Single- and multi-pulse femtosecond laser ablation of optical filter materials

Applied Surface Science 208±209 (2003) 233±237

Single- and multi-pulse femtosecond laser ablation of optical ®lter materials J. KruÈgera,*, M. Lenznera,1, S. Martina, M. Lennerb, C. Spielmannb,2, A. Fiedlerc, W. Kauteka a

Laboratory for Thin Film Technology, Federal Institute for Materials Research and Testing, Unter den Eichen 87, D-12205 Berlin, Germany b Photonics Institute, Vienna University of Technology, Gusshausstr. 27±29, A-1040 Vienna, Austria c Rupp ‡ Hubrach Inferoptics/Laserschutz GmbH, Laubanger 18, D-96052 Bamberg, Germany

Abstract Ablation experiments employing Ti:sapphire laser pulses with durations from 30 to 340 fs (centre wavelength 800 nm, repetition rate 1 kHz) were performed in air. Absorbing ®lters (Schott BG18 and BG36) served as targets. The direct focusing technique was used under single- and multi-pulse irradiation conditions. Ablation threshold ¯uences were determined from a semi-logarithmic plot of the ablation crater diameter versus laser ¯uence. The threshold ¯uence decreases for a shorter pulse duration and an increasing number of pulses. The multi-pulse ablation threshold ¯uences are similar to those of undoped glass material (1 J cm 2). That means that the multi-pulse ablation threshold is independent on the doping level of the ®lters. For more than 100 pulses per spot and all pulse durations applied, the threshold ¯uence is practically constant. This leads to technically relevant ablation threshold values. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Damage; Femtosecond laser ablation; Filter; Incubation; Laser safety; Threshold

1. Introduction The application of ultrashort (subpicosecond) laser pulses for micromachining of different materials shows two major advantages compared to longer pulses: (1) the reduction of the laser ¯uence which is necessary to induce ablation (for ®xed laser wavelength and focusing conditions) [1] and (2) the improvement of the *

Corresponding author. Tel.: ‡49-30-8104-3567; fax: ‡49-30-8104-1827. E-mail address: [email protected] (J. KruÈger). 1 Present address: Katana Technologies GmbH, Albert-EinsteinRing 7, D-14532 Kleinmachnow, Germany. 2 Present address: Department of Physics, University of WuÈrzburg, Am Hubland, D-97074 WuÈrzburg, Germany.

contour sharpness of the laser-generated structures [2]. Therefore, femtosecond laser pulses offer great potential for industrial and medical applications [3]. A ®rst practical use in industry for the trimming of mask defects was reported [4,5]. Laser safety, especially the protection of the human eye and skin, is a prerequisite for an increasing spread of this laser type in factories. Up to now, no values of the maximum permissible exposure (MPE) for the utilization of ultrashort laser pulses are available. Only a few femtosecond pulse laser investigations were performed to test ®lter materials normally used for the protection of the eye against nanosecond laser illumination [6]. Highly absorbing ®lters with glass as the matrix material play a decisive role as the main part of protec-

0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(02)01389-2

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tion equipment (e.g. in laser goggles). In this paper, ablation experiments down to a pulse duration of 30 fs with two types of protection ®lters are presented. The dependence of the ablation threshold ¯uence on pulse duration and number of pulses per spot is investigated. 2. Experimental A Ti:sapphire femtosecond laser (FEMTOPOWER, Femtolasers) was utilized for the ablation experiments in air. It emits 800 nm centre-wavelength pulses with a bandwidth-limited duration of 30 fs at a repetition rate of 1 kHz. Longer pulse durations up to 340 fs were generated by insertion of additional glass material in the beam path maintaining the 30 fs pulse spectrum. The pulse spectrum was determined with a S2000 spectrometer (Ocean Optics) and the pulse duration was measured by means of a dispersion-minimised autocorrelator (Femtolasers). Laser pulse energies were varied using a rotatable half-wave plate in front of the compressor unit of the FEMTOPOWER. The Brewster prisms of the compressor acted as analyser. The pulse energy was measured employing a power meter model 407A (Spectra Physics) and a pyroelectric detector PE-10 with a display unit Nova (Ophir) in the multi- and single-pulse case, respectively. The number of laser pulses per spot was controlled by a pulse counter (Femtolasers) between 1 and 10,000. The target was mounted on a motorized x±y±ztranslation stage (LOT Oriel). The surface of the sample was positioned perpendicular to the direction of the incident laser beam in the focal plane of a silver mirror with a focal length of 500 mm. A Gaussian spatial beam pro®le with a radius (1/e2) of 30 mm in the spot was achieved. Commercially available optical ®lter materials Schott BG18 (thickness 5.3 mm) and Schott BG36 (thickness 2.0 mm) served as targets.

Fig. 1. Light transmission vs. wavelength for the ®lter materials Schott BG18 (solid line) and Schott BG36 (dashed line). The thicknesses of the ®lters are 5.3 mm (BG18) and 2.0 mm (BG36), respectively.

