Tomas Grabec

Dr. Tomas Grabec (Czech Academy of Sciences)

Dr. Tomas Grabec focuses on laser-ultrasonics, wave mechanics, and connected numerical problems.

Modelling of ultrasonic waves was also the topic of his doctoral thesis, defended at the Czech Technical University in Prague in 2021.
Since his student years, Tomas has carried out research at the Institute of Thermomechanics of the Czech Academy of Sciences, where he is a member of Hanus Seiner's team, which develops laser-ultrasonic methods for characterisation of elastically anisotropic materials, ultimately to enhance understanding of martensitic phase transition, which gives alloys promising diverse applications. In addition, Tomas had several fruitful stays at RECENDT GmbH (Research Centre for Non-Destructive Testing based in Linz, Austria), where he collaborated on investigating ultrasonic scattering in polycrystalline materials.
He will bring his expertise in waves and materials characterisation to his upcoming MSCA Fellowship in Matt Clark's team at the University of Nottingham.

Surface Acoustics and Materials Characterisation by Transient Grating Spectroscopy

Optical generation and detection of ultrasound give a fully non-contact route to a material’s elastic response. Transient grating spectroscopy (TGS) is a striking example: two interfering sub-nanosecond pump pulses create a spatially harmonic thermoelastic excitation, launching counterpropagating sur-face acoustic waves that are read out by a heterodyned continuous-wave probe.

Rotating the excitation direction over the sample surface builds detailed frequency-angular maps of the surface response, visualising not only surface acoustic waves, but also bulk-wave-related features with a clarity rarely achieved by conventional ultrasonic methods (Fig. 1) [1]. With the excitation pattern forcing the waves to follow the intended direction, detailed maps of phase velocities can be obtained even for strongly anisotropic materials [2]. As these velocities are set entirely by the elasticstiffness tensor, density and crystallographic orientation, such maps are a rich input for elasticity characterisation through an inverse problem [3].

We address this inverse problem with GuliweRR [4], a MATLAB package computing surface- and guided-wave velocities in anisotropic and layered media using the Ritz approach, paired with anoptimisation module recovering elastic constants from measured velocities. Beyond bulk single crystals,the method has been validated on thin films measured by TGS, where the film thickness can be resolved alongside the elastic constants themselves [5].

Most recently, the procedure has been extended to solve for crystallographic orientation and elastic constants simultaneously—at least for cubic-symmetry materials—removing the need for carefully pre-oriented specimens or multiple characterisation methods. This capability has so far been explored using other methods that map acoustic velocity across a sample—most notably the SRAS method [6]—and is now shown to apply directly to TGS data of a single-crystalline sample, utilising the advantage of high sensitivity (and connected detection of bulk-wave-related features). Although still a fresh development, it points towards a general characterisation workflow taking an unoriented, unknown sample straight to its full stiffness tensor, and is offered here as a direction for discussion.

Figure 1: Transient grating spectroscopy (TGS): (a) optical setup, (b) interference of pump lasers, (c)geometry of the interference pattern forming the thermoelastic source, (d) frequency-angulardispersion map of a Fe3Al single crystal.

References:

[1] T. Grabec et al., arXiv:2510.10696 (2026). doi: 10.48550/arXiv.2510.10696
[2] P. Stoklasov´a et al., Exp. Mech. 61(4), 663–676 (2021). doi: 10.1007/s11340-021-00698-6
[3] P. Stoklasov´a et al., Ultrasonics 56, 381–389 (2015). doi: 10.1016/j.ultras.2014.09.004
[4] T. Grabec, GuliweRR (2026). https://github.com/tomasgrabec/GuliweRR
[5] T. Grabec et al., Ultrasonics 138, 107211 (2024). doi: 10.1016/j.ultras.2023.107211
[6] R.J. Smith et al., Meas. Sci. Technol. 25(5), 055902 (2014). doi: 10.1088/0957-0233/25/5/055902