Stroboscopic X-ray Topography Research Articles

Stroboscopic X-ray imaging of vibrating dislocations excited by 0.58 GHz phonons.

We report on direct imaging, by means of stroboscopic x-ray topography, of phonon-induced dislocation vibrations. X-ray images taken from LiNbO3 crystals excited by 0.58 GHz surface acoustic waves, showed individual acoustic wave fronts as well as their distortions when crossing the dislocation line. The observed contrast is well explained by considering the dynamic deformation field of vibrating dislocation. Comparing simulated deformation maps and x-ray images permitted determination of the local velocities of vibrating dislocations and their viscosity coefficients. We found unexpectedly high velocity values (not far from the speed of sound) and extremely low viscosity coefficients, 2-3 orders of magnitude lower than previously measured in ductile materials.

Visualization of short surface acoustic waves by stroboscopic x-ray topography: analysis of contrast

In this paper the capabilities of stroboscopic x-ray topography to visualize the propagation of short (<10 μm) surface acoustic waves (SAW) in single crystals are analysed. Experiments with LiNbO3 crystals under SAW excitation were carried out at the ID19 beam line of the European synchrotron radiation facility. The developed theoretical analysis of the SAW-induced contrast in Bragg scattering geometry helped to optimize experimental conditions and receive high quality images with well-resolved individual acoustic wave fronts. The basic contrast mechanism is related to the SAW-induced corrugation of atomic planes that causes the focusing of x-rays. The focal distance is directly proportional to the SAW wavelength and inversely proportional to the deformation amplitude at the crystal surface. Simulations of diffraction contrast allowed us to conclude that the best contrast is achieved when the acoustic wave vector is perpendicular to the x-ray scattering plane and the sample/film distance equals 2–4 focal distances.

X-ray imaging of surface acoustic waves generated in semiconductor crystals by an external transducer

We demonstrate successful x-ray-diffraction imaging of high-frequency (0.29-GHz) surface acoustic waves (SAWs) propagating in semiconductor crystals (Si and GaAs) with no deposited electrodes on top of them. Experiments were performed at the ID19 beamline of the European Synchrotron Radiation Facility (Grenoble). We used the stroboscopic x-ray topography technique, in which x-ray bursts coming from the storage ring are synchronized with the excited acoustic waves. This technique is able to visualize individual acoustic wave fronts of traveling SAWs and their distortions caused by phonon scattering. In order to generate SAW in semiconductor crystals the latter were coupled to LiNbO3-based SAW transducers via contact liquids. The maximum SAW transmission is achieved for evanescent waves under optimized coupling conditions at the liquid/sample interface. The technique developed opens a way to directly study phonon interaction with defects in nonpiezoelectric and weakly piezoelectric crystals.

Visualization of surface acoustic wave scattering by dislocations

Stroboscopic X-ray topography at the synchrotron beam line was used to visualize the propagation of a 580 MHz surface acoustic waves (SAW) in LiNbO 3 crystals. For this purpose, the X-ray bursts coming from the synchrotron storage ring with periodicity of 5.68 MHz were synchronized with the SAW frequency in a phase-locked mode. This method allowed us to “stop” the SAW in time and to observe the X-ray diffraction contrast caused by the dynamic deformation field of SAW. The X-ray topographic images showed well-resolved individual acoustic wave fronts of 6 μm SAW as well as their distortions due to SAW scattering by linear dislocations. Some of the images revealed an exceptional contrast of the concentric rings about the dislocation line, which is caused by coherent interaction of the secondary elastic waves. This contrast is similar to the Fresnel zones in optics, and this conclusion is confirmed by direct summation of secondary waves emitted by local elements of a vibrating dislocation string.

X-ray imaging of acoustic wave interaction with dislocations

We used stroboscopic X-ray topography to visualize the propagation of high-frequency (580 MHz), short wavelength (6 μm) surface acoustic waves in LiNbO 3 crystals. The images showed well-resolved individual acoustic wave fronts as well as their distortions caused by scattering on linear dislocations. The observed wave front shapes indicate that a considerable portion of acoustic energy flows along a dislocation line, providing forced vibration of the dislocation string and re-emission of the secondary elastic waves. We also found that the linear dislocations, situated perpendicular to the crystal surface, produce characteristic Fresnel zone contrast in the X-ray images due to an interference of the secondary elastic waves re-emitted by the vibrating dislocations.

