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Top 20 Most Read Articles
September 2006
The 20 articles with the most full-text downloads during the month, in descending order.
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Slice imaging and velocity mapping using a single field Rev. Sci. Instrum. 77, 083101 (2006); http://dx.doi.org/10.1063/1.2222084 (5 pages) Online Publication Date: 2 August 2006
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In this study we demonstrate that it is possible to perform velocity mapping and slice imaging of charged particle products using a single field (two electrodes) improving on the multielectrode geometries of the past. The performance of this new geometry is competitive to the multielectrode ones, presently limited by detector “spot” resolution. For imaging both photoelectrons and photofragments a resolution of ∼ 1% in velocity is achieved without further software manipulation such as event counting. The advantages of this new design are its ability to focus large volumes, its achromaticity (very little sensitivity on photofragment velocities), independence of the laser beam position in the field perpendicular to the extraction field, and compactness. Using this new design we have studied the photodissociation dynamics of pyrrole at 243 nm and show that internal conversion of electronically excited state molecules to the ground state is important even at such low excitation energies.
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Dynamic proportional-integral-differential controller for high-speed atomic force microscopy Rev. Sci. Instrum. 77, 083704 (2006); http://dx.doi.org/10.1063/1.2336113 (7 pages) Online Publication Date: 31 August 2006
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In tapping mode atomic force microscopy, the cantilever tip intermittently taps the sample as the tip scans over the surface. This mode is suitable for imaging fragile samples such as biological macromolecules, because vertical oscillation of the cantilever reduces lateral forces between the tip and sample. However, the tapping force (vertical force) is not necessarily weak enough for delicate samples, particularly for biomolecular systems containing weak inter- or intramolecular interactions. Light tapping requires an amplitude set point (i.e., a constant cantilever amplitude to be maintained during scanning) to be set very close to its free oscillation amplitude. However, this requirement does not reconcile with fast scans, because, with such a set point, the tip may easily be removed from the surface completely. This article presents two devices to overcome this difficulty; a new feedback controller (named as “dynamic proportional-integral-differential controller”) and a compensator for drift in the cantilever-excitation efficiency. Together with other devices optimized for fast scan, these devices enable high-speed imaging of fragile samples.
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Precision and accuracy of thermal calibration of atomic force microscopy cantilevers Rev. Sci. Instrum. 77, 083703 (2006); http://dx.doi.org/10.1063/1.2336115 (6 pages) Online Publication Date: 31 August 2006
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To have confidence in force measurements made with atomic force microscopes (AFMs), the spring constant of the AFM cantilevers should be known with good precision and accuracy, a topic not yet thoroughly treated in the literature. In this study, we compared the stiffnesses of uncoated tipless uniform rectangular silicon cantilevers among thermal, loading, and geometric calibration methods; loading was done against an artifact from the National Institute of Standards and Technology (NIST). The artifact was calibrated at NIST using forces that were traceable to the International System of units. The precision and accuracy of the thermal method were found to be 5% and 10%, respectively. Force measurements taken with different cantilevers can now be meaningfully compared.
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Design and performance of a beetle-type double-tip scanning tunneling microscope Rev. Sci. Instrum. 77, 093701 (2006); http://dx.doi.org/10.1063/1.2336112 (5 pages) Online Publication Date: 12 September 2006
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A combination of a double-tip scanning tunneling microscope with a scanning electron microscope in ultrahigh vacuum environment is presented. The compact beetle-type design made it possible to integrate two independently driven scanning tunneling microscopes in a small space. Moreover, an additional level for coarse movement allows the decoupling of the translation and approach of the tunneling tip. The position of the two tips can be controlled from the millimeter scale down to 50 nm with the help of an add-on electron microscope. The instrument is capable of atomic resolution imaging with each tip.
