Volume 73, Issue 5, May 2002
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
73(2002); http://dx.doi.org/10.1063/1.1470236View Description Hide Description
We have conductedmeasurements at five different thermal neutron wavelengths to determine the transmission characteristics of a tapered monolithic focusing lens with a focal length of 100 mm, suitable for time-of-flightdiffraction. Both the width of the focused beam and the intensity gain of the optic increase as a function of wavelength. We have performed similar measurements on a polychromatic beam on a pulsed neutron source, where the results are subject to background from short wavelength neutrons. The use of a beryllium filter shows the increased effective gain for the longer wavelengths at the expense of an increased focused beam width by a factor of 2.
73(2002); http://dx.doi.org/10.1063/1.1469672View Description Hide Description
We have developed a simple, compact, high power, diode-pumped, intracavity frequency-doubled, laser capable of generating output powers of up to 70 W at 10 kHz, and 16.5 W at 1 kHz. The output beam quality is highly multimode, with an This, combined with the short output pulse duration of 36–60 ns, and the high average power, makes this laser ideal for pumping ultrafast Ti:sapphire laser amplifier systems.
Superheterodyne light beating spectroscopy for Rayleigh–Brillouin scattering using frequency-tunable lasers73(2002); http://dx.doi.org/10.1063/1.1469671View Description Hide Description
We developed a new optical superheterodyne method for light scattering using frequency tunable lasers and succeeded in measuringBrillouinspectra of simple liquids in a wide frequency range from 10 MHz to 3 GHz. In this method the speed of a photodetector and electric circuits does not limit the upper frequency bound, since the superheterodyne detection of the signal allows us to shift the optical beat frequency down to a desired fixed frequency suitable for electric signal processing. Thus an almost ultimate signal-to-noise ratio is realized at any phonon frequency. This is a great advantage over conventional optical beating methods. It is realized by the continuous tunability of the frequency of the laser used as a local oscillator light for superheterodyne detection. Our method provides an extremely high frequency resolution (∼300 kHz), which is determined by the stability of an optical frequency difference between the two lasers under its feedback control. We call this method “superheterodyne Brillouinspectroscopy.” The method has a high potential for studying the dynamics in a wide class of transparent condensed matters.
Direct measurement of laser power through a high numerical aperture oil immersion objective lens using a solid immersion lens73(2002); http://dx.doi.org/10.1063/1.1470231View Description Hide Description
For many laser applications, information on irradiated laser power is important. However, direct measurement of laser power through a high numerical aperture objective lens is difficult in a laser microscope. In this article, we propose a method which use a solid immersion lens (SIL) for such measurements. A laser beam focused by an objective lens is introduced to the flat surface of a SIL, emitted through the spherical surface, and then detected. In this way the divergence of the laser beam is reduced, and as a result the detection efficiency of the laser power increases. From theoretical analysis, a Weierstrass-sphere type SIL was found to be an appropriate thickness for this propose. Transmittance of the SIL is evaluated for several refractive indeces. The validity of this method is confirmed experimentally.
- PARTICLE SOURCES, OPTICS and ACCELERATION, DETECTORS
Beam measurements on the source and low energy beam transport system for the Spallation Neutron Source73(2002); http://dx.doi.org/10.1063/1.1470238View Description Hide Description
The ion source and low energy beam transport (LEBT) section of the front-end systems presently being built at Lawrence Berkeley National Laboratory are required to provide 50 mA of beam current at a 6% duty factor (1 ms pulses at 60 Hz) with a normalized root mean square emittance of less than 0.20 π mm mrad. Experimental results, including emittance, chopping, and steering measurements, on the performance of the ion source and LEBT system operated at the beam parameters demanded will be discussed.
73(2002); http://dx.doi.org/10.1063/1.1464658View Description Hide Description
A compact plasma accelerator concept based on plasma production at a magnetic cusp was designed and fabricated. Plume and discharge characteristics of the device were documented using a Faraday probe and a retarding potential analyzer. The discharge current variations with increasing discharge voltage were nonlinear with the discharge current increasing rapidly with voltage. The device demonstrated the capability of generating ion beamlets (∼80 eV) with downstream peak current densities comparable to that of higher power ion thrusterdevices In general, the device appeared to operate best at very low flow rates. High propellant utilization fractions (ionization fractions) were measured below 1 SCCM of Xe flow (88% at 0.48 SCCM). Floating potential measurements made downstream of the device were used to estimate the downstream electric field. These measurements, which were used to qualitatively assess beam neutralization, indicated beam neutralization does occur downstream of the device. Based on the measurements, the compact plasma accelerator concept could potentially be used as a low energy ion source for propulsion applications or for low energy plasma/materials processing applications.
