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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 22 — Oct. 29, 2007
  • pp: 14524–14529
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Orientation-controllable self-organized microgratings induced in the bulk SrTiO3 crystal by a single femtosecond laser beam

Juan Song, Xinshun Wang, Xiao Hu, Jian Xu, Yang Liao, Haiyi Sun, Jianrong Qiu, and Zhizhan Xu  »View Author Affiliations


Optics Express, Vol. 15, Issue 22, pp. 14524-14529 (2007)
http://dx.doi.org/10.1364/OE.15.014524


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Abstract

Self-organized microgratings were induced in the bulk SrTiO3 crystal by readily scanning the laser focus in the direction perpendicular to the laser propagation axis. The groove orientations of those gratings could be controlled by changing the irradiation pulse number per unit scanning length, which could be implemented either through adjusting the scanning velocity at a fixed pulse repetition rate or through varying the pulse repetition rate at a fixed scanning velocity. This high-speed method for fabrication of microgratings will have many potential applications in the integration of micro-optical elements. The possible formation mechanism of the self-organized microgratings is also discussed.

© 2007 Optical Society of America

1. Introduction

In recent times, there is a rapid development in ultrafast laser technology [1

1. E. V. Pestryakov, A. I. Alimpiev, and V. N. Matrosov, “Prospects for the development of femtosecond laser systems based on beryllium aluminate crystals doped with chromium and titanate ions,” Quantum Elect. 31, 689–696 (2001). [CrossRef]

], the field of femtosecond laser processing is extended to wider applications [2–4

2. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21, 1729–1731 (1996). [CrossRef] [PubMed]

]. Among these applications, periodic micro-/nano-structuring of materials attracts special attention for its potential value in all-optical circuits. Traditionally, one way for fabricating a grating is, to employ a two-beam interference configuration [5

5. B. Tan, Narayanswamy, R. Sivakumar, and K. Venkatakrishnan, “Direct grating writing using femtosecond laser interference fringes formed at the focal point,” J. Opt. A 7, 169–174 (2005). [CrossRef]

], and an alternative way is to scan the focal spot of the laser beam line by line [6

6. Q. Z. Zhao, J. R. Qiu, X. W. Jiang, C. J. Zhao, and C. S. Zhu, “Fabrication of internal diffraction gratings in calcium fluoride crystals by a focused femtosecond laser,” Opt. Express 12, 742–746 (2004). [CrossRef] [PubMed]

]. Obviously, the former method is technically complex because of its high requirement for the spatial and temporal alignment between two laser beams, while the latter one is very time-consuming. In this paper, we have reported a kind of self-organized micrograting induced in the bulk SrTiO3 crystal by a near-infrared femtosecond laser. Here, we have reported a much more simple method for fabricating gratings in the bulk SrTiO3 crystal: by translating the laser focal point perpendicular to the beam propagation direction, a regular grating was self-fabricated in the side face. Although the interior self-organized gratings have been reported inside fused silica glass both in the light propagation direction and in the plane perpendicular to the laser beam axis [7–11

7. C. W. Smelser, D. Grobnic, and S. J. Mihailov, “Generation of pure two-beam interference grating structures in an optical fiber with a femtosecond infrared source and a phase mask,” Opt. Lett. 29, 1730–1732 (2004). [CrossRef] [PubMed]

], to the best of our knowledge, it is the first time that a regular large-area self-organized micrograting is observed in the plane determined by the translation direction and the light propagation direction.

2. Experimental setup

In our experiment, the sample was a piece of SrTiO3 single crystal grown by the flame-fusion method. SrTiO3 crystal, whose high refractive index of 2.34 helps to heavily increase the refractive index contrast with respect to voids fabricated inside it, has considerable potential to function as photonic crystal. The SrTiO3 sample with the dimensions of 10mm×10mm×3mm was polished on the (1 0 0) crystal plane for laser irradiation and then polished on the (0 1 0) crystal plane for microscopic observation. The experimental scheme is shown in Fig. 1 in detail: firstly, the laser beam was tightly focused into SrTiO3 crystal at a fixed depth, and then a straight line was inscribed by translating the laser focus along the y axis in the (1 0 0) crystal plane, perpendicular to the laser beam propagation axis (z-axis). Finally, after laser irradiation, the side facet (y-z plane) was placed under the optical microscope for observation.

