Appl. Phys. A 63, 103 — 107 (1996) Pulse-width influence on the laser-induced structuring of CaF (111) 2 D. Ashkenasi*, H. Varel, A. Rosenfeld, F. Noack, E.E.B. Campbell Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Postfach 1107, D-12474 Berlin, Germany (Fax: #49-30/6392-1229, E-mail: CAMPBELL@mbi.fta-berlin.de) Received: 26 February 1996/Accepted: 27 February 1996 Abstract. We have investigated the morphology of CaF 2 (111) irradiated by 780 nm laser pulses of varying pulse width (200 fs — 8 ns) with fluences above the damage threshold. Large differences can be observed which we relate to the mechanisms and dynamics of defect production in this wide band gap material. The best defined and most controllable ablation is obtained for laser pulse widths of a few picoseconds. For nanosecond and femtosecond pulses strong fracturing of the crystal is observed with damage outside the laser irradiated zone. This has a thermal origin for nanosecond pulses but a non-thermal origin for pulse widths below approximately 1 ps. PACS: 81.40.!z; 81.60.!j The interaction of laser pulses with optically transparent materials has been the subject of some considerable interest for many years [1 — 3]. For practical reasons, it is important to determine the damage thresholds and the damage processes for materials which are extensively used e.g. in laser optics. With the increasing availability of ultra-short laser pulses (with pulse widths on the femtosecond to picosecond time range) it may also be of practical interest to determine whether it is possible to produce controllable and well-defined micrometer-sized structures in transparent materials by means of pulsed laser ablation. This is generally not possible with standard nanosecond lasers. Fluoride crystals are among the most studied transparent materials. One reason for this is their practical importance as optical components in UV laser optics, which is a consequence of their very wide band gaps. Another more fundamental reason is the relative simplicity of the crystal structures which makes them ideal model systems for investigating the basic mechanisms *Also at: Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany involved in the laser damage (or ablation) process. Many fundamental studies on defect formation have been carried out on alkali halides and alkaline earth fluorides [4 — 8] using laser pulses longer than a few tens of picoseconds. The presence of defects and surface states strongly influences the optical properties of the materials leading to enhanced photon absorption, vacancy creation, and finally laser ablation [9]. Two recent studies have investigated the damage threshold for CaF as a function 2 of laser pulse width (q) for photon energies much lower than the band gap [10, 11]. In these studies different behaviour could be observed for three different time regimes: for t520 ps the damage threshold was linearly proportional to q1@2, as one would expect from a thermally dominated process [10]; a plateau was reached for 24q45 ps and, for even shorter pulses, the damage threshold decreased further [11]. In this paper we report morphological studies of the surface damage and ablation structures produced on laser irradiation with pulse widths in these three different time regimes. Quite dramatic differences can be observed which we attribute to different mechanisms related to the dynamics of defect production and relaxation in the crystal. 1 Experimental setup and results The laser used in our experiments was a Ti:Sapphire oscillator-amplifier system based on the chirped-pulseamplification technique (CPA). CPA allowed us to vary the pulse width without significantly changing other important parameters. The laser pulse at a wavelength of 790 nm was focused by an f"58 mm lens onto the sample surface giving an irradiated area of approximately 500 lm2. The experiments were carried out under vacuum ((10~5 mbar) and for single-shot as well as for multipleshot irradiation per site at a repetition rate of 1 and 50 Hz. The CaF samples (Korth, Germany) were polished on 2 both sides. The polished samples have a uniform distribution of mechanically-induced surface defects which ensures, as well as possible, equal conditions for every new 104 Fig. 1. Electron microscope pictures of polished CaF 2 (111) after 50 on 1 shot ablation in vacuum ((10~5 mbar) with laser pulses of different pulse widths (200 fs, 580 fs, 1.8 ps, 3.2 ps and 4.7 ps) at a wavelength of 790 nm and a fluence of 7 J/cm2 . 