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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 13 — Jun. 26, 2006
  • pp: 6207–6212
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Self-focusing without external electric field in BaTiO3

Anne Rausch, Armin Kiessling, and Richard Kowarschik  »View Author Affiliations


Optics Express, Vol. 14, Issue 13, pp. 6207-6212 (2006)
http://dx.doi.org/10.1364/OE.14.006207


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Abstract

We report on the first observation, to our knowledge, of self-focusing without an external electric field in barium titanate crystals under cw laser beam irradiance. This effect we observed at an intensity of 0.2W/cm2 on the 633nm wavelength regime in the case of ordinarily as well as extraordinarily polarized light.

© 2006 Optical Society of America

1. Introduction

Photorefractive, nonlinear optical crystals, such as BaTiO3, are currently used for a wide range of applications in electronics, optics and optoelectronics. So it is very desirable to know how this material acts under laser irradiance, and what interactions between material and light appear. One of the nonlinear optical effects mostly discussed in the last decade is self-focusing. In balance with diffraction this effect can cause soliton formation. Usually an external electric field is applied to change the refractive index of the material by the electro-optic effect, and so to generate self-focusing. This excites in balance with self-diffraction screening solitons [1

1. M. Segev, B. Crosignani, A. Yariv, and B. Fisher, “Spatial solitons in photorefractive media,” Phys. Rev. Lett. 68, 923–926 (1992). [CrossRef] [PubMed]

, 2

2. G. C. Duree, J. L. Shultz, G. J. Salamo, M. Segev, A. Yariv, B. Crosignani, P. D. Porto, E. J. Sharp, and R. R. Neurgaonkar, “Observation of self-trapping of an optical beam due to the photorefractive effect,” Phys. Rev. Lett. 71, 533–536 (1993). [CrossRef] [PubMed]

]. In BaTiO3 a strong self-focusing effect occurred using external fields in the range of 2.8kV/cm [3

3. E. DelRe, M. Tamburrini, and G. Egidi, Bright photorefractive spatial solitons in tilted BaTiO3, presented at the Eleventh Annual Meeting of the [IEEE Lasers and Electro-Optics Society] (LEOS 98), Orlando, Fla., 3–4 December 1998.

, 4

4. J. Andrade-Lucio, M. Iturbe-Castillo, P. Marquez-Aguilar, and R. Ramos-Garcia, “Self-focusing in photorefractive BaTiO3 crystal under external DC electric field,” Opt. Quantum Electron. 30, 829–834 (1998). [CrossRef]

].

But it is also known that photorefractive self-focusing can appear without an external electric field, only due to intrinsic fields and the anisotropic properties of the material [5

5. P. Guenter and J. Huignard, Photorefractive Materials and Their Applications 1 - Fundamental Phenomena (Springer, Berlin, 1988). [CrossRef]

, 6

6. W. Kaenzig, “Space charge layer near the surface of a ferroelectric,” Phys. Rev. 98, 549–550 (1955). [CrossRef]

, 7

7. V. Matusevich, A. Krasnoberski, D. Khmelnitski, A. Kiessling, and R. Kowarschik, “Some aspects of fanning, self-focusing and self-defocusing in a photorefractive Ba0.77Ca0.23TiO3 crystal,” J. Opt. A: Pure Appl. Opt. 6, 507–513 (2003). [CrossRef]

]. Here, also a balance between self-focusing and diffraction can be achieved, which leads to so called photovoltaic solitons [8

8. G. Valley, M. Segev, B. Crosignani, A. Yariv, and M. B. Fejer, “Dark and Bright Photovoltaic Spatial Solitons,” Phys. Rev. A 50, R4457–R4460 (1994). [CrossRef] [PubMed]

, 9

9. J. Liu, “Existence and stability of rigid photovoltaic solitons in an open-circuit amplifying or absorbing photo-voltaic medium,” Phys. Rev. E 68, 0266071–0266077 (2003). [CrossRef]

]. Until now photovoltaic solitons were observed only in LiNbO 3 crystals [10

10. F. Chen, “Optically induced change of refractive indices in LiNbO3,” J. App. Phys. 40, 3389–3396 (1969). [CrossRef]

, 11

11. L. Palfalv, J. Hebling, G. Almasi, A. Peter, and K. Polgar, “Refractive index changes in Mg-doped LiNbO3 caused by photorefraction and thermal effects,” J. Opt. A 5, S280–S283 (2003). [CrossRef]

