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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 1, Iss. 7 — Nov. 1, 2011
  • pp: 1244–1250
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Role of ablation and incubation processes on surface nanograting formation

Feng Liang, Réal Vallée, Daniel Gingras, and See Leang Chin  »View Author Affiliations


Optical Materials Express, Vol. 1, Issue 7, pp. 1244-1250 (2011)
http://dx.doi.org/10.1364/OME.1.001244


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Abstract

The role of ablation and incubation processes in the formation of surface nanogratings with femtosecond pulses were investigated by measuring ablation thresholds and depth of nanogrooves at different pulse to pulse spacings. Our observations indicated that the nanograting formation essentially relies on a laser ablation process which can be modeled by a simple set of equations.

© 2011 OSA

1. Introduction

2. Experiment

A Ti:sapphire chirped-pulse amplification laser system (Spitfire, Spectra-Physics) with a maximum output energy of 2 mJ (1 KHz) at the central wavelength of 800 nm was employed in the experiment. The laser beam was slightly divergent in order to compensate for self-focusing in air and other optical media before reaching the target. The experimental setup is shown in Fig. 1. The laser pulses passing through a neutral density filter and a continuously variable metallic neutral density filter were split in two by a beam splitter (BS). The transmitted part (T = 10%) was directed into a calibrated photodiode which monitored the incident energy for nanograting formation. The reflected part (R = 90%) passing through a half wave plate and an electronic shutter was focused onto the surface of a fused silica block (Corning 7980-UV, surface quality 40–20, wave front λ/4) by a 25X objective (N.A. = 0.5). The pulses were pre-compensated so that they were essentially transform-limited (45 fs) at the fused silica sample which was mounted on a 3D motorized translation stage.

Fig. 1 Experimental Setup. The inset shows the third harmonic signal as a function of the translation distance along x-axis. The rising and falling edges correspond to the opening and closing of the electronic shutter. The gradually reduced third harmonic signal indicates the decrease of the laser intensity due to the displacement of the focal spot from the sample surface.

Based on the fact that the third harmonic signal is critically generated at the interface between a glass surface and air, the sample surface and the x-axis translation stage were precisely aligned perpendicularly to the laser propagation direction by monitoring the retroreflection and the third harmonic signal during the scan. Figure 1 (inset) shows a typical plot of the third harmonic signal from the sample surface as a function of the scanned distance along x-axis. The fast rising and falling edges correspond to the opening and closing of the electronic shutter. The small slope in the third harmonic signal indicates that there is a slight displacement of the focal spot from the sample surface. The decrease of the laser intensity is about 1.7% for a translation distance of 1.5 mm estimated from the linear regression of the third harmonic signal (red line). This decrease is roughly of the order of the RMS of the laser signal whose effect on the nanograting formation can be considered as negligible. After the writing, the sample was etched with 1% HF acid in the ultrasonic bath for about 2 minutes then imaged under a scanning electron microscope (FEI, model Quanta 3D FEG).

3. Ablation threshold for nanograting formation

Fig. 2 (a). Schematic drawing showing the threshold effect of nanograting formation. (b). The SEM pictures for nanograting formation at different pulse energies (90, 100, 110, 120 nJ/pulse) for a given pulse to pulse spacing 10 nm. K: laser propagation direction; S: scan direction; E: electric field
Fig. 3 The overall extent of the nanogratings as a function of incident pulse energy for three different pulse to pulse spacings (5, 10 and 20 nm). Solid lines were fitted according to Eq. (2).

