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Nanoscale rifts and ripples at a periodicity of 130 nm were generated on Si(100) surfaces immersed in water using tightly focused 800 nm 12 fs pulsed 85 MHz laser light at subnanojoule pulse energies. At radiant exposure close to the ablation threshold rifts were typically 20-50 nm in width and 70 nm in depth running perpendicular to the laser polarization. On increase of the irradiance, the rifts broadened and formed periodic ripples, whereas at highest exposure, a random nanoporous surface topology emerged. Rift and ripple formation is explained by laser-induced standing surface plasma waves, which result in periodic variation of dissipation and ablation.

In recent years, high-spatial frequency laser-induced surfaces structures have been generated in a large variety of dielectrics. In silicon subwavelength ripples, some of which featured periodicities below 100 nm, were formed using ultrafast lasers. We demonstrate for Si(100) surfaces that generation of a dense electron-hole plasma in the focal spot of ultrashort-pulsed laser light followed by massive excitation of plasma waves provides an explanation for the formation of such high-spatial frequency surface structures. The applied Drude-like model includes carrier-carrier collisions and is in excellent agreement with the experimentally observed ripple period.

Low mean powers of 1–10 mW are sufficient for material nanoprocessing when using femtosecond laser microscopes. In particular, near infrared 12 fs laser pulses at peak TW/cm2 intensities, picojoule pulse energies, and 85 MHz repetition rate have been employed. Three-dimensional two-photon lithography as well as direct multiphoton ablation have been performed. Subwavelength sub-100 nm cuts have been realized in photoresists, silicon wafers, glass, polymers, metals, and biological targets. When reducing the mean power to the microwatt range, nondestructive two-photon imaging was performed with the same setup taking advantage of the broad laser emission spectrum. Multiphoton microscopes based on low-cost ultracompact sub-20 fs laser sources may become novel nonlinear optical tools for highly precise nanoprocessing and two-photon imaging.

In magnetron sputtered indium-tin-oxide thin films of varying oxygen content, nanostructures were formed using tightly focused high-repetition rate near-infrared sub-15 femtosecond pulsed laser light. At radiant exposure well beyond the ablation threshold, cuts of 280-350 nm in width were generated. Illumination close to the ablation threshold resulted in periodic cuts of typically 20 nm in width at periodicities between 50 nm and 180 nm, as well as single sub-20 nm cuts. Subthreshold exposure, in combination with hydrochloric acid etching, yielded nanowires of 50 nm minimum lateral dimensions.