Volume 81, Issue 5, May 2010
Index of content:
- Optics; Atoms and Molecules; Spectroscopy; Photon Detectors
Wavelength stabilization of a synchronously pumped optical parametric oscillator: Optimizing proportional-integral control81(2010); http://dx.doi.org/10.1063/1.3385684View Description Hide Description
We describe a formal approach to the wavelength stabilization of a synchronously pumped ultrafast optical parametric oscillator using proportional-integral feedback control. Closed-loop wavelength stabilization was implemented by using a position-sensitive detector as a sensor and a piezoelectric transducer to modify the cavity length of the oscillator. By characterizing the frequency response of the loop components, we constructed a predictive model of the controller which showed formally that a proportional-only feedback was insufficient to eliminate the steady state error, consistent with experimental observations. The optimal proportional and integral gain coefficients were obtained from a numerical optimization of the controller model that minimized the settling time while also limiting the overshoot to an acceptable value. Results are presented showing effective wavelength and power stabilization to levels limited only by the relative intensity noise of the pump laser.
Optical sensing and determination of complex reflection coefficients of plasmonic structures using transmission interferometric plasmonic sensor81(2010); http://dx.doi.org/10.1063/1.3405912View Description Hide Description
The combination of interferometry and plasmonic structure, which consists of goldnanoparticle layer, sputter coated silicon oxide spacer layer, and aluminummirror layer, was studied in transmission mode for biosensing and refractive index sensing applications. Because of the interferometric nature of the system, the information of the reflection amplitude and phase of the plasmonic layer can be deduced from one spectrum. The modulation amplitude in the transmission spectrum, caused by the interference between the plasmonic particle layer and the mirror layer, increases upon the refractive index increase around the plasmonic particles due to their coherent backscattering property. Our proposed evaluation method requires only two light sources with different wavelengths for a stable self-referenced signal, which can be easily and precisely tuned by a transparent spacer layer thickness. Unlike the standard localized surface plasmon sensors, where a sharp resonance peak is essential, a broad band plasmon resonance is accepted in this method. This leads to large fabrication tolerance of the plasmonic structures. We investigated bulk and adsorption layer sensitivities both experimentally and by simulation. The highest sensitivity wavelength corresponded to the resonance of the plasmonic particles, but useful signals are produced in a much broader spectral range. Analysis of a single transmission spectrum allowed us to access the wavelength-dependent complex reflection coefficient of the plasmonic particle layer, which confirmed the reflection amplitude increase in the plasmonic particle layer upon molecular adsorption.
81(2010); http://dx.doi.org/10.1063/1.3422256View Description Hide Description
We have successfully observed the vibrational spectra of organic monolayers sandwiched between a liquidHgelectrode and a Si substrate by means of a newly developed reflection absorption (RA) device. The vibrational spectra of organic monolayers between two electrodes can be observed under a certain bias voltage. The monolayers were fabricated by the reaction of hydrogen-terminated Si(111) with 1-octadecene. A metal/insulator/semiconductor structure was prepared using liquidHg as a metalelectrode and the organic monolayer as an insulator. Infrared (IR) light entered from the Si substrate side with an incident angle of 75°. The reflected IR light from the metallic Hg was detected by a mercury-cadmium-telluride detector. We obtained RA spectra using a bare H–Si(111) substrate as a reference. The absorbance of the RA spectrum was comparable with that of the transmission spectrum for the octadecyl-terminated Si(111) without Hg. The C–H stretching modes in the group show blueshifts, and the C–H antisymmetric stretching modes in the are broadened in comparison with the transmission spectrum. Under a certain bias voltage, we observed changes in band shape. We concluded that the variation was due to the temperature increase by resistive heating of the substrate.
SAMRAI: A novel variably polarized angle-resolved photoemission beamline in the VUV region at UVSOR-II81(2010); http://dx.doi.org/10.1063/1.3425778View Description Hide Description
A novel variably polarized angle-resolved photoemission spectroscopy beamline in the vacuum-ultraviolet (VUV) region has been installed at the UVSOR-II 750 MeV synchrotron light source. The beamline is equipped with a 3 m long APPLE-II type undulator with horizontally/vertically linear and right/left circular polarizations, a 10 m Wadsworth type monochromator covering a photon energy range of 6–43 eV, and a 200 mm radius hemispherical photoelectron analyzer with an electron lens of a ±18° acceptance angle. Due to the low emittance of the UVSOR-II storage ring, the light source is regarded as an entrance slit, and the undulator light is directly led to a grating by two plane mirrors in the monochromator while maintaining a balance between high-energy resolution and high photon flux. The energy resolving power and photon flux of the monochromator are typically and , respectively, with a exit slit. The beamline is used for angle-resolved photoemission spectroscopy with an energy resolution of a few meV covering the UV-to-VUV energy range.
