Volume 75, Issue 3, March 2004
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY; PHOTON DETECTORS
75(2004); http://dx.doi.org/10.1063/1.1645654View Description Hide Description
The force exerted by an optical trap on a dielectric bead in a fluid is often found by fitting a Lorentzian to the power spectrum of Brownian motion of the bead in the trap. We present explicit functions of the experimental power spectrum that give the values of the parameters fitted, including error bars and correlations, for the best such fit in a given frequency range. We use these functions to determine the information content of various parts of the power spectrum, and find, at odds with lore, much information at relatively high frequencies. Applying the method to real data, we obtain perfect fits and calibrate tweezers with less than 1% error when the trapping force is not too strong. Relatively strong traps have power spectra that cannot be fitted properly with any Lorentzian, we find. This underscores the need for better understanding of the power spectrum than the Lorentzian provides. This is achieved using old and new theory for Brownian motion in an incompressible fluid, and new results for a popular photodetection system. The trap and photodetection system are then calibrated simultaneously in a manner that makes optical tweezers a tool of precision for force spectroscopy, local viscometry, and probably other applications.
75(2004); http://dx.doi.org/10.1063/1.1646744View Description Hide Description
Narrow-bandwidth, broadly tunable vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) radiation has been generated up to 20 eV by resonance-enhanced four-wave mixing in argon using the five two-photon resonances of argon between 105 000 and corresponding to the optically accessible fine-structure components of the electron configuration. These two-photon resonances were reached by using the output of an excimer laser (157 nm) and the tripled output of a dye laser. The highest VUV/XUV intensities were obtained using the two-photon resonance at in combination with the main excimer line. The conversion efficiency reaches an optimum for photon energies around 16 eV and slowly decreases when the photon energy rises to 20 eV. The use of the argon resonances also facilitates the generation of intense VUV laser radiation around a region that is not easily accessible by four-wave mixing with the commonly used two-photon resonances of krypton (202.3 and 212.5 nm) and xenon (222.6 and 249.6 nm). The bandwidth of the VUV/XUV laser radiation was measured to be less than over the entire range between 11 and 20 eV. The VUV/XUV laser system was used to measure the isotopic shift of the resonance in neon at and fully rotationally resolved photoelectron spectra of around and between 140 000 and Improved values of the first adiabatic ionization energy of and were determined.
Minimization of the acquisition time in energy dispersive x-ray diffraction measurements on disordered systems75(2004); http://dx.doi.org/10.1063/1.1645655View Description Hide Description
The energy dispersive x-ray diffraction (EDXD) technique proved to be a powerful tool for the structural investigations of disordered systems. Although EDXD fulfills all of the characteristics required to study this class of samples, the adopted measurement procedure is not standard. This article focused on the optimization of the experimental setup required in order to minimize the acquisition time. We demonstrate that, in the symmetrical transmission geometry (usually adopted) and under very general experimental conditions, the quantities influencing the total time of measurement depend on only two free parameters, namely the sample thickness and the width of the energy spectrum of the primary beam. For this reason, despite the large number of variables involved in the problem, the final result can be summarized in a three-dimensional plot of the acquisition time as a function of these two independent parameters. Therefore, the plots allow the most convenient choice of the two parameters to minimize the measurement time, at a parity of statistical accuracy. On the other hand, it shows how an inappropriate setting may expand this time by several orders of magnitude. The method is applied to the two systems that have been most thoroughly investigated by EDXD, namely and Hg.
Effective suppression of fluorescence light in Raman measurements using ultrafast time gated charge coupled device camera75(2004); http://dx.doi.org/10.1063/1.1646743View Description Hide Description
A high level of fluorescence background signal rejection was achieved for solid and powder samples by using a combination of simple low-resolution spectrograph and ultrafast gated charge coupled device(CCD)camera. The unique timing characteristics of the CCDcamera match exceptionally well to characteristics of a Ti:sapphire oscillator allowing fast gated light detection at a repetition rate of up to 110 MHz, making this approach superior in terms of the duty cycle in comparison with other time-resolved Raman techniques. The achieved temporal resolution was about 150 ps under 785 nm Ti: sapphire laser excitation. At an average excitation power up to 300 mW there was no noticeable sample damage observed. Hence, the demonstrated approach extends the capabilities of Raman spectroscopy regarding the investigation of samples with a short fluorescence lifetime. The combination of a spectrometer and a gated CCDcamera allows simultaneous study of spectral and temporal characteristics of emitted light. This capability opens an exciting possibility to build a universal instrument for solving multitask problems in applied laser spectroscopy.
