In-situ femtosecond laser pulse characterization and ... - OSA Publishing

In-situ femtosecond laser pulse characterization and compression during micromachining Xin Zhu, Tissa C. Gunaratne, Vadim V. Lozovoy and Marcos Dantus Department of Chemistry, Michigan State University, East Lansing, MI 48824 [email protected]

Abstract: We report on phase measurements and adaptive phase distortion compensation of femtosecond pulses using multiphoton intrapulse interference phase scan (MIIPS) based on second harmonic generation in the plasma generated on the surface of silicon and metals. ©2007 Optical Society of America OCIS codes: (320.5540) Pulse Shaping; (320.7100) Ultrafast Measurement; (320.7110) Ultrafast Nonlinear optics.

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V. V. Lozovoy, and M. Dantus, "Coherent control in femtochemistry," Chemphyschem 6, 1970-2000 (2005). R. Trebino, and D. J. Kane, "Using Phase Retrieval to measure the intensity and phase of ultrashort pulses frequency-resolved optical gating," J. Opt. Soc. Am. A-Opt. Image Sci. Vis. 10, 1101-1111 (1993). C. Iaconis, and I. A. Walmsley, "Spectral phase interferometry for direct electric-field reconstruction of ultrashort optical pulses," Opt. Lett. 23, 792-794 (1998). B. von Vacano, T. Buckup, and M. Motzkus, "Shaper-assisted collinear SPIDER: fast and simple broadband pulse compression in nonlinear microscopy," J. Opt. Soc. Am. B-Opt. Phys. 24, 1091-1100 (2007). I. Pastirk, B. Resan, A. Fry, J. MacKay, and M. Dantus, "No loss spectral phase correction and arbitrary phase shaping of regeneratively amplified femtosecond pulses using MIIPS," Opt. Express 14, 9537-9543 (2006). B. W. Xu, J. M. Gunn, J. M. Dela Cruz, V. V. Lozovoy, and M. Dantus, "Quantitative investigation of the multiphoton intrapulse interference phase scan method for simultaneous phase measurement and compensation of femtosecond laser pulses," J. Opt. Soc. Am. B-Opt. Phys. 23, 750-759 (2006). M. Dantus, V. V. Lozovoy, and I. Pastirk, "MIIPS characterize and corrects femtosecond pulses," Laser Focus World 43, 101 (2007). A. M. Weiner, D. E. Leaird, J. S. Patel, and J. R. Wullert, "Programmable Shaping of Femtosecond Optical Pulses by Use of 128-Element Liquid-Crystal Phase Modulator," IEEE J. Quantum Electron. 28, 908-920 (1992). V. Hommes, M. Miclea, and R. Hergenroder, "Silicon surface morphology study after exposure to tailored femtosecond pulses," App. Surf. Sci. 252, 7449-7460 (2006). R. Stoian, M. Boyle, A. Thoss, A. Rosenfeld, G. Korn, and I. V. Hertel, "Dynamic temporal pulse shaping in advanced ultrafast laser material processing," App. Phys. A-Materials Science & Processing 77, 265-269 (2003). T. C. Gunaratne, X. Zhu, R. Amin, V. V. Lozovoy, and M. Dantus, "Influence of femtosecond pulse shaping on silicon micromachining monitored by laser induced breakdown spectroscopy and surface second harmonic generation," Phys. Rev. B., (in preparation) (2007). R. W. Terhune, P. D. Maker, and C. M. Savage, "Optical Harmonic Generation in Calcite," Phys. Rev. Lett. 8, 404-& (1962). P. S. Pershan, "Nonlinear Optical Properties of Solids - Energy Considerations," Phys. Rev. 130, 919-& (1963). E. Adler, "Nonlinear Optical Frequency Polarization in Dielectric," Physical Review a-General Physics 134, A728-& (1964). N. Bloembergen, "Wave Propagation in Nonlinear Electromagnetic Media," Proc. IEEE 51, 124-& (1963). N. Bloembergen, and Y. R. Shen, "Optical Nonlinearities of a plasma," Phys. Rev. 141, 298-305 (1966). N. G. Basov, V. Y. Bychenkov, O. N. Krokhin, M. V. Osipov, A. A. Rupasov, V. P. Silin, G. V. Sklizkov, A. N. Starodub, V. T. Tikhoncchuk, and A. S. Shikanov, "Second harmonic generation in a laser plasma," Sov. J. Quantum Electron 9, 1081-1102 (1979). D. von der Linde, H. Schulz, T. Engers, and H. Schuler, "Second harmonic generation in plasmas produced by intense femtosecond laser pulses," IEEE J. Quantum Electron. 28, 2388-2397 (1992). T. Engers, W. Fendel, H. Schuler, H. Schulz, and D. von der Linde, "second harmonic generation in plasmas prodused by femtosecond laser pulses," Phys. Rev. A 43, 4564-4567 (1991). A. Terasevitch, C. Dietrich, K. Sokolowski-Tinten, and D. von der Linde, "3/2 harmonic generation by femtosecond laser pulses in steep-gradient plasmas," Phys. Rev. E 68, 026410 (2003).

