Volume 101, Issue 1, 01 January 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/10.1063/1.2403239View Description Hide Description
A model is proposed for describing two-dimensional diffusion of Ti into an initially congruent crystal under a Li-enriched atmosphere created by a mixed two-phase ( and ) powder at elevated temperature [vapor transport equilibration (VTE)]. The influence of VTE treatment on Ti diffusivity is considered in the model. To solve the model, four key input parameters including Li-concentration-dependent Li and Ti diffusivities and two switching times and were determined. Prior to solve the Ti-diffusion model, a one-dimensional VTE model is solved at first to obtain the dynamic concentration depth profile. Both the Li-diffusion and Ti-diffusion models were solved by using finite difference method. Based on secondary-ion-mass spectrometry analysis, the validity of the VTE and Ti-diffusion models as well as the numerical method employed are confirmed. After that, diffusion at of an -wide, -thick Ti strip defined on the surface of a -cut or an -cut substrate was simulated for the VTE duration up to . Based on the numerical results, the Ti-(Li-)diffusion characteristics are discussed in the aspects of (1) the relation of depth and width profile function of Ti concentration to the VTE duration, (2) the substrate cut effect on both the Ti and Li diffusions, (3) the relation of the Ti-concentration depth and half-width to the VTE duration, and (4) the VTE duration dependence of the mean ratio within the Ti-diffused layer.
Propagation of terahertz radiation through random structures: An alternative theoretical approach and experimental validation101(2007); http://dx.doi.org/10.1063/1.2403860View Description Hide Description
A model describing the propagation of terahertz frequency radiation through inhomogeneous materials is proposed. In such materials (e.g., powders or clothing), the size of the scattering centers, their separation, and the wavelength of the radiation are all commensurate. A phase distribution function is used to model the optical properties of a randomly structured transmitting layer. The predictions of the model are compared with exact (Mie) theory for isolated spherical scatterers and with previously published experimental data. Measurements of the transmission of terahertz radiation through a variety of samples in order to validate the present model are also reported. These include arrays of cylinders, textiles, powders, and glass balls. Overall, satisfactory agreement between the experimental data and theoretical predictions is obtained.
Giant Goos-Hänchen displacement enhanced by dielectric film in frustrated total internal reflection configuration101(2007); http://dx.doi.org/10.1063/1.2405737View Description Hide Description
It is predicted that the Goos-Hänchen displacement in the usual frustrated total internal reflection configuration can be resonantly enhanced greatly by coating a dielectric thin film onto the surface of the first prism when the angle of incidence is larger than the critical angle for total reflection at the prism-vacuum interface and is smaller than but close to the critical angle for total reflection at the prism-film interface. Theoretical analysis shows that the displacement of transmitted beam is about half the displacement of reflected beam in the thick limit of the vacuum gap between the two prisms. This is to be compared with the relation in the usual symmetric double-prism configuration that the displacement of transmitted beam is equal to the displacement of reflected beam. Numerical simulations for a Gaussian incident beam of waist width of 100 wavelengths reveal that when the dielectric thin film is of the order of wavelength in thickness, both the reflected and transmitted beams maintain well the shape of the incident beam in the thick limit of the vacuum gap. So largely enhanced displacements would lead to applications in optical devices and integrated optics.
101(2007); http://dx.doi.org/10.1063/1.2402588View Description Hide Description
A self-consistent nonparabolic calculation of the band structure and intersubband absorption in a V-groove-quantum-wire are presented and analyzed with respect to doping concentration, geometry, and temperature. A comparison with a parabolic flatband model shows that both the self-consistency and the nonparabolicity considerably affect subband edges and the intersubband absorption which are studied when both effects are taken into account. The absorption spectra exhibit a prominent peak in the terahertz region (wavelength range ) for the polarization in the direction of the weak confinement , while for polarization in the direction of the strong confinement , it shows two groups of peaks, one in a slightly lower wavelength range than for the polarization, and the second for Technological parameters considered in our analysis can be used to tailor and adjust the absorption spectra for various applications in the terahertz spectral range.
101(2007); http://dx.doi.org/10.1063/1.2405236View Description Hide Description
Simultaneous self-mode locking and self- switching in a diode-pumped intracavity frequency doubled green laser operating at is presented. Nearly 100% modulation depth for self-mode-locked green pulses has been achieved and a maximum average output power of was obtained at an incident pump power of . By using a hyperbolic secant function method, considering the Gaussian spatial distribution of the intracavity photon density and the influences of continuous pump rate, as well as the stimulated radiation lifetime of the active medium and the excited-state lifetime of the saturable absorber, a rate equation model was introduced to reconstruct the -switched and mode-locked lasers. The theoretical results are in good agreement with the experimental results, and the width of the self-mode-locked green pulse was estimated to be about .
