Incident Beam Intensity Research Articles

Photoreflectance studies of surface state density of InAlAs

The surface barrier height and surface Fermi level of InAlAs were investigated via photoreflectance spectra. Surface state density was then determined from the surface barrier height as a function of temperature, illumination power intensity, and intrinsic layer thickness. Results obtained from these three independent approaches all give the same conclusion, that the surface states are distributed over two separate regions within the energy band gap. Closely examining the photovoltage induced by various incident beam intensities revealed that the photovoltage effect is negligible when the illumination power intensity is below 1.0 μW/cm2.

SECOND HARMONIC LIGHT EMISSION FROM PLASMAS PRODUCED WITH 1.053μm LASER

Time-integrated spectra of second harmonic (2ω) light emission from plasmas produced with 1.053μm laser are simultaneously measured at several scattering-angles.By analyzing the relationship between scattering energy and scattering angle,also by analyzing the total 2ω light intensity as a function of incident beam intensity,we found that the Langmuir turbulence,which is produced by cascade of parametric decay instability,plays an important role in the laser-plasma interaction.

Contribution to residual-stress evaluation in high-stress-gradient zones by X-ray diffraction

The non-destructive X-ray diffraction method is usually employed to evaluate stress states at the near surface of materials. In specific zones with high-stress gradients parallel to the surface, like welding joins, areas affected by lasers, or ball-bearing raceways, the classical measurement method is not particularly well suited, as the irradiation spot covers regions of varying stress according to its size. This leads to inaccurate stress evaluations. A new X-ray diffraction measurement and data treatment method is therefore proposed. It is based on longitudinal step-by-step scanning in a fixed direction of the surface. As the acquired data results from the convolution of local diffraction peaks with the incident-beam intensity, an accurate model of the true two-dimensional intensity distribution inside the spot has been developed. Firstly, the true shape and size of the irradiation area is defined. The distribution of the intensity received by the sample surface is then computed. The true local strains and stresses along the scanning direction are finally determined from the acquired peaks using an optimization through a least-squares fitting by a nonlinear function.

Derivative photoelectron holography of As/Si(001)

The new technique of derivative photoelectron holography is used to examine the As-terminated Si(001) surface, which exhibits a two-domain (2×1) reconstruction. Logarithmic derivatives of the photoemission intensity are measured, from which the intensity fine structure function is deduced. Since the logarithmic derivative function is independent of the incident beam intensity and the detection efficiency, most experimental uncertainties are eliminated. The resulting atomic images are in good agreement with expectation.

An Allene-Doped Liquid Argon Ionization Chamber for O and Ne Ions at 100 MeV/n

An allene-doped liquid argon ionization chamber with a sensitive volume of 48 mm×48 mm×40 mm has been constructed and investigated for precise measurement of the heavy ion energies at around 100 MeV/n. An energy resolution of 1% (full width at half maximum [FWHM]) was achieved for the 99 MeV/n 16O and 125 MeV/n 20Ne ion beams. However, the energy resolution was found to deteriorate as the beam intensity increased. To understand this phenomenon, a simple model is proposed, in which a space-charge electric field due to the positive ions left along the track of an incident ion is taken into consideration. The estimated electric field is strong enough to worsen the energy resolution when the incident beam intensity is only 100 particles/s.

Analysis of temporal behavior of beams diffracted by volume gratings formed in photopolymers

The temporal behavior of beams diffracted by volume gratings in photopolymer thin films are measured and analyzed by solution of the diffusion equation for the monomer concentration inside the thin films. Two contributors to the refractive-index change that forms the volume gratings are assumed: One is the phase grating formed by modulation of the monomer concentration, and the other is the phase grating formed by modulation of the density of the polymeric materials. The phase grating that is due to monomer modulation is responsible for the initial fast rise and decay of the diffracted signal, and the phase grating that is due to modulation of density of the polymeric materials is responsible for the slowly rising and then steady signal. The temporal behavior of the diffracted beams is determined by the ratio of magnitudes of the incident beam intensity and the diffusion coefficient.

