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Property study on nickel ion implanted planar waveguide in KTiOAsO4 crystal
Nuclear Instruments and Methods in Physics Research B 194 (2002) 355–358 www.elsevier.com/locate/nimb
Letter to the Editor
Property study on nickel ion implanted planar waveguide in KTiOAsO4 crystal F. Chen
a,*
, X.-L. Wang a, Q.-M. Lu b, G. Fu a, S.-L. Li a, F. Lu a, K.-M. Wang a, D.-Y. Shen c a
Department of Physics, Shandong University, Jinan 250100, China School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China Department of Technical Physics & the Key Laboratory of Heavy Ion Physics, Chinese Ministry of Education, Peking University, Beijing 100871, China b
c
Received 7 December 2001; received in revised form 16 April 2002
Abstract The first planar waveguide in KTiOAsO4 (KTA) crystal by nickel ion implantation is presented. The dark modes are observed and analyzed using the prism-coupling method. The refractive index profile is reconstructed by the reflectivity calculation method. A comparison of the refractive index profile is made between the as-implanted waveguide and one annealed at 240 °C for 50 min. The TRIM 98 code is carried out to simulate the implantation process, which may be helpful to understand the formation of the waveguide in KTA. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.79.Gn; 61.80.Jh Keywords: Optical waveguide; Ion implantation; Nonlinear optical materials
1. Introduction KTiOAsO4 (KTA) is one of the promising crystals for nonlinear optical and electrooptic device applications. The material features large nonlinear optical and electrooptic coefficients in comparison to KTiOPO4 and has added benefit of significantly reduced absorption in the 2–5 lm
*
Corresponding author. E-mail address: [email protected] (F. Chen).
region [1,2]. Moreover, KTA has a higher figure of merit for second harmonic generation and lower ionic conductivity [3]. In the past years ion implantation has become a relatively mature method for the fabrication of waveguides in many optical materials, including crystals, glass, semiconductors and polymers [4–6]. In previous work, MeV light ions, such as He and H, were used as an irradiation source at doses of 1016 ions/cm2 to form waveguides below the surface of some materials within several microns. In this case, a low refractive index ‘‘optical barrier’’ has been produced at the end of the track where most of the displacement damage
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 1 1 4 3 - 6
356
F. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 194 (2002) 355–358
occurs. Such an optical barrier confines the light to a narrow layer or ‘‘optical well’’ (act as a waveguide) between itself and the surface [4]. Resent results suggest that MeV heavier ion implantation may be a more efficient method to form waveguides in optical materials, because in this way implantation at much lower doses (1014 –1015 ions/ cm2 ) is adequate [7,8]. Nickel (Niþ ) ion is a suitable candidate. MeV Niþ implantation has been used to fabricate planar waveguides in quartz, LiNbO3 and Tm:NaY(WO4 )2 [9–11]. In the present work, we report for the first time the formation of 3.0 MeV Niþ ion implanted waveguide in KTA, and analyze the refractive index profiles before and after annealing at 240 °C for 50 min. 2. Experimental details The KTA samples are optically polished and their refractive indices were measured before implantation. The samples were implanted by 3.0 MeV Niþ ions (at a dose of 1 1015 ions/cm2 ) at room temperature. The ion beam was scanned to ensure a uniform implantation over the samples. The beam current was typically set at 300 nA. During the implantation the sample was tilted off by 7° from the beam direction in order to minimize the channeling effect. The prism-coupling method was used to observe the waveguide mode. A laser beam at 633 nm from a He–Ne laser struck at base of a rutile prism, and hence the laser beam was coupled into the waveguide region. A silicon photodetector was used to detect the reflected beam. The prism, waveguide and the photodetector were mounted on a rotary table so that the incident angle of the laser beam could be changed. The intensity of light striking the photodetector was plotted as a function of the incident angle, where a sharp drop in the intensity profile would correspond to a propagation mode. 3. Results and discussion Fig. 1 shows the relative intensity of the light (TE polarized) reflected from the prism versus the effective reflective index (a) nx and (b) ny of the incident light measured in 3.0 MeV Niþ ion im-
Fig. 1. The relative intensity of the light (TE polarized) reflected from the prism versus the effective reflective index (a) nx and (b) ny of the incident light measured in 3.0 MeV Niþ ion implanted waveguides in KTA.
