Index of content:
Volume 71, Issue 2, February 2000
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
71(2000); http://dx.doi.org/10.1063/1.1150203View Description Hide Description
A novel two-beam method is proposed and applied for the first time, to characterize photorefractive damage (PRD) in a quasiphase-matched (QPM) wavelength converter. In the proposed method, irradiation light from a Ti sapphire laser and a broadband probe beam from an erbium-doped fiber amplifier are coupled into a QPM waveguide. The PRD effect caused by the irradiation is studied by monitoring the generated second-harmonic light spectrum of the probe light. It is shown that PRD in the QPM waveguide can be qualitatively characterized by the proposed method, and relevant information relating to the QPM wavelength conversion can be extracted directly.
71(2000); http://dx.doi.org/10.1063/1.1150204View Description Hide Description
We discuss three ways to combine two laser beams with equal linear polarizations and very closely spaced frequencies into a single output beam containing up to 100% of the input power of each beam. One setup, a modified Mach–Zehnder interferometer, is examined in detail; it allows to adjust the combined output power electronically with the help of a simple servo loop. With off-the-shelf optical components we obtained a 98% efficiency.
71(2000); http://dx.doi.org/10.1063/1.1150205View Description Hide Description
The design of a diode laser frequency stabilization system using the Zeeman effect is described. Various regimes of operation are analyzed using the Jones matrix approach. The system is different from the original Joint Institute for Laboratory Astrophysicsdesign in that the magnetic fields are fully contained and thus it can be used in proximity of magnetically sensitive instruments.
71(2000); http://dx.doi.org/10.1063/1.1150206View Description Hide Description
A system is described that employs a diode-pumped Nd:YAG continuous-wave laser source servolocked to a three-mirror optical cavity and an analog detection circuit that extracts the ring-down rate from the exponentially decaying ring-down waveform. This scheme improves on traditional cavity ring-down spectroscopy setups by increasing signal acquisition rates to tens of kilohertz and reducing measurement noise sources. For example, an absorptionspectrum of a weak transition at 1064 nm is obtained in less than 10 s at a spectral resolution of 75 kHz employing a cavity with an empty-cavity ring-down decay lifetime of 2.8 μs and a total roundtrip path length of 42 cm. The analog detection system enables laser frequency scan rates greater than 500 MHz/s. The long-term sensitivity of this system is and the short-term sensitivity is
Time-correlated photon counting technique robust against multiple photon events using a multianode photomultiplier tube71(2000); http://dx.doi.org/10.1063/1.1150207View Description Hide Description
A time-correlated photon counting technique which is robust against multiple photon events has been developed. Utilizing a 16 channel multianode photomultiplier tube, both the time interval between the excitation and the first detection of a fluorescence photon and the number of detected fluorescence photons are measured simultaneously. Based on the time and the number data fluorescence decay parameters can be estimated by maximizing the log likelihood. Results of computer simulations and experiments show that this technique can be successfully applicable at high detection rates of fluorescence photon detection event per excitation on average.
Data acquisition card for fluctuation correlation spectroscopy allowing full access to the detected photon sequence71(2000); http://dx.doi.org/10.1063/1.1150208View Description Hide Description
Typically, fluctuation correlation spectroscopy (FCS) data acquisition cards measure the number of photon events per time interval (i.e., bin)—time mode. Commercial FCS cards combine the bins through hardware in order to calculate the autocorrelation function. Such a design therefore does not yield the time resolvedphotonsequence, but only the autocorrelation of that sequence. A different acquisition method which measures the number of time intervals between photon events has been implemented—photon mode. This method takes advantage of the fact that in FCS the rate of photon counts is much less than the frequency of the clock that is used to determine the temporal location of the photons. By using this new mode of data acquisition, the current card design allows for 25 ns time resolution. The data acquisition card can operate in both time and photon mode and yields the time resolvedsequence of photon arrivals in both cases. Therefore, the data is available for analysis by any method(s), such as but not limited to, autocorrelation, photon counting histogram, and higher order autocorrelation.
- CHARGED PARTICLE SOURCES, OPTICS and ACCELERATION
71(2000); http://dx.doi.org/10.1063/1.1150209View Description Hide Description
We constructed an experimental system to observe nonlinear effects in the Compton scattering. The electron beam is supplied by an electron gun and a photon beam comes from a table-top tera-watt laser system. As a test experiment, we measured the Compton scattering in linear regime and had a good agreement with simulations, indicating that the background level of the system was under control and the detection efficiency was well estimated. The simulation study shows that it is necessary to use a laser beam with an inclined polarization plane to obtain a maximal amount of second harmonics in photon-electron scattering.
71(2000); http://dx.doi.org/10.1063/1.1150210View Description Hide Description
New pulsed magnetic quadrupolelenses have been developed that can be used to focus pulsed ion beams of high rigidity. To reach sufficiently high flux densities we built such quadrupolelenses as optimized steel-free devices. Up to now a flux density of ≈ 12 T has been reached in a quadrupole of 22 mm aperture with a sufficient field quality. This value is higher than or comparable to values reached in superconductingquadrupolelenses or in pulsed plasma lenses.
