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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 4 — Feb. 13, 2012
  • pp: 4389–4396
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Mechanism of nanograting formation on the surface of fused silica

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


Optics Express, Vol. 20, Issue 4, pp. 4389-4396 (2012)
http://dx.doi.org/10.1364/OE.20.004389


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Abstract

Nanograting inscription with a tightly focused femtosecond beam on the surface of fused silica was studied. The width and spacing of grooves are shown to decrease with the increase of the number of overlapped shots in both stationary and scanning cases. We propose a model to explain this behavior, based on both the so-called nanoplasmonic model and the incubation effect.

© 2012 OSA

1. Introduction

Nanograting formation has been intensively studied over the last two decades. Many investigations were conducted to study their formation mechanism as a function of writing parameters such as laser wavelength, pulse energy, repetition rate, polarization, scan speed, and so on [1

1. C. Hnatovsky, R. S. Taylor, P. P. Rajeev, E. Simova, V. R. Bhardwaj, D. M. Rayner, and P. B. Corkum, “Pulse duration dependence of femtosecond-laser-fabricated nanogratings in fused silica,” Appl. Phys. Lett. 87, 014104 (2005). [CrossRef]

7

7. Q. Sun, F. Liang, R. Vallée, and S. L. Chin, “Nanograting formation on the surface of silica glass by scanning focused femtosecond laser pulses,” Opt. Lett. 33, 2713–2715 (2008). [CrossRef] [PubMed]

]. It has been shown that the orientation of nanograting is perpendicular to the linear electric field due to local field enhancement [5

5. 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]

] and that well-shaped nanogratings are created when the incident laser intensity lies slightly above the threshold for nanograting formation [9

9. 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]

]. Several models based on self-organization [2

2. M. Henyk, N. Vogel, D. Wolfframm, A. Tempel, and J. Reif, “Femtosecond laser ablation from dielectric materials: comparison to arc discharge erosion” Appl. Phys. A 69, S355–S358 (1999). [CrossRef]

], interference [3

3. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultra-short light pulses” Phys. Rev. Lett. 91, 247405 (2003). [CrossRef] [PubMed]

, 4

4. M. Huang, F. Zhao, Y. Cheng, N. Xu, and Z. Xu, “Origin of laser-induced near-subwavelength ripples: interference between surface plasmons and incident Laser,” ACS Nano 3, 4062–4070 (2009). [CrossRef] [PubMed]

], nanoplasmonics [5

5. 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]

] and standing wave [8

8. R. Buividas, L. Rosa, R. Šliupas, T. Kudrius, G. Šlekys, V. Datsyuk, and S. Juodkazis, “Mechanism of fine ripple formation on surfaces of (semi)transparent materials via a half-wavelength cavity feedback,” Nanotechnology 22, 055304 (2011). [CrossRef]

] have been proposed. However, none of these models can satisfactorily explain the dependence of the width and spacing of nanoplanes/nanogrooves as a function of the number of overlapped shots [3

3. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultra-short light pulses” Phys. Rev. Lett. 91, 247405 (2003). [CrossRef] [PubMed]

] or pulse to pulse spacing [7

7. Q. Sun, F. Liang, R. Vallée, and S. L. Chin, “Nanograting formation on the surface of silica glass by scanning focused femtosecond laser pulses,” Opt. Lett. 33, 2713–2715 (2008). [CrossRef] [PubMed]

]. In this paper, we present a parallel study of the formation of the nanogratings both in the stationary case (i.e. on a pulse-to-pulse evolution basis) as well as in the scanning case (i.e. as a function of the pulse to pulse spacing). The precise shape of local intensity distribution (i.e. nanoplasmonic effect [5

5. 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]

]) together with the reduction of the ablation threshold (i.e. incubation effect [10

10. 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]

12

12. F. Liang, R. Vallée, D. Gingras, and S. L. Chin, “Role of ablation and incubation processes on surface nanograting formation,” Opt. Mater. Express 1, 1244–1250 (2011). [CrossRef]

]) are shown to be responsible for nanograting formation. A model is proposed accordingly.

