Transient Diffraction Research Articles

Diffraction effects in hydrophone measurements

Theoretical and experimental methodology are presented for accurately determining the effective radii of unfocused, circular plane piston transducers as well as using tone burst hydrophone measurements to verify steady-state theoretical calculations. Experiments using two specially fabricated unfocused, composite piezoelectric transducers demonstrate the validity of the methodology. For spherically focused circular transducers, a simple model is used to estimate the transient diffraction encountered in co-axial flat hydrophone measurements.

Transient diffraction of a plane step pressure pulse by a hard sphere

Transient diffraction of sound pulses by a sphere is a classical problem extensively studied in acoustics (and in EM) for nearly a century. In spite of this volume of work, a satisfactory treatment of the sound field in the neighborhood of the shadow boundary is not available, and it is still one of the unsolved problems of diffraction theory [see comments in F. Friedlander, Sound Pulses (1958), p. 148]. To our knowledge, no treatment of this case has yet appeared. The modal series solution for this situation is presented here. The time histories of a sufficiently large number of terms of the modal series are accurately computed; a task that is possible today due to the existence of better computers and of more sophisticated computational algorithms. The Cesàro summation method [E. Whittaker and G. Watson, Modern Analysis (1965), p. 155] is used to completely eradicate the Gibbs’ phenomenon effect. This approach yields the diffracted field in the entire space-time domain. In particular, a complete treatment of the diffracted sound field is developed for a hard sphere insonified by a step pressure pulse, including the neighborhood of the shadow boundary where, heretofore, no analytical solution exists.

High-frequency temporal structure of laser and phase-conjugated signals in intracavity degenerate four-wave mixing of radiation from electron-beam-controlled discharge CO2 and CO lasers in their active media

The high-frequency temporal structure of probe-laser and phase-conjugated signals, generated in the course of degenerate four-wave mixing of long pulses from CO2 and CO lasers in their own inverted media, was investigated experimentally with nanosecond resolution throughout the whole pulse. Single-frequency (corresponding to one vibrational — rotational transition) and multifrequency pulses of ∼10 — 20 μs and ∼ 200 — 300 μs durations, generated respectively by electron-beam-controlled discharge CO2 and CO lasers, had a complex temporal dynamics with a depth of modulation up to 100%. In one round-trip period (∼100 ns) the pulses from these CO2 and CO lasers consisted of several spikes (free-running mode) or of one spike (self-mode-locking with the aid of a plasma mirror) of τ0.1 ∼10 ns duration. The temporal dynamics of the phase-conjugated signal also had a complex structure, differing from the probe signal structure over longer (in excess of 100 ns) and shorter time intervals. The relative influence of the amplitude and phase mechanisms of the formation of transient diffraction gratings in the active medium, and also the influence of large- and small-scale diffraction gratings, and of the temporal synchronism on the dynamics of the phase-conjugated signal were considered.

Behaviour of radiation defects under the influence of mechanical strain in ion-implanted silicon

The interactions between radiation defects and internal mechanical strain in ion-implanted semiconductor crystals have been investigated by means of Hall-effect measurements, deep-level transient spectroscopy (DLTS) and X-ray diffraction. The mechanical strain had been intentionally introduced into silicon crystals by 320 keV Ge-ion implantation (10 15−3 × 10 16 cm −2) and subsequent annealing. The samples were then subjected to H + - or Si +-ion bombardment for the introduction of radiation defects. Both Hall-effect and DLTS measurements showed a decrease of the defect production rate in a wide dose interval and accelerated annealing of the radiation defects in strained samples compared to unstrained reference samples. The reduction of the defect concentration during implantation and annealing under the influence of strain is supposed to be connected with an energy transfer from the elastic mechanical strain field to the defects.

Transient multiwave mixing and diffraction with nonsinusoidal intensity-dependent index grating in a nonlinear medium

We study the effects of an intensity dependence in the nonlinear index change coefficient on transient and steady-state optical wave mixing and self-diffractions, using the temperature-dependent thermal index change of a nematic liquid crystal as a model material. We show that the nonsinusoidal index grating created by a sinusoidal optical intensity grating has significant influence on the phases and amplitudes of the multiwave mixing processes. Experimental results on the intensity distribution of the multiorder diffractions in a nematic liquid-crystal film near the phase transition temperature are also presented to illustrate the basic effects.

