Picosecond Laser Ultrasonics (PLU)

The theory of the Brillouin scattering of probe laser light by the picosecond acoustic pulse propagating in spatially inhomogeneous medium was developed to support ultrafast opto-acousto-optic experiments intended to depth-profiling of transparent inhomogeneous materials with spatial resolution, which is not limited by the probe light wave length and can be deeply sub-optical (nanometers scale) (see ref.1, ref.2). The developed theory suggests that the latter is controlled by the lengths of the strain fronts in a coherent acoustic pulse (CAP) and their separation, because light is scattered not by the mechanical strain, but by the strain gradients. The theory suggests how the spatial in-depth inhomogeneity of acoustic, optic and acousto-optic parameters of the media could be extracted by conducting time-domain Brillouin scattering (TDBS) experiments, also known as picosecond ultrasonic interferometry, at different angles of probe incidence and measuring both the phase and the amplitude of the transient reflectivity signals. First successful experiments were conducted on revealing the profiles of the sound velocity, of optical refractive index and of photo-elastic constant in sub-micrometer films of nanoporous low-k material with spatial resolution of few tens of nanometers (see ref.). TDBS imaging was later applied to diagnostics of the polycrystalline water and argon ices in the diamond anvil cell (see ref.1, ref. 2, ref.3 , ref. 4). The TDBS provided opportunity to reveal for the first time the depth layering (texture) of the ices at about 20-40 micrometers distances between the anvils (cell thickness) caused by the preferential orientations of the crystallites (see. ref.) and photo-induced motion of the phase front between two different water ice phases (see ref.). The other applications of the TDBS to spatial imaging of the processes were observations of additional curing of low-k films by the two-photon absorption of green laser radiation, when it was used in the TDBS experiments (see ref.).

The TDBS was applied to evaluation of the polycrystalline and damaged (cracked) materials with a grain size exceeding the lateral dimensions of the pump/probe foci (see ref.1, ref.2, ref.3, ref.4). The experiments proved the opportunity to find crystallographic orientation of the individual grains and the inclinations of the inter-grain interfaces in the model polycrystalline ceramic from TDBS measurements with quasi-longitudinal acoustic (QLA) and quasi-transverse acoustic (QTA) photo-generated CAPs (see ref.). The TDBS imaging with quasi-shear acoustic waves provided better sensitivity to crystallographic orientations of the neighbor grains, than with quasi-longitudinal waves also in the high-pressure experiments with water ices (see ref.).

A theory of the TDBS scattering for the Gaussian CAP, heterodyning and scattered light beams, inclined one relative to another, i.e., for the case of non-collinear heterodyning of the acoustically scattered light, was developed (see ref.). The theory suggested that by fitting the TDBS signals, which, in the case of deviations from collinearity of the light beams, strongly depend on the “mismatch” angle and contain simultaneously both Brillouin oscillations and the CAP echoes arriving on the interfaces, it is possible to derive local inclination angle of the interface by a single measurement in a single point. The experiments in the lithium niobate plates destructed under non-hydrostatic pressure loading confirmed these theoretical expectations (see ref.).

The theoretical analysis contra-intuitively demonstrated that the depth of imaging and spectral resolution of the time-domain Brillouin scattering microscopy, with collinearly propagating paraxial sound and light beams, do not depend on the focusing/diffraction of sound (see ref.). The variations of the amplitude of the TDBS signal are only due to the variations of the probe light amplitude caused by light focusing/diffraction. Therefore, the theory explained, why in the experiments the focusing of the CAP-generating pump beam has much less influence on the TDBS microscopy depth of imaging, than the focusing of the probe light beam.

The theoretical developments explained possible existence of “invisible” acoustic frequencies in TDBS experiments (see ref.) and supported the applications of TDBS not to imaging of the materials inhomogeneity but to imaging of the CAPs evolution in homogeneous media, caused either by CAP focusing (see ref.) or by its transformation due to acoustic nonlinearity (see ref.1, ref.2).

First TDBS imaging experiments in the so-called plate geometry were initiated (see ref.). The advantage of this scheme is an independence of the measured Brillouin frequency on the refractive index of a material. Therefore, in a material with an inhomogeneity of optical properties, the imaging of the acoustic velocity can be accomplished independently of the refractive index imaging.

TDBS was applied to imaging of the evolution of a material inhomogeneity with time, opening the path to 4D (3D spacial+temporal).

