The decrease of laser-induced damage threshold (LIDT) when exposed with high number of laser pulses is a well-known phenomenon in dielectrics. In the femtosecond regime this fatigue is usually attributed to the accumulation of laser-induced lattice defects. Little is known about the accumulation mechanisms in oxides used for dielectric coatings. In this work, S-on-1 laser-induced damage threshold test was combined with time-resolved digital holography in order to investigate laser-induced lattice defects in Nb<sub>2</sub>O<sub>5</sub> single layer. The results provided insights into the current understanding of accumulation of laser-induced defects.
In order to correlate laser damaging fluence with the pertinent theoretical considerations, there were many attempts in the past to establish reliable damage predicting criterion. Such criterion then could be used to estimate laser fluence that triggers the damage process in various optical materials. For example, reaching of materials critical property such as - temperature (melting point), - thermoelastic stress, - electron density are good examples. On the other hand, however, it is already clear that damage mechanism is irradiation condition (wavelengths, pulse duration) and material property dependent. There are no physical restrictions of causing damage by reaching critical stress without critical electron density and vice versa. Accordingly, total absorbed energy or absorbed energy density is likely more suited candidate of universal damage criteria as a common denominator for all critical processes. To our best knowledge, it was never estimated experimentally in the vicinity of the damaging fluence of optical materials. In this study, we present a novel approach based on pump- probe digital holographic microscopy that enables quantitative assessment of absorbed energy during the damage process in transparent dielectric media. By using this method, a case study is conducted in fused silica glass with sharply focused infrared laser pulses at 1030 nm central wavelength and 450 fs pulse duration. By doing so we were able to estimate energy fraction of the incident pulse that is needed to trigger optical damage.
A special dielectric edge filter extremely sensitive to any change in refractive indices, layer thicknesses and angle of incidence has been investigated using holographic pump-probe measurements at different intensity values. Different physical processes overlapping in time were found to occur, namely the Kerr effect, free- electron generation and their subsequent trapping. A numerical model was used to reproduce the experimental results and decouple these processes.
Time resolved digital holography (TRDH) is a versatile tool that provides valuable insights into the dynamics of femtosecond damage initiation by providing spatiotemporal information of excited material. However, interpreting of TRDH data in thin film dielectric coatings is rather complicated without appropriate theoretical models that are able to correctly describe underlying nature of damage formation. Therefore, a model based on finite difference time domain (FDTD) method with complete Keldysh theory for nonlinear ionization of atoms and multiple rate equation (MRE) method for conduction band electrons was developed. The model was used to reproduce both temporal and spatial characteristics of TRDH experiment performed on Ta<sub>2</sub>O<sub>5</sub> dielectric coating. Fitted material parameters were then applied to indirectly estimate LIDT of the coating.
Time-resolved investigations of laser-matter interaction processes in dielectric coatings and bulk silica leading to laserinduced damage were performed with high temporal and spatial resolution. Distinct excitation geometries were used to study different aspects of laser matter interaction. Samples were irradiated at the pump fluence levels below and above their single shot laser-induced damage thresholds. The obtained results provide new insights about the sequence of interdependent processes. The fundamental differences between the so called 1-on-1 and S-on-1 damage morphologies are observed and discussed. New data of numerical simulations revealing the nonlinear properties of optical thin films are presented. Increased visibility in time resolved damage detection as well as observation of coherent oscillations in measured signals are introduced and discussed.