Time resolved detection of particle removal from dielectrics on femtosecond laser ablation

doi:10.1016/S0169-4332(97)00613-2

Applied Surface Science

Volumes 127–129, May 1998, Pages 76-80

https://doi.org/10.1016/S0169-4332(97)00613-2 Get rights and content

Abstract

The dynamics of the ablation process in different oxides (amorphous and crystalline quartz, sapphire and MgO) after intensive laser excitation was studied by a combination of the femtosecond pump–probe technique with optical surface scattering. A 120-fs laser pulse (λ=800 nm) was focused onto the sample with a fluence at least twice the single-shot surface damage threshold. A much weaker probe laser pulse optically aligned collinear to the pump beam illuminated the excited area at variable delay times up to 100 ps. We observe a material dependent increase in the scattering signal after a certain delay time which we interpret as the onset of the ablation process. This occurs much earlier for amorphous and crystalline silica than for sapphire or MgO, as a consequence of differences in the electron–phonon coupling strength. The morphology of the irradiated spot on sapphire, amorphous and crystalline quartz is indicative of melting and vaporisation in contrast to that on MgO which clearly shows fracturing.

Introduction

Investigations of the fundamental mechanisms leading to laser ablation have been receiving growing attention in recent years. With the advances in femto- and picosecond laser technology, it is becoming possible to study the initial stages leading to ablation in real time and without the added problems of dealing with the laser–plume interactions which complicate the interpretation of nanosecond laser ablation. However, many details concerning the energy transfer into the lattice after short pulse laser excitation remain poorly understood. In the early pioneering work by Downer et al. [1], pump–probe measurements of the ablation process in Si were carried out. Similar investigations were performed by Bor et al. [2]for UV ablation of polymers and by von der Linde and Schüler [3]for several materials, including dielectrics. In these studies, the temporal development of the reflectivity of the surface after strong excitation was investigated. An increase in the reflectivity could be observed on the timescale of the laser pulse due to plasma formation at the surface. After sufficient energy is deposited into the material, it takes another typical 50 to 100 ps to melt a 20-nm surface layer of silicon using picosecond laser pulses [4]. Recent time resolved studies of carrier dynamics by Petite et al. [5]have demonstrated a significant material dependency in the lifetime of excited electrons in the conduction band of dielectrics. While the electrons remain in or closely below the conduction band for over 50 ps for MgO and sapphire, they are trapped deep into the bandgap after only 150 fs in amorphous and crystalline quartz. The lifetimes are connected to the dynamics of formation of radiation induced defects.

In this paper, we introduce a new pump–probe technique to determine the onset of ablation from the surface. This method should provide us with more insight into the relative electron–phonon coupling strengths of different dielectrics and into the lattice dynamics after intense laser excitation.

Section snippets

Experiment

The experimental set-up used in this work is illustrated in Fig. 1 and is very similar to that described in Ref. [6]. The pump and probe beams are generated by a Ti:Sapphire laser at a wavelength of 800 nm and a pulse duration of 120 fs. The probe beam passes through a delay line (0–200 ps) and is then aligned collinear with the pump beam. Both laser beams are focused with a 75-mm lens onto the target which is mounted on a xy-table. All the experiments were carried out under vacuum conditions

Results and discussion

Fig. 2 presents the results of the scattered laser pulse signal as a function of the delay time between pump and probe pulse. The corresponding surface morphologies can be seen from the optical microscope pictures in Fig. 3. There are significant differences in the results from the different materials. We determine the onset of material removal after only 3 ps for amorphous and crystalline quartz, whereas for sapphire and MgO, the ablation occurs after 12 and 20 ps, respectively.

Amorphous

Conclusion

We have presented a new experimental method with which we can determine the timescale for the removal of material from a laser irradiated surface. The behaviour of four dielectric materials (a-SiO₂, c-SiO₂, c-Al₂O₃, c-MgO) was investigated on single-shot irradiation with 120 fs pulses with a pulse energy of ca. 50 μJ (corresponding to a laser fluence of approximately a factor of two or above the damage threshold). The onset of material removal occurs on a timescale of a few picosecond.

Acknowledgements

This work was financially supported by the BMBF (13N6591, PROBE I).

References (11)

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There are more references available in the full text version of this article.

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As will be shown later, the reflectivity in this ring decays below the initial level in approximately 20 ps and afterwards undergoes a much slower dynamics with very small changes observable up to 500 ps. These results are consistent with the observations by Rosenfeld et al. based on time resolved scattering measurements [15]. While at 500 ps, the ablation process in the central region still continues, at t ≈ 10 ns, all dynamic processes have ended, as can be seen by comparison with an image recorded a few seconds after irradiation (t = ∞).
We have investigated plasma formation and relaxation dynamics induced by single femtosecond laser pulses at the surface of crystalline SiO₂ (quartz) along with the corresponding topography modifications. The use of fs-resolved pump-probe microscopy allows combining spatial and temporal resolution and simultaneous access to phenomena occurring in adjacent regions excited with different local fluences. The results show the formation of a transient free-electron plasma ring surrounding the location of the inner ablation crater. Optical microscopy measurements reveal a 30% reflectivity decrease in this region, consistent with local amorphization. The accompanying weak depression of ≈15 nm in this region is explained by gentle material removal via Coulomb explosion. Finally, we discuss the timescales of the plasma dynamics and its role in the modifications produced, by comparing the results with previous studies obtained in amorphous SiO₂ (fused silica). For this purpose, we have conceived a new representation concept of time-resolved microscopy image stacks in a single graph, which allows visualizing quickly suble differences of the overall similar dynamic response of both materials.
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