Characterization of vacancy-related defects introduced into silicon during heat treatment by deep-level transient spectroscopy and gamma-ray diffraction techniques

Vacancy-related defects introduced into n-Si during annealing or aluminium diffusion at high temperature (1000–1250°C) have been studied. Different ambients (argon, nitrogen, vacuum and chlorine-containing atmosphere) were used to create a vacancy supersaturation during heat treatments. Three deep-level centers whose formation is governed by the presence of vacancies have been identified. They were characterized by the following temperature dependences of the thermal emission rate:e3 = 7.92 × 107T2 × exp(− 0.455/kT),e5 = 2.64 × 106T2 × exp( − 0.266/kT),e7 = 7.26 × 106T2 × exp (− 0.192/kT). The influence of different factors, such as heat-treatment conditions, concentration of oxygen and doping level in initial crystals, on center formation was studied. An asymmetric diffuseγ-ray scattering was observed near the surface of a crystal irradiated by thermal neutrons and annealed in a chlorine-containing atmosphere. This scattering is related to the formation of structural defects of the vacancy type. In the same region of the crystal, the concentration of the E7 center was one order of magnitude higher than that of other deep-level centers. Comparison of theγ-ray diffraction and deeplevel transient spectroscopy (DLTS) data suggests that the formation of the center occurs under the conditions of Si supersaturation with vacancies.

High-efficiency transient phase conjugation by turning on reading beam incident upon steady-state transmission grating in BaTiO 3 crystal

Transient high diffraction efficiency of phase-conjugate output in a photorefractive crystal of BaTiO 3 was measured for the case in which the reading beam illuminates the transmission grating, which has been formed by the two writing beams and in steady-state before the illumination. The experimental results show that the peak value of the diffraction efficiency can be over ten times as large as its stationary value.

Impulse characteristics of smooth objects in bistatic case

The impulse characteristic calculation method for transient diffraction problem on the smooth convex ideally conducting object in the bistatic case is developed. The physical optics approach, which is modified by means of the exception of the terminator breaks provoked by the inadequacy of the surface current description near the light-shadow boundary, is used. Calculated formulae and bistatic impulse characteristic numerical calculation results for the ideally conducting triaxial ellipsoid are given.

Photorefractive p-i-n diode quantum well spatial light modulators

We demonstrate the performance of all-semiconductor photorefractive p-i-n diodes operating in the longitudinal quantum-confined Stark geometry. Low-temperature-grown shallow quantum wells provide high-mobility vertical transport, and potential steps incorporated into the semiconductor buffer layers increase the transit time across the buffer and therefore increase the quantum efficiency for trapping of charge before it is swept out to the doped p-type and n-type contacts. The buffer design and the doped contacts both make all-semiconductor photorefractive devices possible, with peak transient output diffraction efficiencies approaching 3%, but without the need for dielectric insulating layers. We also redefine device speed by making a distinction between transient rise times and frequency response, showing that in these p-i-n devices the update rate is an order of magnitude slower than the inverse rise time.

Pulsed beam diffraction by a perfectly conducting wedge: local scattering models

Develops local scattering models for a collimated wavepacket (pulsed beam, PB) impinging on a perfectly conducting wedge. They are derived from an exact solution that has been derived previously. Unlike the exact solution, the local solutions have explicit space-time forms which parameterize the problem in terms of tractable phenomena that may be extended to noncanonical configurations. This is done in the second half of the paper wherein the local models are extended to accommodate astigmatic wavepackets for which there is no exact solution. The local models developed have the format of the geometrical theory diffraction and of the uniform theory of diffraction (UTD), extended to accommodate PB fields: they are governed by transient diffraction functions (time-domain counterparts of the UTD diffraction coefficient) but also include structure functions that describe the space-time distribution of the scattered wavepacket. They explain, uniformly, the physics of the scattering phenomena as a function of the PB parameters: direction, distance from the edge, spatial collimation, and pulse length.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Deep defect levels and mechanical strain in Ge +-implanted silicon

Electrically active defects and the deformation of the crystal lattice produced by Ge + ion implantation of silicon and their evolution during annealing have been investigated using deep-level transient spectroscopy and X-ray diffraction measurements. Three types of defect levels have been observed. In as-implanted samples and at low annealing temperatures (up to about 600°C) well-known vacancy-related defects levels are observed. At about 500°C levels at E c − 0.53 eV and E c − 0.28 eV, related to defect complexes containing Ge, show up and disappear at about 900–1000°C annealing. In addition, two less dominant defect levels at E c − 0.40 eV And E c − 0.15 eV are observed at high annealing temperatures (500–900°C). They are related to defects at the boundary between recrystallized regions and the rest of the crystal and are not specific for Ge. The annealing behaviour of the mechanical strain reflects the behaviour of vacancy related and Ge-related defects in silicon.

Nonlinear excited-state dynamics of a thin copper phthalocyanine film by femtosecond transient grating spectroscopy

Transient diffraction spectra and their rise and decay dynamics of a thin copper phthalocyanine film (≈ 50 nm thickness) were investigated by femtosecond transient grating spectroscopy using a white-light continuum as a probe pulse. Temporal profiles of first- and second-order diffractions were strongly dependent on the excitation intensity, which was interpreted in terms of exciton-exciton annihilation with a time-dependent rate constant.

Grating competition for charge carriers in photorefractive bismuth silicon oxide

Charge carrier competition between the gratings produced in degenerate four-wave mixing in bismuth silicon oxide reduces steady-state diffraction efficiency, suppresses several processes of transient diffraction efficiency, and changes the concavity of reflection grating development.

