Olga Varlamova

High-resolution investigations of ripple structures formed by femtosecond laser irradiation of silicon

We report on the structural investigation of self-organized periodic microstructures (ripples) generated in Si(100) targets after multishot irradiation by approximately 100-fs to 800-nm laser pulses at intensities near the single shot ablation threshold. Inspection by surface sensitive microscopy, e.g., atomic force microscopy (AFM) or scanning electron microscopy (SEM), and conventional and high-resolution transmission electron microscopy reveal complex structural modifications upon interaction with the laser: even well outside the ablated area, the target surface exhibits fine ripple-like undulations, consisting of alternating crystalline and amorphous silicon. Inside the heavily modified area, amorphous silicon is found only in the valleys but not on the crests which, instead, consist of highly distorted crystalline phases, rich in defects.

Genesis of femtosecond-induced nanostructures on solid surfaces

The start and evolution of the formation of laser-induced periodic surface structures (LIPSS, ripples) are investigated. The important role of irradiation dose (fluence×number of pulses) for the properties of the generated structures is demonstrated. It is shown how, with an increasing dose, the structures evolve from random surface modification to regular sub-wavelength ripples, then coalesce to broader LIPSS and finally form more complex shapes when ablation produces deep craters. First experiments are presented following this evolution in one single irradiated spot.

Self-organized Surface Patterns Originating from Laser-Induced Instability

Self-organized surface pattern formation upon femtosecond laser ablation is considered in framework of an adopted surface erosion model, based on the description for spontaneous pattern formation on ion bombarded surfaces. We exploit the similarity to ion-beam sputtering and extend a corresponding model for laser ablation by including laser polarization. We find that an asymmetry in deposition and dissipation of incident laser energy, related to the laser polarization, results in a corresponding dependence of coefficients in a nonlinear equation of the Kuramoto-Sivashinsky type. We present the surface morphologies obtained by this model for different polarization of the laser beam and discuss a time evolution of the nanopattern. A comparison of numerical results with experimental data shows an excellent qualitative agreement. Our results support the non-linear self-organization mechanism of pattern formation on the surface of solids.

Long-time feedback in self-organized nanostructures formation upon multipulse femtosecond laser ablation

Self-organized nanostructures (ripples) on the target surface after multi-pulse femtosecond laser ablation exhibit, obviously, a positive multi-pulse feedback in the self-organization process. Experiments on different targets (CaF2, Si) investigate this feedback in more detail, in particular its dynamics. The influence of pulse number and time separation between successive pulses on both the size and the complexity of the nanostructures as well as the size of the modified surface area is studied. In addition to a dependence on the coupled dose, confirming incubation effects previously observed on ablation efficiency, both modified area as well as pattern feature size and complexity decrease with increasing pulse-to- pulse delay between 1 ms and 1 s, indicating an unexpectedly long lifetime of the feedback. Further, for silicon, a persisting modification of the crystalline structure is found well beyond the ablation spot, though no apparent change in surface morphology can be seen. Mapping the band-to-band photoluminescence displays a spatially modulated dramatic increase of non-radiative recombination compared to unaffected material.

Long-Time Feedback in the Formation of Self-Organized Nanostructures upon Multipulse Femtosecond Laser Ablation

The self‐organization feedback in nanostructures (ripples) formation upon femtosecond laser ablation is investigated in detail, with particular emphasis on its dynamics. We study the influence of time separation between successive pulses on both the size and complexity of the nanostructures and on the size of the modified surface area. By varying the pulse separation between 1 ms and 1 s (rep. rate between 1 kHz and 1 Hz), we find that both modified area as well as pattern feature size and complexity decrease with increasing pulse‐to‐pulse delay, indicating that the coupling efficiency between laser and target increases with increasing repetition rate. Structure formation resulting from surface instability, induced by the laser impact, suggests that the laser‐induced instability results in a transient increase of absorption probability, slowly decaying in time. The stronger this transient absorption, the better is the coupling for the succeeding pulse, thus resulting in a positive feedback. Experimental r...

Femtosecond laser induced surface instability resulting in self-organized nanostructures

The fundamental mechanisms and dynamics of laser ablation are reviewed, based on experiments with femtosecond laser pulses to exclude secondary effects like the interaction of the incident laser light with the ablation plume or with a target preconditioned during the initial slope of the laser pulse. It is shown that the incident energy drives the target into a state of instability, far from thermodynamic equilibrium. The subsequent ultra-rapid relaxation results in the formation of self-organized regular nanostructures in the irradiated and ablated area.

Femtosecond laser ablation from silicon and ripples formation: Evolution of surface excitation

We report on pump-probe experiments to study the time evolution of surface excitation during femtosecond laser ablation from silicon and its influence on self-organized ripples formation.

Feedback Effect on the Self-Organized Nanostructures Formation on Silicon upon Femtosecond Laser Ablation

The role of multi-pulse feedback in self-organized nanostructure (ripples) formation on silicon surface upon femtosecond laser ablation is investigated. For irradiation at constant intensity and pulse repetition rate, the previously postulated feedback effect of accumulated dose with in¬creasing number of pulses is confirmed and investigated in detail: both the modified surface area as well as the complexity and feature size of generated nanostructures increase with accumulated dose. More interestingly, at constant total incident dose (number of pulses times pulse energy) accumu¬lation and feedback depend strongly on temporal pulse separation. The feedback becomes increas¬ingly weaker with increasing time intervals between successive pulses, involving times up to one second and more before individual pulses act independently. In a first attempt to model this long-lived coupling, we find that conduction band electrons, produced by the preceding laser pulse, can provide, indeed, such feedback by facilitating coupling of subsequent pulses for substantial delays. However, the achieved time span of about a millisecond is still significantly shorter than observed experimentally.