Transmitted Wave Front Research Articles

Geometry effects on the droplet shock-induced cavitation

Assessment of geometry effects affecting shock-induced cavitation within a droplet is investigated for the first time. To do this, we use a thermodynamically well-posed multiphase numerical model accounting for phase compression and expansion, which relies on a finite pressure-relaxation rate formulation and which allow for heterogeneous nucleation. These geometry effects include the shape of the transmitted wave front, which is related to the shock speed to droplet sound speed ratio and the droplet geometry (cylindrical vs spherical). Phenomenological differences between the column and the droplet configurations are presented. In addition, the critical Mach number for cavitation appearance is determined for both cases: between M = 1.8 and M = 2 for the column, and between M = 2 and M = 2.2 for the droplet. Based on the transmitted wavefront geometry, with Mach number varying from 1.6 to 6, two cavitation regimes have been identified, and the transition has been characterized: an exponentially (M < 4.38) and a linearly (M > 4.38) increasing bubble-cloud volume. On more applied aspects, we also investigate the influence of the bubble cloud on the interface disruption and compare the results against the pure liquid droplet test case. A parallel with the technique of effervescent atomization is eventually presented.

Observations of a Flare-ignited Broad Quasiperiodic Fast-propagating Wave Train

Large-scale extreme-ultraviolet (EUV) waves are frequently observed as an accompanying phenomenon of flares and coronal mass ejections (CMEs). Previous studies mainly focused on EUV waves with single wave fronts that are generally thought to be driven by the lateral expansion of CMEs. Using high spatiotemporal resolution multi-angle imaging observations taken by the Solar Dynamics Observatory and the Solar Terrestrial Relations Observatory, we present the observation of a broad quasiperiodic fast-propagating (QFP) wave train composed of multiple wave fronts along the solar surface during the rising phase of a GOES M3.5 flare on 2011 February 24. The wave train transmitted through a lunate coronal hole (CH) with a speed of ∼840 ± 67 km s−1, and the wave fronts showed an intriguing refraction effect when they passed through the boundaries of the CH. Due to the lunate shape of the CH, the transmitted wave fronts from the north and south arms of the CH started to approach each other and finally collided, leading to a significant intensity enhancement at the collision site. This enhancement might hint at the occurrence of interference between the two transmitted wave trains. The estimated magnetosonic Mach number of the wave train is about 1.13, which indicates that the observed wave train was a weak shock. Period analysis reveals that the period of the wave train was ∼90 s, in good agreement with that of the accompanying flare. Based on our analysis results, we conclude that the broad QFP wave train was a large-amplitude fast-mode magnetosonic wave or a weak shock driven by some nonlinear energy release processes in the accompanying flare.

Characterizing the physical limitations of phase-only metasurface design for acoustic holograms

Numerous recent studies have shown that acoustic metasurfaces (AMS) can be used to control reflected or transmitted wave fronts. Acoustic holograms have commonly been created by designing a metasurface that applies local phase changes to a wave front to construct arbitrary pressure fields at one or more image planes. The hologram design process typically features iterative methods, such as the Iterative Angular Spectrum approach. However, metasurfaces that apply phase-only modulation are restricted in the quality of the pressure field reconstruction that can be achieved, especially when attempting to create different holograms at different planes for a single-frequency incident wave. The control of magnitude and phase by an AMS is of significant interest because it enables one to completely define an entire reflected or transmitted wave field in a 3-D region. In the present work, the physical limitations in simultaneously reconstructing multiple single-frequency holograms at different planes via phase-only metasurface design are examined. Methods for overcoming these limitations, including both amplitude and phase control by an acoustic metasurface, are also investigated. Comparisons between simulation and experiment are made for a multi-image reflective hologram system. [S.N.L. is managed and operated by NTESS under DOE NNSA Contract No. DE-NA0003525.]