Two-way optical memory for azobenzene-containing urethane–urea copolymer films

An azobenzene-containing urethane–urea copolymer as a photochromic material has two optical properties for data storage. One is photoisomerization related to the polarization of incident beam. The other is photoablation related to the intensity of incident beam. It seemed that we could separate them with the power or wavelength of the recording beam. Therefore, we studied the characteristics of the phenomena and found that the selective use of the effects is applicable to some types of memory.

Simultaneous measurements of second-harmonic generation and two-photon photoelectric emission from Au

Simultaneous measurements of the second-harmonic generation (SHG) and of the two-photon photoelectric emission (2PPE) responses induced from an Au target by the same incident picosecond laser beam at λ = 532 nm are performed. The effects of a dc electric field, applied between the sample (photocathode) and an anode in order to collect the photoelectrons, are investigated. The dependence on incident beam intensity and polarization of both 2PPE charge and SHG intensity is studied experimentally. Similar comportments are observed, however differences are found between SHG and photoemission concerning their surface specificity and their sensitivity to the heat provided by the laser beam.

Non-linear polarization effects in guest-host liquid crystals

We report on an experimental investigation of the non-linear polarization effect in guest-host nematic liquid crystals (GH-NLCs) with uniaxial alignment. The degree of polarization of a laser beam transmitting the GH-NLCs and an aperture behind them is strongly dependent on the incident beam intensity. The mechanism is discussed on the basis of the laser-induced photothermal self-phase modulation. The origin of the effect is a strong anisotropy of the intensity-induced change or refractive indices.

A Small-Angle X-ray Scattering Apparatus for Studying Biological Macromolecules in Solution

This paper describes the development of a simple laboratory-based small-angle X-ray scattering apparatus for the study of biological macromolecules in solution. The instrument is based on a two-circular-aperture collimation design combined with a conventional rotating-anode Cu Ka X-ray source, a graphite monochromator and a multiwire area detector. The geometry of the collimator, the beam-stop-to-detector distance and the thickness of the platinum foil of the defining aperture have been optimized to reduce background scattering. The effective Q range is from 0.01 to 0.33 Å−1, where Q = (4π sin θ)/λ is the magnitude of the scattering vector, 2θ is the scattering angle and λ is the wavelength of the X-rays. The length of the collimator, the pinhole sizes and the helium-flushed sample-to-detector path can be easily changed depending on the resolution and intensity requirements of an experiment. The diffraction pattern of a polycrystalline pellet of ammonium sulfate mounted about 2.5 cm in front of the beam stop and 40 cm in front of the detector is used to monitor changes in the incident-beam intensity as well as the differences in absorption of X-rays by the sample solutions and the solvents, to ensure correct background subtractions. Data collection is controlled by a computer through a parallel DMA (direct memory access) I/O module. Data collection and reduction software has been developed. The typical data collection time is about 2 h for a 5 mg ml−1 10 kDa protein dissolved in an aqueous solution. Examples of applications of this small-angle X-ray scattering instrument to studying protein size and conformation changes are presented.

Digital Image Acquisition and Presentation for High Resolution SEM

Images obtained from an analog SEM are traditionally viewed and recorded from a cathode-ray tube (CRT). Many laboratories use instant film (e.g. Polaroid #52, #55 instant film) to justify image quality and obtain permanent image quickly. Digital imaging provides an alternative approach for image acquisition and recording. One major advantage of digital SEM is image averaging that allows one to improve the signal-to-noise ratio (SNR) from a noisy quick-scan image to reduce charging. SEM signal yield is proportional to incident beam intensity, image acquisition time or duration of beam interaction with specimen (dwell time). The higher beam intensity, or longer the dwell time, the more signal generated. However, for high-resolution SEM imaging, the beam dose and dwell time are limited by drafting, radiation damage, and contamination. Therefore high-resolution biological SEM images invariably have poor SNR.