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Development of a parallel local oxidation nanolithography instrument Rev. Sci. Instrum. 77, 086106 (2006); http://dx.doi.org/10.1063/1.2336773 (3 pages) Online Publication Date: 22 August 2006
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We have developed an instrument to perform local oxidation nanofabrication processes in parallel. The instrument has three major components, the stamp holder, the sample base, and the supporting frame. The sample base is actuated by three precision screws that enable motion in the three orthogonal directions. Sample base and stamp holder are enclosed and sealed inside a chamber with two inlets to introduce different gases. The chamber is supported by a rigid frame. We show the parallel patterning of silicon oxide features on silicon surfaces by the application of a bias voltage between the sample and the stamp when they are in contact. Arrays of parallel lines separated by 100 nm have been patterned over cm2 regions in one minute.
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Interface circuits for quartz crystal sensors in scanning probe microscopy applications Rev. Sci. Instrum. 77, 083701 (2006); http://dx.doi.org/10.1063/1.2238467 (5 pages) Online Publication Date: 7 August 2006
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Complementary to industrial cantilever based force sensors in scanning probe microscopy (SPM), symmetrical quartz crystal resonators (QCRs), e.g., tuning fork, trident tuning fork, and needle quartz sensors, are of great interest. A self-excitation scheme with QCR is particularly promising and allows the development of cheap SPM heads with excellent characteristics. We have developed a high performance electronic interface based on an amplitude controlled oscillator and a phase-locked loop frequency demodulator applicable for QCR with frequencies from 10 up to 10 MHz. The oscillation amplitude of the sensing tip can be set from thermal noise level up to amplitudes of a tenth of nanometers. The device is small, cheap, and highly sensitive in amplitude and frequency measurements. Important features of the design are grounded QCR, parasitic capacity compensation, bridge schematic, and high temperature stability. Characteristic experimental data of the device and its operation in combination with a commercial SPM and a homemade scanning near-field optical microscope are reported. By using the 1 MHz needle quartz resonator with a standard atomic force microscope tip attached, atomic scale resolution in ambient conditions is achieved. Furthermore, reproducible measurements on very soft materials (Langmuir-Blodgett layers) with a very stiff needle quartz ( ∼ 400 000 N/m) are possible.
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Application of frequency combs in the measurement of the refractive index of air Rev. Sci. Instrum. 77, 083104 (2006); http://dx.doi.org/10.1063/1.2239036 (5 pages) Online Publication Date: 7 August 2006
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We report a new method in the precision measurement of the refractive index of air using a highly unbalanced Michelson interferometer with a femtosecond optical frequency comb as the light source. Standard dry air is filled into a 30 m multipass cell, serving as the long arm of the interferometer, while a short arm acts as the reference path. Both time and frequency domain interferograms are recorded to measure the refractive index of air. The deviation of our experimental results with Edlén’s formula is 1.4×10−9 at 800 nm. Our experiment has a standard error of 5.2×10−9 at fixed parameters (pressure and temperature). This is achieved by putting the multipass cell into a temperature-stabilized box, and also by locking the interferometer path length with a He–Ne laser. We achieved a temperature stabilization of 0.8 mK for 25 h. This corresponds to 0.4 μm multipass cell length change. The locking of the He–Ne interferometer enables us to achieve 7 nm path-length change outside the multipass cell. Combined with accurate measurement of temperature and pressure, we were able to achieve an accuracy of 7.7×10−9.
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Assessing and managing laser system stability for quantum control experiments Rev. Sci. Instrum. 77, 083107 (2006); http://dx.doi.org/10.1063/1.2336193 (5 pages) Online Publication Date: 24 August 2006
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Stable laser operation, which is essential for quantum control experiments as well as many other phase dependent processes, is investigated with respect to the influence of amplitude and spectral phase noise. Simulations are first performed and an easy to implement experimental method is presented to monitor the amplitude and phase stability of an ultrafast laser system. As an illustration of this stability assessment technique, the data monitoring is used to guide the identification and elimination of fluctuations in the laser amplification process. Through a number of practical alterations of the amplifier configuration, the stability of the laser system was greatly and consistently improved. Fluctuations on different time scales were eliminated, with special emphasis given to maintaining a stable spectral phase.
