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May 2011

Volume 82, Issue 5, Articles (05xxxx)

Issue Cover Spotlight Figure

Rev. Sci. Instrum. 82, 053710 (2011); http://dx.doi.org/10.1063/1.3592992 (7 pages)

Rajiv Giridharagopal, Jun Zhang, and Kevin F. Kelly

A tunable microwave-frequency alternating current scanning tunneling microscope can simultaneously image topography and differential capacitance. Here, the differential capacitance image on an organic semiconductor thin film, poly(3-hexylthiophene), on a gold substrate shows spatial variation in carrier concentration.

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back to top Optics; Atoms and Molecules; Spectroscopy; Photon Detectors

Characterizing plasma mirrors near breakdown

Matthias Geissel, Marius S. Schollmeier, Mark W. Kimmel, Patrick K. Rambo, Jens Schwarz, Briggs W. Atherton, and Erik Brambrink

Rev. Sci. Instrum. 82, 053101 (2011); http://dx.doi.org/10.1063/1.3585981 (7 pages)

Online Publication Date: 9 May 2011

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Experiments dedicated to the characterization of plasma mirrors with a high energy, single shot short-pulse laser were performed at the 100 TW target area of the Z-Backlighter Facility at Sandia National Laboratories. A suite of beam diagnostics was used to characterize a high energy laser pulse with a large aperture through focus imaging setup. By varying the fluence on the plasma mirror around the plasma ignition threshold, critical performance parameters were determined and a more detailed understanding of the way in which a plasma mirror works could be deduced. It was found, that very subtle variations in the laser near field profile will have strong effects on the reflected pulse if the maximum fluence on the plasma mirror approaches the plasma ignition threshold.
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29.27.Fh Beam characteristics

Compact fiber-coupled terahertz spectroscopy system pumped at 800 nm wavelength

Frank Ellrich, Tristan Weinland, Daniel Molter, Joachim Jonuscheit, and René Beigang

Rev. Sci. Instrum. 82, 053102 (2011); http://dx.doi.org/10.1063/1.3587070 (7 pages) | Cited 5 times

Online Publication Date: 10 May 2011

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Photonic terahertz (THz) technology using femtosecond (fs) lasers has a great potential in a wide range of applications, such as non-destructive testing of objects or spectroscopic identification of chemical substances. For industrial purposes, a THz system has to be compact and easily implementable into the particular application. Therefore, fiber-coupled THz systems are the key to a widespread use of THz technology. In order to have flexible THz emitters and detectors near infrared fs light pulses have to be sent through optical fibers of considerable length. As a consequence, the fiber's dispersion has to be compensated for and nonlinear effects in the fiber have to be minimized. A fiber-based THz time-domain spectroscopy system of high stability, flexibility, and portability is presented here.
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07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques
42.62.-b Laser applications
42.81.Dp Propagation, scattering, and losses; solitons
42.81.Qb Fiber waveguides, couplers, and arrays

Measurement of the refractive index dispersion of As2Se3 bulk glass and thin films prior to and after laser irradiation and annealing using prism coupling in the near- and mid-infrared spectral range

N. Carlie, N. C. Anheier, Jr., H. A. Qiao, B. Bernacki, M. C. Phillips, L. Petit, J. D. Musgraves, and K. Richardson

Rev. Sci. Instrum. 82, 053103 (2011); http://dx.doi.org/10.1063/1.3587616 (7 pages) | Cited 3 times

Online Publication Date: 10 May 2011

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The prism coupling technique has been utilized to measure the refractive index in the near- and mid-IR spectral region of chalcogenide glasses in bulk and thin film form. A commercial system (Metricon model 2010) has been modified with additional laser sources, detectors, and a new GaP prism to allow the measurement of refractive index dispersion over the 1.5–10.6 μm range. The instrumental error was found to be ±0.001 refractive index units across the entire wavelength region examined. Measurements on thermally evaporated AMTIR2 thin films confirmed that (i) the film deposition process provides thin films with reduced index compared to that of the bulk glass used as a target, (ii) annealing of the films increases the refractive index of the film to the level of the bulk glass used as a target to create it, and (iii) it is possible to locally increase the refractive index of the chalcogenide glass using laser exposure at 632.8 nm.
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42.70.Ce Glasses, quartz
42.79.Wc Optical coatings
78.35.+c Brillouin and Rayleigh scattering; other light scattering
42.79.Bh Lenses, prisms and mirrors
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
78.66.Jg Amorphous semiconductors; glasses

