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Measurement of Femtosecond Laser-Induced Damage and Ablation Thresholds in Dielectrics

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Measurement of Femtosecond Laser-Induced Damage and Ablation Thresholds in Dielectrics Appl Phys A DOI 10.1007/s00339-009-5077-6 Measurement of femtosecond laser-induced damage and ablation thresholds in dielectrics N. Sanner · O. Utéza · B. Bussiere · G. Coustillier · A. Leray · T. Itina · M. Sentis Received: 2 September 2008 / Accepted: 15 December 2008 © Springer-Verlag 2009 Abstract The paper is focused on the importance of ac- peculiarity of using femtosecond pulses for processing ma- curate determination of surface damage/ablation thresh- terials is the possibility to separate in time the energy depo- old of a dielectric material irradiated with femtosecond sition (heating of electrons during the laser pulse) and the laser pulses. We show that different damage characteriza- damage appearance (energy relaxation occurring after the tion techniques and data treatment procedures from a single pulse). The energy is deposited in the material by nonlin- experiment provide complementary physical results char- ear absorption of photons via multiphoton or tunneling ef- acterizing laser–matter interaction. We thus compare and fects, followed by an avalanche mechanism leading to strong discuss two regression techniques, well adapted to the ionization. This free-electron plasma initially enhances light measurement of laser ablation threshold, and a statistical absorption, until a critical density is reached leading to a approach giving the laser damage threshold and further in- metal-like behavior of the dielectric. Then electron transport formation concerning the deterministic character of fem- and different energy relaxation channels eventually lead- tosecond damage. These two measurements are crucial for ing to damage and ablation (plasma expansion and/or mat- laser micromachining processes and high peak-power laser ter vaporization) are likely to occur in a quite complex technology in general. combination. The precise knowledge of these processes is crucial for the comprehensive understanding of the exper- · · PACS 42.70.CE 61.80.Ba 42.62.Eh imental observations and for predicting the damage and/or ablation thresholds of a material in the frame of micro- machining process development and high peak-power laser 1 Introduction technology in general. Part of the answer could be provided by parametric studies of laser-induced damage (and/or ab- Femtosecond lasers are unique tools for micro- (nano-) lation) thresholds (LIDT/LIAT) with various experimental machining materials like transparent dielectrics, providing conditions (wavelength, pulse duration, polarization, num- benefits in terms of minimal invasiveness (reduced “Heat- Affected Zone”) and precision over longer laser pulses. The ber of pulses, material bandgap). However, even for similar experiments from different authors [1–6] reporting surface LIDT/LIAT measurements for fused silica samples irradi- N. Sanner () · O. Utéza · B. Bussiere · G. Coustillier · ated with single shot, ∼100 fs, 800 nm pulse, a large scat- A. Leray · T. Itina · M. Sentis tering of results, from 2 to 12 J/cm2, exists. Unfortunately, Laboratoire LP3, UMR 6182 CNRS—Université the dispersion of these measurements (F ≈ 10 J/cm2) is de la Méditerranée, Campus de Luminy, case 917, 13288 Marseille cedex 9, France largely superior to the absolute surface LIDT/LIAT value, e-mail: [email protected] thus preventing the precise determination of accurate data. Fax: +33-4-91829289 This issue is even more critical in the context of femtosecond laser-dielectric interaction as it is supposed to be extremely B. Bussiere Amplitude Technologies, 2 rue du Bois Chaland, CE2926, deterministic owing to its highly nonlinear nature [7]. In ad- 91029 Evry cedex, France dition, this particular feature is one of the main reasons why N. Sanner et al. ultrashort material processing is now acknowledged as a rel- evant technology for applications requiring a high level of accuracy. In particular, the spatial extent of the processed zone can be limited to the laser spot area for which the local fluence exceeds the material threshold, enabling to reach [8] or even beat [7] the diffraction-limited beam surface when the LIDT is surpassed only in the central region of the fo- cused Gaussian beam distribution. This sharp ‘threshold’ ef- fect, arising from the highly nonlinear character of absorp- tion, is of prime interest for emerging nanomorphing appli- Fig. 1 Experimental setup. M: mirrors; Pol.: reflective Brewster po- cations, and is only obtained for pulse energies very close larizer; BD: beam dump; L:lensf = 100 mm to the material ablation threshold. The precise and reliable determination of material thresholds is then a crucial issue 2 Experimental setup for both fundamental and applicative