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October 15, 2010 / Vol. 35, No. 20 / OPTICS LETTERS 3417 Materials processing with a tightly focused femtosecond laser vortex pulse Cyril Hnatovsky,1,* Vladlen G. Shvedov,1,2 Wieslaw Krolikowski,1 and Andrei V. Rode1 1 Laser Physics Centre, Research School of Physics and Engineering, The Australian National University, Canberra Australian Capital Territory 0200, Australia 2 Department of Physics, Taurida National University, Simferopol 95007 Crimea, Ukraine *Corresponding author: [email protected] Received June 30, 2010; revised September 2, 2010; accepted September 5, 2010; posted September 20, 2010 (Doc. ID 130950); published October 8, 2010 In this Letter we present the first (to our knowledge) demonstration of material modification using tightly focused single femtosecond laser vortex pulses. Double-charge femtosecond vortices were synthesized with a polarizationsingularity beam converter based on light propagation in a uniaxial anisotropic medium and then focused using moderate- and high-NA optics (viz., NA ¼ 0:45 and 0.9) to ablate fused silica and soda-lime glass. By controlling the pulse energy, we consistently machine micrometer-size ring-shaped structures with <100 nm uniform groove thickness. © 2010 Optical Society of America OCIS codes: 320.2250, 140.3390, 320.7160, 140.3440, 140.3300. Femtosecond laser vortex pulses provide an opportunity to investigate the effects of the optical angular momentum on photoionization processes [1] and also to obtain information on how the ring-shaped intensity distribution of vortex pulses affects the light-matter interaction. Unfortunately, such studies pose a significant challenge owing to technical difficulties hindering the synthesis of high-power broadband laser vortex beams. The traditional methods utilizing spiral phase plates and holograms are inherently chromatic and therefore require the introduction of correcting elements in order to compensate for the topological charge dispersion occurring in polychromatic pulses. Uncompensated charge dispersion leads to a poor quality of the beam and the impossibility of using it for precision laser materials processing [2]. Compensation schemes, however, come at the expense of making the beam-shaping setups significantly more complicated and sensitive to alignment and remain either bandwidth limited, or low throughput, or unsuitable for operation with high-intensity laser pulses ([3] and references therein). So far, none of the proposed compensation techniques have been tested or used in laser micromachining. Recently we have synthesized high-quality doublecharge femtosecond vortex beams by using polarization singularities associated with the beam propagation in uniaxial birefringent crystals [4]. In this Letter we employ this powerful technique to generate double-charge femtosecond laser vortex beams and use them for reproducible submicrometer structuring of fused silica (SiO2 ) and soda-lime glass samples. To the best of our knowledge, this is the first report on materials processing using tightly focused femtosecond vortex pulses. It was shown in [4] that a circularly polarized femtosecond Gaussian beam propagating along the optical axis of a uniaxial crystal is converted into a superposition of two polarization states with opposite handednesses. The state with the handedness opposite to that of the input beam carries a double-charge optical vortex (l ¼ 2), whereas the state with the handedness coinciding with that of the input beam represents a nonvortex beam. After the crystal, the wave propagating in free space becomes a super0146-9592/10/203417-03$15.00/0 position of independent solutions of the scalar wave equation ð∇2⊥ þ 4πiλ−1 ∂z ÞΨ ¼ 0, and the vortex solution can be written as Ψl¼2 ¼ Ψzþδ − Ψz−δ , where Ψzδ ¼ ðr 2 þ w20 ξzδ ÞGzδ expð2iφÞr −2 , Gzδ ¼ ðE=ξzδ Þ expð−r 2 = ðw20 ξzδ ÞÞ, and ξzδ ¼ 1 þ iðz δÞλ=ðπw20 Þ. E is a constant electric field amplitude, λ is the wavelength in vacuum, w0 is the 1=e2 beam waist radius, and δ denotes the axial shift of the waists of the ordinary and extraordinary beams with respect to each other due to double refraction in the crystal. For paraxial beams, the shift is approximated by δ ¼ dðn2o − n2e Þ=ð2n2e no Þ, where no and ne stand for the ordinary and extraordinary refractive index, respectively, and d is the thickness of the crystal along the beam propagation direction z. The focusing of such beams was studied theoretically in [5]. In our experiments we used the output beam of a Clark-MXR femtosecond Ti:sapphire amplifier with a central wavelength at λ ¼ 775 nm. Figure 1(a) shows the experimental setup for the generation of doublecharge femtosecond vortex beams (see [3]). For consistency, the