Measurement of optical-beat frequency in a photoconductive terahertz-wave generator using microwave higher harmonics

Kengo Murasawa, Koki Sato, and Takehiko Hidaka

Rev. Sci. Instrum. 82, 053104 (2011); http://dx.doi.org/10.1063/1.3589859 (7 pages)

Online Publication Date: 11 May 2011

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A new method for measuring optical-beat frequencies in the terahertz (THz) region using microwave higher harmonics is presented. A microwave signal was applied to the antenna gap of a photoconductive (PC) device emitting a continuous electromagnetic wave at about 1 THz by the photomixing technique. The microwave higher harmonics with THz frequencies are generated in the PC device owing to the nonlinearity of the biased photoconductance, which is briefly described in this article. Thirteen nearly periodic peaks in the photocurrent were observed when the microwave was swept from 16 to 20 GHz at a power of −48 dBm. The nearly periodic peaks are generated by the homodyne detection of the optical beat with the microwave higher harmonics when the frequency of the harmonics coincides with the optical-beat frequency. Each peak frequency and its peak width were determined by fitting a Gaussian function, and the order of microwave harmonics was determined using a coarse (i.e., lower resolution) measurement of the optical-beat frequency. By applying the Kalman algorithm to the peak frequencies of the higher harmonics and their standard deviations, the optical-beat frequency near 1 THz was estimated to be 1029.81 GHz with the standard deviation of 0.82 GHz. The proposed method is applicable to a conventional THz-wave generator with a photomixer.
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07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment

Continuous measurement of the arrival times of x-ray photon sequence

Qiurong Yan, Baosheng Zhao, Lizhi Sheng, and Yong'an Liu

Rev. Sci. Instrum. 82, 053105 (2011); http://dx.doi.org/10.1063/1.3592333 (5 pages)

Online Publication Date: 20 May 2011

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In order to record x-ray pulse profile for x-ray pulsar-based navigation and timing, this paper presents a continuous, high-precision method for measuring arrival times of photon sequence with a common starting point. In this method, a high stability atomic clock is counted to measure the coarse time of arrival photon. A high resolution time-to-digital converter is used to measure the fine time of arrival photon. The coarse times and the fine times are recorded continuously and then transferred to computer memory by way of memory switch. The pulse profile is obtained by a special data processing method. A special circuit was developed and a low-level x-ray pulse profile measurement experiment system was setup. The arrival times of x-ray photon sequence can be consecutively recorded with a time resolution of 500 ps and the profile of x-ray pulse was constructed. The data also can be used for analysis by many other methods, such as statistical distribution of photon events per time interval, statistical distribution of time interval between two photon events, photon counting histogram, autocorrelation and higher order autocorrelation.
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97.60.Gb Pulsars
06.30.Ft Time and frequency
42.50.Ar Photon statistics and coherence theory
84.40.Ua Telecommunications: signal transmission and processing; communication satellites
02.50.-r Probability theory, stochastic processes, and statistics

The use of dynamic light scattering and Brownian microscopy to characterize protein aggregation

Ye Li, Vassiliy Lubchenko, and Peter G. Vekilov

Rev. Sci. Instrum. 82, 053106 (2011); http://dx.doi.org/10.1063/1.3592581 (8 pages) | Cited 2 times

Online Publication Date: 26 May 2011

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Dynamic light scattering (DLS) is often used to monitor aggregation in protein solutions. Here, we explore the veracity of the aggregate sizes, size distribution widths, concentrations, and lifetime resulting from DLS. We use as an example a solution of the protein lysozyme in which dense liquid clusters of radius about 100 nm reproducibly exist. We compare the results of DLS to those of Brownian microscopy. We show that because of the sixth power dependence of the scattered light intensity on the size of the scatterers, DLS overestimates the mean size of the clusters. The factor of overestimation depends on the shape of the size distribution and is ∼1.6 × in the studied solution. The related underestimate of the cluster concentration is ∼10 ×. The CONTIN algorithm, often employed to process DLS data, may, in some instances, produce non-physical results. We put forth an alternative method to determine the aggregates’ sizes, concentrations, and volume fractions. We show that DLS yields a reliable width of the cluster size distribution only if the cluster concentration is above 109 cm−3 and their volume fraction is above 10−6. DLS yields a lower bound of the cluster lifetime, which may be orders of magnitude lower than the real one.
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87.15.M- Spectra of biomolecules
87.15.bk Structure of aggregates
87.15.nr Aggregation
87.50.wf Biophysical mechanisms of interaction

