Study of bottom-up column formation in bulk silicon initiated at silicon-air and silicon-glass interfaces by ultrafast laser processing

Through-bulk columnar structures perpendicular to the surface of a 1 mm thick monocrystalline silicon sample are achieved using ultrashort pulsed laser radiation at a wavelength of around 1.6 μm. A range of pulse parameters is systematically investigated to evaluate the probability of column formation and to optimize column growth throughout the bulk of silicon. Fabrication is performed with a glass substrate as support below the sample and in air in order to assess the influence of substrate support on the modification probability. The results indicate that the absence of a support substrate promotes columnar growth over a wider range of parameter combinations compared to substrate-supported configurations, and that the transition from column formation to no column formation is transient with a probabilistic behavior of the modifications in the transition region. Furthermore, pulse duration is found to play a critical role for the modification probability such that the lack of permanent modification observed at 550 fs pulse duration can be mitigated by employing longer pulses of 2.5 ps. Under optimized writing conditions, continuous cylindrical columns with lengths of up to 1 mm can be produced at writing speeds up to 20 μm/s.