Ablation-cooled material removal with ultrafast bursts of pulses

Journal name:
Nature
Volume:
537,
Pages:
84–88
Date published:
DOI:
doi:10.1038/nature18619
Received
Accepted
Published online
Corrected online

The use of femtosecond laser pulses allows precise and thermal-damage-free removal of material (ablation) with wide-ranging scientific1, 2, 3, 4, 5, medical6, 7, 8, 9, 10, 11 and industrial applications12. However, its potential is limited by the low speeds at which material can be removed1, 9, 10, 11, 13 and the complexity of the associated laser technology. The complexity of the laser design arises from the need to overcome the high pulse energy threshold for efficient ablation. However, the use of more powerful lasers to increase the ablation rate results in unwanted effects such as shielding, saturation and collateral damage from heat accumulation at higher laser powers6, 13, 14. Here we circumvent this limitation by exploiting ablation cooling, in analogy to a technique routinely used in aerospace engineering15, 16. We apply ultrafast successions (bursts) of laser pulses to ablate the target material before the residual heat deposited by previous pulses diffuses away from the processing region. Proof-of-principle experiments on various substrates demonstrate that extremely high repetition rates, which make ablation cooling possible, reduce the laser pulse energies needed for ablation and increase the efficiency of the removal process by an order of magnitude over previously used laser parameters17, 18. We also demonstrate the removal of brain tissue at two cubic millimetres per minute and dentine at three cubic millimetres per minute without any thermal damage to the bulk9, 11.

At a glance

Figures

  1. Principles of ablation-cooled removal of a material by laser.
    Figure 1: Principles of ablation-cooled removal of a material by laser.

    a, Schematic representation of the ablation process for low (traditional regime, left diagrams) and high (ablation-cooled regime, right diagrams) repetition rates. Temperature profiles are illustrated for t = τ1 (i), which is shortly after the arrival of the first pulse for both cases; for t = τ2 (ii), which is before (shortly after) the arrival of the second (last) pulse for the low-repetition-rate (high-repetition-rate) laser; and for t = τ3 (iii), which is shortly after the arrival of the last pulse for the low-repetition-rate laser. The colouration of the target material is based on simulation results shown in b at the indicated time intervals of τ1, τ2 and τ3. b, Calculated evolution of the temperatures at the surface (solid lines) and below (at a depth of 30 times the optical penetration depth) the surface (dotted lines) for repetition rates of 10 MHz (blue lines) and 1,600 MHz (black lines). The pulse energies and number of pulses are the same for both cases. The higher repetition rate results in substantially lower temperatures below the surface due to ablation cooling. c, Expanded view of the shaded section of the plot in b. d, Calculated evolution of the surface temperature (dashed lines) and amount of ablated material (solid lines) for repetition rates of 100 MHz (green lines), 400 MHz (blue lines) and 1,600 MHz (red lines). The ablation rate remains approximately the same when the product of the pulse energy and repetition rate is maintained. The spikes in the surface temperatures precisely indicate the arrival of pulses, which are not shown explicitly for clarity. e, Experimental set-up for direct confirmation of the ablation-cooling effect. f, The measured temperature increase that is induced on thermoelectric module 1 (the target material; solid lines) and thermoelectric module 2 (attached to the coverslip that collects a portion of the ablated particles; dashed lines, values have been multiplied by three to aid comparison with ΔTtarget) with the laser operating in the ablation-cooled regime (blue lines) and in the traditional regime (red lines).

  2. Scaling down of the pulse energy with increasing repetition rate.
    Figure 2: Scaling down of the pulse energy with increasing repetition rate.

    a, b, Volumes (symbols) of Cu (a) and Si (b) ablated by a single burst of pulses as a function of total incident energy and fluence for different intraburst repetition rates. The predictions of the toy model for the lowest and highest repetition rates in the ablation-cooled regime are also shown (solid lines). c, d, Ablation efficiency in terms of number of atoms of Cu (c) and Si (d) ablated per incident photon as a function of pulse energy and pulse fluence for different repetition rates. The legend applies to all panels. The lower and upper limits to the data correspond to the ablation threshold and available laser energy, respectively. In all panels the sample size for each data point is 20, where the centre values represent the mean and the error bars represent the standard deviation. Coloured symbols highlight the onset of the ablation-cooled regime and (beyond 108 MHz) the inverse scaling of the pulse energy with repetition rate in the ablation-cooled regime.

  3. Ablation of hard and soft tissues.
    Figure 3: Ablation of hard and soft tissues.

    a, b, Laser removal of a section of human dentine obtained in the traditional regime (a, 1 kHz uniform repetition rate) and in the ablation-cooled regime (b, 1.7 GHz intraburst repetition rate). Although both ablation cooling and traditional ultrafast processing avoid thermal damage at sufficiently low average powers, the ablation-cooled regime achieves approximately six times more ablation despite using pulse energies that are about 12 times lower. c, d, When the (uniform or intraburst, respectively) repetition rate, average power and scanning speed are simultaneously increased by a factor of 25, the traditional regime of ultrafast processing results in thermal damage (c; Supplementary Video 4), whereas the ablation-cooled regime completely avoids thermal effects and achieves an ablation speed of 3 mm3 min−1, despite using a pulse energy that is 25 times lower (d; Supplementary Video 5). The insets in ad show laser scanning microscope characterizations of the ablated holes. e, f, Histological images corresponding to about 1 mm3 sections, which were removed from a rat brain with the laser operating at an average power of 600 mW in the traditional regime (e), showing presence of thermal damage, and in the ablation-cooled regime (f), showing no major thermal damage. g, Ablation-cooled laser removal of brain tissue at an average power of 2.7 W, achieving an ablation speed of 2 mm3 min−1 and showing no major thermal damage. h, Bright-field optical image of a bovine cornea from which a flap was removed following ablation-cooled laser processing of a section 0.4 mm below the surface. Inset, optical coherence tomography image of the section indicated by the rectangle.