75(2004); http://dx.doi.org/10.1063/1.1646739View Description Hide Description
On the basis of an optical Kerr-gate method, we have constructed a high performance system for femtosecond spectral snapshots to measure ultrafast behavior of time-dependent spectrum of weak luminescence. Optical components and their configurations have been investigated to optimize the system. We have measured nonlinear refractive indices and two-photon absorption coefficients of cubic crystals and glasses by the z-scan technique in order to evaluate their performances for an optical Kerr gate. has been found to be highly efficient and suitable as a Kerr-gate material. Simulations based on the Gaussian-beam propagation were performed to find appropriate configurations to obtain high time resolution. Observation for time-resolved luminescencespectra of β-carotene has revealed that time resolution of 200 fs is achieved in a wide range of wavelengths, which is limited by the pulse width of the gating light.
75(2004); http://dx.doi.org/10.1063/1.1646737View Description Hide Description
Plasma mirrors are devices capable of switching very high laser powers on subpicosecond time scales with a dynamic range of 20–30 dB. A detailed study of their performance in the near-field of the laser beam is presented, a setup relevant to improving the pulse contrast of modern ultrahigh power lasers (TW–PW). The conditions under which high reflectivity can be achieved and focusability of the reflected beam retained are identified. At higher intensities a region of high specular reflectivity with rapidly decreasing focusability was observed, suggesting that specular reflectivity alone is not an adequate guide to the ideal range of plasma mirror operation. It was found that to achieve high reflectivity with negligible phasefront distortion of the reflected beam the inequality must be met sound speed, time from plasma formation to the peak of the pulse). The achievable contrast enhancement is given by the ratio of plasma mirrorreflectivity to cold reflectivity.
- NUCLEAR PHYSICS, FUSION and PLASMAS
75(2004); http://dx.doi.org/10.1063/1.1646740View Description Hide Description
A new diagnostic for the National Spherical Torus Experiment is described whose function is to measure ion rotation and temperature at the plasma edge. The diagnostic is sensitive to C III, C IV, and He II intrinsic emission, covering a radial region of 15 cm at the extreme edge of the outboard midplane. Thirteen chords are distributed between toroidal and poloidal views, allowing the toroidal and poloidal rotation and temperature of the plasma edge to be simultaneously measured with 10 ms resolution. Combined with the local pressure gradient and EFIT reconstructed magnetic field profile, the edge flow gives a measure of the local radial electric field.
75(2004); http://dx.doi.org/10.1063/1.1647696View Description Hide Description
Application of microwavereflectometry to study Alfvén wave resonances in the TCABR tokamak is described. A microwavereflectometer was used to register plasma densityoscillations driven by the excited Alfvén waves, under the condition of the spectrum scanned by a controlled plasma density rise. It is shown that when the position of the local Alfvén resonance which is defined by the relation is close to the plasma zone where the microwave signal is reflected, the high-frequency modulation of the output signal of the reflectometer at the rf generator frequency increases. This method can give information about the localization of the rf power deposition zone in Alfvén waveplasma heating and current drive experiments. It also allows finding the plasma current profile in the region of the rf power deposition.
75(2004); http://dx.doi.org/10.1063/1.1646733View Description Hide Description
The instrumental factors and measuring conditions affecting neutron emission spectrometry measurements of tokamakplasmas are described and analyzed. The measured energy broadening and shift of the neutron emission is used to deduce ion temperature and toroidalplasma rotation velocity representing average (effective) values for the nonuniform plasma volume viewed. Analytical expressions are derived for the relationship between the line-volume integrated effective temperature and the radial profile for the case of thermal plasmas with isotropic neutron emission; effects on due to spectral broadening from the radial dependence were also considered. The analysis method presented here is applied to high quality data obtained with the magnetic proton recoil neutron spectrometer installed at Joint European Torus for measurements of deuterium–tritium plasmas. Similarly, cases of anisotropic neutron emission were quantitatively assessed.
75(2004); http://dx.doi.org/10.1063/1.1646732View Description Hide Description
We present the design of a modular, robust, high-current switch for controlling discharge current in the large plasma device (LAPD) at UCLA. As developed, 10 insulated gate bipolar transistor units are connected in parallel to provide 24 kA current-carrying capacity. Associated snubber and protection circuits are described in detail. The switch is typically operated at a 1 Hz repetition rate, with up to a 2% duty cycle. It has proven to be quite reliable, to date having pulsed approximately times, with a net throughput of about 2 billion Coulombs. This type of switch not only has an application in such a plasma source, but can also be used to switch large currents for a variety of applications. The design is modular and the switch can be made smaller or larger depending on the desired current rating.
