Volume 81, Issue 10, October 2010
Index of content:
- Optics; Atoms and Molecules; Spectroscopy; Photon Detectors
81(2010); http://dx.doi.org/10.1063/1.3480647View Description Hide Description
We describe a high-performance time-resolved fluorescence (HPTRF) spectrometer that dramatically increases the rate at which precise and accurate subnanosecond-resolved fluorescence emission waveforms can be acquired in response to pulsed excitation. The key features of this instrument are an intense , high-repetition rate (10 kHz), and short (1 ns full width at half maximum) laser excitation source and a transient digitizer (0.125 ns per time point) that records a complete and accurate fluorescence decay curve for every laser pulse. For a typical fluorescent sample containing a few nanomoles of dye, a waveform with a signal/noise of about 100 can be acquired in response to a single laser pulse every 0.1 ms, at least times faster than the conventional method of time-correlated single photon counting, with equal accuracy and precision in lifetime determination for lifetimes as short as 100 ps. Using standard single-lifetime samples, the detected signals are extremely reproducible, with waveform precision and linearity to within 1% error for single-pulse experiments. Waveforms acquired in 0.1 s (1000 pulses) with the HPTRF instrument were of sufficient precision to analyze two samples having different lifetimes, resolving minor components with high accuracy with respect to both lifetime and mole fraction. The instrument makes possible a new class of high-throughput time-resolved fluorescence experiments that should be especially powerful for biological applications, including transient kinetics, multidimensional fluorescence, and microplate formats.
81(2010); http://dx.doi.org/10.1063/1.3488357View Description Hide Description
A Bruker Optics IFS 125HR Fourier transform spectrometer (FTS) and the Laboratoire de Physique Moléculaire pour l'Atmosphère et l'Astrophysique retrieval algorithm were adapted for ground based atmospheric measurements. As one of the major instruments of the experimental research platform QualAir, this FTS is dedicated to study the urban air composition of large megacity such as Paris. The precise concentration measurements of the most important atmospheric pollutants are a key to improve the understanding and modeling of urban air pollution processes. Located in the center of Paris, this remote sensing spectrometer enables to monitor many pollutants. Examples for and CO are demonstrating the performances of this new experimental setup.
Trace gas detection based on off-beam quartz enhanced photoacoustic spectroscopy: Optimization and performance evaluation81(2010); http://dx.doi.org/10.1063/1.3480553View Description Hide Description
A gas sensor based on off-beam quartz enhanced photoacoustic spectroscopy was developed and optimized. Specifically, the length and diameter of the microresonator tube were optimized, and the outer tube shape is modified for enhancing the trace gas detection sensitivity. The impact of the distance between the quartz tuning fork and an acoustic microresonator on the sensor performance was experimentally investigated. The sensor performance was evaluated by determining the detection sensitivity to vapor in ambient air at normal atmospheric pressure. A normalized noise equivalent absorption coefficient of was achieved.
81(2010); http://dx.doi.org/10.1063/1.3492154View Description Hide Description
In this paper, a simple nondestructive method is described to obtain the refractive index profile of a gradient refractive index rod lens by means of optical coherence tomography. The approach exploits the fact that optical coherence tomography provides a direct measurement of the optical path of the light traveled through a gradient refractive index rod lens. The refractive index profile for a gradient refractive index rod lens is retrieved by iterative fitting of the optical path calculated by the ray tracing method with that experimentally measured using optical coherence tomography. The measured refractive index profile is in good agreement with theory.
81(2010); http://dx.doi.org/10.1063/1.3494616View Description Hide Description
We present two implementations of photon counting time-multiplexing detectors for near-infrared wavelengths, based on Peltier cooled InGaAs/InP avalanche photodiodes. A first implementation is motivated by practical considerations using only commercially available components. It features 16 bins, pulse repetition rates of up to 22 kHz, and a large range of applicable pulse widths of up to 100 ns. A second implementation is based on rapid gating detectors, permitting dead times below 10 ns. This allows one to realize a high dynamic-range 32 bin detector, able to process pulse repetition rates of up to 6 MHz for pulse widths of up to 200 ps. Analysis of the detector response at 16.5% detection efficiency reveals a single-shot energy resolution on the attojoule level.
