Showing papers in "Review of Scientific Instruments in 1998"

High power ultrafast lasers

Abstract: In this article, we review progress in the development of high peak-power ultrafast lasers, and discuss in detail the design issues which determine the performance of these systems. Presently, lasers capable of generating terawatt peak powers with unprecedented short pulse duration can now be built on a single optical table in a small-scale laboratory, while large-scale lasers can generate peak power of over a petawatt. This progress is made possible by the use of the chirped-pulse amplification technique, combined with the use of broad-bandwidth laser materials such as Ti:sapphire, and the development of techniques for generating and propagating very short (10–30 fs) duration light pulses. We also briefly summarize some of the new scientific advances made possible by this technology, such as the generation of coherent femtosecond x-ray pulses, and the generation of MeV-energy electron beams and high-energy ions.

Optical tweezer arrays and optical substrates created with diffractive optics

Abstract: We describe a simple method for creating multiple optical tweezers from a single laser beam using diffractive optical elements. As a demonstration of this technique, we have implemented a 4×4 square array of optical tweezers—the hexadeca tweezer. Not only will diffractively generated optical tweezers facilitate many new experiments in pure and applied physics, but they also will be useful for fabricating nanocomposite materials and devices, including photonic bandgap materials and optical circuit elements.

Cavity enhanced absorption and cavity enhanced magnetic rotation spectroscopy

Abstract: It is experimentally demonstrated that a narrow band continuous wave (cw) light source can be used in combination with a high-finesse optically stable cavity to perform sensitive, high-resolution direct absorption and optical rotation spectroscopy in an amazingly simple experimental setup, using ideas from the field of cavity ring down spectroscopy Light from a scanning narrow band cw laser is coupled into the cavity via accidental coincidences of the laser frequency with the frequency of one of the multitude of modes of the cavity The absorption and polarization rotation information is extracted from a measurement of the time-integrated light intensity leaking out of the cavity as a function of laser wavelength

Phase measurement of light absorption and scatter in human tissue

Abstract: Analog and digital technologies are presented for precise measurement of propagation delay of photons from source and detector placed on portions of the human body. The goal of the apparatus design is to quantify absorption (μa) and scattering (μs′) induced by biological pigments and biological structures, respectively. Body tissues are highly scattering with a mean distance between scatterers of less than a mm (at 700–850 nm). Significant absorption is mainly due to 5%–10% of the tissue volume occupied by blood. Measurement of μa and μs′ is done by both time and frequency domain equipment. This article focuses upon frequency domain equipment because of its simplicity, reduced noise bandwidth, versatility, and the strong analogy to very high frequency/ultrahigh frequency communication devices, particularly those using phase modulation. Comparisons are made of homodyne and heterodyne systems together with evaluation of single and multiple side band systems, with particular emphasis on methods for multiplexed optical and radio frequencies by frequency encoding or time-sharing technologies. The applications of these phase modulation systems to quantitative brain and muscle blood oximetry, functional activity of the forebrain, and other important problems of medical science, are presented.

Quantitative microwave near-field microscopy of dielectric properties

Abstract: A theoretical model analysis for a recently developed scanning evanescent microwave microscope has been performed. The result enables a quantitative microscopy of local complex dielectric constant profiles for dielectric materials. Various experiments were performed and found to be in good agreement with the theoretical results. The estimation of intrinsic resolution of the microscope suggests that nanometer spatial resolution is achievable. System analysis gives a limiting sensitivity of about δe/e∼1×10−5.

Mass spectrometric characterization of a high-field asymmetric waveform ion mobility spectrometer

Abstract: Ion mobility spectrometry (IMS) has become an important method for the detection of many compounds because of its high sensitivity and amenability to miniaturization for field-portable monitoring; applications include detection of narcotics, explosives, and chemical warfare agents. High-field asymmetric waveform ion mobility spectrometry (FAIMS) differs from IMS in that the electric fields are applied using a high-frequency periodic asymmetric waveform, rather than a dc voltage. Furthermore, in FAIMS the compounds are separated by the difference in the mobility of ions at high electric field relative to low field, rather than by compound to compound differences in mobility at low electric field (IMS). We report here the first cylindrical-geometry-FAIMS interface with mass spectrometry (FAIMS-MS) and the MS identification of the peaks observed in a FAIMS compensation voltage (CV) spectrum. Using both an electrometer-based-FAIMS (FAIMS-E) and FAIMS-MS, several variables that affect the sensitivity of ion detection were examined for two (polarity reversed) asymmetric waveforms (modes 1 and 2) each of which yields a unique spectrum. An increase in the dispersion voltage (DV) was found to improve the sensitivity and separation observed in the FAIMS CV spectrum. This increase in sensitivity and the unexpected dissimilarity in modes 1 and 2 suggest that atmospheric pressure ion focusing is occurring in the FAIMS analyzer. The sensitivity and peak locations in the CV spectra were affected by temperature, gas flow rates, operating pressure, and analyte concentration.

