Rippled area formed by surface plasmon polaritons upon femtosecond laser double-pulse irradiation of silicon: the role of carrier generation and relaxation processes

Abstract

The formation of laser-induced periodic surface structures (LIPSS, ripples) upon irradiation of silicon with multiple irradiation sequences consisting of femtosecond laser pulse pairs (pulse duration 150 fs, central wavelength 800 nm) is studied numerically using a rate equation system along with a two-temperature model accounting for one-and twophoton absorption and subsequent carrier diffusion and Auger recombination processes. The temporal delay between the individual equal-energy fs-laser pulses was varied between 0 and ∼ 4 ps for quantification of the transient carrier densities in the conduction band of the laser-excited silicon. The results of the numerical analysis reveal the importance of carrier generation and relaxation processes in fs-LIPSS formation on silicon and quantitatively explain the two time constants of the delay dependent decrease of the Low-SpatialFrequency LIPSS (LSFL) area observed experimentally. The role of carrier generation, diffusion and recombination are quantified individually.

Surface structures induced by ultrashort laser pulses: Formation mechanisms of ripples and grooves

“…The temporal variation of the free-carrier density, N e , induced by the laser pulse is described by the twotemperature model for the free-carriers temperature T e and the lattice temperature T l coupled to free-carrier dynamics equation [22,30,32]: …”

Anisotropy modulations of femtosecond laser pulse induced periodic surface structures on silicon by adjusting double pulse delay

“…This observation of the LIPSS morphology modulation signifies a strong influence by the time delays of the double fs pulse train on LIPSS structuring. The fabrication of the LIPSS on silicon is considered to be caused by excitation of SPP at the air-silicon interface when the material turns from a semiconducting into a metallic state [8,9,30,31]. Upon high-intensity laser irradiation of silicon with fs laser pulses, electrons can be promoted from the valence (VB) to the conduction band (CB) via linear and nonlinear optical absorption effects.…”

“…The process of ablation and damage can be described in terms of the excitation and relaxation of the conduction band electrons (CBEs). As a universal phenomenon in the field of laser ablation, the relaxation of CBEs (mainly carrier diffusion and Auger recombination) [30,31] generated by the first leading pulse of the double pulses is crucial to the whole ablation process and hence affects the resulting ablated rippled area. Here, the decrease in the continuously ablated area which occurs with the increase in the pulse delays can be ascribed to CBE relaxation.…”

“…Therefore, with the increase in pulse delay, the decreasing number of CBEs reduces the energy coupling by the second pulse to the transiently excited material, leading to the decay of the ablation, which results in the reducing of the rippled area. More recently, the CBEs generation and their relaxation is theoretically researched with sequences of double fs laser pulse, for large double pulse delays, carriers relaxation (mainly carrier diffusion and Auger recombination), which affect the SPP excitation, contributes to the rapid decrease of the ripple area [30,31]. In order to comprehensively investigate the anisotropy modulation of the LIPSS at given pulse delays, a couple of double-fs-pulse-train-induced ripple structures are fabricated under different polarization directions.…”

“…This anisotropic geometrical morphologies modulation effect based on the double pulse delay can be interpreted by the reducing deposited pulse energy as the increases of the double pulse delay. The free electron recombination strongly impacts the free electron density [19,30,31], which affects the resulted pulse energy absorption on surface of the sample. As mentioned above, the carrier relaxation increases with the increase of the pulse delay.…”