Acoustic Strain Waves Research Articles

Shift-current-induced strain waves inLiNbO3mapped by femtosecond x-ray diffraction

The response of the crystal lattice to an electric shift current induced via the two-photon bulk-photovoltaic effect in a lithium niobate (${\mathrm{LiNbO}}_{\text{3}}$) crystal is directly mapped by femtosecond x-ray diffraction. Acoustic strain waves of large amplitude are generated by piezoelectric coupling to the current-related polarization while other mechanisms such as anharmonic phonon-phonon couplings and electron-phonon coupling through deformation potentials play a minor role. A striking variation of the strain wave speed occurs as a function of the relative orientation between the crystal's $c$-axis, the direction of the current flow, and the polarization of the incident pump pulse. The observed behavior is relevant for a large class of ferroelectrics.

Shear Modulus Decomposition Algorithm in Magnetic Resonance Elastography

Magnetic resonance elastography (MRE) is an imaging modality capable of visualizing the elastic properties of an object using magnetic resonance imaging (MRI) measurements of transverse acoustic strain waves induced in the object by a harmonically oscillating mechanical vibration. Various algorithms have been designed to determine the mechanical properties of the object under the assumptions of linear elasticity, isotropic and local homogeneity. One of the challenging problems in MRE is to reduce the noise effects and to maintain contrast in the reconstructed shear modulus images. In this paper, we propose a new algorithm designed to reduce the degree of noise amplification in the reconstructed shear modulus images without the assumption of local homogeneity. Investigating the relation between the measured displacement data and the stress wave vector, the proposed algorithm uses an iterative reconstruction formula based on a decomposition of the stress wave vector. Numerical simulation experiments and real experiments with agarose gel phantoms and human liver data demonstrate that the proposed algorithm is more robust to noise compared to standard inversion algorithms and stably determines the shear modulus.

Interferometric detection of acoustic shock waves

We excite high-amplitude longitudinal coherent acoustic strain waves in a chromium film deposited on a thin sapphire slab, by 100-fs pulses from a 1-kHz Ti:sapphire regenerative amplifier. The bipolar strain pulse that is launched into the crystal, gradually transforms into a shock wave when attenuation by thermal phonons is weaker than the nonlinear action. We detect such shock waves at the opposite side of the slab by probing the reflection of a second chromium film in a standard reflection geometry, with a common-path interferometer to resolve both the induced amplitude and phase changes. We first discuss the interferometer and its standard performance. We then present the measurements on propagating shock waves, and analyze the results on a qualitative level.

Strain propagation in nanolayered perovskites probed by ultrafast x-ray diffraction

Strain propagation in a perovskite heterostructure grown on a ${\mathrm{SrTiO}}_{3}$ (STO) substrate is studied by ultrafast x-ray diffraction. Femtosecond displacive phonon excitation in a ${\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_{3}∕{\mathrm{SrRuO}}_{3}$ (PZT/SRO) film launches acoustic strain waves propagating into the STO substrate. We demonstrate a two-step time evolution of diffracted x-ray intensity which originates from different interfering contributions to the (004) Bragg peak of the STO substrate. Analysis by dynamical x-ray diffraction theory gives the absolute strain $\mathrm{\ensuremath{\Delta}}a∕{a}_{0}$ in a wide range of optical pump fluences. Ultrafast transient strain as small as $\mathrm{\ensuremath{\Delta}}a∕{a}_{0}=2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ is determined in this way.

Magnetic resonance elastography: Non-invasive mapping of tissue elasticity

Magnetic resonance elastography (MRE) is a phase-contrast-based MRI imaging technique that can directly visualize and quantitatively measure propagating acoustic strain waves in tissue-like materials subjected to harmonic mechanical excitation. The data acquired allows the calculation of local quantitative values of shear modulus and the generation of images that depict tissue elasticity or stiffness. This is significant because palpation, a physical examination that assesses the stiffness of tissue, can be an effective method of detecting tumors, but is restricted to parts of the body that are accessible to the physician’s hand. MRE shows promise as a potential technique for ‘palpation by imaging’, with possible applications in tumor detection (particularly in breast, liver, kidney and prostate), characterization of disease, and assessment of rehabilitation (particularly in muscle). We describe MRE in the context of other recent techniques for imaging elasticity, discuss the processing algorithms for elasticity reconstruction and the issues and assumptions they involve, and present recent ex vivo and in vivo results.

Magnetic resonance imaging of transverse acoustic strain waves.

We describe a phase contrast based MRI technique with high sensitivity to cyclic displacement that is capable of quantitatively imaging acoustic strain waves in tissue-like materials. A formalism for considering gradient waveforms as basis functions to measure arbitrary cyclic motion waveforms is introduced. Experiments with tissue-like agarose gel phantoms show that it is possible to measure small cyclic displacements at a submicron level by an appropriate choice of the applied gradient basis function and to use this capability to observe the spatial and temporal pattern of displacements caused by acoustic strain waves. The propagation characteristics of strain waves are determined by the mechanical properties of the media. It is therefore possible to use this technique to noninvasively estimate material properties such as elastic modulus.

Magnetic Resonance Elastography by Direct Visualization of Propagating Acoustic Strain Waves

A nuclear magnetic resonance imaging (MRI) method is presented for quantitatively mapping the physical response of a material to harmonic mechanical excitation. The resulting images allow calculation of regional mechanical properties. Measurements of shear modulus obtained with the MRI technique in gel materials correlate with independent measurements of static shear modulus. The results indicate that displacement patterns corresponding to cyclic displacements smaller than 200 nanometers can be measured. The findings suggest the feasibility of a medical imaging technique for delineating elasticity and other mechanical properties of tissue.