Formation of Nanostructures on a Ti:sapphire Surface Irradiated by Femtosecond Laser Pulses and the Influence on Reflection Reduction

We measured the femtosecond laser ablation threshold of Ti:sapphire, which may be important for forming nanostructures with a femtosecond laser (515 nm, 150 fs, 5 kHz). An ablation threshold of 1.51 J/cm2 at 1000 pulses for Ti:sapphire was estimated from the laser fluence dependence of the crater diameter. A nanostructure was produced on the Ti:sapphire surface below the ablation threshold fluence. The nanostructures caused a reflection reduction of 2.0% at a wavelength of 400 nm, which agreed well with the calculation for a root mean square surface roughness of 100 nm.