Volume 73, Issue 6, June 2002
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY; PHOTON DETECTORS
Few-cycle mid-infrared pulse generation, characterization, and coherent propagation in optically dense media73(2002); http://dx.doi.org/10.1063/1.1480464View Description Hide Description
The generation of four- to five-cycle mid-infrared pulses using a single-stage potassiumniobateoptical parametric amplifier(OPA) is demonstrated. The OPA is pumped by a gain-switched Ti:Sapphire regenerative amplifier and is seeded with the near-infrared portion of a white light continuum. The OPA is continuously tunable from 2700 to 4700 nm, and maintains sub-65 fs pulses that are fully characterized using cross-correlation frequency-resolved optical gating (XFROG). These are the shortest near-transform-limited pulses reported over this large infrared spectral range. This apparatus is used to measure ultrafast vibrational responses, specifically, pulse profiles modified by the free-induction decay (FID) of O–H oscillators. Both the intensity and instantaneous phase of these pulses are determined after traversing samples of isotopically diluted water (HDO in using XFROG, representing a new application of the XFROG technique to the study of (dipolar) molecular responses. Pronounced beating of the trailing edges of FID-modified pulses is observed even below optical densities of one, and the details of these features are found to depend on the chirp of the field used to generate the FID. These results indicate that discretion should be used in the selection of sample conditions for nonlinear infrared spectroscopic measurements.
73(2002); http://dx.doi.org/10.1063/1.1477607View Description Hide Description
The HUT reference spectrometer was modified for measuring specular reflectance in the wavelength range of 300 to 850 nm. The instrument is based on a diffraction-gratingmonochromator, reflecting optics, sample control mechanics and detection systems with linear responsitivities. Relative standard uncertainties between 0.14% and 0.32% were estimated for the reflectance measurements. For spectral reflectance between 1.5% and 15%, the results of test measurements using samples with known reflectances confirm that for all geometries the relative deviations are less than 0.36%. A set of ultraviolet (UV)-interference filters was measured in the UVB wavelength range, and the results are used as a part of filter radiometer characterization.
- PARTICLE SOURCES, OPTICS and ACCELERATION; PARTICLE DETECTORS
A high sensitivity electron momentum spectrometer with simultaneous detection in energy and momentum73(2002); http://dx.doi.org/10.1063/1.1473223View Description Hide Description
A new apparatus for electron–electron coincidence experiments has been developed to examine molecular orbital patterns quantitatively by electron momentum spectroscopy. Using a spherical analyzer and position-sensitive detectors, it enables one to measure energy and angular correlations between the two outgoing electrons simultaneously. The design and performance of the apparatus is reported together with results on Ar to show extensive improvements in coincidence count rates and statistical precision, covering a wide range of binding energies and momenta.
73(2002); http://dx.doi.org/10.1063/1.1472467View Description Hide Description
This article describes the construction and optimization of a Langmuir–Taylor detector for lithium, using a rhenium ribbon. The absolute detection probability of this very sensitive detector is measured and the dependence of this probability with oxygen pressure and surface temperature is studied. Sources of background signal and their minimization are also discussed in detail. A comparison between our data concerning the response time of the detector and literature values is given. A theoretical analysis has been made: this analysis supports the validity of the Saha–Langmuir law to relate the ionization probability to the work function. Finally, the rapid variations of the work function with oxygen pressure and temperature are explained by a chemical equilibrium model.
- NUCLEAR PHYSICS, FUSION and PLASMAS
73(2002); http://dx.doi.org/10.1063/1.1475348View Description Hide Description
The implementation of an innovative technique for measuring the propagation of intense laser pulses through plasma channels is described. At high laser intensities, temporally resolved stimulated Raman backscattering can be used to diagnose both the electron density and the laser intensity inside the plasma channel, observations which are not possible using other techniques. This diagnostic is demonstrated in experiments using an open-ended capillary in which a plasma channel was created. The plasma channel was generated using either an electrical discharge or laser ablation by a second laser pulse.
Differential method for the real time measurement of the diamagnetic β and internal inductance in Tore Supra73(2002); http://dx.doi.org/10.1063/1.1475350View Description Hide Description
A differential method for real time measurements of the diamagnetic energy and poloidal β has been successfully installed on the Tore Supra tokamak. This diagnostic makes use of concentric diamagnetic loops compensated by precise measurements of toroidal current variations. This diamagneticmeasurement is now routinely used in the feedback system to control the internal inductance of the Tore Supra plasma using non-inductive current drive generated by lower hybrid waves.