breakthroughs. The problem of laser damage and ablation measurement The experimental setup is presented in Fig. 1.Thelaser in femtosecond regime has already been addressed in the source is a commercial S-pulse system from Amplitude Sys- literature. There are ex situ investigations of the diameter, tèmes, delivering 450 fs (controlled by a second-order auto- depth and morphology of damages by AFM [9], SEM [3], correlator), 1 kHz, 200 µJ pulses at 1025 nm. The intensity optical miscroscopy [10] or profilometry [11]. On the other distribution is Gaussian with a M2 factor equal to 1.3. The 2 hand, a multitude of in situ procedures are applied like time- M value was determined by studying the beam propaga- = of-flight [12], light scattering [13], time-resolved plasma tion with a long focal lens (f 300 mm) and a CCD Spiri- formation [14], time-resolved interference [15, 16], plasma con beam analyzer. The beam is linearly polarized, allow- radiation [17, 18], or transient reflectivity [9, 14]. Never- ing simple energy adjustment by means of a half-wave plate combined with a reflective polarizer used at the Brewster theless, there is no general agreement on the definition of angle. The incident beam on the sample is then s-polarized, thresholds (melting, damage, and ablation), and on the meth- whatever the tuning of energy. For small values of energy, ods of measurements (with their different detection limits). calibrated neutral density filters are added, in order to benefit Moreover, one observes a large variety of experimental se- from small and precise energy increment which is required tups, laser beam and material parameters (chemical material for an accurate LIDT determination. The beam is expanded composition, surface state, etc.). As a consequence, it ap- with an afocal system providing a beam radius w = 4mm pears unavoidable to observe a large scattering of threshold at 1/e2, and is focused onto the surface of the sample with a values, which makes the comparisons between experiments standard plano-convex BK7 lens of 100 mm focal length. very delicate. The target consists of the most widely studied transpar- In this paper, we present different techniques for sur- ent dielectric material, i.e. fused silica (Heraeus HOQ310, face LIDT/LIAT measurements used in the experiments thickness 2 mm and diameter 25 mm). To compare the sur- with femtosecond laser pulses. The proposed techniques are face LIDT/LIAT results deduced from the three methods based on a single experimental setup but different post- presented below, and because these thresholds may depend mortem analysis and data treatment. Assumptions respon- on surface imperfections (scratches, cracks, grooves, etc.), sible for systematic errors are considered. We show that in- roughness, exact chemical composition or contamination, all formation on laser ablation and/or laser damage threshold experiments are performed on the same SiO2 sample pol- ished with standard optical quality. The sample is mounted of a material are preferentially obtained, depending on the on a three-axis computer-controlled translation stage and its applied technique and procedure of treatment of the experi- position is carefully adjusted by combined energy-scan and mental data. z-scan procedures, allowing to precisely locate the surface The paper is organized as follows. Section 2 describes at the waist position with an accuracy much better than the the setup configuration including precise description of the Rayleigh range, ensuring the accuracy of the measurements. laser source, the diagnostics and the experimental protocol. As an example, for the 100 mm focal lens used in this ex- The appropriate definitions of material damage and abla- periment, the half Rayleigh range is ∼350 µm and the final tion thresholds are discussed in the same section. Then we z-scan step amounts to 10 µm. A far-field imaging system is present in Sect. 3 the measurement and data exploitation also implemented. This system consists of the focusing op- techniques for surface LIDT/LIAT determination, which are tics itself combined with a CCD camera and its objective, compared and discussed in Sect. 4. providing real-time visualization of the target surface with Measurement of femtosecond laser-induced damage and ablation thresholds in dielectrics a high magnification. The target surface is illuminated with tor is measured and subtracted from the recovered single- incoherent grazing red light (Stocker Yale Specbright LED shot amplifier energy value. Note the pulse energy ratio, −9 spotlight centered at 630 nm), so that the imaging system E1shot,oscillator/Esingle-shot,amplifier ∼ 10 , is largely inferior is the equivalent of an in situ dark-field microscope, which to any material modification threshold, even under multi- collects the reflected and scattered light coming from per- shot irradiation [13, 19]. The pre- and post-pulses are mini-
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