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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 10 — May. 12, 2008
  • pp: 6768–6773
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Ion exchange in KTiOPO4 crystals irradiated by Copper and Hydrogen ions

Ruifeng zhang, Fei Lu, Jie Lian, Hanping Liu, Xiangzhi Liu, Qingming Lu, and Hongji Ma  »View Author Affiliations


Optics Express, Vol. 16, Issue 10, pp. 6768-6773 (2008)
http://dx.doi.org/10.1364/OE.16.006768


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Abstract

Cs+-K+ ion exchanges were produced on KTiOPO4 crystals which is prior irradiated by Cu+ can H+ ions. The energy and dose of implanted Cu+ ions are 1.5 MeV and 0.5×1014 ions/cm2, and that of H+ are 300 keV and 1×1016 ions/cm2, respectively. The temperature of ions exchange is 430°C, and the time range from 15 minutes to 30 minutes. The prism coupling method is used to measure the dark mode spectra of the samples. Compared with results of ion exchange on the sample without irradiations, both the number of guided mode and its corresponding effective refractive index are decreased. The experimental results indicate that the ion exchange rate closely related with the lattice damage and the damage layers formed in the depth of maximum nuclear energy deposition act as a barrier to block the ions diffuse into the sample and the concentration of defects can modify the speed of ion exchange.

© 2008 Optical Society of America

1. Introduction

Potassium titanyl phosphate (KTiOPO4, KTP) is one of the most attractive nonlinear optical crystals for its excellent nonlinear and electro-optic properties, e.g. high nonlinear coefficient, wide transparency range, broad angular and thermal acceptances, which make KTiOPO4 widely used in many optical applications [1

1. J. D. Bierlein and H. Vanherzeele, “Potassium titanyl phosphate: properties and new applications,” J. Opt. Soc. Am. B 6, 622–633 (1989). [CrossRef]

,2

2. M. Roth, N. Angert, M. Tseitlin, and A. Alexandrovski, “On the optical quality of KTP crystals for nonlinear optical and electro-optic applications,” Opt. Mater. 16, 131–136 (2001). [CrossRef]

]. One of the more promising applications is used in nonlinear optic applications such as second harmonic generation (SHG) and optical parametric oscillators (OPOs) [3

3. M. J. Jongerius, R. J. Bolt, and N. A. Sweep, “Blue second-harmonic generation in waveguides fabricated in undoped and scandium-doped KTiOPO4,” J. Appl. Phys. 75, 3316–3325 (1994). [CrossRef]

,4

4. Liviu Neagu, Constantin Ungureanu, Razvan Dabu, Aurel Stratan, Constantin Fenic, and Laurentiu Rusen, “Compact eye-safe laser sources based on OPOs with KTP or PPKTP crystals,” Opt. Laser Technol. 39, 973–979 (2007). [CrossRef]

]. In recent years, quasi-phase matching (QPM) in periodically poled KTP crystal was used to realize high conversion efficiency SHG and miniaturization of the blue/green source [5

5. J. Hellström, V. Pasiskevicius, H. Karlsson, and F. Laurell, “High-power optical parametric oscillation in largeaperture periodically poled KTiOPO4,” Opt. Lett. , 25, 174–176 (2000). [CrossRef]

]. The relatively high efficiency of SHG was attributed to domain reversal periodically.

Researchers have developed many techniques to make the domain reversal periodically. Q. Chen and W. P. Risk first reported the QPM waveguide device fabricated in KTP using an electric field poling technique [6

6. Q. Chen and W. P. Risk, “Periodic poling of KTiOPO4 using an applied electric field,” Electron. Lett. 30, 1516–1517 (1994). [CrossRef]

]. J. D. Bierlein and co-workers produced the periodically domain reversion structure in KTP for the first time by an ion exchange method [7

7. C. J. Van Poel, J. D. Bierlein, and J. B. Brown, “Efficient type I blue second-harmonic generation in periodically segmented KTiOPO4 waveguides,” Appl. Phys. Lett. 57, 2074–2076 (1990). [CrossRef]

