Laser-induced microexplosions in transparent materials: microstructuring with nanojoules

Transcription

1 Laser-induced microexplosions in transparent materials: microstructuring with nanojoules Chris B. Schaffer, André Brodeur, Nozomi Nishimura, and Eric Mazur * Harvard University, Department of Physics and Division of Engineering and Applied Sciences, Cambridge, MA 0238 ABSTRACT We tightly focus femtosecond laser pulses in the bulk of a transparent material. The high intensity at the focus causes nonlinear absorption of the laser energy, producing a microscopic plasma and damaging the material. The tight external focusing allows high intensity to be achieved with low energy, minimizing the effects of self-focusing. We report the thresholds for breakdown and critical selffocusing in fused silica using -fs pulses at both 400-nm and 800-nm wavelength. We find that permanent damage can be produced with only nj (25 nj) for 400-nm (800-nm) pulses, and that the threshold for critical self-focusing is 40 nj for the 400-nm pulses and 580 nj for the 800-nm pulses. The critical self-focusing thresholds are more than an order of magnitude above the breakdown thresholds, confirming that self-focusing does not play a dominant role in the damage formation. This lack of self-focusing allows a straightforward interpretation of the wavelength and bandgap dependence of bulk breakdown thresholds. The energies necessary for material damage are well within the range of a cavity-dumped oscillator, allowing for precision microstructuring of dielectrics with a high repetition-rate laser that is roughly one-third the cost of an amplified system. Keywords: laser-induced breakdown, optical damage, laser micromachining, nonlinear absorption mechanisms, self-focusing, femtosecond laser-matter interactions. INTRODUCTION Several groups have investigated the use of femtosecond lasers for materials processing and micromachining. 5 Compared to longer-pulse lasers, femtosecond lasers offer increased precision and minimized thermal damage to the material, thus enabling new machining processes. Femtosecond lasers have been used to microstructure a wide variety of materials, including metals, semiconductors, and insulators. In transparent materials, femtosecond lasers offer the potential for micromachining both on the surface and inside the bulk of the material with no damage to the surface. 6,7 In this paper we will focus on the interaction of femtosecond pulses with bulk transparent materials. When a high-intensity ultrashort laser pulse interacts with a transparent material, laser energy can be absorbed through nonlinear processes such as multiphoton absorption, 8 tunneling ionization, 8 and avalanche ionization. 9, This absorption produces a hot electron-ion plasma in the region where the energy is absorbed. When the absorption occurs at the surface of the material, the result is ablation. 3 If the laser is focused in the bulk of the material, the intensity at the focus can cause absorption of the laser energy inside the sample rather than at the surface. With tight external focusing, the absorption forms a microscopic, hot plasma at the laser focus, in the bulk of the material. This plasma then expands into the surrounding material, forming a cavity inside the sample. Previously we have reported the production of 200-nm diameter damage structures inside transparent solids using this technique. 6,7 In addition, we have studied the dynamics of the plasma expansion, and the thresholds for damage in several materials. 2 In this paper, we report on the production of microscopic damage inside fused silica with laser energies of only tens of nanojoules. Energies in this range are readily available from cavity-dumped 3,4 and long-cavity 5 laser oscillators. With these laser systems, our techniques could allow for bulk and surface microstructuring of transparent materials without an amplified laser. Furthermore, we determine that self-focusing can be neglected in our tight-focusing geometry, allowing threshold inten- * mazur@physics.harvard.edu; WWW:

