MATERIAL PROCESSING WITH FEMTOSECOND LASER PULSES. A. Rosenfeld, D. Ashkenasi, E.E.B. Campbell*, M. Lorenz, R. Stoian, H. Varel.

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1 MATERIAL PROCESSING WITH FEMTOSECOND LASER PULSES A. Rosenfeld, D. Ashkenasi, E.E.B. Campbell*, M. Lorenz, R. Stoian, H. Varel Max-Born-Institut für Nichtlineare Optik und Kurzzeitpunktspektroskopie Rudower Chaussee 6, 89 Berlin, Germany *Gothenburg University & Chalmers University of Technology, S-96 Gothenburg, Sweden Abstract Ultrashort laser pulses have considerable potential for micron and sub-micron structuring of several materials. The lower energy impact, the reduction of thermal damage, the elimination of laser-plume interaction, and the exploitation of nonlinear optical effects all contribute to a strong improvement when compared to results using pulse widths in the nanosecond range. Different phenomena are observed, related to three fluence regimes. (i) Fluence» damage threshold fluence (F th ). In this regime it is possible to produce long channels with a high aspect ratio and little residual damage and stress in the material. (ii) Fluence F th. Here, two distinct ablation phases can be observed. A gentle and a strong ablation phase. The gentle phase leads to controlled melting and vaporisation of material. At higher laser intensities (or after a sufficient number of incubation pulses) the strong ablation phase is observed which we relate to phase explosion. The damage threshold drops dramatically during the first laser shots, due to defect incubation. This has important consequences for applications, such as laser machining and for the lifetime of optical components(iii) Fluence < F th. Self-focusing of the laser light due to the Kerr effect can occur in the dielectric materials for ps pulses. This leads to the possibility of direct writing of micro-structures in the material bulk without producing damage on the entrance or exit surfaces.. Introduction Material removal caused by high intensity laser pulses has been demonstrated to be a powerful tool in surface micro-patterning and structuring of a range of materials [,]. There are a number of advantages in using lasers for micro-machining or structuring: single-step processing, high flexibility, direct writing of structures by moving the laser beam at speeds much greater than can be obtained with mechanical tools, no contamination of the material being processed and the extreme focusing that is possible, leading to highly localised treatment of materials with a spatial resolution of better than µm. However, this high spatial resolution normally cannot be achieved using standard ns lasers due to strong thermal effects which occur in the material and the destructive influence of the plasma which is formed above the surface. With optically transparent, dielectric materials one requires a very high laser intensity with ns pulses in order to obtain sufficient energy absorption in the material to observe macroscopic material removal. The thermal stresses that are built up in the material during, and after, the relatively long laser pulses lead to extensive cracking and exfoliation and, in some cases, produce a spectacular explosive destruction of the sample. Most of the work available until now on the interaction of intense laser pulses with optically transparent materials has concentrated on determining the laser intensities at which damage can be seen to occur in the material. This is of considerable relevance for the further development of high peak power / short pulse laser systems since damage to optical components is often the limiting factor in the performance of these lasers. However, a broad introduction of ultrashort-pulse lasers for

2 industrial manufacturing still suffers from the present complicated and costly arrangement of these systems. The development of new compact laser systems designed also for industrial needs and the advances made in material processing with ultrashort laser pulses (in many cases even under atmosphere conditions) will stimulate the use of the femtosecond technology outside research and development. There are many advantages of using ultra-short laser pulses with pulse duration below ca. ps. Much less energy input is required to produce the same amount of material removal Thermal damage around the irradiated area is considerably reduced Multi-photon excitation can be exploited to achieve smaller structures There is no laser interaction with the ablated particles Non-linear optical processes in the dielectric material can be utilised to produce novel material processing possibilities This chapter concentrates on the processes occurring when dielectric materials are irradiated with ultra-short pulses in the near-ir region (79-8 nm) of the spectrum. This is the wavelength range produced by the Ti:Sapphire laser, the most commonly available amplified fs-laser system. The photon energy is much lower than the band gap for all the materials discussed here. By using the method of chirped pulse amplification [3] it is possible to change the pulse duration without significantly altering other parameters such as the geometrical beam profile making possible a detailed comparison of effects due solely to the pulse duration. Basically, the laser processing of dielectrics with ultra-short pulses in the infrared can be divided into three major fluence regimes for the following applications []:. the high fluence regime, F/F th >> to drill channels with a high aspect ratio [5];. the intermediate fluence regime F/F th > to generate pockets and surface (periodic) patterns [6,7], and 3. the low fluence regime F/F th < to produce micro structures inside the sample or on the rear side [8,9]. A few examples of recently generated micro structures in fused silica (a-sio ) and sapphire (c-al O 3 ) are depicted in Fig.. A = spot size high fluence surface bulk intermediate fluence A surface processing threshold F th normal diffraction length low fluence self focusing at high power Fig.. Schematic diagram of the laser beam profile focused on the entrance surface of a transparent dielectric material. The laser pulses are coming from the top of the diagram. The intensity distribution of each laser pulse is also indicated with reference to the surface damage threshold indicated by the dashed, horizontal line. a) high fluence used to produce high aspect ratio channels (b) intermediate fluence for pockets and surface patterning (c) low fluence, below surface damage threshold, for bulk modifications and exit surface structuring utilising non-linear selffocusing (Kerr effect). The three experimental examples shown in the diagram have diameters in the range -3 µm and were produced in fused silica at 8 nm. deep channels surface patterns micro pockets bulk modifications rear micro holes