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Laser-scanning confocal vibrometer microscope: Theory and experiments Rev. Sci. Instrum. 77, 083702 (2006); http://dx.doi.org/10.1063/1.2336103 (11 pages) Online Publication Date: 18 August 2006
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In this article we present a new laser-scanning confocal microscope for vibration measurements in microscopic mechanical and biological structures. Our system can map out-of-plane vibrations with picometer amplitude resolution while the locus transverse and depth resolutions are in the submicrometer regime. We have achieved this performance by using the measurement beam of a heterodyne laser-Doppler vibrometer as the scanned laser beam of a confocal microscope. The power of the heterodyne carrier is a measurement of the detected light intensity. Therefore, the laser-scanning confocal laser-Doppler vibrometer microscope (CVM) can also be used as common confocal microscope to image and measure geometries of three-dimensional structures. The focus of this article is on the analysis of the transverse resolution, which cannot be adopted from other techniques. Our realized CVM allows vibration measurements up to 20 MHz. The measured minimum 1/e2-power spot diameter of 730 nm enables vibration analysis of submicrometer structures. In this article we discuss the fundamental limits of this new microscope type and we demonstrate the results of our realized system.
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Compact system for high-speed velocimetry using heterodyne techniques Rev. Sci. Instrum. 77, 083108 (2006); http://dx.doi.org/10.1063/1.2336749 (8 pages) Online Publication Date: 24 August 2006
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We have built a high-speed velocimeter that has proven to be compact, simple to operate, and fairly inexpensive. This diagnostic is assembled using off-the-shelf components developed for the telecommunications industry. The main components are fiber lasers, high-bandwidth high-sample-rate digitizers, and fiber optic circulators. The laser is a 2 W cw fiber laser operating at 1550 nm. The digitizers have 8 GHz bandwidth and can digitize four channels simultaneously at 20 GS/s. The maximum velocity of this system is ∼ 5000 m/s and is limited by the bandwidth of the electrical components. For most applications, the recorded beat frequency is analyzed using Fourier transform methods, which determine the time response of the final velocity time history. Using the Fourier transform method of analysis allows multiple velocities to be observed simultaneously. We have obtained high-quality data on many experiments such as explosively driven surfaces and gas gun assemblies.
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Calibration of atomic‐force microscope tips Rev. Sci. Instrum. 64, 1868 (1993); http://dx.doi.org/10.1063/1.1143970 (6 pages)
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Images and force measurements taken by an atomic‐force microscope (AFM) depend greatly on the properties of the spring and tip used to probe the sample’s surface. In this article, we describe a simple, nondestructive procedure for measuring the force constant, resonant frequency, and quality factor of an AFM cantilever spring and the effective radius of curvature of an AFM tip. Our procedure uses the AFM itself and does not require additional equipment. |
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Dynamic atomic force microscopy operation based on high flexure modes of the cantilever Rev. Sci. Instrum. 77, 096105 (2006); http://dx.doi.org/10.1063/1.2348634 (4 pages) Online Publication Date: 12 September 2006
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We show the interest of the high flexure modes of vibration for amplitude-controlled atomic force microscopy (AFM). In connection with AFM working conditions, we define the stabilization time threshold of the oscillating sensor. We show experimentally that, in both air and vacuum, the stabilization time decreases appreciably when the order of the flexure mode of the cantilever increases. Under ambient conditions, this increases the possible scan speeds by about one order of magnitude. Under vacuum and using standard sensors, the amplitude-controlled conditions are satisfied for harmonics equal to or higher than the second. Morphology imaging is then obtained. Thus, high flexure mode AFM easily extends the well known amplitude-controlled operations from ambient to vacuum environment, which allows new AFM applications.