planted waveguides in KTA. When the light is coupled into the waveguide region, a lack of reflected light would result in a dip. As we can see, there are three and four modes observed for nx and ny , respectively. However, only the first dip seems much sharp while others become broader. This means that the first mode is real propagation one, in which the light could be well confined. When this mode has been excited, the light propagating along the length of the sample will result in a bright point at the sample end face, and the point
F. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 194 (2002) 355–358
is much close to the front surface of the sample. The broader dips may correspond to leaky modes, which are often the result of the multiple reflectivity at the interface between the waveguide and substrate. When the leaky modes have been excited, the propagating light also resulted in a bight point at the sample end face, but the point is located in the middle of the end face, or much close to the inverse of the sample. To study the stability of the waveguide, the sample is annealed at 240 °C for 50 min in air and similar dark mode property is obtained. Reflectivity calculation method (RCM) [12] is used to reconstruct the refractive index profile of the KTA waveguide before and after the annealing at 240 °C for 50 min in air. In such a simulation, we first choose an analytic function for the index profile characterized by several parameters, which is constructed from a skewed flat-topped Gaussian peak, with a sloping base plateau; second, calculate the mode indices of this hypothetical profile by RCM; third, compare these theoretical mode index values with the experimental values obtained from the experimental measurement; finally, keep changing the parameters to alter the index profile shape until the theoretical mode indices match the experimental ones within a satisfactory error. The final profile is therefore assumed to be the optimum shape for the given mode index data. Figs. 2(a) and (b) indicate the refractive index profiles of nx and ny based on RCM for the waveguide, respectively. For nx , before and after annealing, it is found that the maximum decrease (barrier) of refractive index is about 1.5% and 0.96% but the minimum of the index decrease (waveguide region) was about 1.3% and 0.78%, respectively. For ny , similar phenomenon is observed. It seems that the annealing could reduce the refractive index of the waveguide region as well as that of the barrier. Table 1 lists the comparison of the measured mode indices with fitted values of the indices based on RCM. It is found that the measured effective refractive index is in good agreement with the calculated values better than 103 . Since the refractive index profile is very important for the waveguide, we use TRIM 98 (transport of ions in matter) code to simulate the process of the 3.0 MeV Niþ implantation into
357
Fig. 2. The refractive index profiles of (a) nx and (b) ny based on RCM for the waveguide before and after the annealing at 240 °C for 50 min in air, respectively. The solid and dashed lines represent the index profiles of the waveguide before and after the annealing.
KTA in order to obtain some useful information [13]. Fig. 3 shows the comparison of the nuclear energy loss as a function of the penetration depth (vacancy distribution) with the refractive index profiles of nx and ny . We find the shapes of vacancy distribution and the index profiles are similar to a certain extent. This may suggest that the damage induced by the nuclear energy deposition is one important factor influencing the refractive index profile of the waveguide. But it should be noted that the peak positions of the refractive index profiles are much deeper than that of the vacancy
358
F. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 194 (2002) 355–358
Table 1 The comparison of the measured mode indices with fitted values of the indices based on RCM Before the annealing
After the annealing
Exp.
Calc.
Exp.
Calc.
nx 1.78380 1.76380 1.73060
1.78377 0.00002 1.76387 0.00006 1.73070 0.0001
1.78690 1.76720 1.73290
1.78696 0.00006 1.76707 0.0001 1.73358 0.0006
ny 1.78410 1.76520 1.73250 1.68550
1.78501 0.0005 1.76484 0.0004 1.73264 0.0001 1.68533 0.0001
1.78910 1.77070 1.73840 1.69200
1.78983 0.0007 1.77020 0.0005 1.73878 0.0004 1.69205 0.00005
Error
Error
4. Summary The KTA waveguide is formed by 3.0 MeV Niþ ion implantation at a dose of 1 1015 ions/cm2 at room temperature. The refractive index profiles of nx and ny are reconstructed for the waveguide before and after the annealing at 240 °C for 50 min using RCM. The TRIM 98 simulation suggests that the nuclear energy deposition may be an important factor for the formation of the waveguide.
Acknowledgements This work is supported by the National Natural Science Foundation of China (grant no. 10035010) and the MOE Key Laboratory of Heavy Ion Physics of Peking University.
References
Fig. 3. Comparison of the nuclear energy loss as a function of the penetration depth (vacancy distribution) with the refractive index profiles of nx and ny .
distribution. This may be due to the radiationenhanced diffusion (RED) of defects occurred during the process of the implantation. The collision cascades and the following localized energy deposition cause high concentration of vacancies, which sometimes result in greater defect movement than would be expected. This suggests that the RED of the defects could be over the scale of the collision cascades itself and the ion track [14]. Similar phenomena have been reported in MeV Niþ ion implanted LiNbO3 and Tm:NaY(WO4 )2 waveguides [10,11]. Nevertheless, a detailed understanding of such phenomena still needs further investigation.
