Etching peculiarities in sapphire induced by double-pulse irradiation with variable delay and crossed polarisation

This manuscript presents the results of the intra-volume modifications and etching peculiarities of bulk crystalline sapphire (& alpha;-AL2O3) induced by double-pulse femtosecond laser radiation with a variable delay between pulses. The selective etching of sapphire was tested using various combinations of double-pulse irradiation. It was demonstrated that by varying the inter-pulse delay from -10 ps to 10 ps, the enhanced etching rate of the sapphire microchannels could be achieved compared to the single-pulse processing. It was demonstrated that there is an asymmetrical etching behaviour versus inter-pulse delay, depending on what polarisation of the first pulse relative to the scan direction is, (positive and negative pulse delays) when etching only in hydrofluoric acid (HF). In order to etch the entire structure, a combination of HF and a solution of H3PO4: H2SO4 (3:1) was used. The isotropic etching properties of H3PO4: H2SO4 made it possible to release items from the crystalline portion of the sapphire.