Ripples revisited: non-classical morphology at the bottom of femtosecond laser ablation craters in transparent dielectrics

doi:10.1016/S0169-4332(02)00450-6

Applied Surface Science

Volumes 197–198, 30 September 2002, Pages 891-895

https://doi.org/10.1016/S0169-4332(02)00450-6 Get rights and content

Abstract

Studying femtosecond laser ablation of wide band gap insulators, with BaF₂ as a representative, we observe a complex structure of fine ripples at the bottom of the ablated crater, which are oriented perpendicular to the beam polarization. A second, wider periodic structure oriented parallel to the beam polarization appears superimposed on the first one. To check the idea of an interference pattern translating into ripples, a controlled interference was created in the target in a non-collinear two-beam experiment. However, no signature of it was observed in the ablated spot. This calls the classical interpretation for ripples formation into question. More likely, we assume that the ripples structures are due to self-organization structure formation during the relaxation of the highly non-equilibrium surface after explosive positive ion emission.

Introduction

Laser induced periodic surface structures have been intensely studied on metals, semiconductors and dielectrics for more than three decades [1], [2], [3], [4], [5]. A large variety of wavelengths and pulses ranging from CW to picosecond duration were used, and more or less the structures created seemed to be the result of an interference between the incident laser beam and a surface wave (polariton) randomly scattered on the surface asperities. Corresponding to the interference the intensity distribution along the surface should result in a modulation of the ablation efficiency, leading to crests and valleys called ripples.

Commonly, ripples have been found growing linear and parallel on the surface, oriented perpendicular to the electric field (E) of the incident beam. Also a second type parallel to E has been found at an increased number of pulses [3]. They could be a result of a single laser shot, though they are observed only after multiple shots, when their modulation depth is increased. The higher laser intensity, the higher modulation depth. The ripple period, in most cases, has been related to Rayleigh’s diffraction condition, being given by the laser wavelength (λ) and the angle of incidence measured from the normal to the surface (α): $Λ= λ 1± sin α$ where the negative and positive signs correspond to forward and backward scattering, respectively.

In this paper we try to combine the surface structuring obtained with our previous results on femtosecond ablation of BaF₂ [6], [7], [8]. In short, we had shown that multiphoton ionization produced an electrostatically unstable surface leading to an explosive emission of positive ions, with a mean kinetic energy of 100 eV. The energy distribution corresponds to a temperature of 1 eV. The whole ablation process appears to be electrostatically driven, the role of thermal processes being thus negligible. But this comes against the idea of the ripple formation which corresponds to a phase transition.

To test the ripple pattern cause we produced a controlled interference by crossing two non-collinear femtosecond beams under a small angle and examine what happens when two temporally overlapped pulses from beams polarized alike or with crossed polarizations come together to produce ripples. As a check for the interference condition, we used the self-diffraction from the corresponding transient index grating [9].

Section snippets

Experiment

Femtosecond laser ablation was performed on single crystalline, fresh cleaved BaF₂ targets (≈1 mm thick). We used an amplified Ti:sapphire laser system (800 nm wavelength, 120 fs pulse duration, 1 kHz repetition rate). The targets were placed under ultra-high vacuum (≈10⁻⁹ mbar). Firstly, we verified the ripples characteristics in terms of shape, orientation and period, for a single incident beam. Secondly, we performed a non-collinear two-beam experiment and produced ablated spots under multiple

Ablation with a single beam

For checking the structure characteristics we performed femtosecond laser ablation of BaF₂ at an angle of incidence >2°. The ablated spots were produced each by accumulation of ≈45,000 pulses at a moderate laser intensity (0.8–0.9×10¹³ W/cm²). As can be seen in Fig. 2, we obtained a well-developed fine modulation perpendicular to the beam polarization with a period of ≈230 nm. We found less pronounced ripples grown parallel to the electric field of the incident beam (period ≈600–900 nm).

Ablation with two beams

Working

Discussion

Already our single beam experiments appear to be not in agreement with the classical ripples model: the fine ripples structure with a period of about 230 nm (Fig. 2) is in obvious contradiction to Eq. (1), allowing only Λ≥λ/2=400 nm as the smallest structure. Moreover, as shown earlier [10], the ripples period depends on the laser intensity rather than on the wavelength: the higher the intensity, the larger the minimum spacing. Further, if the classical ripples model would match our case, we

Conclusions

Two types of structures were observed in femtosecond ablation of BaF₂ and they were correlated with the electric field of the incident beam. However, their period as far does not match the ripple stated period. We observed no trace in the ripple pattern of a transient index grating created to test if the surface changes as a result of the intensity variation along it. We, therefore, consider being possible that ripples are a result of the surface relaxation by self-organization.

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