Polarization-dependent effects of refractive index change associated with photoisomerization investigated with Z-scan technique101(2007); http://dx.doi.org/10.1063/1.2402582View Description Hide Description
The Z-scan technique is used to investigate the effects of refractive index change (RIC) associated with photoisomerization for two cases: in which the sample is pumped and probed by the same elliptically polarized light, and in which the sample is pumped by a linearly polarized light and probed by another linearly polarized light. Both experimental and theoretical results show that for the first case, this effect depends upon the polarization of the light; that is, when the polarization changes continuously from circularly polarization to linearly polarization, the effects of RIC becomes stronger. For the second case, the effect is similar to the effects of saturable Kerr effect, whose nonlinear coefficient is measured.
101(2007); http://dx.doi.org/10.1063/1.2405738View Description Hide Description
We directly measure the gain and threshold characteristics of three quantum dot laser structures that are identical except for the level of modulation doping. The maximum modal gain increases at fixed quasi-Fermi level separation as the nominal number of acceptors increases from 0 to 15 to 50 per dot. These results are consistent with a simple model where the available electrons and holes are distributed over the dot, wetting layer, and quantum well states according to Fermi-Dirac statistics. The nonradiative recombination rate at fixed quasi-Fermi level separation is also higher for the -doped samples leading to little increase in the gain that can be achieved at a fixed current density. However, we demonstrate that in other similar samples, where the difference in the measured nonradiative recombination is less pronounced, doping can lead to a higher modal gain at a fixed current density.
Theoretical study on high-speed modulation of Fabry-Pérot and distributed-feedback quantum-dot lasers: -factor-limited bandwidth and eye diagrams101(2007); http://dx.doi.org/10.1063/1.2407259View Description Hide Description
This paper presents a theoretical study of the high-speed modulation response of Fabry-Pérot (FP) and distributed-feedback (DFB)quantum-dot lasers based on the rate equation models, making reference to available experimental data. We show that the -factor-limited maximum modulation bandwidth increases with the maximum optical gain and that there is an optimum cavity loss to maximize the bandwidth at a given maximum gain, enabling us to design the bandwidth of FP lasers as well as DFB lasers with and without a phase shift. We present modulation wave forms of FP quantum-dot lasers to indicate that the maximum modal gain of is sufficient for eye opening, which explains the recent success of modulation of the quantum-dot laser with ten dot layers in the active region having the maximum modal gain of . We show a design for low-driving-current operation by shortening the cavity length with the optimum cavity loss maintained by the high-reflectivity coating.
Achieving accuracy in spectroradiometric measurements of temperature in the laser-heated diamond anvil cell: Diamond is an optical component101(2007); http://dx.doi.org/10.1063/1.2402587View Description Hide Description
We present theoretical calculations of the optical effects of the diamond anvil on the accuracy of spectroradiometric temperature measurements made with the laser-heated diamond anvil cell. Considering the dual effects of wavelength-dependent index of refraction and wavelength-dependent absorbance, we find that systematic errors can be minimized by using low numerical aperture optics and by characterizing wavelength-dependent absorbance for each anvil as part of system response measurement. Quantitatively, failure to observe these guidelines can lead to systematic errors of hundreds of degrees. This physical effect may be one part of the origin of the discrepancy of experimental measurements of the melting temperature of iron at megabar pressures.
101(2007); http://dx.doi.org/10.1063/1.2409311View Description Hide Description
Emission characteristics from GaAlAs–GaAs–GaAlAs Gunn devices of different lengths placed in Fabry-Pérot cavities have been investigated. The emission from the device of length of is stimulated in nature, whereas for the other two devices of lengths of 100 and , the emission is spontaneous. Lasing action in such devices, when biased above the threshold of negative differential resistance, arises from the band to band recombination of impact-ionized nonequilibrium electron hole pairs created within the propagating high field Gunn domains. Quantitative results evaluated using nonequilibrium semiconductor-statistics show that reduction in the length of the device leads to (a) a shift of the gain peak towards higher energies and (b) an increase in the average nonequilibrium carrier temperature. As a result, lasing in very short devices is inhibited and only ultra-bright-spontaneous emission is observed.
101(2007); http://dx.doi.org/10.1063/1.2426916View Description Hide Description
Transmission studies for multiple heterostructures consisting of two kinds of single-negative materials inserted with defects are presented. The results show that multiple-channeled filters can be obtained by adjusting the period number and thicknesses of defects. These structures provide an excellent way to select useful multiple-channeled optical signals from a stop gap, and it is useful in optical device applications.