First experimental observation in the PLU experiments on nanometers-thick layer on a substrate, of an oscillation frequency, which is much lower than eigen frequencies of the layer was reported [see ref.]. This frequency was identified as an observation of mass-on-spring oscillation (a motion of a thin metal layer on adhesion bond). Advanced models of the mechanical interfaces were developed to support theoretically the PLU experiments, conducted in different Laboratories in Asia and Europe:

1. by Prof. Chi-Kuang Sun (National Taiwan University) - the generation of CAPs, at frequencies over 1 THz, by depositing a single-layer absorbing material (graphene) on the surface of a transparent substrate [see ref.] (in these experiments the thermal expansion of the adhesion bond was in the origin of thermoelastic contribution to the generated CAP), - probing hydrophilic interface of solid/liquid-water by nanoultrasonics [see ref.]
2. by Prof. Thomas Dekorsy and Dr. Mike Hettich (Konstanz University) - confined viscoelastic polymer nano-layers [see ref. 1, ref. 2], adhesion in Al/Si membranes [see ref. 1, ref. 2] and in rolled-up GaAs/InGaAs multilayer tubes [see ref.],
3. by Prof. Andrey Akimov (Nottingham University) and Dr. Samuel Raetz (Le Mans University) - GHz frequencies elastic coupling of van der Waals nanolayers to the substrate and in the heterostructures [see ref. 1, ref. 2]. The theory supporting the experiments [see ref.], revealed that adhesive coupling of van der Walls layers to periodic nanogratings leads to hybridization of photo-generated GHz flexural waves in the layer with mass-on-spring vibrations.

Very fruitful, for the extraction of quantitative information from the experimental data, were the theories suggested for several PLU experiments on elasticity of an assembly of disordered nanoparticles interacting via either van der Waals – bonded or covalent-bonded in a coating layer [see ref.] and on the influence of nanocontacts nature on the mechanical properties of a silica nanoparticle assembly [see ref] which were conducted by Prof. Pascal Ruello and Dr. Gwenaelle Vaudel in the frame of the contracts with French industry. 

In laser ultrasonics (LU) I was always interested not only in bulk acoustic waves, but, perhaps, even more, in surface acoustic waves (as well as interface waves and Lamb waves). Starting from Moscow State University (MSU), I either initiated or supported theoretically, or both, LU projects related to surface and guided acoustic waves in Heidelberg, in Leuven and in Sapporo. For the picosecond laser ultrasonics, I had for a long time an idea (a theory published in 1990 and 1992) to efficiently generate by ultrafast lasers GHz frequency SAWs in normally cut superlattices (SLs). The idea was to grow the semiconductor/dielectric SL of alternating light-absorbing and transparent layers and, then, to cleave it along the growth direction to get a periodically structured surface for monitoring of GHz SAWs. When working on this theory (for the future, with no chances at that time to be supported in MSU) I was excited by two things. First of all, the carriers photo-generated by the inter-band pump light absorption would be confined laterally in the light-absorbing semiconductor layers, their lateral diffusion would be suppressed, leading to enhanced generation efficiency of SAWs. Second, by using the SLs with a period of single-digit nanometers it could be possible to monitor the SAWs at sub-THz and even THz frequencies. We have started the realization of the project on “cleaved SLs” in Le Mans only in 2020, when we received Post-Doctoral Fellowships, first, from the Institute of Acoustics of Le Mans University (LMU) and, then, the Marie-Sklodowska-Curie Fellowship of the European Commission. However, before this, I accumulated an experience with GHz SAWs all-optical monitoring with the nanostructures of sub-optical periodicity, by initiating, designing and theoretically supporting in 2011-2015 the project on surface metal gratings in the Laboratory of Prof. Thomas Dekorsy in Konstanz University. To my knowledge, the records, resulted from the realization of this project (optical monitoring about 90 GHz generalized Rayleigh waves [see ref.], and 197 GHz generalized Lamb waves [see ref.] are not bitten in journal publications until now.