topological charge in our experiments was l ¼ þ2. The cross section of the generated vortex beam consists of a series of concentric rings, as shown in the inset of Fig. 1(a). The number of rings is determined by the input beam diameter at L1, the optical power of L1, and the crystal parameters [3]. In our experiments the beam expander, comprising L1 and L2, was adjusted to allow only the first bright ring of the vortex beam to enter the objective O1 (i.e., the first dark ring coincides with the entrance aperture of O1) to be used for material modification. To compare the simulated and actual behavior of a femtosecond vortex beam, we mapped the intensity distribution in the focal region of O1 with an imaging objective O2 having a NA significantly higher than that of O1, viz., NA ¼ 0:9 (Nikon M Plan 100×) versus NA ¼ 0:45 (Olympus LUCPlan FLN 20×) [see Fig. 1(b)]. The images were captured with a CCD camera and further analyzed using ImageJ software. The analysis shows that the peakto-valley intensity variation along the maximum of the first bright ring, which, according to our measurements, has a diameter of ∼2 μm at the focus, does not exceed © 2010 Optical Society of America 3418 OPTICS LETTERS / Vol. 35, No. 20 / October 15, 2010 15%. The average intensity along the first ring at the focus (i.e., z ¼ 0) is ∼1:2 and ∼2 times higher than those measured at z ¼ 5 μm and z ¼ 10 μm, respectively. Based on these numbers, the “confocal parameter” of the generated beam is ∼20 μm, which is ∼3 times larger than that of a Gaussian beam with a 2 μm waist diameter. The observed elongated tubular focus agrees well with the simulations shown in Fig. 1(b), which were performed for O1 with NA ¼ 0:45 and λ ¼ 775 nm. Our simulations also show that the radial intensity distribution of a vortex beam is well approximated by an expression describing the Laguerre–Gaussian pl beam (i.e., p zeros in the radial direction and 2l angular nodes; l also determines the topological charge of a beam): I¼ 2jljþ1 r 2jlj expð−2r 2 =ðw0 jξjÞ2 Þ P; jlj!π ðw0 jξjÞ2ðjljþ1Þ where P is the total power and ξ ¼ 1 þ izλ=ðπw20 Þ. According to this expression, the maximum intensity Fig. 1. (a) Setup for the generation of double-charge femtosecond vortex pulses and materials processing with objective O1. BF, beam filter viz., an aperture ∼1 mm in diameter located ∼5 m from the beam converter BC; λ=4, achromatic quarterwave plate; L1, negative lens (−50 mm); CR, 10-mm-long c-cut CaCO3 crystal; L2, positive lens (þ125 mm); PBS, polarization beam splitter; IB and VB, CCD images (6:4 mm × 4:2 mm) of the incident and vortex beams, respectively; NB, nonvortex beam that is removed from the system; S, glass sample. (b) Simulated (top) and experimental (bottom) intensity distributions of a focused polarization-singularity vortex beam in the x–z and x–y planes. O2, imaging objective; CCD images show the intensity distribution in the x–y plane at the focus of O1 with NA ¼ 0:45. I¼ 2jljjlj expð−jljÞ P jlj!πðw0 jξjÞ2 is achieved at r 2l ¼ ðw0 jξjÞ2 jlj=2. For a vortex with l ¼ 2 and r 2 ¼ w0 jξj the peak intensity is by a factor of 2e−2 lower than for a Gaussian beam (i.e., l ¼ 0) with the same 1=e2 radius. In experiments we explored two focusing regimes using 200 fs (FWHM after BC) linearly polarized doublecharge vortex pulses. In the first case, O1 had a moderate NA of 0.45 (Olympus LUCPlan FLN 20×). The beam was focused onto the surface of a fused silica (i.e., SiO2 ) sample. Figure 2 shows topographic images of the ablation rings produced with a single pulse at ∼150 nJ (after O1), which were recorded with a Wyko NT9100 surface profiler providing a 0:45 μm lateral and a subnanometer vertical resolution. The depth of the ablation ring craters is ∼40 nm, and the profile is uniform along the grooves. The lateral dimensions of the craters agree with the intensity distribution in the focal region of O1 shown in Fig. 1(b). The ∼0:2 μm3 volume of ablated silica agrees with the amount of material removal per unit energy in air of 1:5 μm3 =μJ reported in [6]. In the second set of experiments, samples of soda-lime glass were irradiated using O1 with a high NA of 0.9 (Nikon M Plan 100×). After irradiation the samples were examined under a Hitachi 4300SE/N scanning electron microscope (SEM). The SEM images in Fig. 3(a) show the ablation rings produced at different pulse energies. From the leftmost image corresponding to the lowest energy, one can see that (i) the diameter of the ablation ring is ∼1 μm, which is half of the value obtained with NA ¼ 0:45, and (ii) the width of the ablated annular groove is less than 100 nm. According to measurements performed with a Veeco MultiMode scanning probe microscope, the grooves are ∼90 nm deep, thus exhibiting a ∼1:1 depth-to-height aspect ratio. The production of such narrow and deep grooves is facilitated by the sharp intensity distribution across the ring. At higher pulse energies, the ablation signature is much more pronounced, but it still preserves an annular shape with sharp and clean edges. It is also noteworthy that ablation of material performed using a single vortex pulse is highly reproducible [Fig. 3(c)]. Fig. 2. (Color online) Surface profile of a SiO2 sample ablated with double-charge single ∼150 nJ femtosecond vortex pulses using NA ¼ 0:45 focusing optic. Ablation rings are separated by 5 μm. October 15, 2010 / Vol. 35, No. 20 / OPTICS LETTERS Fig. 3. Ablation of soda-lime glass with double-charge single femtosecond vortex pulses using an NA ¼ 0:9 focusing optic. (a) Ablation rings produced at a pulse energy of ∼100, ∼150, ∼250, and ∼700 nJ from left to right. (b) Cavities produced at a pulse energy of ∼1:5 μJ. (c) Region of a 200 μm × 200 μm array produced at a pulse energy of ∼250 nJ. Ablation rings are separated by 5 μm. An interesting effect is observed when the pulse energy is increased to ∼1:5 μJ and the laser focus is positioned ∼2 μm subsurface [Fig. 3(b)]. In this case we clearly observe the formation of deep cavities on the axis of the focused beam where the residual central light intensity is negligible [7]. The ∼1:5 μm deep cavity in the end of the row has opened, whereas the cavities to the left of it are still covered with material to a different extent. The top shell of the cavity seen in the third image, which has almost detached from the substrate, is strongly charging and therefore looks pronouncedly bright on the SEM image. We suggest that the formation of nanocavities with a single femtosecond vortex pulse is caused by the implosion phenomenon. By focusing a femtosecond vortex pulse very tightly, we concentrate the electromagnetic energy inside a micrometer-size toroidal volume. In our experiments the peak intensity at the focus is nominally (i.e., aberration-free focusing) ∼4:5 × 1014 W=cm2 , which exceeds by far the ionization threshold (i.e., ∼1013 W=cm2 ) of glass. In the regions where the light intensity exceeds the ionization threshold, the material is rapidly atomized and converted into a toroidally shaped 3419 electron plasma. The volume of the ∼1 μm diameter, 1.5μm-long plasma toroid (see Fig. 3) is estimated at ∼1 μm3 , which translates into a ∼1 MJ=cm3 electron thermal energy density (assuming a 50% pulse energy deposition efficiency) [8]. Within a few picoseconds after the pulse, the electrons give their energy to the lattice and two shock waves (SWs) emerge from the energy deposition zone: one wave propagates toward the center of the toroid, whereas the other wave propagates into the surrounding material. The estimated ∼1 MJ=cm3 electron thermal energy density corresponds to a ∼0:5 TPa pressure created inside the material that drives the SWs [8]. This pressure is well above the Young modulus for soda-lime glass, viz., ∼20 GPa. At a certain point the SWs convert into sound waves. The stopping distance can be estimated from the condition that the internal energy inside the volume confined within the SW fronts is comparable to the absorbed pulse energy [8]. The stopping distance for our quasi-cylindrical geometry is estimated at ∼1 μm, which exceeds the radius of the toroid. Therefore, the converging SW can reach the axis of the toroid and raise pressure inside that region dramatically. The implosion phase is followed by the explosion of the ultracompressed material near the surface, which leads to the formation of cavities. Further research and detailed modeling are necessary to ascertain the origin of this novel phenomenon. We acknowledge financial support from the National Health and Medical Research Council of Australia and the Australian Research Council. We also acknowledge Dr. F. Brink and Dr. H. Chen for their assistance in obtaining the SEM images and K. Vu for his assistance in measuring depth profiles of the ablated samples. References 1. A. Picón, J. Mompart, J. R. Vázquez de Aldana, L. Plaja, G. F. Calvo, and L. Roso, Opt. Express 18, 3660 (2010). 2. R. Ling-Ling, Q. Shi-Liang, and G. Zhong-Yi, Chin. Phys. B 19, 034204 (2010). 3. Y. Tokizane, K. Oka, and R. Morita, Opt. Express 17, 14518 (2009). 4. V. G. Shvedov, C. Hnatovsky, W. Krolikowski, and A. V. Rode, Opt. Lett. 35, 2660 (2010). 5. A. Volyar and T. Fadeyeva, Opt. Spectrosc. 96, 96 (2004). 6. A. Ben-Yakar and R. L. Byer, J. Appl. Phys. 96, 5316 (2004). 7. Y. Iketaki, T. Watanabe, N. Bokor, and M. Fujii, Opt. Lett. 32, 2357 (2007). 8. E. G. Gamaly, S. Juodkazis, K. Nishimura, H. Misawa, B. Luther-Davies, L. Hallo, P. Nicolai, and V. T. Tikhonchuk, Phys. Rev. B 73, 214101 (2006).