Modeling and characterization of quantum dot lasers grown using metal organic chemical vapor deposition101(2007); http://dx.doi.org/10.1063/1.2409612View Description Hide Description
We report on the lasing characteristics of three- and five-stack quantum dot(QD) lasers grown by metal organic chemical vapor deposition. By increasing the number of stacked dot layers to 5, lasing was achieved from the ground state at for device lengths as short as (no reflectivity coatings). The unamplified spontaneous emission and ratio as a function of injection current were also investigated. While the five-stack QD lasers behaved as expected with ratios of prior to lasing, the three-stack QD lasers, which lased from the excited state, exhibited -ratio values as high as 4. A simple model was developed and indicated that high ratios can be generated by three nonradiative recombination pathways: (i) high monomolecular recombination within the wetting layer, (ii) Auger recombination involving carriers within the QDs (“unmixed” Auger), and (iii) Auger recombination involving both the QD and wetting layer states (“mixed” Auger), which dominate once the excited and wetting layer states become populated.
- PLASMAS AND ELECTRICAL DISCHARGES
101(2007); http://dx.doi.org/10.1063/1.2404583View Description Hide Description
The discharge voltage was measured for 15 different metallic target materials at constant current before and after plasmaoxidation in order to understand its behavior during reactive magnetron sputtering.Plasmaoxidation of the target surface was achieved by sputtering the target in pure oxygen. The discharge voltage measured in pure argon is characteristic for each kind of metallic target and is related to the ion induced secondary electron emission (ISEE) coefficient of the target material. Based on this relation a value for the ISEE coefficient of the oxidized target surface can be calculated. Two distinct groups can be discerned: for one group the ISEE coefficient of the oxidized target surface is larger than the ISEE coefficient of the metal, while the opposite behavior is noticed for the second group. This difference seems to find its origin in the reduction behavior of the oxides under ion bombardment, since the ISEE coefficient of the oxide can be related to the simulated degree of reduction of the oxide. It is shown that the ISEE coefficient of the reduced oxides decreases with increasing oxygen content in the target. This is confirmed experimentally by sputtering in pure argon reduced titanium oxide targets with a known composition.
Local retarding field for ions towards a positively biased substrate in plasma and its application to soft ion-bombardment processing101(2007); http://dx.doi.org/10.1063/1.2402972View Description Hide Description
A potential rise acting as a local retarding field for drifting ions has been observed in front of a positively biased electron-absorbing substrate in a downstream, electron cyclotron resonance plasma. Increasing positive substrate bias shifted the plasma potential upward and decreased the sheath potential to a certain minimum depending upon pressure , to satisfy a current balance between the substrate holder and the surrounding wall. For the minimal sheath potential, the incident ion flux to the substrate was shown to decrease with increasing bias due to prevention of ion arrival by the retarding field. Soft ion bombardment at the minimal sheath potential in a hydrogen plasma was then used to etch silicon wafer surfaces as in vacuo pretreatment for nanocrystalline diamonddeposition. The highest diamond particle density of the order of was finally obtained on a clean and smooth surface with minimal damage formed by low energy , high flux ion irradiation.
101(2007); http://dx.doi.org/10.1063/1.2403965View Description Hide Description
Time integrated measurements of the angular distributions of fusion products and x rays in a small dense plasma focus machine are made inside the discharge chamber, using passive detectors. The machine is operated at with a stored energy of and a deuterium filling pressure of . Distributions of protons and neutrons are measured with CR-39 Lantrack® nuclear track detectors, on chips, thick. A set of detectors was placed on a semicircular Teflon® holder, away from the plasma column, and covered with Al filters, thus eliminating tritium and helium-3 ions, but not protons and neutrons. A second set was placed on the opposite side of the holder, eliminating protons. The angular distribution of x rays is also studied within the chamber with TLD-200 dosimeters. While the neutronangular distributions can be fitted by Gaussian curves mounted on constant pedestals and the proton distributions are strongly peaked, falling rapidly after , the x-ray distributions show two maxima around the axis, presumably as a result of the collision of a collimated electron beam against the inner electrode, along the axis.
101(2007); http://dx.doi.org/10.1063/1.2401659View Description Hide Description
The nickel atom density was measured in an inductively coupled argon plasma with an internal Ni coil, as a function of pressure and power, using optical absorption spectroscopy.Nickel atoms were sputtered from the coil and from a separate Ni target under optional target bias. A fraction of the atoms was ionized in the high-density plasma. The gas temperature was determined by analyzing the rovibrational spectra of the second positive system of nitrogen actinometer gas. The electron density was determined by optical emission spectroscopy in combination with a global model. For a pressure of and coil power of , the Ni atom density ranged from , increasing strongly with pressure. The Ni atom density first increased with power but saturated at high power levels. The measuredNi atom density agreed fairly well with the predictions of a global model, in particular, at the higher pressures. The model also predicted that the ion density greatly increased at higher powers and pressures. Applying bias on the target electrode increased the Ni atom density by 60%.
