This paper examines how femtosecond laser-induced periodic surface structures (LIPSS) are formed on three classes of materials—metals, semiconductors, and dielectrics—when they are irradiated in air by linearly polarized femtosecond laser pulses with pulse durations of roughly 30–150 fs and wavelengths near 800 nm. The authors combine experiments and theory to explain how the periodic structures depend on material type, fluence, pulse number, and transient changes in optical properties during and after laser excitation. Their central claim is that LIPSS formation cannot be understood only from the static optical properties of a material; instead, the ultrafast excited state of the material plays a crucial role, especially in semiconductors and dielectrics.

The introduction explains that LIPSS have attracted growing attention because they offer a relatively simple single-step nanostructuring method for modifying surface properties such as optical response, mechanical behavior, and chemical functionality. The paper distinguishes between two main categories: LSFL (low-spatial-frequency LIPSS), whose periods are close to the laser wavelength, and HSFL (high-spatial-frequency LIPSS), whose periods are much smaller than the wavelength. In strongly absorbing materials such as metals and semiconductors, LSFL typically form perpendicular to the laser polarization and are commonly explained by interference between the incoming laser beam and a surface electromagnetic wave, often involving surface plasmon polaritons (SPPs). By contrast, HSFL are less well understood, and their origin is still debated; possible explanations include second-harmonic effects, plasmonic modes, and self-organization processes.

Experimentally, the study uses Ti:sapphire femtosecond laser systems operating around 790–800 nm with repetition rates of 10 Hz or 1 kHz. The researchers irradiate three representative materials with very different band gaps: titanium as a metal, single-crystal silicon as a semiconductor, and fused silica as a dielectric. In some experiments, they also generate double femtosecond pulses with a Michelson interferometer so that the delay between two nearly equal pulses can be controlled from negative to positive picoseconds. The irradiated surfaces are then analyzed by optical microscopy and scanning electron microscopy, with Fourier-transform methods used to quantify ripple spacing.

For titanium, the results show two distinct surface morphologies depending on the fluence near the ablation threshold. At a peak fluence of about 0.13 J/cm², the surface develops LSFL perpendicular to the polarization, and the measured periods are around 510–670 nm, which is close to the laser wavelength. At a slightly lower fluence of about 0.09 J/cm², the researchers observe HSFL parallel to the polarization in the region surrounding the center of the irradiated spot, with periods as small as 70–90 nm. This is one of the most striking findings of the paper, because it demonstrates that sub-100 nm periodic structures can be produced on a metal surface in air using a simple one-step process, without requiring vacuum or liquid environments. The authors interpret the LSFL in metals through interference involving a laser-driven surface wave, but they note that the physical origin of the HSFL on titanium is still unresolved and may involve additional effects such as surface oxidation.

For silicon, the paper focuses on why the ripple period becomes sub-wavelength and why it changes with the number of laser pulses. The authors argue that there are two separate mechanisms. First, during the femtosecond pulse, the laser excites a large number of electrons into the conduction band, temporarily changing silicon from a semiconductor into something more metal-like. By combining the Sipe theory of LIPSS with a Drude model, they show that when the carrier density becomes sufficiently high, SPP excitation becomes possible, and this changes the expected ripple period. In other words, the ripple spacing is influenced by the optical properties of the transient excited state, not just by the unexcited silicon. Second, with repeated laser pulses, the surface itself becomes more strongly modulated, and this evolving grating topography changes the resonance conditions for laser coupling, causing the LSFL period to shift to smaller values. The experiments support this explanation: as the pulse number increases, the average LSFL period decreases from roughly 770 nm down to about 560 nm. This means that in silicon, the final ripple spacing is controlled both by ultrafast carrier excitation and by cumulative surface evolution under multipulse irradiation.

The analysis of fused silica is especially important because it gives direct evidence for the role of transient optical-property changes. In ordinary multipulse experiments, fused silica can show HSFL with periods of about 200–400 nm, oriented perpendicular to the polarization, when the fluence is just above the damage threshold. At higher fluence, the material instead forms LSFL with periods between 500 and 800 nm, oriented parallel to the polarization. However, the most revealing experiments use double pulses with controlled delays and orthogonal polarizations. In this setup, the authors find that the first pulse determines the orientation of the final surface pattern. Even when the second pulse arrives shortly afterward and has a different polarization, the permanent LSFL orientation follows the polarization of the pulse that arrived first. This shows that the surface enters a highly sensitive transient state immediately after the first pulse, and that the second pulse interacts with a material that is no longer in its original dielectric condition.

The paper also shows that the delay time between the two pulses changes the ripple period itself. When the delay is very short, around 0.17 ps, the LSFL period is close to 800 nm, approximately the laser wavelength. As the delay increases to more than 2 ps, the period rapidly decreases and saturates near 550 nm, which is close to λ/n for fused silica. The authors interpret this as evidence that the optical response of the material evolves extremely quickly after excitation. At very short delays, the laser-generated free-electron plasma makes the material behave in a more metal-like way. As time passes and the excited state relaxes, the material returns toward more dielectric-like behavior. Several physical processes may contribute to this transition, including increased transient reflectivity, reduced energy deposition from the second pulse, electron–phonon relaxation, self-trapped exciton formation, electron diffusion, and plasma shielding. This part of the study is especially significant because it provides a direct experimental proof that transient excited-state dynamics are central to femtosecond LIPSS formation.

Overall, the paper concludes that LIPSS formation is not governed by a single universal mechanism. In metals like titanium, LSFL can be reasonably described by interference with surface electromagnetic waves, while HSFL remain less clearly explained. In semiconductors like silicon, both transient carrier excitation and pulse-number-dependent surface evolution determine the ripple period. In dielectrics like fused silica, the experiments clearly show that ultrafast transient states control both the orientation and spacing of the resulting periodic structures. The broader message of the paper is that temporal pulse shaping, especially with ultrashort double-pulse or pulse-sequence techniques, offers a powerful new method for controlling nanoscale surface patterning. This makes femtosecond laser processing not only a tool for creating ripples, but also a platform for actively engineering surface nanostructures by manipulating the material’s ultrafast excitation dynamics.