Instrument reflections and scene amplitude modulation in a polychromatic microwave quadrature interferometer75(2004); http://dx.doi.org/10.1063/1.1646738View Description Hide Description
A polychromatic microwave quadrature interferometer has been characterized using several laboratory plasmas. Reflections between the transmitter and the receiver have been observed, and the effects of including reflection terms in the data reduction equation have been examined. An error analysis which includes the reflections, modulation of the scene beam amplitude by the plasma, and simultaneous measurements at two frequencies has been applied to the empirical database, and the results are summarized. For reflection amplitudes around 10%, the reflection terms were found to reduce the calculated error bars for electron density measurements by about a factor of 2. The impact of amplitude modulation is also quantified. In the complete analysis, the mean error bar for high-density measurements is 7.5%, and the mean phase shift error for low-density measurements is
- MICROSCOPY and IMAGING
Nondestructive high spatial resolution imaging with a 60 GHz near-field scanning millimeter-wave microscope75(2004); http://dx.doi.org/10.1063/1.1646735View Description Hide Description
We demonstrate a nondestructive millimeter-wave surface imaging technique using a near-field scanning millimeter-wave microscope (NSMM) with a resonant standard waveguide probe at an operation frequency A chemically etched metallic probe tip was coupled to the resonant rectangular waveguide. By properly tuning the tunable resonator and the probe tip we could improve sensitivity and spatial resolution of the NSMM. By measuring the change of the quality factor in the near-field zone, near-field scanning millimeter-wave images of patterned metalfilms and thin films were obtained with a spatial resolution better than 1 μm. We observed the dependence of the current density NSMM images of patterned indiumtin oxide films on the bias currents.
Removing interference and optical feedback artifacts in atomic force microscopy measurements by application of high frequency laser current modulation75(2004); http://dx.doi.org/10.1063/1.1646767View Description Hide Description
Atomic force microscopymeasurements based on optical beam deflection can seriously be affected by two specific types of artifacts. Disturbances of the first type are caused by interference on the quadrant photodiode between the beam reflected directly from the cantilever and stray light from the sample surface. The second type is optical feedbackeffects caused by light scattering from the surface back into the laser cavity. We demonstrate that both types can be significantly reduced by application of high frequency laser current modulation. Residual noise is dominated by electronic and mechanical noise, shot noise, and noise caused by the thermal motion of the cantilever.
75(2004); http://dx.doi.org/10.1063/1.1646742View Description Hide Description
We present a tool dedicated to potentiometric measurements over a length longer than the usual scanning length of local probes techniques, with a submicronic resolution, suitable for use in cryogenic environment. The measurement is carried out by bringing a conducting tip in contact with the sample surface. We describe in detail the connecting procedure and the inertial system used to move the sample. To illustrate the possibilities given by this system, we have measured the instantaneous voltage profile modified by Joule heating of a metallic bridge crossed by a high current density, and we have probed the potential profile across a normal-superconducting junction.
- CONDENSED MATTER; MATERIALS
75(2004); http://dx.doi.org/10.1063/1.1645652View Description Hide Description
Synchrotron x-ray diffractionmeasurements in quartz crystals of different thickness excited by standing acoustic waves were carried out at the Advanced Photon Source of Argonne National Laboratory. We demonstrated the ability to significantly modify the quartz rocking curves for 20–25 keV x rays by changing the shear wave parameters in the frequency range between 15 and 105 MHz. Dynamic deformation introduced into the crystal lattice by acoustic waves resulted in a remarkable broadening of the rocking curves. The broadening effect strongly depends on the strength of the ultrasound, which can be easily regulated by changing the acoustic amplitude or frequency near the resonance. The maximum rocking curve broadening reached 17 times, which corresponds to the wavelength band, when used as a monochromator or analyzer for 20–25 keV x rays. The initial rocking curve shape is restored by sweeping the acoustic frequency within a 50–100 kHz range near the resonance. The tunable broadening effect allows effective manipulation of x-ray intensities in time domain. Time-resolved x-ray diffractionmeasurements under a 19.6 MHz acoustic wave excitation were performed by synchronizing the acoustic wave and x-ray burst periodicity. We used the fact that twice per period the standing wave produces a zero net deformation across the crystal thickness. By introducing an oscillating delay to the acoustic excitation, we were able to effectively change the phase of the acoustic wave relative to the x-ray burst periodicity. The x-ray diffraction intensity was strongly affected by tuning the timing of the x-ray arrivals to the minimum or maximum acoustic deformation. A deep modulation of x rays was observed in a wide frequency range between 0.1 Hz and 1 MHz, which certifies that acoustically excited quartz crystals can potentially be used as slow and fast x-ray modulators with high duty cycle.
