Fig. 1.

Accessing ultrafast laser energy and plasma density inside silicon. a Simplified schematic of the measurement methods: Two customized infrared microscopes, each composed of an objective, a tube lens and an InGaAs camera, are positioned laterally and along the optical axis. Ultrashort probe pulses illuminate the interaction region for lateral imaging of transient microplasmas and permanent modifications. Beam profiling at the exit surface of the sample is performed to retrieve, by a z-scanning procedure, the 3D distributions of the delivered laser fluence inside silicon. b Cross-sections of fluence distributions for ultrashort pulses focused in silicon with a numerical aperture of 0.45: The beam focus is positioned at a depth of 1 mm inside silicon as shown by the dotted white lines. Measurements and simulations are compared for increasing pulse energies up to 350 nJ as labeled for each row (the label “linear” stands for a sub-nanojoule energy ensuring the absence of nonlinear effects). All distributions are normalized to their maximum fluence F _max