Branched hole drilling in silica glass by continuous-wave laser backside irradiation

We demonstrated branched hole drilling of glass using two laser beams based on continuous-wave laser backside irradiation (CW-LBI). Two laser beams were aimed from orthogonal directions and adjusted to cross each other in the glass. A stainless-steel foil was attached on one side of the glass to initiate plasma. The illumination of one laser beam heated the foil and formed plasma in the vicinity of the foil. The plasma propagated along the laser beam toward a light source and branched at the crossing point of the two laser beams. When the plasma reached the glass surface, the plasma was ejected and a branched hole was obtained. The shape of the branched hole was similar to that of the plasma. The depth and diameter of the branched hole was 210 and 15 μm, respectively. By changing the beam positions and defocus distance, the shape of the branched hole was changed.