scanning electron microscopy point toward the high film deposition rates. X-ray diffraction spectra verify the deposition of amorphous carbon a-C films which identifies no crystalline peak.
A comparison between detailed and configuration-averaged collisional-radiative codes applied to nonlocal thermal equilibrium plasmas101(2007); http://dx.doi.org/10.1063/1.2713085View Description Hide Description
A collisional-radiative model describing nonlocal-thermodynamic-equilibrium plasmas is developed. It is based on the HULLAC (Hebrew University Lawrence Livermore Atomic Code) suite for the transitions rates, in the zero-temperature radiation field hypothesis. Two variants of the model are presented: the first one is configuration averaged, while the second one is a detailed level version. Comparisons are made between them in the case of a carbonplasma; they show that the configuration-averaged code gives correct results for an electronic temperature (or higher) but fails at lower temperatures such as . The validity of the configuration-averaged approximation is discussed: the intuitive criterion requiring that the average configuration-energy dispersion must be less than the electron thermal energy turns out to be a necessary but far from sufficient condition. Another condition based on the resolution of a modified rate-equation system is proposed. Its efficiency is emphasized in the case of low-temperature plasmas. Finally, it is shown that near-threshold autoionization cascade processes may induce a severe failure of the configuration-average formalism.