2 sities to be determined. This knowledge of the intensity enables the wavelength and bandgap dependences of the breakdown threshold to be compared with theoretical predictions, and the dominant ionization mechanism to be determined. 2. BREAKDOWN THRESHOLD To measure the breakdown threshold, we use the scattering technique shown in Fig.. A -fs laser pulse is focused beneath the surface of the sample with a 0.65 numerical aperture (NA) microscope objective. By spatially expanding the gaussian input beam and selecting only the central region, we ensure a near flat-field input to the microscope objective and therefore use the full numerical aperture. The laser pulse is focused 70 µm beneath the surface of the material, where the microscope objective is designed to have minimal aberrations. Compared to our previous work, 6,7,2 the focusing in these experiments is both tighter (we utilize the full NA) and suffers fewer aberrations. To determine the breakdown threshold we illuminate the pumped region of the sample with a He:Ne laser and block the directly transmitted He:Ne light. When breakdown occurs, the material is damaged and we detect the He:Ne light scattered around the beam block by the damage spot. The femtosecond laser energy is measured with a calibrated photodiode (not shown in Fig. ). We find that the breakdown threshold for fused silica is nj for 400- nm pulses and 25 nj for 800-nm pulses. The damage produced with energies near the breakdown threshold is too small or does not yield sufficient contrast to be seen under an optical microscope. fs pump objective sample objective He:Ne laser beam block detection Fig.. Scattering setup for measuring laser breakdown thresholds in transparent materials. To corroborate the scattering data, we measured the transmission of the tightly-focused pulses through the sample. The transmission of 400-nm and 800-nm, -fs pulses through fused silica as a function of laser energy is shown in Fig. 2. For both wavelengths, the transmission drops abruptly just above the breakdown threshold measured by the scattering technique, indicating the onset of strong absorption. 3. VISIBLE DAMAGE MORPHOLOGY Because the damage produced near threshold cannot be resolved optically, we studied the morphology of damage produced by pulses with above-threshold energies. We find there is a sharp threshold (the visible damage threshold) for the production of damage that can be seen under a high-power microscope. For both 400-nm and 800-nm pulses, a single laser shot with only 40 nj of energy produces a visible change. Figure 3 shows an optical image of the damage produced with -fs, 800-nm laser pulses with 50 nj of energy. The photograph was taken in reflection using a.4 NA oil-immersion microscope objective. The horizontal spacing between damage spots is 5 µm, and the diameter of the damage structures is about 500 nm in the image. This diameter is only an upper limit, however, as it is at the resolution limit of the microscope. Visible damage similar to those shown in Fig. 3 were, in previous work, produced using slower focusing and more laser energy. 6 Scanning electron 6 and atomic force 7 microscopy of those damage spots revealed a 200-nm diameter. The inset in Fig. 3 shows a side-view of a single damage structure. The structure is about 2 µm long, indicating that the visible damage is elongated along the laser beam propagation direction. 4. CRITICAL SELF-FOCUSING To establish the role of self-focusing in our experiments we measured the threshold for white-light continuum generation in a slow-focusing geometry (0.20-m focal length). It has been shown that, in such a geometry, the threshold for critical self-focusing corresponds to the threshold for white-light continuum generation. 6 9 The correspondence of these two thresholds allows us to determine the critical power for self-focusing from a measurement of the white-light continuum threshold. Note that selffocusing does not depend on the external focusing geometry, but only on the peak power; 20 thus the critical self-focusing threshold is the same for fast and slow external focusing. We find that the threshold for critical self-focusing is 40 nj for the