73(2002); http://dx.doi.org/10.1063/1.1476715View Description Hide Description
A new x-raydiagnostic has been commissioned at the OMEGA laser facility at the University of Rochester. It is a transmission curved crystal spectrometer designed primarily to characterize the hot-electron energy distribution of laser generated plasmas by registering the continuum x-rayspectrum produced by these hot plasmas from 12 to 60 keV. The diagnostic package is assembled in a linear configuration to ride in a standard instrument insertion module. The instrument consists of a nosecone with a blast shield, spectrometer, electronic imager, drive electronics, and battery. The instrument is connected to the external diagnostic processor and control unit by trigger and data fiber optic cables. Time integrated spectra from various targets have been registered with high sensitivity from single shots of the OMEGA laser.
Slit-wire camera, a new diagnostics method for measurement of small scale high-energy density structures in hot plasmas73(2002); http://dx.doi.org/10.1063/1.1480462View Description Hide Description
A new technique has been developed for the detection of small scale plasma objects, particularly in the x-ray and extreme ultraviolet region. The technique is based on a new device, called a slit-wire (S-W) camera. The S-W camera uses a combination of a slit, to provide one-dimensional imaging with large effective aperture, and a set of micron size wires across the slit to provide information about the emitting source based upon blocking of the incident light signal. A series of simultaneous slit-wire images of the same discharge, each with a different filter material, allows the characteristic size of the emitting plasma to be determined over a wide spectral range. Time resolved measurements of the plasma radius of a capillary discharge plasma and time integrated measurements of the hot-spot size in a plasma focus discharge are presented to illustrate the technique.
Millimeter-wave two-dimensional imaging optical system for interferometer of the GAMMA 10 tandem mirror73(2002); http://dx.doi.org/10.1063/1.1473224View Description Hide Description
A millimeter-wave optical system for plasma diagnostics for measuring the two-dimensional distribution of electron density in the Tsukuba GAMMA 10 tandem mirror has been developed. Two-dimensional images of plasma density of a cross-sectional area of in the machine axis direction were successfully measured with the optical system. The position at which the present imaging system is installed corresponds to the plug region, and the plasma radius is relatively narrow (less than 100 mm). Information about the density in the area is valuable in the study of confining-potential formation. This optical system was developed for a millimeter-wave phase-imaging interferometer and was designed with the ray tracing method to reduce aberration. It was evaluated experimentally by measuringimages of point sources and phase images of dielectric targets. The receiving optical system from the plasma to detector array was designed with a spatial resolution of 28.7 mm and a magnification of 0.30. Ray tracing indicated that the transverse spherical aberration was 78% of the sampling interval of 5.0 mm on an image plane and that even for the worst image point the distortion was 10% of the sampling interval. In the design of the optical system for plasma diagnostics, the off-focus effect due to plasma size (plasma diameter: 200 mm) with electron density profile is proposed and considered.
73(2002); http://dx.doi.org/10.1063/1.1480456View Description Hide Description
The x-ray spectra from an electron cyclotron resonance argon plasma produced in a compact magnetic trap, containing a tungsten target, are reported. The maximum attainable x-ray energy was recorded for different magnetic field configurations and energies higher than 300 keV were found. The maximum x-ray energy is strongly peaked near the change of the resonance surface topology from a one sheet hyperboloid to a two sheet hyperboloid. This observation is explained theoretically. Different regimes of x-ray production are described. Two distinct pressure ranges where x rays are produced were found. The characteristic lines of metals, e.g., from the target (tungsten), protection (lead), are clearly resolved. The radiography of an integrated circuit is shown as an example of possible use of the x-raysource for nondestructive control.
- MICROSCOPY and IMAGING
73(2002); http://dx.doi.org/10.1063/1.1476719View Description Hide Description
We present an original surface forces apparatus which enables us to measure the interaction forces between any solid surfaces such as, e.g., metallic surfaces, opaque surfaces, or rough surfaces. The relative displacement of the surfaces is measured with a capacitive sensor. The forces are measured by a stiff and highly sensitive interferometric sensor. The measurements are performed in a dc to 100 Hz bandwidth. This feature allows us to study the mechanical response of a nanometric confined medium to rapid strain variations in the linear regime. An example of nanorheological measurement of dodecane confined in a nanometric gap is given at the end of this article.