3 The Gaussian laser beam is focused on the entrance surface of the material. The focus area is used to define the fluence (J/cm ). For laser fluences much larger than the ablation threshold (Fig. (a)) it is possible to drill high aspect ratio channels. For laser fluences at or slightly above the ablation threshold fluence (F th ) surface patterning (ripple formation) and pocket formation is seen (Fig. (b)). A strong dependence of the observed phenomena on the number of laser shots is indicative of strong incubation effects. Two ablation phases are apparent: a gentle ablation phase where very little material is removed per laser shot and a strong ablation phase which shows some indications of phase explosion. Finally, for fluences below the surface damage threshold (Fig. (c)), the high intensities associated with the ultra-short laser pulses induce non-linear optical effects in the transparent material leading to self-focusing of the laser pulse and the occurrence of micro-explosions in the bulk interior. The size and position of this damage structure produced in the bulk can be controlled by adjusting the laser parameters (pulse duration, pulse energy and number of laser shots). When self-focusing occurs close to the exit surface of the material ablation can be observed leaving very smooth conical structures. These three different fluence regimes are discussed in detail in the following three sections. The materials chosen for the experiments are classed as having either strong or intermediate electron-phonon coupling strengths [7]. It will be shown that this parameter along with the dynamics of defect production plays a very important role in the ultra-short pulse laser ablation process. Furthermore we will show the dynamics of the material removal process experimentally determined by scattering measurements.. F» F th : High Aspect Ratio Channels. Results and Discussion Once the strong ablation phase has set in it is possible to ablate long, narrow channels with high aspect ratios []. Some examples are shown in Fig. for ablation of SiO with fs pulses (79 nm, J/cm ) as a function of number of laser shots. One can clearly see a characteristic narrowing of the holes from the diameter produced at the entrance surface of the sample to a diameter much smaller than the laser focus diameter after approximately -3 µm. This diameter then remains constant until the bottom of the hole is reached. The holes shown in Fig. were produced under vacuum ( -3 mbar). The maximum hole depth that can be reached depends on the ambient pressure and the laser fluence. It does not seem to depend to any significant extent on the laser pulse duration within the range investigated [,]. µm 8 6 µm Fig.. Long and thin channels (length up to.8 mm at 5 laser shots and diameter of ca. µm) generated in fused silica with ultra short laser pulses of a pulse duration = fs, wavelength = 79 nm, repetition rate = 6 Hz, fluence = J/cm (8 µj) for different shot numbers as indicated in the figure. Pressure at target was < - mbar. N = shots side view

