Femtosecond dynamics of highly excited dielectric thin films

Abstract

Highly excited wide-gap dielectrics are well suited to study the evolution of non-equilibrium conduction band electrons in solids. Compared to semiconductors, the measurable quantities in femtosecond pump-probe experiments – transient reflection (R) and transmission (T) – are less affected by processes such as gap shrinkage and band filling, simplifying the data interpretation. The measurements were performed on three oxides (TiO2, Ta2O5 and HfO2) thin films (~ 500 nm) with different band gaps (3.3 eV, 3.8 eV, 5.1 eV). Pulses at 800 nm and 25 fs duration excited the samples to ~60% of the threshold for dielectric breakdown. Besides their technical importance as optical coatings, we chose thin films to avoid self-focusing. The difficulty when working with thin films is the presence of standing waves (Fabry-Perot effects) of pump and probe, strongly influencing the transmission and reflection behavior after excitation. An algorithm to retrieve the change of the complex dielectric constant, ∆ε(t)= ∆εR(t) − i∆ε(t), from measured ∆R(t) and ∆T(t) data was developed [1]. This algorithm calculates the optical response of the film due to the periodic excitation by the pump through multi-photon absorption and impact ionization. Detailed studies were performed to characterize and maximize the stability of the algorithm with respect to experimental uncertainties. Figure 1 shows the so obtained time dependent dielectric function for two samples.