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Rev. Sci. Instrum. 77, 083901 (2006); http://dx.doi.org/10.1063/1.2221539 (9 pages) Online Publication Date: 7 August 2006
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The design and performance of an x-ray photoelectron spectroscopy (XPS)–scanning tunneling microscopy (STM) surface analysis system for studying nanostructured materials are described. The analysis system features electron spectroscopy methods (XPS and Auger electron spectroscopy) in addition to a variable temperature STM. With the analytical methods of the system, surface chemical analysis as well as surface morphology down to atomic resolution can be obtained. The system also provides facilities for sample cleaning, annealing, gas dosing, depth profiling, and surface modifications by sputtering and evaporation. Controlled gas exposures from ultrahigh vacuum to atmospheric pressures in the adjustable temperature range of 120–1100 K can be carried out in different chambers. A fast entry air lock allows the transfer of samples and STM tips into the system without air exposures. The surface analysis system uses a common sample holder in all five chambers which are independently pumped and separated from each other by gate valves. Thus, it is possible to make all sample preparations and experiments in situ under well-defined conditions as illustrated by the formation and characterization of strained, self-assembled nano-oxides on Cu(100).
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Rev. Sci. Instrum. 76, 061101 (2005); http://dx.doi.org/10.1063/1.1927327 (12 pages) Online Publication Date: 26 May 2005
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Nanoelectromechanical systems (NEMS) are drawing interest from both technical and scientific communities. These are electromechanical systems, much like microelectromechanical systems, mostly operated in their resonant modes with dimensions in the deep submicron. In this size regime, they come with extremely high fundamental resonance frequencies, diminished active masses,and tolerable force constants; the quality (Q) factors of resonance are in the range Q ∼ 103–105—significantly higher than those of electrical resonant circuits. These attributes collectively make NEMS suitable for a multitude of technological applications such as ultrafast sensors, actuators, and signal processing components. Experimentally, NEMS are expected to open up investigations of phonon mediated mechanical processes and of the quantum behavior of mesoscopic mechanical systems. However, there still exist fundamental and technological challenges to NEMS optimization. In this review we shall provide a balanced introduction to NEMS by discussing the prospects and challenges in this rapidly developing field and outline an exciting emerging application, nanoelectromechanical mass detection.
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Rev. Sci. Instrum. 77, 083109 (2006); http://dx.doi.org/10.1063/1.2234850 (6 pages) Online Publication Date: 25 August 2006
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The effects of small amounts of energy delivered at times before the peak intensity of ultrahigh-intensity ultrafast-laser pulses have been a major obstacle to the goal of studying the interaction of ultraintense light with solids for more than two decades now. We describe implementation of a practical double-plasma-mirror pulse cleaner, built into a f = 10 m null telescope and added as a standard beamline feature of a 100 TW laser system for ultraintense laser-matter interaction. Our measurements allow us to infer a pulse-height contrast of 5×1011—the highest contrast generated to date—while preserving ∼ 50% of the laser intensity and maintaining excellent focusability of the delivered beam. We present a complete optical characterization, comparing empirical results and numerical modeling of a double-plasma-mirror system.
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Photoacoustic imaging in biomedicine Rev. Sci. Instrum. 77, 041101 (2006); http://dx.doi.org/10.1063/1.2195024 (22 pages) Online Publication Date: 17 April 2006
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Photoacoustic imaging (also called optoacoustic or thermoacoustic imaging) has the potential to image animal or human organs, such as the breast and the brain, with simultaneous high contrast and high spatial resolution. This article provides an overview of the rapidly expanding field of photoacoustic imaging for biomedical applications. Imaging techniques, including depth profiling in layered media, scanning tomography with focused ultrasonic transducers, image forming with an acoustic lens, and computed tomography with unfocused transducers, are introduced. Special emphasis is placed on computed tomography, including reconstruction algorithms, spatial resolution, and related recent experiments. Promising biomedical applications are discussed throughout the text, including (1) tomographic imaging of the skin and other superficial organs by laser-induced photoacoustic microscopy, which offers the critical advantages, over current high-resolution optical imaging modalities, of deeper imaging depth and higher absorption contrasts, (2) breast cancer detection by near-infrared light or radio-frequency–wave-induced photoacoustic imaging, which has important potential for early detection, and (3) small animal imaging by laser-induced photoacoustic imaging, which measures unique optical absorption contrasts related to important biochemical information and provides better resolution in deep tissues than optical imaging.