2. Experiment

We used two Ti-sapphire lasers, Coherent RegA 9000 (beam ‘A’) and Spectra-Physics Spit-fire (beam ‘B’) for writing nanogratings in the stationary and scanning cases, respectively. The central wavelengths of both laser systems are 800 nm. Note that the laser beam with a different pulse duration would slightly change the ablation threshold and plasma density during the interaction with transparent materials [13

13. A. Tien, S. Backus, H. Kapteyn, M. Murnane, and G. Mourou, “Short-pulse laser damage in transparent materials as a function of pulse duration,” Phys. Rev. Lett. 82, 3883–3886 (1999). [CrossRef]

]. In our experiment, the laser beam was focused by an microscope objective (Melles Griot 25X, N.A. = 0.5) onto the surface of a fused silica plate (Corning 7980) mounted on a 3D motorized translation stage. A circular variable metallic ND filter was used to control the incident laser energy. The focal spot diameters (1/e2 of the fluence profile) and transform limited pulse durations (FWHM) were 2.4 μm, 80 fs for beam ‘A’ and 2.6 μm, 45 fs for beam ‘B’, respectively. Single shot mode was used in the stationary case. A series of ablation spots was created with different number of shots per site. The delay between two successive shots was about 2 seconds. In the scanning case, nanogratings were written by translating the sample perpendicular to the laser propagation direction. After the writing, the samples were imaged under a scanning electron microscope (SEM, FEI Quanta 3D FEG).

3. Results and discussion

Fig. 1 The shot-to-shot evolution of nanogrooves at 90 nJ/pulse. The dashed lines indicate the location for cross-section roughly.

In the scanning case, the nanogratings were written as a function of pulse to pulse spacing d for a pulse energy of 100 nJ/pulse and with laser polarization either perpendicular or parallel to the scan direction (Figs. 2(a) and 2(b)). In both cases, the width and spacing of the grooves increase significantly with d. However, slight damage is observed for d ≤ 20 nm with the laser polarization parallel to the scan direction (Fig. 2(b)).

Fig. 2 Nanograting formation as a function of d at 100 nJ/pulse. K: laser propagation direction; S: scan direction; E: laser polarization direction.

Fig. 3 (a) Local intensity distribution of the first shot (normalized with respect to the incident peak intensity). (b) and (c) Local intensity distribution along y-axis and x-axis. (Simulated with plasma density: 2.5 × 1021/cm3; single shot ablation threshold: 3.95J/cm2; pulse energy: 90 nJ/pulse, pulse width: 80 fs; focal spot diameter: 2.4 μm.)
Fig. 4 Local intensity distribution as a function of the number of laser shots (top) and the corresponding ideal patterns in the (x,y) plane (bottom). The red line is the normalized incident laser intensity. These plots are obtained with the following parameters: plasma density: 2.5 × 1021/cm3 for all nanogrooves; ablation threshold for (a) 3.95 J/cm2; (b) 3.46 J/cm2; (c) 3.05 J/cm2; (d) 2.95 J/cm2; pulse energy: 90 nJ/pulse, pulse width: 80 fs; focal spot diameter: 2.4 μm.

Fig. 5 Schematic drawing showing the modification of local intensity for the case of laser polarization parallel to the scan direction. The self-repetition of increase of the local intensity and decrease of the ablation threshold at the leading side-maximum in (b) is the driver for ordered grating formation.
Fig. 6 Evolution of width (a) and spacing (b) of nanogrooves at 100nJ/pulse with laser polarization parallel to the scan direction. The red curve in (b) corresponds to the simulation performed with the following parameters: plasma density: 2.5 × 1021/cm3; pulse energy: 106 nJ/pulse, pulse width: 42 fs; focal spot diameter: 2.56 μm and the ablation fluence is following: Fd = 3.06 + (3.89 – 3.06)exp(−0.034(1.28/d – 1)) (see Ref. [12])

4. Conclusion

In summary, we have derived a model based on nanoplasmonics that accounts for the formation of periodic nanogrooves at the surface of glass upon exposure to ultrashort pulses. The evolution of the local intensity distribution from shot to shot, together with the reduced ablation threshold, essentially governs this nanograting formation. In particular, the local field side-maxima appearing along the laser polarization axis are shown to play a key role in triggering the nanoplane formation.