(wavelength of the Ti:sapphire laser). Applying Lambert±Beer's law, one obtains absorption coef®cients of a > 22 cm 1 (BG18) and a ˆ 29 cm 1 (BG36), respectively. At l ˆ 800 nm, a sequence of two BG36 ®lters results in a transmission <10 5 which would make it a candidate for laser protection systems like BG18. However, for the utilization of the very short pulses with a duration of 30 fs, the absorption window of BG36 is (in contrast to BG18) too narrow for a reliable protection against the laser light showing a bandwidth of 50 nm. A transmission of about 5% through a 4 mm thick BG36 ®lter can be found at l ˆ 775 nm and l ˆ 835 nm, respectively (Fig. 2).

3. Results and discussion For the samples Schott BG18 and Schott BG36, the (small-signal) ®lter transmission in dependence on the wavelength is depicted in Fig. 1. Both ®lters exhibit a comparatively low transmission at l ˆ 800 nm

Fig. 2. Emission spectrum of the 30 fs laser (solid line) and transmission spectrum after passing a 4 mm thick Schott BG36 ®lter (dashed line).

J. KruÈger et al. / Applied Surface Science 208±209 (2003) 233±237

Fig. 3. Scheme of laser-induced damage on the samples (top) and corresponding spatial Gaussian ¯uence pro®le along x-axis (bottom). F0 denotes the maximum laser ¯uence, Fth is the ablation threshold ¯uence and D marks the diameter of the ablated area.

In the context of this paper, damage is a synonym for ablation. For the ®lter materials, the measurement of the diameters D of the laser-damaged areas (Fig. 3) was done with an optical microscope (Reichert-Jung, Polyvar). The laser ¯uence suf®cient to ablate the material (for a ®xed pulse duration t and number of pulses per spot N) is named threshold ¯uence Fth. The relative error of the ¯uence-determination amounts to 20%. A one-dimensional spatial Gaussian ¯uence distribution F(x) according to Fig. 3 can be written as   2x2 F…x† ˆ F0 exp (1) w2

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Fig. 4. Squared diameter (D2) vs. maximum laser ¯uence F0 for the ablation of Schott BG18: t ˆ 30 fs, N ˆ 100. The low-¯uence data (full symbols) were used for the ®t (straight line).

yields a Gaussian beam radius of w ˆ 25 mm. The extrapolation to D2 ˆ 0 delivers a damage threshold ¯uence of Fth ˆ 0:8 J cm 2. Only the low-¯uence data points (full symbols) were utilized for the determination of w and Fth. For the high-¯uence values (hollow symbols), a deviation of the crater diameters from the linear regression is observed and may be due to a variation of the spatial beam pro®le from an ideal Gaussian one. For both ®lter materials, series of ablation experiments were performed with a varying number of pulses per spot N, keeping the pulse duration t constant (Figs. 5 and 6). For all pulse durations in the range between 30 and 340 fs, the ablation threshold ¯uence decreases with increasing number of pulses

with the Gaussian beam radius w. With F…x ˆ D=2† ˆ Fth , Eq. (1) can be transformed into a relation between the crater diameter D and the maximum laser ¯uence F0:   F0 D2 ˆ 2w2 ln (2) Fth Eqs. (1) and (2) are also valid if the laser ¯uence F is replaced by the pulse energy E. Taking into account Eq. (2), ablation threshold ¯uences Fth were obtained from a semi-logarithmic plot of the squared diameter of the ablated area versus laser ¯uence. As an example, Fig. 4 shows the result for the treatment of Schott BG18 with 30 fs pulses and 100 shots per spot. The slope of the straight line

Fig. 5. Ablation threshold ¯uence Fth vs. number of pulses N for the ablation of Schott BG18. Parameter: pulse duration t.

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Fig. 6. Ablation threshold ¯uence Fth vs. number of pulses N for the ablation of Schott BG36. Parameter: pulse duration t.

Fig. 7. Ablation threshold ¯uence Fth vs. pulse duration t for the ablation of Schott BG18. Parameter: number of pulses N.

per spot. This behaviour can be attributed to incubation, i.e. the material undergoes a modi®cation during the ®rst pulses and is ablated by subsequent illumination [3]. For Schott BG18, the ablation threshold changes by a factor of 2 during the ®rst 100 pulses (Fig. 5). For N > 100, a saturation of the Fth(N)-dependence can be noted at ¯uences of the order of 1 J cm 2. The ®lter Schott BG36 shows a performance similar to that of BG18 (Fig. 6). In contrast to BG18, the spreading of the single-pulse thresholds is larger and reaches from 1.8 J cm 2 (30 fs) to 5.9 J cm 2 (340 fs). For N > 100 pulses, again the threshold ¯uence tends to a unique limit of 1 J cm 2. Technically relevant multi-pulse ablation threshold values of the order of 1 J cm 2 could be determined for both absorbing ®lters and a pulse duration range from 30 to 340 fs. The relation between ablation threshold ¯uence Fth and pulse duration t is depicted in Fig. 7 (Schott BG18) and Fig. 8 (Schott BG36). For t < 230 fs, an increasing ablation threshold ¯uence was found with rising pulse duration for both materials. An analogous dependency was reported for the ablation of transparent dielectrics [7±9]. For these samples, laser-induced breakdown was discussed in such a manner that a critical electron density in the conduction band has to be reached for damage [10]. Two processes, avalanche and multiphoton ionisation, lead to the accumulation of electrons [11]. A shorter pulse duration results in an enhanced probability for multiphoton absorption. Therefore, the seed electrons for