In Vitro Visualization of Ultrasonic Wave Fronts Interacting with Heel Bones Using Refracto-Vibrometry

Ultrasonic measurements of the heel bone (calcaneus) are used commonly for osteoporosis screening. Pulses emitted by an ultrasound transducer are incident on the calcaneus, and the transmitted wave fronts are detected with a separate transducer. In the current study, full field videos were obtained using refracto-vibrometry of ultrasonic pulses interacting human calcaneus samples in an in vitro environment. Pulses were emitted by a 500 kHz Panametrics V303 transducer. The measurement beam from a Polytec PSV-400 scanning laser Doppler vibrometer laser was directed through a water tank towards a stationary retroreflective surface. Acoustic wave fronts (density variations) which pass through the measurement laser cause variations in the integrated optical path length. The time-varying signals detected by the vibrometer at numerous scan points were used to determine the time evolution of ultrasonic wave fronts. The resulting videos enable visualization of the propagating wave fronts and the backscattered and transmitted wave fronts. These videos enable direct investigation of wave front distortion due to reflection, refraction and diffraction effects

Optical imaging of propagating ultrasonic wave fronts resulting from ultrasonic pulses incident on heel bones using refracto-vibrometry

Ultrasonic measurements of the heel bone (calcaneus) are used commonly for osteoporosis screening. Pulses emitted by an ultrasound transducer are incident on the calcaneus, and the transmitted wave fronts are detected with a separate transducer. In the current in-vitro study, full field videos of propagating ultrasonic wave fronts incident on a calcaneus sample, along with transmitted and backscattered waves were obtained using refracto-vibrometry. Pulses were emitted by a 500 kHz Panametrics V303 transducer. To optically detect ultrasonic wave fronts, the measurement beam from a Polytec PSV-400 scanning laser Doppler vibrometer laser was directed through a water tank towards a stationary retroreflective surface. Acoustic wave fronts (density variations) which pass through the measurement laser cause variations in the integrated optical path length between the vibrometer and retroreflector. The time-varying signals detected by the vibrometer at numerous scan points were used to determine the time evolution of ultrasonic wave fronts. The resulting videos enable visualization of the propagating wave fronts incident on the calcaneus and the backscattered and transmitted wave fronts. These videos enable direct investigation of wave front distortion due to reflection, refraction and diffraction effects for pulses transmitted through the calcaneus during ultrasonic heel scanning.

Digital holographic microscopy for detection of Trypanosoma cruzi parasites in fresh blood mounts

An off-axis holographic microscope, in a transmission mode, calibrated to automatically detect the presence of Trypanosoma cruzi in blood is developed as an alternative diagnosis tool for Chagas disease. Movements of the microorganisms are detected by measuring the phase shift they produce on the transmitted wave front. A thin layer of blood infected by Trypanosoma cruzi parasites is examined in the holographic microscope, the images of the visual field being registered with a CCD camera. Two consecutive holograms of the same visual field are subtracted point by point and a phase contrast image of the resulting hologram is reconstructed by means of the angular spectrum propagation algorithm. This method enables the measurement of phase distributions corresponding to temporal differences between digital holograms in order to detect whether parasites are present or not. Experimental results obtained using this technique show that it is an efficient alternative that can be incorporated successfully as a part of a fully automatic system for detection and counting of this type of microorganisms.

Scaling limits for wave pulse transmission and reflection operators

The random paraxial wave equation is revisited to take into account not only random forward scattering, but also random backscattering. In this paper we are interested in the transmitted wave fronts and also wave fronts reflected by a strong interface buried in a random medium. In the weakly heterogeneous regime the reflected and transmitted wave fields are characterized by reflection and transmission operators that are the solutions of Itô–Schrödinger diffusion models. These models allow for the computations of the Wigner distributions and the autocorrelation functions of the reflected and transmitted waves. They also fully take into account the fact that the waves travel through the same medium during the propagation to and from the interface, which induces an increase of the beam radius and of the correlation radius, and also predict the enhanced backscattering effect in the backscattered direction.

Coherent Diffractive Imaging Using Phase Front Modifications

We introduce a coherent diffractive imaging technique that utilizes multiple exposures with modifications to the phase profile of the transmitted wave front to compensate for the missing phase information. This is a single spot technique sensitive to both the transmission and phase shift through the sample. Along with the details of the method, we present results from the first proof of principle experiment. The experiment was performed with 6.0 keV x rays, in which an estimated spatial resolution of 200 nm was achieved.