Although applications of metal gratings to SAWs had been reported earlier, we managed to increase the efficiency of SAWs laser generation, by combining in phase thermoelastic expansion of metallic lines with contraction of Si substrate (irradiated between the metal lines) due to deformation-potential mechanism. We were able to detect higher order generalized Rayleigh and Lambs modes in these phononic structures/samples and achieve the detection of quasi-monochromatic GHz SAWs generated in a periodic nanostructure at a sharp edge of a metal film (non-periodic detector, however, providing an access to large wave vectors of SAWs) deposited several micrometers aside. The end of these investigations was close to the time of Prof. Thomas Dekorsy departure from Konstanz University. Therefore, the experiments on the monitoring GHz surface CAPs (and not quasi-periodic wave packets) were shifted to Hokkaido University (Prof. O. B. Wright and DR. Osamu Matsuda). The experiments on all-optical monitoring of SAWs, generated at one nanometers-width and micrometers-length metal line and propagating to another spatially-separated one, were started around 2017 [see ref.]. The lines were either additionally periodically structured along their length (to get profit from the plasmon resonances) or not. However, because of the limitations with metal nano-patterning in Hokkaido University the frequencies of the monitored SAWs were up to 15 GHz only and in most of the experiments the frequency spectra were strongly structured because of significant role of the strong mechanical resonances of gold wires deposited on Si, which were influencing both spectral transformation functions (optoacoustic in generation and acousto-optic in detection). Therefore, we designed new nanostructures with aluminum wires of optimal height, deposited in the grooves on Si surface, to suppress acoustic resonances. We have preliminary experimental results in Hokkaido University, confirming that we are moving in the right direction for monitoring Rayleigh CAPs with broad and smooth frequency-spectrum, but still in the frequency range around or below 10 GHz, because of technical difficulties in structures preparation.

In our first experiments in Le Mans on the SLs, grown, characterized and cleaved by Prof. Hong Lu in Nanjing University [see ref.] we monitored the generalized Rayleigh SAW at 40 GHz (generation, detection, ballistic propagation from one SLs to another). We think that we have preliminary experimental results, demonstrating diffusive propagation of these SAWs in these structures, i.e., due to a parabolic and not a linear relation between their frequencies and wave numbers in the vicinity of the Brillouin zone center (k=0) of the periodic nanostructure (where they are monitored). We presented the diffusive SAWs at the conferences, but decided not to include in the first journal publication before confirming their observation in SLs with other (shorter) periodicity. For supporting the above described experiments, I developed a theory of the generation and detection in cleaved SLs of the generalized Rayleigh waves, which explicitly takes into account that the latters contain laterally homogeneous (non-periodic component) in their mode structure. The theory predicts that laterally averaged part of laser excitation contributes to their generation, while their detection can be dominated by a laterally averaged response of the SLs to their propagation and can be detected by heterodyning acoustically-scattered light in the direction normal to the surface. Speaking, differently we are optically monitoring k=0 (near k=0) motions in a vicinity of the Brillouin zone center. For example, the homogeneous component of the SAW is detectable due to laterally averaged homogeneous component of the photoelastic parameter. Additionally, the interaction of periodic component of SAW with periodic component of photoelastic parameter, also contributes to k=0 detection, because of k-k=0. The theory, describing the expected different dominant contributions to the generation and detection processes is available in the Supplementary Information to [see ref.].

Recently the developed theory suggested opportunity of a detection of sub-optical wavelength SAWs by the evanescent components of the diffracted probe laser field under the sub-optical metal grating [see ref.].

In our next series of the experiments with shorter (about 20 nm) period SLs (grown, characterized and cleaved by Dr. Aristide Lemaitre, Dr. Martina Morassi and Prof., Daniel Lanzillotti-Kimura in University Paris-Sacley), we monitored at sub-THz frequencies all acoustical waves, which were accessible by our probe light incident on the cleaved surface (the generalized Rayleigh waves at 160 GHz, shear and longitudinal waves carrying energy along the surface at frequencies 180-200 GHz and 300 GHz, respectively) [see ref.]. We also conducted the experiments with the cleaved SLs, on which ~2 nm thick transparent films were deposited immediately after the cleavage to protect surface from possible oxidation processes, which are potentially increasing the surface roughness. We are rather close to submit these experimental results, presented only at the conferences, for journal publication: we have drafted experimental part of the manuscript and currently approaching a confident theoretical explanation for the broad band 180-200 GHz of the detected quasi-shear waves both analytically and through numerical modelling of their generation and propagation/focusing in an elastically anisotropic (cubic) medium. COMSOL modelling of acoustic waves propagation in the cleaved SLs and also in the non-periodic cleaved structures, containing individual layers for the generation and detection of CAPs, is conducted for about 5 years already by my collaborator Dr. Serhii Kukhtaruk in V.E. Lashkaryov Institute of Semiconductor Physics in Kyiv.

