We present new features in femtosecond LIPSS on Silicon and CaF 2 , obtained after a low number of pulses near the ablation threshold. In contrast to the general case of multi-pulse LIPSS, where a strong dependence of the structures on the laser polarization is observed, single-shot exposition of silicon at about the ablation threshold results in a concentric pattern of very regular sub-wavelength ripples following the oval shape of the irradiated spot, without any reference to the laser

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... ation. Further, a low contrast (± 5 nm) center (at highest irradiation density of the Gaussian beam profile) is half surrounded by a high contrast (± 20 nm) region towards the edge -without any change in ripples periodicity. In the high contrast region several perfectly round conical holes are observed, about 50 nm deep and in diameter comparable to the ripples spacing. These holes are surrounded by a ridge about 20 nm high. For about 100 pulses well below the ablation threshold on CaF 2 , no ripples structure can be produced, yet. Nevertheless, the irradiated spot is clearly visible in the SEM picture by a change of SEM contrast. All over and even around the spot, spherical bumps are visible with a rather narrow size-distribution of about 2 µm diameter. They almost appear like re-condensed nanoparticles which is, however, not in agreement with the negligible amount of ablated material (ablation depth < 1 nm). At higher pulse numbers (≈ 2,000), these bumps persist in the outer range of the ablated spot, whereas in the central region, ripples occur.