The behaviour of sapphire under intense two-colour excitation by picosecond laser

Abstract

Ultrashort pulse lasers are evidencing their benefits in the processing of transparent materials. Sapphire is one of the most attractive engineering materials today. It is hard and, therefore, difficult to machine mechanically to the required shape. Laser dicing is one of the promising techniques for sapphire separation. Two-pulse two-colour irradiation was applied to initiate free-shape cutting of the material. Two collinear laser beams with wavelengths of 1064 and 355 nm, pulse duration of 10 ps and inter-pulse delay of 0.1 ns were combined to induce intra-volume modifications (directional cracks) in sapphire for wafer separation. The photon energy of both beams is well below the band gap, and various channels of the multi-photon excitation were involved in the process. Significant enhancement in the modification area was experimentally observed when intensities of focused infrared and ultraviolet beams were within narrow ranges. We discuss the resonant laser–sapphire interaction mechanisms, leading to up to four times higher excitation of the material involving multiple photons and energetic levels of intrinsic defects in the band-gap. The energy level schemes of colour centres involved in two-step multi-photon absorption in sapphire under intensive laser irradiation have been prepared.