Of course, by extending the project on surface Rayleigh CAPs monitoring with metallic lines (rods) in Hokkaido, we tried to achieve generation and detection of GHz Rayleigh CAPs using the cleaved nanostructures. After epitaxial growth on the substrate of a designed non-periodic sequence of transparent and opaque nanometers-thick layers with hundreds of nanometers separation between them, a cleavage along the direction of growth produces the nanostructured surface for all-optical experiments with GHz CAPs. The samples were grown and characterized by Dr. Jerome Wolfman and Dr. Beatrice Negulecu in University of Tours. In these experiments we used structures prepared from oxides to avoid oxidation of the cleaved (or cut by ion-beam) surface. With a single layer of a metallic oxide (SRO, LSMO) for CAPs generation, which is confined between two layers of transparent oxides (STO, BSTO), we managed to rise a record in laser-based wide-frequency-band Rayleigh CAPs from below 10 GHz (controlled by laser focusing) to 40 GHz (controlled by the thickness of light absorbing layer). Additional advantage of the oxides, for the substrate and the layers, are very tiny differences in their acoustic impedances. Therefore, the photo-generated CAPs are reflected only on the edge of the sample (which in our experiments is either mechanically free or loaded by non-oxide material). In both cases, in the absence of the reflection at the interfaces of oxides, it is rather easy to identify the monitored longitudinal and Rayleigh CAPs via their arrival times. We drafted the manuscript based on CAPs identification through their arrivals times and the durations (suggested by analytical theory and numerical modelling), presented the results at the conferences [see ref. 1, ref. 2], but we are delaying its submission, because our current theories of their detection are not reproducing the detected signals close-enough. We are currently continuing to work on this topic with our collaborators.

Summary

The theories proposing different opportunities to generate coherent shear hypersound in the bulk of anisotropic materials or by acoustic mode conversion at surfaces and interfaces and to detect them via depolarized probe light scattering were developed for picosecond laser ultrasonics. An important physical insight in the opportunity to generate shear waves in the bulk of cubic crystals exhibiting isotropic thermal expansion has been obtained. The theory of the generation by laser-induced thermo-elastic gratings of plane inhomogeneous shear acoustic wave, which is a particular purely shear mode of bulk elastic motion, was proposed. The first successful experiments on generation of shear acoustic pulses of picosecond duration by different physical mechanisms (thermoelastic and inverse piezoelectric effects) were initiated and conducted in metals and semiconductors near their surfaces which had been disoriented relative to the axes of crystallographic symmetry. 

Based on the experience, accumulated in MSU with ns lasers, the experiments on generating of picosecond shear CAPs by fs-laser-induced inverse piezoelectric effect in GaAs with an external electric field applied along the surface were initiated in Hokkaido University. Prof. Oliver Wright had at that time an extremely strong Post-Doc, Dr. David Hurley, who, however, failed to achieve the goals. Later, we tried this approach in Le Mans University and failed. In both case, the problem was, presumably in preparing good electrical contacts. However, Dr. David Hurley was, finally, successful in the first ever realization of picosecond shear CAPs through mode conversion of the laser-generated picosecond longitudinal CAP in reflection, when incident from the isotropic metal film on anisotropic metal substrate [ seeref. 1,ref. 2]. Following the developed theory, an optimal orientation of the single-crystal Zn substrate in combination with polycrystalline Al film was designed, as well as an optimal angle of probe incidence (which should not be normal to the shear wave front for photoelastic sensitivity to shear in isotropic medium) and an orientation of probe incidence plane and polarization relative to shear polarization. Our next achievements in Oliver’s Lab was launching shear CAPs in transparent isotropic SiO₂ film by transmission into SiO₂ of the CAPs generated in a designed Zn single crystal substrate and generation of quasi-shear CAPs by laser-induced screening of the built-in field in disoriented GaAs [see ref.1, ref.2].