A simple extended-cavity diode laser

Abstract: Operating a laser diode in an extended cavity which provides frequency-selective feedback is a very effective method of reducing the laser’s linewidth and improving its tunability. We have developed an extremely simple laser of this type, built from inexpensive commercial components with only a few minor modifications. A 780 nm laser built to this design has an output power of 80 mW, a linewidth of 350 kHz, and it has been continuously locked to a Doppler-free rubidium transition for several days.

Rotating-compensator multichannel ellipsometry: Applications for real time Stokes vector spectroscopy of thin film growth

Abstract: A multichannel spectroscopic ellipsometer based on the rotating-compensator principle was developed and applied to measure the time evolution of spectra (1.5–4.0 eV) in the normalized Stokes vector of the light beam reflected from the surface of a growing film. With this instrument, a time resolution of 32 ms for full spectra is possible. Several advantages of the rotating-compensator multichannel ellipsometer design over the simpler rotating-polarizer design are demonstrated here. These include the ability to: (i) determine the sign of the p-s wave phase-shift difference Δ, (ii) obtain accurate Δ values for low ellipticity polarization states, and (iii) deduce spectra in the degree of polarization of the light beam reflected from the sample. We have demonstrated the use of the latter spectra to characterize instrument errors such as stray light inside the spectrograph attached to the multichannel detector. The degree of polarization of the reflected beam has also been applied to characterize the time evo...

High-frequency demodulation of multi-photon fluorescence in hyper-Rayleigh scattering

Abstract: A novel technique for suppression of the multi-photon fluorescence contribution in second-order nonlinear optical hyper-Rayleigh scattering experiments is described. The technique takes advantages of the demodulation and the phase shift in the frequency domain of the time-delayed (multi-photon) fluorescence in the time domain. We demonstrate the effectiveness of demodulation at high modulation frequencies of the fundamental laser beam by determining the molecular second-order nonlinear polarizability for a reference molecule under fluorescent conditions. The value that was obtained for crystal-violet in methanol with 9,10-diphenylanthracene added as a centrosymmetric fluorophore compares very well with the values that were previously obtained. The possibility of complete suppression of all fluorescence, based on phase-sensitive measurements in quadrature with the fluorescence, is also discussed.

Microwave reflectometry for magnetically confined plasmas

Abstract: This paper is about microwave reflectometry -- a radar technique for plasma density measurements using the reflection of electromagnetic waves by a plasma cutoff. Both the theoretical foundations of reflectometry and its practical application to the study of magnetically confined plasmas are reviewed in this paper. In particular, the role of short-scale density fluctuations is discussed at length, both as a unique diagnostic tool for turbulence studies in thermonuclear plasmas and for the deleterious effects that fluctuations may have on the measurement of the average plasma density with microwave reflectometry.

Improved two-dimensional product imaging : the real-time ion-counting method

Abstract: A novel ion-counting method for significantly improving the spatial resolution and detection sensitivity of two-dimensional product imaging in molecular beam experiments is presented. The method makes use of real-time digital image processing to retrieve, threshold, and determine the local maximum of each ion hitting a microchannel plate assembly. The current version can process data at rates up to 3.07 Mbyte/s, and methods for accelerating this rate are proposed.

Performance of an energy-compensated three-dimensional atom probe

Abstract: A wide acceptance angle first-order reflectron lens has been incorporated into a three-dimensional atom probe (3DAP) to provide improved mass resolution. This new 3DAP instrument is capable of resolving isotopes in the mass spectrum, with resolutions better than m/Δm=500 full width at half maximum and 250 full width at 10% maximum. However, use of a reflectron for energy compensation within an imaging system means that improvements in mass resolution result in degradation of the spatial resolution. This article addresses the detailed design of the energy compensated 3DAP, and the minimization and compensation of chromatic aberrations in the imaging performance of the instrument. Some applications of the new instrument are included to illustrate its capabilities in the atomic-scale analysis of engineering alloys.