the avalanche ionisation are generated more deterministically resulting in a lower ablation threshold. For Schott BG18 and t > 230 fs, the ablation threshold drops with increasing pulse duration in contrast to Schott BG36. The falling threshold with increasing pulse duration cannot be understood in terms of the above-mentioned argumentation. A speculative explanation could be a formation of defect states with a minimum generation time [12]. If the pulse duration exceeds this time, they can be produced during the laser pulse. These defects with energy levels within the bandgap give rise to ef®cient linear absorption that takes place for the remaining pulse. A comparison of ablation threshold values of absorbing ®lters with those of transparent dielectrics [8] is presented in Fig. 9. Taking into account different N, the ablation thresholds of the absorbing ®lters do

Fig. 8. Ablation threshold ¯uence Fth vs. pulse duration t for the ablation of Schott BG36. Parameter: number of pulses N.

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transparent glass suggesting a dominant effect of the matrix of the ®lters for the damage behaviour. Acknowledgements

Fig. 9. Ablation threshold ¯uence Fth vs. pulse duration t for the ablation of barium aluminium borosilicate glass (Corning# 7059) [8], Schott BG18 and Schott BG36. Parameter: number of pulses N.

not differ from those of a transparent material. This ®nding leads to the conclusion that the (transparent) silicate glass matrix of the ®lters [13] plays the key role for the ablation behaviour. 4. Conclusions Ablation experiments with Ti:sapphire laser pulses (30±340 fs, 800 nm, 1 kHz) on target materials Schott BG18 and Schott BG36 were conducted in air. The ablation threshold ¯uence decreases with increasing number of pulses per spot due to incubation. The ®lter material Schott BG18 exhibits threshold ¯uences for one pulse per spot between 1.3 J cm 2 (30 fs) and 2.2 J cm 2 (340 fs). For Schott BG36, single-pulse thresholds ranging from 1.8 Jcm 2 (30 fs) to 5.9 J cm 2 (340 fs) are observed. Both ®lters show technically relevant multi-pulse ablation threshold values of the order of 1 J cm 2 for pulse durations between 30 and 340 fs. Absorbing ®lters feature nearly equal ablation thresholds like a

We gratefully acknowledge ®nancial support by the German Federal Ministry of Education and Research in the framework of the project ``Safety for Applications of Femtosecond Laser Technology'' (SAFEST) (BMBF-Projektverband Femtosekundentechnologie). M. Lenner and C. Spielmann would like to thank Femtolasers Produktions GmbH for bene®t. References [1] S. KuÈper, M. Stuke, Appl. Phys. B 44 (1987) 199. [2] R. Srinivasan, E. Sutcliffe, B. Braren, Appl. Phys. Lett. 51 (1987) 1285. [3] D. BaÈuerle, Laser Processing and Chemistry, third ed., Springer, Berlin, 2000. [4] R. Haight, D. Hayden, P. Longo, T. Neary, A. Wagner, Proc. SPIE 3546 (1998) 477. [5] Y. Shani, I. Melnick, S. Yoffe, Y. Sharon, K. Lieberman, H. Terkel, Proc. SPIE 3546 (1998) 112. [6] W. Koschinski, A. Schirmacher, E. Sutter, J. Laser Appl. 10 (1998) 126. [7] W. Kautek, J. KruÈger, M. Lenzner, S. Sartania, C. Spielmann, F. Krausz, Appl. Phys. Lett. 69 (1996) 3146. [8] M. Lenzner, J. KruÈger, S. Sartania, Z. Cheng, C. Spielmann, G. Mourou, W. Kautek, F. Krausz, Phys. Rev. Lett. 80 (1998) 4076. [9] A.-C. Tien, S. Backus, H. Kapteyn, M. Murnane, G. Mourou, Phys. Rev. Lett. 82 (1999) 3883. [10] N. Bloembergen, IEEE J. Quant. Electr. QE-10 (1974) 375. [11] B.C. Stuart, M.D. Feit, A.M. Rubenchik, B.W. Shore, M.D. Perry, Phys. Rev. Lett. 74 (1995) 2248. [12] D. Ashkenasi, H. Varel, A. Rosenfeld, F. Noack, E.E.B. Campbell, Nucl. Instrum. Methods Phys. Res., B 122 (1997) 359. [13] W. Vogel, Glaschemie, 3. Au¯age, Springer, Berlin, 1992 (in German).