73(2002); http://dx.doi.org/10.1063/1.1472465View Description Hide Description
A new method of assessing the magnitude of the magnetic field in a transmission electron microscope using a permanent magnetic material is described. The approach is versatile and simple enough to be implemented for certain scientific or engineering situations in which the exact calibration of the magnetic field in the microscope column using a small Hall probe may be a problem. We have applied this approach to obtain the magnetic fieldcalibration inside a JEM 3000F field emissionelectron microscope as a function of the objective lens potential. In the course of this in situcalibration it was not necessary to disassemble the microscope or interrupt its operation. The procedure used is versatile and accurate enough to measuremagnetic fields up to 20 kOe in SI units) as was confirmed by subsequent Hall-probe field measurement of the same electron microscope. The calibration method described in this article does not require any special Hall-probe holder adaptations and can be applied to any transmission electron microscope or similar instrument. To illustrate the utility of the technique and its results, quantitative analysis of magnetic domain images obtained with Lorentz microscopy during magnetic reversal of a sample subjected to an in situmagnetic field in the JEM3000F microscope is presented and discussed.
Atomic force microscope detector drift compensation by correlation of similar traces acquired at different setpoints73(2002); http://dx.doi.org/10.1063/1.1475352View Description Hide Description
The atomic force microscopemeasures surface topography by maintaining a certain cantilever deflection or vibration amplitude as the cantilever is scanned over a sample surface. The desired cantilever deflection or amplitude is referred to as the setpoint, and is maintained by moving the sample toward or away from the cantilever. The signal from the cantilever deflection detector has a real component, due to cantilever deflection, and a drift component due to various sources of drift. We present a method of eliminating the drift component by sensing and correcting it in real time. Our method involves automatically changing the setpoint so as to maintain a certain set difference in the relative feature richness of two traces taken with slightly offset setpoints. We show how the system maintains a setpoint only 70 mV above minimum, perturb it with a gentle blow of air that causes 200 mV of detector drift, and observe its recovery within
73(2002); http://dx.doi.org/10.1063/1.1477608View Description Hide Description
This article describes the design of a dual-beam optical tweezers (OT) instrument which, in contrast to conventional single-beam OT, directly measures the change in light momentum flux when a trapped object experiences a force. Consequently, no local calibration is needed to measure the force acting on a trapped particle. The instrument has a high trapping efficiency and forces up to 200 pN can be measured. In addition, the above-mentioned system operates in conjunction with a three-dimensional steerable single-beam OT.
73(2002); http://dx.doi.org/10.1063/1.1480459View Description Hide Description
An atomic force microscope for nanocantilevers measuring from a few 100 nm to a few μm in length was implemented. The natural frequencies of the nanocantilevers lie in the range of 1 MHz to 1 GHz, and optical detection schemes adapted to their size and frequency range was selected. A helium neon laser with a beat frequency of 890 MHz was used as the laser source. The beat was shifted to 1090 MHz by an acousto-optical-modulator, and used as the carrier for heterodyne laser Dopplermeasurement. This enabled velocity measurement up to around 100 MHz. The probe beam of the Dopplerinterferometer was guided to the nanocantilever by a single mode polarization-maintaining optical fiber terminated by a collimating lens, a quarter wave plate, and a focusing lens. Reflected light was collected by the same optics and mixed with the reference beam. Self-excitation of the nanocantilever at its lowest natural frequency was implemented for an amplitude of 1 at 36 MHz. The Q factor of the cantilever was 8000. Noise effective amplitude of the Dopplerinterferometer was smaller than 10 above 10 MHz. Frequency detection was possible for a nanowire measuring 100 nm in width.
- CONDENSED MATTER; MATERIALS
73(2002); http://dx.doi.org/10.1063/1.1473222View Description Hide Description
A device to probe the molecular structure of materials next to a solid interface in a thin film geometry has been developed. The device can produce controlled thicknesses as small as 1 μm with parallelity better than 0.003°. We have shown that the thickness and the parallelity of the film produced between two optical surfaces can be quantified using white light and monochromatic light interferometry, respectively. In addition, this apparatus allows the study of these films in a static state or under shear using spectroscopic techniques involving transmission or reflection measurements.
