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Notes: In this article, the nonlinear characteristics of magnetostatic forward volume wave (MSFVW)-based guided-wave magnetooptic Bragg cell modulators in bismuth-substituted yttrium iron garnet-gadolinium gallium garnet waveguides using nonuniform bias magnetic field are reported. First, the dispersion characteristics of the MSFVW under nonuniform bias magnetic field are analyzed, and the explicit expression for its bandwidth is determined. The transmission measurements of the MSFVW show that owing to the nonuniform magnetic field, the bandwidth is significantly increased. Next, the results of noncollinear magnetooptic (MO) Bragg diffraction experiments using the MSFVW in the frequency range from 2.0 to 4.0 GHz are presented. Two types of nonlinear process, namely, the four-magnon decay and modulation instabilities, are observed. However, the MO experiments at the carrier frequency of 2.85, 3.10, and 3.25 GHz suggest that the decay instabilities did not play a significant role in the MO interaction because of the larger degree of phase mismatch induced by the satellite waves generated during the nonlinear processes. We find that despite the presence of the decay instabilities, the MO Bragg diffraction characteristics still comply with that predicted by the coupled-mode theory before the nonlinear processes evolve into the modulation instabilities. Once the four-magnon modulation instabilities set in at the threshold powers, the MO Bragg diffraction will incur a drop in diffraction efficiency by as much as 9%. This feature results from perturbation of the satellite waves of smaller wave numbers induced in the modulation instabilities that lead to the MO phase mismatch. A model is established to describe the combined contributions of the initial MSFVW and the excited satellite waves associated with the modulation instabilities to the MO Bragg diffraction characteristics. © 2000 American Institute of Physics.

Notes: Spatially, temporally, and angularly resolved collinear collective Thomson scattering was used to diagnose the excitation and damping of a relativistic-phase-velocity self-modulated laser wakefield. The excitation of the electron plasma wave was observed to be driven by Raman-type instabilities. The damping is believed to originate from both electron beam loading and modulational instability. The collective Thomson scattering of a probe pulse from the ion acoustic waves, resulting from modulational instability, allows us to measure the temporal evolution of the plasma temperature. The latter was found to be consistent with the damping of the electron plasma wave. © 2000 American Institute of Physics.

Notes: The electron beam generated in a self-modulated laser-wakefield accelerator is characterized in detail. A transverse normalized emittance of 0.06 π mm mrad, the lowest ever for an electron injector, was measured for 2 MeV electrons. The electron beam was observed to have a multicomponent beam profile and energy distribution. The latter also undergoes discrete transitions as the laser power or plasma density is varied. In addition, dark spots that form regular modes were observed in the electron beam profile. These features are explained by analysis and test particle simulations of electron dynamics during acceleration in a three-dimensional plasma wakefield. © 1999 American Institute of Physics.

Notes: In this paper we discuss recent progress in using the Camassa–Holm equations to model turbulent flows. The Camassa–Holm equations, given their special geometric and physical properties, appear particularly well suited for studying turbulent flows. We identify the steady solution of the Camassa–Holm equation with the mean flow of the Reynolds equation and compare the results with empirical data for turbulent flows in channels and pipes. The data suggest that the constant α version of the Camassa–Holm equations, derived under the assumptions that the fluctuation statistics are isotropic and homogeneous, holds to order α distance from the boundaries. Near a boundary, these assumptions are no longer valid and the length scale α is seen to depend on the distance to the nearest wall. Thus, a turbulent flow is divided into two regions: the constant α region away from boundaries, and the near wall region. In the near wall region, Reynolds number scaling conditions imply that α decreases as Reynolds number increases. Away from boundaries, these scaling conditions imply α is independent of Reynolds number. Given the agreement with empirical and numerical data, our current work indicates that the Camassa–Holm equations provide a promising theoretical framework from which to understand some turbulent flows. © 1999 American Institute of Physics.

Notes: Phase control and the increase of the available free energy are two basic mechanisms that enhance the efficiency of a cyclotron autoresonance maser (CARM) device through linearly tapering the externally applied magnetic field. Based on these two mechanisms, a nonlinearly profiled magnetic field, as formed by adding a positively three-quarter sine profile onto a uniform magnetic field, applied on CARM is proposed in this study. Numerical results show that applications of the nonlinearly profiled magnetic field can raise the efficiency of CARM to around 50% over a wide range of frequency detunings. The reduction of efficiency sensitivity to the beam velocity spread can also be achieved for the increase of the available free energy with the use of the nonlinearly profiled magnetic field. Furthermore, the nonlinearly profiled magnetic field is also far more effective and practical than the linearly tapered magnetic field in efficiency enhancement. © 1995 American Institute of Physics.