4 This is illustrated in Fig. 3 for SiO for two different laser fluences and three different pulse durations at an ambient pressure of -3 mbar. Some mechanical stress is apparent in Fig. around the top of the holes where the narrowing can be seen. This stress is reduced as the pulse duration is reduced []. It is also strongly dependent on the intensity distribution of the laser beam profile []. The examples shown in Fig. were obtained with a Gaussian beam profile. Careful beam alignment using a combination of apertures and lenses to achieve a tophat profile can lead to channels of high quality with very little evidence of mechanical stress even in extremely brittle materials like MgF []. The formation of the channels has been attributed to multiple reflection of the light at the channel walls and does not appear to be dependent upon self-focusing or channelling of the laser pulse []. depth [µm] 5 5 SiO - ablation at λ = 795 nm fs ps ps 6 J/cm ; J/cm ; Fig. 3. Ablation depth vs. number of laser shots at a wavelength = 79 nm (repetition rate = Hz) under vacuum conditions of < - mbar using three different laser pulse durations:.,.3 and.3 ps. Triangle: J/cm, circle: 6 J/cm number of shots 3 qu795a - 3. F F th : Two Ablation Phases and the Role of Defects.. Damage Threshold There have been a number of studies of the laser-induced breakdown or damage fluence in optically transparent materials as a function of pulse duration [-6]. Stuart et al. [3] determined the damage threshold for fused silica and CaF with pulses in the range 7 fs to ns (λ = 53 nm). Their definition of the damage threshold was the value at which visible permanent modifications to the surface could be seen with a Nomarski microscope. In contrast to Du et al. they did not observe an increase in the damage threshold for pulse durations < ps. However, they did observe deviation from τ / behaviour for τ < ps with the rate of the decrease of the damage threshold decreasing as the pulse duration was reduced. Their results are shown in Fig.. The deviation from τ / behaviour occurs for pulse duration on the order of the electron-phonon coupling time which is the relevant time-scale for energy transfer to the lattice, even when the pulse duration lies considerably below this value. Both groups interpreted their results in terms of theoretical models in which electrons, initially produced by multi-photon ionisation by the short laser pulses, are further heated resulting in avalanche ionisation and rapid plasma formation. In later experiments the continuously decreasing trend found by Stuart et al. was confirmed by single-shot measurements on both fused silica and CaF using a variety of ex-situ and in-situ techniques, including plasma emission to determine the breakdown or damage threshold at a wavelength of 79 nm [5]. The use of multiple laser

5 shot threshold for silica and 5 shot threshold Fig.. Multi-shot damage thresholds for silica and calcium fluoride as a function of laser pulse width (53 nm). The full lines through the data points are fits to the model (discussed in text). Adapted from []. The gray circles and open squares are one and five shot thresholds for fused silica adapted from [6]. shots (reported by Stuart et al. [3]) serves simply to decrease the absolute value of the damage threshold due to incubation effects as discussed below. Stuart et al., for example, exposed their samples to 6 laser shots at many different fluence levels for a given pulse duration to identify the threshold value, which they defined as irreversible modification on the surface optically observable. There is no discussion as to what degree the number of laser shots between and 6 change the threshold. This data is very important for applications, such as for estimating the lifetime of optical components and for laser processing. Fig. 5 illustrates the surface damage threshold in different materials plotted semilogarithmically over laser shot numbers N and determined at different pulse durations of. ps and two different focal spot sizes (/e ) on the surface: 5 and 5 µm, depending on the focal length of the two lenses used in this study. Within the experimental uncertainty the fluence threshold levels are spot size independent. fluence [J/cm ] 3 SiO - fs, 8 nm spot size = 5 µm spot size = 5 µm number of laser pulses N fig. fluence [J/cm ] 5 3 Al O 3 gentle; strong a). ps number of laser pulses N Fig. 5 Semilog-arithmic plot of the surface threshold versus shot numbers in fused silica, CaF and Al O 3 determined at a laser wavelength of 8 nm. Solid line from fit following Eq.. fluence [J/cm ] CaF 3. ps. ps. ps. ps (bulk) number of laser shots N cath g fluence [J/cm ] gentle; strong b).3 ps number of laser pulses N fig.