Appendix

The sphere of slightly over dense plasma induced at the laser peak modifies the electric field local distribution (see Fig. 3). From the external (e) and internal (i) electric potentials Ve=Elrcos(θ)+ɛ1ɛ+2R03r2Elcos(θ) and Vi=3rɛ+2Elcos(θ)[14

14. M. A. Plonus, Applied Electro-Magnetics (McGraw-Hill, 1978).

], one can derive the moduli of the external Ee and internal Ei fields in response to the laser field El:
Ee=|Verr^1rVeθθ^|=(A02+B02)El,
(1)
Ei=|Virr^1rViθθ^|=C0El
(2)
where A0=(1+2ɛ1ɛ+2R03r3)cos(θ), B0=(ɛ1ɛ+2R03r31)sin(θ), and C0=3ɛ+2, respectively, are local field modification factors resulting from the plasma. The subscript ‘0’ is introduced to identify this zone which will become the central groove and ɛ′ = ɛi/ɛe (ɛi = −0.1538, ɛi = 2.1025). We note that the ablation zone is small so that the electric field can be considered as constant over it. We therefore assume in the following that the plasma density in the ablation zone is constant and uniformly distributed. Thus, we have fixed in our model the value of the plasma density to 2.5 × 1021/cm3 corresponding to a slightly over dense plasma. This value was actually found to better account for our experimental data although any value between 2 and 3 × 1021/cm3 lead to reasonable agreement with the experiment. The θ is the polar angle with respect to x-axis and R0 is the radius of the central ablation zone as depicted on Fig. 7. The internal dielectric constant ɛi=n2e2Nplmeɛ0(ωl2+(1/τc)2) was calculated according to the Drude model [15

15. A. Q. Wu, I. H. Chowdhury, and X.-F. Xu, “Femtosecond laser absorption in fused silica: numerical and experimental investigation,” Phys. Rev. B 72, 085128 (2005). [CrossRef]

] where n = 1.45 is the refractive index of the sample, e is the electron charge, Npl is the plasma density, me = 0.635m is the effective electron mass, m is the electron mass, ɛ0 is the vacuum permittivity, ωl is the laser frequency, and τc = 23.3 fs is the electron collision time. The initial value of R0 can be simply calculated by R0=w02ln(Il/Ith)/2, where Ith is the single shot ablation threshold and Il is the laser intensity distribution. To simplify the analysis we restrict ourselves in the following to the evolution of the local intensity distribution along the x-axis. By setting θ = 0, we obtain:
Ie,0=A02Il,Ii,0=C02IlIlocal,0=Ie,0+Ii,0
(3)
We note that the local intensity distribution Ilocal,0 has two side-maxima. With the increase of the number of shots, a pair of new ablation zones (with half width R1) will be created at distance s1 from the central groove once the reduced ablation threshold is exceeded (see Fig. 7). The presence of this new pair of ablation zones will in turn affect the local intensity distribution according to a new set of local field modification factors. Iteratively, new pairs of ablation zones will be created with modification factors taking the general form:
An+=[1+2ɛ1ɛ+2Rn3(xsn)3],|xsn|>Rn,An=[1+2ɛ1ɛ+2Rn3(x+sn)3],|x+sn|>Rn,Cn+=Cn=C0=3ɛ+2,|x±sn|Rn,n1
(4)
where Rn and sn are generalized from R1 and s1 defined in Fig. 7. The integer ‘n’ thus stands for the nth pair of grooves whereas ‘+/−’ refers to the groove on the right/left hand side of the central groove (as labeled in Fig. 4). Thus, the local intensity distributions in the presence of multiple grooves become:
Ie,n=(An+2+An2)Ilocal,n1Ii,n=(Cn+2+Cn2)Ilocal,n1Ilocal,n=Ie,n+Ii,n,n1
(5)
By applying the standard recursive method for the local intensity distribution, sn, Rn and Ilocal,n are iteratively determined. In the scanning case (with electric field parallel to scan direction), we assume it is the laser that is moved instead of the sample. The moving laser is defined as: Il=I0exp[2(x(N1)d)2/w02], where N is the pulse number. The new groove is created only by the leading side-maximum whose distribution is mainly governed by its adjacent groove:
Ie,n=An2Il,|xsn|>Rn,n0
(6)
where: An=[1+2ɛ1ɛ+2Rn3(xsn)3]. The initial position of the first groove s0 is set to 0. During the scanning, the amplitude of leading side-maximum is boosted which leads, through incubation, to a reduction of ablation threshold in the corresponding area. Ablation followed by groove formation occur once this side-maximum exceeds the reduced ablation threshold (s1 and R1 are thus determined). A new leading side-maximum then arises which eventually lead to the onset of a new groove in such a way that the process repeats itself until the beam scanning stops. Because of our limited knowledge of shot-to-shot ablation threshold, the width of grooves is difficult to calculate. In the simulation, the width of grooves was set in agreement with the experimental results (Fig. 1 and Fig. 6(a)) and the previously reported average reduced ablation threshold [12

