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Neutron depth profiling study of lithium niobate optical waveguides
Nuclear Instruments and Methods in Physics Research B 141 (1998) 498±500
Neutron depth pro®ling study of lithium niobate optical waveguides P. Kolarova a, J. Vacõk b, J. Spirkov a-Hradilov a a
a,*
, J. Cerven a
b
Department of Inorganic Chemistry, Institute of Chemical Technology, Technicka 5, 166 28 Prague, Czech Republic b z near Prague, Czech Republic Nuclear Physics Institute, Academy of Sciences of the Czech Republic, 250 68 Re
Abstract The relation between optical properties and the structure of proton exchanged and annealed proton exchanged optical waveguides in lithium niobate was studied using the mode spectroscopy and neutron depth pro®ling methods. We have found a close correlation between the lithium depletion and the depth pro®le of the extraordinary refractive index. The form of the observed dependence between Li depletion and refractive index depends on the fabrication procedure by which the waveguide was prepared but it is highly reproducible for specimens prepared by the same procedure. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 78.20.Wc; 78.70.Nx Keywords: Lithium niobate; Proton exchange; Optical waveguide; Neutron depth pro®ling
1. Introduction The proton exchange (PE) technique has become a widely used method of fabrication of optical waveguides in single crystalline wafers of lithium niobate. The process consists of two steps: reaction of the substrate with a molten proton source, typically an acid (such as adipic acid), which involves the out-diusion of lithium ions (Li ) and the diusion of protons (H ) into a bulk substrate. This one-for-one exchange gives in a large increase of the extraordinary refractive index ne and a small decrease of the ordinary refractive * Corresponding author. Fax: +420 2 311 2206; e-mail: jarmila.hradilova@(ovscht.cz.
index no of the altered layer. The increase of the ne is due to the waveguiding properties of proton exchanged lithium niobate. For the waveguide formation only partial proton exchange is necessary, so that the resulting composition of the exchanged layer is Lil-x Hx NbO3 , where x stands for a portion of the exchanged ions. A second fabrication step, in the form of thermal annealing, is typically required to eliminate the ne instabilities and to restore desirable optical properties. The annealed proton exchanged (APE) waveguides are widely used in sophisticated optoelectronic structures. The relationship between the ne and the actual composition of the waveguiding layer is of particular interest for determining the optimum fabrication procedure. This paper is primarily concerned
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 1 6 0 - 8
P. Kol arova et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 498±500
499
with correlations between the depth pro®le of the extraordinary refractive index ne and the distribution of lithium in the waveguiding layers, obtained using the neutron depth pro®ling (NDP) analytical method. The preliminary results presented here illustrate an application of the NDP method in the characterization of lithium niobate optical waveguides. 2. Experimental Waveguides were fabricated by immersing Xand Z-cuts of lithium niobate into molten adipic acid [1] (either pure or doped one with a small amount of Li-adipate) for 4 h at 213°C. The as-exchanged waveguides were then annealed in air for typically 1 ± 1.5 h at 350°C. The optical properties of the fabricated samples were measured by the mode spectroscopy (prism coupling method) at 0.633 lm and their refractive index pro®les were reconstructed from the measured eective-index spectra using the inverse WKB method [2]. Lithium depth pro®les were determined using the neutron depth pro®ling (NDP) method based on the 6 Li(n,a)3 H nuclear reaction induced by thermal neutrons [3].
Fig. 1. NDP Li-depletion depth pro®les of as-exchanged Z-cuts fabricated from (a) pure and (b) Li-doped (containing 1.5 mol% of Li-adipate) adipic acid for 4 h at 213°C. The samples were measured 45 min after the proton exchange. Both waveguides supported three modes.