6 This behavior is expected for ultra-short laser pulses, since the energy deposition remains strongly localized, in contrast to ns laser pulses. During the first 5 laser pulses we obtain a dramatic until 7 % decrease in damage threshold. In the case of CaF we obtain an expected pulse duration dependency of the plot. For longer pulse duration the threshold is at the first 5 shots higher then for shorter laser pulse duration. For Al O 3 is clearly seen the difference between the strong and the gentle etch phase in the dependence of the threshold on the shot number. Incubation effects in dielectric materials can be greatly influenced by the excitation and generation of conduction band electrons which will eventually lead to an accumulation of defect sites. The primary (resonantly enhanced) multi-photon excitation will lead to a production of electron-hole pairs on a sub- fs time-scale. These states have a lifetime between 5 fs and several ps [7] before forming self-trapped excitons and Frenkel-pairs. A small fraction of these Frenkel-pairs may not recombine and stabilize to F-centers[8], introducing additional energy levels and excitation routes for the next laser shot. The relative change in the laser-induced defect concentration will decrease with increasing shot numbers until finally reaching a point of saturation in the dielectric. The reduction in damage threshold is therefore less pronounced while going to higher shot numbers. In such a case, irradiation at a fluence below a minimum level would require an infinite numbers of pulses to initiate the defect accumulation and, hence, activate macroscopic damage. If we assume that the relative change Fth = Fth ( N, τ p ) Fth ( N, τ p ) is proportional to F th ( N, τ p), N: laser shot numbers and τ P : pulse duration, we can describe the laser shot number dependency of the surface damage threshold F ( N, τ ) in the following straightforward way [9]: th p [ ] F ( N, τ ) = F (, τ ) + F (, τ ) F (, τ ) e th p th p th p th p k ( N ) Here, F th (, τ P ) is the single shot threshold and k characterizes the strength in the defect accumulation. The larger k is, the fewer laser shots are necessary to obtain a minimum damage threshold Fth (, τ p ) at infinite number of laser shots. In Fig. 5 the solid lines are the calculated curves obtained from the fit of the threshold behavior of based on Eq.... Surface Morphology and Material Removal In all dielectrics studied to date there is a very characteristic development of the surface structure with number of laser shots and/or laser pulse intensity for pulse durations on the order of ps or lower. Laser irradiation in a fluence or intensity regime just above the surface damage threshold induces two different ablation phases dependent upon the laser intensity or number of pulses (incubation). This has been studied in most detail for sapphire [6,,] but has also been clearly seen in CaF and SiO []. The behaviour is very different from what is known with ns laser irradiation. Sapphire has been extensively investigated with ns pulses due to its many useful mechanical, optical and electrical properties [3-9]. Results have been reported on the quality of the ablation structures, the optical emission from the ablation plume, the energy deposition at the surface and the threshold fluence for laser ablation. With both UV and IR ns laser pulses the main mechanism appears to be electronic sputtering with no direct evidence for thermal ablation. An electronic mechanism is supported by the observation of an excessive yield of AlO molecules with high translational temperatures but low rotational and vibrational temperatures when using excimer laser pulses close to threshold. Ablation of sapphire with 3 ps laser pulses at 66 nm resulted in cleanly ablated features with few micro-cracks around the rim []. Two ablation, or sputtering, regimes were observed in ()

7 these experiments at a fluence of J/cm : a gentle, slow material removal for the first few laser shots followed by a much faster explosive sputtering in which ten times as much material per pulse was removed. The ejection of molten droplets was also observed during the explosive phase, in strong contrast to the ns results []. The behaviour at 79 nm in the pulse duration range between fs and 5 ps has been studied in detail by our own group [6] and confirms the presence of two distinct ablation regimes, both showing strong evidence of melting and thermal ablation. Scanning electron microscope (SEM) pictures of surface structures produced on sapphire after and 5 laser shots (79 nm) for two different pulse durations are shown in Fig. 6. The fluence levels chosen for the shot comparison are about a factor of two (left column) and four (middle column) above the determined surface damage threshold. The pictures shown in Fig. 6(a) and (d) demonstrate that during the first few laser shots at these relatively low fluences the processed area stays very smooth (with the exception of some ripple structures that can be seen at the edge of the irradiated area and will be discussed further below). As the laser fluence is increased the affected area on the sapphire surface increases but still stays within the (/e ) beam area. The onset of the second, much stronger, ablation phase, which leaves a rougher surface, is observed when the laser intensity exceeds a given critical value. This can be clearly seen in the middle column of Fig. 6. For subps pulses the crossover from gentle to strong ablation occurs at a much earlier stage, even considering the lower surface damage threshold. µm µm µm N = (a) µm N = (b) µm N = 5 (c) µm Fig. 6. SEM pictures of the structures produced on sapphire for two different pulse durations (79 nm): fs (top) and.3 ps (bottom). (a): N =, 3. Jcm -, (b) N =, 6. Jcm -, (c) N = 5, 9.6 Jcm -, (d) N =, 6. Jcm -, (e) N =,.8 Jcm -, (f) N = 5, 9.6 Jcm - N = N = N = 5 (d) (e) (f) A cross-section through the structure in Fig. 6(e) taken with an atomic force microscope (AFM) is shown in Fig. 7. The depth of the structure changes dramatically from a smooth plane with a depth of 6 nm and a mean roughness of ca. nm on the edges to a steep-sided central hole of µm diameter and over µm deep. The extent of damage around the ablated hole for strong ablation decreases with decreasing pulse duration. Strong ablation can also occur on the first laser shot if the pulse has sufficient intensity [6]. Careful comparison of the diameters of the structures produced under single-shot conditions was able to give an

