Coherent acoustic phonons and ultrafast carrier dynamics in hetero-epitaxial BaTiO₃–BiFeO₃ films and nanorods

Abstract

The desire for multifunctional devices has driven significant research toward exploring multiferroics, where the coupling between electric, magnetic, optical, and structural order parameters can provide new functionality.

Evaluation of Optical and Acoustical Properties of Ba1−xSrxTiO3 Thin Film Material Library via Conjugation of Picosecond Laser Ultrasonics with X-ray Diffraction, Energy Dispersive Spectroscopy, Electron Probe Micro Analysis, Scanning Electron and Atomic Force Microscopies

“…In particular, the stronger photoelasticity of the SRO for the probe light was reported in comparison with the rather common values of the photoelastic coefficients of the STO [59]. In our samples, the contribution to ∆R/R from the CAP reflected by SRO transducer is expected to be nearly completely suppressed due to the well-known close-to-perfect matching of the longitudinal acoustic impedance of SRO to that of STO and BaTiO 3 (BTO) [46,49,64]. This expected, nearly perfect matching of the BST0.4 film, of the SRO optoacoustic generator and of the STO substrate is the reason for the absence of any pronounced temporal oscillations corresponding to the vibrational eigen modes of the film and of the optoacoustic transducer in the signals presented in Figure 5c [28,30,31,65,66].…”

Ultrafast dynamical charge-lattice coupling in rare-earth nickelate thin films studied by time-resolved terahertz spectroscopy

“…The oscillations appear at a time when this acoustic strain reaches the interface between the film and substrate. As the acoustic phonons are the carriers of heat and sound in the material, we can roughly estimate the sound velocity in the films by relation as: t = L vs , where t is the time at which the oscillations appear, L is the film thickness, and v s is the sound velocity [20]. For the L25ENO and P50ENO films, v s was found to be 3.75 km s −1 and 3 km s −1 , respectively.…”

“…[21]. Damping of the acoustic phonon modulation could be due to the transmission of the acoustic waves to the substrate, which acts as a heat sink and depends on how well the acoustic impedances are matched between the film and substrate [14,20]. Here, the difference in the speed of sound can have several reasons, for example, the different wavelengths of pump and probe [22], uncertainty in film thickness, disorder, epitaxial strain, etc.…”

“…Acoustic strain developed (as demonstrated in previous section) in these metallic films can be transduced into an electrical signal by an output transducer. This technique suggests an edge over the conventional SAW devices as the acoustic strain is solely generated by an optical pump pulse, and, thus, eradicates the need to fabricate the SAW device [20]. Being an optical technique, this allows for better dynamical manipulation of acoustic strain in time and can be used to achieve higher and different frequencies by hetero-epitaxial tuning of metallicity of nickelates films.…”