3 transmission (a) 400 nm fs fused silica breakdown threshold visible damage critical self-focusing 0. (b) breakdown threshold visible damage transmission 800 nm fs fused silica critical self-focusing laser energy (µj) Fig. 2. Transmission of tightly-focused, -fs pulses through fused silica (a) at 400-nm and (b) at 800-nm. The thresholds for breakdown, visible damage, and critical self-focusing are indicated. 400-nm pulses and 580 nj for the 800-nm pulses. These energies are more than an order of magnitude above the breakdown thresholds we measured using the scattering technique, indicating that self-focusing does not play a dominant role in the damage formation. This lack of self-focusing allows the spot size of the laser at the focus to be calculated with confidence. Thus intensity of the laser pulse can be known, allowing a straightforward interpretation of the wavelength and bandgap dependence of bulk breakdown thresholds. 5. DISCUSSION We discuss two issues: First, what is the utility of our techniques for microstructuring? Second, what new physics can we learn from the fact that we achieve breakdown without self-focusing? While the damage produced near threshold may find use in some microstructuring applications (perhaps where a small, localized index change is desired) most applications will probably use the structures formed at energies above the visible damage threshold. We have demonstrated binary three-dimensional data storage using such structures. 6 Other potential applications include the manufacture of three-dimensional diffractive optical elements and internal engraving of transparent materials. In this work, we produced optically-visible damage (shown in Fig. 3) with only 40 nj of incident laser energy. Let us consider how the visible damage threshold can be lowered further. Whether visible damage is produced or not depends on how much energy is deposited in the microscopic focal volume inside the sample. From Fig. 2 we see that at the visible damage threshold the transmission of both the 400-nm and 800-nm laser pulses is about 75%, indicating that about 25% or nj of the incident laser energy is deposited. 2 This nj produces damage with sufficient size and refractive-index change to be resolved optically and to be useful for microstructuring applications. If the absorption of the laser pulse could be increased,

4 Fig. 3.Top-view optical image of micro-damage produced in fused silica using -fs, 800-nm, 50- nj laser pulses. Inset shows a side-view of a single damage structure. the required nj would be deposited with less incident laser energy, and the visible damage threshold would be lowered. Increased absorption can be achieved by using shorter laser pulses, tighter focusing, or smaller bandgap materials. In these three cases the ionization is enhanced for a given energy and the absorption of the laser energy by the material is increased. We estimate that using 30-fs, 800-nm laser pulses and 0.65-NA focusing, visible damage could be achieved with 20 nj of incident laser energy (i.e. 50% absorption). State-of-the-art cavity-dumped laser oscillators could easily deliver 30-fs, 20-nJ pulses to the sample, allowing micromachining without an amplified laser. Furthermore, the repetition rate could be as high as MHz, providing high machining speeds. In our experimental geometry the breakdown thresholds are more than an order of magnitude smaller than the critical self-focusing thresholds (see Fig. 2). 8 Without self-focusing, we can calculate the spot size and peak intensity at threshold with some confidence. At the -nj (25-nJ) breakdown threshold, the spot size (FWHM) is 0.25 µm (0.5 µm) giving a peak intensity of.3 4 W/cm 2 (8. 3 W/cm 2 ) for the 400-nm (800-nm) pulses. From these intensities, we can calculate the Keldysh parameter. 8 A Keldysh parameter smaller than about.5 indicates tunneling ionization is the dominant ionization mechanism, while a large Keldysh parameter indicates multiphoton absorption dominates. We find the Keldysh parameter is.7 for the 400- nm pulses and. for the 800-nm pulses. While it is clear that with 400-nm pulses we are closer to the multiphoton ionization regime, and with the 800-nm pulses we are more in the tunneling regime, we cannot unambiguously determine which mechanisms are responsible. The ionization rates calculated for our peak intensities indicate that with 400-nm pulses the multiphoton ionization alone achieves a critical density plasma, the plasma density usually assumed to be necessary for breakdown. For the 800-nm pulses, the tunneling ionization rate is