Surface structure determination with X-ray standing waves

The X-ray standing-wave technique is reviewed in its application to surface investigations. Element- specific, structural analysis can be performed with high spatial resolution even for low surface coverages. We present several examples. Interface structure determination is discussed briefly. The fundamental principles and possible variations of the technique, the experimental requirements and the quality of structural information obtained from standing-wave measurements are described in detail.     

The spin probe technique in the EPR-imaging of structurally heterogeneous media

The application of the spin-probe technique for the study of spatial distributions by means of EPR-imaging is of great interest for structurally heterogeneous systems with a different mobility of probes, or for systems with a high concentration gradient. Diferent molecular mobilities of spin probes in the sample provoke the existence of a set of probe lineshapes complicating essentially the task of reconstructing the spatial distribution of centers. The methods and approaches enabling the spatial probe distributions to be obtained without additional measurements are worked out and tested with a set of lines of different shapes being present in the sample. As an application, processes of polypropylene destruction under loading and wheat grain swelling in water have been investigated using TEMPO as the nitroxide spin probe.     

Phase-shift interferometry combined with surface plasmon resonance effect for two-dimensional bio-surface analysis

The phase-shift interferometry combined with surface plasmon resonance (SPR) effect has been studied as a novel technique used to analyze the bio-surface, which measures the spatial phase variation of SPR reflected light. The spatial sensitivity of the SPR imaging sensor is improved over the conventional SPR imaging systems based on optical intensity.     

Pressure-sensitive paint as a distributed optical microphone array

Pressure-sensitive paint is presented and evaluated in this article as a quantitative technique for measurement of acoustic pressure fluctuations. This work is the culmination of advances in paint technology which enable unsteady measurements of fluctuations over 10 kHz at pressure levels as low as 125 dB. Pressure-sensitive paint may be thought of as a nano-scale array of optical microphones with a spatial resolution limited primarily by the resolution of the imaging device. Thus, pressure-sensitive paint is a powerful tool for making high-amplitude sound pressure measurements. In this work, the paint was used to record ensemble-averaged, time-resolved, quantitative measurements of two-dimensional mode shapes in an acoustic resonance cavity. A wall-mounted speaker generated nonlinear, standing acoustic waves in a rigid enclosure measuring 216 mm wide, 169 mm high, and 102 mm deep. The paint recorded the acoustic surface pressures of the (1,1,0) mode shape at approximately 1.3 kHz and a sound pressure level of 145.4 dB. Results from the paint are compared with data from a Kulite pressure transducer, and with linear acoustic theory. The paint may be used as a diagnostic technique for ultrasonic tests where high spatial resolution is essential, or in nonlinear acoustic applications such as shock tubes.     

Local second-harmonic generation enhancement on gold nanostructures probed by two-photon microscopy

The localization of surface second-harmonic generation (S-SHG) enhancements from granular gold structures that exhibit local plasmon resonance was investigated. A two-photon microscopy technique was used to perform high spatial resolution S-SHG imaging. The magnitude and the spatial density of S-SHG enhancement confined in submicroscopic regions are strongly dependent on the morphology of the gold's surface. Polarization measurements of local S-SHG responses reveal the local field anisotropy in enhancement regions and furthermore prove the incoherent and strongly depolarized nature of the emission, which is attributed to ultrafast fluctuations of the enhancement location in the focal volume.     

Digital correlation of grainy shadow images for surface profile measurement

An optical method, combining the projection of a structured light pattern with the digital correlation technique, is discussed in this paper. This method allows measurement of full-field profile on object surface of about 20 mm square size. Tests on a rotated plane surface have been performed in order to quantify the method capabilities. It is shown that this technique leads to an accuracy of about 1 μm for a spatial resolution of around half a millimeter. Profile measurements of a micro-engraved object and at the crack tip of a polymer sample are also presented. Results give proof that this technique remains efficient even in presence of important height slope.     

Implementation of the direct demodulation method for natural gamma ray spectral logging

Spectrum analysis of natural gamma ray spectral logging (SGR) data is a critical part of surface information processing systems. Due to the low resolution, which is an inherent weakness of SGR, and the low signal-to-noise ratio problem of logging measurements, SGR is usually treated with a low confidence level. The Direct Demodulation (DD) method is an advanced technique to solve modulation equations interactively under physical constraints. It has higher sensitivity and spatial resolution than the traditional methods and can effectively suppress the logging noise. Based on standard count rate spectral data obtained from the China Offshore Oil Logging Company SGR Calibration Facility, this paper presents the application of the DD method to gamma-ray logging. The results are compared with four traditional algorithmic methods, showing that the DD method is a credible choice, with higher sensitivity and higher spatial resolution in gamma-ray log interpretation. The Point-Spread-Function of the Shengli Oil Logging Company's natural gamma ray spectroscopy instrument is obtained for the first time. The quantities of various radionuclides in their calibration pits are also obtained. The DD method was applied successfully to gamma-ray logging, offering a new option for SGR logging algorithm selection.     