]. Another way to obtain self-focusing without external field is the use of high laser intensities, which leads to thermal focusing effects. In barium titanate as well as in SBN this effect is observed with short laser pulses in the range of 102…103W/mm2 [11

11. L. Palfalv, J. Hebling, G. Almasi, A. Peter, and K. Polgar, “Refractive index changes in Mg-doped LiNbO3 caused by photorefraction and thermal effects,” J. Opt. A 5, S280–S283 (2003). [CrossRef]

, 12

12. M. Horowitz, R. Daisy, O. Werner, and B. Fischer, “Large thermal nonlinearities and spatial self-phase modulation in SrxBa1-xNb2O6 and BaTiO3 crystals,” J. Appl. Phys. 17, 475–477 (1992).

].

In this letter we present experimental data, which show that self-focusing without external field occurs also in BaTiO3, using only an intensity of 0.2W/cm2. It is demonstrated that the effect depends on the crystal doping and cut, and on the polarization direction of the laser beam as well.

2. Experimental Set-Up

The experiments on investigations of the optical induced changes of the diameter of a laser beam were performed at room temperature. Poled single crystals of ferroelectric BaTiO 3 that belong to the point group 4mm were used. BaTiO3 possesses large linear electro-optic coefficients [13

13. D. Nolte, Photorefractive effects and materials (Kluwer Acad. Publ., Boston, 1995).

] and a large spontaneous polarization. The samples lay clamp free on a board. The source of light was a He-Ne-laser with λ=633nm, whose beam first passed an intensity filter IF and a Glan-Thompson-polarizer P like it is shown in Fig. 1. The light was focused on the front face of the crystal K by a micro-objective O with a focus diameter of about 4µm. The beam at the rear face of the crystal was imaged by a telescope system L on a CCD-camera C with a reproduction scale of M=2.

An intensity of 0.2Wcm-2 was used. During the whole measurement the intensity of the laser beam behind the crystal was proofed to be constant.

Fig. 1. Experimental set-up: La: He-Ne-laser, I: intensity filter, P: Glan-Thompson-polarizer, O: micro-objective, K: crystal, L: telescope system, C: CCD-matrix.

3. Experimental Results

The samples of BaTiO3, used in the experiments (see table 1), were nominal pure or doped with Rh or Co. The typical run of transmission of these three types of doping in dependence on the wavelength is shown in Fig. 2.

Studying the effect of self-focusing it is remarkable that the undoped crystals showed a much stronger focusing effect than rhodium or cobalt doped crystals under the above described conditions (Fig. 3).

Table 1. Samples (1–19) of the barium titanate crystals with edge lengths a/b/c, color, absorption coefficients α at 633nm, doping and cut

table-icon
View This Table
Fig. 2. Dependence of the crystals transmission on the wavelength.

Another important parameter is the cut of the crystals. We analyzed two 45°cut samples and six 0°cut samples of undoped barium titanate. Although self-focusing of the laser beam in 0°cut samples could be observed, the effect was stronger in the 45°cut samples, like it is shown in Fig. 4.

Fig. 3. Comparison of self-focusing in nominal pure, Rh and Co doped crystals. The beam diameter w at the rear face of the crystal is normalized at the output diameter w at the beginning of the measurement.
Fig. 4. Comparison of self-focusing in 0° and 45°cut crystals of undoped barium titanate. The beam diameter w at the rear face of the crystal is normalized at the output diameter w at the beginning of the measurement.

Therefore, to investigate the dependence of the self-focusing effect on the polarization direction of the beam a undoped 45°cut BaTiO3 crystal (number 19 in the table above) was used. The crystals size, the direction of the optical axis and the two used polarization directions are shown in Fig. 5.

Fig. 5. Direction of polarizations of the laser beam and edge length of the crystal that was used in experiment.
Fig. 6. Beam diameter at
I=Imaxe2
in dependence on time in the case of ordinarily and extraorinarily polarized light. The beam diameter w at the rear face of the crystal is normalized at the output diameter w at the beginning of the measurement.

In the case of ordinarily polarized light, there was an external white light source, as background illumination, necessary to achieve a self-focusing effect. Using extraordinarily polarized light, no dependence on this white light illumination could be observed.

