Fig. 1: Critical density plasmas for localized modification writing.
From: In-chip critical plasma seeds for laser writing of reconfigurable silicon photonics systems
a Illustration of the critical-plasma seeding concept using synchronized femtosecond and picosecond pulses to achieve highly localized internal structuring of silicon. b Ultrafast infrared microscopy images of microplasmas produced inside Si with 200-fs pulses focused at NA = 0.85 (snapshots at 20-ps delay) and measured minimum transmission levels in the plasma images as function of pulse energy. The correspondingly peak free-carrier densities (calculated using a Drude model) are always found on the plasma front (dashed line on the images) and reach values approaching the critical plasma density (Nc ≈ 4.6 × 1020 cm−3).