Improved diffusion measurement in heterogeneous systems using the magic asymmetric gradient stimulated echo (MAGSTE) technique

A magic asymmetric gradient stimulated echo (MAGSTE) sequence was recently proposed to improve molecular diffusion measurements in the presence of spatially varying background gradients. Its effectiveness has been demonstrated previously with simulated background gradients and in phantoms that contain bulk susceptibility differences. In this study, we investigated the MAGSTE technique in microscopically heterogeneous systems, and compared it with the conventional bipolar pulsed gradient stimulated echo (bPGSTE) sequence. We demonstrated that the MASGTE measurements, compared to the bPGSTE method, varied significantly less when the diffusion encoding/decoding interval (delta) was changed. In addition, the MAGSTE technique provided good characterization of the surface area-to-volume ratio for heterogeneous systems investigated in this study. In sum, this study showed that the MAGSTE technique provided diffusion measurements superior to those of the bPGSTE sequence, especially in the presence of severe heterogeneous background gradients.     

Narrow-band BoxCARS applied to CO2 laser discharges

Summary CARS (Coherent Anti-Stokes Raman Scattering) has developed into a powerful tool for studying molecular systems. One of its possibilities is to derive vibrational and rotational temperatures as well as concentrations of molecules from measurements of the energy level population differences. A very good spatial resolution of CARS technique is one of its important advantages. This feature has been utilized for making spatially resolved measurements of the vibrational and rotational temperatures of N₂ in a d.c.-excited transverse-flow CO₂ laser discharge. Apart from that also spectra of CO₂, CO and O₂ in the discharge have been taken, which allowed us to evaluate the spatial distributions of those components in the discharge. Additionally first investigations of a microwave-excited CO₂ laser module have been performed for comparison. Paper presented at the ?XI European CARS Workshop?, Florence, Italy, 23–25 March, 1992.     

Determination of the spatial distribution of focused laser radiation from the shape of the Stark resonance profile

A method of determining the spatial distribution of focused laser radiation from the shape of the resonance maximum in the ion yield, which is due to the ionization of atoms via a resonance induced by the dynamic Stark effect, is proposed. This method gives the same results as the technique based on photometric measurements. The results of the measurements of the spatial distribution are compared with the results of calculation for focused Gaussian laser beams.     

Elastic waves in graded-composition systems

We study the transverse and sagittal elastic waves in graded-composition systems, having a parabolic or linear grading, by means of the full transfer matrix technique and the surface Green function matching method. We obtain the velocities of the different waves as a function of the thickness of the structure, and we compare the variation of the velocities of these waves for the different profiles employed in our calculations. The spatial localization of the different modes is also considered.     

AES studies of surface composition of AgCu alloys

The influence of pretreatments like sputtering, annealing and cleaving on the surface composition is important in quantitative Auger electron spectroscopy. The present paper deals with this influence for the binary alloy system AgCu with 100, 95, 80, 60, 50, 40, 20, 5, and 0 at% Ag. Sputtering causes an enrichment of Cu on the surface due to the higher sputter coefficient of Ag. In contrast cleaving yields an enrichment of Ag due to the lower tensile strength of Ag. The surface composition obtained by scribing is used as standard for the bulk composition because this technique is independent of parameters like sputter coefficient or tensile strength. The results are compared with previous measurements of homogeneous alloy systems.     

Optical studies of fast plasma transport in Si

We present two new optical techniques for the investigation of the transport properties of ambipolar plasmas in semiconductors. The first method is a time-of-flight technique: Using surface doping with shallow impurities which provide a characteristic bound exciton emission we introduce optically active spatial markers into the thin Si wafers investigated. Carrier pairs are excited at the undoped surface of the wafer by short laser pulses. From time-resolved studies of the bound exciton emission we obtain average velocity values for the ambipolar transport through the sample. The most attractive feature of the time-of-flight method compared to other spatially resolved measurements is the combination of very high spatial resolution (submicron range) with a high sensitivity. The second method used to study the plasma transport is based on a time-resolved investigation of the Mott transition between the electron-hole plasma and free excitons in Si. Using well-established values for the Mott transition and the temporal evolution of the plasma and the free exciton emissions we obtain values for the velocity with which the plasma expands to the Mott density as functions of the excitation power and the temperature. The advantage of this method is the lack of any need for spatial resolution. Possible extensions of both methods are discussed.     

Spatial domain multiplexing: A new dimension in fiber optic multiplexing

A novel multiplexing technique for fiber optic communications has been developed that supports multiple channels of optical energy inside an optical fiber by confining each individual channel to a unique spatial location. These channels can operate at exactly the same wavelength as well as differing wavelengths. The basic operating principle and experimental results for spatial domain multiplexed fiber optic communication systems is presented here. This technique adds a new dimension to currently available multiplexing schemes and has the potential to increase the bandwidth of existing and futuristic optical fiber systems by multiple folds.     

