Volume 81, Issue 9, September 2010
Index of content:
- Optics; Atoms and Molecules; Spectroscopy; Photon Detectors
81(2010); http://dx.doi.org/10.1063/1.3474226View Description Hide Description
A method has been established that generates values spaced according to a mathematical function, specifically the logarithm function that can be applied to a stepper motor. Here, it is applied to yield logarithmically spaced time delay points for subnanosecond interferometric time-resolved experiments using a stepper motor controlled translation stage. Application of this method is discussed in terms of three input parameters: the optical delay stage time resolution,; the time of maximum delay, ; and the desired number of data points, . The method improves the efficiency of interferometric time-resolved data collection while providing data collection effective to determine decay parameters. In principle, this technique could be generalized to any mathematical function.
81(2010); http://dx.doi.org/10.1063/1.3480551View Description Hide Description
The optical configuration of a Fabry–Pérot interferometer is uncomplicated. This has already been applied in different measurement systems. For the displacement measurement with the Fabry–Pérot interferometer, the result is significantly influenced by the tilt angles of the measurement mirror in the interferometer. Hence, only for the rather small measuring range, the Fabry–Pérot interferometer is available. The goal of this investigation is to enhance the measuring range of Fabry–Pérot interferometer by compensating the tilt angles. To verify the measuring characteristic of the self-developed Fabry–Pérot interferometer, some comparison measurements with a reference standard have been performed. The maximum deviation of comparison experiments is less than in the traveling range of 30 mm. The experimental results show that the Fabry–Pérot interferometer is highly stable, insensitive to environment effects, and can meet the measuring requirement of the submicrometer order.
81(2010); http://dx.doi.org/10.1063/1.3475689View Description Hide Description
We present the AS-2 Attosecond Beamline at the Joint Laboratory for Attosecond Physics of the Max-Planck-Institut für Quantenoptik and Ludwig-Maximilians-Universität for time resolved pump/probe experiments with attosecond resolution. High harmonic generation and subsequent filtering of the generated extreme ultraviolet (XUV) continuum by means of metal filters and XUV multilayermirrors serve for the generation of isolated attosecond laser pulses. After high harmonic generation, the remaining fundamental laser pulse is spatially separated from the attosecond XUV pulse, to what is to our knowledge for the first time, by means of a perforated mirror in a Mach–Zehnder interferometer. Active stabilization of this interferometer guarantees the necessary temporal resolution for tracking attosecond dynamics in real time. As a proof-of-principle, photoelectron streaking experiments are performed and experimental techniques for their realization are summarized. Finally we highlight the potential of the presented beamline system for future experiments in comparison with previously demonstrated attosecond beamlines.
81(2010); http://dx.doi.org/10.1063/1.3484189View Description Hide Description
A new method based on the double-beam polarization balance bridge technique for measuring the ultralow total loss, including absorption, scattering, and reflection in ultrahigh antireflection mirror is presented. By rotating the polarizer before and after the insertion of a mirror into one beam out of the Wollaston prism to adjust the intensities of the two beams to be equal, the total loss can be obtained by recording the angle variation of the polarizer. The factors affecting the measurement precision of the technique and the main sources of errors are investigated in detail. The total loss measurement precision of the mirror is 10 ppm and the zero drift is 10 ppm at 632.8 nm.
- Particle Sources, Optics and Acceleration; Particle Detectors
Radial current high power dummy load for characterizing the high power laser triggered transformer-type accelerator81(2010); http://dx.doi.org/10.1063/1.3480546View Description Hide Description
A radial-current aqueous resistive solution load was applied to characterize a laser triggered transformer-type accelerator. The current direction in the dummy load is radial and is different from the traditional load in the axial. Therefore, this type of dummy load has smaller inductance and fast response characteristic. The load was designed to accommodate both the resistance requirement of accelerator and to allow optical access for the laser. Theoretical and numerical calculations of the load’s inductance and capacitance are given. The equivalent circuit of the dummy load is calculated in theory and analyzed with a PSPICE code. The simulation results agree well with the theoretical analysis. At last, experiments of the dummy load applied to the high power spiral pulse forming line were performed; a quasisquare pulse voltage is obtained at the dummy load.