Fig. 1. The schematic graph for femtosecond laser inducing self-organized microgratings in the bulk SrTiO3 crystal.

3. Experimental results

Figure 2 shows a typical self-organized grating induced in the bulk SrTiO3 crystal by employing the method mentioned above. The specific experimental parameters were as follows: laser pulses with the energy of 45 μJ were tightly focused through a 100× microscope objective (NA=0.9) into the SrTiO3 crystal. The laser focus was located at about 200 μm below the sample surface. The scanning velocity was set as 200 μm/s and the laser pulse repetition rate was fixed at 1 kHz. The light beam propagated from left to right. As shown in Fig. 2, except for a few nonparallel grooves at the first beginning, a well regular grating was self-formed with the grooves orientated nearly perpendicular to the laser propagation axis. It is easy for us to extend the size of the grating simply by increasing the scanning distance of the laser beam.

Fig. 2. A typical self-assembled micrograting induced at the focal depth of 200 μm beneath the sample surface by translating the laser focus along the direction perpendicular to the laser beam axis.

We further investigated the influence of the scanning velocities on the formation of the self-assembled microgratings as displayed in Fig. 3. Six different scanning velocities (300 μm/s, 250 μm/s, 200 μm/s, 150 μm/s, 100 μm/s and 50 μm/s) were chosen for comparison. The laser parameters and the focusing conditions were the same as that in Fig. 2. The inset of Fig. 3 shows the schematic of the characterization of the grating structure, where S denotes the scanning direction, g represents the groove orientation, and ϴ accounts for the angle between them. We define that ϴ is positive when the grating vector g situates on the left of the scanning direction vector S and negative otherwise. It is clear that with decreasing the scanning velocity, ϴ turns gradually from negative value to positive value. Interestingly, there exists a critical scanning velocity so that ϴ nearly approaches zero.

Fig. 3. The dependence of the grating structure on the scanning velocity at a fixed laser pulse repetition rate.

To further verify that the change of the groove orientation with the scanning velocity is an essential physical phenomenon but not a pseudomorph caused by the movement error, we carried out another group of experiments where the stage scanning velocity was fixed at a constant value of 80μm/s while the laser repetition rates were chosen as 166 Hz, 333 Hz, and 500 Hz respectively. The pulse energy used was also 45 μJ, and the focal depth was still 200 μm beneath the sample surface. As shown in Fig. 4, although the scanning velocity was kept unchanged in each case, the induced self-assembled microgratings have very distinct orientation angles ϴ.

Fig. 4. The influence of the laser pulse repetition rate on the grating structures at a fixed scanning velocity.

Based on two groups of experimental results described above, we conclude that the groove orientation can be varied by changing the irradiation pulse number per unit scanning length.

4. Discussion

Some kinds of self-organized periodic structures inside transparent dielectrics have been reported previously [9–11

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

]. They employed the same transverse direct-writing method as we did. Both groups pointed out that even if the laser focus was fixed inside the samples without translation, there still appeared periodic nanostructures in the cross sections both along and perpendicular to the laser beam propagation direction. Instead, in our case, if the laser focus was fixed, just a self-aligned void array appeared along the laser propagation axis [12

12. J. Song, X. S. Wang, J. Xu, H. Y. Sun, Z. Z. Xu, and J. R. Qiu, “Microstructures induced in the bulk of SrTiO3 crystal by a femtosecond laser,” Opt. Express 15, 2341–2347 (2007). [CrossRef] [PubMed]

] and additional translation of laser focus is needed to induce self-organized periodic microstructures of our kind. In addition, the groove orientations in their cases were controlled by changing the polarization state of the laser beam while in our case by adjusting the pulse number per unit scanning length. Obviously, different formation mechanisms dominated the formation of the self-organized periodic structures.

Fig. 5. A typical void string induced in the bulk SrTiO3 crystal by tightly focusing fs laser pulses through a 100× microscope objective.
Fig. 6. The schematic of a void-moving model.