73(2002); http://dx.doi.org/10.1063/1.1470234View Description Hide Description
We present results of a systematic study of persistent, or residual, images that occur in charged-coupled device(CCD)detectors. A phenomenological model for these residual images, also known as “ghosting,” is introduced. This model relates the excess dark current in a CCD after exposure to the number of filled impurity sites which is tested for various temperatures and exposure times. We experimentally derive values for the cross section, density, and characteristic energy of the impurity sites responsible for the residual images.
- NUCLEAR PHYSICS, FUSION and PLASMAS
An emissive probe with a rhenium filament for measuring plasma potential in a radio frequency oxygen plasma73(2002); http://dx.doi.org/10.1063/1.1469674View Description Hide Description
Plasma potential measurements in a rf inductively coupled oxygen plasma were carried out using an emissive probe with rhenium filaments. Rhenium was chosen because of the remarkable electrical conductivity of its oxide which is times higher than tungsten oxide. Using 75 μm diam filaments hot-wire emissive probe measurements of an oxygen plasma potential were performed in pressures of 3–100 mTorr. Due to surfacecontaminants, filament conditioning was performed in order to allow electron emission. Analysis of time-averaged current–voltage curves yielded the maximum and minimum of the rf fluctuation in the plasma. Axial profiles of the plasma potential fluctuations at 3.3 mTorr show a nearly linear decrease with the distance from the wall, and demonstrate the feasibility for further investigation of oxygen plasmas using rhenium filaments.
73(2002); http://dx.doi.org/10.1063/1.1463714View Description Hide Description
This article describes the use of neural networks (NNs) for the on-line computation of the radiated power in JET. The NNs have been trained using a database of about 120 discharges, for which the emitted power had been calculated via tomographic inversion of JET bolometric signals. In addition to the bolometric data, elongation and triangularity have been used as input to the NN, since these provide useful complementary information. Dedicated NNs have been designed for the determination of the total radiated power, the power from the bulk, and from the divertor region. All the NNs have been tested with a set of about 30 discharges with positive results. Moreover, the NNs can operate at full sampling speed and are therefore suited to follow edge localized modes and other rapid phenomena. The sensitivity of the NNs to failures in the input signals has also been tested, proving their robustness. Their possible use in feedback applications is finally briefly discussed.
73(2002); http://dx.doi.org/10.1063/1.1463716View Description Hide Description
A technique has been developed to separately obtain emission spectra from individual ion–molecule and electron–ion recombination reactions. This involves using a flowing afterglow to create the reaction producing the emissions of interest and pulse modulating a gas into the afterglow that reacts with only the ion species causing that reaction in the plasma. In this way, photon emissions have been detected where other interfering emissions from the ionization source and from other reaction processes, are more than an order of magnitude larger. The technique has been tested with an plasma to which OCS had been added. In this case, with pulsed modulation of into the plasma, emissions can be isolated from reactions and from the electron recombination of its ion products, as well as from similar reactions involving (generated from the reaction with The technique is such that the majority of the data acquisition time is devoted to collecting data where there is significant signal with little time being used in wavelength regions where there is no significant signal. In these scans, photon counting times are varied to give all signal peaks the same signal to noise ratio independent of intensity and thus the same statistical significance. This is a considerable improvement over standard spectral scanning approaches. Critical features of the technique are evaluated and the means of optimizing the scan are discussed.
