Transient reflectivity and transmission changes during plasma formation and ablation in fused silica induced by femtosecond laser pulses

Transcription

1 Appl Phys A (2008) 92: DOI /s z Transient reflectivity and transmission changes during plasma formation and ablation in fused silica induced by femtosecond laser pulses D. Puerto W. Gawelda J. Siegel J. Bonse G. Bachelier J. Solis Received: 12 October 2007 / Accepted: 9 April 2008 / Published online: 27 May 2008 Springer-Verlag Berlin Heidelberg 2008 Abstract We have studied the plasma formation and ablation dynamics in fused silica upon irradiation with a single 120 fs laser pulse at 800 nm by using fs-resolved pumpprobe microscope. It allows recording images of the laserexcited surface at different time delays after the arrival of the pump pulse. This way, we can extract both the temporal evolution of the surface reflectivity and transmission, at 400 nm, for different spatial positions in the spots (and thus for different local fluences) from single series of images. At fluences well above the visible ablation threshold, a fast and large increase of the reflectivity is induced by the formation of a dense free-electron plasma. The maximum reflectivity value is reached within 1.5 ps, while the normalized transmission decreases within 400 fs. The subsequent temporal evolution of both transient reflectivity and transmission are consistent with the occurrence of surface ablation. In addition, the time-resolved images reveal the existence of a free-electron plasma distribution surrounding the visible ablation crater and thus formed at local fluences below the ablation threshold. The lifetime of this sub-ablation plasma D. Puerto ( ) W. Gawelda J. Siegel J. Solis Laser Processing Group, Instituto de Óptica, CSIC, Serrano 121, Madrid, Spain puerto@io.cfmac.csic.es J. Bonse Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Strasse 2A, Berlin, Germany G. Bachelier Laboratoire de Spectrométrie Ionique et Moléculaire, UMR 5579 CNRS Université Claude Bernard-Lyon 1, Bâtiment Alfred Kastler, 43 Boulevard du 11 Novembre 1918, Villeurbanne Cedex, France is 50 ps, and its maximum electron density amounts to cm 3. PACS Re Ce Mf Jm Ba Ds 1 Introduction During the last decade numerous research groups have studied the interaction of ultrashort laser pulses with materials in order to clarify the mechanisms underlying laserinduced structural modifications. Laser ablation is of particular interest due to its many fields of application such as microstructuring [1] or sub-surface nonlinear processing of dielectrics [2]. Information regarding its mechanisms has been obtained, for instance, by studying the influence of laser pulse duration [1, 3 7] and material parameters [6 10] on the ablation threshold, as well as dynamics of the ablation process [8 10]. In case of femtosecond (fs) laserdielectric interaction, ablation occurs as a consequence of a complex sequence of processes, triggered by multiphoton ionization, which generates seed electrons for other electron multiplying processes such as impact ionization. Very high carrier densities (>10 21 electrons/cm 3 ) may be induced, causing dielectric breakdown (formation of a dense absorbing plasma) and eventually leading to the ablation of a thin surface layer. We have studied the dynamics of plasma formation and ablation in fused silica. Using fs time-resolved microscopy, we can image the temporal evolution and spatial distribution of the material upon irradiation with single fs laser pulse. The same technique has been used very successfully