The interest to shear waves in picosecond laser ultrasonics was continuous and has not disappeared until now. A theory of the GHz inhomogeneous plane shear waves generation by laser-grating was suggested. The grating is created on the surface of isotropic metal sample, while shear waves are launched by the mode-conversion of the plane inhomogeneous longitudinal modes, thermoelastically generated by laterally periodic laser-induced heating of the sample, in their reflection from the mechanically free surface. The theory predicted the dispersion of plane inhomogeneous waves and explained some features of the pulsed signals detected in earlier experiments with laser gratings, conducted in MIT [ see ref.]. It was extended to the case of the interface of metal and transparent coating [see ref.]. With Dr. Thomas Pezeril we obtained fellowship for a PhD student to start the experiments. However, an experimental scheme with two orthogonal laser gratings (to facilitate the detection of shear via probe mode conversion), suggested by Dr. Pezeril and put in the mathematical frame by me, was not successful. Therefore, later we started with Dr. Osamu Matsuda from Hokkaido University (who was visiting Le Mans University for 1-2 months each year to do experiments) the project on generation of the laterally inhomogeneous plane acoustic waves by laser-irradiated metal gratings [see ref.1, ref.2]. Differently speaking, a periodically laterally modulated in amplitude plane-front wave is just a superposition of two plane waves emitted by a periodic structure in two opposite diffraction orders. It was demonstrated that metal gratings assist not only in the generation but also in the detection via TDBS of the acoustic CAPs emitted in different diffraction orders. The obtained data contain multiple frequency components, which are interpreted by considering all possible angles for the Brillouin scattering of light achieved through multiplexing of the propagation directions of light and coherent sound by the metallic grating. Brillouin scattering with metallic gratings, operating in reflection mode, provided access to wide range of acoustic frequencies from minimal to maximal possible values in a single experimental optical configuration for the directions of probe light incidence and scattered light detection. This was achieved by monitoring the backward and forward Brillouin scattering processes in parallel. Then [see ref.], the direction of probe light incidence and its polarization relative to the sample surface as well as the orientation of the metallic grating were specifically chosen for efficient Brillouin scattering of the probe light from shear phonons propagating in the elastically isotropic materials in the direction of the first diffraction order. Our progress was interrupted by Covid19, however, now, we have already the experimental results where periodic square lattice of the metallic absorbers deposited on the sample surface facilitates the monitoring of the shear CAPs by the TDBS [see ref. ]. Potential applications include measurements of the acoustic dispersion, simultaneous determination of sound velocity and optical refractive index, and evaluation of samples with a single direction of possible optical access. I hope that in the future the tested approaches could be realized with laser interference patterns, replacing the metallic gratings.

In our other projects in Le Mans University with Prof. Pascal Ruello, devoted to shear CAPs, we generated ps quasi-shear CAPs by the inter-band absorption the fs laser pulses in BiFeO₃ and followed their interaction with probe laser pulses in the TDBS experimental configuration [ see ref. 1,ref. 2]. Rather recently, Prof. Pascal Ruello, who was from the beginning a driving engine in our experiments with BiFeO₃, initiated the experiments on detection of the quasi-shear CAPs in this material by time-resolved X-rays diffraction [see ref. ]. I also supported theoretically two experiments on the laser-generation of the quasi-shear modes in crystals, conducted in Dortmund Technical University [ see ref. 1, ref. 2] by the teams of Dr. Alexey Scherbakov and Prof. Ilya Akimov, respectively, both supervised by Prof. Manfred Bayer. Much earlier, we developed a theory, which predicts the opportunity to generate picosecond shear coherent acoustic pulses (CAPs) on the surface ferroelectric crystal by the effect of electrostriction [see ref.]. Polarization of shear CAP can be controlled by the polarization of the pump laser pulse. Shear CAPs can be also generated via mechanism of the inverse piezoelectric effect by the THz electromagnetic pulse, which is excited due to demodulation (rectification) of the ultrafast laser pulse at the surface of the nonlinear optical crystal [see ref. 1, ref.2].

In our experiments in Le Mans University, devoted to monitoring of the generalized Rayleigh waves at sub-THz frequencies in the normally-cut superlattice (SL) [see ref.], we revealed in the transient reflectivity signals a dominance of the photo-generated shear waves, transporting energy beneath the surface (along and at small angle to it). This is achieved because their propagation direction (normal to SL layering) is the focusing direction of quasi-shear waves in the cubic materials constituting the SL while at mechanically free cleaved surface of the SL the shear strain is absent. We also revealed thermoelastic generation of the ps shear CAPs by fs laser pulses on the front surface of an isotropic metallic rough plate and their detection on the opposite surface. The proposed theory suggests that shear CAP generation is dominantly due to mode conversion of the photo-generated longitudinal CAPs in reflection from the rough free surface. Shear CAP arrival on the rare surface is detected mostly due to its mode-conversion in reflection into a longitudinal CAP, although roughness of rare surface potentially provides conditions (angles of probe incidence and probe polarizations) for direct photoelastic detection of shear as well. There are in some of our experiments with rough plates also the signatures of the photo-generated Rayleigh waves. Our results obtained on rough surfaces were presented at the conference [ see ref.].