- MICROSCOPY and IMAGING
73(2002); http://dx.doi.org/10.1063/1.1470710View Description Hide Description
A couple of experimental techniques have been implemented to an aperturenear-field scanning optical microscopy (NSOM) to obtain reliable measurement of sample dichroism on the local scale. First, a method to test NSOM tapered fiber probes toward polarization conservation into the near optical field is reported. The probes are characterized in terms of the in-plane polarization of the near field emerging from their aperture, by using a thin dichroic layer of chromophore molecules, structured along stretched polymeric chains, to probe such polarization when approached in the near-field region of the probe. Second, to assure that the light intensity coupled in the fiber is polarization independent, an active system operating in real time has been realized. Such combination of techniques allowed quantitative imaging of local dichroism degree and average orientation by means of dual-phase lock-in demodulation of the optical signal. Translation of the coupled light polarization state in the near field has been observed for one-half of the tested probes. For the others, the tip acts as a polarizer, and therefore showed it was not suitable for polarization modulation NSOM measurements.
73(2002); http://dx.doi.org/10.1063/1.1470233View Description Hide Description
We demonstrate high-speed near-field scanning optical microscopy using a tapered fiber tip that is inserted in a perforated quartz-crystal resonator. With this high-frequency dithering probe, we have obtained near-field optical images at the scanning speed of 1.3 mm/s and it takes only 0.5 s to image the surface area of for pixels without any compromise of spatial lateral resolution. As an application, we have obtained sequential scanning images of the diffusing microspheres in water, which shows slow change of the sample configuration in the time scale of several minutes.
- CONDENSED MATTER; MATERIALS
A portable ultrahigh vacuum apparatus for the production and in situ characterization of clusters and cluster-assembled materials73(2002); http://dx.doi.org/10.1063/1.1470226View Description Hide Description
We present and discuss the design and operation of a compact ultrahigh vacuum compatible apparatus for the production and deposition of supersonic cluster beams. The apparatus is equipped with a pulsed microplasma cluster source capable of providing supersonic beams of high stability and intensity. The cluster mass distribution can be analyzed by a two-stage time-of-flightmass spectrometer. The transportability and versatility of the apparatus make the system well suited to perform in situ studies on both gas phase clusters and cluster assembled materials using different characterization facilities. The performances of the system have been tested by scanning tunneling microscopy and photoemissionspectroscopy experiments on cluster-assembled carbon films.
A facility for characterizing the dynamic mechanical behavior of thin membranes for microelectromechanical systems73(2002); http://dx.doi.org/10.1063/1.1470227View Description Hide Description
A bulge testingsystem capable of applying static and dynamic loads to thin film membranes is described. The bulge tester consists of a sealed cavity, filled with a fluid, bounded on the bottom by a circular stainless steel diaphragm and on the top by the thin film membrane of interest. An actuator is used to apply either a static or a periodic force to the stainless steel diaphragm. The force is transmitted through the water to the thin film membrane. This facility provides for both accelerated lifetime testing and simulated service environment testing. The thin film membranes tested are composite stacks consisting of thin films of silicon,glass, metallic electrodes, and lead-zirconate-titanate. Pressure and deflection of a membrane are acquired simultaneously during loading. An image capture system coupled with an interferometer provides the means to capture interferograms of deflected membranes during both static and dynamic testing conditions. Images are then postprocessed to construct deflection versus pressure relationships, which can be used to extract materials’ properties. Accelerated lifetime testing is performed by subjecting the thin film membranes to cyclic loading at strain levels 45%–90% of the static failure strains. In simulated service environment testingthin film membranes are subjected to cyclic loading over a range of frequencies. For a given applied force, as the resonant frequency is approached the dynamic behavior of the thin film structures vary significantly from that observed for static loading. At resonance the deflection of a thin film membrane is almost three times that of a statically deflected membrane subjected to the same applied force.
Optical fiber interferometer for measuring the coefficient of piezoelectric thin films with compensation of substrate bending73(2002); http://dx.doi.org/10.1063/1.1463713View Description Hide Description
An optical fiberinterferometer for measuring the coefficient of piezoelectric samples is described. Its configuration is based on the Mach–Zehnder interferometer, and a double incidence on the thin-film samples successfully suppresses the undesirable bending effect of the substrate. Detection of the small displacement is based on an active homodyne scheme. Results are reported for a bulk piezoelectric transducer(PZT) sample and a PZT thin-film incorporated in a microactuator.