Time-correlated single photon counting imager for simultaneous magnetic resonance and near-infrared mammography

Abstract: We describe a near infrared (NIR) imager for mammography, designed to work simultaneously with a magnetic resonance (MR) scanner. The imager employs two pulsing laser diodes, with average power of 25 μW, at 780 and 830 nm. The two wavelengths are time multiplexed into 24 source fibers. The detection part consists of eight parallel time-correlated photon-counting channels with overall counting capacity of 106 photons/s. We use long optical fibers to avoid interference with the magnetic field. Specially designed coupling plates, for breast soft compression, bear both the MR radio-frequency coils and the optical source and detector fibers. Capillaries containing water and copper sulfate mark the position of the plates on the MR images for accurate coregistration of NIR and MR images. Instrument compatibility has been successfully tested with volunteers in the MR scanner. The use of gallium arsenide photomultiplier tubes has allowed penetration depths of 10 cm in the human breast. Imaging algorithms, based on the analytical modeling of photon propagation in inhomogeneous media, have been applied successfully to image 0.8-mm-diam absorbing and scattering cylindrical perturbations in transmittance geometry of breast-like phantoms.

Characterization of a cryogenically cooled high-pressure gas jet for laser/cluster interaction experiments

Abstract: We have developed and carried out detailed characterization of a cryogenically cooled (34–300 K), high-pressure (55 kTorr) solenoid driven pulsed valve that has been used to produce dense jets of atomic clusters for high intensity laser interaction studies. Measurements including Rayleigh scattering and short pulse interferometry show that clusters of controlled size, from a few to >104 atoms/cluster can be produced from a broad range of light and heavy gases, at average atomic densities up to 4×1019 atoms/cc. Continuous temperature and pressure control of the valve allows us to vary mean cluster size while keeping the average atomic density constant, and we find that many aspects of the valves behavior are consistent with ideal gas laws. However, we also show that effects including the build up of flow on milliseconds time scales, the cooling of gas flowing into the valve, and condensation of gas inside the valve body at temperatures well above the liquefaction point need to be carefully characterized in order to decouple the operation of the jet from the laser interaction physics.

A miniature capacitance dilatometer for thermal expansion and magnetostriction

Abstract: A very small capacitive sensor for measuring thermal expansion and magnetostriction of small and irregular shaped samples has been developed. A capacitive method with tilted plates is used. The tilted plate capacitance formula is used for the calculation of the capacitor gap, the calibration is performed by measuring the signal of a standard material. The active length of the sample can be less than 1 mm. The absolute resolution is about 1 A. All mechanical connections of the dilatometer are carried out by tiny Cu–Be springs, enabling the small force on the sample to be adjusted (50–500 mN) and no additional sample fixing is necessary. The cell has been tested in the temperature range 0.3–200 K and in static magnetic fields up to 15 T. The zero signal of the dilatometer has been determined by measuring a silver sample. The correct operation and reproducibility has been verified by measuring the thermal expansion of Cu. The thermal expansion and magnetostriction of a DyCu2 single crystal has been determined. The advantage of this method compared to specific heat measurements is that a large temperature range can be covered with one equipment. This high static and dynamic range of sample length, temperature, and magnetic field suggests a number of possible applications, like the investigation of crystal field effects on the magnetoelastic properties of single crystals or structural phase transitions.

Low cost PC based scanning Kelvin probe

Abstract: We have developed a novel, low cost, scanning Kelvin probe (SKP) system that can measure work function (wf) and surface potential (sp) topographies to within 1 meV energy resolution. The control and measurement subcomponents are PC based and incorporate a flexible user interface, permitting software control of major parameters and allowing easy user implementation via automatic setup and scanning procedures. We review the mode of operation and design features of the SKP including the digital oscillator, the compact ambient voice-coil head-stage, and signal processing techniques. This system offers unique tip-to-sample spacing control (to within 40 nm) which provides a method of simultaneously imaging sample height topographies and is essential to avoid spurious or “apparent” wf changes due to scanning-induced spacing changes. We illustrate SKP operation in generating high resolution wf/sp profiles of metal interfaces (as a tip characterization procedure) and operational electronic devices. The SKP potenti...

A computer-based digital feedback control of frequency drift of multiple lasers

Abstract: We report a method to monitor and control laser frequencies with an optical cavity and a digital feedback system. A frequency-stabilized He–Ne laser provides the reference that is transferred to several other lasers using a scanning Fabry–Perot cavity. A personal computer-based multifunction data acquisition system generates the scan wave form, and reads the detector outputs synchronously with the cavity scan. The computer determines the positions of all of the peaks in the scan, and generates output signals to control the laser frequencies. It also provides a visual display of cavity spectra. We have successfully used the setup to achieve a long-term lock of the lasers for magneto-optical trapping of radioactive francium atoms.