[1] L.K. Cheng, L.T. Cheng, J.D. Bierlein, F.C. Zumsteg, Appl. Phys. Lett. 62 (1993) 346. [2] W.J. Wang, S.S. Jiang, X.R. Huang, Appl. Phys. Lett. 68 (1996) 25. [3] J.D. Bierlein, H. Vanherzeele, Appl. Phys. Lett. 54 (1989) 783. [4] P.D. Townsend, P.J. Chandler, L. Zhang, Optical effects of ion implantation, Cambridge University, Cambridge, 1994. [5] J. Gazecki, J.M. Kubica, M. Zamora, G.K. Reeves, C.M. Johnson, M.C. Ridgway, Thin Solid Films 340 (1999) 233. [6] W. Hong, H.-J. Woo, H.-Wm. Choi, Y.-S. Kim, G.-D. Kim, Appl. Surf. Sci. 169 (2001) 428. [7] H. Hu, F. Lu, F. Chen, B.R. Shi, K.M. Wang, D.-Y. Shen, J. Appl. Phys. 89 (2001) 5224. [8] H. Hu, F. Lu, F. Chen, B.R. Shi, K.M. Wang, D.-Y. Shen, Appl. Opt. 40 (2001) 3759. [9] F. Chen, H. Hu, F. Lu, B.R. Shi, J.H. Zhang, K.M. Wang, D.Y. Shen, X.M. Wang, Opt. Commun. 190 (2001) 153. [10] H. Hu, F. Chen, F. Lu, J.H. Zhang, J.T. Liu, K.M. Wang, B.R. Shi, D.Y. Shen, X.M. Wang, Chin. Phys. Lett. 18 (2001) 242. [11] F. Chen, H. Hu, K.M. Wang, F. Lu, B.R. Shi, F.X. Wang, Z.X. Cheng, H.C. Chen, D.Y. Shen, Opt. Commun. 200 (2001) 179. [12] P.J. Chandler, F.L. Lama, Opt. Acta 33 (1986) 127. [13] J.F. Ziegler, J.P. Biesack, U. Littmark, Stopping and Ranges of Ions in Matter, Pergamon, New York, 1985. [14] G.W. Arnold, P. Mazzoldi, Ion Beam Modification of Insulators, Elsevier, Amsterdam, 1987.
Letter to the Editor
Property study on nickel ion implanted planar waveguide in KTiOAsO4 crystal F. Chen
a,*
, X.-L. Wang a, Q.-M. Lu b, G. Fu a, S.-L. Li a, F. Lu a, K.-M. Wang a, D.-Y. Shen c a
Department of Physics, Shandong University, Jinan 250100, China School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China Department of Technical Physics & the Key Laboratory of Heavy Ion Physics, Chinese Ministry of Education, Peking University, Beijing 100871, China b
c
Received 7 December 2001; received in revised form 16 April 2002
Abstract The first planar waveguide in KTiOAsO4 (KTA) crystal by nickel ion implantation is presented. The dark modes are observed and analyzed using the prism-coupling method. The refractive index profile is reconstructed by the reflectivity calculation method. A comparison of the refractive index profile is made between the as-implanted waveguide and one annealed at 240 °C for 50 min. The TRIM 98 code is carried out to simulate the implantation process, which may be helpful to understand the formation of the waveguide in KTA. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 42.79.Gn; 61.80.Jh Keywords: Optical waveguide; Ion implantation; Nonlinear optical materials
1. Introduction KTiOAsO4 (KTA) is one of the promising crystals for nonlinear optical and electrooptic device applications. The material features large nonlinear optical and electrooptic coefficients in comparison to KTiOPO4 and has added benefit of significantly reduced absorption in the 2–5 lm
*
Corresponding author. E-mail address: [email protected] (F. Chen).
