Physical mechanisms of ultrafast optoacoustic transformation in solids

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).