Table 1. Measured Beam Radii, Threshold Energies and the Computed Threshold Fluences for Different Pulse to Pulse Spacings

table-icon
View This Table

The good agreement between experimental data and the fit based on Eq. (2) supports our assumption that nanograting formation relies on a laser ablation process. This assumption is further supported by the depth of material removed in the process. Figure 4 shows the cross-section of the nanogratings written with pulse energy 100 nJ at different pulse to pulse spacing. For this experiment the laser polarization is parallel to the scan direction. To produce those pictures, the nanogratings were first filled by platinum deposition (on the right side of each picture) and then etched down by a focused ion beam (FIB) to expose the cross-section of the gratings. Note that the nanogrooves on the right side are completely filled with platinum while those on the left are partially filled due to the diffusion of the platinum during the deposition. The purpose of the deposition of the platinum before etching is to avoid the so-called curtain effect [15

15. L. A. Giannuzzi and F. A. Stevie, Introduction to Focused Ion Beams (Springer Science + Business Media, Inc., 2005). [CrossRef]

] so that the depth of the nanogrooves resulting from the material removal by the laser can be precisely measured. The curtain effect can be seen in the lower half on the left side of the pictures where the nanogrooves are just partially filled with platinum. With the decrease of pulse to pulse spacing (from 100 nm to 20 nm), the pulse overlap is thus increased. This results in an increase of the ablation rate because of the decrease of the ablation threshold and/or the increase of the effective absorption coefficient [10

10. C. Phipps, Laser Ablation and Its Applications (Springer, 2006).

]. An increase of the depth of removed material is thus expected and experimentally observed (from 363 nm to 444 nm), in agreement with an ablation process.

Fig. 4 Nanogratings written at pulse energy 100 nJ at different pulse to pulse spacings. K: laser direction; E: electric field; S: scan direction. d: pulse to pulse spacing. (a). d = 100 nm, depth = 363 nm. (b). d = 80 nm, depth = 418 nm. (c). d = 40 nm, depth = 428 nm. (d). d = 20 nm, depth = 444 nm.

4. Incubation

As shown in Fig. 5, the nanograting formation ablation threshold drops dramatically for small wz/d corresponding to low effective pulse number, but it rapidly becomes level as the number of overlapping pulses increases. The lower limit of the nanograting formation threshold is probably due to the saturation of the laser-induced defects [4

4. A. Rosenfeld, M. Lorenz, R. Stoian, and D. Ashkenasi, “Ultrashort-laser-pulse damage threshold of transparent materials and the role of incubation,” Appl. Phys. A 69, S373–S376 (1999). [CrossRef]

], i.e., the ablation threshold cannot be decreased further by decreasing the pulse to pulse spacing. Through the fit (red curve) according to Eq. ( 3), the single and infinity shot ablation thresholds Fss = 3.89 J/cm2 and F0 = 3.06 J/cm2 are derived together with the value of k (see below). This represents threshold reduction of about 25%. Experimentally we start to observe laser ablation under the SEM at 3.98 ± 0.11 J/cm2 with single shot and no modification at 2.99 ± 0.11 J/cm2 with pulse numbers up to 1000 shots. These two experimental values are close to those (Fss and F0) derived by the fitting, which suggests that the modified dielectric incubation equation is good for nanograting formation. The derived incubation factor k = 0.034 indicates that incubation is weak yet very important in the nanograting formation.

Fig. 5 The incubation curve for nanograting formation. The ablation threshold drops dramatically and levels off at low and high effective pulse number (wz/d), respectively.

According to our experimental observation (section 3 and Fig. 2(b)), the nanograting would be damaged if the incident pulse fluence is about 20% higher than the ablation threshold whose value is influenced by the incubation effect. Therefore, in order to write a good nanograting and avoid damage, the incident pulse fluence should be always kept within the 20% range above the ablation threshold. In other words, the incident pulse fluence should be adjusted in terms of the pulse to pulse spacing based on the modified incubation equation (Eq. (3)).This should be applicable to writing nanograting on the surface of other transparent dielectrics.

5. Conclusion

We have demonstrated that nanograting formation on the surface of fused silica is basically a laser ablation process. The ablation threshold fluence is influenced by the incubation effect which in turn depends on the pulse to pulse spacing. The overall extent of the nanograting follows a logarithmic behavior with pulse fluence and the ablation depth increases with the increase of the effective pulse number within the focal spot. To obtain good gratings without damage, the incident pulse fluence should be carefully selected in terms of the pulse to pulse spacing because of the incubation effect. A modified incubation equation was proposed to describe the role of the incubation process in nanograting formation.

