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

J R Hortensius,A Sasani,A D Caviglia,E Bousquet,D Afanasiev

doi:10.1038/s41535-020-00297-z

J R Hortensius, A Sasani + Show 3 more

Open Access

https://doi.org/10.1038/s41535-020-00297-z

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Abstract

Strain engineering has been extended recently to the picosecond timescales, driving ultrafast metal–insulator phase transitions and the propagation of ultrasonic demagnetization fronts. However, the nonlinear lattice dynamics underpinning interfacial optoelectronic phase switching have not yet been addressed. Here we perform time-resolved all-optical pump-probe experiments to study ultrafast lattice dynamics initiated by impulsive light excitation tuned in resonance with a polar lattice vibration in LaAlO3 single crystals, one of the most widely utilized substrates for oxide electronics. We show that ionic Raman scattering drives coherent rotations of the oxygen octahedra around a high-symmetry crystal axis. By means of DFT calculations we identify the underlying nonlinear phonon–phonon coupling channel. Resonant lattice excitation is also shown to generate longitudinal and transverse acoustic wave packets, enabled by anisotropic optically induced strain. Importantly, shear strain wave packets are found to be generated with high efficiency at the phonon resonance, opening exciting perspectives for ultrafast material control.

Highlights

Epitaxy can be used to impose misfit strain capable of altering the properties of materials
As strain is naturally related to dynamics of the crystal lattice, here we study light-induced ultrafast lattice dynamics directly in an LaAlO3 substrate
We show that impulsive optical excitation at the photon energy tuned in resonance with a polar stretching of the Al–O bonds drives a non-polar rotational mode of oxygen octahedra via ionic Raman scattering[19]

Summary

INTRODUCTION

Epitaxy can be used to impose misfit strain capable of altering the properties of materials. An inherently different approach is to use ultrashort pulses of light that are tuned in resonance with an infrared-active atomic vibration of a substrate, in order to transform the structural and electronic properties of an epitaxial thin film[14]. This mechanism, applied extensively to insulating lanthanum aluminate (LaAlO3) substrates, governs ultrafast metal–insulator transitions[14], ultrasonic magnetic dynamics[17], and sonic lattice waves[18] in various thin films of strongly correlated oxides. These results uncover an hitherto unknown microscopic feature of ultrafast strain engineering that opens wide perspectives for material control via optically tunable strain

RESULTS AND DISCUSSION

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