After reaching the stable focussed state, the beam diameter didn’t change within 10 hours. As expected, the wave-guiding canal in the crystal remained stable for month if the crystal isn’t illuminated and not heated [14

14. G. Bacher, M. Chiao, G. Dunning, M. Klein, C. Nelson, and B. Wechsler, “Ultralong dark decay measurements in BaTiO3,” Opt. Lett. 21, 18–20 (1996). [CrossRef] [PubMed]

]. To avoid the influence of previously written refraction index changes, before starting the investigation, the crystal had to be illuminated homogeneously with light, for instance by a white light emitting diode. Thus, the charge distribution became homogenized, older stored changes of the refractive index were erased. With a diode current of 28mA, it took about 30 minutes to get the crystal in its original state.

4. Conclusion

It could be shown that in BaTiO3 a self-focusing effect appears at a laser beam intensity of 0.2W/cm2 without applying an external electric field. This effect can be observed by use of ordinarily polarized light as well as extraordinarily polarized light. The greatest focusing we achieved in undoped crystals with 45°cut and using extraordinarily polarized light. The focused state remains stable in darkness about 10 hours.

Acknowledgments

We would like to thank the Deutsche Forschungsgemeinschaft for financial support within the Forschergruppe “Nichtlineare raum-zeitliche Dynamik in dissipativen und diskreten optischen Systemen”.

References and links

1.

M. Segev, B. Crosignani, A. Yariv, and B. Fisher, “Spatial solitons in photorefractive media,” Phys. Rev. Lett. 68, 923–926 (1992). [CrossRef] [PubMed]

2.

G. C. Duree, J. L. Shultz, G. J. Salamo, M. Segev, A. Yariv, B. Crosignani, P. D. Porto, E. J. Sharp, and R. R. Neurgaonkar, “Observation of self-trapping of an optical beam due to the photorefractive effect,” Phys. Rev. Lett. 71, 533–536 (1993). [CrossRef] [PubMed]

3.

E. DelRe, M. Tamburrini, and G. Egidi, Bright photorefractive spatial solitons in tilted BaTiO3, presented at the Eleventh Annual Meeting of the [IEEE Lasers and Electro-Optics Society] (LEOS 98), Orlando, Fla., 3–4 December 1998.

4.

J. Andrade-Lucio, M. Iturbe-Castillo, P. Marquez-Aguilar, and R. Ramos-Garcia, “Self-focusing in photorefractive BaTiO3 crystal under external DC electric field,” Opt. Quantum Electron. 30, 829–834 (1998). [CrossRef]

5.

P. Guenter and J. Huignard, Photorefractive Materials and Their Applications 1 - Fundamental Phenomena (Springer, Berlin, 1988). [CrossRef]

6.

W. Kaenzig, “Space charge layer near the surface of a ferroelectric,” Phys. Rev. 98, 549–550 (1955). [CrossRef]

7.

V. Matusevich, A. Krasnoberski, D. Khmelnitski, A. Kiessling, and R. Kowarschik, “Some aspects of fanning, self-focusing and self-defocusing in a photorefractive Ba0.77Ca0.23TiO3 crystal,” J. Opt. A: Pure Appl. Opt. 6, 507–513 (2003). [CrossRef]

8.

G. Valley, M. Segev, B. Crosignani, A. Yariv, and M. B. Fejer, “Dark and Bright Photovoltaic Spatial Solitons,” Phys. Rev. A 50, R4457–R4460 (1994). [CrossRef] [PubMed]

9.

J. Liu, “Existence and stability of rigid photovoltaic solitons in an open-circuit amplifying or absorbing photo-voltaic medium,” Phys. Rev. E 68, 0266071–0266077 (2003). [CrossRef]

10.

F. Chen, “Optically induced change of refractive indices in LiNbO3,” J. App. Phys. 40, 3389–3396 (1969). [CrossRef]

11.

L. Palfalv, J. Hebling, G. Almasi, A. Peter, and K. Polgar, “Refractive index changes in Mg-doped LiNbO3 caused by photorefraction and thermal effects,” J. Opt. A 5, S280–S283 (2003). [CrossRef]

12.

M. Horowitz, R. Daisy, O. Werner, and B. Fischer, “Large thermal nonlinearities and spatial self-phase modulation in SrxBa1-xNb2O6 and BaTiO3 crystals,” J. Appl. Phys. 17, 475–477 (1992).

