We report on the experimental study of microstructures fabricated on the surface of fused silica by two femtosecond laser pulses, a tightly focused 266 nm beam followed by a loosely focused 800 nm beam. By setting the fluence of each pulse below the damage threshold, visible microstructures are fabricated using the combined beams. Our results suggest that the ultraviolet pulse generates seed electrons through multiphoton absorption, and the near-infrared pulse utilizes these electrons to cause damage by avalanche ionization.

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We report on the experimental results of 300 nm features generated on fused silica using a near-infrared (IR) femtosecond laser pulse initiated by an ultraviolet (UV) pulse. With both pulses at a short (~60 fs) delay, the damage threshold of the UV pulse is only 10% of its normal value. Considerable reduction of UV damage threshold is observed when two pulses are at ± 1.3 ps delay. The damage feature size of the combined pulses is similar to that of a single UV pulse. A modified rate equation model with the consideration of defect states is used to help explain these results. This concept can be applied to shorter wavelengths, e.g. XUV and X-ray, with the required fluence below their normal threshold.

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