Laser-induced ultrasonic waveform derivation and transition from a point to a homogeneous illumination of a plate

Abstract

Ultrasound modeling, being an established practice, is used to study the fundamentals of light-matter interactions. Although much has been published on the matter, pressure and thermal expansion induction mechanisms in laser ultrasonics have rarely been combined, as they should, in a single ultrasonic source while the effects of its size variation have only been shown to a limited extent. In the paper, we unite these light-matter interaction mechanisms, with inclusion of lateral optical forces, into a single laser-stimulated source as it is observed in nature. With a laser pulse as a manipulable source, we simulate the multifaceted workings of light-matter interactions by exposing the distinct transients originating from different source localities as generated by different induction mechanisms. We also present a transition of simulated ultrasonic waveforms in the epicentral point on the surface of a solid plate opposite from the source while it is expanded from a point to a quasi-limitless extent for pressure and thermal expansion generation regimes. The model utilizes geometric probability theory together with Huygens' superposition principle and temporal convolutions to construct the desired waveforms out of individual Green's functions. We show how the ultrasound generation regimes stem out of a single source and how its size together with energy and momentum transfers during the light-matter interactions affect the induced ultrasonic transients.

Isolated detection of elastic waves driven by the momentum of light

“…Results from the FEM simulations are further supported by calculations using a statistically enhanced Green’s function formalism (GFF) 33 , 36 , which combines statistically weighted Green’s functions to incorporate the illumination and detection area distributions and their relative positions into an area-to-area transfer function of the substrate. In addition to validating FEM simulation results, GFF simulations are used for two purposes.…”

“…The statistically enhanced GFF method is used to simulate the light-induced (thermo)elastic source of ultrasonic waves and their subsequent propagation and reverberation in the substrate 33 . It solves similar thermoelastic equations as given in Eqs.…”

“…In general, areal distributions of both interactions and their relative positions are incorporated, by means of geometric probability 44 , into a normalized area-to-area weight function ψ ( v , w ). This is combined with a continuous Green’s function field g ( v , w , t ), comprised of a multitude of neighboring Green’s functions, to form the area-to-area transfer function 33 . From it, the general displacement waveforms are calculated as …”

“…Displacements have been measured with sufficient spatial (1 mm lateral and 40 fm vertical) and temporal (0.2 μs) resolution to visualize elastic wave propagation. To determine the origin of the measured waveforms, we use ab initio modelling of momentum deposition 20 , 21 and material deformation 11 , 33 . Predicted waveforms match the shape and amplitude of those measured in the experiments.…”

Compact Laser Driver for Ultrasonic Arbitrary Position and Width Pulse Sequences Generation

“…More mechanisms produce ultrasonic waves in plasma regime: light pressure, thermal expansion, electrostriction, and material ablation recoil [9], [10], [29]. Very high amplitude signals, into nonlinear ranges of ultrasound, can be obtained [10], [30].…”