73(2002); http://dx.doi.org/10.1063/1.1464653View Description Hide Description
A simple optical multiple reflection system is developed with two cylindrical concave mirrors at an appropriate spacing. The two cylindrical mirrors have different focal lengths and their principal sections are orthogonal. The alternate focusing of the two cylindrical mirrors at different direction keep the reflecting spots small. The reflecting spots fall on Lissajous patterns on the cylindrical mirrors. The mathematics for this optical system is described and the calculated coordinates of beam spots are very close matches of the experimental observations. The cylindrical mirroroptical system is easy to construct and align, with a suitable method for obtaining long optical paths and a large number of passes in small volumes. In a photoacoustic spectrometer the beam family enhance the effective power in the photoacousticcell and thus the signal-to-noise ratio of photoacoustic signal. An experimental result for photoacoustic spectrum of HDSe gas is given.
A new tool for measuring polar magneto-optical Kerr hysteresis curves at high fields and low temperatures73(2002); http://dx.doi.org/10.1063/1.1468683View Description Hide Description
The design of a unique probe to measure the magneto-optical (MO) Kerr rotation and MO Kerr ellipticity of thin films and multilayers at high magnetic fields (0–25 T) and low temperatures (2–325 K) is described. The design of the probe is based on direct optics. Magnetic iron was used to screen the optical components from the stray field of the magnet. The equipment has sensitivity better than 0.25 mdegree at 632.8 nm and the residual background that results from the Faraday rotation in the optical components is smaller than 2 mdegree/T. The probe measures the polar MO hysteresis curve of samples with dimensions from 0.5–2 cm. Preliminary results on a 9 monolayer iron film and a single-crystalline FePt sample show the performance of this new characterization tool. This new MO magnetometer can be used in one of the resistive magnets of the National High Magnetic Field Laboratory in Tallahassee and is available for all internal and external users.
Noncontact technique for determining viscosity from the shape relaxation of ultrasonically levitated and initially elongated drops73(2002); http://dx.doi.org/10.1063/1.1470228View Description Hide Description
We present a technique that can determine the viscosity of highly viscous liquids, particularly, undercooled liquids that exist at temperatures below their freezing points. The technique involves levitation of a liquid drop using an ultrasonic standing wave, and elongation of the drop by rotating it beyond the point of bifurcation. The elongated drop is then allowed to be restored to its original shape by surface tension driven relaxation. The time-dependent shape parameters of the relaxing drop are related to the viscosity through a relaxation model. In addition, this technique can also determine the surface tension that has a known relationship with the angular velocity at the bifurcation point. The feasibility of the technique is demonstrated by performing the measurement using sucrose solutions as a model liquid drop. The obtained viscosity values show a good correlation with those determined by a falling ball method.
73(2002); http://dx.doi.org/10.1063/1.1470235View Description Hide Description
Real-time monitoring of the dielectric properties of polymer melts and filled polymer melts has been carried out during extrusion. The measurements are obtained using a dielectric cell that is placed directly in line with the extruder machine. The dielectric cell consists of interdigitating electrodes that are deposited on the inside of a ceramic ring that is electrically insulated and temperature controlled to the set point of extrusion. As the processed resin passes through the ring, its permittivity and conductivity are measured. The spatial sensitivity of the cell was determined experimentally and was biased to the resin flowing near the electrodes. Using the spatial sensitivity function, we examined the time profile of the transition from one composition to another during extrusion. We demonstrate the operation of the cell during the processing of polystyrene filled with aluminum oxide and calcium carbonate and of polyethylene-ethyl vinyl acetate copolymer filled with montmorillonite clay.
73(2002); http://dx.doi.org/10.1063/1.1472468View Description Hide Description
A method for directly monitoring two-beam coupling (2BC) in photorefractive materials is presented. This method allows accurate measurement of the gain and loss in the probe and pump beams, respectively, even in situations where substantial power is diffracted into higher order beams (i.e., for large grating spacings). Modulation of the pump beam at a frequency significantly greater than the grating decay rate, coupled with synchronous detection of the modulated probe beam signal, provides this information, automatically correcting for diffractive losses. The probe beam gain and pump beam loss are obtained without further correction. 2BC measurements are made on dye-doped photorefractive polymer-supported liquid crystalfilms to demonstrate the advantages of this method. The measurements are made for small grating spacings, in which little high-order diffraction is observed, and for large grating spacings for which there are several diffraction orders.