Precision and accuracy in film stiffness measurement by Brillouin spectroscopy

M. G. Beghi, F. Di Fonzo, S. Pietralunga, C. Ubaldi, and C. E. Bottani

Rev. Sci. Instrum. 82, 053107 (2011); http://dx.doi.org/10.1063/1.3585980 (11 pages) | Cited 1 time

Online Publication Date: 27 May 2011

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The interest in the measurement of the elastic properties of thin films is witnessed by a number of new techniques being proposed. However, the precision of results is seldom assessed in detail. Brillouin spectroscopy (BS) is an established optical, contactless, non-destructive technique, which provides a full elastic characterization of bulk materials and thin films. In the present work, the whole process of measurement of the elastic moduli by BS is critically analyzed: experimental setup, data recording, calibration, and calculation of the elastic moduli. It is shown that combining BS with ellipsometry a fully optical characterization can be obtained. The key factors affecting uncertainty of the results are identified and discussed. A procedure is proposed to discriminate factors affecting the precision from those affecting the accuracy. By the characterization of a model transparent material, silica in bulk and film form, it is demonstrated that both precision and accuracy of the elastic moduli measured by BS can reach 1% range, qualifying BS as a reference technique.
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46.80.+j Measurement methods and techniques in continuum mechanics of solids

A two-reflection divergent differentiating critical angle refractometer

Wenping Guo, Min Xia, Wei Li, Jie Dai, and Kecheng Yang

Rev. Sci. Instrum. 82, 053108 (2011); http://dx.doi.org/10.1063/1.3593501 (8 pages) | Cited 3 times

Online Publication Date: 27 May 2011

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A new critical angle refractometer (CAR) for high accuracy refractive index measurement of liquid has been developed. The instrument improves the accuracy by two reflections in an elongated parallelogram prism, and acquires the angular reflectivity without any angle scanning parts through introduction of a point source with a divergent beam and a charge coupled device. In addition, it employs a simple and robust measurement method that gets the critical angle by differentiating the angular reflectivity. Through investigating absorbing media with absorption index κ (the imaginary part of refractive index) from 0 to 10−2.1, the theoretical calculation shows that the proposed two-reflection CAR would outperform the traditional one-reflection CAR on lowering the principal error from the differentiation method and improving the ability of getting the critical angle. By testing two typical liquids–salt-water solution and milk, the preliminary experiment indicates that this two-reflection divergent differentiating critical angle refractometer is feasible and of high accuracy.
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07.60.Hv Refractometers and reflectometers
42.79.Bh Lenses, prisms and mirrors
back to top Particle Sources, Optics and Acceleration; Particle Detectors

Performance of the Argonne National Laboratory electron cyclotron resonance charge breeder

R. Vondrasek, A. Kolomiets, A. Levand, R. Pardo, G. Savard, and R. Scott

Rev. Sci. Instrum. 82, 053301 (2011); http://dx.doi.org/10.1063/1.3586765 (6 pages) | Cited 5 times

Online Publication Date: 12 May 2011

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An electron cyclotron resonance charge breeder for the Californium rare ion breeder upgrade (CARIBU), a new radioactive beam facility for the Argonne Tandem Linac Accelerator System (ATLAS), has been constructed and commissioned. Charge breeding efficiencies up to 15.6% have been realized for stable beams with a typical breeding time of 10 ms/charge state. The CARIBU system has been undergoing commissioning tests utilizing a 100 mCi 252Cf fission source. A charge breeding efficiency of 14.8 ± 5% has been achieved for the first radioactive beam of 143Cs27+.
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29.27.Bd Beam dynamics; collective effects and instabilities
29.20.Ej Linear accelerators
29.38.-c Radioactive beams
07.77.Ka Charged-particle beam sources and detectors
29.20.Ba Electrostatic accelerators