8 indication of the multi-photon order of the initial excitation step. The dimensions of the holes produced during gentle ablation are consistent with a -photon excitation whereas the strong ablation structures indicate a much higher order of approximately 6 [6]. + µm µm - µm µm section analysis > µm 6 nm µm µm 6 µm Fig. 7. Section analysis after AFM analysis of a sapphire pocket processed with N = (79 nm).3 ps,.8 Jcm - (shown as SEM picture in Fig. 6(e)) The onset of the gentle ablation phase was thus attributed to the low-order excitation of surface defect states providing sufficient carrier electrons in a thin surface region to lead to a gentle heating during the remainder of the laser pulse and subsequent vaporisation of the material. The order of the initial stage in the strong ablation for single-shot experiments is consistent with the direct multi-photon ionisation of electrons from the valence to the conduction band. The onset of the strong ablation phase after a given number of incubation pulses at lower laser intensities will be due to the increasing density of bulk defect states allowing a lower order excitation of electrons to the conduction band. When a sufficient concentration of seed electrons is produced early in the pulse rapid heating of the bulk due to collisional ionisation followed by ablation will take place. The defects may be produced either by multi-photon excitation with IR photons or by lower order excitation by visible or UV photons produced by self-phase modulation in the bulk, as discussed above Ripples It is possible to generate a periodic pattern (ripples) throughout the irradiated area during the gentle ablation phase. Fig. 8 gives two examples of ripples, on sapphire and amorphous SiO, generated with normal incidence fs pulses at a fluence of J/cm. The ripples produced on sapphire were always oriented perpendicular to the electric field vector of the laser polarisation with a spacing corresponding to the wavelength of the laser light [6]. The modulation depth was on the order of a few tens of nanometres, similar to the depth of material removed per laser shot at the corresponding pulse energy and duration. Ripple formation was first observed by Birnbaum [3] after ruby-laser irradiation of various semiconductor surfaces. Since then they have been discussed extensively in the literature. Bäuerle has given a useful summary of the present state of knowledge []. Ripples originate from the interference between the incident laser light and the scattered wave along the interface. For normal incidence the period of the interference pattern is given by λ/n where n is the refractive index of the material.

9 For fs irradiation of dielectrics, illustrated in Fig. 8, the spacing seen for the ripples is clearly λ rather than λ/n. This indicates that the behaviour changes from dielectric to metallic in the surface region of the material during the interaction with the ultra-short laser pulses [6]. The ripples have only been observed on dielectrics with ultra-short laser pulses after a certain number of incubation shots during the gentle ablation phase. They have not yet been observed on single-shot irradiation. There may be a contribution to this effect of metallisation of the clear ripple formation on sapphire for pulse durations as long as 5 ps (the longest pulse durations investigated in our experiments) [6]. c-al O 3 µm.5 µm -.5 µm section analysis.8 µm µm Fig. 8. AFM-pictures and section analysis of ripple pattern on sapphire (top) and fused silica (bottom). Laser processing was conducted under following conditions: wavelength = 8 nm, pulse duration = fs, fluence = J/cm ; shots for sapphire and 3 shots for fused silica. a-sio µm section analysis.7 µm µm - µm µm The situation is somewhat different for SiO. It was not possible to observe a clearly defined periodic pattern in amorphous or crystalline SiO for pulse durations of fs or longer. One possible explanation for the difference between the two materials is the lifetime of the electrons in the conduction band. The carrier lifetimes have been investigated by Petite and co-workers [3] for SiO, MgO and Al O 3. There is clearly a strong difference between the behaviour of SiO and the other two materials. The free carrier density falls off rapidly in thsurface region due to stoichiometry changes, however, no direct evidence for this has been found to date. The main effect is attributed to the rapid build-up of electrons in the conduction band during the first part of the laser pulse. As discussed above, these electrons come predominantly from the excitation of defect states during the gentle ablation phase. A certain incubation period is needed in order to build-up a sufficient density of defect states in the surface region which can supply a high enough free electron density during the ultra-short laser pulse to lead to metallic behaviour. (The repetition rate in these experiments was - Hz). Ripples are most clearly observed shortly before the onset of the strong ablation phase. They cannot be observed after the strong ablation phase has been initiated since that involves the explosive removal of bulk material, thus destroying any surface effects. We have observed clear ripple formation on sapphire for pulse durations as long as 5 ps (the longest pulse durations investigated in our experiments) [6].