Optical pumping-induced spatio-temporal modifications to propagation, polarization and intensity of laser beams in sodium vapour

Circularly polarized laser beams propagating through sodium vapour and tuned to the buffer-gas-broadened atomic transition can optically pump sodium atoms into a non-absorbing ground state. This causes an intensity-dependent refractive index gradient along as well as transverse to the laser beam propagation direction, giving rise to a number of nonlinear spatio-temporal intensity and polarization pattern creating processes. In the case of a single circularly polarized laser beam we have observed self-focusing and defocusing, the transformation of the incident Gaussian beam intensity profiles into ring profiles, a large shift of about 5 GHz of the maximum of the absorption profile when suitable magnetic fields are applied and the deflection of a beam by the inhomogeneous transverse magnetic field of a current-carrying wire. When two beams of opposite circular polarization are superimposed, astonishing effects such as the mutual deflection of both beams (beam bouncing), the mutual extinction of both beams (beam switching), the separation of initially overlapping beams (beam splitting) and the mutual attraction of both beams (beam attraction) can be observed. While most of the effects can be well described for the stationary state by a to atomic transition model, the correct description of the dynamics requires the consideration of all hyperfine states.

X-ray absolute intensity measurement at HASYLAB ultrasmall angle x-ray scattering beamline BW4

In order to obtain small angle scattering intensities in absolute units, measurements of the incident beam intensity and the intensity transmitted through the sample are required. For this purpose the scattering intensity of a calibrated polyethylene (LUPOLEN) standard is used. For the determination of the absolute photon flux and the correction of the scattering patterns obtained with a two dimensional position sensitive detector we use the detector response function. A set of data measured subsequently at the standard and at a given sample can be evaluated in a program that calculates a pattern in absolute units corrected by means of the detector response. This program also calculates the absolute photon flux, determines automatically the center coordinates of the incident beam, and calculates the scattering vectors that belong to the different positions on the detector.

Detection mechanism of an optical evanescent field using a noncontact mode atomic force microscope with a frequency modulation detection method

By using the noncontact atomic force microscope with a frequency modulation detection method, the force gradient induced by the optical evanescent field was detected in a high vacuum. We succeeded in measuring the exponential distance dependence of the force gradient induced by the optical evanescent field. Furthermore, we investigated the incident beam intensity and bias voltage dependence of the force gradient induced by the optical evanescent field. We confirmed that the detection mechanism is not photothermal effect but the surface photovoltage effect.

An Incident-Beam Monitor for Use in Protein Crystallography at a Synchrotron Source

A compact incident-beam monitor has been developed for use in synchrotron protein crystallography at the National Synchrotron Light Source beamline X8C. Incident-beam intensity is monitored by measurement of the radiation scattered from a thin polymer film into a PIN diode. For improved statistical accuracy, the scattering film can be replaced by a thin metal foil. Fluorescent radiation emitted by the metal foil increases the beam-monitoring signal up to 50 times relative to that produced by scattering alone. With this design, the forward-scattered radiation is restricted to an area not larger than the beam stop, minimizing excessive background radiation. Support materials have been optimized so that no unwanted edge effects are present between 6 and 19.5 keV, making the detector useful for monitoring incident-beam intensities over the wide range of absorption edges often associated with multiwavelength anomalous diffraction (MAD) experiments. Accurate incident-beam monitoring also simplifies optimization of the X-ray beam through the diffractometer collimation after each new electron orbit is established in the synchrotron.