Acknowledgments

This work is supported by the Natural Sciences and Engineering Research Council of Canada, Le Fonds Québécois de la Recherche sur la Nature et les Technologies, Canada Research Chairs, Canada Foundation for Innovation, Canadian Institute for Photonic Innovations, and Ministère du Développement économique, de l’Innovation et de l’Exportation. The authors appreciate Mr. S. Gagnon and Mr. M. Martin for the technical support.

References and links

1.

D. K. Sardar, M. F. Becker, and R. M. Walser, “Multipulse laser damage of GaAs surfaces,” J. Appl. Phys. 62, 3688–3693 (1987). [CrossRef]

2.

Y. Jee, M. F. Becker, and R. M. Walser, “Laser-induced damage on single-crystal metal surfaces,” J. Opt. Soc. Am. B 5, 648–659 (1988). [CrossRef]

3.

D. Ashkenasi, M. Lorenz, R. Stoian, and A. Rosenfeld, “Surface damage threshold and structuring of dielectrics using femtosecond laser pulses: the role of incubation,” Appl. Surf. Sci. 150, 101–106 (1999). [CrossRef]

4.

A. Rosenfeld, M. Lorenz, R. Stoian, and D. Ashkenasi, “Ultrashort-laser-pulse damage threshold of transparent materials and the role of incubation,” Appl. Phys. A 69, S373–S376 (1999). [CrossRef]

5.

M. Lenzner, J. Krüger, W. Kautek, and F. Krausz, “Incubation of laser ablation in fused silica with 5-fs pulses,” Appl. Phys. A 69, 465–466 (1999). [CrossRef]

6.

X. Liu, D. Du, and G. Mourou, “Laser ablation and micromachining with ultrashort laser pulses,” IEEE J. Quantum Electron. 33, 1707–1716 (1997). [CrossRef]

7.

F. Liang, Q. Sun, D. Gingras, R. Vallée, and S. L. Chin, “The transition from smooth modification to nanograting in fused silica,” Appl. Phys. Lett. 96, 101903 (2010). [CrossRef]

8.

Y. Barad, H. Eisenberg, M. Horowitz, and Y. Silberberg, “Nonlinear scanning laser microscopy by third harmonic generation,” Appl. Phys. Lett. 70, 922–924 (1997). [CrossRef]

9.

F. Liang, Q. Sun, R. Vallée, and S. L. Chin, “Multiple refocusing characterization and critical power measurement using third harmonic generation at interface,” Appl. Phys. B 99, 235–239 (2009). [CrossRef]

10.

C. Phipps, Laser Ablation and Its Applications (Springer, 2006).

11.

V. R. Bhardwaj, E. Simova, P. P. Rajeev, C. Hnatovsky, R. S. Taylor, D. M. Rayner, and P. B. Corkum, “Optically produced arrays of planar nanostructures inside fused silica,” Phys. Rev. Lett. 96, 057404 (2006). [CrossRef] [PubMed]

12.

X. Guo, R. Li, Y. Hang, Z. Xu, B. Yu, Y. Dai, B. Lu, and X. Sun, “Coherent linking of periodic nano-ripples on a ZnO crystal surface induced by femtosecond laser pulses,” Appl. Phys. A 94, 423–426 (2009). [CrossRef]

13.

J. Liu, “Simple technique for measurements of pulsed Gaussian-beam spot sizes,” Opt. Lett. 7, 196–198 (1982). [CrossRef] [PubMed]

14.

N. Sanner, O. Utéza, B. Bussiere, G. Coustillier, A. Leray, T. Itina, and M. Sentis, “Measurement of femtosecond laser-induced damage and ablation thresholds in dielectrics,” Appl. Phys. A 94, 889–897 (2009). [CrossRef]

15.