105 spot. If cleaved samples are used one can not guarantee the homogeneity of the surface and one has enhanced damage at or near steps [12]. We employed two complementary methods for analysis of the surface after irradiation: electron microscopy which can give detailed information on changes that have occured to the sample surface and atomic force microscopy which can provide detailed information on the depth and height of structures. Figure 1 shows some typical electron microscope pictures obtained after irradiation of the sample with 50 laser shots at a fluence of 7 J/cm2. This fluence is approximately a factor 2 — 3 higher than the threshold damage fluence for pulse widths in the range shown (5 ps — 200 fs) [11]. For the two shortest pulse widths (200 fs and 580 fs) one can see strong fracturing at the edge of and beyond the irradiated area with no indication of melting. On increasing the pulse width to a few picoseconds one obtains a much smoother, well-defined ablation in the area exposed to the laser pulse. This area is delineated by a narrow melt edge which can be clearly seen in the enlargement of the 1.8 ps structure in Fig. 1. There are no signs of fracture except for the small ((2 lm) rectangular shaped sub-structures in the centre and a small crack at the left-hand side of the irradiated area for t"1.8 ps. These effects seem to decrease on going to slightly longer pulses. Figure 2 shows the situation with 8 ns pulses. A fluence of 50 J/cm2 was used which is close to the damage threshold fluence for this pulse width. Here one sees very strong fracturing with damage occuring far beyond the irradiated area (the geometrical pulse profiles were very similar over the entire pulse width range investigated). Again, as for the fs pulses, we see no evidence of melting. The nanosecond results are in good agreement with previous single-shot studies using 248 nm excimer laser pulses (14 ns) [12]. In this work extensive fracturing along the natural cleavage planes could be observed leading to the formation of large tiles of various shapes which may be partly or entirely removed Fig. 2. Electron microscope pictures of polished CaF (111) after 50 2 on 1 shot ablation in vacuum ((10~5 mbar) with laser pulses of 8 ns at a wavelength of 790 nm and a fluence of 50 J/cm2 from the surface and which show no signs of melting. Dickinson observed similar effects with ns pulses for single-shot damage of MgO and NaCl [13]: fracture with no signs of melting. Atomic force pictures of the ablated structures obtained after 5 laser shots are shown in Fig. 3 for two different pulse widths (200 fs and 4.7 ps). The fluence was chosen in each case to be a factor of three higher than the in [11] determined threshold damage fluence. The much smoother structure for 4.7 ps is immediately apparent and seems to closely mirror the geometrical profile of the laser pulse. For 200 fs pulses the damage around the hole is less apparent than in the electron microscope pictures but can still be seen. 2 Discussion CaF is a so-called type II material with very strong 2 electron-phonon coupling which implies a high rate for self-trapping of excitons and/or holes after laser excitation [8, 14, 15]. There are two main excitation mechanisms using photons with energies well below the band gap: (a) multi-photon absorption leading to the formation of defects (via electron-hole pairs) such as self-trapped excitons and Frenkel defects and (b) single photon absorption between occupied and empty defect states or of free electrons in the conduction band [2, 16]. The latter will lead to most of the local heating of the crystal. In our case of surface damage it is generally believed that single photon absorption in the conduction band is more likely to lead to photoexcitation than to efficient local heating [2, 17]. The mechanisms leading to macroscopic material damage and ablation for different laser pulse widths will thus depend very much on the kind of defects produced, the time range for their production and their lifetime. The nanosecond results of Gogoll et al. [12] were interpreted as being due to the build-up of shear stress due to lateral temperature variations in the irradiated volume. This leads to thermoelastic displacement along the cleavage planes which gives rise to the strong fracturing observed in the experiments. No melting is observed at the fluences investigated because the defect formation rate and thus the heat input is too low for melting. The temperature rise is, however, still large enough to produce the shear stress leading to fracturing. For excitation with ps pulses there is a much larger probability for the formation of defects (electron-hole pairs) due to multiphoton absorption. These electron-hole pairs are very efficiently converted to self-trapped excitons (STE) on a time range of about 1 ps [8]. These have a lifetime of a few ls [9, 15]. If the critical defect density for strong single photon absorption (approx. 