Effect of radical species density and ion bombardment during ashing of extreme ultralow- interlevel dielectric materials101(2007); http://dx.doi.org/10.1063/1.2405123View Description Hide Description
The significance of ion impact and radical species density on ash-induced modification of an extreme ultralow- interlevel dielectric (ILD) material in a patterned single damascene structure exposed to and dual frequency capacitive discharges is determined by combining plasma diagnostics, modeling of the ion angular distribution function, and material characterization such as angle resolved x-ray photoelectron spectroscopy. Radical species density was determined by optical emission actinometry under the same conditions and in the same reactor in a previous study by the present authors. ILD modification is observed and correlated with changes in the plasma for a range of pressures, bias powers , and percent Ar in the source gas (0%, 85%). For the discharge, extensive modification of the ILD sidewall was observed for significant ion scattering conditions, whereas minimal modification of the ILD sidewall was observed under conditions of minimal or no ion scattering. Further, for an identical increase in the O-radical density (∼ an order of magnitude), a different degree of modification was induced at the ILD trench bottom surface depending on whether pressure or percent Ar was used to increase the radical density. The different degrees of modification seemingly correlated with the relative changes in the ion current for increasing pressure or percent Ar. For the discharge, reduced damage of the ILD sidewall and trench bottom surfaces was observed for increasing pressure (increasing N-radical density) and decreasing ion current to both surfaces. It is, thus, proposed that the mechanism for modification of the porous ILD is dominated by the creation of reactive sites by ion impact under the present conditions. A detailed discussion of the results which support this proposal is presented.
101(2007); http://dx.doi.org/10.1063/1.2404471View Description Hide Description
Laser-induced fluorescence-dip spectroscopy was used to measure the spatial distribution of electric fields around a biased cylindrical probe in an argon rf discharge. Two-dimensional maps of the field profiles were obtained around the probe as functions of argon pressure, discharge power, probe bias, and phase of the rf cycle. Analysis of the radial dependence of the electric fields indicated that the ion density decreased in the vicinity of the probe for all cases, contrary to the orbit motion limited regime typically employed for Langmuir probe analysis. We also observed perturbation of the plasma by the presence of the probe that extended many times the measurable length scale of the sheath into the plasma around the probe. Electric field distributions were also measured as functions of distance from the plasma sheath boundary. Coupling between the bounding sheath fields of the plasma and those formed around the biased probe was observed, even at distances greater than length scales of either sheath.
Gas temperature and electron temperature measurements by emission spectroscopy for an atmospheric microplasma101(2007); http://dx.doi.org/10.1063/1.2409318View Description Hide Description
A microplasma suitable for material processing at atmospheric pressure in argon and argon-oxygen mixtures is being studied here. The microplasma is ignited by a high voltage dc pulse and sustained by low power at . the mechanisms responsible for sustaining the microplasma require a more detailed analysis, which will be the subject of further study. Here it is shown that the microplasma is in nonequilibrium and appears to be in glow mode. The effect of power and oxygen content is also analyzed in terms of gas temperature and electron temperature. Both the gas temperature and the electron temperature have been determined by spectral emission and for the latter a very simple method has been used based on a collisional-radiative model. It is observed that power coupling is affected by a combination of factors and that prediction and control of the energy flow are not always straightforward even for simple argon plasmas. Varying gas content concentration has shown that oxygen creates a preferential energy channel towards increasing the gas temperature. Overall the results have shown that combined multiple diagnostics are necessary to understand plasmacharacteristics and that spectral emission can represent a valuable tool for tailoring microplasma to specific processing requirements.
101(2007); http://dx.doi.org/10.1063/1.2409566View Description Hide Description
Enhancements in rates of ionization,dissociation, and current in the dc saddle field (DCSF) glow discharge are studied using the direct Monte Carlo method (DMCM). The DCSF consists of a planar semitransparent anode positioned between two planar cathodes, producing a symmetric electric field that serves to lengthen the path of electrons. Simulations of current versus anode transparency of a discharge agree very well with experimental results reported previously. Numerical results of the DCSF discharge are also presented. At typical operating conditions (, cathode-anode spacing, anode transparency of 0.8), DMCM predicts a significant increase in the dissociation rate at pressures below in comparison to the dc diode. In consideration of its use for plasma enhanced chemical vapor deposition, the efficiency with which the DCSF operates at low pressures makes it attractive for the production of thin films whose qualities are sensitive to gas phase reactions.