To support the above conjecture, the following experiment was carried out. The femtosecond laser pulses with the energy of 30 μJ were tightly focused into the SrTiO3 crystal at a focal depth of 200 μm beneath the sample surface, and the scanning velocity was set as 250 μm/s. Two lines were written along the axis perpendicular to the laser beam axis, but difference between them is that one was written along the y+ direction and the other was written along the y- direction. The induced grating structures are presented in Fig. 7, where red arrows indicate the scanning directions. It is clear that the groove orientations in two cases are both tilted and well symmetric with respect to the laser propagation direction (z axis). This symmetry could be easily understood by employing our void-moving model, just as Fig. 8 demonstrates.

Fig. 7. Two different grating structures induced by scanning the laser focus in two opposite directions.
Fig. 8. The schematic of how the symmetry is formed.

5. Conclusion

In this paper, we have reported a kind of self-assembled micrograting induced in the bulk SrTiO3 crystal by translating a tightly-focused laser beam along the axis perpendicular to the laser propagation direction. The groove orientation of the micrograting could be controlled by changing the laser pulse number per unit scanning length. A void-moving model is proposed to qualitatively explain the formation mechanism of the grating as well as the variation of the groove orientation. This method for fabrication of gratings has the advantages of being mask free, high efficiency, selectivity and localizability, and it has high potential for applications in fabricating large-area gratings inside transparent dielectrics.

Acknowledgment

This work was supported by the National Basic Research Program of China (No. 2006CB806000).

References and links

1.

E. V. Pestryakov, A. I. Alimpiev, and V. N. Matrosov, “Prospects for the development of femtosecond laser systems based on beryllium aluminate crystals doped with chromium and titanate ions,” Quantum Elect. 31, 689–696 (2001). [CrossRef]

2.

K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21, 1729–1731 (1996). [CrossRef] [PubMed]

3.

H. Sun, Y. Xu, S. Juodkazis, K. Sun, M. Watanabe, S. Matsuo, H. Misawa, and J. Nishii, “Arbitrary-lattice photonic crystals created by multiphoton microfabrication,” Opt. Lett. 26, 325–327 (2001). [CrossRef]

4.

J. Qiu, C. Zhu, T. Nakaya, J. Si, F. Ogura, K. Kojima, and K. Hirao, “Space-selective valence state manipulation of transition metal ions inside glasses by a femtosecond laser,” Appl. Phys. Lett. 79, 3567–3569 (2001). [CrossRef]

5.

B. Tan, Narayanswamy, R. Sivakumar, and K. Venkatakrishnan, “Direct grating writing using femtosecond laser interference fringes formed at the focal point,” J. Opt. A 7, 169–174 (2005). [CrossRef]

6.

Q. Z. Zhao, J. R. Qiu, X. W. Jiang, C. J. Zhao, and C. S. Zhu, “Fabrication of internal diffraction gratings in calcium fluoride crystals by a focused femtosecond laser,” Opt. Express 12, 742–746 (2004). [CrossRef] [PubMed]

7.

C. W. Smelser, D. Grobnic, and S. J. Mihailov, “Generation of pure two-beam interference grating structures in an optical fiber with a femtosecond infrared source and a phase mask,” Opt. Lett. 29, 1730–1732 (2004). [CrossRef] [PubMed]

8.

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]

9.

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

10.

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

11.

C. Hnatovsky, E. Simova, P. P. Rajeev, D. M. Rayner, P. B. Corkum, and R. S. Taylor, “Femtosecond laser writing of porous capillaries inside fused silica glass,” Opt. Lett. 32, 1459–1461 (2007). [CrossRef] [PubMed]

12.

J. Song, X. S. Wang, J. Xu, H. Y. Sun, Z. Z. Xu, and J. R. Qiu, “Microstructures induced in the bulk of SrTiO3 crystal by a femtosecond laser,” Opt. Express 15, 2341–2347 (2007). [CrossRef] [PubMed]

13.