10 The situation is somewhat different for SiO. It was not possible to observe a clearly defined periodic pattern in amorphous or crystalline SiO for pulse durations of fs or longer. One possible explanation for the difference between the two materials is the lifetime of the electrons in the conduction band. The carrier lifetimes have been investigated by Petite and co-workers [3] for SiO, MgO and Al O 3. There is clearly a strong difference between the behaviour of SiO and the other two materials. The free carrier density falls off rapidly in the case of SiO with a decay time on the order of 5 fs, whereas it stays practically constant on the time-scale of the experiments (3 ps) for the other two materials. Longer time-scale studies have shown a decay time of about ps for Al O 3 and 5 ps for MgO [3]. The fast decay for SiO is attributed to the rapid formation of self-trapped excitons. The ripple experiments [,6] provide additional support for the lifetime measurements of Petite et al. [3]. It is only possible to observe this type of ripple formation (with a spacing of λ) when the laser pulse duration is shorter than the lifetime of free electrons in the conduction band of the material... Dynamics of the Ablation Process Femtosecond pump-probe techniques have been applied to the time-resolved detection of surface morphology changes after laser irradiation [3]. The measurement principle is shown schematically in Fig. 9. The pump and probe beams were generated by a Ti:Sapphire laser (79 nm) and were of fs duration. The studies were carried out for single-shot conditions at a fluence of J/cm (pump pulse) under vacuum. The signal of the scattered light from the much less intense probe pulse (far below the damage threshold of all the materials investigated), delayed by a variable time delay (- ps), was detected by a long distance microscope, tilted under an angle of 3 o, coupled to a photomultiplier. This set-up can detect a change in scattering signal when the surface is modified due to melting (decrease in surface roughness giving a lower scattering signal) or macroscopic material removal (increase in surface roughness). Laser Isc Fig. 9. measurement principle of the scattered light pump-probe investigation. I sc is the intensity of the scattered light. V abl is the ablated volume. Vabl If we assume, that the intensity of the scattered light (I sc ) is proportional to the ablated volume (V abl ), therefore also proportional to the number of the ablated particles (N abl ) then it follows that N abl = N - n(t), where N is the number of the total ablated particles and n(t) is the number of particles leave the excited area at time t. The transient response of number n of particles ablated dn(t) can be written dn(t)/dt = -kn(t). We obtain n(t) = N e -k(t-t) for t t

11 and if t < t, t is the onset of the process. The value k the ablation strength, the larger k the smaller is the ablation time. For the scattering signal we then obtain the following equation I sc = A(- e -k(t-t)), (A = a N ) () Results for the measured and fitted scattering curves are shown in Fig. (left column) for four different oxides. In order to observe a change in the scattering signal the surface roughness will need to change by approximately 5 % compared to the non-irradiated polished surface. Under our conditions, if we assume an average particle velocity of 3 - m/s this gives an offset of approximately ps in the time-scale for onset of particle removal. The corresponding surface morphologies after the single shot exposure can be seen from the optical microscope pictures in the right column of Fig.. scatterin signal (arb. u.) scattering signal (arb. u.) scattering signal [arb. u.] 3 SiO Pump 5 µj Probe 9 µj A ±. t ±.3795 k.55 ±.68, delay (ps) c - SiO Pump 5 µj Probe 9 µj Al O 3 Pump 5,5 µj Probe 9 µj A.589 ±.363 t -.37 ± k.66 ±.6, delay [ps] A.77 ±.8936 t ±.997 k.37 ±.7, delay [ps] Fig.. Scattered light signal (arb. units) of probe laser pulse vs. Delay time (log scale) for four different oxides: fused SiO, crystalline SiO, sapphire Al O 3 and MgO (left column Solid line from fit following Eq. ) and optical microscope pictures (right column) after single-shot illumination of the surfaces each representing typical results of morphological change caused by intense excitation (pump laser pulse = 5 µj, corresponding to a fluence of -3 times above threshold. scattering signal [arb. u.] MgO Pump 5 µj Probe 9 µj A.36 ±.35 t ±.8 k.73 ±.5, delay [ps] There are significant differences in the scattering signal for the different materials. Ablation onsets after only ca. ps (3 ps ps offset) for amorphous and crystalline quartz whereas for sapphire and MgO the ablation occurs after 9 and 6 ps respectively. Quartz is a material with a very strong electron-phonon coupling strength [33], in contrast to sapphire and MgO which