Numerical removal of ring artifacts in microtomography

Abstract: Defective and insufficient calibrated detector elements introduce ring or half-circle artifacts in microtomographic image reconstructions. A computationally efficient numerical filter is presented which suppresses these defects by taking advantage of the particular appearance of ring artifacts in the Fourier transforms of the recorded sinograms. The performance of the filter is demonstrated on experimental data taken with high-energy synchrotron radiation in phase-contrast (outline) mode.

A novel single-crystal adsorption calorimeter and additions for determining metal adsorption and adhesion energies

Abstract: A new microcalorimeter for measuring heats of adsorption on clean single-crystal surfaces is described, and its operational characteristics are presented. The principle is similar to that pioneered by David King’s group: A pulse of gas from a molecular beam adsorbs on an ultrathin single crystal’s surface, causing a measurable transient heat input and temperature rise. Our novel heat detector is a 9 μm thick pyroelectric polymer ribbon, which is mechanically driven to make a gentle mechanical/thermal contact to the back of the single-crystal sample during measurements. Advantages include use of thicker samples (1 μm), sample preparation at very high temperatures, and potential measurements at cryogenic temperatures. A novel chopped molecular beam of metal vapor and a method of correcting for absorbed radiation from the hot effusion cell are also described. This system is applied to study the heats of adsorption of metals on clean, well-defined and single-crystalline surfaces as a detailed function of cove...

Automated optical liquid film thickness measurement method

Abstract: A nonintrusive, automated, optical film thickness measurement technique has been developed to be used with a wide range of fluids and flow configurations. In this method, light is reflected from the surface of a liquid film flowing over a transparent wall. This reflected light generates an image on the outside of the wall which is captured and digitized using a charge coupled device camera and framegrabber card in a desktop computer. The image is processed to determine the positions of the reflected light rays, with which the film thickness is calculated. The entire process is automated, allowing for the collection of 600 data points in about 4 min using a personal computer with a 486 microprocessor. Film thicknesses on the order of 0.01 mm may be determined using inexpensive components, with the possibility of greater precision using more advanced imaging equipment. An automated calibration procedure allows for the determination of the necessary physical parameters, so the index of refraction of the test fluid or the test section wall need not be known a priori. Static liquid measurements agree to within 2.2% of measurements made using the needle-contact method. Film thickness data are presented for both round and square test sections operating under annular, two-phase flow conditions with air and water.

Quasi-optical cw mm-wave electron spin resonance spectrometer

Abstract: We describe a novel cw millimeter-wave electron spin resonance (ESR) spectrometer designed to operate in the frequency range of 80–200 GHz and in the temperature range of 2.5–300 K, which may be easily scaled to higher frequencies. The spectrometer uses a bimodal reflection cavity coupled to a circular corrugated guide and uses Gaussian quasi-optics for most of the front-end signal processing. This technique has very low insertion loss and allows a number of sophisticated measurement techniques to be employed including induction operation, which significantly reduces the effect of microphonics and stray reflections. A number of examples are given illustrating the sensitivity of the instrument and the advantages of using ESR at high fields.

A precision piezodriven micropositioner mechanism with large travel range

Abstract: A micropositioning stage with large travel range has been designed and built. The stage combines a piezoelectric driving element, flexure pivoted multiple Scott–Russell linkage, and a parallel guiding spring. Quality engineering techniques are used to optimize the configuration of the device in order to achieve the maximum displacement gain and the minimum angular deviation. A simple open-loop compensator is applied to reduce the hysteresis of the dynamic response of the stage. The experiment shows that the stage achieved a vacuum-compatible device with a travel of greater than 100 μm, a resolution of 0.04 μm, and an angular deviation of less than 31.1 μrad. The first natural frequency of the stage is 80 Hz and the settling time is approximately 50 ms. Compared with the uncontrolled condition, the controlled hysteresis is reduced significantly.

Argon ion laser-induced fluorescence with diode lasers

Abstract: Diode lasers have been used for ion temperature measurements in ArII plasmas by finding new laser-induced fluorescence (LIF) schemes suited to the present range of available wavelengths. The new LIF schemes require excitation at 664, 669, and 689 nm, all near industry-standard wavelengths. Conventional LIF measurements performed by dye lasers in ArII use 611.66 nm in vacuum, shorter than any commercially available red diode laser line, and depend on the population of the 3d′ 2G9/2 metastable state. The metastable state density of the conventional LIF scheme was found to be larger than the populations of the other metastable states by an order of magnitude or less. A master oscillator power amplifier diode laser was used both in a Littman–Metcalf cavity and as an optical amplifier for a low power diode laser which was in a Littman–Metcalf cavity. Both systems provided intensity of up to 500 mW, continuously tunable over 10 nm centered at 666 nm, and were used to obtain high resolution ion velocity distribu...