Notes: We recently reported a new spray technique called ultrasound-modulated two-fluid (UMTF) atomization and the pertinent "resonant liquid capillary wave (RLCW) theory" based on linear models of Taylor-mode breakup of capillary waves. In this article, flow visualizations of liquid jets near the nozzle tip are presented to verify the central assumption of the RLCW theory that the resonant liquid capillary wave in UMTF atomization is initiated by the ultrasound at the nozzle tip. Specifically, a bright band beneath the nozzle tip was seen in ultrasonic and UMTF atomization separately, but not in two-fluid atomization. The bright band can be attributed to scattering of laser light sheet by the capillary waves generated by the ultrasound on the intact liquid jet. As the capillary wave travels downstream in the direction of airflow, it is amplified by the air blowing around it and eventually collapsed into drops. Therefore, the jet breakup time can be determined by dividing the measured band length with the capillary wave velocity. The breakup times thus determined for water and glycerol/water jets are twice the values predicted by the modified Taylor's model with a sheltering parameter, and are one order of magnitude shorter than those in conventional two-fluid atomization. Furthermore, the images of the spray in the proximity of the nozzle tip obtained by 30 ns laser pulses are consistent with the drop sizes obtained 2.3–6 cm downstream from the nozzle tip by 13 s time average of continuous laser light. Also reported in this article is the good agreement between the measured viscosity effects on the drop-size and size distribution in UMTF atomization and those on the relative amplitude growth rates of capillary waves at different wavelengths predicted by Taylor's model as a result of its inclusion of higher order terms other than the first in viscosity. These new findings have led to the conclusion that UMTF atomization occurs via Taylor-mode breakup of capillary waves; secondary atomization and drop coalescence are negligible. Further, UMTF atomization offers a means to control the drop-size and size distribution of two-fluid atomization for uniform drop formation. © 1999 American Institute of Physics.

Notes: Photoreflectance is used to investigate the band gap, built-in electric field, and surface Fermi level of a series of lattice-matched In0.52Al0.48As surface-intrinsic n+ structures having different undoped layer thicknesses. Experimental results indicate that, although the built-in electric field depends on the undoped layer thickness, there is a range of thickness within which the surface Fermi level is weakly pinned. From the dependence of electric field and surface Fermi level on the undoped layer thickness, we can determine that the surface states distribute over two separate regions within the energy band gap. The densities of the surface states are evaluated as well. Moreover, the dependence of the built-in electric field on undoped layer thickness is converted into the dependence of surface state density on the surface Fermi level in order to theoretically and exactly calculate the energy spectrum of the surface state density using a Guassian distribution function. The center and width of the distribution near the conduction band are obtained from the fitting parameters. © 2001 American Institute of Physics.

Notes: We present a novel semiconductor quantum well (QW) structure consisting of alternating (In,Ga)As and Ga(P,As,Sb) layers grown pseudomorphically on a GaAs substrate by all-solid-source molecular beam epitaxy. The band gap of the QW is determined by the thickness and composition of both types of layers and can be varied from 1.1 to 1.55 μm. Calculations show that the observed strong room-temperature photoluminescence in this wavelength range can be explained by a type-II transition in the QW. Structural investigations by reflection high-energy electron diffraction, transmission electron microscopy, and secondary ion mass spectroscopy confirm a triple layer structure with laterally modulated composition. Photoluminescence measurements reveal a linewidth of 50 meV at 1.3 μm and a luminescence decay time of 240 ps. Our investigations demonstrate the feasibility of this materials system for vertical cavity surface-emitting lasers and other optoelectronic devices on GaAs. © 2000 American Institute of Physics.

Notes: Tantalum-related thin films containing different amounts of nitrogen are sputter deposited at different argon-to-nitrogen flow rate ratios on (100) silicon substrates. Using x-ray diffractometry, transmission electron microscopy, composition and resistivity analyses, and bending-beam stress measurement technique, this work examines the impact of varying the nitrogen flow rate, particularly on the crystal structure, composition, resistivity, and residual intrinsic stress of the deposited Ta2N thin films. With an adequate amount of controlled, reactive nitrogen in the sputtering gas, thin films of the tantalum nitride of nominal formula Ta2N are predominantly amorphous and can exist over a range of nitrogen concentrations slightly deviated from stoichiometry. The single-layered quasi-amorphous Ta2N (a-Ta2N) thin films yield intrinsic compressive stresses in the range 3–5 GPa. In addition, the use of the 40-nm-thick a-Ta2N thin films with different nitrogen atomic concentrations (33% and 36%) and layering designs as diffusion barriers between silicon and copper are also evaluated. When subjected to high-temperature annealing, the single-layered a-Ta2N barrier layers degrade primarily by an amorphous-to-crystalline transition of the barrier layers. Crystallization of the single-layered stoichiometric a-Ta2N (Ta67N33) diffusion barriers occurs at temperatures as low as 450 °C. Doing so allows copper to preferentially penetrate through the grain boundaries or thermal-induced microcracks of the crystallized barriers and react with silicon, sequentially forming {111}-facetted pyramidal Cu3Si precipitates and TaSi2 Overdoping nitrogen into the amorphous matrix can dramatically increase the crystallization temperature to 600 °C. This temperature increase slows down the inward diffusion of copper and delays the formation of both silicides. The nitrogen overdoped Ta2N (Ta64N36) diffusion barriers can thus be significantly enhanced so as to yield a failure temperature 100 °C greater than that of the Ta67N33 diffusion barriers. Moreover, multilayered films, formed by alternately stacking the Ta67N33 and Ta64N36 layers with an optimized bilayer thickness (λ) of 10 nm, can dramatically reduce the intrinsic compressive stress to only 0.7 GPa and undergo high-temperature annealing without crystallization. Therefore, the Ta67N33/Ta64N36 multilayered films exhibit a much better barrier performance than the highly crystallization-resistant Ta64N36 single-layered films. © 2000 American Institute of Physics.

Notes: We describe the construction of a high voltage electric arc puller for controllable fabrication of bent near-field optical fiber probes. Various probes with bent angles ranging from 30° to 75° and bent lengths between 600 and 900 μm were successfully produced. The tip diameters achieved are between 100 and 200 nm. These bent type probes can be made into cantilevered probes that can be used for any dynamic mode atomic force microscope, and make the construction of a scanning near-field optical microscope easily attainable.© 1998 American Institute of Physics.