12 both fall into the category of materials with intermediate electron-phonon coupling strengths [33]. The larger the electron-phonon coupling the more energy can be transferred from the electrons to the phonons per collision, therefore, the energy transfer process into the lattice is quicker. The ablation process can only begin once sufficient energy has been transferred to the lattice. Thus, in quartz, a large proportion of the electronic excitation energy must be very efficiently transferred to the lattice on a time-scale on the order of 5 fs after which selftrapped excitons and permanent colour centres are formed [3]. (Note that the pulse duration in these experiments is fs.) The energy transferred to the lattice on this time-scale is sufficient to generate the removal of particles on a time-scale of - ps. The surfaces of both quartz samples are smooth after ablation indicating that melting and vaporisation is the dominant ablation process. As discussed above, the laser ablation of sapphire with ultra-short IR pulses is predominantly of a thermal nature and the material removal mechanism is thus very similar to the quartz situation. The reason for the longer delay before the onset of scattering is presumably related to the longer time-scale for electron-phonon coupling. The data for MgO are shifted to longer times than for sapphire, indicating a weaker coupling constant, and the scattering intensity is much lower. One possible interpretation for the reduced scattering signal may be obtained with the help of Fig. (right column, microscocopic pictures). The morphology of the irradiated spot on the MgO sample is very different from the other three materials and shows a clear surface fracturing, similar to the effects observed on CaF. Very similar behaviour has been seen after single-shot irradiation of polished MgO () with a ns 8 nm excimer laser [3]. The edges of the rectangular structures lie along the [] cleavage planes of the crystal. It would thus seem that a considerable fraction of the absorbed energy is trapped into defects rather than converted to heat energy in this case. This leads to the build-up of stress in the material that finally manifests itself in the formation of macroscopic cracks. The time-scale for the development of these cracks is correlated to the velocity of sound propagation in solids and therefore be much longer than for the thermal removal of material and would not be observed on the time-scale of the pump-probe experiments. The scattering results for CaF are very similar to MgO [35].. 3 F < F th : Self-Focusing When the laser beam is focused on the surface at a fluence below damage fluence, no damage on the entrance surface can be observed even under conditions, where bulk modifications are generated several µm below the surface. Fig. depicts some examples of laser generated bulk structures in a-sio due to the self-focusing of the laser beam at three different pulse durations, a pulse energy of 3.5 µj and shot numbers running from N = 5 to 5. The modification depth z M is defined as the distance from the entrance surface at which the modification starts, as indicated by the horizontal thin lines in Fig.. As additional laser pulses act on the bulk modification starting at a specific modification depth z M, the damage adds up forming a damage track with increasing length towards the surface. A very clear dependence of z M on the laser pulse energy is seen with z M decreasing for increasing pulse energy and decreasing pulse duration. A similar dependence was obtained for the other samples. In cases, where the first damage point is generated - µm below the surface, only a few laser shots are necessary to obtain a violent ablation feature at the surface. Additional studies are necessary to unambiguously discriminate between surface and bulk accumulation properties at high shot numbers. A closer inspection of the bulk structures, as to be seen in the insert of Fig., reveals a conglomeration of smaller structures reminiscent of micro-explosions. It is possible to utilize the self-focusing effects to produce controlled micrometer sized modifications in bulk. The size and depth of the structures can be varied by varying the laser pulse duration and/or energy. To determine short pulse laser induced bulk

13 damage thresholds and to investigate the capabilities of nonlinear optical effects to generate controllable bulk microstructures, we conducted this study on self focusing in different wide band-gap materials with picosecond and sub-picosecond laser pulses. Fig. 7 presents the results of the reciprocal modification depths z M - versus the square root of the laser power P for a- SiO (a) and c-sio (b) at different pulse widths. For catastrophic self focusing a linear dependency is expected in this kind of plot[36], as it is the case also for the experimental depths of modifications. A linear fit to the experimental data yields the critical laser power for self-focusing, P cr. The value of P cr for a-sio obtained from the fits is practically identical (3.6 MW) for the pulse duration range of. to. ps. The same is true for c-sio where P cr =.85 MW for. and.8 ps. Included in Fig. as dotted lines are the expected dependencies for Fig. Microscopic side view of bulk modifications in a mm thick a-sio sample. The three different pulse widths,.7,. and. ps, demonstrate different modification depths, 35, 5 and µm respectively, from which the damage begins after approx. shots. Additional laser pulses contribute to the length of the damage track moving toward the surface. 5 a-sio 5 c-sio /z M [mm] ps. ps.8 ps. ps C.S.F.-theorie: P cr = 3.6 MW (a-sio ) /z M [mm] - 3. ps.8 ps C.S.F theorie: P cr =.6 MW (c-sio ) P / [MW] / P / [MW] / Fig. Plot of reciprocal modification depth vs. square root of laser power for (a) a-sio and (b) c-sio. Lines are linear fit to the experimental data. The dotted line represents the expected catastrophic self focusing depth assuming (a) P cr = 3.6 MW for a-sio and (b) P cr =.85 MW for c-sio.

