Single‐Lens Counter‐Focused Ultrashort Laser Pulses for Enhanced Microplasma Formation Inside Silicon

The polarization‐dependent diffraction efficiency inherent to liquid‐crystal spatial light modulators (SLM) is exploited to create coaxial double‐focus laser beams. The axial separation and peak‐intensity balance between the beam foci are independently controlled by actively tuning the encoded quadratic phase hologram applied to the SLM and adjusting the input laser polarization. With this and exploiting the high reflectivity of silicon, internal micro‐interactions with counterpropagating infrared pulses have become accessible by only using a single focusing element. By comparatively probing ultrafast nonlinear excitation dynamics, we show that the added angular light components and the dual‐pulse nature of the cooperative interactions under the proposed configuration, immediately lead to an enhancement of the local excitation conditions in bulk silicon. We report on the achievement of near‐critical plasma densities reaching up to , a value inaccessible by conventional focusing. This opens the door to plasma‐optics experiments and material transformations at increased energy density levels, with three‐dimensional controllability inside silicon and other narrow‐bandgap materials.