Applied Surface Science

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1 Applied Surface Science 258 (2012) Contents lists available at SciVerse ScienceDirect Applied Surface Science j our nal ho me p age: ate/apsusc Plasma dynamics and structural modifications induced by femtosecond laser pulses in quartz J. Hernandez-Rueda a,, D. Puerto a,b, J. Siegel a, M. Galvan-Sosa a, J. Solis a a Laser Processing Group, Instituto de Óptica, C.S.I.C., Serrano 121, E Madrid, Spain b Nanophotonics Technology Center, Universitat Politècnica de València, Valencia, Spain a r t i c l e i n f o Article history: Available online 13 December 2011 Keywords: Pump-probe Quartz Femtosecond laser Plasma Dynamics Microscopy Fused silica a b s t r a c t We have investigated plasma formation and relaxation dynamics induced by single femtosecond laser pulses at the surface of crystalline SiO 2 (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 SiO 2 (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 Elsevier B.V. All rights reserved. 1. Introduction Structuring of dielectrics is becoming a key application of ultrashort laser pulses with numerous applications [1]. The basic idea is to exploit the large optical bandgap of the material with respect to the low photon energy of commercial infrared femtosecond lasers. In that way, only multi-photon and impact ionization are possible processes for laser energy to be deposited in the material, which enables particularly surface structuring with very fine details and sharp edges [2,3]. Despite this conceptually simple processing scheme, several limitations have been found in terms of surface roughness, topography and minimum feature size attainable, which depend on the specific material [4 6] and processing conditions used [7]. In this sense, it is clear that not only energy deposition plays an important part in surface processing but also the variety of energy relaxation channels that are specific to each material. A convenient way to monitor the laser-induced excitation/relaxation processes is the assessment of the free electron plasma formed upon energy deposition and its temporal evolution. Given the strong dependence of the optical properties of a dielectric material on the density of free electrons, optical pumpprobe techniques are ideal tools for this purpose, which have the Corresponding author. address: javihr@io.cfmac.csic.es (J. Hernandez-Rueda). added benefit to distinguish between free electron plasmas and plasmas produced upon surface ablation [8]. These techniques, originally single-point, have further evolved into pump-probe microscopy, providing additional information on the transient spatial electron distribution of an excited surface region [9,10]. This enables to spatially locate and temporally resolve a number of characteristic transient features and relate them to the final morphologies produced in a certain material. In particular, fs-microscopy is able to quantify local transient electron densities [10,11], distinguish regions of strong ablation from regions where Coulomb explosion occurs, and visualize the ejection of molten material [6]. This technique has been used to analyze the structural transformation dynamics upon fs laser surface irradiation in fused silica, where it was possible to identify a narrow local fluence window in which a transient plasma was formed below the ablation threshold, triggering local densification and surface depression of the material, which is accompanied by a local increase in refractive index [6,10,11]. These density and refractive changes are consistent with those observed focusing fs laser pulses inside fused silica as done in writing of embedded optical waveguides [12]. The present work focuses on investigating the plasma dynamics and material changes in the crystalline counterpart of fused silica, quartz. We have studied the temporal and spatial plasma evolution and related it to the surface topography measured after irradiation and the permanent changes in optical reflectivity. The results, clearly different from those observed in fused silica, are discussed /$ see front matter 2011 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 9390 J. Hernandez-Rueda et al. / Applied Surface Science 258 (2012) in terms of densification and defect formation. A new representation concept of the 4-D data stack obtained from fs microscopy (2-D spatial, temporal and reflectivity) is introduced to visualize subtle differences in the behavior of quartz and fused silica. 