]. In this approach, the KTP crystal was immersed in RbNO3/Ba(NO3)2 melt for a certain time to permit rubidium ions to enter KTP substrate in exchange for potassium ions. In the exchange region, the index of refractive is increased by the replacement of K+ by Rb+, and the direction of ferroelectric polarization reversal by the replacement of K+ by Ba2+ [8

8. F. Laurell, M. G. Roelofs, W. Bindloss, H. Hsiung, A. Suna, and J. D. Bierlein, “Detection of ferroelectric domain reversal in KTiOPO4 waveguides,” J. Appl. Phys. 71, 4664–4670 (1992). [CrossRef]

]. However, there are some limitations for this technique according to Ref. [9

9. Q. Chen and W. P. Risk, “High efficiency quasi-phase matched frequency doubling waveguides in KTiOPO4 fabricated by electric field poling,” Electron. Lett. 32, 107–108 (1996). [CrossRef]

]. For example, the region of domain-inverted is usually shallower than the refractive index increased region; the waveguide fabricated by ion exchange is a multimode waveguide which means the processing parameters can not give good domain inversion and an optimized waveguide synchronously.

Ion implantation may be an effective method to overcome the shortcomings. Ion irradiation can generate lattice damage layer in the depth region of high nuclear energy deposition, which may act as a barrier for further ion exchange. The depth of the damage layer is dependent on the energy of the incident ions which can be controlled accurately [10–12]. Therefore, the ion implantation can be used in the ion exchange process to modify depth of the waveguide to coincide with the domain inversion region.

In this paper, the Cs-K ion exchanges were performed in KTP crystals after the crystal were implanted by heavy (Cu) and light (H) ions. The dark mode spectra of the waveguide were measured by a prism coupling method. The difference between Cs-K ion exchanges fabricated in pure KTP crystal and the crystals implanted by Cu+ or H+ ions were investigated.

2. Experiments

Samples of z-cut KTiOPO4 with dimensions of 5×7×1.5 (x×y×z mm3) were provided by the School of Chemistry and Chemical Engineering, Shandong University. The samples were optically polished and cleaned before implantation. The energies of H+ and Cu+ ions are 300 keV and 1.5 MeV, and the doses of them are 1×1016 ions/cm2 and 0.5×1014 ions/cm2, respectively. The ion implantations were performed in a 1.7 mV tandem accelerator at Peking University at room temperature. In order to avoid channeling, the samples were tilted 7° from the beam direction. Subsequently all the samples were exposed to a CsNO3 melt at T=430°C for a time 15 minutes and 30 minutes respectively to create Cs+-K+ exchange waveguides. The dark mode spectra of the samples were measured by a conventional prism coupling method with a Model 2010 Prism Coupler (Metricon, USA). The parameters in experiments are list in Table 1 detailed.

Table 1. Experiment parameters for samples suffered H+ and Cu+ irradiation and Cs ion exchange

table-icon
View This Table

3. Results and discussion

It seems that implantation of ions could usually result in relatively high damage, or amorphization, in crystals. In order to get a better understanding for the ion implantation process, we used the TRIM’2003 (transport and range of ions in matter) code to simulate the process of Cu+ and H+ implanted into KTP [13

13. Ziegler J F, Biesack J P, and Littmark U, “Computer code TRIM,” http://www.srim.org.

].

Fig. 1. The damage induced by the Cu+ and H+ implantation based on TRIM’2003 vs. the penetration depth

Figure 1 shows the damage induced by the Cu+ and H+ implantation based on TRIM’2003 as a function of the penetration depth. The peak position of the damage profile induced by Cu+ and H+ ion implantation is at about 0.9 and 2.35 µm below the waveguide surface. To remove point defects induced by implantation, all the samples were annealed at 200°C for 40 min. Subsequently, a standard ion exchange process for 15 or 30 minutes was carried out on sample 2#. As a reference, sample 1# suffered from the same ion exchange process except ion implantation. There are four and six guide modes observed for the virgin sample 1# exchange in CsNO3 melt for 15 and 30 minutes, respectively, and the refractive index profiles of sample 1# in x direction were reconstructed by iWKB method [14