not sufficient to reach the critical density, indicating that there is some avalanche ionization which increases the carrier density. We estimate two to three generations occur in the avalanche. In future work, we will vary both the wavelength of the laser light and the bandgap of the material to observe, more clearly, different ionization regimes and the transitions between them. 6. CONCLUSIONS We have demonstrated the capability for three-dimensional microstructuring of transparent solids using tightly-focused femtosecond laser pulses with only 40 nj of laser energy. State-of-the-art cavity-dumped and long-cavity Ti:Sapphire laser oscillators produce energies in this range at high repetition rates. Combining our technique with this laser technology opens the door to micromachining without amplified laser systems, greatly decreasing the cost and complexity of the laser. We measured the thresholds for both bulk optical breakdown and critical self-focusing in fused silica for femtosecond laser pulses of 400-nm and 800-nm wavelength. For both wavelengths, the self-focusing threshold was more than an order of magnitude larger than the

5 breakdown threshold, indicating that the external focusing, not self-focusing, determines the spot size. Because we can know the spot size of the laser, we can determine the intensities necessary for breakdown. This will allow for bandgap and wavelength dependence studies of breakdown thresholds to yield information about the fundamental ionization mechanisms at work, and to observe transitions between different ionization regimes. As a first step, we have indications that for femtosecond pulses in fused silica, multiphoton absorption dominates with 400-nm pulses while tunneling and avalanche ionization dominate at 800 nm. ACKNOWLEDGMENTS This work was supported by a Materials Research Science and Engineering Center grant. C.B.S. acknowledges a National Defence Science and Engineering Fellowship. A.B. acknowledges support by the Natural Sciences and Engineering Research Council of Canada. REFERENCES. B.C. Stuart, M.D. Feit, S. Herman, A.M. Rubenchik, B.W. Shore, and M.D. Perry, J. Opt. Soc. Am. B 3, 459 (996); Phys. Rev. B 53, 749 (996). 2. D. Du, X. Liu, and G. Mourou, Appl. Phys. B 63, 67 (996). 3. M. Lenzner, J. Kruger, S. Sartania, Z. Cheng, Ch. Spielmann, G. Mourou, W. Kautek, F. Krausz, Phys. Rev. Lett. 80, 4076 (998). 4. B.N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, and A. Tunnermann, Appl. Phys. B 63, 9 (996). 5. D. von der Linde and H. Schuler, J. Opt. Soc. Am. B 3, 26 (996). 6. E.N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T.-H. Her, J.P. Callan, and E. Mazur, Opt. Lett. 2, 2023 (996). 7. E.N. Glezer and E. Mazur, Appl. Phys. Lett. 7, 882 (997). 8. L.V. Keldysh, Sov. Phys. JETP 20, 307 (965). 9. N. Bloembergen, IEEE J. Quantum Electron. QE-, 375 (974).. M. Sparks, D.L. Mills, R. Warren, T. Holstein, A.A. Maradudin, L.J. Sham, E. Loh, Jr., and D.F. King, Phys. Rev. B 24, 359 (98).. E.N. Glezer, C.B. Schaffer, N. Nishimura and E. Mazur, Opt. Lett. 22, 87 (997). 2. C.B. Schaffer, N. Nishimura, and E. Mazur, to appear in Proc. SPIE A. Baltuska, Z. Wei, M.S. Pshenichnikov, D.A. Wiersma, and Robert Szipocs, Appl. Phys. B 65, 75 (997). 4. M.S. Pshenichnikov, W.P. de Boeij, and D.A. Wiersma, Opt. Lett. 9, 572 (994). 5. S.H. Cho, B.E. Bouma, E.P. Ippen, J.G. Fujimoto, Summaries of Papers Presented at the Conference on Lasers and Electro-Optics 998 Technical Digest Series 6, 559 (998). 6. P.B. Corkum and C. Rolland, IEEE J. Quantum Electron. 25, 2634 (989). 7. A. Brodeur and S.L. Chin, Phys. Rev. Lett. 80, 4406 (998); A. Brodeur and S.L. Chin, to appear in J. Opt. Soc. Am. B. 8. By critical self-focusing we refer to the self-focusing experienced by a laser pulse at the critical power for self-focusing. 9. The intensity increases abruptly when critical self-focusing occurs leading to significant ionization of the material. The rapid change in the index of refraction caused by this ionization, together with self-phase modulation, leads to the production of a white-light continuum. 20. J.H. Marburger, Prog. Quantum Electron. 4, 35 (975). 2. The loss of laser energy due to reflection and scattering is small, so the absorption is approximately one minus the transmission.