Femtosecond laser induced emission of coherent terahertz pulses from ruthenium thin films

We demonstrate emission of terahertz pulses from Ru thin films through a second-order non-linear optical process. Ru deposited on different substrates showed different THz emission properties. For Ru on glass, a strong laser-induced increase in THz emission amplitude is observed when exposing the sample to optical powers above a certain threshold. We provide evidence that at these powers laser-induced oxidation occurs, resulting in an increased slope of the linear dependence of the THz-electric-field amplitude on pump power. The THz-electric-field, in this case, is mainly polarized along the sample surface, pointing in the same direction everywhere on the surface. In contrast to Ru on glass, the electric-field amplitude of the THz pulses emitted by Ru on sapphire and on CaF
2
shows a simple single linear dependence on pump power. Surprisingly, it is polarized orthogonal to the sample surface. However, in this case thermal oxidation enhances the emission and introduces an additional polarization component along the sample surface. This component also points in the same direction everywhere on the sample surface, similar to as-deposited Ru on glass. Although the precise THz generation mechanism remains an open question, our results show a strong correlation between emission strength and the degree of oxidation. Furthermore, they highlight the importance of the interfaces, i.e. both the choice of substrate and the chemical composition of the top surface in THz emission experiments, showing how the knowledge of the state of the sample surface is crucial for the interpretation of THz emission experiments from (non-magnetic) metal surfaces.