14. J. M. White and P. F. Heidrich, “Optical waveguide refractive index profiles determined from measurement of mode indices: a simple analysis,” Appl. Opt. 15, 151–155 (1976). [CrossRef] [PubMed]

]. According to a typical diffusion distribution, the effective diffusion depths of Cs+ ions are about 3.51 µm for 15 min and 3.62 µm for 30 min respectively. Fig. 2 shows the measured dark-mode spectrum of the sample 2#, when the TE polarized light was used. As a reference, the dark mode spectrum of sample 1# exchange for 15 min is also shown in this figure. An obvious difference is found in Fig. 2.

Fig. 2. The measured relative intensity of reflected TE polarized light from the prism versus the effective refractive index for sample 2#. (a) sample 1# exchange for 15 minutes; (b) sample 2# as implanted; (c) sample 2# exchange for 15 minutes; (d) sample 2# exchange for 30 minutes.

There are no effective guide modes observed on Fig. 2 for sample 2#. The refractive index at the surface are 1.7622 and 1.7629, respectively; both are larger than the refractive index of substrate KTP (1.7621), and larger than that of the sample with only implantation (1.7614). It is also apparent that the index increased with the exchange time. The results indicate that although the Cs+-K+ exchanges take place reliably, the ion exchange rate decreases dramatically because the exchange channel is disturbed due to the lattice damage caused by Cu+ irradiation.

Fig. 3. (a) The dark mode spectra of sample 2# and 3# after ion exchange for 15 and 30 min at 430°C. (b) The reconstructed refractive index profile of sample 3# after ion exchange for 30 min (the solid line), “★” represents the effective index of the guide mode.

Since an annealing treatment at a moderate temperature has an effect of repairing the damaged crystal structure [15

15. Fei Lu, Fengxiang Wang, Wei Li, Jianhua Zhang, and Keming Wang, “Annealing behavior of barriers ion-implanted LiNbO3 and LiTaO3 planar waveguide,” Appl. Opt. 38, 5122–5126 (1999). [CrossRef]

], a post-irradiation annealing were performed for sample 3#, which suffered the same Cu+ implantation as sample 2#. After an additional annealing at 430°C for 30 min, sample 3# was ion exchanged at the same conditions as sample 2#. The dark mode spectra of sample 3# in x direction after ion exchange for 15 min and 30 min were shown in Fig. 3(a). It can be seen that only one effective guided mode is found in both spectra of sample 3#, the corresponding effective refractive indices are 1.7619 and 1.7626, respectively. Compared with that of sample 2#, we can see that these values are larger than the effective index 1.7607 and 1.7616, corresponding to the ‘dip’ on the spectra of sample 2#, respectively, which implied that an annealing is helpful to repair the Cs+-K+ exchange channel in the near surface region, where the lattice damage ratio is relative low. While in the region of damage peak, the ion exchange may be blocked due to the serious lattice disorder. Based on this analysis, it is reasonable to consider the possible index profile of sample 3# consist of two parts: one is an exponential decay distribution or a Gaussian distribution in near surface, which may be governed by diffusion profile; the other is a reduced index barrier at the lattice damage peak. The BPM method was used to calculate the effective index of waveguide mode based on this model, and the results show that the profile of Gaussian plus reduced index barrier can provide a good coincide with the measured results. The corresponding profile is given in Fig. 3(b) (the solid line) and the measured effective indices of the guide mode were marked with ‘★’ in this figure. The index profile of sample 3# is quite different from that of sample 1#. Due to the existing of the barrier, the increase of refractive index is confined in a very thin layer at the surface.