Study on the effect of measuring methods on incident photon-to-electron conversion efficiency of dye-sensitized solar cells by home-made setup81(2010); http://dx.doi.org/10.1063/1.3488456View Description Hide Description
An experimental setup is built for the measurement of monochromatic incident photon-to-electron conversion efficiency (IPCE) of solar cells. With this setup, three kinds of IPCE measuring methods as well as the convenient switching between them are achieved. The setup can also measure the response time and waveform of the short-circuit current of solar cell. Using this setup, IPCE results of dye-sensitized solar cells (DSCs) are determined and compared under different illumination conditions with each method. It is found that the IPCE values measured by AC method involving the lock-in technique are sincerely influenced by modulation frequency and bias illumination.Measurements of the response time and waveform of short-circuit current have revealed that this effect can be explained by the slow response of DSCs. To get accurate IPCE values by this method, the measurement should be carried out with a low modulation frequency and under bias illumination. The IPCE values measured by DC method under the bias light illumination will be disturbed since the short-circuit current increased with time continuously due to the temperature rise of DSC. Therefore, temperature control of DSC is considered necessary for IPCE measurement especially in DC method with bias light illumination. Additionally, high bias light intensity is found to decrease the IPCE values due to the ion transport limitation of the electrolyte.
Green and ultraviolet pulse generation with a compact, fiber laser, chirped-pulse amplification system for aerosol fluorescence measurements81(2010); http://dx.doi.org/10.1063/1.3488363View Description Hide Description
We use a compact chirped-pulse amplified system to harmonically generate ultrashort pulses for aerosolfluorescence measurements. The seed laser is a compact, all-normal dispersion, mode-locked Yb-doped fiber laser with a 1050 nm center wavelength operating at 41 MHz. Average powers of more than 1.2 W at 525 nm and 350 mW at 262 nm are generated with pulse durations. The pulses are time-stretched with high-dispersion fiber, amplified by a high-power, large-mode-area fiber amplifier, and recompressed using a chirped volume holographic Bragg grating. The resulting high-peak-power pulses allow for highly efficient harmonic generation. We also demonstrate for the first time to our knowledge, the use of a mode-locked ultraviolet source to excite individual biological particles and other calibration particles in an inlet air flow as they pass through an optical chamber. The repetition rate is ideal for biofluorescence measurements as it allows faster sampling rates as well as the higher peak powers as compared to previously demonstrated Q-switched systems while maintaining a pulse period that is longer than the typical fluorescence lifetimes. Thus, the fluorescence excitation can be considered to be quasicontinuous and requires no external synchronization and triggering.
81(2010); http://dx.doi.org/10.1063/1.3470126View Description Hide Description
A method of analyzingFaraday rotation data from pulsed magnetic field measurements is described. The method uses direct least-squares elliptical fitting to measured data. The least-squares fit conic parameters are used to rotate, translate, and rescale the measured data. Interpretation of the transformed data provides improved accuracy and time-resolution characteristics compared with many existing methods of analyzingFaraday rotation data. The method is especially useful when linear birefringence is present at the input or output of the sensing medium, or when the relative angle of the polarizers used in analysis is not aligned with precision; under these circumstances the method is shown to return the analytically correct input signal. The method may be pertinent to other applications where analysis of Lissajous figures is required, such as the velocity interferometer system for any reflector (VISAR) diagnostics. The entire algorithm is fully automated and requires no user interaction. An example of algorithm execution is shown, using data from a fiber-based Faraday rotationsensor on a capacitive discharge experiment.
Spatially resolvable optical emission spectrometer for analyzing density uniformity of semiconductor process plasma81(2010); http://dx.doi.org/10.1063/1.3488104View Description Hide Description
We proposed a spatially resolved optical emission spectrometer (SROES) for analyzing the uniformity of plasma density for semiconductor processes. To enhance the spatial resolution of the SROES, we constructed a SROES system using a series of lenses, apertures, and pinholes. We calculated the spatial resolution of the SROES for the variation of pinhole size, and our calculated results were in good agreement with the measuredspatial variation of the constructed SROES. The performance of the SROES was also verified by detecting the correlation between the distribution of a fluorine radical in inductively coupled plasmaetch process and the etch rate of a film on a silicon wafer.
Ultrasensitive nanoparticle detection using a portable whispering gallery mode biosensor driven by a periodically poled lithium-niobate frequency doubled distributed feedback laser81(2010); http://dx.doi.org/10.1063/1.3499261View Description Hide Description
We demonstrate a significant reduction in the limit of label-free detection of individual viral-sized nanoparticles in aqueous solution through the use of a frequency doubled telecom laser constructed from a distributed feedback-periodically poled lithium-niobate (DFB-PPLN) union. By driving a whispering gallery modebiosensor near a wavelength of 650 nm with this device we have detected real-time adsorption steps for particles 36 nm in radius with a signal to noise ratio of 8. The noise equivalent detection limit is (17 nm radius). This new lower limit is attributed to the ultralow resonance wavelength noise associated with the use of the DFB-PPLN device.