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Received 7 Aug 2007; revised 28 Sep 2007; accepted 5 Nov 2007; published 20 Nov 2007

26 November 2007 / Vol. 15, No. 24 / OPTICS EXPRESS 16061

21. N. D. Whitbread, J. A. R. Williams, J. S. Roberts, I. Bennion, and P. N. Robson, "Optical autocorrelator that used a surface-emitting second-harmonic generator on (211)B GaAs," Opt. Lett. 19, 2089-2091 (1994). 22. E. J. Canto-Said, P. Simon, C. Jordan, and G. Marowsky, "Surface second-harmonic generation in Si(111) for autocorrelation measurments of 248 nm femtosecond pulses," Opt. Lett. 18, 2038-2040 (1993). 23. W. L. Kruer, The physics of laser plasma interactions (Addison-Wesley Publishing Co., 1988).

1. Introduction Phase characterization and automated pulse compression through adaptive phase distortion compensation of femtosecond laser pulses is important for reproducible nonlinear optical applications using femtosecond lasers [1]. Currently the characterization of femtosecond pulses is usually carried out by autocorrelation, frequency resolved optical gating (FROG) [2], spectral interferometry for direct electric field reconstruction (SPIDER) [3] or a relatively new method called shaper assisted collinear SPIDER (SAC-SPIDER) [4]. These methods depend on the mode quality of the beam and typically are relatively difficult to set up because they require the overlap of two or more beams in space and in time. Here we use multiphoton intrapulse interference phase scan (MIIPS) [5-7], which is a highly accurate single beam characterization and dispersion compensation method that is not dependent on beam quality. [8]. This system, which uses an adaptive pulse shaper, allows us to explore the effect of pulse shaping (spectral phase) and reproducibility in advanced ultrafast laser material processing [9, 10]. During MIIPS, a series of calibrated phase functions are introduced and the spectra of a nonlinear optical process, typically second harmonic generation (SHG), obtained for the reference phases are used to analytically measure the spectral phase distortions of the pulse. The MIIPS program calculates the phase distortions analytically and uses the adaptive pulse shaper to compensate them. Here we scan a well calibrated function f(ω)=αsin{γ(ω−ω0)-δ(ω)} where α = π, γ is bandwidth of the pulse, ω0 is the carrier frequency and δ(ω) determines the position of the phase mask with respect to the spectrum of the pulse. Phase measurement is based on the fact that the second harmonic spectrum has a maximum, ωmax(δ),where local phase distortions for the phase modulated pulse are minimal, i.e. when f″(ω)+φ″(ω)→0 where φ is the spectral phase of the pulse. By scanning δ in the calibrated phase function f(δ,ω) and measuring the position of the corresponding second harmonic spectrum maximum ωmax(δ), we can directly calculate the unknown phase distortion φ using the experimentally measured function δmax(ω), the formula φ″(ω)=αγ2×sin[γ(ω-ω0)-δmax(ω)] and double integration in the frequency domain. Details of the MIIPS method can be found in a comprehensive paper [6]. Figure 1 presents SHG-FROG and MIIPS traces for pulses (through a 40x, 0.6 NA objective) that are both compressed to transform limited (top) and uncompressed (bottom). For transform limited pulses, δmax(ω) is a linear function that produces straight parallel features separated by π. Dispersions introduced by the objective, and other optics, result in changes in the spacing and slope of the features. Pulse compression is accomplished by eliminating the phase distortions (including higher order terms) by applying the negative of the phase measured using MIIPS. When the shaper introduces –φ(ω), transform limited pulses are obtained, as shown in Fig. 1 (top).