For comparison of the effect of lattice damage on the ion exchange, sample 4# was irradiated by light ions (H) before Cs+-K+ exchange were performed. The lattice damage induced by H+ irradiation was simulated by TRIM’2003 and was also shown in Fig. 1 to compare with that of Cu+ ion implantation. The damage induced by light ions was quite different from that of heavy ions [16

16. Ke-Ming WANG, Fei LU, Ming-Qi MENG, Bo-Rong SHI, Wei LI, Feng-Xiang WANG, Ding-Yu SHEN, and Nelson CUE, “Optical waveguide of MeV Hydrogen ion implantation KTiOPO4,” Jpn. J. Appl. Phys. 37, L1055–L1057 (1998). [CrossRef]

,17

17. Thomas Opfermann, Thomas Höche, and Werner Wesch, “Radiation damage in KTiOPO4 by ion implantation of light elements,” Nucl. Instrum. Methods in Phys. Res. B 166–167, 309–313 (2000). [CrossRef]

]. The defects caused by H+ ion implantation are concentrated at the end of the ion track and few lies at the surface region. After annealing at 200°C for 40 min for removing the point defects induced by implantation, Cs ion exchange was carried out at 430°C for 15 and 30 min. The dark mode spectra obtained by prism coupling method for the sample 4# were shown in Fig. 4.

Fig. 4. The dark mode spectra of sample 4# after ion exchange for 15 minutes and 30 minutes when TE polarized light were used. (a) sample 1# after ion exchange for 15 min; (b) sample 4# after exchange for 15 min; (c) sample 4# after exchange for 30 min.

From the results it can be seen that the mode distribution for exchange 15 min is only a bit different from that of sample without H+ implantation. However, the result for 30 min exchange shows a different tendency, the index on surface decreases noticeably. The possible explanation is schematically shown in Fig. 5(a), the dot line and solid line corresponds to the profile of lattice damage and Cs+ diffusion, respectively. In the near surface region, the lattice is almost “untouched” by the H+ implantation (see Fig. 5(a.1)). The ion exchange near the sample surface is hardly affected by the lattice damage, which concentrates only in a narrow region at about 2.35 µm below surface according to Fig. 1. The damage layer could act as a shallow barrier, but it can not block the Cs diffusion completely, see Fig. 5(a.2). With the extension of exchange time, the aggregate H+ ion will diffuse and the strain will be released. Along with the release of strain, the lattice disorder will extend to both sides and tend to be flat (see Fig. 5(a.3)). The lattice disorder will result in a low exchange rate on the surface. At the same time the Cs+ ions will diffuse into deep region of the sample because of the gradient of Cs+ density. The indices profiles according to iWKB method are given in Fig. 5(b), which show a good consistence with above analysis.

Fig. 5. (a) The sketch about relations between the lattice damage distribution (the dot line) and relative refractive index (the solid line) (1) without irradiation; (2) exchanged for 15 min after H+ implanted; (3) exchange for 30 min after H+ implantation. (b) The refractive index profiles reconstructed by iWKB method for sample 4#.

4. Conclusion

Acknowledgement

This work is supported by the National Natural Science Foundation of China (Grant No. 10735070) and the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20070422078).

References and links

1.

J. D. Bierlein and H. Vanherzeele, “Potassium titanyl phosphate: properties and new applications,” J. Opt. Soc. Am. B 6, 622–633 (1989). [CrossRef]

2.

M. Roth, N. Angert, M. Tseitlin, and A. Alexandrovski, “On the optical quality of KTP crystals for nonlinear optical and electro-optic applications,” Opt. Mater. 16, 131–136 (2001). [CrossRef]

3.

M. J. Jongerius, R. J. Bolt, and N. A. Sweep, “Blue second-harmonic generation in waveguides fabricated in undoped and scandium-doped KTiOPO4,” J. Appl. Phys. 75, 3316–3325 (1994). [CrossRef]

4.

Liviu Neagu, Constantin Ungureanu, Razvan Dabu, Aurel Stratan, Constantin Fenic, and Laurentiu Rusen, “Compact eye-safe laser sources based on OPOs with KTP or PPKTP crystals,” Opt. Laser Technol. 39, 973–979 (2007). [CrossRef]

5.

J. Hellström, V. Pasiskevicius, H. Karlsson, and F. Laurell, “High-power optical parametric oscillation in largeaperture periodically poled KTiOPO4,” Opt. Lett. , 25, 174–176 (2000). [CrossRef]

6.

