Fabrication of millimeter-long structures in sapphire using femtosecond infrared laser pulses and selective etching

•If sapphire is irradiated along the direction perpendicular to the polarization of the light, the irradiated lines do not show a smooth single amorphized cross-section, but rather a series of vertical amorphized parallel nanolines.•The ideal window of pulse repetition rate is between 0.100 MHz and 1 MHz. Below this range the structures do not show a single and constant cross-section. Upon using a repetition rate higher than this range, the modified lines also show irregular cross-sections.•If a pulse energy of more than 234 nJ for pulse repetition rates of f = 0.001 to 1 MHz is used, the focus is split in multiple foci.•The presence of cracks prevent the formation of hollow stacks of microchannels and limits its size to 64 laterally overlapping channels.