Laser nonlinear-optical probing of silicon/SiO2 interfaces: Surface stress formation and relaxation

A nonlinear optical technique based on optical second harmonic generation in reflection is shown to provide information on the surface layer structure of semiconductor crystals, thin films and layered systems. The second harmonic intensity is sensitive to inhomogeneous stress in centrosymmetric materials via spatial selection rules and the appearance of an electric dipole contribution to the second order nonlinear optical susceptibility. The technique is used to monitor mechanical stress relaxation in the SiO₂/Si interface during several annealing procedures.     

Optimisation using measured Green's function for improving spatial coherence in acoustic measurements

Aberrating materials can degrade acoustic measurements by distorting the acoustic wavefront and causing acoustic speckle (as opposed to speckle noise which is a manifestation of coherent backscatter). The amplitude and phase fluctuations associated with acoustic speckle can introduce considerable measurement uncertainty which is difficult to deal with. This paper demonstrates a new technique which optimises the spatial distribution of the generation of the ultrasound to compensate for the aberration. This technique uses experimentally measured Green's functions to allow the calculation of the field resulting from the generation wavefront during optimisation. The technique is used to improve the accuracy of velocity measurements in a steel sample using 82 MHz SAW waves. This is achieved by optimising for improved spatial coherence in the measurement region which suppresses the speckle noise. Experimental evidence of acoustic aberration arising from grain structure is shown for steel and aluminium and the measured Green's function optimisation technique is shown to overcome the resulting acoustic speckle. The technique was performed using the Adaptive Optical Scanning Acoustic Microscope (AOSAM) at Nottingham University, UK.     

Multispectral imaging of paintings by optical scanning

In the last few years, there has been a growing interest on multi-spectral imaging as non-destructive technique for painting diagnostics. Providing spectral and colorimetric characterization of the whole paint layer, such a technique is suitable to document the conservation state of an artwork. In this work we present a scanning system for multi-spectral imaging of paintings in the 380–800 nm spectral region, specifically developed to overcome most of the problems related to traditional detection systems based on CCD or Vidicon camera. The device is composed of a spectrometer for contact-less single-point spectral measurements, moved by two orthogonally mounted translation stages. It scans an area of 1 m² with 16 dots/mm² spatial resolution and 10 nm spectral resolution. Spectral reflection factor and tristimulus value measurements were carried out on coloured ceramic tiles and the results were compared to the corresponding certified values. We also present the first application to an ancient painting.     

A combined study of surface roughness in polycrystalline aluminium during uniaxial deformation using laser-induced photoemission and confocal microscopy

A new technique integrating measurements obtained by photo-stimulated electron (PSE) emission and scanning laser confocal microscopy (SLCM) has been developed to characterize the deformation of commercially pure aluminium in uniaxial plastic strain. Real time, in situ PSE signals provide details about the evolution and propagation rates of surface events during the deformation process. High-resolution SLCM measurements yield details describing the relative magnitude and spatial distribution of the surface features. During homogeneous deformation, uniform generation of surface area produces a monotonic increase in PSE intensity whereas necking induces a saturation condition. Analysis of the surface area created by the deformation revealed that the rate of generation correlates well with the PSE intensity. The consistencies in the data acquired with these two techniques signify that they yield complementary information and that the combination provides essential details about the deformation process in a material with low hardness such as an aluminium alloy.     

Discrete periodic melting point observations for nanostructure ensembles

We report a study of the thermodynamic properties of indium clusters on a SiN (x) surface during the early stages of thin film growth using a sensitive nanocalorimetry technique. The measurements reveal the presence of abnormal discontinuities in the heat of melting below 100 degrees C. These discontinuities, for which temperature separation corresponds to a spatial periodicity equal to the thickness of an indium monolayer, are found to be related to the atomic "magic numbers," i.e., the number of atoms necessary to form a complete shell of atoms at particle surface.     

Experiments of ocean surface waves and underwater target detection imaging using a slit Streak Tube Imaging Lidar

This letter describes a flash Lidar imaging technique that uses a streak tube camera as a receiver to detect short scale surface wave fields for oceanographic, underwater target and coastal measurements. The range and intensity imaging of short scale sea-waves and underwater targets can be supplied by this system through light reflected from the surface of the sea and targets. Through a surface scattering model, more accurate range and intensity information can be extracted. The technique can be applied to surface wave measurements and underwater target detection with high resolution. The feasibility of this technique is demonstrated by applying the technique to data acquired by shipborne Streak Tube Imaging Lidar (STIL) in the Yellow Sea and also the East and South China Seas.