Volume 72, Issue 6, June 2001
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
Boolean algebra operations performed on optical bits by the generation of holographic fields through second-order nonlinear interactions72(2001); http://dx.doi.org/10.1063/1.1364665View Description Hide Description
Boolean algebra operations such as AND or XOR are performed on optical bits encoded as amplitude modulations in two wave fronts that are made interact in a crystal endowed with second-order nonlinearity. If the corresponding wave fields are at the same frequency ω and the crystal is tuned for the phase-matched generation of 2ω, we show that the generated wave front reconstructs a holographicimage containing the outputs of the desired operations. Since a nearly diffraction-limited optical resolution can be easily achieved in the holographicimage at 2ω, a correspondingly high density of data is encodeable in the wave front at ω playing the role of object wave front. The experiments demonstrate the feasibility of the operation of a parallel half-adder performing the sum of multiple data with a one-digit binary number, which is encoded in the reference wave front.
72(2001); http://dx.doi.org/10.1063/1.1373670View Description Hide Description
We demonstrate that two-frequency laser light can be generated by rapid current modulation of a diode laser which is injection locked to a separate frequency-stabilized diode laser. First-order sidebands containing at least 2% of the power in the carrier are produced for modulation This configuration is a convenient and inexpensive alternative to using two separate frequency-stabilized diode lasers or an electro-opticmodulator.
72(2001); http://dx.doi.org/10.1063/1.1369640View Description Hide Description
A highly sensitive intracavity millimeter-wave spectrometer was developed for the investigation of the absorption spectra of van der Waals complexes in a supersonic jet. The key element of the spectrometer is a tunable oscillator, called OROTRON, which generates the millimeter-wave radiation through the interaction of an electron beam with the electromagnetic field of a high quality Fabry–Perot resonant cavity. This cavity consists of a movable spherical mirror and a fixed planar mirror with the periodic structure imprinted on its surface. The electron beam moves along the periodic structure of the plane mirror. This part separated from the rest of the resonator by a mica foil is kept under ultrahigh vacuum conditions. The molecular jet is injected by a pulsed valve into the other part of the resonator. The absorption in the jet is sensitively detected by measuring the electric current in a special collector circuit of the OROTRON. The spectral purity of the OROTRON radiation is 10–15 kHz providing the capability of sub-Doppler spectral resolution without phase locking. An increase in sensitivity of a factor of about 100 in comparison with the usual single pass arrangement was evaluated from the measurements of the absorption lines of the CO rare isotopomers, the Ar–CO and Ne–CO van der Waals complexes. The high sensitivity, wide spectral range, and simple tunability of the spectrometer make it a very efficient tool for the searching of weakly absorbing species in a jet.
Resolution enhancement in the magnetic bottle photoelectron spectrometer by impulse electron deceleration72(2001); http://dx.doi.org/10.1063/1.1367364View Description Hide Description
We present a modified time of flight magnetic bottle photoelectron spectrometer (MBPES), designed to operate with mass-selected ion beams. It is based on a configuration in which the ion beam is coaxial with the electron flight tube. High resolution combined with high efficiency is achieved by applying impulse deceleration at the electron detachment zone immediately after detachment. The main effect of the impulse is to improve the resolution of the spectrometer by subtracting the ion velocity from the electron velocity. The impulse can also counteract the spread in the flight times caused by the alignment of the electrons in the magnetic bottle. Unlike previous designs of the MBPES, ultimate resolution is achieved without any loss in signal intensity. Resolutions of 8, 25, and 48 meV were obtained for 0.28, 1.22, and 2.11 eV electrons, respectively, with 60% collection efficiency. This design also allows easy switching between high-resolution high-yield and polarization-sensitive photoelectron spectroscopy modes. Our simulations indicate that, with careful attention to details, sub-meV resolution can be obtained using this approach.