Microsecond time-resolved Fourier transform infrared analytics in a low pressure glow discharge reactor81(2010); http://dx.doi.org/10.1063/1.3492094View Description Hide Description
A low pressure glow discharge reactor has been designed to allow time-resolved infrared spectroscopic investigation of the discharge zone in practical conditions. The benefits of such reactor are demonstrated through the study of the evolution in the infrared spectra of gas mixture at the microsecond time-scale. It has been shown that the spectra are greatly affected by the electrical discharge in the region, where the asymmetric stretch mode of falls. The molecules are excited through a collision with excited molecules, where the transfer of energy occurs by a resonant effect. The mechanisms involved are reversible and following plasma pulses.
A new imaging method for understanding chemical dynamics: Efficient slice imaging using an in-vacuum pixel detector81(2010); http://dx.doi.org/10.1063/1.3489890View Description Hide Description
The implementation of the Timepix complementary metal oxide semiconductor pixel detector in velocity map slice imaging is presented. This new detector approach eliminates the need for gating the imaging detector. In time-of-flight mode, the detector returns the impact position and the time-of-flight of charged particles with 12.5 ns resolution and a dynamic range of about . The implementation of the Timepix detector in combination with a microchannel plate additionally allows for high spatial resolution information via center-of-mass centroiding. Here, the detector was applied to study the photodissociation of at 452 nm. The energy resolution observed in the experiment was and is limited by the experimental setup rather than by the detector assembly. All together, this new compact detector assembly is well-suited for slice imaging and is a promising tool for imaging studies in atomic and molecular physics research.
- Particle Sources, Optics and Acceleration; Particle Detectors
A double bend achromat lattice for the Pohang Light Source to reduce emittance and increase number of insertion devices81(2010); http://dx.doi.org/10.1063/1.3488362View Description Hide Description
The Pohang Light Source (PLS) storage ring is a synchrotron light source with the emittance of 18.9 nm at 2.5 GeV and has delivered vacuum ultraviolet and soft x-rays during the past 15 years. We investigate a lattice design for the 3 GeV ring for an upgrade project that keeps the existing tunnel. We investigate a double bend achromat (DBA)structure that provides the reduction of emittance by a factor of 3 and the increase of the number of straight section by a factor of 2 than the existing PLS lattice. We present several characteristics on the beam dynamics,dynamicaperture, and optics matching in the low-emittance lattice which includes squeezed space between magnets. Present PLS lattice has 12 long straight sections of 6.8 m long and the lattice is modified to provide the additional 12 short straight sections of 3.7 m long by eliminating a bending magnet in the middle of the cell of the present triplet bending achromat lattice. Thus, the new lattice consists of a total of 24 straight sections that consist of and long straight sections, which can provide the spaces for the 4- and 2-m-long insertion devices. We present the design results in detail for a DBA lattice in 281.82 m long circumference. It is shown that the emittance of 6.2 nm in the lattice can be achieved by allowing nonzero dispersions in the straight sections. The lattice provides high brilliance at the photon energy of a few 10 keV that meets the requirements by synchrotron radiation users; however, it may require a strong focusing and become sensitive to machine errors and effects of insertion devices. Thus, we investigated the dynamicaperture in the lattice by a simulation method and achieved an optimal tune under the strength of sextupole magnets of for the low-emittance ring. We also performed the lattice tunings to restore the optics due to the errors in the low-emittance ring. In result, our designed lattice shows a good optimization in terms of emittance, brilliance, and circumference as a light source for a 3 GeV.
Scintillating screens sensitivity and resolution studies for low energy, low intensity beam diagnostics81(2010); http://dx.doi.org/10.1063/1.3488123View Description Hide Description
In order to investigate the limits of scintillating screens for beam profile monitoring in the ultra-low energy, ultra-low intensity regime, CsI:Tl, YAG:Ce, and a Tb glass-based scintillating fiber optic plate (SFOP) were tested. The screens response to 200 and 50 keV protonbeams with intensities ranging from a few picoampere down to the subfemtoampere region was examined. In the following paper, the sensitivity and resolution studies are presented in detail for CsI:Tl and the SFOP, the two most sensitive screens. In addition, a possible use of scintillators for ultra-low energy antiproton beam monitoring is discussed.
81(2010); http://dx.doi.org/10.1063/1.3495965View Description Hide Description
We report on a new setup developed for neutron scattering experiments in periodically alternating magnetic fields at the sample position. The assembly consisting of rf generator, amplifier, wide band transformer, and resonance circuit. It allows to generate homogeneous acmagnetic fields over a volume of a few and variable within a wide range of amplitudes and frequencies. The applicability of the device is exemplified by ac polarized neutron reflectometry (PNR): a new method established to probe remagnetization kinetics in soft ferromagnetic films. Test experiments with iron films demonstrate that the ac field within the accessible range of frequencies and amplitudes produces a dramatic effect on the PNR signal. This shows that the relevant ac field parameters generated by the device match well with the scales involved in the remagnetization processes. Other possible applications of the rf unit are briefly discussed.
