Istvan A. Veres

Characterization of broadband fiber optic line detectors for photoacoustic tomography

The frequency response of fiber optic line detectors is investigated in the presented paper. An analytical model based on oblique scattering of elastic waves is used to calculate the frequency dependent acousto-optical transfer functions of bare glass optical and polymer optical fibers. From the transfer functions the transient response of fibers detectors to photoacoustically excited spherical sources is derived. Photoacoustic tomography is simulated by calculating the temporal response of arrays of fiber optic line detectors and subsequent image reconstruction. The results show that the choice of the fiber material is of significant importance and influences the quality of imaging.

Complex band structures of two dimensional phononic crystals: Analysis by the finite element method

In this work, the calculation of complex band structures of two-dimensional bulk phononic crystals (2DPCs) is discussed by the finite element method. A modification of the classical ω(k) approach—calculating the unknown frequencies for a real wave number—is modified to a k(ω) solution, which allows the evaluation of complex wave numbers for real frequencies. The dispersion relation of a 2DPC in a square lattice is presented and it is shown that the problem reduces to a polynomial eigenvalue problem with quadratic and quartic eigenvalue problems in the Γ−X−M−Γ directions. The developed method is applied for solid-vacuum PCs made of isotropic materials consisting cylindrical holes. Complex dispersion diagrams are calculated with various Poissons ratios and the mode shapes of the propagating and the evanescent modes are presented. The significance of the complex bands is discussed.

Broadband evolution of phononic-crystal-waveguide eigenstates in real- and k-spaces

Control of sound in phononic band-gap structures promises novel control and guiding mechanisms. Designs in photonic systems were quickly matched in phononics, and rows of defects in phononic crystals were shown to guide sound waves effectively. The vast majority of work in such phononic guiding has been in the frequency domain, because of the importance of the phononic dispersion relation in governing acoustic confinement in waveguides. However, frequency-domain studies miss vital information concerning the phase of the acoustic field and eigenstate coupling. Using a wide range of wavevectors k, we implement an ultrafast technique to probe the wave field evolution in straight and L-shaped phononic crystal surface-phonon waveguides in real- and k-space in two spatial dimensions, thus revealing the eigenstate-energy redistribution processes and the coupling between different frequency-degenerate eigenstates. Such use of k-t space is a first in acoustics, and should have other interesting applications such as acoustic-metamaterial characterization.

Focusing and subwavelength imaging of surface acoustic waves in a solid-air phononic crystal

Focusing and subwavelength imaging of surface acoustic waves (SAWs) through a phononic crystal flat lens are discussed in the presented work. Experimental and numerical wave fields are obtained in the time-domain by an optical technique and by numerical simulations. Spatial distributions of the acoustic field are accessed using a temporal Fourier transform. The revealed focusing of the elastic waves in the first band of the crystal is governed by the concave equifrequency contour of the leaky-Rayleigh wave. The spatial distributions of the experimental and numerical acoustic fields also unfold subwavelength imaging of SAWs. Numerical simulations show that the imaging quality can be improved by embedding the flat lens into a medium with higher wave velocity.

Numerical modeling of thermoelastic generation of ultrasound by laser irradiation in the coupled thermoelasticity

Highlights ► Numerical modeling of ultrasound in the coupled thermoelasticity is discussed. ► Thermoelastic generation of ultrasound by laser irradiation is modeled. ► The influence of the coupling in the generalized thermoelasticity is investigated. ► For ultra high frequency waves (∼100 GHz) strong attenuation is observed. ► Laser-generated wave fields are presented in isotropic and hexagonal media.

Spatial and temporal frequency domain laser-ultrasound applied in the direct measurement of dispersion relations of surface acoustic waves

We present a laser-ultrasound measurement technique which combines adjustable spatial and temporal modulation of the excitation laser beam. Our method spreads the intensity of an amplitude modulated continuous wave laser over a micro-scale pattern on the sample surface to excite surface acoustic waves. The excitation pattern consists of parallel, equidistant lines and the waves generated from the individual lines interfere on the sample surface. Measurement is done in the spatial-temporal frequency domain allowing the direct determination of dispersion relations. The technique performs with high signal-to-noise-ratios and low peak power densities on the sample.

Characterization of the spatio-temporal response of optical fiber sensors to incident spherical waves

A theoretical framework is presented for calculating the opto-acoustic response of optical fiber ultrasound sensors with several layers of coating. A harmonic point source generating a spherical wavefront with arbitrary position and frequency is assumed. The fiber is acoustically modeled by a layered cylinder on which spherical waves are scattered. The principle strains on the fiber axis are calculated from the scattering of the acoustic waves and used in a strain-optic model to calculate the phase shift of the guided modes. The theoretical results are compared to experimental data obtained with a sensing element based on a π-phase-shifted fiber Bragg grating and with photoacoustically generated ultrasonic signals.

Point source in a phononic grating: stop bands give rise to phonon-focusing caustics

We use locally-excited gigahertz surface phonon wavepackets in microscopic line structures of different pitches to reveal profound anisotropy in the radiation pattern of a point source in a grating. Time-domain data obtained by an ultrafast optical imaging technique and by numerical simulations are Fourier transformed to obtain frequency-filtered real-space acousticfield patterns and k-space phononic band structure. The numerically-obtained k-space images are processed to reveal an intriguing double-horn structure in the lowest-order group-velocity surface, which explains the observed non-propagation sectors bounded by caustics, noted at frequencies above the bottom of the first stop band. We account for these phonon-focusing effects, analogous to collimation effects previously observed in two- and three-dimensional lattices, with a simple

Vibrational modes of GaAs hexagonal nanopillar arrays studied with ultrashort optical pulses

We investigate the vibrational modes of a triangular array of anisotropic, hexagonal GaAs nanopillars on a GaAs substrate through ultrafast changes in optical reflectivity. By comparison with simulations, we identify GHz resonances, mode shapes, and damping. In addition, by varying the pillar diameter, height, and pitch, we distinguish collective and localized modes. A proper understanding of substrate-attached nanostructure dynamics will lead to better characterization of nanosensors based on perturbations to vibrational resonances.

Stability analysis of second- and fourth-order finite-difference modelling of wave propagation in orthotropic media

The stability of the finite-difference approximation of elastic wave propagation in orthotropic homogeneous media in the three-dimensional case is discussed. The model applies second- and fourth-order finite-difference approaches with staggered grid and stress-free boundary conditions in the space domain and second-order finite-difference approach in the time domain. The numerical integration of the wave equation by central differences is conditionally stable and the corresponding stability criterion for the time domain discretisation has been deduced as a function of the material properties and the geometrical discretization. The problem is discussed by applying the method of VonNeumann. Solutions and the calculation of the critical time steps is presented for orthotropic material in both the second- and fourth-order case. The criterion is verified for the special case of isotropy and results in the well-known formula from the literature. In the case of orthotropy the method was verified by long time simulations and by calculating the total energy of the system.