In situ four-point conductivity and Hall effect apparatus for vacuum and controlled atmosphere measurements of thin film materials73(2002); http://dx.doi.org/10.1063/1.1475349View Description Hide Description
An ultrahigh vacuum (UHV) chamber equipped with a fixture for in situ four-point Van Der Pauw conductivity and Hall effect measurements has been constructed and attached to a multichamber thin film synthesis and characterization system. The combined systems allow for film synthesis and characterization of microstructure,chemical composition, morphology, and electronic transportproperties without air exposure. The four-point measurement fixture features spring-loaded probes for electrical contacts and temperature measurement and a sample docking mechanism designed to minimize probe damage to the films. The electronics were designed for measurement of high resistance samples. Measurements can be made at sample temperatures from 25 to in selected gas environments from UHV to atmospheric pressure. The design and performance of the system are reported, and representative results on the electronic transportproperties of n-type Si (100) and tungsten oxide films on sapphire are presented.
73(2002); http://dx.doi.org/10.1063/1.1476716View Description Hide Description
We have demonstrated that piezoelectrically driven, squeeze mode, tubular reservoir liquid drop generation, originally developed as a “drop-on-demand” method for ejection of microdrops of liquids or suspensions, can successfully operate with dry powder. Spherical silverpowder with maximum particle diameter of 20 μm was loaded into and ejected from a 100 μm orifice glass dropper with a flat piezoelectric disk driver. Time of flight experiments were performed to optimize the dropper operation and to determine the size and velocity of the ejected particles. It was found that at certain values of the amplitude, duration, and repetition rate of the voltage pulses applied to the piezoelectric disk, one can eject powder clots of a stable size, comparable with the dropper orifice diameter. In contrast to the operation with a liquid, a clot is not ejected at each pulse, but quasiperiodically with an interval corresponding to thousands of pulses. The application for injection of atoms into helium buffer gas at cryogenic temperatures is discussed.
Multispeckle diffusing-wave spectroscopy: A tool to study slow relaxation and time-dependent dynamics73(2002); http://dx.doi.org/10.1063/1.1476699View Description Hide Description
A multispeckle technique for efficiently measuring correctly ensemble-averaged intensity autocorrelation functions of scattered light from nonergodic and/or nonstationary systems is described. The method employs a charge coupled device(CCD)camera as a multispeckle light detector and a computer-based correlator, and permits the simultaneous calculation of up to 500 correlation functions, where each correlation function is started at a different time. The correlation functions are calculated in real time and are referenced to a unique starting time. The multispeckle nature of the CCDcamera detector means that a true ensemble average is calculated; no time averaging is necessary. The technique thus provides a “snapshot” of the dynamics, making it particularly useful for nonstationary systems where the dynamics are changing with time. Delay times spanning the range from 1 ms to 1000 s are readily achieved with this method. The technique is demonstrated in the multiple scattering limit where diffusing-wave spectroscopytheory applies. The technique can be combined with a two-cell technique that can measure faster decay times. The combined technique can measure delay times from 10 ns to 1000 s. The method is peculiarly well suited for studying aging processes in soft glassy materials, which exhibit both short and long relaxation times, nonergodic dynamics, and slowly evolving transient behavior.
73(2002); http://dx.doi.org/10.1063/1.1475351View Description Hide Description
We describe the design and operation of a temperature controlled Couette shear cell for small angle neutron scattering (SANS) studies of complex fluids under flow. This design incorporates a vapor barrier, which prevents sample evaporation to relatively high shear rates. This cell enables the investigation of systems which are highly sensitive to evaporation. Over the duration of a Couette SANS measurement composition phase transitions due to evaporation can be misinterpreted as true shear-induced transformations. We give a brief report of recent experiments performed on one such system: bicontinuous sponge phases for which the surfactantmembrane constituents are Cetylpyridinium chloride and hexanol. These clearly demonstrate the limitations of previous designs and the utility of the vapor barrier in measurements of a predicted shear induced sponge to lamellar phase transition. Using this cell we also describe and test a simple and effective way to put SANS data taken in the tangential Couette configuration on an absolute scale.