81(2010); http://dx.doi.org/10.1063/1.3405983View Description Hide Description
The complete spectral characterization of ultrashort pulses is demonstrated with a new diagnostic called Spectral Interferometry Resolved in Time Extra Dimensional. This method, based on spectral shearing interferometry, is self-referenced and self-calibrated. It yields directly to an interferogram pattern displaying an intuitive representation of the derivative of the spectral phase. No iterative algorithm is needed for phase measurement making this method suitable for real time and easy characterization. This technique is highlighted by the spectral phase characterization of pulses out of a folded nondispersive line and the pulse shape is compared with a trace recorded with an intensity autocorrelator.
81(2010); http://dx.doi.org/10.1063/1.3427218View Description Hide Description
During the past decade, the application of ambient pressurephotoemission spectroscopy (APPES) has been recognized as an important in situ tool to study environmental and materials science, energy related science, and many other fields. Several APPES endstations are currently under planning or development at the USA and international light sources, which will lead to a rapid expansion of this technique. The present work describes the design and performance of a new APPES instrument at the Advanced Light Source beamline 9.3.2 at Lawrence Berkeley National Laboratory. This new instrument, Scienta R4000 HiPP, is a result of collaboration between Advanced Light Source and its industrial partner VG-Scienta. The R4000 HiPP provides superior electron transmission as well as spectromicroscopy modes with spatial resolution in one dimension and angle-resolved modes with simulated 0.5° angular resolution at 24° acceptance. Under maximum transmission mode, the electron detection efficiency is more than an order of magnitude better than the previous endstation at beamline 9.3.2. Herein we describe the design and performance of the system, which has been utilized to record spectra above 2 mbar.
81(2010); http://dx.doi.org/10.1063/1.3429950View Description Hide Description
A Fabry–Pérot enhanced electro-optic sensing system that utilizes a drive-current-tuned wavelength laser diode is presented. An electro-optic prober made of crystal with an asymmetric Fabry–Pérot cavity is used in this system. To lock the wavelength of the laser diode at resonant condition, a closed-loop power control scheme is proposed. Experiment results show that the system can keep the electro-optic prober at high sensitivity for a long working time when the closed-loop control function is on. If this function is off, the sensitivity may be fluctuated and only one-third of the best level in the worst case.
81(2010); http://dx.doi.org/10.1063/1.3422263View Description Hide Description
In this paper a new approach to analyzedissociative recombination product distance distributions measured at ion storage rings is presented. This approach is based on an Abel transform and the validity is demonstrated for the imaging data obtained from the CRYRING experiments on the dissociative recombination of and . The reported method is generally valid for the data analysis of high resolution merged beams experiments such as DESIREE where isotropic fragment distributions can be expected.
A precise high-resolution near infrared continuous wave cavity ringdown spectrometer using a Fourier transform based wavelength calibration81(2010); http://dx.doi.org/10.1063/1.3422254View Description Hide Description
A wavelength calibration technique is described, which is based on a combination of a Fourier transform wavelength meter and a distributed feedback laser locked to a molecular transition as a frequency marker in the spectrum. The technique provides a reliable wavelength scale to be used in high resolution continuous wave cavity ringdown spectroscopy without need for stabilization of the probe laser and accurately known molecular transitions in the scanned wavelength range. Due to a continuous reference measurement, ambient influences on the laser sources are effectively suppressed. As an example, we measured highly resolved cavity ringdown spectra of isotopomers and determined the line strength of several absorption lines at a wavelength around 1687 nm. A near infrared wavelength precision of and an absolute accuracy on the order of was readily achieved. The general concept is easy to implement and can be further refined by using additional reference lasers, thus holding the potential of even higher wavelength accuracy.
81(2010); http://dx.doi.org/10.1063/1.3386580View Description Hide Description
We describe an ultraviolet (uv) diode laser system for cooling trapped ions. Using four stages of thermoelectric cooling, 10 mW of light at 369.5 nm is obtained by cooling a 373.4-nm uv diode to approximately . Frequency stabilization is provided by a diffraction grating mounted in the Littrow configuration which allows for a mode-hop free tuning range of . In order to avoid water condensation, the diode laser and associated optics are placed inside an evacuated chamber. Saturated absorption spectroscopy utilizing an Yb hollow cathode lamp is performed. This laser system is currently being used to cool single ions in an experiment whose ultimate goal is to look for modern variation of the fine-structure constant.