The design and performance of an effusive gas source of conical geometry for photoionization studies72(2001); http://dx.doi.org/10.1063/1.1373667View Description Hide Description
The design, construction, and performance of an effusive gas source of conical geometry is reported. This gas jet enables experiments that require the gas and photon beams to be essentially collinear and has a focusing multicapillary array with the central portion left free for the photon beam. In total, the source comprises 90 “tubes” in three layers and has been designed by modeling free molecular gas flow and optimizing for highest gas density ∼2–3 mm from the jet’s exit plane. The nature of the gas flow through the source and its focusing properties are investigated theoretically and experimentally.
- PARTICLE SOURCES, OPTICS and ACCELERATION
72(2001); http://dx.doi.org/10.1063/1.1372169View Description Hide Description
We describe a high flux source of cold (80 K) metastable helium atoms. The source employs a direct current nozzle discharge which produces in excess of atoms/steradian/s. Liquid nitrogen cooling of the discharge source yields atomic velocities below 900 Such a source has practical applications for experiments concerned with laser cooling and trapping of metastable helium atoms.
- NUCLEAR PHYSICS, FUSION and PLASMAS
72(2001); http://dx.doi.org/10.1063/1.1370557View Description Hide Description
A National Ignition Facility (NIF) core diagnostic instrument has been designed and will be fabricated to record x-ray spectra in the 1.1–20.1 keV energy range. The High-Energy Electronic X-Ray (HENEX) instrument has four reflection crystals with overlapping coverage of 1.1–10.9 keV and one transmission crystal covering 8.6–20.1 keV. The spectral resolving power varies from approximately 2000 at low energies to 300 at 20 keV. The spectrum produced by each crystal is recorded by a modified commercial dental x-raycharge coupled devicedetector with a dynamic range of at least 2500.
High-resolution x-ray crystal spectrometer/polarimeter at torus experiment for technology oriented research-9472(2001); http://dx.doi.org/10.1063/1.1370558View Description Hide Description
A high-resolution x-raypolarimeter has been installed at the torus experiment for technology oriented research (TEXTOR-94). The instrument consists of two spectrometers in Johann geometry, utilizing the dependence of Bragg reflection on the polarization. The dispersion plane of the so-called horizontalspectrometer coincides with the midplane of the tokamak, whereas the dispersion plane of the so-called verticalspectrometer lies perpendicular to the midplane of the tokamak. Both spectrometers operate with cylindrically bent quartz crystals of dimensions with a spacing of 4.9130 Å. The radii of curvature of the horizontal and vertical spectrometer crystals are 3820 mm and 4630 mm, respectively. The detectors are multiwire proportional counters having a large entrance window of with a high count rate capability of up to and a spatial resolution of 0.4–0.65 mm, depending on the count rate. The unique data acquisition system makes it possible to record up to 8192 spectra per TEXTOR-94 discharge to study fast events and impurity transport. The instruments are sensitive to spurious amounts of argon in hydrogen or deuterium discharges and aim for x-raypolarizationmeasurements and other core plasma diagnostics like electron temperature, ion temperature,plasma rotation, impurity transport, and relative abundance of The first measurements of toroidal rotation speed, ion and electron temperatures show a very good agreement with charge exchange recombination spectroscopy (XRS) and electron cyclotron emission (ECE) diagnostics available at TEXTOR-94. The measured values of the ratio are higher than that predicted from coronal equilibrium. The ratio depends strongly upon the electron density both for ohmic and neutral beam injection cases. This suggests that the charge state distribution is also dependent on diffusion and charge exchange processes in the plasmas.