2. Experimental The sample used in this study is commercial quartz (crystalline SiO 2, VM-TIM Germany) with a bandgap of 8.4 ev. The laser irradiation and fs-microscopy setup is described in detail elsewhere [11]. Briefly, the sample is exposed in air to a single, s-polarized pump pulse (800 nm, 120 fs), incident at an angle of 53. The spot has a Gaussian intensity distribution and a 1/e 2 diameter of 66 m 37 m. Before reaching the sample, a fraction of the pump pulse is split off, frequency-doubled and injected into the illumination port of a home-built microscope, to provide homogeneous illumination (400 nm probe beam). A long-working distance objective lens (NA 0.42) and a tube lens form an image of the sample surface on the chip of a 12 bit CCD camera, which detects the reflected probe light. A narrow band-pass filter centered at 400 nm, blocking scattered pump light and plasma light emission, is placed in front of the CCD camera. The time delay between pump and probe pulses can be adjusted by means of a motorized optical delay line. A stack of images at different temporal delays is recorded, covering a delay range from 100 fs up to 10 ns. The temporal response of the system has been optimized by minimizing the probe pulse duration with a pulse prism compressor in the probe beam arm and using cross correlation measurements performed with a two-photon absorption InGaP detector located at the position of the sample plane. These measurements lead to an optimal cross correlation value of 200 fs (vs. the pump pulse duration of 120 fs). After irradiation, the sample has been inspected with a white light microscope (WLM) for analyzing local changes in surface reflectivity and an optical interferometric microscope (WLI) for studying the surface topography of the laser-irradiated regions. 3. Results and discussion Fig. 1 shows a set of time-resolved reflectivity images of the sample surface at different delay times t after the arrival of the pump laser pulse with a peak fluence of F = 7.5 J/cm 2. At t = 100 fs, a slight increase in surface reflectivity within the irradiated region can be observed. This increase is consistent with the formation of a free electron plasma, produced via multi-photon and impact ionization. At t 1 ps, the reflectivity increase reaches its peak value and the full spatial extension of the laser excited region can be appreciated. The reflectivity peaking at this delay, occurring much later than the arrival of the pump pulse, points to a very significant increase in the free carrier density occurring after the absorption of the pump pulse, which can only be interpreted in terms of a delayed impact ionization process in absence of inverse bremsstrahlung. It is worth recalling that the system temporal response has been optimized and characterized by cross-correlation to 200 fs. Recent calculations in dependence on the increasing free electron density, using a kinetic approach, support our findings and report a continuing increase in the free electron density due to impact ionization even after the end of the pulse [13]. A more detailed discussion about delayed impact ionization and plasma relaxation will be given when comparing the behavior of quartz and fused silica. At t = 10 ps, the central region of the excited area shows a pronounced drop in reflectivity, below the steady state reflectivity of the non-excited material. This feature is characteristic for the onset of surface ablation in dielectrics [6,14]. Surrounding the ablating region, a ring of increased reflectivity shows that in this region the free-electron plasma has not yet fully relaxed. 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 = ). The central part, in which ablation has occurred, has a dark, speckled appearance. In comparison, the annular region, which has only experienced plasma formation and relaxation without ablation, manifests as a grey ring, featuring a change in the local optical properties of the material. For quantitative data analysis and representation we have conceived a new representation scheme, capable of visualizing in a single graph all relevant information of the data stack obtained from fs microscopy (spatial resolution, temporal resolution and reflectivity). To this end, the transient images have been normalized with respect to a reference image recorded before irradiation and then multiplied by the steady-state reflectivity of quartz at the probe wavelength (R = 0.036). The signal-to-noise ratio of the data extracted from the images has been improved by performing radial averaging, taking into account the elliptic shape of