<title>Laser micromachining of transparent fused silica with 1-ps pulses and pulse trains</title>

Abstract

Ablation rates and etched-surface morphology of fused silica has been studied with 1-ps Nd:glass laser pulses in a regime of near-diffraction-limited spot size. Shallow holes of 1.7- micrometers diameter were too small for the formation of laser- induced periodic-surface structures. Atomic-force and scanning-electron microscopy showed that reproducible etch depth and moderately smooth surfaces are attainable for low fluences of 5.5 - 45 J/cm<SUP>2</SUP>--the `gentle' ablation regime. Etch depth progressed linearly with the number of laser pulses until the onset of surface swelling and shock- induced microcracks after a critical number N<SUB>c</SUB> of laser pulses, scaling as N<SUB>c</SUB> equals 1.7 + 80/F (fluence F in J/cm<SUP>2</SUP>). Below this limit--for accumulated etch depths less than approximately 2 micrometers --3D surface structuring with sub-micron precision is possible with picosecond-laser pulses. In the strong ablation regime (F &gt; 45 J/cm<SUP>2</SUP>), surface morphology was poor and microcracking developed within 2 - 4 pulses. These shock-induced microcracking effects were eliminated when a mode-locked train of approximately 400 identical 1-ps pulses, each separated by 7.5 ns, was applied. Very smooth and deep (approximately 30- micrometers ) holes of 7 - 10-micrometers diameter were excised at a total fluence of approximately 100 kJ/cm<SUP>2</SUP>, establishing a new means for rapid and precise micromachining of fused silica and other brittle materials.