Ion optical design of a collinear laser-negative ion beam apparatus

C. Diehl, K. Wendt, A. O. Lindahl, P. Andersson, and D. Hanstorp

Rev. Sci. Instrum. 82, 053302 (2011); http://dx.doi.org/10.1063/1.3587617 (7 pages) | Cited 4 times

Online Publication Date: 12 May 2011

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An apparatus for photodetachment studies on atomic and molecular negative ions of medium up to heavy mass (M ≃ 500) has been designed and constructed. Laser and ion beams are merged in the apparatus in a collinear geometry and atoms, neutral molecules and negative ions are detected in the forward direction. The ion optical design and the components used to optimize the mass resolution and the transmission through the extended field-free interaction region are described. A 90° sector field magnet with 50 cm bending radius in combination with two slits is used for mass dispersion providing a resolution of M/ΔM≅800 for molecular ions and M/ΔM≅400 for atomic ions. The difference in mass resolution for atomic and molecular ions is attributed to different energy distributions of the sputtered ions. With 1 mm slits, transmission from the source through the interaction region to the final ion detector was determined to be about 0.14%.
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41.75.Cn Negative-ion beams
42.25.Bs Wave propagation, transmission and absorption
42.15.Eq Optical system design
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
42.62.-b Laser applications

Numerical studies of transient gain reduction process in a multi-wire proportional chamber

Ken Katagiri, Takuji Furukawa, and Koji Noda

Rev. Sci. Instrum. 82, 053303 (2011); http://dx.doi.org/10.1063/1.3585866 (7 pages)

Online Publication Date: 19 May 2011

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A gain reduction process caused by successive beam irradiation in a multi-wire proportional chamber was numerically investigated to clarify the relations between the gas gain variation and the ion density distribution. A numerical code was developed based on a two-dimensional drift-diffusion model in order to evaluate the ion and electron density distributions and the electric field variation caused by the space charge effect. In order to consider the gain reduction process which occurs under the high rate and successive irradiation, the simulations were performed for the time period of ∼10–100 μs, which is much longer than the time required for ions to travel from an anode to a cathode. The numerical simulation results showed that for the low gas gain regime of ∼10, quasi-stationary density distribution of the ions was formed by the high-rate beams of ∼108 − 1010 particles per second, and that the transient variation of the gas gain became constant after establishment of the quasi-stationary ion density distributions.
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29.40.Cs Gas-filled counters: ionization chambers, proportional, and avalanche counters

A compact electron cyclotron resonance proton source for the Paul Scherrer Institute's proton accelerator facility

C. Baumgarten, A. Barchetti, H. Einenkel, D. Goetz, and P. A. Schmelzbach

Rev. Sci. Instrum. 82, 053304 (2011); http://dx.doi.org/10.1063/1.3590777 (7 pages) | Cited 2 times

Online Publication Date: 25 May 2011

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A compact electron cyclotron resonance proton source has been developed and installed recently at the Paul Scherrer Institute's high intensity proton accelerator. Operation at the ion source test stand and the accelerator demonstrates a high reliability and stability of the new source. When operated at a 10 − 12 mA net proton current the lifetime of the source exceeds 2000 h. The essential development steps towards the observed performance are described.
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29.20.-c Accelerators
52.50.Qt Plasma heating by radio-frequency fields; ICR, ICP, helicons
back to top Nuclear Physics, Fusion and Plasmas

Deconvolution of Thomson scattering temperature profiles

R. Scannell, M. Beurskens, P. G. Carolan, A. Kirk, M. Walsh, T. O’Gorman, and T. H. Osborne

Rev. Sci. Instrum. 82, 053501 (2011); http://dx.doi.org/10.1063/1.3581230 (8 pages) | Cited 7 times

Online Publication Date: 5 May 2011

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Deconvolution of Thomson scattering (TS) profiles is required when the gradient length of the electron temperature (Te) or density (ne) are comparable to the instrument function length (ΔR). The most correct method for deconvolution to obtain underlying Te and ne profiles is by consideration of scattered signals. However, deconvolution at the scattered signal level is complex since it requires knowledge of all spectral and absolute calibration data. In this paper a simple technique is presented where only knowledge of the instrument function I(r) and the measured profiles, Te, observed(r) and ne, observed(r), are required to obtain underlying Te(r) and ne(r). This method is appropriate for most TS systems and is particularly important where high spatial sampling is obtained relative to ΔR.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
41.20.Jb Electromagnetic wave propagation; radiowave propagation
06.20.fb Standards and calibration
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma

Evaluation of an electrostatic dust removal system with potential application in next-step fusion devices

F. Q. L. Friesen, B. John, C. H. Skinner, A. L. Roquemore, and C. I. Calle

Rev. Sci. Instrum. 82, 053502 (2011); http://dx.doi.org/10.1063/1.3587619 (4 pages) | Cited 2 times

Online Publication Date: 17 May 2011

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The ability to manage inventories of carbon, tritium, and high-Z elements in fusion plasmas depends on means for effective dust removal. A dust conveyor, based on a moving electrostatic potential well, was tested with particles of tungsten, carbon, glass, and sand. A digital microscope imaged a representative portion of the conveyor, and dust particle size and volume distributions were derived before and after operation. About 10 mm3 volume of carbon and tungsten particles were moved in under 5 s. The highest driving amplitude tested of 3 kV was the most effective. The optimal driving frequency was 210 Hz (maximum tested) for tungsten particles, decreasing to below 60 Hz for the larger sand particles. Measurements of particle size and volume distributions after 10 and 100 cycles show the breaking apart of agglomerated carbon and the change in particle distribution over short timescales (<1 s).
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06.30.Bp Spatial dimensions (e.g., position, lengths, volume, angles, and displacements)
07.60.Pb Conventional optical microscopes

Determination of radial location of rotating magnetic islands by use of poloidal soft x-ray detector arrays in the STOR-M tokamak

M. Dreval, C. Xiao, S. Elgriw, D. Trembach, S. Wolfe, and A. Hirose

Rev. Sci. Instrum. 82, 053503 (2011); http://dx.doi.org/10.1063/1.3593110 (8 pages) | Cited 2 times

Online Publication Date: 26 May 2011

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A technique is presented for determining the radial location of the rotating magnetic islands in the STOR-M tokamak by use of soft x-ray (SXR) detector arrays. The location is determined by examining the difference in the integrated SXR emission intensities through two adjacent lines of sight. A model for calculating dependence of the line integrated SXR emission intensity on the radius, the mode numbers and the magnetic island geometry, has been developed. The SXR difference signal shows phase inversion when the impact parameter of the line of sight sweeps across the magnetic islands. Experimentally, the difference SXR signals significantly reduce noise and suppress the influence of background plasma fluctuations through common mode rejection when a dominant mode exists in the STOR-M tokamak. The radial locations of the m = 2 magnetic islands have been determined under several experimental conditions in the STOR-M discharges. With the decrease in the tokamak discharge current and thus the increase of the safety factor at the edge, the radial location of the m = 2 magnetic islands has been found to move radially inward.
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52.70.La X-ray and γ-ray measurements
29.40.-n Radiation detectors
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
back to top Microscopy and Imaging

A piezo-thermal probe for thermomechanical analysis

Angelo Gaitas, Sachi Gianchandani, and Weibin Zhu

Rev. Sci. Instrum. 82, 053701 (2011); http://dx.doi.org/10.1063/1.3587624 (4 pages) | Cited 5 times

Online Publication Date: 10 May 2011

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Thermomechanical analysis (TMA) is widely used to characterize materials and determine transition temperatures and thermal expansion coefficients. Atomic-force microscopy (AFM) microcantilevers have been used for TMA. We have developed a micromachined probe that includes two embedded sensors: one for measuring the mechanical movement of the probe (deflection) and another for providing localized heating. The new probe reduces costs and complexity and allow for portability thereby eliminating the need for an AFM. The sensitivity of the deflection element ((ΔR/R)/deflection) is 0.1 ppm/nm and its gauge factor is 3.24. The melting temperature of naphthalene is measured near 78.5  °C.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
07.79.Lh Atomic force microscopes
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
81.16.-c Methods of micro- and nanofabrication and processing

The FAST module: An add-on unit for driving commercial scanning probe microscopes at video rate and beyond