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Fig. 1. Comparison of SHG-FROG and MIIPS traces for compressed (TL) and uncompressed pulses. The two sets of measurements were made using the same laser pulses, a 40x, 0.6 NA microscope objective and a SHG crystal. The dashed lines coincide with the MIIPS features observed for TL pulses and are used as a guide to the eye.

Here we demonstrate in-situ characterization and adaptive dispersion compensation of femtosecond laser pulses using the surface second harmonic generation (SSHG) from three different substrates, silicon, copper and aluminum. This study is motivated by analysis of the microscopic morphology of ablated holes using shaped laser pulses in our laboratory, where we found that TL pulses create the sharpest borders[11]. SSHG was first reported by Terhune [12] in 1962 in calcite and further investigated theoretically by Pershan [13], Adler [14] and later, Bloembergen [15, 16] in silicon and germanium. Although some of these early experiments are focused on SSHG generation below the ablation threshold, some reports deal with the SSHG generation in plasma produced by laser matter interaction above the ablation threshold [17-20]. Von der Linde and coworkers have carried out experimental and theoretical studies on SHG generated in plasma produced by femtosecond laser pulses[18, 19]. In our experiments we used normal incidence and collected the SSHG beam confocally in order to both mimic the micromachining environment, where this technique has great potential, and to avoid signal fluctuation due to surface roughness. It has been shown before that SSHG signal can be utilized in autocorrelation to measure femtosecond pulses [21, 22]. The main advantage of SSHG-MIIPS is that pulse characterization and optimization are done in-situ, after a high numerical aperture microscope objective. This ensures that phase distortions introduced by the optics are corrected and that the optimum phase is introduced for achieving optimum micromachining at the sample without modifications to the machining setup. This approach does not need a non-linear SHG crystal with phase matching requirements (carrier frequency and bandwidth dependence), which tend to be expensive and prone to laser damage. 2. Experimental section The experimental setup, shown in Fig. 2, includes a regeneratively amplified Ti:sapphire laser (Spitfire-Spectra Physics) seeded with 100 MHz oscillator (KM Labs, 45 nm FWHM) after compensating for phase distortions using a MIIPS-enabled pulse shaper [5] that is also used to apply arbitrary phase functions. The output laser pulses from the amplifier are centered at 800 nm with 750 μJ/pulse at 1 kHz with pulse duration of 35 fs. A fraction of this output was spatially filtered with a 100 μm pinhole located within an up-collimating Keplerian telescope with 300 mm and 600 mm lenses. The output of the telescope was directed to a 0.6 numerical #86220 - $15.00 USD

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aperture 40X objectives (Plan Fluor ELWD, Nikon) which focused the laser beam at normal incidence onto the sample, which was mounted on motorized X-Y stages. The stages moved after every laser pulse in order to supply fresh surface for each laser shot. Here we are used a confocal arrangement to mitigate the effects that are arising from surface imperfections. The SSHG signal was collected and focused to a high sensitivity fiber coupled spectrometer (QE65000, Ocean Optics).

Fig. 2. Schematic of the experimental setup. L1, L2 and L3 are 300, 600 and 50 mm focal length lenses. P pinhole, BS beams splitter that transmits 800 nm and reflects 300-600 nm.

3. Results and discussion The emission spectra from a Si substrate are plotted for three laser intensities in Fig. 3. At low fluence (