12. F. Liang, R. Vallée, D. Gingras, and S. L. Chin, “Role of ablation and incubation processes on surface nanograting formation,” Opt. Mater. Express 1, 1244–1250 (2011). [CrossRef]

] was also used.

Fig. 7 Evolution of local intensity distribution along x-axis as a function of the number of pulses.

Acknowledgments

This work is supported by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation and the Canadian Institute for Photonic Innovations. We thank Mrs S. Gagnon, M. Martin and D. Gingras for the technical support and Dr. Q.Q. Wang for the helping in the experiment.

References and links

1.

C. Hnatovsky, R. S. Taylor, P. P. Rajeev, E. Simova, V. R. Bhardwaj, D. M. Rayner, and P. B. Corkum, “Pulse duration dependence of femtosecond-laser-fabricated nanogratings in fused silica,” Appl. Phys. Lett. 87, 014104 (2005). [CrossRef]

2.

M. Henyk, N. Vogel, D. Wolfframm, A. Tempel, and J. Reif, “Femtosecond laser ablation from dielectric materials: comparison to arc discharge erosion” Appl. Phys. A 69, S355–S358 (1999). [CrossRef]

3.

Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultra-short light pulses” Phys. Rev. Lett. 91, 247405 (2003). [CrossRef] [PubMed]

4.

M. Huang, F. Zhao, Y. Cheng, N. Xu, and Z. Xu, “Origin of laser-induced near-subwavelength ripples: interference between surface plasmons and incident Laser,” ACS Nano 3, 4062–4070 (2009). [CrossRef] [PubMed]

5.

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]

6.

W. Yang, E. Bricchi, P. G. Kazansky, J. Bovatsek, and A. Y. Arai, “Self-assembled periodic sub-wavelength structures by femtosecond laser direct writing,” Opt. Express 14, 10117–10124 (2006). [CrossRef] [PubMed]

7.

Q. Sun, F. Liang, R. Vallée, and S. L. Chin, “Nanograting formation on the surface of silica glass by scanning focused femtosecond laser pulses,” Opt. Lett. 33, 2713–2715 (2008). [CrossRef] [PubMed]

8.

R. Buividas, L. Rosa, R. Šliupas, T. Kudrius, G. Šlekys, V. Datsyuk, and S. Juodkazis, “Mechanism of fine ripple formation on surfaces of (semi)transparent materials via a half-wavelength cavity feedback,” Nanotechnology 22, 055304 (2011). [CrossRef]

9.