2 804 D. Puerto et al. in the pioneering work of von der Linde and Sokolowski- Tinten, e.g., [11], although mostly in metals and semiconductors, and only on a few occasions in dielectrics [8, 9]. In a recent work [12], we reported a study of the ablation process in fused silica using fs-resolved microscopy, in reflection mode. It was there shown that for fluences within a certain interval, a transient plasma distribution surrounding the visible ablation crater is formed. This sub-ablation plasma is accompanied by an increased reflectivity value after irradiation, consistent with a local increase of the material refractive index of In the present study, we have extended our previous investigation by employing additional measurements of optical transmission. In addition, we have estimated the electron density induced at different local fluences during the experiment by comparing the experimental values to calculations of normalized transmission and reflectivity obtained by using the Drude model for a free electron gas. 2 Experimental The fs time-resolved imaging apparatus used in this work is based on combination of a pump-probe technique and wide-field microscopy in order to obtain the temporal evolution of both the reflectivity and the transmission of the excited surface with high spatial resolution (approx. 1 µm). A schematic diagram of the experimental setup is depicted in Fig. 1. Further details on the reflection-mode of this fs microscope can be found elsewhere [13]. The pump-probe microscope employs a single 120 fs pump pulse (selected from 100 Hz pulse train by a synchronized electro-mechanical shutter) at 800 nm, which irradiates an unexposed surface region of the sample in air. The s-polarized pump laser beam is focused onto the sample at an angle of incidence of 54 to an elliptical spot size of 75.9 µm 50.8 µm(1/e 2 -diameter of a Gaussian intensity distribution). This Gaussian spot size has been obtained using the method described by Liu [14] and has been done separately in a low band gap material (Ge 2 Sb 2 Te 5 films with E g = 0.5 ev) in order to exclude nonlinear effects. At a variable and controllable time delay, a low-intensity probe pulse at 400 nm illuminates the sample surface at normal incidence. In the transmission measurements, we use a lens (f = 300 mm) to illuminate the sample from behind. The time delay between both pulses can be tuned by means of either a motorized translation stage in the pump path (up to 1 ns delay) or manually by extending the probe optical path (up to 22 ns). We use a long working distance microscope objective MO (20, N.A. = 0.42) and a tube lens to image the sample surface onto a CCD camera (12-bit). A bandpass filter centered at 400 nm (Filter3 in Fig. 1) isused to block the scattered pump light and optical plasma emission. The effective temporal response of the setup has been estimated to be 400 fs for transmission measurements and 800 fs for the reflectivity configuration. The reason for this Fig. 1 Femtosecond dual-mode wide-field pump-probe microscope. The reflection mode is drawn with solid lines, whereas the transmission configuration is sketched with dashed lines. Both pump and probe path lengths are synchronized by means of either manual or motorized delay stages. A polarizing beamsplitter (PBS1) reflects a small fraction of the fundamental beam into the probe beam, which is then frequency-doubled (400 nm). The sample rests on a three-axis motorized sample holder. Additional photodiodes (PD1-2) allow for precise pump fluence determination and further diagnostics. Filter1 removes the residual 800 nm light from the probe beam, whereas Filter2 blocks the reflected 400 nm from the pump beam

3 Transient reflectivity and transmission changes during plasma formation and ablation 805 Fig. 2 Upper row (a) (c): Time-resolved surface transmission images (λ probe = 400 nm) of fused silica at different delay times (see labels) after exposure to a pump pulse (λ pump = 800 nm, 120 fs, 11.9 J/cm 2 ). Lower row (d) (f): Time-resolved surface reflectivity images are recorded at the same experimental conditions as the transmission data. The intensity is encoded in a linear grey scale with an optimized contrast in each case. (g) Temporal evolution of the outer horizontal radii of the bright region (a, r b ) and dark region (", r d ) of the surface reflectivity images. The radius of the visible ablation crater (,r a ) is only pronounced at longer time delays in the ns range. The frame size of the images (a) (f) is µm 2 difference stems from the additional group velocity dispersion (GVD) experienced by the probe pulse inside the MO in reflection mode. The sample used in this study is fused silica ( Lithosil by Schott) with a measured band gap of 7.2 ev, similar to the one used in Ref. [12]. 3 Results and discussion The temporal evolution of reflectivity and transmission of the excited material upon a single pulse laser irradiation is shown in Fig. 2. Figures 2a c show transmission images of the sample surface at three different delays after irradiation with a pump pulse having a peak fluence of 11.9 J/cm 2,well above the ablation threshold. The corresponding surface reflectivity images (at the same time delays and pump fluence) are shown in Figs. 2d e. The images at t = 1.5 ps delay, i.e., Figs. 2a and f, show a strong decrease in transmission accompanied by a concomitant reflectivity increase within the irradiated region, which we attribute to the formation of a dense free-electron plasma at the surface [8 10]. At 10 ps delay, the reflectivity image (Fig. 2e) shows a central dark region, which indicates the occurrence of surface ablation [8, 9], whereas in transmission, the dark region shows only minor changes (Fig. 2b). Interestingly, in Fig. 2e, a bright and relatively wide outer ring persists, having a reflectivity higher than that of the unexposed surface. In contrast, the corresponding transmission image at 10 ps (Fig. 2b) shows no clear sign of two spatially distinct regimes within the irradiated region. In Figs. 2c and f, the images at t = (several seconds after the pump pulse) show the final visible ablation craters. A comparison of the images depicted in Figs. 2e f indicates the existence of a transient free-electron plasma region that extends beyond the visible surface ablation crater in agreement with thee results reported in Ref. [12]. The threshold fluence value for the formation of this free electron plasma is found to be F e-plasma = 4.9J/cm 2. In order to compare our fluences values to literature data, which generally refers to normal incidence of the laser beam, the higher Fresnel reflectivity due to the elevated angle of incidence (54 ) in our measurement has to be considered, leading to apparently higher threshold values. The correction factor for the elevated angle can be estimated [15] under our conditions as C angle = (1 R s (54 ))/(1 R s (0 )) Using this factor we obtain F e-plasma(0 ) = 4.4 J/cm 2 and thus consistent with the single-pulse ablation thresholds reported by Varel et al. [6] (5.8J/cm 2 using Nomarski microscopy and 5.3 J/cm 2 using plasma emission) and the one reported by Tien et al. [4] (3.7J/cm 2 using Nomarski microscopy). These findings are confirmed by analyzing the dependence of the horizontal radius r b of the bright region of increased reflectivity together with the horizontal radius r d of the dark inner region, both as a function of time delay. The results are

