Laser-induced Writing of Submicron Surface Relief Gratings in Fused Silica on the Fly

Abstract

The hybrid laser processing method LIBWE (laser-induced backside wet etching) allows the defined micro and submicron structuring of transparent materials like fused silica at small laser fluences and with high quality. In this study the etching of fused silica with mercury as liquid absorber is demonstrated using an excimer laser (λ = 248 nm, 25 ns pulses). The high absorption coefficient of liquid metals enlarges the absorption of the UV laser radiation so that despite the high reflectivity the threshold for etching was measured to be 0.75 J/cm2. With rising laser fluence the etch rate linearly increases in the whole laser fluence range and reaches a value of 650 nm/pulse at a laser fluence of about 10 J/cm2. For the etching of 2D periodic sub-micron structures into fused silica, four interfering laser beams were used generated by projection of a chessboard-like diffraction phase mask. In conjunction with liquid-metal LIBWE, submicron surface relief gratings with regular dot patterns have been realized on planar and prestructured samples in a one-step direct fabrication process. The large etch rates at LIBWE by means of the liquid metal enable a highspeed processing; gratings with a period of 800 nm and a depth up to 60 nm can be etched on the fly with only one laser pulse.