Detection of picosecond strain pulses generated in VO2 films upon ultrafast phase transition
Picosecond acoustics utilizes short strain pulses generated by pulsed lasers to determine and affect properties of various media, from solids to biological cells. This represents a challenge in the field - to generate strong enough picosecond strain pulses with reduced heat dissipation. We show that a medium with a first-order ultrafast insulator-metal transition, such as vanadium dioxide VO2, may facilitate addressing this challenge. We used a 35 nm and 100 nm VO2 films grown epitaxially by PLD on a r-cut Al2O3 substrate in M1 orientation ('XRD' data). On the other side of the substrate, a 30 nm Cr layer was deposited allowing photoelastic detection of generated pulses. We further characterized the phase transition in the samples. Static transition was characterized by heating the sample and monitoring reflectance at 1028 nm. The ultrafast trasition was characterized using femtosecond pump-probe by varying pump fluence and monitoring reflectivity change vs. pump-probe delay (See 'Characterization of phase transition' data). We also measured optical reflectivity and transmission from the samples on 1028 nm and derived optical absorption in them at temperatures below and above the static phase transition (See 'Static optic measurements' data). The laser source used for all but XRD experiments is a 170-fs Yb:KGd(WO4)2 regenerative amplifier with 5 kHz repetition rate and a central photon energy of 1.2 eV. A conventional picosecond acoustics setup was used with two beams taken from the same laser source. One beam was incident on the VO2 film acting as a pump to generate a strain pulse, the other one passing via a motorized delay line was directed on the Cr film and served as a probe. We monitored the intensity of a reflected probe pulse as a function of the time delay at 295 K and 350 K (See 'Acoustics pump-probe' data). Due to non-linear nature of strain pulse propagation in sapphire, longer pulse duration is one of the main markers for strong initial amplitude of the generated pulse. We derived photo-generated strain amplitude from the pulse duration. Plotting it versus light energy absorbed in VO2 film (See 'Photogenerated strain vs absorbed energy' data), we obtained a distinct deviation of the monotonous increase between the PIPT threshold and saturation which cannot be described by thermoelastic or deformation potential strain-generation mechanisms. We ascribe this to the contribution from the photo-induced phase transition. We also calculated the temperature rise in VO2 film associated with generation of the strain pulse for two initial phases of VO2 demonstrating effectiveness of utilization of the phase transition (See 'Calculated temperature rise vs absorbed energy' data).