Ultrafast Insulator-Metal Transition in VO2 Nanostructures Assisted by Picosecond Strain Pulses

Abstract

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.

Unraveling the Mott-Peierls intrigue in vanadium dioxide

“…Within our model description, the effect of a uniaxial tensile strain would be taken into account by adding to the Hamiltonian Eq. (1) terms like: −F 1 X 2 1 or −F 2 X 2 2 (F 1 , F 2 > 0), depending on the direction of the applied stress [167][168][169]. In presence of such terms, the saddle points observed in Fig.…”

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

“…The increase of the fraction of the excited material undergoing the PIPT from 0 to 1 as the absorbed energy increases from J T to J S is related to a distribution of nucleation sites in the sample versus energy which was approximated by the Gaussian error function centered at J 0 = (J T + J S )/2, with a dispersion parameter σ 0 . Due to the various inhomogeneities in VO 2 [40][41][42] we also take into account Gaussian distributions of J T and J S with narrower dispersions σ T and σ S , respectively. Then the general expression for the strain generated upon excitation of the VO 2 in the insulating phase takes a form (see Supplementary Note 5 for details):…”

“…Designing VO 2 nanostructures with sharp PIPT, i.e., with threshold and saturation fluences close to each other, W T ≈ W S 52 , would allow strain generation with negligible lattice heating when working at T ≈ T c 49 since the energy in this case is fully spent for the PIPT excitation. It would further enable fine tuning of the generated strain pulses' parameters by switching PIPT on or off using various means, such as varying excitation fluence in a narrow range, applying voltage 53 , or strain 42,[54][55][56] . Using transducers grown on differently oriented sapphire or other substrates may enable control over the direction in which the largest nonthermal strain generation occurs thus opening a pathway for a further optimization.…”

Ultrafast strain engineering and coherent structural dynamics from resonantly driven optical phonons in LaAlO3

“…Notable examples include the enhancement of ferroelectric and ferromagnetic order 1 and even the engineering of artificial multiferroics at room temperature 2 . Whereas static strain engineering is a well-established paradigm [3][4][5] , ultrafast strain engineering has emerged only recently as an effective method to manipulate functional properties of oxides 6,7 , control collective excitations [8][9][10][11] , induce changes in the band topology 12,13 , and drive optoelectronic phase switching 14,15 . The generation of strain pulses traditionally relies on opto-acoustic conversion processes either in the functional material itself or in opto-acoustic transducers, often involving electronic excitation 16 .…”