Measurement of the integral refractive index and dynamic cell morphometry of living cells with digital holographic microscopy

We have developed a digital holographic microscope (DHM), in a transmission mode, adapted to the quantitative study of cellular dynamics. Living cells in culture are optically probed by measuring the phase shift they produce on the transmitted wave front. The high temporal stability of the phase signal, equivalent to lambda/1800, and the low acquisition time (~20micros) enable to monitor cellular dynamics processes. An experimental procedure allowing to calculate both the integral refractive index and the cellular thickness (morphometry) from the measured phase shift is presented. Specifically, the method has been applied to study the dynamics of neurons in culture during a hypotonic stress. Such stress produces a paradoxical decrease of the phase which can be entirely resolved by applying the methodological approach described in this article; indeed the method allows to determine independently the thickness and the integral refractive index of cells.

High-energy, high-average-power laser with Nd:YLF rods corrected by magnetorheological finishing

A high-energy, high-average-power laser system, optimized to efficiently pump a high-performance optical parametric chirped-pulse amplifier at 527 nm, has been demonstrated. The crystal large-aperture ring amplifier employs two flash-lamp-pumped, 25.4-mm-diameter Nd:YLF rods. The transmitted wave front of these rods is corrected by magnetorheological finishing to achieve nearly diffraction-limited output performance with frequency-doubled pulse energies up to 1.8 J at 5 Hz.

Design of a nonnull interferometer for aspheric wave fronts.

The presence of highly aspheric wave fronts in an interferometer leads to a need for system calibration, and this calibration requirement affects the design of the interferometer. Dynamic range, vignetting, and the ability to characterize components all must be considered during the design stages. The interferometer must be designed with respect to wave-front propagation as opposed to reference sphere aberrations. A nonnull interferometer for measurement of aspheric transmitted wave fronts has been developed, and the design process is described. Transmitted wave fronts for a conformal window and a progressive-addition bifocal lens have been measured to demonstrate the applicability of the system to aspheric testing.

Applications of the wave packet method to resonant transmission and reflection gratings

Scattering of femtosecond laser pulses on resonant transmission and reflection gratings made of dispersive (Drude metals) and dielectric materials is studied by a time-domain numerical algorithm for Maxwell's theory of linear passive (dispersive and absorbing) media. The algorithm is based on the Hamiltonian formalism in the framework of which Maxwell's equations for passive media are shown to be equivalent to the first-order equation, ∂Ψ/ ∂t= HΨ , where H is a linear differential operator (Hamiltonian) acting on a multi-dimensional vector Ψ built of the electromagnetic inductions and auxiliary matter fields describing the medium response. The initial value problem is then solved by means of a modified time leapfrog method in combination with the Fourier pseudospectral method applied on a non-uniform grid that is constructed by a change of variables and designed to enhance the sampling efficiency near medium interfaces. The algorithm is shown to be highly accurate at relatively low computational costs. An excellent agreement with previous theoretical and experimental studies of the gratings is demonstrated by numerical simulations using our algorithm. In addition, our algorithm allows one to see real time dynamics of long living resonant excitations of electromagnetic fields in the gratings in the entire frequency range of the initial wide band wave packet as well as formation of the reflected and transmitted wave fronts.

Interferometric measurement of the refractive-index distribution in plastic lenses by use of computed tomography.

A nondestructive measurement method that makes possible the measurement of a three-dimensional refractive-index distribution of any shape plastic lens is presented. In this method, a Mach-Zehnder interferometer and shearing interferometer are combined into a single optical system and are used selectively. Interference fringes of a test object that is immersed in matching liquid are detected at various rotation angles. And transmitted wave fronts are calculated with these interference fringes. Finally, the refractive-index distribution is obtained by computed tomography analysis. In addition, accurate control of the matching liquid temperature makes it possible to measure the absolute refractive index of the test object. This system has good performance with a measurement accuracy of 10(-4) or better peak to valley.

Diffractive Alvarez lens

A diffractive Alvarez lens is demonstrated that consists of two separate phase plates, each having complementary 16-level surface-relief profiles that contain cubic phase delays. Translation of these two components in the plane of the phase plates is shown to produce a variable astigmatic focus. Both spherical and cylindrical phase profiles are demonstrated with good accuracy, and the discrete surface-relief features are shown to cause less than lambda/10 wave-front aberration in the transmitted wave front over a 40 mm x 80 mm region.

