Spatio-temporal laser shaping for internal structuring of semiconductors (Invited Paper)

To address the controllability challenges of intense femtosecond laser pulses in semiconductors, we investigate a range of spatial and temporal optimization strategies to advance three-dimensional laser-writing technologies. We introduce an elegant approach for in-volume interactions using tightly focused, counter-propagating infrared pulses through a single lens. The cooperative effect between the two beams enhances bulk excitation conditions within silicon. To achieve highly localized and reliable processing, we employ a multi-timescale irradiation scheme in which pre-ionizing femtosecond pulses generate critical plasma seeds that synchronize with subsequent writing pulses. This approach enables isotropic, ∼1-µm-sized modifications inside silicon. Notably, it also leads to a substantial improvement in the controllability of material transformations, enabling unique demonstrations such as rewritable optical memories (exceeding 100 writing/erasure cycles) and graded-index functionalities. These results highlight promising pathways toward precise 3D laser processing solutions and the fabrication of reconfigurable, monolithic silicon-based devices.