4 806 D. Puerto et al. plotted in Fig. 2g. We observe that r b remains approximately constant ( 24 µm) up to a few ns time delays. For longer delays, the observed ablation crater has a slightly smaller radius (r a 18 µm). In contrast, r d is formed not earlier than at t = 4 ps and extends rapidly towards the outer region, reaching its transient maximum extension (r d,max 19 µm) at t = 25 ps, before it shrinks again. The fact that r d,max is comparable to r a further supports the interpretation that the formation of the dark region in the reflectivity images is caused by surface ablation. The transmission images support this observation, although the optical contrast in less pronounced than in the case of the reflectivity. The comparison of spatial dimensions of the visible ablation crater after the irradiation (i.e., t = ) with the transient transmission image at 10 ps (see dashed horizontal lines in Figs. 2c and b) confirms that the latter one is larger and spatially overlaps with r b in the reflectivity images. Considering the Gaussian spatial intensity profile of the pump laser beam at the sample, we can extract from both sequences of images (Fig. 2) the temporal evolution of the reflectivity and the transmission, i.e., R(t) and T(t),fora given fluence. These results are shown in Figs. 3a b for local fluences of 11.9 and 6.4 J/cm 2, respectively. The corresponding spatial locations for both reflectivity and transmission measurements are indicated in the inset of Fig. 3. The transmission values shown in Figs. 3a b were normalized to 1, whereas the reflectivity is given in absolute units. At the peak fluence (11.9 J/cm 2 ), we observe a rapid five-fold increase of the reflectivity from R 0 = 0.036, corresponding to the unexposed material, to a maximum value R max = 0.23 at t = 1.5 ps. This is accompanied by a simultaneous decrease of the normalized transmission from T 0 = 1.0 tot min = 0.2 within 400 fs. The initial evolution of both transient signals is due to the same phenomenon, namely the formation of dense freeelectron plasma at the surface. The subsequent reflectivity decrease indicates a decrease of the plasma electron density due to expansion (ablation), reaching values below R 0 at 7 ps delay. This low reflectivity level (R = 0.01) remains constant between 10 and 500 ps and is related most likely to the shielding of the surface caused by the ablating material. After the initial ultrafast decrease, the normalized transmission signal shows no significant changes on a timescale up to 40 ps. At longer delays, the transient transmission gradually increases up to the initial level T 0 within approximately 2 ns (data not shown here). Similarly, the reflectivity recovers values near R 0 on a timescale of a few nanoseconds, suggesting that the ablation plume has sufficiently expanded and thus decreased its density, which facilitates probing of the visible ablation crater. The final recovery of reflectivity and transmission values very close to the initial ones is consistent with a shallow Fig. 3 Transient reflectivity and transmission curves extracted from time-resolved images at two different local fluences, namely 11.9 J/cm 2 (a) and 6.4 J/cm 2 (b), whose spatial positions are shown in the inset figure. Thehorizontal lines indicate the initial reflectivity (dashed) and normalized transmission (dash-dot) levels crater depth below 150 nm compared to a much larger horizontal crater diameter ( 36 µm) and relatively smooth final state of the surface. While the overall behavior at high-fluence excitation is consistent with previously reported results [8 10, 16] at lower local fluences, i.e., 6.4 J/cm 2 (Fig. 3b), we observe striking differences in the temporal evolution of both signals. Here, we clearly observe a longer rise time in the transient reflectivity signal, reaching R max = 0.1 in 3 ps. The subsequent reflectivity decay is slower at this fluence ( 17 ps), and no presence of reflectivity values below R 0 is detected, indicating the absence of surface ablation. The transient transmission decreases within 500 fs reaching the minimum T min = 0.4, and the subsequent recovery to T 0 is now shorter. In order to estimate the electron densities induced in our experiments, we have calculated the electron plasma reflectivity and transmission as a function of the electron density (n e ), using the Drude model for a free electron gas [17]. Considering the normal incidence of the probe beam, the reflectivity at the air dielectric interface for a given density of free carriers is given by the Fresnel equation [18]: R = k 1 k 2 2, (1) k 1 + k 2