Friedrich Esch, Carlo Dri, Alessio Spessot, Cristina Africh, Giuseppe Cautero, Dario Giuressi, Rudi Sergo, Riccardo Tommasini, and Giovanni Comelli

Rev. Sci. Instrum. 82, 053702 (2011); http://dx.doi.org/10.1063/1.3585984 (7 pages) | Cited 2 times

Online Publication Date: 11 May 2011

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multimedia


See Also: Publisher's Note

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We present the design and the performance of the FAST (Fast Acquisition of SPM Timeseries) module, an add-on instrument that can drive commercial scanning probe microscopes (SPM) at and beyond video rate image frequencies. In the design of this module, we adopted and integrated several technical solutions previously proposed by different groups in order to overcome the problems encountered when driving SPMs at high scanning frequencies. The fast probe motion control and signal acquisition are implemented in a way that is totally transparent to the existing control electronics, allowing the user to switch immediately and seamlessly to the fast scanning mode when imaging in the conventional slow mode. The unit provides a completely non-invasive, fast scanning upgrade to common SPM instruments that are not specifically designed for high speed scanning. To test its performance, we used this module to drive a commercial scanning tunneling microscope (STM) system in a quasi-constant height mode to frame rates of 100 Hz and above, demonstrating extremely stable and high resolution imaging capabilities. The module is extremely versatile and its application is not limited to STM setups but can, in principle, be generalized to any scanning probe instrument.
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07.79.Cz Scanning tunneling microscopes

A modular designed ultra-high-vacuum spin-polarized scanning tunneling microscope with controllable magnetic fields for investigating epitaxial thin films

Kangkang Wang, Wenzhi Lin, Abhijit V. Chinchore, Yinghao Liu, and Arthur R. Smith

Rev. Sci. Instrum. 82, 053703 (2011); http://dx.doi.org/10.1063/1.3585986 (7 pages) | Cited 1 time

Online Publication Date: 11 May 2011

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A room-temperature ultra-high-vacuum scanning tunneling microscope for in situ scanning freshly grown epitaxial films has been developed. The core unit of the microscope, which consists of critical components including scanner and approach motors, is modular designed. This enables easy adaptation of the same microscope units to new growth systems with different sample-transfer geometries. Furthermore the core unit is designed to be fully compatible with cryogenic temperatures and high magnetic field operations. A double-stage spring suspension system with eddy current damping has been implemented to achieve ⩽5 pm z stability in a noisy environment and in the presence of an interconnected growth chamber. Both tips and samples can be quickly exchanged in situ; also a tunable external magnetic field can be introduced using a transferable permanent magnet shuttle. This allows spin-polarized tunneling with magnetically coated tips. The performance of this microscope is demonstrated by atomic-resolution imaging of surface reconstructions on wide band-gap GaN surfaces and spin-resolved experiments on antiferromagnetic Mn3N2(010) surfaces.
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07.79.Cz Scanning tunneling microscopes

Development of a mobile magnetic resonance imaging system for outdoor tree measurements

Takeshi Kimura, Yuto Geya, Yasuhiko Terada, Katsumi Kose, Tomoyuki Haishi, Hiroshi Gemma, and Yoshihiko Sekozawa

Rev. Sci. Instrum. 82, 053704 (2011); http://dx.doi.org/10.1063/1.3589854 (5 pages) | Cited 4 times

Online Publication Date: 17 May 2011

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By combining a 0.3 T permanent magnet with flexible rotation and translation mechanism, a probe with a local electromagnetic shielding, several electrical units, a mobile lift, and an electric wagon, a mobile magnetic resonance imaging (MRI) system was developed for outdoor tree measurements. 2D cross-sectional images of normal and diseased branches of a pear tree were acquired for measurements of T1, T2, proton density, and apparent diffusion constant (ADC). The ADC map clearly differentiated diseased from normal branches. A whole-day measurement of the ADC map demonstrated that microscopic water flow in the normal branch changed proportionally with solar radiation. Therefore, we have concluded that our mobile MRI system is a powerful tool for studies of plants in outdoor environments.
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07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques

Atomic resolution ultrafast scanning tunneling microscope with scan rate breaking the resonant frequency of a quartz tuning fork resonator