Videos

  1. Animated version of the schematic description of ablation cooling in Figure 1
    Video 1: Animated version of the schematic description of ablation cooling in Figure 1
    The colouration is based on a mapping of temperature profiles as determined from simulations for the case of 800-MHz pulses (ablation cooling regime) and 10-MHz pulses (outside of the ablation cooling regime). The time and depth coordinates are normalized to characteristic values, as defined in the section below on numerical simulations. While the simulations are 1D, we have taken the liberty of assuming that the profile stays the same in drawing cross-sectional temperature profiles for simplicity. This approximation will not, however, change the main features of the result and the movie is intended to be an illustration, and not an accurate representation of the simulation results.
  2. Shot during material processing experiments using 100-MHz intra-burst repetition rate pulses and 25-kHz uniform repetition rate results on a steel target with the same power.
    Video 2: Shot during material processing experiments using 100-MHz intra-burst repetition rate pulses and 25-kHz uniform repetition rate results on a steel target with the same power.
    The major difference in the sound and plasma brightness is indicative of and consistent with roughly order of magnitude increases in ablation rates in the ablation-cooling regime.
  3. 3D representation of the laser-processed brain tissue
    Video 3: 3D representation of the laser-processed brain tissue
    This video shows a 3D representation of the laser-processed brain tissue obtained through micro-CT imaging.
  4. Dentine processing experiment
    Video 4: Dentine processing experiment
    This video shows the dentine processing experiment using the traditional regime with same power and duration as in Supplementary Video 5. The results of this experiment are shown in Figure 3c.
  5. Dentine processing experiment
    Video 5: Dentine processing experiment
    This video shows the dentine processing experiment using the ablation-cooled regime with same power and duration as in Supplementary Video 4. The results of this experiment are shown in Figure 3d.

Change history

Corrected online 29 September 2016
The x-axis numbering in Fig. 2c and 2d was corrected.

References

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Author information

Affiliations

  1. Department of Electrical and Electronics Engineering, Bilkent University, Ankara 06800, Turkey

    • Can Kerse,
    • Denizhan K. Kesim,
    • Önder Akçaalan &
    • Fatih Ömer Ilday
  2. Department of Physics, Bilkent University, Ankara 06800, Turkey

    • Hamit Kalaycıoğlu,
    • Parviz Elahi &
    • Fatih Ömer Ilday
  3. Department of Mechanical Engineering, Bilkent University, Ankara 06800, Turkey

    • Barbaros Çetin
  4. FiberLAST, Inc., Ankara 06531, Turkey

    • Seydi Yavaş
  5. Nanotechnology and Nanomedicine Department, Hacettepe University, Ankara 06800, Turkey

    • Mehmet D. Aşık
  6. ASELSAN, Ankara 06150, Turkey

    • Bülent Öktem
  7. Menlo Systems GmbH, Am Kloperspitz 19a, Martinsried 82152, Germany

    • Heinar Hoogland &
    • Ronald Holzwarth
  8. Lehrstuhl für Laserphysik, Department Physik, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen 91058, Germany

    • Heinar Hoogland

Contributions

C.K., H.K. and F.Ö.I. designed the research and interpreted the results. H.K., P.E., S.Y., Ö.A., and C.K. developed the laser systems. H.H. and R.H. developed a high-repetition-rate fibre oscillator. C.K., D.K.K. and B.Ö. performed the laser processing experiments. B.Ç. and C.K. developed the numerical models. M.D.A. carried out brain slicing and histological examinations.

Competing financial interests

F.Ö.I., C.K. and H.K. declare competing financial interests due to a pending patent application regarding the technique outlined in this Letter.

Corresponding author

Correspondence to:

Reviewer Information Nature thanks K. Mitra and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author details

Supplementary information

Video

  1. Video 1: Animated version of the schematic description of ablation cooling in Figure 1 (288 KB, Download)
    The colouration is based on a mapping of temperature profiles as determined from simulations for the case of 800-MHz pulses (ablation cooling regime) and 10-MHz pulses (outside of the ablation cooling regime). The time and depth coordinates are normalized to characteristic values, as defined in the section below on numerical simulations. While the simulations are 1D, we have taken the liberty of assuming that the profile stays the same in drawing cross-sectional temperature profiles for simplicity. This approximation will not, however, change the main features of the result and the movie is intended to be an illustration, and not an accurate representation of the simulation results.
  2. Video 2: Shot during material processing experiments using 100-MHz intra-burst repetition rate pulses and 25-kHz uniform repetition rate results on a steel target with the same power. (6.39 MB, Download)
    The major difference in the sound and plasma brightness is indicative of and consistent with roughly order of magnitude increases in ablation rates in the ablation-cooling regime.
  3. Video 3: 3D representation of the laser-processed brain tissue (9.07 MB, Download)
    This video shows a 3D representation of the laser-processed brain tissue obtained through micro-CT imaging.
  4. Video 4: Dentine processing experiment (693 KB, Download)
    This video shows the dentine processing experiment using the traditional regime with same power and duration as in Supplementary Video 5. The results of this experiment are shown in Figure 3c.
  5. Video 5: Dentine processing experiment (506 KB, Download)
    This video shows the dentine processing experiment using the ablation-cooled regime with same power and duration as in Supplementary Video 4. The results of this experiment are shown in Figure 3d.

PDF files

  1. Supplementary Information (16.3 MB)

    This file contains Supplementary Text and Data, Supplementary Figures 1-47, Supplementary Tables 1-5, Supplementary Video legends and additional references (see Contents for more details).

Additional data