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]

10.

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]

11.

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]

12.

F. Liang, R. Vallée, D. Gingras, and S. L. Chin, “Role of ablation and incubation processes on surface nanograting formation,” Opt. Mater. Express 1, 1244–1250 (2011). [CrossRef]

13.

A. Tien, S. Backus, H. Kapteyn, M. Murnane, and G. Mourou, “Short-pulse laser damage in transparent materials as a function of pulse duration,” Phys. Rev. Lett. 82, 3883–3886 (1999). [CrossRef]

14.

M. A. Plonus, Applied Electro-Magnetics (McGraw-Hill, 1978).

15.

A. Q. Wu, I. H. Chowdhury, and X.-F. Xu, “Femtosecond laser absorption in fused silica: numerical and experimental investigation,” Phys. Rev. B 72, 085128 (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 Microfabrication

History
Original Manuscript: November 23, 2011
Revised Manuscript: December 21, 2011
Manuscript Accepted: December 23, 2011
Published: February 8, 2012

Citation
Feng Liang, Réal Vallée, and See Leang Chin, "Mechanism of nanograting formation on the surface of fused silica," Opt. Express 20, 4389-4396 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-4-4389


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References

  1. C. Hnatovsky, R. S. Taylor, P. P. Rajeev, E. Simova, V. R. Bhardwaj, D. M. Rayner, and P. B. Corkum, “Pulse duration dependence of femtosecond-laser-fabricated nanogratings in fused silica,” Appl. Phys. Lett.87, 014104 (2005). [CrossRef]
  2. M. Henyk, N. Vogel, D. Wolfframm, A. Tempel, and J. Reif, “Femtosecond laser ablation from dielectric materials: comparison to arc discharge erosion” Appl. Phys. A69, S355–S358 (1999). [CrossRef]
  3. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultra-short light pulses” Phys. Rev. Lett.91, 247405 (2003). [CrossRef] [PubMed]
  4. M. Huang, F. Zhao, Y. Cheng, N. Xu, and Z. Xu, “Origin of laser-induced near-subwavelength ripples: interference between surface plasmons and incident Laser,” ACS Nano3, 4062–4070 (2009). [CrossRef] [PubMed]
  5. 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]
  6. W. Yang, E. Bricchi, P. G. Kazansky, J. Bovatsek, and A. Y. Arai, “Self-assembled periodic sub-wavelength structures by femtosecond laser direct writing,” Opt. Express14, 10117–10124 (2006). [CrossRef] [PubMed]
  7. Q. Sun, F. Liang, R. Vallée, and S. L. Chin, “Nanograting formation on the surface of silica glass by scanning focused femtosecond laser pulses,” Opt. Lett.33, 2713–2715 (2008). [CrossRef] [PubMed]
  8. R. Buividas, L. Rosa, R. Šliupas, T. Kudrius, G. Šlekys, V. Datsyuk, and S. Juodkazis, “Mechanism of fine ripple formation on surfaces of (semi)transparent materials via a half-wavelength cavity feedback,” Nanotechnology22, 055304 (2011). [CrossRef]
  9. 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]
  10. 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]
  11. 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]
  12. F. Liang, R. Vallée, D. Gingras, and S. L. Chin, “Role of ablation and incubation processes on surface nanograting formation,” Opt. Mater. Express1, 1244–1250 (2011). [CrossRef]
  13. A. Tien, S. Backus, H. Kapteyn, M. Murnane, and G. Mourou, “Short-pulse laser damage in transparent materials as a function of pulse duration,” Phys. Rev. Lett.82, 3883–3886 (1999). [CrossRef]
  14. M. A. Plonus, Applied Electro-Magnetics (McGraw-Hill, 1978).
  15. A. Q. Wu, I. H. Chowdhury, and X.-F. Xu, “Femtosecond laser absorption in fused silica: numerical and experimental investigation,” Phys. Rev. B72, 085128 (2005). [CrossRef]

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