1021 cm~3 [10]) is reached during the first part of the laser pulse (1 — 2 ps) the remainder of the pulse can efficiently excite between excitonic states which, followed by rapid conversion of electronic to vibrational energy, leads to a rapid heating of the crystal producing melting and vapourisation of the material. Since we have a Gaussian laser beam, the temperature rise at the edges is lower than in the centre giving the characteristic melt edge (Fig. 1). Due to the low heat conductivity in these materials there is no 106 Fig. 3. Atomic force microscope pictures as surface height profile (left) and as section analysis (right) of polished CaF (111) after 5 on 1 shot 2 ablation in vacuum ((10~5 mbar) with laser pulses at a wavelength of 790 nm: pulse width: 200 fs, fluence 10 J/cm2 (top); 4.7 ps, 15 J/cm2 (bottom) observable thermal damage outside the area that directly interacts with the laser beam. For excitation with laser pulses with q41 ps one will have an even larger probability for the multiphoton excitation of electron-hole pairs during the laser pulse. These will convert to self-trapped excitons on a timescale of 1 ps as discussed above, i.e. for very short pulses the self-trapped excitons will be formed after the pulse. There is no remaining laser pulse to produce rapid heating of the crystal via single photon absorption from the defect states. However, the localised excitons produced after the laser pulse will weaken the bonds in the crystal [9] and strongly deform the lattice. If they have been produced with sufficient density without being thermally removed, the lattice deformation will cause immense stress in the crystal thus inducing severe fracturing and explosive non-thermal removal of material. Perhaps still somewhat speculative, the above considerations are able to explain the experimental findings very satisfactorily. Based on our present knowledge on defect formation, the above considerations should be valid for similar type-II materials, e.g. MgF . It is 2 interesting to note, that the optically induced defect formation does not only account for simple damage but with a proper choice of pulse width and fluence may in future play a major role for high quality structuring of wide-gap materials. Further experiments are in progress to test the suggested mechanisms for ablation in the ps and fs regimes. Acknowledgements. We would like to thank S. Gogoll and Prof. E. Matthias (both of the F.U. Berlin) for some stimulating discussions. Financial support from the BMBF (13N6591/1) is gratefully acknowledged. 107 References 1. C.R.Giuliano: Appl. Phys. Lett. 5, 137 (1964) 2. E. Matthias, R. Dreyfus: In Photoacoustic, Photothermal and Photochemical Processes at Surfaces and ¹hin Films, ed. by P. Hess, Topics Curr. Phys., Vol. 47 (Springer, Berlin, Heidelberg 1989) p. 89 3. R.M. Wood: ¸aser Damage in Optical Materials (Adam Hilger, Bristol 1986) 4. N.H. Tolk, M.M. Traum, J.C. Tully, T.E. Madey (eds): Desorption Induced by Electronic ¹ransitions, DIE¹ I, Springer Ser. Chem. Phys., Vol. 24 (Springer Berlin, Heidelberg 1983) 5. W. Brenig, D. Menzel (eds): Desorption Induced by Electronic ¹ransitions, DIE¹ II, Springer Ser. Surf. Sci., Vol 4 (Springer, Berlin, Heidelberg 1985) 6. R.H. Stulen, M.L. Knotek (eds): Desorption Induced by Electronic ¹ransitions, DIE¹ III, Springer Ser. Surf. Sci., Vol 13 (Springer, Berlin, Heidelberg 1988) 7. N. Itoh: In Defects in Insulating Crystals, ed. by V.M. Tuchkevich, K.K. Shvarts (Springer, Berlin, Heidelberg 1981) pp. 343 8. R.T. Williams: Opt. Eng. 28, 1024 (1989) 9. R.F. Haglund Jr., N. Itoh: In ¸aser Ablation, Springer Ser. Mater. Sci., Vol. 28, ed. by J.C. Miller (Springer, Berlin, Heidelberg 1994) 10. B.C. Stuart, M.D. Feit, A.M. Rubenchik, B.W. Shore, M.D. Perry: Phys. Rev. Lett. 74, 2248 (1995) 11. H. Varel, D. Ashkenasi, A. Rosenfeld, R. Herrmann, F. Noack, E.E.B. Campbell: Appl. Phys. A 62, 293 (1996) 12. S. Gogoll, E. Stenzel, H. Johansen, M. Reichling, E. Matthias, Nucl. Instrum. Methods Phys. Res. B (1996) (in press) 13. R.L. Webb, L.C. Jensen, S.C. Langford, J.T. Dickinson: J. Appl. Phys. 74 2323; 2338 (1993) 14. N. Itoh, K. Tanimura: Opt. Eng. 28, 1028 (1989) 15. N. Itoh, K. Tanimura: J. Phys. Chem. Solids 51 717 (1990) 16. S.C. Jones, P. Braunlich, R.T. Casper, X.-A. Shen, P. Kelly: Opt. Eng. 28, 1039 (1989) 17. E. Matthias: private communication (1996) .