Stroboscopic x-ray topography in crystals under 10-μm-surface acoustic wave excitation

High-frequency stroboscopic x-ray topography is developed with the goal to visualize surface acoustic wave propagation. For this purpose the resonant frequencies of LiNbO3-based surface acoustic wave devices (290 and 355 MHz) are synchronized with the frequency of x-ray flashes (5.68 MHz) delivered by the European Synchrotron Radiation Facility (ESRF, Grenoble). X-ray topographs, taken in such a manner, revealed periodic contrast caused by a “frozen” acoustic deformation field. This technique allows for visualization of individual wavefronts of traveling acoustic waves having a wavelength in a 10 μm range. X-ray stroboscopic topographs showed a weak wavefront distortion due to the scattering of acoustic waves on linear dislocations. Secondary spherical waves were observed as a result of the strong acoustic wave interaction with submicron size density perturbations.

Visualization of 10 μm surface acoustic waves by stroboscopic x-ray topography

X-ray topographs under acoustic wave excitation were taken from LiNbO3-based surface acoustic wave devices with He-implanted waveguide layers, using monochromatized synchrotron radiation from the European Synchrotron Radiation Facility (ESRF), Grenoble. Measurements were performed in the stroboscopic mode, i.e., by synchronizing the electron bunch frequency with the resonant frequency of the acoustic wave excitation in the 300 MHz range. These x-ray diffraction images showed plane acoustic wave propagation through the LiNbO3 crystals as well as a weak wavefront distortion due to the scattering on dislocation deformation fields. In some images secondary spherical waves were observed as a result of the strong acoustic wave interaction with the submicron size density perturbations.

Pendellö/sung fringes in stroboscopic X-ray section topography of weakly excited quartz resonators

The theory for slightly distorted crystals [Kato (1963). J. Phys. Soc. Jpn, 18, 1785-1791] is applied to the case of quartz AT-cut resonators to interpret stroboscopic section topographs. Features in the diffraction process such as ray trajectories, phase changes and intensities of diffracted waves are drawn from this theory. Furthermore, it is shown that extra Pendellö/sung fringes appearing on section topographs with increasing acoustic vibration can be directly related to the vibration amplitude within a precision of about 0-3 Å. Experimental images are compared with Kato's theoretical profiles and simulated images obtained by solving Takagi's [Acta Cryst. (1962), 15, 1311-1312] equations. The results lead to the conclusion that stroboscopic X-ray topography is a precise method to measure the vibration amplitude in weakly excited resonators.

Stroboscopic X-ray topography of quartz resonators

Stroboscopic X-ray topography has been performed to observe the different states of acoustic vibrations in quartz resonators with a time resolution of 1 ns. These experiments have been made at LURE (Orsay, France) using the pulsed light delivered by the DCI storage ring. The studied resonators have shown that the vibration is affected by the nature, the density and the distribution of the defects.

Stroboscopic X-ray topography of quartz resonators

Stroboscopic X-ray topography has been performed to observe the different states of acoustic vibrations in quartz resonators with a time resolution of 1 ns. These experiments have been made at LURE (Orsay, France) using the pulsed light delivered by the DCI storage ring. The studied resonators have shown that the vibration is affected by the nature, the density and the distribution of the defects.

Stroboscopic synchrotron-X-radiation topography and its application to the imaging of travelling surface acoustic waves

Images of travelling Rayleigh waves on surface acoustic wave (SAW) devices have recently been obtained using high frequency stroboscopic X-ray topography. When the excited SAW was phase locked to the pulsed synchrotron X-radiation emitted from the Daresbury SRS operating in single bench mode, well defined, “time frozen” images of the strains associated with the propagating wavefronts were obtained. The instrumentation and experimental requirements for these and similar stroboscopic experiments using multiple bunches are described. Examples of results of work on LiNbO 3 and quartz SAW devices are given and the extension to more general stroboscopic X-ray scattering experiments is discussed.

Stroboscopic topography with nanoseconds time resolution

Stroboscopic X-ray topography experiments have been carried out to observe acoustic vibrations in single crystal AT quartz using the synchrotron radiation of the storage ring DORIS as a pulsed light source. Our results for the first time demonstrate that this technique is able to achieve nanoseconds time resolution in topographic experiments. The contrast of dislocation images in a vibrating crystal is briefly discussed.

Direct imaging of travelling Rayleigh waves by stroboscopic X-ray topography

We report here the first successful experiments which exploit the time structure and single bunch mode of operation of the Daresbury Synchrotron Radiation Source (SRS). Rayleigh waves travelling on the surface of a piezoelectric crystal have been imaged by stroboscopic X-ray topography, in which the generation of the waves is synchronized with X rays emitted by the orbiting electrons in the storage ring. Interactions between the Rayleigh waves and microscopic crystalline defects have been observed and provide new insights into the factors affecting surface acoustic wave (SAW) device operation. It has been demonstrated that this novel technique has considerable potential for the study of periodic phenomena in crystals.