Molybdenum cell for x-ray diffraction measurements of fluid alkali metals at high temperatures and high pressures75(2004); http://dx.doi.org/10.1063/1.1646769View Description Hide Description
We have developed a sample cell for x-ray diffraction measurements of fluid alkali metals at high temperatures and high pressures. All parts of the cell are made of molybdenum which is resistant to the chemical corrosion of alkali metals. Single crystalline molybdenum disks electrolytically thinned down to 40 μm were used as the walls of the cell through which x rays pass. The crystal orientation of the disks was controlled in order to reduce the background from the cell. All parts of the cell were assembled and brazed together using a high-temperature Ru–Mo alloy. Energy dispersive x-ray diffraction measurements have been successfully carried out for fluid rubidium up to 1973 K and 16.2 MPa. The obtained demonstrates the applicability of the molybdenumcell to x-ray diffraction measurements of fluid alkali metals at high temperatures and high pressures.
75(2004); http://dx.doi.org/10.1063/1.1647695View Description Hide Description
An instrument for the synthesis and analysis of nanoparticles and ultrathin films of Pu and other actinides has been constructed. To facilitate the production of nanoscale specimens of these materials,pulsed laserablation was chosen for the deposition process. The highly toxic and radioactive nature of these materials created a challenging safety environment that needed to be addressed before any work could begin. Particular attention has been paid in this respect towards our future work with plutonium, though the design structure presented here would work equally well with other nonradioactive toxic or reactive materials. The analytical capabilities of the instrument include in situphotoelectron spectroscopy, low energy electron diffraction, scanning tunneling microscopy, and scanning tunneling spectroscopy. The instrument design and first results will be discussed.
Synthesis of aluminum nitride films by plasma immersion ion implantation–deposition using hybrid gas–metal cathodic arc gun75(2004); http://dx.doi.org/10.1063/1.1646741View Description Hide Description
Aluminum nitride(AlN) is of interest in the industry because of its excellent electronic, optical, acoustic, thermal, and mechanical properties. In this work, aluminum nitridefilms are deposited on silicon wafers (100) by metal plasma immersion ion implantation and deposition (PIIID) using a modified hybrid gas–metal cathodic arc plasma source and with no intentional heating to the substrate. The mixed metal and gaseous plasma is generated by feeding the gas into the arc discharge region. The deposition rate is found to mainly depend on the Al ion flux from the cathodic arc source and is only slightly affected by the flow rate. The AlN films fabricated by this method exhibit a cubic crystalline microstructure with stable and low internal stress. The surface of the AlN films is quite smooth with the surface roughness on the order of 1/2 nm as determined by atomic force microscopy, homogeneous, and continuous, and the dense granular microstructures give rise to good adhesion with the substrate. The N to Al ratio increases with the bias voltage applied to the substrates. A fairly large amount of O originating from the residual vacuum is found in the samples with low N:Al ratios, but a high bias reduces the oxygen concentration. The compositions, microstructures and crystal states of the depositedfilms are quite stable and remain unchanged after annealing at 800 °C for 1 h. Our hybrid gas–metal source cathodic arc source delivers better AlN thin films than conventional PIIID employing dual plasmas.
75(2004); http://dx.doi.org/10.1063/1.1645656View Description Hide Description
Soft x-ray absorption spectroscopy is a powerful probe of surface electronic and geometric structure in metals, semiconductors, and thin films. Because these techniques generally require ultrahigh vacuum, corresponding studies of volatile liquid surfaces have hitherto been precluded. We describe the design and implementation of an x-rayexperiment based on the use of liquid microjets, permitting the study of volatile liquid surfaces under quasi-equilibrium conditions by synchrotron-based spectroscopy. The liquid microjet temperatures are also characterized by Raman spectroscopy, which connects our structural studies with those conducted on liquid samples under equilibrium conditions. In recent experiments, we have observed and quantified the intermolecular surface relaxation of liquid water and methanol and have identified a large population of “acceptor-only” molecules at the liquid water interface.
75(2004); http://dx.doi.org/10.1063/1.1645634View Description Hide Description
A procedure is described to build a slotted-plate device to directly measure the static yield stress of a suspension. The technique involves a balance as well as a linear motion platform. A slotted plate is suspended from the balance and is vertically positioned in a suspension sample which stands on the platform. The yield stress is determined from the point where initially constant slope of the force versus platform travel distance changes. Wall effects associated with the original plate yield-stress instrument were minimized by opening a series of slots on the plates. Yield-stress values of both high-concentration (40, 50, 60, and 70 wt % and low-concentration (2, 3, and 5 wt % bentonite) aqueous suspensions were measured. Very small yield-stress values (down to could be determined with our slotted-plate device. Other methods, such as the popular vane technique, could not measure yield-stress values of the bentonite suspensions for concentrations less than 7 wt %, corresponding to a yield stress of 1.29 Pa.