region [1,2]. Moreover, KTA has a higher figure of merit for second harmonic generation and lower ionic conductivity [3]. In the past years ion implantation has become a relatively mature method for the fabrication of waveguides in many optical materials, including crystals, glass, semiconductors and polymers [4–6]. In previous work, MeV light ions, such as He and H, were used as an irradiation source at doses of 1016 ions/cm2 to form waveguides below the surface of some materials within several microns. In this case, a low refractive index ‘‘optical barrier’’ has been produced at the end of the track where most of the displacement damage
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 1 1 4 3 - 6
356
F. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 194 (2002) 355–358
occurs. Such an optical barrier confines the light to a narrow layer or ‘‘optical well’’ (act as a waveguide) between itself and the surface [4]. Resent results suggest that MeV heavier ion implantation may be a more efficient method to form waveguides in optical materials, because in this way implantation at much lower doses (1014 –1015 ions/ cm2 ) is adequate [7,8]. Nickel (Niþ ) ion is a suitable candidate. MeV Niþ implantation has been used to fabricate planar waveguides in quartz, LiNbO3 and Tm:NaY(WO4 )2 [9–11]. In the present work, we report for the first time the formation of 3.0 MeV Niþ ion implanted waveguide in KTA, and analyze the refractive index profiles before and after annealing at 240 °C for 50 min. 2. Experimental details The KTA samples are optically polished and their refractive indices were measured before implantation. The samples were implanted by 3.0 MeV Niþ ions (at a dose of 1 1015 ions/cm2 ) at room temperature. The ion beam was scanned to ensure a uniform implantation over the samples. The beam current was typically set at 300 nA. During the implantation the sample was tilted off by 7° from the beam direction in order to minimize the channeling effect. The prism-coupling method was used to observe the waveguide mode. A laser beam at 633 nm from a He–Ne laser struck at base of a rutile prism, and hence the laser beam was coupled into the waveguide region. A silicon photodetector was used to detect the reflected beam. The prism, waveguide and the photodetector were mounted on a rotary table so that the incident angle of the laser beam could be changed. The intensity of light striking the photodetector was plotted as a function of the incident angle, where a sharp drop in the intensity profile would correspond to a propagation mode. 3. Results and discussion Fig. 1 shows the relative intensity of the light (TE polarized) reflected from the prism versus the effective reflective index (a) nx and (b) ny of the incident light measured in 3.0 MeV Niþ ion im-
Fig. 1. The relative intensity of the light (TE polarized) reflected from the prism versus the effective reflective index (a) nx and (b) ny of the incident light measured in 3.0 MeV Niþ ion implanted waveguides in KTA.
planted waveguides in KTA. When the light is coupled into the waveguide region, a lack of reflected light would result in a dip. As we can see, there are three and four modes observed for nx and ny , respectively. However, only the first dip seems much sharp while others become broader. This means that the first mode is real propagation one, in which the light could be well confined. When this mode has been excited, the light propagating along the length of the sample will result in a bright point at the sample end face, and the point
F. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 194 (2002) 355–358
is much close to the front surface of the sample. The broader dips may correspond to leaky modes, which are often the result of the multiple reflectivity at the interface between the waveguide and substrate. When the leaky modes have been excited, the propagating light also resulted in a bight point at the sample end face, but the point is located in the middle of the end face, or much close to the inverse of the sample. To study the stability of the waveguide, the sample is annealed at 240 °C for 50 min in air and similar dark mode property is obtained. Reflectivity calculation method (RCM) [12] is used to reconstruct the refractive index profile of the KTA waveguide before and after the annealing at 240 °C for 50 min in air. In such a simulation, we first choose an analytic function for the index profile characterized by several parameters, which is constructed from a skewed flat-topped Gaussian peak, with a sloping base plateau; second, calculate the mode indices of this hypothetical profile by RCM; third, compare these theoretical mode index values with the experimental values obtained from the experimental measurement; finally, keep changing the parameters to alter the index profile shape until the theoretical mode indices match the experimental ones within a satisfactory error. The final profile is therefore assumed to be the optimum shape for the given mode index data. Figs. 2(a) and (b) indicate the refractive index profiles of nx and ny based on RCM for the waveguide, respectively. For nx , before and after annealing, it is found that the maximum decrease (barrier) of refractive index is about 1.5% and 0.96% but the minimum of the index decrease (waveguide region) was about 1.3% and 0.78%, respectively. For ny , similar phenomenon is observed. It seems that the annealing could reduce the refractive index of the waveguide region as well as that of the barrier. Table 1 lists the comparison of the measured mode indices with fitted values of the indices based on RCM. It is found that the measured effective refractive index is in good agreement with the calculated values better than 103 . Since the refractive index profile is very important for the waveguide, we use TRIM 98 (transport of ions in matter) code to simulate the process of the 3.0 MeV Niþ implantation into
357
Fig. 2. The refractive index profiles of (a) nx and (b) ny based on RCM for the waveguide before and after the annealing at 240 °C for 50 min in air, respectively. The solid and dashed lines represent the index profiles of the waveguide before and after the annealing.