A simulation package for the signal pathway in absorbing media under linear perturbation.

This paper describes a simple simulation of the signal pathway in an ultrasonic A-scan. It includes transient diffraction acoustic field, transducer response, and frequency dependent absorption in the field medium. The approach selected here aim to develop digital computer simulations of the physical processes that underlie these effects. The soft tissue is considered as a viscoelastic medium and the relaxation theory has been used to calculate the absorption data and dispersion in the tissue. The simulation is used to estimate the degradation in range of resolution due to wave absorption.

Transient diffraction and precursorlike effects in vacuum

We predict the existence of a specific wave object that arises when a spatially limited electromagnetic wave with an abrupt leading edge propagates in a vacuum. This object demonstrates a precursorlike behavior, since the diffraction spreading of the field proceeds behind it. The transverse structure and the field strength at the leading edge of the object remain unchanged during propagation, whereas the object’s energy decreases as z−1 (z is the distance of propagation) because of diffractive erosion of its trailing edge. We show that this phenomenon resembles to a certain extent the Sommerfeld precursor formation in a temporally dispersive medium.

Recording of transient gratings using the short lived bacteriorhodopsin photocycle intermediates

Bacteriorhodopsin, a light‐transducing protein in the purple membrane of Halobacterium halobium, has unique properties that make it a potentially attractive material for use in optical imaging and processing. A transient diffraction grating based on the K intermediate of the bacteriorhodopsin photocycle has been produced on the nanosecond time scale. The diffraction efficiency, which would be sufficient for pattern recognition for example, was measured and the intensity dependence assessed.

In situx-ray studies of rapid crystallization of amorphousNiZr2

We have studied the crystallization of amorphous ${\mathrm{NiZr}}_{2}$, using time-resolved x-ray scattering, for crystallization times varying from several minutes to tens of milliseconds. At relatively low temperatures, where the kinetics of transformation are slow, crystallization of the equilibrium ${\mathrm{NiZr}}_{2}$ phase occurs by time-dependent nucleation and growth. As the transformation temperature is increased, an ever greater fraction of a transient metastable crystal phase is formed, before the equilibrium phase is reached. We believe that the transient phase is in fact a precursor to the formation of the equilibrium phase at all temperatures. By rapidly quenching samples containing this transient phase, we have, by x-ray and electron diffraction, identified the crystal structure as belonging to the space group ${\mathit{O}}_{\mathit{h}}^{7}$, with a lattice parameter of 12.61\ifmmode\pm\else\textpm\fi{}0.01 \AA{}. The transient phase is believed to be formed because it is structurally closer to the amorphous than the equilibrium phase, thereby requiring less atomic rearrangements to form. By considering the formation of the ${\mathit{O}}_{\mathit{h}}^{7}$ structure in similar (transition-metal)-${\mathrm{Zr}}_{2}$ alloys, we deduce that the stability of the transient phase is closely related to the filling of the electronic d band.

Aberrated foci in the time and frequency domains and experiments with acoustical diffraction catastrophes.

Properties of wave fields at and near foci are considered for situations in which the amplitude is limited by the shape of an initial wave front rather than its aperture or initial dimensions. Such aberrated foci are relevant to high-frequency scattering from curved surfaces and to understanding the response of source and receiver arrays. Caustics classified by generic catastrophes are considered such as produced by random phase perturbations or by aberrations of imaging systems [M. V. Berry and C. Upstill, Prog. Opt. 18, 258 (1980)]. Experiments were carried out in which ultrasonic transients and tone bursts were reflected from a curved metal sheet in water such that the resulting caustic surface was a cusp which opens up generally transverse to the propagation direction of the reflected wave front [C. K. Frederickson, Ph.D. dissertation, Washington State University (1991)]. The results illustrate several more general properties of transient responses and diffraction patterns near weak foci. For example, the singular part of the travel time surface describing the merging of echoes has a shape like that of the next higher cuspoid catastrophe: a swallowtail catastrophe. Conditions for neglecting the effects of apertures on catastrophe diffraction patterns are examined. [Work supported by ONR.]

Perturbation analysis of transient diffraction efficiency of photorefractive materials

Using a perturbational analysis, we solve the Kukhtarev equation to analyze the transient process of holographic write-in and read-out in a photorefractive material. The diffraction efficiency as a function of various write and read conditions is discussed. Results show that the diffraction efficiency may be designed to either increase or decrease initially during the reading process. Furthermore, the diffracted light is frequency-shifted according to the applied external electric field.

Transient multiple diffraction rings induced by ultrafast laser from chinese tea

Transient multiple diffraction rings associated with self-defocusing from chinese tea solution in ethanol are observed during the time of laser “turn on”. A second harmonic of cw mode-locked Nd:YAG laser was used. The phenomenon is shown to be the result of laser induced refractive-index change. Diffraction ring patterns under different conditions are compared. The dependence of ultrafast laser induced fluorescence on the polarization of laser beams is also observed. A tentative explanation is given.