Q. Chen and W. P. Risk, “Periodic poling of KTiOPO4 using an applied electric field,” Electron. Lett. 30, 1516–1517 (1994). [CrossRef]

7.

C. J. Van Poel, J. D. Bierlein, and J. B. Brown, “Efficient type I blue second-harmonic generation in periodically segmented KTiOPO4 waveguides,” Appl. Phys. Lett. 57, 2074–2076 (1990). [CrossRef]

8.

F. Laurell, M. G. Roelofs, W. Bindloss, H. Hsiung, A. Suna, and J. D. Bierlein, “Detection of ferroelectric domain reversal in KTiOPO4 waveguides,” J. Appl. Phys. 71, 4664–4670 (1992). [CrossRef]

9.

Q. Chen and W. P. Risk, “High efficiency quasi-phase matched frequency doubling waveguides in KTiOPO4 fabricated by electric field poling,” Electron. Lett. 32, 107–108 (1996). [CrossRef]

10.

L. Laversenne, P. Hoffmann, M. Pollnau, P. Moretti, and J. Mugnier, “Designable buried waveguides in sapphire by proton implantation,” Appl. Phys. Lett. 85, 5167–5169 (2004). [CrossRef]

11.

Fei Lu, Tingting Zhang, Gang Fu, Xuelin Wang, Keming Wang, Dingyu Shen, and Hongji Ma, “Investigation and analysis of a single-mode waveguide formed by multienergy-implanted LiNbO3,” Opt. Express 13, 2256–2262 (2005). [CrossRef] [PubMed]

12.

Feng Chen, Yang Tan, Lei Wang, Dong-Chao Hou, and Qing-Ming Lu, “Optical channel waveguides with trapezoidal-shaped cross sections in KTiOPO4 crystal fabricated by ion implantation,” Appl. Surf. Sci. 254, 1822–1824 (2008). [CrossRef]

13.

Ziegler J F, Biesack J P, and Littmark U, “Computer code TRIM,” http://www.srim.org.

14.

J. M. White and P. F. Heidrich, “Optical waveguide refractive index profiles determined from measurement of mode indices: a simple analysis,” Appl. Opt. 15, 151–155 (1976). [CrossRef] [PubMed]

15.

Fei Lu, Fengxiang Wang, Wei Li, Jianhua Zhang, and Keming Wang, “Annealing behavior of barriers ion-implanted LiNbO3 and LiTaO3 planar waveguide,” Appl. Opt. 38, 5122–5126 (1999). [CrossRef]

16.

Ke-Ming WANG, Fei LU, Ming-Qi MENG, Bo-Rong SHI, Wei LI, Feng-Xiang WANG, Ding-Yu SHEN, and Nelson CUE, “Optical waveguide of MeV Hydrogen ion implantation KTiOPO4,” Jpn. J. Appl. Phys. 37, L1055–L1057 (1998). [CrossRef]

17.

Thomas Opfermann, Thomas Höche, and Werner Wesch, “Radiation damage in KTiOPO4 by ion implantation of light elements,” Nucl. Instrum. Methods in Phys. Res. B 166–167, 309–313 (2000). [CrossRef]

OCIS Codes
(160.3130) Materials : Integrated optics materials
(160.4330) Materials : Nonlinear optical materials
(230.7390) Optical devices : Waveguides, planar

ToC Category:
Materials

History
Original Manuscript: December 21, 2007
Revised Manuscript: March 10, 2008
Manuscript Accepted: April 14, 2008
Published: April 28, 2008

Citation
Ruifeng Zhang, Fei Lu, Jie Lian, Hanping Liu, Xiangzhi Liu, Qingming Lu, and Hongji Ma, "Ion exchange in KTiOPO4 crystals irradiated by Copper and Hydrogen ions," Opt. Express 16, 6768-6773 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-10-6768