KTA in order to obtain some useful information [13]. Fig. 3 shows the comparison of the nuclear energy loss as a function of the penetration depth (vacancy distribution) with the refractive index profiles of nx and ny . We find the shapes of vacancy distribution and the index profiles are similar to a certain extent. This may suggest that the damage induced by the nuclear energy deposition is one important factor influencing the refractive index profile of the waveguide. But it should be noted that the peak positions of the refractive index profiles are much deeper than that of the vacancy
358
F. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 194 (2002) 355–358
Table 1 The comparison of the measured mode indices with fitted values of the indices based on RCM Before the annealing
After the annealing
Exp.
Calc.
Exp.
Calc.
nx 1.78380 1.76380 1.73060
1.78377 0.00002 1.76387 0.00006 1.73070 0.0001
1.78690 1.76720 1.73290
1.78696 0.00006 1.76707 0.0001 1.73358 0.0006
ny 1.78410 1.76520 1.73250 1.68550
1.78501 0.0005 1.76484 0.0004 1.73264 0.0001 1.68533 0.0001
1.78910 1.77070 1.73840 1.69200
1.78983 0.0007 1.77020 0.0005 1.73878 0.0004 1.69205 0.00005
Error
Error
4. Summary The KTA waveguide is formed by 3.0 MeV Niþ ion implantation at a dose of 1 1015 ions/cm2 at room temperature. The refractive index profiles of nx and ny are reconstructed for the waveguide before and after the annealing at 240 °C for 50 min using RCM. The TRIM 98 simulation suggests that the nuclear energy deposition may be an important factor for the formation of the waveguide.
Acknowledgements This work is supported by the National Natural Science Foundation of China (grant no. 10035010) and the MOE Key Laboratory of Heavy Ion Physics of Peking University.
References
Fig. 3. Comparison of the nuclear energy loss as a function of the penetration depth (vacancy distribution) with the refractive index profiles of nx and ny .
distribution. This may be due to the radiationenhanced diffusion (RED) of defects occurred during the process of the implantation. The collision cascades and the following localized energy deposition cause high concentration of vacancies, which sometimes result in greater defect movement than would be expected. This suggests that the RED of the defects could be over the scale of the collision cascades itself and the ion track [14]. Similar phenomena have been reported in MeV Niþ ion implanted LiNbO3 and Tm:NaY(WO4 )2 waveguides [10,11]. Nevertheless, a detailed understanding of such phenomena still needs further investigation.
[1] L.K. Cheng, L.T. Cheng, J.D. Bierlein, F.C. Zumsteg, Appl. Phys. Lett. 62 (1993) 346. [2] W.J. Wang, S.S. Jiang, X.R. Huang, Appl. Phys. Lett. 68 (1996) 25. [3] J.D. Bierlein, H. Vanherzeele, Appl. Phys. Lett. 54 (1989) 783. [4] P.D. Townsend, P.J. Chandler, L. Zhang, Optical effects of ion implantation, Cambridge University, Cambridge, 1994. [5] J. Gazecki, J.M. Kubica, M. Zamora, G.K. Reeves, C.M. Johnson, M.C. Ridgway, Thin Solid Films 340 (1999) 233. [6] W. Hong, H.-J. Woo, H.-Wm. Choi, Y.-S. Kim, G.-D. Kim, Appl. Surf. Sci. 169 (2001) 428. [7] H. Hu, F. Lu, F. Chen, B.R. Shi, K.M. Wang, D.-Y. Shen, J. Appl. Phys. 89 (2001) 5224. [8] H. Hu, F. Lu, F. Chen, B.R. Shi, K.M. Wang, D.-Y. Shen, Appl. Opt. 40 (2001) 3759. [9] F. Chen, H. Hu, F. Lu, B.R. Shi, J.H. Zhang, K.M. Wang, D.Y. Shen, X.M. Wang, Opt. Commun. 190 (2001) 153. [10] H. Hu, F. Chen, F. Lu, J.H. Zhang, J.T. Liu, K.M. Wang, B.R. Shi, D.Y. Shen, X.M. Wang, Chin. Phys. Lett. 18 (2001) 242. [11] F. Chen, H. Hu, K.M. Wang, F. Lu, B.R. Shi, F.X. Wang, Z.X. Cheng, H.C. Chen, D.Y. Shen, Opt. Commun. 200 (2001) 179. [12] P.J. Chandler, F.L. Lama, Opt. Acta 33 (1986) 127. [13] J.F. Ziegler, J.P. Biesack, U. Littmark, Stopping and Ranges of Ions in Matter, Pergamon, New York, 1985. [14] G.W. Arnold, P. Mazzoldi, Ion Beam Modification of Insulators, Elsevier, Amsterdam, 1987.