RESUMEN
Results concerning the controllable ablation of nano-layered thin films (NLTF) by femtosecond laser pulses are presented. Investigated samples were titanium-aluminum bilayers, deposited on a silicon substrate, with the top titanium or aluminum layer of variable thickness on the surface. Irradiation was done in ambient air with single femtosecond laser pulses under standard laboratory conditions. The samples were analyzed by complementary methods of optical and scanning electron microscopy and optical profilometry, exhibiting laser-fluence-dependent ablative removal either of the top layer or the entire bilayer or even partial ablation of the underlying silicon substrate. The removal (spallation) threshold fluences for the topmost layer are scalable versus its thickness almost irrespectively of its material, being rather selective for the Ti-coated samples and much less selective for the Al-coated samples. The removal of the entire bilayers was found to be strongly influenced by electronic properties of the underlying metallic layer, dictating the NLTF-Si adhesion, heat conduction, and capacity in the NLTFs toward the NLTF-Si interface and beyond, as well as by their thermophysical characteristics, e.g., almost twice higher melting temperature and enthalpy for Ti. As a result, precise fs-laser machining of the entire NLTFs is pronounced and selective for the samples with the fusible Al at the low-adhesion Al-Si interfaces, compared with the incomplete NLTF removal from the high-adhesion and refractory Ti-Si interfaces.
RESUMEN
Nonlinear interaction of the powerful terahertz radiation pulse with metal nanofilm is investigated. Quantitative analysis of the nonlinear interaction is illustrated by an example of an aluminum film of a thickness of several tens of nanometers. Rapid heating of the conduction electrons and the lattice causes significant rise of the effective electron collision frequency. This implies a noticeable change in reflectivity and an increase of transmissivity by an order of magnitude.
RESUMEN
We present a time-domain solution of the equations describing low-frequency radiation generation when a metal surface is irradiated by a non-damaging femtosecond optical pulse of s-polarized radiation. Ponderomotive force, drag current, and temperature gradient along the surface are found to be sources of the terahertz radiation. A comparison of these sources under typical experimental conditions reveals that both the drag current and the thermal gradient give similar contribution to the terahertz signal. The ponderomotive force effect is several times less than these two and exists only during the incident pulse.