Transient effects and delay time in the dynamics of resonant tunneling

We consider an analytic solution of the time-dependent Schr\"odinger equation with the initial condition \ensuremath{\psi}(x,0)=exp(ikx) along -\ensuremath{\infty}0 to investigate the time evolution for x>0 of the wave function in a double-barrier resonant structure at resonance. For typical parameters of the structure we find that the single-resonance approximation is valid from a few tenths of the corresponding lifetime onward. Very short times require the contribution of many far away resonances. The buildup time along the internal region takes a few lifetimes. At birth the transmitted wave front is blurred; however, for long times it becomes well defined and moves with classical velocity yielding a delay time \ensuremath{\tau}\ensuremath{\approx}2\ensuremath{\hbar}/\ensuremath{\Gamma} as in the stationary-phase treatment.

Laser mirrors with variable reflected intensity and uniform phase shift: design process

Optical coatings with circularly symmetric graded reflectance are used as laser mirrors in unstable resonators. A proper design of such coatings permits any maximum central reflectance to be obtained along with a null external reflectance. Different design approaches are discussed, and an optimized design that gives negligible distortion of the reflected and transmitted wave front is proposed. Coatings with super-Gaussian and step reflectance profiles are examined as two different solutions for improving laser beam quality.

Interferometric stress birefringence measurement

We describe a sensitive method for measuring the stress birefringence of an optical window that utilizes a phase-measuring Fizeau interferometer incorporating a variable retarder and a nonpolarizing beam splitter. When we test a material in an interferometer cavity, the wave front transmitted through the material is deviated by the surfaces, inhomogeneity, and birefringence of the material. Birefringence causes the transmitted wave front to have different optical path difference (OPD) profiles for the vertical and horizontal orientations of linear polarization. Subtracting these OPD profiles reveals the amount of phase difference between the fast and slow axes of the material. Hence, birefringence may be calculated. Phase-measurement techniques and a computer-controlled interferometer employing a variable liquid-crystal retarder provide a fully automated instrument for measuring stress birefringence. The theoretical derivation, discussion of the instrument, and experimental results are presented.

Polarization aberration analysis of the advanced x-ray astrophysics facility telescope assembly

<p>The advanced x-ray astrophysics facility (AXAF) telescope consists of six concentric paraboloid-hyperboloid pairs of mirrors that operate near grazing incidence. Because of the substantial polarization effects at large angles of incidence there has been concern regarding the feasibility of doing polarimetry near the telescope focal plane. The primary mirror is shown to act as a tangentially directed half-wave linear retarder and to almost completely depolarize the linearly polarized component of the light. The secondary mirror introduces an additional half-wave of linear retardance. The two-mirror telescope assembly acts as a tangentially directed one-wave linear retarder. Each mirror depolarizes alone but together the two-mirror assembly preserves the polarization state. The net instrumental polarization effects are small and polarimetry is feasible with AXAF.</p><p>The polarization aberration function for the telescope is derived by using Jones calculus. Polarization aberration functions are used to calculte the effect of instrumental polarization on the transmitted wave front and the polarization state that is due to the primary mirror and the telescope assembly.</p>

Simulated annealing of binary holograms for the interconnection of single-mode structures

The simulated annealing algorithm is applied to the design of binary holographic optical elements for the connection of single-mode structures. The system energy used with the simulated annealing algorithm is the absolute square of the overlap between the transmitted wave front and the modal characteristics of the receiving device; the system variables are the binary states of the hologram pixels. The plausibility of the obtained binary patterns is discussed for examples involving plane and spherical modal characteristics. The ability of the annealing process to avoid local minimum trapping is demonstrated.

Ultrasonic Bragg imaging of flaws

A superficial crack on an aluminum plate becomes an internal flaw when covered by a second plate. Ultrasonic waves are caused to resonate between the external surfaces of the plates and produce transmission maxima characteristic of Lamb modes in the plates. The transmitted wave fronts in a liquid, modified by the presence of the flaw, are now used to diffract light at the Bragg angle. The Bragg diffraction orders contain images of the ultrasonic wave fronts and, hence, can be used to image the internal flaws. Characteristic flaws are shown and resolution limits are discussed. [Research supported by the U.S. Office of Naval Research and by the UT-ORNL Science Alliance.]