81(2010); http://dx.doi.org/10.1063/1.3405839View Description Hide Description
We designed a near infrared tunable resonance Raman spectroscopy system based on a tandem of thick volume Bragg gratings (VBGs). VBGs are here the constituents of two light filtering units: a tunable laser line filter (LLF) and a tunable notch filter (NF). When adapted in a micro-Raman setup with a single stage monochromator (1800 gr/mm grating), the tandem of LLF and NF allowed measurements of Raman signals down to . The good performance and fast tunability of the VBG Raman system was demonstrated on a sulfur powder and on a bulk single-walled carbon nanotube sample through a series of 22 Stokes and anti-Stokes spectra recorded at excitation wavelengths between 800 and 990 nm. The main drawbacks of the setup are the limited spectral range to the near infrared and the small angular acceptance of the filters , which causes mainly attenuation problems with the NF. The impact of the main limitations is discussed and solutions are provided.
Transportable distance measurement system based on superheterodyne interferometry using two phase-locked frequency-doubled Nd:YAG lasers81(2010); http://dx.doi.org/10.1063/1.3428729View Description Hide Description
We describe a transportable distance measurementsystem based on synthetic wavelength interferometry. Two frequency-doubled Nd:yttrium aluminum garnet lasers at 532 nm are used to generate a synthetic wavelength of . A nonpolarizing interferometric system has been set up to eliminate polarization cross-talk issue. A superheterodyne detection was performed to measure the synthetic phase and to determine absolute distances. The capability to achieve fringe interpolation of has been demonstrated and an agreement in distance measurement at the level has been achieved, compared to an optical interferometric 3 m long displacement bench.
- Particle Sources, Optics and Acceleration; Particle Detectors
81(2010); http://dx.doi.org/10.1063/1.3397322View Description Hide Description
For liquid-scintillator neutrino detectors of kiloton scale, the transparency of the organic solvent is of central importance. The present paper reports on laboratory measurements of the optical scattering lengths of the organic solvents phenylxylylethane, linear alkylbenzene (LAB), and dodecane, which are under discussion for next-generation experiments such as (Sudbury Neutrino Observatory), HanoHano, or LENA (Low Energy Neutrino Astronomy). Results comprise the wavelength range of 415–440 nm. The contributions from Rayleigh and Mie scattering as well as from absorption/re-emission processes are discussed. Based on the present results, LAB seems to be the preferred solvent for a large-volume detector.
81(2010); http://dx.doi.org/10.1063/1.3429941View Description Hide Description
In situ measurements of the dynamics and composition of space plasmas have greatly improved our understanding of the space environment. In particular, mass spectrometers that use a combination of electrostatic analyzers and time-of-flight systems can identify revealing dynamic and compositional characteristics of ions, and thus constrain their sources and the physical processes relevant for their transport. We demonstrate an optimized design of a linear-electric-field time-of-flight technology that can be used to obtain a high signal to noise: ions that follow an energy-isochronous oscillation within the instrument impact an emissive plate and cause secondary electrons to be sent toward the detector, triggering a high-resolution measurement. By focusing these secondary electrons to a central area on a position-sensitive anode, their signals are separated from ions and neutrals that do not experience energy-isochronous motion. Using their impact positions, the high mass resolution measurements are easily distinguished from other signals on the detector, leading to very favorable signal-to-noise ratios. This optimization provides an improvement to existing technologies without increasing the instrument size or complexity, and uses a novel time-of-flight circuit that combines timing and position information from many signals and ions.
Instrumentation for the in situ control of carrier recombination characteristics during irradiation by protons81(2010); http://dx.doi.org/10.1063/1.3429944View Description Hide Description
Instrument and methods for the remote and in situcontrol of carrier recombination parameters during irradiation by protons of energy in the range of 3–8 MeV are presented. Direct techniques for measurements and separation of carrier recombination and trapping/generation characteristics based on the analysis of microwave probed photoconductivity transients during exposure on protons of different energies and irradiations at different temperatures are described. Simultaneously, a spectroscopy of activation energy of dominant traps has been performed before and just after irradiation by temperature scans of variation in the recombination parameters.
