# Special Issue on Laser Ultrasonics
## 2 From Applied to Fundamental Research  
This Special Issue was launched to provide a platform for the presentation of recent research achievements in all areas of laser ultrasonics. This issue welcomed papers from across this diverse field, from the evaluation of fundamental light-matter interactions at femtosecond temporal and nanometer spatial scales and studies of coherent phonon interactions with other excitations and particles (excitons, magnons, electrons, or bubbles) to the nondestructive testing, industrial and biomedical applications of laser ultrasonics. Topics of interest for this issue included, but were not limited to, new laser ultrasonic generation and/or detection techniques and instrumentation, signal processing approaches and inverse analysis, materials characterization and nondestructive evaluation (NDE), industrial applications, simulation and theory, and fundamental acousto-optic and wave propagation phenomena.  
The papers in this Special Issue contain the latest research on laser ultrasonics. There were 14 papers submitted to this issue and 10 papers were published. While the papers cover a wide range of applied and fundamental topics, the majority of papers (70%) are more oriented towards novel applications of laser ultrasonics as might be expected for a Special Issue in _Applied Sciences_.  
Here, we present a brief overview of the manuscripts that were selected for publication. The first seven papers of the Special Issue are applications oriented. [PERSON] et al. [1] present a review of principles and applications of photoacoustic imaging and laser ultrasonic imaging. The review covers techniques developed in the Laser Optoacoustic Laboratories in the Institute on Laser and Information Technologies of the Russian Academy of Sciences, the National University of Science and Technology MISiS, and the International Laser Center of the Lomonosov Moscow State University. The applications discussed include combined photoacoustic and laser ultrasonic imaging for biologicalobjects, and tomographic laser ultrasonic imaging of solids. [PERSON] et al. [2] demonstrate the application of laser ultrasonics for nondestructive testing in additive manufacturing (AM) processes. [PERSON] et al. [3] developed a laser ultrasonic approach to remotely characterize the elastic and thermal properties of a coating-substrate structure used to protect fuel claddings in lead-bismuth eutectic cooled nuclear reactors. [PERSON] et al. [4] present a study focusing on the application of the S\({}_{0}\) Lamb plate mode generated by symmetric laser excitation on the plate edge to the detection and localization of defects in an aluminum plate immersed in water. [PERSON] et al. [5] describe the application of laser ultrasonic inspection to deformation and cracking caused by residual stress with application in laser welding technology. [PERSON] et al. [6] report the first application of nonlinear longitudinal acoustic pulses generated and detected by lasers for nondestructive diagnostics of material thermal aging. [PERSON] et al. [7] applied the transient grating method to study the behavior of surface acoustic waves in nanowire-based composite structures.  
The last three papers in this Special Issue cover fundamental research in laser ultrasonics. [PERSON] et al. [8] studied the propagation of ultrasonic waves in lysozyme solutions characterized by different degrees of aggregation and networking. [PERSON] et al. [9] report the variation in the longitudinal acoustic velocity and the optical refractive index in thin films deposited on different substrates across the rhombohedral to tetragonal structural phase transition induced by element substitution. Finally, [PERSON] et al. [10] demonstrate how semiconductor saturable absorber mirrors can be characterized non-destructively by their coherent acoustic response in the sub-THz range.