Nerijus Šiaulys

Direct holographic imaging of ultrafast laser damage process in thin films

Dynamic process of femtosecond laser-induced damage formation in dielectric thin films is reconstructed from a series of time-resolved images. Ta2O5 single-layer coatings of four different thicknesses have been investigated in transmission mode by means of time-resolved off-axis digital holography. Different processes overlapped in time were found to occur; namely, the Kerr effect, free-electron generation, ultrafast lattice heating, and shockwave generation. The trends in contribution of these effects are qualitatively reproduced by numerical models based on electron-rate equations and Drude theory, which take into account transient changes in the films and interference effects of the pump and probe pulses.

Holographic study of ultrafast optical excitation in GaN film induced by nonlinear propagation of light

The dynamics of pulse transformation and free-carrier generation associated with the nonlinear propagation of 1030 nm and 300 fs light pulses in Gallium nitride film has been studied by use of pulsed digital holography with 25 fs temporal resolution. Domination of the Kerr effect at the leading part and electron plasma at the trailing part of the pulse has been identified from the reconstructed spatiotemporal phase-contrast images. Experimental results have been supported by the simplified numerical model capable to reproduce the main features of a dynamic process.

In situ study of two interacting femtosecond filaments in sapphire.

Interaction dynamics of two copropagating femtosecond light filaments in sapphire crystal is studied by means of time-resolved off-axis digital holographic and shadowgraphic microscopy with 22 fs temporal and 1 μm spatial resolution. In particular, we demonstrate that filament interaction originates from instantaneous modification of the refractive index resulting from the Kerr effect. Fusion, splitting, and even suppression of the resulting plasma channels induced by interacting filaments was observed by varying time delay between the input pulses. Free electron channels were reconstructed in the form of the time-lapse movie with unprecedented spatial and temporal resolution.

Time-resolved Digital Holography For Nonlinear Refraction Index Measurements

Traditional methods of nonlinear refraction index (n 2) measurements assume it as a constant measure. Keeping in mind its possible variation in time we explore an alternative approach of time-resolved digital holography for such measurements.

What time-resolved measurements tell us about femtosecond laser damage?

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.

Application of time-resolved digital holographic microscopy to study femtosecond damage process in thin films

An imaging of strongly excited thin film dielectric coating is done by the means of femtosecond time-resolved off-axis digital holography (TRDH). Ta2O5 single layer coating have been investigated at different time moments in transmission mode. The evolving damage process was recorded in series of microscopic amplitude and phase contrast images. Different processes were found to occur and namely: Kerr effect, free-electron generation, ultrafast lattice heating and shock wave generation. The trends in electronic contribution are qualitatively reproduced by the theoretical model while the other effects require additional studies.