- Nuclear Physics, Fusion and Plasmas
The x-ray calibration facility of the laser integration line in the 0.9–10 keV range: The high energy x-ray source and some applications81(2010); http://dx.doi.org/10.1063/1.3427236View Description Hide Description
The laser integration line (LIL) located at CEA-CESTA is equipped with x-ray plasma diagnostics using different kinds of x-ray components such as filters, mirrors, crystals, detectors, and cameras. The CEA-DAM of Arpajon is currently developing x-raycalibration methods and carrying out absolute calibration of LIL x-ray photodetectors. To guarantee LIL measurements, detectors such as x-raycameras must be regularly calibrated close to the facility. A new x-ray facility is currently available to perform these absolute x-raycalibrations. This paper presents the x-ray tube based high energy x-ray source delivering x-ray energies ranging from 0.9 to 10 keV by means of an anode barrel. The purpose of this source is mainly to calibrate LIL x-raycameras but it can also be used to measure x-ray filter transmission of plasma diagnostics. Different x-ray absolute calibrations such as x-ray streak and framing camera yields, x-raycharge-coupled device quantum efficiencies, and x-ray filter transmissions are presented in this paper. A x-ray flat photocathodedetector sensitivity calibration recently performed for a CEA Z-pinch facility is also presented.
Obtaining point spread function of penumbral encoding aperture with “expectation maximization” algorithm based on matched source-image pair experiment81(2010); http://dx.doi.org/10.1063/1.3427208View Description Hide Description
A source penumbral image reconstruction method with linear mapping principle for geometrical optics is established. The ideal binary point spread function (PSF) can be obtained using a geometrical optics model. The system PSF with certain sharpness was obtained using a Monte Carlo (MC) model. Considering other factors besides the transportation of the x -rays or particles (fusionneutrons) in the penumbral encodingaperture in MC model, such as the scattering background and the systematic error, the PSF from MC model “source-image pair matching” experiments with a large area standard oval shape source were processed. A method for correcting and calibrating the PSF by the expectation maximization adaptive algorithm was established and the optimized PSF with sharpness was achieved. This is more consistent with the real system PSF despite the increased noise level of the two-dimensional PSF and large irregularity in the PSF profile.
New interpolation algorithm for evaluation a spatial profile of plasma densities in radio frequency discharges81(2010); http://dx.doi.org/10.1063/1.3427220View Description Hide Description
A new interpolation to estimate a spatial profile of plasma densities of unmeasured regions for two-dimensional probe diagnostics is proposed. Our algorithm is based on the plasmadiffusioncharacteristics. From a comparison with a well known interpolation, Modified Shepard, it has been found that our method gives more accurate plasma density profile, especially at the edge region of the measurement substrate, and improves computational efficiency
Plasma column displacement measurements by modified Rogowski sine-coil and Biot–Savart/magnetic flux equation solution on IR-T1 tokamak81(2010); http://dx.doi.org/10.1063/1.3422385View Description Hide Description
The modified Rogowski sine-coil (MRSC) has been designed and implemented for the plasma column horizontal displacement measurements on small IR-T1 tokamak. MRSC operation has been examined on test assembly and tokamak. Obtained results show high sensitivity to the plasma column horizontal displacement and negligible sensitivity to the vertical displacement; linearity in wide, ±0.1 m, range of the displacements; and excellent, 1.5%, agreement with the results of numerical solution of Biot–Savart and magnetic flux equations.
81(2010); http://dx.doi.org/10.1063/1.3429207View Description Hide Description
The static (unloaded) electrical parameters of a capacitor bank are of utmost importance for the purpose of modeling the system as a whole when the capacitor bank is discharged into its dynamic electromagnetic load. Using a physical short circuit across the electromagnetic load is usually technically difficult and is unnecessary. The discharge can be operated at the highest pressure permissible in order to minimize current sheet motion, thus simulating zero dynamic load, to enable bank parameters, static inductance, and resistance to be obtained using lightly damped sinusoid equations given the bank capacitance. However, for a plasma focus, even at the highest permissible pressure it is found that there is significant residual motion, so that the assumption of a zero dynamic load introduces unacceptable errors into the determination of the circuit parameters. To overcome this problem, the Lee model code is used to fit the computed current trace to the measuredcurrent waveform. Hence the dynamics is incorporated into the solution and the capacitor bank parameters are computed using the Lee model code, and more accurate static bank parameters are obtained.