A versatile automated sample changer for texture measurements on the high pressure-preferred orientation neutron diffractometer81(2010); http://dx.doi.org/10.1063/1.3485035View Description Hide Description
An automated sample changer with an Eulerian cradle for neutron texture measurements is described. This device has been measuring over 2300 texture and almost 400 powder samples at ambient conditions since it became operational in 2002 for use in the high pressure-preferred orientation diffractometer at the LANSCE neutron scattering facility. Operation for almost a decade resulted in sustained enhancements of mechanics, electronics, and software which significantly improved reliability and resiliency. We also describe in this paper our platform independent computer program POD2K which we use to create publication quality pole figure plots for texture samples.
- Nuclear Physics, Fusion and Plasmas
81(2010); http://dx.doi.org/10.1063/1.3478338View Description Hide Description
A radio frequency (rf) compensation design using auxiliary double probes connected in parallel with a main measurement probe was developed for Langmuir probediagnostics. This probe structure can reduce the sheath impedance of the main probe. In our probe design, the sheathcapacitance of the probe can be increased and its sheath resistance can be decreased with increasing dc bias differential voltage between the auxiliary double probes. The I-V characteristic curve and electron energy distribution functions measured by our probe system had sufficient rf compensation performance in inductively coupled plasmas.
81(2010); http://dx.doi.org/10.1063/1.3482070View Description Hide Description
We present the experimental details and results from a low energy but high repetition rate compact plasma capillary source for extreme ultraviolet and soft x-ray research and applications. Two lengths of capillary are mounted in two versions of a closely related design. The discharge operates in 1.6 and 3.2 mm inner diameter alumina capillaries of lengths 21 and 36 mm. The use of water both as dielectric and as coolant simplifies the compact low inductance design with nanosecond discharge periods. The stored electrical energy of the discharge is approximately 0.5 J and is provided by directly charging the capacitor plates from an inexpensive insulated-gate bipolar transistor in or less. We present characteristic argon spectra from plasma between 30 and 300 Å as well as temporally resolved x-rayenergy fluence in discrete bands on axis. The spectra also allow the level of ablated wall material to be gauged and associated with useful capillary lifetime according to the chosen configuration and energy storage. The connection between the electron beams associated with the transient hollow cathode mechanism, soft x-ray output, capillary geometry, and capillary lifetime is reported. The role of these e-beams and the plasma as measured on-axis is discussed. The relation of the electron temperature and the ionization stages observed is discussed in the context of some model results of ionization in a non-Maxwellian plasma.
Development of a thermal neutron detector based on scintillating fibers and silicon photomultipliers81(2010); http://dx.doi.org/10.1063/1.3480995View Description Hide Description
We propose a technique for thermal neutrondetection, based on a converter placed in front of scintillating fibers readout by means of siliconphotomultipliers. Such a technique allows building cheap and compact detectors and dosimeters, thus possibly opening new perspectives in terms of granular monitoring of neutron fluxes as well as space-resolved neutrondetection.
81(2010); http://dx.doi.org/10.1063/1.3480556View Description Hide Description
A method to infer the current density distribution in the current sheath of a plasma focus discharge from a magnetic probe is formulated and then applied to experimental data obtained in a 1.1 kJ device. Distortions on the magnetic probe signal caused by current redistribution and by a time-dependent total discharge current are considered simultaneously, leading to an integral equation for the current density. Two distinct, easy to implement, numerical procedures are given to solve such equation. Experimental results show the coexistence of at least two maxima in the current densitystructure of a nitrogen sheath.
81(2010); http://dx.doi.org/10.1063/1.3482056View Description Hide Description
Images of plasma from a fast-framing camera are the most intuitive and reliable data to understand the status of tokamak plasmas. Generation, movement, size variation, and disruptions of plasmas can be clearly observed from these images. However, it is not a simple task to obtain the exact positions and sizes of plasmas using these images because measured two-dimensional images inevitably lose the three-dimensional position information of their targets. In this paper, the exact positions and sizes of plasmas have been calculated from tangentially viewed images using the toroidal symmetry of the plasma in the Korea SuperconductingTokamak Advanced Research (KSTAR) device. The positions and sizes of the plasmas have been manually determined with displayed cursors on images and automatically determined using a correlation factor with calculated virtual plasma images as well. These two results correspond well with each other. The estimated positions have also been compared with those obtained from magnetic diagnosticsanalysis.