Summary

The theory describing different nonlinear phenomena possible in photoacoustic conversion due to drift of photo-generated charge carriers and diffusion of electron-hole pairs in semiconductors was proposed [see ref. 1,ref 2, ref 3] and applied for the explanation of some experimental observations [see ref 1, ref 2, ref. 3,ref 4, ref 5, ref 6, ref 7]. The theory of the nonlinear acoustic phenomena in piezoelectric semiconductors attributes the most important nonlinear phenomena to interaction of the photo-generated free charge carriers with external and built-in electric fields and to photo-induced screening of these fields. The theory was successfully applied to explanation of coherent hypersound generation in piezoelectric semiconductor superlattices [see ref.], as well as in p- and n-doped semiconductors [see ref 1ref 2]. The nonlinear optoacoustic phenomena at the time scale of the Maxwell relaxation time and at the spatial scale of Debye screening length were experimentally observed in non-doped piezoelectric semiconductor [see ref.]. The theory attributed the physical origin of these observations to the nonlinear dynamics of photo-induced THz Dember electric field and the transition from non-ambipolar flow to ambipolar diffusion of the photoexcited electron-hole plasma with increasing laser fluence.

Summary

Theory of picosecond acoustic pulses generation by femtosecond laser pulses in metals was developed [see ref. 1, ref. 2, ref. 3]. It provided an important physical insight that the characteristic time of energy transfer from the non-Fermi distributed electrons to phonons depends not only on the time of electron-phonon scattering but also on the time of electron-electron scattering [see ref.]. General theory of bulk acoustic wave generation by inter-band light absorption in the semiconductors was proposed for laser ultrasonics [see ref.]. The theory provided understanding of the possible role of different physical mechanism of optoacoustic conversion including, in addition to thermo-elasticity, deformation potential mechanism driven by photo-induced modification of the electrons and holes concentration and the mechanism of the inverse piezoelectric effect driven by the transient electric fields created due to the separation of the photo-generated electrons and holes. The theory provided also insight that supersonic diffusion of the electron-hole plasma can lead to broadening of the acoustic pulses generated by lasers [ see ref. 1, ref. 2]. Most of the theoretical predictions were confirmed by laser ultrasonics and picosecond laser ultrasonics experiments [see ref. 1, ref. 2, ref. 3,ref. 3].

Of course, I was interested not only in nonlinear manifestations of the different laser/matter interactions in the photo-generated acoustic waves. I was and I am interested in the individual physical mechanisms of optoacoustic conversion. The experiments, revealing deformation potential mechanism and inverse piezoelectric effect, were, first, conducted by the methods of Laser Ultrasonics (LU) with ns lasers [experim ]. For supporting these experiments, I published (alone or with my PhD students in MSU) multiple purely theoretical papers, analyzing optoacoustic signals in different geometries and conditions (like including surface recombination processes [ see ref. 1, ref. 2] or photoexcitation of such “exotic” waves as shear horizontal Lamb waves [see ref.] and Gulyaev-Bluestein (acousto-electric) surface waves in piezoelectric semiconductors [see ref. 1, ref. 2]). I analyzed the generation of the ultrashort acoustic pulses in metals by the photo-excited electrons via deformation-potential mechanism (electron pressure) [see ref.] before I started to write theories for the ps experiments with metals by Prof. Oliver Wright. Later, I supported theoretically the experiments of Prof. Chi-Kuang Sun (National Taiwan University) on CAPs, generated by electron pressure in two-dimensional electron gas [see ref.]. I was analyzing opportunity to generate via electrostriction ultrashort longitudinal and shear CAPs [see ref.]. For the experiments on sub-THz coherent vibrations in van der Waals MoSe₂/WSe₂ hetero-bilayers, conducted by my Post-Doctoral student Changhiu Li in Dortmund Technical University (DTU), we theoretically estimated with Prof. Andrey Akimov (Nottingham University) possible contributions to their photoexcitation of light pressure and Coulomb forces, photo-induced by the splitting of the photo-generated electrons and holes between two layers (in addition to deformation potential, thermoelasticity, ….) [see ref.]. In the future, I am very interested to spend time, as I soon as I have it, on the things, which I poorly understand, like a possible strong anisotropy of the deformation potential mechanism and symmetry of some magnetoacoustic effects, in order to contribute more efficiently to some research activities, in which I am currently involved by the colleagues from Dortmund Technica University (DTU).