Nonadiabatic scanning calorimeter

Abstract: A high-resolution computerized calorimeter capable of fully automatic operation in either ac or relaxation modes is described. Emphasis is given to a new version of the relaxation technique in which the heater power is ramped linearly in time. This improvement results in superior performance and convenience in studying both first- and second-order phase transitions and allows quantitative evaluation of latent heats as well as pretransitional heat capacity variations. Examples are given for the use of this calorimeter in the study of liquid crystal phase transitions.

A scanning transmission x-ray microscope for materials science spectromicroscopy at the advanced light source

Abstract: Design and performance of a scanning transmission x-ray microscope (STXM) at the Advanced Light Source is described This instrument makes use of a high brightness undulator beamline and extends the STXM technique to new areas of research After 25 years of development it is now an operational tool for research in polymer science, environmental chemistry, and magnetic materials

Use of a nanoscale Kelvin probe for detecting wear precursors

Abstract: A version of a Kelvin probe that has been developed for an atomic force microscope (AFM), here referred to as a nano-Kelvin probe, is used to create maps of surface potential for study of samples that have been abraded by an AFM tip. The focus is on the wear at very low loads that involve the absence of wear debris and/or wear scars. Wear scars at higher loads, where significant damage to surface has occurred, have also been studied for reference purposes. Samples studied include single crystal aluminum, alumina, gold, and silicon. It is shown that even in cases where there is little or no damage to the surface, as observed by topography scans of an AFM, there is often a large change in the potential at the surface of the sample. The change in surface potential is believed to be the result of chemical and structural changes in the first few nanometers of the sample. We have shown that, even in the case of “zero wear” (that is, no visible deformation of the surface), there can be a significant change in th...

A new type of electrostatic ion trap for storage of fast ion beams

Abstract: A new technique for trapping of fast (keV) ion beams is presented. The trap, which is electrostatic, works on a principle similar to that of optical resonators. The main advantages of the trap are the possibility to trap fast beams without need of deceleration, the well-defined beam direction, the easy access to the trapped beam by various probes, and the simple requirement in terms of external beam injection. Results of preliminary experiments related to the radiative cooling of molecular ions are also reported.

A scanning force microscope with atomic resolution in ultrahigh vacuum and at low temperatures

Abstract: We present a new design of a scanning force microscope (SFM) for operation at low temperatures in an ultrahigh vacuum (UHV) system. The SFM features an all-fiber interferometer detection mechanism and can be used for contact as well as for noncontact measurements. Cooling is performed in a UHV compatible liquid helium bath cryostat. The design allows in situ cantilever and sample exchange at room temperature; the subsequent transport of the microscope into the cryostat is done by a specially designed transfer mechanism. Atomic resolution images acquired at various temperatures down to 10 K in contact as well as in noncontact mode are shown to demonstrate the performance of the microscope.

Streaming metal plasma generation by vacuum arc plasma guns

Abstract: We have developed several different embodiments of repetitively pulsed vacuum arc metal plasma gun, including miniature versions, multicathode versions that can produce up to 18 different metal plasma species between which one can switch, and a compact high-duty cycle well-cooled version, as well as a larger dc gun. Plasma guns of this kind can be incorporated into a vacuum arc ion source for the production of high-energy metal ion beams, or used as a plasma source for thin film formation and for metal plasma immersion ion implantation and deposition. The source can also be viewed as a low-energy metal ion source with ion drift velocity in the range 20–200 eV depending on the metal species used. Here we describe the plasma sources that we have developed, the properties of the plasma generated, and summarize their performance and limitations.

Magnetic measurements on the TCV tokamak

Abstract: The TCV Tokamak was designed to create a large variety of plasma shapes. Such a large flexibility requires high precision magnetic measurements with a good spatial coverage. This article gives a detailed description of the magnetic sensor geometry, fabrication, calibration, the associated electronics, and the diagnostic operation and monitoring. A substantial effort has been made to quantify the precision in the measurements and a novel method has been developed to derive corrections in the sensor position and calibration which optimise the consistency of the entire measurement set. Accuracy of 0.5 mWb in the poloidal flux and 1 mT in the magnetic field with a position error of a few mm have been achieved.