Quanfeng Li and Qingyou Lu

Rev. Sci. Instrum. 82, 053705 (2011); http://dx.doi.org/10.1063/1.3585200 (4 pages)

Online Publication Date: 18 May 2011

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We present an ultra-fast scanning tunneling microscope with atomic resolution at 26 kHz scan rate which surpasses the resonant frequency of the quartz tuning fork resonator used as the fast scan actuator. The main improvements employed in achieving this new record are (1) fully low voltage design (2) independent scan control and data acquisition, where the tuning fork (carrying a tip) is blindly driven to scan by a function generator with the scan voltage and tunneling current (IT) being measured as image data (this is unlike the traditional point-by-point move and measure method where data acquisition and scan control are switched many times).
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07.79.Cz Scanning tunneling microscopes
07.60.Pb Conventional optical microscopes
07.05.Hd Data acquisition: hardware and software

Three-view stereoscopy in dusty plasmas under microgravity: A calibration and reconstruction approach

Michael Himpel, Birger Buttenschön, and André Melzer

Rev. Sci. Instrum. 82, 053706 (2011); http://dx.doi.org/10.1063/1.3589858 (9 pages) | Cited 3 times

Online Publication Date: 18 May 2011

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A three-camera stereoscopy setup is presented that allows to reconstruct the trajectories of particles in dusty plasmas under microgravity. The calibration procedure for the three-camera setup takes the special circumstances into account that occur in close-range imaging of small particles. Additionally, a reconstruction algorithm is presented that is based on the epipolar geometry and delivers the essential particle correspondences. Further improvements are achieved by analyzing the dynamic particle behavior. Two applications of our calibration and reconstruction procedure are presented: A two-dimensional dust structure in the laboratory with a large percentage of hidden particles, and particles inside the void of a dust cloud under microgravity.
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52.27.Lw Dusty or complex plasmas; plasma crystals
42.30.Va Image forming and processing
06.20.fb Standards and calibration
52.25.Fi Transport properties
52.70.Kz Optical (ultraviolet, visible, infrared) measurements

Errors in particle tracking velocimetry with high-speed cameras

Yan Feng, J. Goree, and Bin Liu

Rev. Sci. Instrum. 82, 053707 (2011); http://dx.doi.org/10.1063/1.3589267 (7 pages) | Cited 8 times

Online Publication Date: 20 May 2011

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Velocity errors in particle tracking velocimetry (PTV) are studied. When using high-speed video cameras, the velocity error may increase at a high camera frame rate. This increase in velocity error is due to particle-position uncertainty, which is one of the two sources of velocity errors studied here. The other source of error is particle acceleration, which has the opposite trend of diminishing at higher frame rates. Both kinds of errors can propagate into quantities calculated from velocity, such as the kinetic temperature of particles or correlation functions. As demonstrated in a dusty plasma experiment, the kinetic temperature of particles has no unique value when measured using PTV, but depends on the sampling time interval or frame rate. It is also shown that an artifact appears in an autocorrelation function computed from particle positions and velocities, and it becomes more severe when a small sampling-time interval is used. Schemes to reduce these errors are demonstrated.
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06.30.Gv Velocity, acceleration, and rotation
07.07.Hj Display and recording equipment, oscilloscopes, TV cameras, etc.

Low axial drift stage and temperature controlled liquid cell for z-scan fluorescence correlation spectroscopy in an inverted confocal geometry

Edward S. Allgeyer, Sarah M. Sterling, David J. Neivandt, and Michael D. Mason

Rev. Sci. Instrum. 82, 053708 (2011); http://dx.doi.org/10.1063/1.3590017 (6 pages)

Online Publication Date: 20 May 2011

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A recent iteration of fluorescence correlation spectroscopy (FCS), z-scan FCS, has drawn attention for its elegant solution to the problem of quantitative sample positioning when investigating two-dimensional systems while simultaneously providing an excellent method for extracting calibration-free diffusion coefficients. Unfortunately, the measurement of planar systems using (FCS and) z-scan FCS still requires extremely mechanically stable sample positioning, relative to a microscope objective. As axial sample position serves as the inherent length calibration, instabilities in sample position will affect measured diffusion coefficients. Here, we detail the design and function of a highly stable and mechanically simple inverted microscope stage that includes a temperature controlled liquid cell. The stage and sample cell are ideally suited to planar membrane investigations, but generally amenable to any quantitative microscopy that requires low drift and excellent axial and lateral stability. In the present work we evaluate the performance of our custom stage system and compare it with the stock microscope stage and typical sample sealing and holding methods.
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06.20.fb Standards and calibration