72(2001); http://dx.doi.org/10.1063/1.1368858View Description Hide Description
A novel technology is suggested for making solid hydrogen shells around impurity pellets to be injected into plasmas of fusion devices with a view to looking into its transport properties. In proof-of-principle tests, a 3 mm long and 3 mm diameter cylindrical solid hydrogen shell was formed around a 0.2 mm diameter globular pellet at a temperature of 8–11 K within 5 min and accelerated in a pipe-gun barrel up to 1 km/s.
laser based two-volume collective scattering instrument for spatially localized turbulence measurements72(2001); http://dx.doi.org/10.1063/1.1376655View Description Hide Description
We describe and demonstrate a two-volume collective scattering system for localized measurements of plasma turbulence. The finite crossfield correlation length of plasma turbulence combined with spatial variations in the magnetic field direction are used to obtain spatially localized turbulence measurements at the Wendelstein 7-AS fusion experiment. By comparing with localized measurements obtained using the single wide beam technique [Truc et al., Rev. Sci. Instrum. 63, 3716 (1992)] we provide a proof-of-principle demonstration of the two-beam method. An optimized configuration with large toroidal separation of the measurement volumes is predicted to be capable of providing spatial resolution better than 5 cm.
72(2001); http://dx.doi.org/10.1063/1.1357231View Description Hide Description
A Monte Carlo simulation of tritium β particle motion in a matrix material indicates that the probability of escape through the material surface is greater than 50% for near-surface tritium, but drops quite rapidly with increasing depth. Beyond an areal density depth of the decline is close to exponential, with an e-folding length of approximately the exact value varying slightly with the material. The sensitivity of the tritium imaging technique will decrease at a similar rate with increasing depth of tritium. Experimentally, the image intensity of a tritium-implanted graphite specimen was observed to decrease exponentially with coating thickness as an aluminum layer was evaporated onto its surface, with an exponent that was within 30% of the predicted value. The Monte Carlo simulation also indicated that the limit of lateral resolution of the tritium imaging technique will be slightly less than the depth of the tritium, subject always to Recknagel’s limit of resolution due to “chromatic aberration” of the electron optics, which is of the order of 200 nm. If tritium is uniformly distributed through the material, surface tritium so dominates image formation that Recknagel’s limit inevitably applies.
- BASIC PHENOMENA
72(2001); http://dx.doi.org/10.1063/1.1369628View Description Hide Description
The typical response of the x-ray converter material to the passage of a high-powered relativistic electron beam is vaporization and rapid dispersal. The effect of this dispersal on subsequent pulses for multi-pulse radiography is the collective effects on the propagation of the electron beam through the expanding plasma and the reduced number of electron to photon interactions. Thus, for the dual-axis radiographichydrodynamic test facility, the converter material must either be replaced or confined long enough to accommodate the entire pulse train. Typically the 1-mm-thick high Z and full density converter material is chosen to give peak dose and minimum radiographic spot. For repeated pulses we propose a modified converter, constructed of either low density, high Z material in the form of foam or of foils spaced over ten times the axial thickness of the standard 1 mm converter. The converter material is confined within a tube to impede outward motion in radius outside the beam interaction region. We report single-pulse experiments which measure the dose and spot size produced by the modified converter and compare them to similar measurements made by the standard converter. For multiple pulses over a microsecond time scale, we calculate the radial and axial hydrodynamicflow to study the material reflux into the converter volume and the resultant density decrease as the electron beam energy is deposited. Both the electron transport through the expanding low density plasma and beam in the higher density material are modeled. The x-ray source dose and spot size are calculated to evaluate the impact of the changing converter material density distribution on the radiographic spot size and dose. The results indicate that a multiple-pulse converter design for three or four high-power beam pulses is feasible.
- MICROSCOPY and IMAGING
72(2001); http://dx.doi.org/10.1063/1.1370563View Description Hide Description
A soft x-ray spectromicroscope for scanning transmission x-ray microscopy (STXM) and scanning photoelectron microscopy (SPEM) has been installed at the first undulator-radiation beamline at the Pohang Light Source. The spatial resolution of the spectromicroscope is 0.4 μm in the vertical direction and 0.5 μm in the horizontal direction, with a focused photon flux of photons/s. The operational energy ranges for the STXM and the SPEM are 250–1000 eV and 400–1000 eV, respectively, with spectral resolving power of 1000–3000. The setup details of the spectromicroscope and the SPEM application results of semiconductor devices are presented.
