Internal Structuring of Silicon by Ultrafast Laser Irradiation

Summary form only given. An important challenge in the field of three-dimensional (3D) ultrafast laser processing is to achieve permanent modifications in the bulk of silicon (Si) and narrow-gap materials. Attempts by increasing the energy of infrared femtosecond pulses with conventional laser machining configurations have failed. This paper concentrates on the limitations experienced in femtosecond interactions at 1300-nm wavelength to fully identify their origins. We present experimental investigations on the 3D reconstruction of the nonlinear pulse propagation and ionization of Si under tight focusing configurations. By comparison with simulations, we quantify the strong nonlinear and plasma effects in the pre -focal region causing the limitations . With an extrapolation on the energy density that could be delivered with hyper -NA values (up to 3.5 in Si), we show that solid immersion focusing provides a solution to achieve ultrafast optical breakdown in Si [5]. By repeatedly illuminating the center of a Si sphere with pulses focused at apparent NA near 3, we exceed the breakdown threshold with sub -100 fs pulses. This leads to a highly -localized negative change of the refractive index as measured with an infrared phase microscopy arrangement. Beyond this proof -of -concept demonstration, the experiment can be translated with the astigmatic solid -immersion lens (ASIL) configuration consisting in focusing the laser radiation through an hemispherical Si sample while ensuring an appropriate optical contact with a planar wafer. This makes possible to achieve similar modifications in Si wafers with a long working distance focusing objective of modest NA (<;0.3). It represents a critical step towards the fabrication of 3D photonic microdevices in silicon by a laser writing method.