Large non-thermal contribution to picosecond strain pulse generation using the photo-induced phase transition in VO2

Abstract

Picosecond strain pulses are a versatile tool for investigation of mechanical properties of meso-and nano-scale objects with high temporal and spatial resolutions. Generation of such pulses is traditionally realized via ultrafast laser excitation of a light-to-strain transducer involving thermoelastic, deformation potential, or inverse piezoelectric effects. These approaches unavoidably lead to heat dissipation and a temperature rise, which can modify delicate specimens, like biological tissues, and ultimately destroy the transducer itself limiting the amplitude of generated picosecond strain. Here we propose a non-thermal mechanism for generating picosecond strain pulses via ultrafast photo-induced first-order phase transitions (PIPTs). We perform experiments on vanadium dioxide VO 2 films, which exhibit a firstorder PIPT accompanied by a lattice change. We demonstrate that during femtosecond optical excitation of VO 2 the PIPT alone contributes to ultrafast expansion of this material as large as 0.45%, which is not accompanied by heat dissipation, and, for excitation density of 8 mJ cm −2 , exceeds the contribution from thermoelastic effect by a factor of five.

Shear wave generation by mode conversion in picosecond ultrasonics: Impact of grain orientation and material properties

“…The polarization of laser light was selected to obtain the largest TA amplitude. A time domain experimental data are presented in Figure 3A for (10,5,24) orientated grain (Euler angles 1 = 63. 4 • , = 25 • ).…”

“…The impact of laser light polarization on the signal amplitude has been studied previously. 30 An example of FTA wave amplitude versus polarization angle Ω is shown in Figure 3C on grain (10,5,24). The interaction stems from both the elastic and photoelastic anisotropy in the substrate.…”

“…Picosecond ultrasonics is a specific example of a broader class of optical pump‐probe spectroscopies that are often used to study nonequilibrium processes in solids, in particular for photovoltaic and optoelectronic applications 17,18 . Picosecond ultrasonics utilizes an ultrashort laser pulse to generate ultrasonic waves with nanoscale wavelength through various mechanisms of laser‐material interactions 19‐24 . The longitudinal wave is frequently utilized because it has the largest amplitude 19 .…”

“…17,18 Picosecond ultrasonics utilizes an ultrashort laser pulse to generate ultrasonic waves with nanoscale wavelength through various mechanisms of laser-material interactions. [19][20][21][22][23][24] The longitudinal wave is frequently utilized because it has the largest amplitude. 19 Although usually much | 2789 WANG ANd KHAFIZOV weaker in amplitude, the excitation of shear waves extends the characterization from only longitudinal direction to transverse directions and thus expands the range of measurable physical properties from experiments.…”

Electron-hole asymmetry in electron systems with orbital degeneracy and correlated hopping

“…Classic materials to manifest the Mott transition are a family of vanadium Magneli phases, which have recently seen renewed attention [12]. In these compounds not only the metal-to-insulator transition at increasing temperatures is observed but also doping of cation subsystem or application of the external pressure determine electrical properties.…”