72(2001); http://dx.doi.org/10.1063/1.1369631View Description Hide Description
We have developed a local probe technique in order to realize photoluminescence maps with submicron resolution at temperatures as low as 25 K. To this end a closed cycle He cryostat has been modified in order to damp mechanical vibrations to avoid spatial resolution losses. Both the optical laser pump and the collected signal are fiber-optic coupled. Photoluminescence maps are provided by a motorized translation stage that scans the microscope objective over the sample surface. The overall resolution of the microphotoluminescence (μ-PL) system is ∼500 nm, by considering the contributions of the laser focused spot size the cryostat vibrations, and the motorized stage resolution. The system is described and two low temperature μ-PL experiments on quantum wires and quantum dotnanostructures are presented and discussed.
72(2001); http://dx.doi.org/10.1063/1.1372165View Description Hide Description
A variable-temperature ultrahigh vacuum (UHV) scanning tunneling microscope(STM) was designed and tested. Design details and initial results are presented. The STM is directly attached to the cold face of a continuous flow cryostat which is mounted into a two-chamber UHV system. A significant advantage of this system in comparison to many others is, that samples can be cooled down to base temperature of 6.5 K within very short times of below 2 h. This feature not only increases the potential sample throughput, it also allows to cycle the sample temperature within the regime below 20 K without losing track of given sample locations. The instrument was tested by imagingAu layers on graphite. The vertical stability at low temperature was found to be below 3 pm. Images recorded at 6.5 K show crystalline Au islands and the reconstruction with atomic resolution. Using a resistive heater, the sample temperature was adjusted between 6.5 and 20 K. After an equilibration time of 15 min, the displacement due to the temperature change remained below 150 nm. Scanning tunnelingspectroscopy on Au(111) grains resolves the Au(111) surface state.
72(2001); http://dx.doi.org/10.1063/1.1369637View Description Hide Description
We describe a sensor for use in a scanning capacitancemicroscope (SCM) that is capable of “zeptofarad” capacitancemeasurement resolution in a 1 Hz bandwidth with a peak-to-peak sense voltage on the probe tip of no more than 300 mV. This sensitivity is based on experimental data and simulation results that are in excellent agreement. The complete sensor incorporates an oscillator (phase locked to a 10 MHz crystal oscillator), a coupled transmission line resonator, an amplifier, and a peak detector. The resonator is fabricated from copper-clad, low-loss dielectric material and its size is such that it is easily incorporated with a scanning probe microscope. The sensor’s use in the SCM enables capacitance resolution that has not previously been possible while retaining the instrumental advantages of imaging at low sense voltages. The performance of this sensor is discussed and compared to alternative scanning capacitancemicroscopy methodologies.
72(2001); http://dx.doi.org/10.1063/1.1370562View Description Hide Description
Despite the existence of a variety of optical filters for the separation of spectral components in a multicolor image, a filter that is both continuously wavelength tunable and capable of high spectral resolution while preserving the spatial integrity of a two-dimensional image is not currently available. We present, in this article, the introduction of a novel optical filtering concept that permits the development of such a system. Both the concept and its implementation in an optical-fiber-based prototype that converts two-dimensional images to a one-dimensional array followed by interconversion for image reconstruction are presented. The performance of the prototype is analyzed using both a xenon arc lamp as a standard broadband illuminationsource as well as He–Ne and Ar lasers as sources of coherent radiation. An unoptimized throughput efficiency of approximately 30% and a bandwidth of 6 Å without spectral leakage or spatial crosstalk is obtained over the entire investigated tuning range from 430 to 807 nm. Potential applications of such an optical filtration system, with wavelength tunability on the angstrom scale and potential spatial resolutions in the micrometer range, using suitable optical imaging are discussed.