Characterization of Ceramic Materials with BIGDIFF: A Synchrotron Radiation Debye‐Scherrer Powder Diffractometer

Experiments have been conducted to evaluate the potential of the BIGDIFF Debye–Scherrer diffractometer (radius of 573 mm) for characterizing ceramic materials using synchrotron radiation. The instrument has been tested at a wavelength of 0.1538 nm (1.5378), under in vacuo conditions, with standard reference materials and an alumina‐matrix ceramic specimen using capillary‐mounted (diameter of 0.5 mm) powdered material. The diffraction patterns that have been recorded with imaging plates over a period of 15 min provide phase detectability that is clearly superior to Bragg‐Brentano laboratory X‐ray diffractometry data that is collected in 1 h. The superiority of BIGDIFF for the analysis of phase composition becomes very pronounced for trace phases (,1%). The sharper definition of the Bragg peaks with BIGDIFF synchrotron radiation data also leads to improved estimates of nonlinear residual strain and crystallite size data. The superior performance of BIGDIFF is principally due to (i) the intensity of the synchrotron radiation incident beam that is achieved with high collimation and monochromaticity (E/ΔE 104), (ii) the large ratio of the camera radius to the capillary radius, and (iii) the spatial resolution and dynamic range of the imaging plates.

Steady State Photoconductivity of a Polymer Blend

The spectral responses of steady state photoconductivity of a polymer blend, poly[2-methoxy,5-(2′-ethyl-hexoxy)-1,4-phenylene vinylene](MEH-PPV) and poly[1,3-propanedioxy-1,4-phenylene-1,2-ethenyllene-(2,5-bis(trimethylsilyl)-1,4-phenylene)-1,2-etheny lene-I,4-phenylene](called B-polymer) are investigated. Two peaks of photocurrent(PC) at 2.16 eV and 3.01 eV are observed under continuous illumination. The peak at 3.01 eV is rather broad and has a strong polarity dependence on the applied voltage, which could be due to the rectified microjunction formation in the polymer blend. The incident beam intensity dependences of the two peaks in PC are I pc ∝ I α with α = 1.05. It indicates that the monomolecular recombination kinetics is important in this polymer blend. The observed spectral response of PC is discussed in comparison with the optical absorption as well as the photoluminescence spectra reported earlier in the same polymer blend.

Two-dimensional analysis of the power transfer between crossed laser beams

The power transfer between crossed laser beams made possible by an ion-acoustic wave is studied. A simple formula is derived for the steady-state power transfer, which depends on two dimensionless parameters: the ratio of the incident beam intensities and the normalized beamwidth. Numerical simulations show that the transient power transfer is larger than the steady-state power transfer and usually oscillates in time. The convective depletion of the higher-frequency beam saturates the power transfer more quickly than the damping of the ion-acoustic wave.

The Fragmentation of 510 MeV/Nucleon Iron-56 in Polyethylene. I. Fragment Fluence Spectra

The fragmentation of 510 MeV/nucleon iron ions in several thicknesses of polyethylene has been measured. Non-interacting primary beam particles and fragments have been identified and their LETs calculated by measuring ionization energy loss in a stack of silicon detectors. Fluences, normalized to the incident beam intensity and corrected for detector effects, are presented for each fragment charge and target. Histograms of fluence as a function of LET are also presented. Some implications of these data for measurements of the biological effects of heavy ions are discussed.

The complementary use of X-ray and neutron diffraction in the study of crystals

Neutrons and X-rays interact differently with atoms in crystals. While X-rays primarily give information on electron distributions, neutrons report on nuclear positions, and, through the spin interaction, are sensitive to magnetic structure. These and other differences have been exploited for many years in, for example, X-N difference studies and in determining magnetic structure. The major differences in available X-ray and neutron incident-beam intensities have also influenced the ways in which the two probes are exploited; not only are neutron sources inherently weaker, but this disadvantage is heightened by the weaker neutron-nucleus interaction. Advances in sources of both types, coupled with developments in instrumentation, have enabled not only the relative strengths to be exploited more effectively, but also some of the respective weaknesses in both techniques to be at least partially overcome. After outlining the main relevant advantages of X-rays and neutrons, and specifically of pulsed spallation neutron sources, this paper will discuss some of the scientific areas in which these various advantages are being increasingly exploited with advanced sources and instrumentation. Although the examples focus in particular on studies of structure and disorder in powder samples, including work at high pressures, some attention is given to hydrogen location in, and diffuse scattering from, single crystals. Finally, a personal forward look towards possible future developments is offered.