Control of multiphoton and avalanche ionization using an ultraviolet-infrared pulse train in femtosecond laser micro-/nano-machining of fused silica

We report on the experimental results of micro-and nanostructures fabricated on the surface of fused silica by a train of two femtosecond laser pulses, a tightly focused 266 nm (ultraviolet, UV) pulse followed by a loosely focused 800 nm (infrared, IR) pulse. By controlling the fluence of each pulse below the damage threshold, micro-and nanostructures are fabricated using the combined beams. The resulting damage size is defined by the UV pulse, and a reduction of UV damage threshold is observed when the two pulses are within similar to 1 ps delay. The effects of IR pulse duration on the UV damage threshold and shapes are investigated. These results suggest that the UV pulse generates seed electrons through multiphoton absorption and the IR pulse utilizes these electrons to cause damage by avalanche process. A single rate equation model based on electron density can be used to explain these results. It is further demonstrated that structures with dimensions of 124 nm can be fabricated on the surface of fused silica using 0.5 NA objective. This provides a possible route to XUV (or even shorter wavelength) laser nano-machining with reduced damage threshold.