81(2010); http://dx.doi.org/10.1063/1.3488354View Description Hide Description
A U.S./Russian collaboration of accelerator scientists was directed to the development of high averaged-current and high-quality (emittance ; energy spread ) 1.75 MeV proton beams to produce active interrogation beams that could be applied to counterterrorism. Several accelerator technologies were investigated. These included an electrostatic tandem accelerator of novel design, a compact cyclotron, and a storage ring with energy compensation and electron cooling. Production targets capable of withstanding the beam power levels were designed, fabricated, and tested. The cyclotron/storage-ring system was theoretically studied and computationally designed, and the electrostatic vacuum tandem accelerator at BINP was demonstrated for its potential in active interrogation of explosives and special nuclear materials.
- Nuclear Physics, Fusion and Plasmas
Measurements of the fast electron bremsstrahlung emission during electron cyclotron resonance heating in the HL-2A tokamak81(2010); http://dx.doi.org/10.1063/1.3488966View Description Hide Description
A fast electron bremsstrahlung (FEB) diagnostic technique based on cadmium telluride (CdTe)detector has been developed recently in the HL-2A tokamak for measurements of the temporal evolution of FEB emission in the energy range of 10–200 keV. With a perpendicular viewing into the plasma on the equatorial plane, the hard x-ray spectra with eight different energy channels are measured. The discrimination of the spectra is implemented by an accurate spectrometry. The system also makes use of fast digitization and software signal processing technology. An ambient environment of neutrons, gammas, and magnetic disturbance requires careful shielding. During electron cyclotron resonance heating, the generation of fast electrons and the oscillations of electron fishbone (e-fishbone) have been found. Using the FEB measurement system, it has been experimentally identified that the mode strongly correlates with the electron cyclotron resonance heating produced fast electrons with 30–70 keV.
81(2010); http://dx.doi.org/10.1063/1.3491837View Description Hide Description
The Shanghai Synchrotron Radiation Facility (SSRF) is the first third-generation synchrotron facility in China and operated at an electron energy of 3.5 GeV. One of the seven beamlines in the first construction phase is devoted to soft x-ray spectromicroscopy and is equipped with an elliptically polarized undulator light source, a plane gratingmonochromator, and a scanning transmission x-ray microscope end station. Initial results reveal the high performance of this beamline, with an energy resolving power estimated to be over 10 000 at the argon L-edge and a spatial resolution better than 30 nm.
81(2010); http://dx.doi.org/10.1063/1.3488458View Description Hide Description
An avalanche photodiode based (APD)detector for the visible wavelength range was developed for low light level, high frequency beam emission spectroscopy (BES) experiments in fusion plasmas. This solid state detector has higher quantum efficiency than photomultiplier tubes, and unlike normal photodiodes, it has internal gain. This paper describes the developed detector as well as the noise model of the electronic circuit. By understanding the noise sources and the amplification process, the optimal amplifier and APD reverse voltage setting can be determined, where the signal-to-noise ratio is the highest for a given photon flux. The calculations are compared to the absolute calibration results of the implemented circuit. It was found that for a certain photon flux range, relevant for BES measurements , the new detector is superior to both photomultipliers and photodiodes, although it does not require cryogenic cooling of any component. The position of this photon flux window sensitively depends on the parameters of the actual experimental implementation (desired bandwidth, detector size, etc.) Several detector units based on these developments have been built and installed in various tokamaks. Some illustrative results are presented from the 8-channel trial BES system installed at Mega-Ampere Spherical Tokamak (MAST) and the 16-channel BES system installed at the Torus Experiment for Technology Oriented Research (TEXTOR).
81(2010); http://dx.doi.org/10.1063/1.3500319View Description Hide Description
Plasmaelectrolyticoxidation (PEO) has attracted widespread attention owing to the simplicity of operation and the excellent properties of the formed coating. However, wider applications of PEO have been limited due to the high power consumption. This work describes the design and performance of a novel technique named shorter distance PEO (SD-PEO), which is intended for lowering the energy consumption. The key feature of the method is the application of grid cathode to eliminate the gaseous envelope effect and to block of the exchange of charge carries during SD-PEO process. Compared to PEO carried out at a normal electrode distance, e.g., 50 mm, both the voltage drop and the joule heat consumed in the electrolyte at a shorter distance, e.g., of 5 mm (SD-PEO) are relatively small. Consequently, the energy consumption rendered by the novel SD-PEO method may decrease by more than 25%. Our results reveal that SD-PEO is a low energy-consumption microarc oxidation technique with more potential in industry applications.