W. Watanabe and K. Itoh, “Motion of bubble in solid by femtosecond laser pulses,” Opt. Express 10, 603–608 (2002). [PubMed]

OCIS Codes
(220.4000) Optical design and fabrication : Microstructure fabrication
(320.2250) Ultrafast optics : Femtosecond phenomena
(050.6875) Diffraction and gratings : Three-dimensional fabrication

ToC Category:
Diffraction and Gratings

History
Original Manuscript: August 13, 2007
Revised Manuscript: September 20, 2007
Manuscript Accepted: October 9, 2007
Published: October 19, 2007

Citation
Juan Song, Xinshun Wang, Xiao Hu, Jian Xu, Yang Liao, Haiyi Sun, Jianrong Qiu, and Zhizhan Xu, "Orientation-controllable self-organized microgratings induced in the bulk SrTiO3 crystal by a single femtosecond laser beam," Opt. Express 15, 14524-14529 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-22-14524


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References

  1. E. V. Pestryakov, A. I. Alimpiev, and V. N. Matrosov, "Prospects for the development of femtosecond laser systems based on beryllium aluminate crystals doped with chromium and titanate ions," Quantum Elect. 31, 689-696 (2001). [CrossRef]
  2. K. M. Davis, K. Miura, N. Sugimoto and K. Hirao, "Writing waveguides in glass with a femtosecond laser," Opt. Lett. 21, 1729-1731 (1996). [CrossRef] [PubMed]
  3. H. Sun, Y. Xu, S. Juodkazis, K. Sun, M. Watanabe, S. Matsuo, H. Misawa, and J. Nishii, "Arbitrary-lattice photonic crystals created by multiphoton microfabrication," Opt. Lett. 26, 325-327 (2001). [CrossRef]
  4. J. Qiu, C. Zhu, T. Nakaya, J. Si, F. Ogura, K. Kojima, and K. Hirao, "Space-selective valence state manipulation of transition metal ions inside glasses by a femtosecond laser," Appl. Phys. Lett. 79, 3567-3569 (2001). [CrossRef]
  5. B. Tan, Narayanswamy, R. Sivakumar and K. Venkatakrishnan, "Direct grating writing using femtosecond laser interference fringes formed at the focal point," J. Opt. A 7, 169-174 (2005). [CrossRef]
  6. Q. Z. Zhao, J. R. Qiu, X. W. Jiang, C. J. Zhao, and C. S. Zhu, "Fabrication of internal diffraction gratings in calcium fluoride crystals by a focused femtosecond laser," Opt. Express 12, 742-746 (2004). [CrossRef] [PubMed]
  7. C. W. Smelser, D. Grobnic, and S. J. Mihailov, "Generation of pure two-beam interference grating structures in an optical fiber with a femtosecond infrared source and a phase mask," Opt. Lett. 29, 1730-1732 (2004). [CrossRef] [PubMed]
  8. 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]
  9. Y. Shimotsuma, P. G. Kazansky, J. R. Qiu, and K. Hirao, "Self-organized nanogratings in glass irradiation by ultrashort light pulses," Phys. Rev. Lett. 91, 247405 (2003). [CrossRef] [PubMed]
  10. W. J. Yang, E. Bricchi, P. G. Kazansky, J. Bovatsek and A. Y. Arai, "Self-assembled periodic subwavelength structures by femtosecond laser direct writing," Opt. Express 14, 10117-10124 (2006). [CrossRef] [PubMed]
  11. C. Hnatovsky, E. Simova, P. P. Rajeev, D. M. Rayner, P. B. Corkum, and R. S. Taylor, "Femtosecond laser writing of porous capillaries inside fused silica glass," Opt. Lett. 32, 1459-1461 (2007). [CrossRef] [PubMed]
  12. J. Song, X. S. Wang, J. Xu, H. Y. Sun, Z. Z. Xu, and J. R. Qiu, "Microstructures induced in the bulk of SrTiO3 crystal by a femtosecond laser," Opt. Express 15, 2341-2347 (2007). [CrossRef] [PubMed]
  13. W. Watanabe and K. Itoh, "Motion of bubble in solid by femtosecond laser pulses," Opt. Express 10, 603-608 (2002). [PubMed]

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