Optical probing of ultrafast laser-induced solid-to-overdense-plasma transitions

Understanding the solid target dynamics resulting from the interaction with an ultrashort laser pulse is a challenging fundamental multi-physics problem involving atomic and solid-state physics,plasma physics,and laser physics.Knowledge of the initial interplay of the underlying processes is essential to many applications ranging from lowpower laser regimes like laser-induced ablation to high-power laser regimes like laser-driven ion acceleration.Accessing the properties of the so-called pre-plasma formed as the laser pulse’s rising edge ionizes the target is complicated from the theoretical and experimental point of view,and many aspects of this laser-induced transition from solid to overdense plasma over picosecond timescales are still open questions.On the one hand,laser-driven ion acceleration requires precise control of the pre-plasma because the efficiency of the acceleration process crucially depends on the target properties at the arrival of the relativistic intensity peak of the pulse.On the other hand,efficient laser ablation requires,for example,preventing the so-called“plasma shielding”.By capturing the dynamics of the initial stage of the interaction,we report on a detailed visualization of the pre-plasma formation and evolution.Nanometer-thin diamond-like carbon foils are shown to transition from solid to plasma during the laser rising edge with intensities<10^(16)W/cm^(2).Single-shot near-infrared probe transmission measurements evidence sub-picosecond dynamics of an expanding plasma with densities above 10^(23)cm^(−3)(about 100 times the critical plasma density).The complementarity of a solid-state interaction model and kinetic plasma description provides deep insight into the interplay of initial ionization,collisions,and expansion.

Modeling of the 3D spatio-temporal thermal profile of joule-class Yb3+-based laser amplifiers

Thermal profile modification of an active material in a laser amplifier via optical pumping results in a change in the material’s refractive index,and causes thermal expansion and stress,eventually leading to spatial phase aberrations,or even permanent material damage.For this purpose,knowledge of the 3D spatio-temporal thermal profile,which can currently only be retrieved via numerical simulations,is critical for joule-class laser amplifiers to reveal potentially dangerous thermal features within the pumped active materials.In this investigation,a detailed,spatio-temporal numerical simulation was constructed and tested for accuracy against surface thermal measurements of various endpumped Yb^3+-doped laser-active materials.The measurements and simulations show an excellent agreement and the model was successfully applied to a joule-class Yb3+-based amplifier currently operating in the POLARIS laser system at the Friedrich-Schiller-University and Helmholtz-Institute Jena in Germany.