13.

D. Nolte, Photorefractive effects and materials (Kluwer Acad. Publ., Boston, 1995).

14.

G. Bacher, M. Chiao, G. Dunning, M. Klein, C. Nelson, and B. Wechsler, “Ultralong dark decay measurements in BaTiO3,” Opt. Lett. 21, 18–20 (1996). [CrossRef] [PubMed]

OCIS Codes
(160.4330) Materials : Nonlinear optical materials
(190.4870) Nonlinear optics : Photothermal effects
(190.5330) Nonlinear optics : Photorefractive optics
(190.5940) Nonlinear optics : Self-action effects

ToC Category:
Nonlinear Optics

History
Original Manuscript: March 30, 2006
Revised Manuscript: June 8, 2006
Manuscript Accepted: June 13, 2006
Published: June 26, 2006

Citation
Anne Rausch, Armin Kiessling, and Richard Kowarschik, "Self-focusing without external electric field in BaTiO3," Opt. Express 14, 6207-6212 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-13-6207


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References

  1. M. Segev, B. Crosignani, A. Yariv and B. Fisher, "Spatial solitons in photorefractive media," Phys. Rev. Lett. 68,923-926 (1992). [CrossRef] [PubMed]
  2. G. C. Duree, J. L. Shultz, G. J. Salamo, M. Segev, A. Yariv, B. Crosignani, P. D. Porto, E. J. Sharp and R. R. Neurgaonkar, "Observation of self-trapping of an optical beam due to the photorefractive effect," Phys. Rev. Lett. 71,533-536 (1993). [CrossRef] [PubMed]
  3. E. DelRe, M. Tamburrini, and G. Egidi, Bright photorefractive spatial solitons in tilted BaTiO3, presented at the Eleventh Annual Meeting of the [IEEE Lasers and Electro-Optics Society] (LEOS 98), Orlando, Fla., 3 - 4 December 1998.
  4. J. Andrade-Lucio, M. Iturbe-Castillo, P. Marquez-Aguilar and R. Ramos-Garcia, "Self-focusing in photorefractive BaTiO3 crystal under external DC electric field," Opt. Quantum Electron. 30,829-834 (1998). [CrossRef]
  5. P. Guenter and J. Huignard, Photorefractive Materials and Their Applications 1 - Fundamental Phenomena (Springer, Berlin, 1988). [CrossRef]
  6. W. Kaenzig, "Space charge layer near the surface of a ferroelectric," Phys. Rev. 98,549-550 (1955). [CrossRef]
  7. V. Matusevich, A. Krasnoberski, D. Khmelnitski, A. Kiessling and R. Kowarschik, "Some aspects of fanning, self-focusing and self-defocusing in a photorefractive Ba0.77Ca0.23TiO3 crystal," J. Opt. A: Pure Appl. Opt. 6,507-513 (2003). [CrossRef]
  8. G. Valley, M. Segev, B. Crosignani, A. Yariv and M. B. Fejer, "Dark and bright photovoltaic spatial solitons," Phys. Rev. A 50,R4457-R4460 (1994). [CrossRef] [PubMed]
  9. J. Liu, "Existence and stability of rigid photovoltaic solitons in an open-circuit amplifying or absorbing photovoltaic medium," Phys. Rev. E 68,0266071-0266077 (2003). [CrossRef]
  10. F. Chen, "Optically induced change of refractive indices in LiNbO3," J. Appl. Phys. 40,3389-3396 (1969). [CrossRef]
  11. L. Palfalv, J. Hebling, G. Almasi, A. Peter and K. Polgar, "Refractive index changes in Mg-doped LiNbO3 caused by photorefraction and thermal effects," J. Opt. A 5,S280-S283 (2003). [CrossRef]
  12. M. Horowitz, R. Daisy, O. Werner and B. Fischer, "Large thermal nonlinearities and spatial self-phase modulation in SrxBa1−xNb2O6 and BaTiO3 crystals," J. Appl. Phys. 17,475-477 (1992).
  13. D. Nolte, Photorefractive effects and materials (Kluwers Academic Publishers, Boston, 1995).
  14. G. Bacher, M. Chiao, G. Dunning, M. Klein, C. Nelson and B. Wechsler, "Ultralong dark decay measurements in BaTiO3," Opt. Lett. 21,18-20 (1996). [CrossRef] [PubMed]

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