Direct observation of ultrafast thermal and non-thermal lattice deformation of polycrystalline Aluminum film

TL;DR

This study uses femtosecond electron diffraction to observe ultrafast thermal and non-thermal lattice deformations in polycrystalline aluminum, revealing different dynamics depending on excitation wavelength.

Contribution

It provides direct experimental evidence of non-thermal lattice deformation in aluminum under ultrafast excitation, distinguishing it from thermal effects.

Findings

Non-thermal deformation occurs faster than thermal processes.

Different excitation wavelengths induce distinct lattice dynamics.

Coherent phonons are generated under both excitation conditions.

Abstract

The dynamics of thermal and non-thermal lattice deformation of nanometer thick polycrystalline aluminum film has been studied by means of femtosecond (fs) time-resolved electron diffraction. We utilized two different pump wavelengths: 800 nm, the fundamental of Ti: sapphire laser and 1250 nm generated by a home-made optical parametric amplifier(OPA). Our data show that, although coherent phonons were generated under both conditions, the diffraction intensity decayed with the characteristic time of 0.9+/-0.3 ps and 1.7+/-0.3 ps under 800 nm and 1250 nm excitation, respectively. Because the 800 nm laser excitation corresponds to the strong interband transition of aluminum due to the 1.55 eV parallel band structure, our experimental data indicate the presence of non-thermal lattice deformation under 800 nm excitation, which occurs on a time-scale that is shorter than the thermal processes dominated by electron-phonon coupling under 1250 nm excitation.