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.

Strain engineering is a powerful technology which exploits stationary external or internal stress of specific spatial distribution for controlling the fundamental properties of condensed materials and nanostructures. This advanced technique modulates in space the carrier density and mobility, the optical absorption and, in strongly correlated systems, the phase, e.g. insulator/metal or ferromagnetic/paramagnetic. However, while successfully accessing nanometer length scale, strain engineering is yet to be brought down to ultrafast time scales allowing strain-assisted control of state of matter at THz frequencies. In our work we demonstrate a control of an optically-driven insulator-to-metal phase transition by a picosecond strain pulse, which paves a way to ultrafast strain engineering in nanostructures with phase transitions. This is realized by simultaneous excitation of VO2 nanohillocks by a 170-fs laser and picosecond strain pulses finely timed with each other. By monitoring the transient optical reflectivity of the VO2, we show that strain pulses, depending on the sign of the strain at the moment of optical excitation, increase or decrease the fraction of VO2 which undergoes an ultrafast phase transition. Transient strain of moderate amplitude ~0.1% applied during ultrafast photoinduced non-thermal transition changes the fraction of VO2 in the laserinduced phase by ~1%. By contrast, if applied after the photo-excitation when the phase transformations of the material are governed by thermal processes, transient strain of the same amplitude produces no measurable effect on the phase state.

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We report on experimental picosecond acoustic studies of an ultrafast photoinduced insulator-to-metal and structural transition in VO2 nanostructures epitaxially grown on Al2O3 substrates with different orientations. Applying a pump-probe technique with combined excitation of a sample with picosecond strain and femtosecond laser pulses we demonstrate that dynamical strain of moderate amplitude of 0.1% has a pronounced impact on ultrafast photoinduced phase transition in VO2 nanohillocks. This enables novel path for controlling such transitions at picosecond and nanometer scales. Our experiments also allowed characterizing elastic and photo-elastic properties of the photo-induced metallic phase in VO2 and to relate them to the properties of the equilibrium phase. Furthermore, we demonstrate the generation of picosecond strain pulses upon laser-induced excitation of thin epitaxial VO2.