the excited region. The so-obtained radial reflectivity profile for a given delay has been scaled to the long axis of the ellipse and represents a quantitative measurement of the transient spatial reflectivity distribution. The method to represent these multiple reflectivity profiles for a large number of delay values is to create a 2-D image, the vertical axis corresponding to the spatial position (radial profile of a single image), the horizontal axis to the temporal evolution (images at different delay times) and the pixel value in color scale to the reflectivity value at this position and delay. Such a graph is shown in Fig. 2 for the study performed in quartz. It shows that maximum reflectivity values of R = 0.24 are obtained, almost a factor of seven above the steady-state reflectivity, which confirms the existence of a very dense free electron plasma in the spot center within a time window of about 2 3 ps (red region). This dense plasma in the spot center is followed by an ablation process, lasting up to almost 10 ns and which ends in a permanent reflectivity decrease. The dashed horizontal lines mark the spatial extension of the ablation crater, thus confirming the existence of a less dense plasma outside the crater, having a longer lifetime than in the spot center (see also to Fig. 1, 10 ps). For comparison we have included in Fig. 2 the corresponding representation of the data earlier obtained in fused silica [10]. Overall, the spatial and temporal behaviors, as well as the maximum reflectivity values achieved are very similar. However, this representation allows visualizing subtle differences in the dynamics. For instance, the plasma lifetime outside the ablation crater is shorter in the case of quartz and the ablation process lasts longer and induces a significant permanent change in reflectivity. Two features of the carrier dynamics, which are common to both materials, deserve a further comment: the observation of a maximum reflectivity level delayed with respect to the absorption of the pump pulse, and the relatively slow carrier relaxation dynamics observed. These features have been observed before by us [6,10,11] and other authors [16] in several dielectrics, although a common model to explain them has not yet been proposed. A comparison of the present results to those provided for fused silica in Ref. [6] shows that in the sub-ablative regime (at the edge of the irradiated spot), the electronic density achieved in both silica and quartz is approximately cm 3. Surface ablation effects, as indicated in Ref. [6], impede performing an accurate determination of the maximum plasma density induced above the ablation threshold, which will most likely reach

3 J. Hernandez-Rueda et al. / Applied Surface Science 258 (2012) Fig. 1. Time-resolved reflectivity images of the surface of quartz upon irradiation with a single fs laser pulse (F = 7.5 J/cm 2 ) obtained with fs microscopy. The delay of the illumination pulse with respect to the pump pulse is indicates in the labels. The contrast of each image has been optimized individually. values of or above cm 3. In bulk semiconductors (including narrow and wide band-gap ones) it is well known that for carrier densities n e > cm 3, close to the Mott transition, Coulomb screening effects already start playing a determinant role in the carrier dynamics [17 19]. For a free electron density of cm 3 the Debye length ( D,e ) of the plasma (assuming a carrier kinetic energy of 10 ev) is less than 3 Å, reducing strongly the scattering rate of hot free and valence band electrons. As a consequence a strong slowdown in the rate of carrier generation by impact ionization would take place, enabling the hot free carriers to act as an energy reservoir, which will relax delayed with respect to the termination of the pump pulse for initial carrier densities above cm 3. The other consequence of strong Coulomb screening effects is the suppression of exciton formation as an (ultrafast) carrier energy relaxation mechanism [20]. This occurs when the average distance between particles becomes comparable to the exciton diameter (typically about 1 10 nm depending on its binding energy) and the mutual interactions lead to a screening of the Coulombian binding between electron and hole [18]. The blocking of (selftrapped) exciton formation for carrier densities above cm 3 has been recently observed by Grojo et al. [21] in fused silica. The authors observed a excited carrier population decay with a lifetime of 180 fs, consistent with the formation of self-trapped excitons (STE) and in agreement with Ref. [20], only for n e < cm 3. However, they observed much longer values (up to ns) when the injected carrier density exceeded cm 3. This result is consistent with our description of the process and clearly indicates that Coulomb screening effects play a crucial role for both impact ionization and carrier excitation/relaxation phenomena at electron densities above to cm 3. At even higher densities, in the ablative regime, other effects like band-gap shrinkage and Auger recombination will also contribute to the excited carrier dynamics [22]. In particular Auger recombination (with a recombination rate of a few ps) is expected to dominate for n e > cm 3. Above the ablation threshold both, the maximum reflectivity values observed (see above) and the plasma decay, are limited by ablation and a gradual decay is not observed. In order to correlate the information obtained from plasma dynamics with the surface changes induced in quartz we have performed a systematic measurement of the topography changes induced at increasing peak fluences. To this end we have fixed a delay time of t = 10 ps, which is optimum to distinguish between the ablating region and the sub-ablative plasma-excited region Fig. 2. Representation of the spatial plasma distribution (vertical axis) as a function of delay time (horizontal axis) for quartz at F = 7.5 J/cm 2 (left) and fused silica at F = 11.9 J/cm 2 (right), the latter obtained from the data published in Ref. [10]. The color scale indicates the reflectivity value at each point and delay. The method to obtain these representations from a data stack obtained from fs microscopy is explained in the text. The horizontal lines mark the spatial extension of the ablation crater. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

4 9392 J. Hernandez-Rueda et al. / Applied Surface Science 258 (2012) Fig. 3. Comparison of time-resolved reflectivity images at a delay t = 10 ps (top row), with the corresponding depth profiles of the irradiated regions (bottom row) for a fluence above the ablation threshold (left) and below the ablation threshold (right). and performed topography measurements of each spot. It is worth remarking at this point that we consider ablation to occur in a given spatial location if the reflectivity around this temporal delay falls below that of non-excited surface due to ejection of ionized material, following the definition of ablation onset given by Sokolowski-Tinten et al. [14]. Fig. 3 shows the results obtained for two selected fluences. For a fluence above ablation threshold, the time-resolved image shows the characteristic central dark disk due to ablation and a surrounding bright plasma ring below threshold. The corresponding depth profile demonstrates that the surface topography shows two regimes; a main crater with a depth of almost 40 nm, surrounded by a smaller surface depression in the region corresponding to sub-ablative plasma-excitation. For a fluence just below ablation threshold, the time-resolved image shows a plasma disk and the corresponding topography profile reveals that the plasma-induced surface depression is 15 nm. A similar plasma-induced surface depression has been reported for fused silica and was explained by local densification of the amorphous matrix [10]. In that case, it was possible to confirm this hypothesis by steady-state reflectivity measurements, yielding a local reflectivity increase of a few percent, in agreement with the increase in refractive index upon densification. For the present study in quartz we have also performed such measurements and the results are shown and compared to those obtained in fused silica in Fig. 4. It can be seen that also in quartz, there is a spatial coincidence of the spatially depressed region surrounding Fig. 4. Comparison of steady-state reflectivity images (R WLM) with the corresponding depth profile images (D WLI) (upper row) and the extracted cross sections of both (bottom row). The left column shows the data for quartz and the right one for fused silica, the latter obtained from the data published in Ref. [10]. The fluence values used in each case are those quoted in Fig. 2.