- CONDENSED MATTER; MATERIALS
72(2001); http://dx.doi.org/10.1063/1.1370561View Description Hide Description
To reduce the large absorption effect in diamond anvil pressurecells for soft x rays, perforated anvils employed as diamond-backing plates (DBPs) used in conjunction with miniature anvils (MAs) made of 1/100 carat diamonds were tested for high-pressure efficacy. Static pressures beyond 100 GPa were generated using a piston/cylinder cell having 0.2 mm culets. Tests were carried out in 0.1 mm cavities drilled in a Re gasket, using Ar samples and ruby chips for manometry. Except for a single failure of a 0.3 mm culet MA, no damage was detected in the DBPs drilled with truncated conical holes tapering from 0.3 mm to 1, 1.5, and 2 mm diameter. Another arrangement in which one anvil was partially drilled leaving a 0.5 mm thick wall behind the culet achieved 100 GPa. Detailed discussions are given concerning the benefits of the DBP/MA cells for high-pressure studies with soft x rays and for background reduction in Raman, IR, UV, and Mössbauer spectroscopies.
72(2001); http://dx.doi.org/10.1063/1.1367362View Description Hide Description
We have built an ellipsoidal display analyzer (EDA) for angle-resolved photoelectron spectroscopy and related techniques. The instrument is an improved version of a design by Eastman et al. [Nucl. Instrum. Methods 172, 327 (1980)] and measures the angle-resolved intensity distribution of photoelectrons at fixed energy Such two-dimensional cuts through the Brillouin zone are recorded using a position-sensitive detector. The large acceptance angle (Δθ=43° in the polar direction and Δφ=360° in the azimuthal direction) leads to a collection efficiency which exceeds that of conventional hemispherical analyzers by a factor of about 3000. Using ray-tracing calculations we analyze the electron optical properties of the various analyzer components and optimize their arrangement. This minimizes distortions and aberrations in the recorded images and greatly improves the performance compared to previous realizations of this analyzer. We present examples demonstrating the performance of the analyzer and its versatility. Using a commercial He-discharge lamp we are able to measure complete angular distribution patterns in less than 5 s. The energy and angular resolution are meV and Δθ=1.2°, respectively. Complete stacks of such cuts through the Brillouin zone at different kinetic energies E can be acquired automatically using custom software. The raw data are processed leading to a three-dimensional set of photoelectron intensity versus binding energy E and wave vector From this all relevant information, like the dispersion relations along arbitrary directions of the Brillouin zone or Fermi-surface maps, can then be computed. An additional electron gun enables low-energy electron diffraction, Auger electron spectroscopy, and electron energy-loss spectroscopy. Switching between electrons and photons as the excitation source is possible without any movement of the sample or analyzer. Because of the high acquisition speed it is possible to study the electronic structure of solids as a function of an external parameter (i.e., temperature) or to make animated movies showing, for example, the evolution of electronic states in reciprocal space. After installation of this EDA at a synchrotron providing tunable photon energy, the full power of the instrument will come into play by adding techniques like constant final state or constant initial state spectroscopy, and x-ray photoelectron diffraction.
Simple, accurate, and precise measurements of thermal diffusivity in liquids using a thermal-wave cavity72(2001); http://dx.doi.org/10.1063/1.1372677View Description Hide Description
A simple methodology for the direct measurement of the thermal wavelength using a thermal-wave cavity, and its application to the evaluation of the thermal diffusivity of liquids is described. The simplicity and robustness of this technique lie in its relative measurement features for both the thermal-wave phase and cavity length, thus eliminating the need for taking into account difficult-to-quantify and time-consuming instrumental phase shifts. Two liquid samples were used: distilled water and ethylene glycol. Excellent agreement was found with reported results in the literature. The accuracy of the thermal diffusivity measurements using the new methodology originates in the use of only difference measurements in the thermal-wave phase and cavity length. Measurement precision is directly related to the corresponding precision on the measurement of the thermal wavelength.