Influence of Thermal Finite Diffusion On the Generation and Propagation of the Laser-Induced Ultrasound Longitudinal Waves for Strong Optical Absorber Materials

In this work, to consider the heat flux before solving coupled differential equations for Laser-Induced Ultrasound (LIU) longitudinal waves, we first obtained the exact solutions of a 1D boundary value problem for the Maxwell-Cattaneo- Vernotte (MCV) heat equation for a two-layer system in the frequency domain and then, the temperature solutions are the LIU pressure waves sources in the respective 1D boundary value problem for the wave equation. Theoretically, as a function of the position and frequency, the LIU pressure sample is affected by the temperature second derivative, suggesting that the main LIU source is the heat flux. Moreover, by varying the characteristic heat lag time of the MCV equation <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">( τ q ) </tex> it is found that the LIU pressure remains constant if <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">τ q ≤ τ ₚ </tex> , with a behavior very close to the case when <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">τ q = 0 </tex> . Substantial changes become important if <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">τ q ≥ τ ₚ </tex> , this means that pulse laser works as a bandpass filter. Here. <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">τ ₚ </tex> is the laser pulse time.