14 the catastrophic self focusing depths. For a given pulse duration there seems to be a fixed ration between modification and theoretical catastrophic self focusing depth, for which we at this time can not offer a plausible explanation. For the shortest pulses used in the experiment (.7 ps) for a-sio we see a clear increase in the value of P cr extracted using this procedure (7 MW). We observed a similar increase in the critical self focusing power at sub ps laser pulses for CaF, and LiF. First estimations demonstrate that the group velocity dispersion above. ps is not strong enough to influence the self focusing significantly[9]. It is possible that the behavior may be explained by invoking the onset of defocusing effects due to the presence of free plasma electrons produced by multi-photon ionization. This will give a lower non-linear refractive index giving a larger value for P cr and should be highly nonlinear for our conditions (photon energy much lower than the band gap). The reason for the increasing gradients with increasing laser pulse width for ps pulses, seen in Fig., is also unclear. Further work is in progress to determine the underlying physical mechanisms for the observed behavior. We take the average of the P cr values found, using the above procedure, for pulse duration s greater than ps and use this to determine n. The results are summarized in Table for four different materials (a-sio, c-sio, c-al O 3, CaF ) and compared with literature values obtained by determination of beam distortion in the far field [37] and degenerate three-wave mixing. The results we obtain are in excellent agreement with the literature values. The only discrepancy we find is for a-sio. MATERIAL P cr / (MW) n / ( - m W - ) n / ( -3 esu) n / ( -3 esu) LIT. a-sio 3.6 ±..7 ±..6 ±..85 ±.3 [38].6 ±.5 (a) [37].6 ±.5 (b) [37] c-sio.85 ±. 3. ±.9. ±.7 o:. ±.7 [38] e:.6 ±.7 [38] c-al O 3. ±.. ±.. ±. o:.3 ±.8 [38] e:.3 ±. [38] CaF 5.5 ±.6.5 ±.3.39 ±.8.3 ±.6 [38]. Table : Critical self-focusing power thresholds P cr and Kerr coefficients n determined from straight line fits to plots of the inverse modification depth z M - vs. square root of the laser intensity P / (averaged over the results for pulse durations > ps) for different dielectric materials [9]. The last column compiles the Kerr coefficients n from the literature determined by beam deflection [37] {(a).6 µm; (b).53 µm} and by degenerative three-wave mixing [38] (.6 µm). o: ordinary and e: extraordinary orientation. The value we obtain is about a factor of.5 smaller than from degenerate three-wave mixing [38] but agrees excellently with the value obtained from the beam distortion measurements [37]. One possible explanation for the differences in the literature values for a-sio is that small Kerr contributions of resonant origin could increase the measured n reported from the three-wave mixing experiments which employed ns pulses [38]. If the bulk modification focus z M is moved close to the exit surface of the material it is possible to generate micro-structures on the exit surface without producing any visible damage in the bulk or at the entrance surface [,6]. The technique has a number of advantages compared to direct writing of structures by focusing on the entrance surfaces at fluences higher than the

15 damage threshold: (i) The material removal occurs in the same direction as the laser pulse and thus avoids deposition on the laser optics; (ii) much less energy is introduced to the material, minimising unwanted energy dissipation effects and damage that is perhaps not readily visible; and (iii) a relatively large beam spot size on the entrance surface can invoke structures with diameters on the order of µm on the exit surface, hence avoiding a plasma breakdown at the air-surface interface. Fig. 3 shows some examples of exit-surface structures. The ablated surface is generally extremely smooth and in some occasions even noticeably smoother than the original highly polished surface. Studies of the development of the hole structures with increasing number of laser shots indicate that significant stoichiometry changes are occurring on the surface before ablation begins []. This is much clearer than any indications of changes on the entrance surface close below the damage threshold. These phenomena are the subject of ongoing investigations. 5 µm µm AFM.6 µm 6.5 µm - 6 nm Fig. 3. Exit structures on sapphire (top) and CaF (bottom) mm thick (UV) window material after laser pulses at 79 nm and pulse durations around. ps. Laser beam was focused on the front side at a fluence of. J/cm (ca. 3 times below damage threshold). -. µm 3. CONCLUSION We have demonstrated the applicability of femtosecond laser systems for the micro machining of dielectric materials, showing different examples of highly localized and fairly stress-free micro structures generated in fused silica and sapphire, such as thin channels, periodic patterns, micro holes and bulk modifications. The surface damage threshold is not only related to the pulse duration but demonstrates strong laser shot number dependencies which we relate to an increase in the absorption cross section due to the accumulation of laser induced generation of defects and successfully describe with a very simple model. We have shown that it is possible to produce controlled micrometer-sized modifications in bulk transparent materials using ps laser pulses at laser fluence in the focused spot size at the entrance surface which are lower than the surface damage threshold. The size and depth of the structures can be varied by varying the laser pulse duration and pulse energy. The effects are shown to be due to selffocusing of the laser beam. The experimental determination of the modification depths as a function of the laser power provides a very straightforward method of obtaining the value of the non-linear Kerr coefficient, n which is independent of the laser focus size. We compare these results with those obtained by direct focusing of fs laser pulses inside the bulk material, where incubation and self focusing effect play a less important role.

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