Reconstruction of a scanned topographic image distorted by the creep effect of a Z scanner in atomic force microscopy

Cheolsu Han and Chung Choo Chung

Rev. Sci. Instrum. 82, 053709 (2011); http://dx.doi.org/10.1063/1.3590778 (8 pages) | Cited 2 times

Online Publication Date: 20 May 2011

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We analyzed the illusory slopes of scanned images caused by the creep of a Z scanner in an atomic force microscope (AFM) operated in constant-force mode. A method to reconstruct a real topographic image using two scanned images was also developed. In atomic force microscopy, scanned images are distorted by undesirable effects such as creep, hysteresis of the Z scanner, and sample tilt. In contrast to other undesirable effects, the illusory slope that appears in the slow scanning direction of an AFM scan is highly related to the creep effect of the Z scanner. In the controller for a Z scanner, a position-sensitive detector is utilized to maintain a user-defined set-point or force between a tip and a sample surface. This serves to eliminate undesirable effects. The position-sensitive detector that detects the deflection of the cantilever is used to precisely measure the topography of a sample. In the conventional constant-force mode of an atomic force microscope, the amplitude of a control signal is used to construct a scanned image. However, the control signal contains not only the topography data of the sample, but also undesirable effects. Consequently, the scanned image includes the illusory slope due to the creep effect of the Z scanner. In an automatic scanning process, which requires fast scanning and high repeatability, an atomic force microscope must scan the sample surface immediately after a fast approach operation has been completed. As such, the scanned image is badly distorted by a rapid change in the early stages of the creep effect. In this paper, a new method to obtain the tilt angle of a sample and the creep factor of the Z scanner using only two scanned images with no special tools is proposed. The two scanned images can be obtained by scanning the same area of a sample in two different slow scanning directions. We can then reconstruct a real topographic image based on the scanned image, in which both the creep effect of the Z scanner and the slope effect of the sample have been eliminated. The slope effect of the sample should be eliminated so as to avoid further distortion after removal of the creep effect. The creep effect can be removed from the scanned image using the proposed method, and a real topographic image can subsequently be efficiently reconstructed.
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42.30.Wb Image reconstruction; tomography

Antenna-based ultrahigh vacuum microwave frequency scanning tunneling microscopy system

Rajiv Giridharagopal, Jun Zhang, and Kevin F. Kelly

Rev. Sci. Instrum. 82, 053710 (2011); http://dx.doi.org/10.1063/1.3592992 (7 pages)

Online Publication Date: 26 May 2011

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The instrumental synthesis of high resolution scanning tunneling microscopy (STM) with the ability to measure differential capacitance with atomic scale resolution is highly desirable for fundamental metrology and for the study of novel physical characteristics. Microwave frequency radiation directed at the tip-sample junction in an STM system allows for such high-resolution differential capacitance information. This ability is particularly critical in ultrahigh vacuum environments, where the additional parameter space afforded by including a capacitance measurement would prove powerful. Here we describe the modifications made to a commercial scanning tunneling microscope to allow for broad microwave frequency alternating current scanning tunneling microscopy (ACSTM) in ultrahigh vacuum conditions using a relatively simple loop antenna and microwave difference frequency detection. The advantages of our system are twofold. First, the use of a removable antenna on a commercial STM prevents interference with other UHV processes while providing a simple method to retrofit any commercial UHV-STM with UHV-ACSTM capability. Second, mounting the microwave antenna on a translator allows for specific tuning of the system to replicate experimental conditions between samples, which is particularly critical in sensitive systems like organic thin films or single molecules where small changes in incident power can affect the results. Our innovation therefore provides a valuable approach to give nearly any commercial STM, be it an ambient or UHV system, the capability to measure atomic-scale microwave studies such as differential capacitance or even single molecule microwave response, and it ensures that experimental ACSTM conditions can be held constant between different samples.
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07.79.Cz Scanning tunneling microscopes
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment
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