71(2000); http://dx.doi.org/10.1063/1.1150211View Description Hide Description
Reactions with a heavy projectile incident on a light target can be used for the efficient in-flight production of secondary radioactive beams. An overview of this technique is given using data on beams produced via the and reactions. With primary beam currents of 100 pnA, intensities of up to on target were achieved. Using this beam, the reaction was measured.
71(2000); http://dx.doi.org/10.1063/1.1150212View Description Hide Description
A plasma-cathode electron gun based on a moderate pressure (>5 mTorr) cold-cathode discharge and a high perveance, multiaperture accelerator was previously developed at Hughes Research Laboratories and produced electron beam currents of up to 1 kA at voltages of over 200 kV for pulse lengths of 100 μs. This gun was limited in pulse repetition frequency and duty by the gas-puff system that provided adequate gas pressure in the hollow cathode to operate the glow discharge while keeping the pressure in the beam transport region sufficiently low. We describe a new plasma cathode electron gun (PCE gun) that eliminates this problem by replacing the glow-discharge plasma generator in the electron gun by a low-pressure thermionic discharge in a magnetic multipole confinement chamber. Proper design of the plasma generator and electrical circuit provides high electron-current densities to the accelerator structure at very low gas pressure The static gas pressure permits the pulse repetition frequency to be very high (>1.5 kHz demonstrated) with electron beam currents up to 200 A at voltages up to 120 kV demonstrated. The design and performance of the PCE gun, along with several models used to predict and scale the performance, are discussed.
- NUCLEAR PHYSICS, FUSION and PLASMAS
71(2000); http://dx.doi.org/10.1063/1.1150213View Description Hide Description
A repetitive injector with a new porous pellet generator providing steady-state plasma fueling has been developed for fusion devices. The pellet generator can produce an unlimited number of pellets at 0.5–0.9 Hz. Several thousands of hydrogen pellets of 2.5 mm diameter and 2–4 mm length were formed in different modes and accelerated to 0.8–1.1 km/s with an average 70% reliability. The pellet generator has no moving units and contains only 3–5 pellets as reserve fuel for steady-state operation, so the injector design is compatible with and attractive for future tritium operation. A detailed design description and test results are presented.
71(2000); http://dx.doi.org/10.1063/1.1150214View Description Hide Description
New features of the emissive probe characteristic curves in an inductive plasma source were observed in the mode operation. More than the two inflection points in the derivative of the characteristic curves are found when the harmonics of the input frequency (13.56 MHz) are present. The pairs of inflection points are symmetric about dc plasma potential.
- BASIC PHENOMENA
71(2000); http://dx.doi.org/10.1063/1.1150215View Description Hide Description
A nonintrusive capacitive probe diagnostic is developed to estimate the spatio-temporal charge density variation of corona discharge. Tikhonov regularization is used to calculate the charge density from measured potential. A good time resolution and restricted space resolution in charge density is achieved. The axial electric field due to space charge is also estimated by considering the discharge to be of finite radius and with uniformly distributed charge density along the radial direction. Space charge wave front movement, as predicted by existing theories, is noticed. Constraints of present technique and scope for further improvements are discussed.
71(2000); http://dx.doi.org/10.1063/1.1150216View Description Hide Description
Results of numerical simulation of two- and three-electrode spark gaps operating with pure nitrogen gas are compared. Our result show that in both cases the discharge process evolves via two stages very similar to that was reported in nitrogen glow discharge studies. First, the primary electrons produce positive nitrogen ions; second, the produced ions can disturb the applied field, and can create the space charge region. The undulations observed are because of the secondary effects, in particular, the cathode-directed streamers. The delay time for the three-electrode spark gap is more than that of the two-electrode discharge. Current growth is faster for the two-electrode spark gap in comparison with the three-electrode one. The oscillations are more enhanced for the case of the three-electrode spark gap. Our simulation method provides both temporal and spatial information concerning the operation of such switches. The reported results are helpful in design and optimization of both the two- and three-electrode spark gaps.
- MICROSCOPY and IMAGING
71(2000); http://dx.doi.org/10.1063/1.1150217View Description Hide Description
Measurement of the light emission spectrum from a scanning tunneling microscope (STM) requires a long exposure time due to its extremely low intensity, and thermal drift of the tip during the exposure time limits the spatial resolution. To improve the resolution, a computer controlled servomechanism that locks the STM tip over a target position has been developed. We have measured the light emission spectra from individual nanometer scale structures on an evaporated Au film with and without this mechanism, and demonstrated the effectiveness of the servomechanism.