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Novel capacitor discharging system Rev. Sci. Instrum. 77, 084701 (2006); http://dx.doi.org/10.1063/1.2336188 (5 pages) Online Publication Date: 22 August 2006
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A relatively simple high voltage discharge system has been designed for capacitor pulse discharge with peak current around 1.5 kA with a time to peak in the microsecond range. The circuit is novel in that all low voltage electronics are powered from the high voltage supply, and the system operates as a relaxation oscillator. The system can be used to study the degradation of the capacitors as a result of pulse discharge.
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Visualization of drop-on-demand inkjet: Drop formation and deposition Rev. Sci. Instrum. 77, 085101 (2006); http://dx.doi.org/10.1063/1.2234853 (8 pages) Online Publication Date: 2 August 2006
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An apparatus developed for visualizing drop-on-demand (DOD) drop formation and impaction on substrates is described. Using a pulsed laser, a low-speed charge coupled device camera, and signal generators, an imaging system based on flash photography is shown to be able to obtain sharp images with a temporal resolution of 200 ns and a spatial resolution of 0.81 μm/pixel. Several steps are taken to minimize the “first drop problem” so that excellent reproducibility is achieved; drop formation is reproducible with a positional variation of 1 μm. The visualization system coupled with a motorized stage allows imaging of the impaction of micron-size drops on surfaces. A wave form generator and an amplifier are used to produce the required wave form for a given printhead and ejected liquid. Demonstration of the system for study of DOD micron-size drop formation and impaction on a substrate is presented. The effects of signal wave form on DOD drop formation are demonstrated using a Trident printhead driven by four different signal wave forms. The effect of surface wettability on micron-size drop impaction is shown using hydrophilic and hydrophobic surfaces.
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Alternating high-voltage biasing for terahertz large-area photoconductive emitters Rev. Sci. Instrum. 77, 083111 (2006); http://dx.doi.org/10.1063/1.2336764 (5 pages) Online Publication Date: 31 August 2006
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High-voltage biasing is necessary for efficient generation of terahertz radiation using large-area photoconductive emitters and for electric-field-oriented charge-transfer studies. Coherent detection of terahertz pulses allows ac biasing to be the basis of modulation for lock-in detection. Biasing emitters with an ac field also removes the need for a complete conduction path. The experimental advantages of this approach along with a simple resonant method of generating the high-voltage bias applicable to higher-repetition-rate (up to a few hundred kilohertz) regeneratively amplified systems are described.
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Heat-and-pull rig for fiber taper fabrication Rev. Sci. Instrum. 77, 083105 (2006); http://dx.doi.org/10.1063/1.2239033 (5 pages) Online Publication Date: 18 August 2006
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We describe a reproducible method of fabricating adiabatic tapers with 3–4 μm diameter. The method is based on a heat-and-pull rig, whereby a CO2 laser is continuously scanned across a length of fiber that is being pulled synchronously. Our system relies on a CO2 mirror mounted on a geared stepper motor in order to scan the laser beam across the taper region. We show that this system offers a reliable alternative to more traditional rigs incorporating galvanometer scanners. We have routinely obtained transmission losses between 0.1 and 0.3 dB indicating the satisfactory production of adiabatic tapers. The operation of the rig is described in detail and an analysis on the produced tapers is provided. The flexibility of the rig is demonstrated by fabricating prolate dielectric microresonators using a microtapering technique. Such a rig is of interest to a range of fields that require tapered fiber fabrication such as microcavity-taper coupling, atom guiding along a tapered fiber, optical fiber sensing, and the fabrication of fused biconical tapered couplers.
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