5 J. Hernandez-Rueda et al. / Applied Surface Science 258 (2012) the main crater with a region of change in reflectivity. However, as opposed to what is observed in fused silica, the reflectivity decreases strongly in this region, which could be explained by a refractive index decrease. Yet, if the mechanism induced would be a refractive index change alone, it would be in contradiction with the observed topography, which shows a depression. As a consequence, we interpret the experimental results as a combination of two effects: (a) Surface depression, caused by a gentle material removal process (Coulomb explosion) in a way similar to what has been suggested to occur in sapphire (Ref. [6]). (b) Local amorphization, induced underneath the layer removed by Coulomb explosion. The extraordinarily large decrease in the final reflectivity in this region ( R exp = 30%, c.f. Fig. 4) matches well the strong refractive index decrease for a crystalline to amorphous transition, which can be calculated as R amorphization = 24%, using the refractive indices at 460 nm (center wavelength of the white light microscope). Other structural changes, apart from amorphization, would not be able to cause such a strong change in reflectivity. Moreover, since amorphization by single fs laser pulses has been reported before inside bulk quartz [23], it is very likely that this transformation can also be induced at the surface of the material. 4. Conclusions We have investigated the plasma dynamics and material changes in quartz upon irradiation with single fs laser pulses. The results are in many aspects similar to those obtained in its amorphous counterpart, fused silica. In both materials, a transient plasma is produced at fluences below the ablation threshold leading to a surface depression. However, whereas in fused silica the surface reflectivity increases slightly due to a permanent local increase in the refractive index, in quartz the reflectivity decreases strongly, which we attribute local amorphization. The depression observed in quartz is explained by gentle material removal via Coulomb explosion as observed in other crystalline materials such as sapphire. As for the plasma dynamics, subtle differences are observed for the lifetime of the plasma-excited non-ablating region, being shorter in quartz than in fused silica, and for the duration of the ablating region, being longer in quartz. These and other, more subtle differences, can be appreciated by means of a new representation concept of the data stack obtained from fs microscopy, which enables visualizing the spatial distribution and temporal evolution of the plasma together with its corresponding reflectivity value in a single graph. Acknowledgements This work has been partially supported by the Spanish TEC project. J.H.-R. acknowledges a grant under the same project. D.P. acknowledges a grant of the Spanish Ministry of Science and Education under TEC References [1] H. Misawa, S. Juodkazis, 3D Laser Microfabrication, Wiley-VCH Verlag GmbH & Co., [2] A.P. Joglekar, H. Liu, E. Meyhöfer, G. Mourou, A.L. Hunt, Proc. NAS 101 (2004) [3] L. Englert, B. Rethfeld, L. Haag, M. Wollenhaupt, C. Sarpe-Tudoran, T. Baumert, Opt. Exp. 15 (2007) [4] F. Watanabe, D.G. Cahill, B. Gundrum, R.S. Averback, J. Appl. Phys. 100 (2006) [5] A. Ben-Yakar, A. Harkin, J. Ashmore, R.L. Byer, H.A. Stone, J. Phys. D 40 (2007) [6] D. Puerto, J. Siegel, W. Gawelda, M. Galvan-Sosa, L. Ehrentraut, J. Bonse, J. Solis, J. Opt. Soc. Am. B 27 (2010) [7] D. Wortmann, J. Gottmann, Appl. Phys. A 93 (2008) 197. [8] I.H. Chowdhury, A.Q. Wu, X. Xu, A.M. Weiner, Appl Phys. A 81 (2005) [9] D. von der Linde, H. Schueler, J. Opt. Soc. Am. B 13 (1996) [10] J. Siegel, D. Puerto, W. Gawelda, G. Bachelier, J. Solis, L. Ehrentraut, J. Bonse, Appl. Phys. Lett. 91 (2007) [11] D. Puerto, W. Gawelda, J. Siegel, J. Bonse, G. Bachelier, J. Solis, Appl. Phys. A 92 (2008) [12] K.M. Davis, K. Miura, N. Sugimoto, K. Hirao, Opt. Lett. 21 (1996) [13] B. Rethfeld, H. Krutsch, D.H.H. Hoffmann, Contrib. Plasma Phys. 50 (2010) [14] K. Sokolowski-Tinten, J. Bialkowski, A. Cavalleri, M. Boing, H. Schueler, D. von der Linde, Proc. SPIE 3343 (1998) 46. [15] A. Rosenfeld, D. Ashkenazi, H. Varel, M.##E.E. Wähmer, B. Campbell, Appl. Surf. Sci (1998) [16] I.H. Chowdhury, A.Q. Wu, X. Xu, A.M. Weiner, Appl. Phys. A 81 (2005) [17] G.B. Norris, K.K. Baraj, Phys. Rev. B 26 (1982) [18] P. Nemec, P. Maly, M. Niki, K. Nitsch, Appl. Phys. Lett. 76 (2000) [19] R. Huber, F. Tauser, A. Brodschelm, M. Bichler, G. Abstriter, A. Leitenstorfer, J. Lumin (2001) [20] F. Quéré, S. Guizard, P. Martin, G. Petite, O. Gobert, P. Maynadier, M. Perdix, Appl. Phys. B 68 (1999) [21] D. Grojo, M. Gertsvolf, S. Lei, T. Barillot, D.M. Rayner, P.B. Corkum, Phys. Rev. B 81 (2010) [22] J.P. Callan, A.M.T. Kim, L. Huang, E. Mazur, Chem. Phys. 251 (2000) [23] T. Gorelik, M. Will, S. Nolte, A. Tuennermann, U. Glatzel, Appl. Phys. A 76 (2003) 309.