- Microscopy and Imaging
A procedure to determine the optimum imaging parameters for atomic/molecular resolution frequency modulation atomic force microscopy81(2010); http://dx.doi.org/10.1063/1.3477995View Description Hide Description
We propose a general procedure to determine the optimum imaging parameters (spring constant and oscillation amplitude) to obtain the optimum resolution in frequency modulationatomic force microscopy. We calculated the effective signal-to-noise ratio for various spring constants and oscillation amplitudes, based on the measurement of frequency shift and energy dissipation versus tip-sample distance curves, to find the optimum. We applied this procedure for imaging a lead phthalocyanine (PbPc) thin film on a substrate, and found that the optimum parameters were about 5 N/m and 20 nm, respectively. An improved signal-to-noise ratio was attained in a preliminary experiment using parameters which were close to the calculated optimum.
A new method of Q factor optimization by introducing two nodal wedges in a tuning-fork/fiber probe distance sensor81(2010); http://dx.doi.org/10.1063/1.3480557View Description Hide Description
We report on a new method of achieving and optimizing a high Q factor in a near-field scanning optical microscope (NSOM) by introducing two nodal wedges to a tuning-fork/fiber probe distance sensor and by selecting a vibrational mode of the dithering sensor. The effect of the nodal wedges on the dynamical properties of the sensor is theoretically analyzed and experimentally confirmed. The optimization achieved by the proposed method is understood from the vibration isolation and the subsequent formation of a local vibration cavity. The optimal condition is found to be less susceptible to the variation of the fiber tip length. This method allows effective NSOM measurement of samples placed even in aqueous solution.
Coating of tips for electrochemical scanning tunneling microscopy by means of silicon, magnesium, and tungsten oxides81(2010); http://dx.doi.org/10.1063/1.3484191View Description Hide Description
Different combinations of metal tips and oxide coatings have been tested for possible operation in electrochemicalscanning tunneling microscopy. Silicon and magnesium oxides have been thermally evaporated onto gold and platinum-iridium tips, respectively. Two different thickness values have been explored for both materials, namely, 40 and 120 nm for silicon oxide and 20 and 60 nm for magnesium oxide. Alternatively, tungsten oxide has been grown on tungsten tips via electrochemical anodization. In the latter case, to seek optimal results we have varied the pH of the anodizing electrolyte between one and four. The oxide coated tips have been first inspected by means of scanning electron microscopy equipped with microanalysis to determine the morphological results of the coating. Second, the coated tips have been electrically characterized ex situ for stability in time by means of cyclic voltammetry in 1 M aqueous KCl supporting electrolyte, both bare and supplemented with complex at 10 mM concentration in milliQ water as an analyte. Only the tungsten oxide coatedtungsten tips have shown stable electrical behavior in the electrolyte. For these tips, the uncoated metal area has been estimated from the electrical current levels, and they have been successfully tested by imaging a gold grating in situ, which provided stable results for several hours. The successful tungsten oxide coating obtained at has been assigned to the form.
Precise and millidegree stable temperature control for fluorescence imaging: Application to phase transitions in lipid membranes81(2010); http://dx.doi.org/10.1063/1.3483263View Description Hide Description
We present the design of a custom temperature-controlled chamber suitable for water or oil immersion fluorescence microscopy and its application to phase behavior in lipid bilayer vesicles. The apparatus is self-contained and portable, suitable for multiuser microscopy facilities. It offers a higher temperature resolution and stability than any comparable commercial apparatus, on the order of millidegrees. We demonstrate the utility of the system in the study of miscibility transitions in model membranes. The temperature-dependent phase behavior of model membrane systems that display liquid-ordered phase coexistence with the liquid-disordered phase is relevant to understanding the existence of heterogeneities in biological cell plasma membranes, ubiquitously termed “lipid rafts.”
81(2010); http://dx.doi.org/10.1063/1.3484140View Description Hide Description
We report on the design and performance of a high stability scanning confocal microscope for optical microscopy at low temperatures. By scanning the beam in a cold objective lens system, we achieve wide fields of view without compromising image quality. Photoluminescence from single nitrogen-vacancy centers in high purity diamond is used to illustrate the imaging and stability performance of the microscope.
