Exceeding the bulk modification threshold of silicon with hyper-focused infrared femtosecond laser pulses

Ultra-short breakdown in the bulk of transparent materials has been intensively investigated in the last years, especially in dielectrics [1]. Whereas three-dimensional (3D) femtosecond laser micromachining in dielectrics is highly advanced, it remains extremely challenging in narrow bandgap materials such as silicon (Si). Recent numerical and experimental investigations show that only an underdense microplasma can be generated inside the bulk of crystalline Si by two-photon absorption [2]. The energy deposition inside the material is drastically limited by significant losses in the prefocal region and strong plasma effects. We provide in this study an experimental evidence of this optical limitation by 3D-imaging of the beam propagation with 60-fs, 1300-nm pulses focused 1 mm under the Si-surface. Even for high Numerical Aperture (NA) (up to 0.65), we observe a strict clamping of the delivered energy far below the level needed for material breakdown. For comparison, the horizontal line in Fig.1.a is the measured fluence threshold for surface breakdown, which can be taken as the minimal target for breakdown in the bulk.