L. A. Giannuzzi and F. A. Stevie, Introduction to Focused Ion Beams (Springer Science + Business Media, Inc., 2005). [CrossRef]

OCIS Codes
(140.3390) Lasers and laser optics : Laser materials processing
(140.3440) Lasers and laser optics : Laser-induced breakdown
(220.4241) Optical design and fabrication : Nanostructure fabrication
(050.6624) Diffraction and gratings : Subwavelength structures

ToC Category:
Laser Materials Processing

History
Original Manuscript: August 19, 2011
Revised Manuscript: October 5, 2011
Manuscript Accepted: October 9, 2011
Published: October 13, 2011

Citation
Feng Liang, Réal Vallée, Daniel Gingras, and See Leang Chin, "Role of ablation and incubation processes on surface nanograting formation," Opt. Mater. Express 1, 1244-1250 (2011)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-1-7-1244


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References

  1. D. K. Sardar, M. F. Becker, and R. M. Walser, “Multipulse laser damage of GaAs surfaces,” J. Appl. Phys.62, 3688–3693 (1987). [CrossRef]
  2. Y. Jee, M. F. Becker, and R. M. Walser, “Laser-induced damage on single-crystal metal surfaces,” J. Opt. Soc. Am. B5, 648–659 (1988). [CrossRef]
  3. D. Ashkenasi, M. Lorenz, R. Stoian, and A. Rosenfeld, “Surface damage threshold and structuring of dielectrics using femtosecond laser pulses: the role of incubation,” Appl. Surf. Sci.150, 101–106 (1999). [CrossRef]
  4. A. Rosenfeld, M. Lorenz, R. Stoian, and D. Ashkenasi, “Ultrashort-laser-pulse damage threshold of transparent materials and the role of incubation,” Appl. Phys. A69, S373–S376 (1999). [CrossRef]
  5. M. Lenzner, J. Krüger, W. Kautek, and F. Krausz, “Incubation of laser ablation in fused silica with 5-fs pulses,” Appl. Phys. A69, 465–466 (1999). [CrossRef]
  6. X. Liu, D. Du, and G. Mourou, “Laser ablation and micromachining with ultrashort laser pulses,” IEEE J. Quantum Electron.33, 1707–1716 (1997). [CrossRef]
  7. F. Liang, Q. Sun, D. Gingras, R. Vallée, and S. L. Chin, “The transition from smooth modification to nanograting in fused silica,” Appl. Phys. Lett.96, 101903 (2010). [CrossRef]
  8. Y. Barad, H. Eisenberg, M. Horowitz, and Y. Silberberg, “Nonlinear scanning laser microscopy by third harmonic generation,” Appl. Phys. Lett.70, 922–924 (1997). [CrossRef]
  9. F. Liang, Q. Sun, R. Vallée, and S. L. Chin, “Multiple refocusing characterization and critical power measurement using third harmonic generation at interface,” Appl. Phys. B99, 235–239 (2009). [CrossRef]
  10. C. Phipps, Laser Ablation and Its Applications (Springer, 2006).
  11. V. R. Bhardwaj, E. Simova, P. P. Rajeev, C. Hnatovsky, R. S. Taylor, D. M. Rayner, and P. B. Corkum, “Optically produced arrays of planar nanostructures inside fused silica,” Phys. Rev. Lett.96, 057404 (2006). [CrossRef] [PubMed]
  12. X. Guo, R. Li, Y. Hang, Z. Xu, B. Yu, Y. Dai, B. Lu, and X. Sun, “Coherent linking of periodic nano-ripples on a ZnO crystal surface induced by femtosecond laser pulses,” Appl. Phys. A94, 423–426 (2009). [CrossRef]
  13. J. Liu, “Simple technique for measurements of pulsed Gaussian-beam spot sizes,” Opt. Lett.7, 196–198 (1982). [CrossRef] [PubMed]
  14. N. Sanner, O. Utéza, B. Bussiere, G. Coustillier, A. Leray, T. Itina, and M. Sentis, “Measurement of femtosecond laser-induced damage and ablation thresholds in dielectrics,” Appl. Phys. A94, 889–897 (2009). [CrossRef]
  15. L. A. Giannuzzi and F. A. Stevie, Introduction to Focused Ion Beams (Springer Science + Business Media, Inc., 2005). [CrossRef]

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