A low-temperature ultrahigh vacuum scanning tunneling microscope with a split-coil magnet and a rotary motion stepper motor for high spatial resolution studies of surface magnetism71(2000); http://dx.doi.org/10.1063/1.1150218View Description Hide Description
We present the design of a new ultrahigh vacuum scanning tunneling microscope(STM) which operates at inside the bore of a 2.5 T superconducting split-coil magnet. The tip/sample region can easily be controlled visually, thus allowing safe and fast exchange of samples and tips while the microscope stays at low temperatures. A newly developed rotary motion stepper motor is presented which allows rotation of the sample by >270° about an axis perpendicular to the tip axis. This feature allows metal or molecular beam evaporation normal to the sample surface. Even more important, by means of this device tip and sample can be brought into a parallel or antiparallel magnetic configuration thus opening a novel approach to the study of magnetic phenomena on an atomic length scale. In addition, measurements of the magneto-optical Kerr effect can be carried out without removing the sample from the STM. Also a new tip exchange mechanism is described. The microscopic and spectroscopic performance of the new instrument is illustrated on Au(111)/mica, on Tb(0001)/W(110), and on Gd(0001)/W(110).
71(2000); http://dx.doi.org/10.1063/1.1150219View Description Hide Description
The design of a simple, variable temperature fluid cell for an atomic force microscope is presented. The stage is based on a thermoelectric heating/cooling element, which allows control of sample and fluid temperature from −5 to 130 °C. The stage is stable enough to image at molecular resolution almost throughout the range of accessible temperature and can be used for imaging in either gas or liquids. This allows the molecular scale investigation of surface phase transitions and chemical kinetics at solid/liquid interfaces by varying the temperature. As an example, we present results of temperature-induced phase transitions in self-assembledsurfactant aggregates at solid/liquid interfaces.
Dynamics of a piezoelectric tuning fork/optical fiber assembly in a near-field scanning optical microscope71(2000); http://dx.doi.org/10.1063/1.1150220View Description Hide Description
Factors leading to a decrease in the resonance quality (the factor) of quartz microtuning fork/optical fiber assemblies used as sensing elements in near-field scanning optical microscopes were considered using a simple elastomechanical analysis. Experiments to test the predictions of the analysis were carried out and strategies for recovering high factors were proposed and tested. Three major factors affecting the magnitude of the factor are discussed. The first is the stiffness imparted to the tine of the microtuning fork by the optical fiber attached to it; the second is the location of the attachment point of the fiber along the tine; the third is the resonant vibrational excitation of the fiber tip which acts as an energy dissipative channel. For tapping mode operation using a standard 125 μm diameter fiber, the large longitudinal stiffness of the fiber results in a dramatic -factor degradation. This effect can be overcome by reducing the diameter of the fiber cladding, and by slightly bending the fiber. Under these conditions, bending rather than longitudinal stretching dominates the fiber dynamics. The effective bending force constant for a thinned fiber is predicted to be proportional to A sharp upturn in the factor is observed for consistent with this prediction. The effective stiffness and mass of the fiber are also expected to scale approximately as where is the distance from the point of attachment of the fiber to the fork’s base. Hence, the factor can be improved further by attaching the fiber closer to the tuning fork’s base. Vibrational coupling between the tuning fork and the probe tip can result in a substantial -factor degradation for tips of a certain size. By taking these insights into consideration, we were able to construct tapping mode tuning fork/optical fiber assemblies with factors of up to 9000.
71(2000); http://dx.doi.org/10.1063/1.1150221View Description Hide Description
A technique has been developed to measure two-dimensional (2D) charge distributions on dielectricsurfaces in real time. A thin layer of polymer-dispersed liquid crystal(PDLC) placed between a dielectric substrate and a conducting plane undergoes a transition from an opaque to a transparent state due to the electric field of the surface charge. The cell is uniformly illuminated from the front and the transmitted light, which is proportional to the local charge density, is imaged using a charge coupled device camera. Due to the finite resistivity of the liquid crystal, however, the static field from the charges must be modulated in time in order to produce a sustained transition inside the PDLC. The modulation is produced by placing a rotating metal chopper near the surface. The magnitude and sign of the charge are measured by calibrating the response of the PDLC to a series of bias voltages. The technique has been used to image surfaces as large as 3 mm×5 mm with 30 μm spatial resolution in 333 ms.
- CONDENSED MATTER; MATERIALS
71(2000); http://dx.doi.org/10.1063/1.1150222View Description Hide Description
A conventional nuclear magnetic resonance(NMR) spectrometer with an original low-frequency limit of 2 MHz and equipped with an electromagnet is rebuilt to allow performing NMR experiments at resonance frequencies down to 100 kHz. The instrument is intended for accurate field-dependent spin relaxation studies. The preamplifier and the duplexer are completely replaced by new designs that provide low noise figure, large gain and fast recovery. The receiver, the transmitter, and the magnet power supply are modified to operate at low frequency (receiver and transmitter) and at low current (power supply). The performance of the instrument is demonstrated by NMR experiments in the 500 kHz–2 MHz region and is compared to that of instruments based on direct current-superconducting quantum interference device and operated on room-temperature samples.