5 Transient reflectivity and transmission changes during plasma formation and ablation 807 Fig. 4 Calculations of electron plasma normalized transmission (solid line) and reflectivity (dashed line) as a function of the electron density n e, in fused silica using a Drude model, i.e., see (1) (2). The solid circles indicate the calculated electron density, i.e., cm 3, corresponding to the experimentally observed reflectivity and transmission values, i.e., compare to Fig. 3b where k 1 and k 2 are given by k 2 1 = ω2 με = (nω/c) 2 and k 2 2 = (ω/c)2 [n 2 (ω p /ω) 2 (1 + i/ωτ) 1 ] and ω 2 p = n e e 2 /mε 0, with ω being the frequency of the probe beam, n = 1.47 the refractive index of the fused silica at the probe beam wavelength, c the vacuum speed of light, ε 0 is the vacuum dielectric permittivity, and m and e are the electron mass and charge, respectively. The damping constant τ represents the e e scattering time that has been reported to be inversely proportional to n e [19] (i.e.,τ = τ 1 n c /n e ). The value n c = ε 0 m ω 2 pump /e2 is the critical electron density related to the frequency of the pump beam ω pump, which in our conditions is n c = cm 3. On the other hand, the transmission of a dielectric with a given density of free carries is given by [20]: T = 4Re{ ε}/ ε (2) with ε = 1 [n e /n c (1 + i/ωτ)] being the complex Drude dielectric function [21]. The results of both simulations are shown in Fig. 4. The calculated values of the sample transmission were normalized to unity in order to allow for direct comparison with the experimental values, whereas the reflectivity was calculated in absolute units. Here we used τ 1 as a fitting parameter to obtain a maximum reflectivity value equal to R max (6.4 J/cm 2 ) = 0.1 at 3 ps. The obtained value is τ 1 = 1.76 fs. From Fig. 4 we can estimate the maximum electron density reached in the outer annular region, which is approximately cm 3 at a reflectivity value of R At this density, the calculated transmission (T 0.59) is consistent with the experimental value at t = 3ps(T 0.6). Moreover, the minimum observed in transmission (T 0.39) is well reproduced by the calculation (T 0.38). In order to describe the results of the peak transient reflectivity and transmission (11.9 J/cm 2 ), a τ 1 = 3.7 fs has to be used [12]. Still, in this fluence regime, the model might be too simple to provide reliable estimation of n e due to the complexity given by the ablation process. The decay time of the reflectivity in the outer region is not affected by ablation ( t 17 ps) and thus corresponds to the overall decay time of the dense electron plasma, including all possible relaxation channels, i.e., phonon emission and scattering, as well as electron trapping and exciton formation and nonradiative recombination [22 25]. Interestingly, we observe a slower recovery in the normalized transmission as compared to the reflectivity. This difference corresponds to the time required for the electron density to decrease from cm 3 (where the reflectivity is barely affected by the plasma whereas transmission is strongly affected (c.f. Fig. 4b) to cm 3 (where also stops being affected by the electron plasma). 4 Conclusions We have studied the temporal evolution of electron plasma formation and ablation in fused silica upon irradiation with single fs laser pulses. Our dual-mode setup allows recording both the surface transmission and reflectivity, which yields an important complementary information on the studied process. Moreover, we have quantified the lifetime and estimated the density of the transient electron plasma produced upon laser irradiation below the visible ablation threshold both from transient reflectivity and transmission measurements. Acknowledgements This work has been partially supported by the Spanish TEC project and by the EU in the frame of the TMR project FLASH (MRTN-CT ). D.P. acknowledges a grant of the Spanish Ministry of Science and Education. W.G. and J.B acknowledge the I3P-CSIC Program contracts (co-funded by the European Social Fund). G.B. acknowledges a contract in the frame of the TMR Project FLASH. We are grateful to C. Dorronsoro for his help with the image analysis. References 1. M. Lenzner, Int. J. Mod. Phys. B 13, 1559 (1999) 2. K. Hirao, T. Mitsuyu, J. Si, J. Qiu (eds.), Active Glass for Photonic Devices (Springer, Berlin, 2001) 3. D. Du, X. Liu, G. Korn, J. Squier, G. Mourou, Appl. Phys. Lett. 64, 3071 (1994) 4. A.-C. Tien, S. Backus, H. Kapteyn, M. Murnane, G. Mourou, Phys.Rev.Lett.82, 3883 (1999) 5. B.C. Stuart, M.D. Feit, A.M. Rubenchik, B.W. Shore, M.D. Perry, Phys.Rev.Lett.74, 2248 (1995) 6. H. Varel, D. Ashkenasi, A. Rosenfeld, R. Herrmann, F. Noack, E.E.B. Campbell, Appl. Phys. A: Mater. Sci. Process. 62, 293 (1996) 7. T.Q. Jia, Z.Z. Xu, R.X. Li, D.H. Feng, X.X. Li, C.F. Cheng, H.Y. Sun, N.S. Xu, H.Z. Wang, J. Appl. Phys. 95, 5166 (2004) 8. D.vonder Linde,H.Schueler,J.Opt.Soc.Am.B13, 2216 (1996)

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