81(2010); http://dx.doi.org/10.1063/1.3478001View Description Hide Description
We present an imaging system based on light emitting diode(LED)illumination that produces multispectral optical images of the human ocular fundus. It uses a conventional fundus camera equipped with a high power LED light source and a highly sensitive electron-multiplying charge coupled devicecamera. It is able to take pictures at a series of wavelengths in rapid succession at short exposure times, thereby eliminating the image shift introduced by natural eye movements (saccades). In contrast with snapshot systems the images retain full spatial resolution. The system is not suitable for applications where the full spectral resolution is required as it uses discrete wavebands for illumination. This is not a problem in retinal imaging where the use of selected wavelengths is common. The modular nature of the light source allows new wavelengths to be introduced easily and at low cost. The use of wavelength-specific LEDs as a source is preferable to white light illumination and subsequent filtering of the remitted light as it minimizes the total light exposure of the subject. The system is controlled via a graphical user interface that enables flexible control of intensity, duration, and sequencing of sources in synchrony with the camera. Our initial experiments indicate that the system can acquire multispectral image sequences of the human retina at exposure times of 0.05 s in the range of 500–620 nm with mean signal to noise ratio of 17 dB (min 11, std 4.5), making it suitable for quantitative analysis with application to the diagnosis and screening of eye diseases such as diabetic retinopathy and age-related macular degeneration.
Atomic force microscopy imaging using a tip-on-chip: Opening the door to integrated near field nanotools81(2010); http://dx.doi.org/10.1063/1.3477996View Description Hide Description
We describe in detail how atomic force microscopy(AFM) images can be routinely achieved with macroscopic silicon-based chips integrating mesoscopic tips, paving the way for the development of new near field devices combining AFM imaging with any kind of functionality integrated on a chip. The chips have been glued at the end of the free prong of 100 kHz quartztuning forks mounted in Qplus configuration. Numerical simulations by modal analysis have been carried out to clarify the nature of the vibration modes observed in the experimental spectra. It is shown that two low frequency modes can be used to drive the system and scan the surface with a great stability in amplitude modulation as well as in frequency modulationAFM under ultrahigh vacuum. The AFM capabilities are demonstrated through a series of examples including phase and dissipation contrast imaging, force spectroscopy measurements, and investigations of soft samples in weak interaction with the substrate. The lateral resolution with the tips grown by focused ion beam deposition already matches the one achieved in standard amplitude modulation mode AFM experiments.
- Condensed Matter; Materials
81(2010); http://dx.doi.org/10.1063/1.3469783View Description Hide Description
A broadband electron spin resonance spectrometer is described which operates at frequencies between 4 and 40 GHz and can be used in superconducting magnets. A tunable cylindrical cavity is connected to a vector network analyzer via coaxial cables, and the radiation is fed into the cavity by a coupling loop. No field modulation is employed. Resonance frequencies below 14 GHz are obtained by inserting dielectrics with different permittivities into the cavity. The setup allows for measurements with the microwavemagnetic field either parallel or perpendicular to the external field.
Integrating in situ high pressure small and wide angle synchrotron x-ray scattering for exploiting new physics of nanoparticle supercrystals81(2010); http://dx.doi.org/10.1063/1.3480558View Description Hide Description
Combined small and wide angle synchrotron x-ray scattering (SAXS and WAXS) techniques have been developed for in situhigh pressure samples, enabling exploration of the atomic structure and nanoscale superstructure phase relations. These studies can then be used to find connections between nanoparticle surfaces and internal atomic arrangements. We developed a four-axis control system for the detector, which we then employed for the study of two supercrystals assembled from 5 nm and 10 nm Aunanoparticles. We optimized the x-ray energy and the sample-to-detector distance to facilitate simultaneous collection of both SAXS and WAXS. We further performed in situhigh pressure SAXS and WAXS on a cubic supercrystal assembled from 4 nm wurtzite-structure CdSenanoparticles. While wurtzite-structure CdSenanoparticles transform into a rocksalt structure at 6.2 GPa, the cubic superstructure develops into a lamellarlike mesostructure at 9.6 GPa. Nanoparticle coupling and interaction could be enhanced, thus reducing the compressibility of the interparticle spacing above . At , the wurtzite-to-rocksalt phase transformation results in a noticeable drop of interparticle spacing. Above 6.2 GPa, a combined effect from denser CdSenanoparticle causes the interparticle spacing to expand. These findings could be related to a series of changes including the surface structure, electronic and mechanical properties, and strain distribution of CdSe under pressure. This technique opens the way for exploring the new physics of nanoparticles and self-assembled superlattices.