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References

  1. J. D. Bierlein and H. Vanherzeele, "Potassium titanyl phosphate: properties and new applications," J. Opt. Soc. Am. B 6, 622-633 (1989). [CrossRef]
  2. M. Roth, N. Angert, M. Tseitlin, and A. Alexandrovski, "On the optical quality of KTP crystals for nonlinear optical and electro-optic applications," Opt. Mater. 16, 131-136 (2001). [CrossRef]
  3. M. J. Jongerius, R. J. Bolt, and N. A. Sweep, "Blue second-harmonic generation in waveguides fabricated in undoped and scandium-doped KTiOPO4," J. Appl. Phys. 75, 3316-3325 (1994). [CrossRef]
  4. L. Neagu, C. Ungureanu, R. Dabu, A. Stratan, C. Fenic, and L. Rusen, "Compact eye-safe laser sources based on OPOs with KTP or PPKTP crystals," Opt. Laser Technol. 39, 973-979 (2007). [CrossRef]
  5. J. Hellström, V. Pasiskevicius, H. Karlsson, and F. Laurell, "High-power optical parametric oscillation in large-aperture periodically poled KTiOPO4," Opt. Lett. 25, 174-176 (2000). [CrossRef]
  6. Q. Chen and W. P. Risk, "Periodic poling of KTiOPO4 using an applied electric field," Electron. Lett. 30, 1516-1517 (1994). [CrossRef]
  7. C. J. Van Poel, J. D. Bierlein, and J. B. Brown, "Efficient type I blue second-harmonic generation in periodically segmented KTiOPO4 waveguides," Appl. Phys. Lett. 57, 2074-2076 (1990). [CrossRef]
  8. F. Laurell, M. G. Roelofs, W. Bindloss, H. Hsiung, A. Suna, and J. D. Bierlein, "Detection of ferroelectric domain reversal in KTiOPO4 waveguides," J. Appl. Phys. 71, 4664-4670 (1992). [CrossRef]
  9. Q. Chen and W. P. Risk, "High efficiency quasi-phase matched frequency doubling waveguides in KTiOPO4 fabricated by electric field poling," Electron. Lett. 32, 107-108 (1996). [CrossRef]
  10. L. Laversenne, P. Hoffmann, M. Pollnau, P. Moretti, and J. Mugnier, "Designable buried waveguides in sapphire by proton implantation," Appl. Phys. Lett. 85, 5167-5169 (2004). [CrossRef]
  11. F. Lu, T. Zhang, G. Fu, X. Wang, K. Wang, D. Shen, and H. Ma, "Investigation and analysis of a single-mode waveguide formed by multienergy-implanted LiNbO3," Opt. Express 13, 2256-2262 (2005). [CrossRef] [PubMed]
  12. F. Chen, Y. Tan, L. Wang, D.-C. Hou, and Q.-M. Lu, "Optical channel waveguides with trapezoidal-shaped cross sections in KTiOPO4 crystal fabricated by ion implantation," Appl. Surf. Sci. 254, 1822-1824 (2008). [CrossRef]
  13. J. F. Ziegler, J. P. Biesack, and U. Littmark "Computer code TRIM," http://www.srim.org.
  14. J. M. White and P. F. Heidrich, "Optical waveguide refractive index profiles determined from measurement of mode indices: a simple analysis," Appl. Opt. 15, 151-155 (1976). [CrossRef] [PubMed]
  15. F. Lu, F. Wang, W. Li, J. Zhang, and K. Wang, "Annealing behavior of barriers ion-implanted LiNbO3 and LiTaO3 planar waveguide," Appl. Opt. 38, 5122-5126 (1999). [CrossRef]
  16. K.-M. Wang, F. Lu, M.-Q. Meng, B.-R. Shi, W. Li, F.-X. Wang, D.-Y. Shen,  and N. Cue, "Optical waveguide of MeV Hydrogen ion implantation KTiOPO4," Jpn. J. Appl. Phys. 37, L1055-L1057 (1998). [CrossRef]
  17. T. Opfermann, T. Höche, and W. Wesch, "Radiation damage in KTiOPO4 by ion implantation of light elements," Nucl. Instrum. Methods in Phys. Res. B 166-167, 309-313 (2000). [CrossRef]

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