and at the same time the refractive index gain Dne 0.12 for the PE waveguide is reduced to Dne 0.05 at the surface. The number of supported modes has increased from 4 PE to 6 APE. Similar specimen eects are observed on the Li depth pro®les measured by the NDP method (Fig. 3). The annealing results in a massive Li transport towards the surface and a corresponding increase of the waveguiding layer depth from 1.2 lm PE to 7.3 lm APE (see Fig. 2). The rather complicated form of the Li depth pro®le in the 1 lm thick sur-
3. Results NDP depth pro®les of the as-exchanged waveguides in the Z-cuts produced using pure and Lidoped adipic acid are presented in Fig. 1. One can see that with pure adipic acid practically all Li atoms are leached from the surface layer about 1 lm thick. Treatment in Li-doped adipic acid leads only to a partial proton exchange, which is indicated by the higher Li concentration in the surface layer. The approximately constant Li concentration to a depth of about 0.5 lm may be due to the diusion of atoms from the Li-rich reaction mixture into the specimen. The eects of the post-exchange annealing on the ne depth pro®le and on the lithium distribution in the waveguiding layers, are shown in Figs. 2 and 3 for the X-cut waveguide. After annealing, the original step-like depth pro®le of ne is smeared
Fig. 2. Depth pro®les of the extraordinary refractive index ne for as-exchanged PE and post-exchange annealed APE waveguides in a X-cut crystal. The PE specimen was prepared by proton exchange in adipic acid +0.5% Li-adipate mixture for 4 h at 213°C and the APE specimen by subsequent annealing for 1. 5 h at 350°C.
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P. Kol arova et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 498±500
Fig. 3. NDP Li-depth pro®les of the (a) as-exchanged PE and (b) post-exchange annealed APE waveguides of Fig. 2.
Fig. 4. Relations between the extraordinary refractive index ne and lithium depletion (measured in the depths of particular modes) for as-exchanged PE and post-exchange annealed APE waveguides in the Z-cut. The PE specimen was prepared by proton exchange in pure adipic acid for 4 h at 213°C and the A-PE specimen by subsequent annealing for 1.5 h at 350°C. The numbers stand for the depth of the particular modes (in lm).
face layer of APE (Fig. 3) may be due to Li exchange between the specimen's surface and the Li-rich reaction mixture. To demonstrate more clearly the strong correlation between ne and the Li content in the specimen surface layer, the dependences of the ne versus Li depletion for PE and APE specimens are shown in Fig. 4. It was demonstrated in several experiments that the particular form of the ne vs. the Li depletion curve depends strongly on the specimen preparation (e.g. composition of reaction mixture and temperatures of the proton exchanging and annealing).
It appears that three factors are responsible for observed changes of ne namely: (a) the actual concentration of Li, (b) the PE induced stress and (c) the concentration of the incorporated hydrogen. The discussion of the last two factors is out of the scope of this short note. More complete data and their interpretation will be published elsewhere.
4. Conclusions
Acknowledgements
Using mode spectroscopy and a neutron depth pro®ling method, a strong correlation between ne and Li depletion was found in lithium niobate based waveguides, prepared by proton exchange and post-exchange annealing. The post-exchange annealing results in a Li redistribution, a corresponding reduction of the surface ne . and an increase in the waveguiding layer depth. The optical properties and the structure of the waveguides examined depend strongly on the fabrication procedure but, for the specimens prepared under the same conditions, they are reproducible.
This research has been supported by the Grant Agency of the Czech Republic under the contract no. 102/96/1610. References [1] P. Kol arova, J. Hradilov a, P. Nov ak, J. Schr ofel, J. Vacõk, J. Ctyrok y, PHOTONICS '95, Eur. Opt. Soc. Ann. Meet. Dig. Ser. 2A, Prague, 13±26 August 1995, p. 151. [2] J.M. White, P.F. Heinrich, Appl. Opt. 15 (1976) 151. [3] J. Vacõk, J. Cerven a, V. Hnatowicz, V. Havr anek, D. Fink, Acta Phys. Hungar. 75 (1994) 369±372.
Neutron depth pro®ling study of lithium niobate optical waveguides P. Kolarova a, J. Vacõk b, J. Spirkov a-Hradilov a a
a,*
, J. Cerven a
b
Department of Inorganic Chemistry, Institute of Chemical Technology, Technicka 5, 166 28 Prague, Czech Republic b z near Prague, Czech Republic Nuclear Physics Institute, Academy of Sciences of the Czech Republic, 250 68 Re
Abstract The relation between optical properties and the structure of proton exchanged and annealed proton exchanged optical waveguides in lithium niobate was studied using the mode spectroscopy and neutron depth pro®ling methods. We have found a close correlation between the lithium depletion and the depth pro®le of the extraordinary refractive index. The form of the observed dependence between Li depletion and refractive index depends on the fabrication procedure by which the waveguide was prepared but it is highly reproducible for specimens prepared by the same procedure. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 78.20.Wc; 78.70.Nx Keywords: Lithium niobate; Proton exchange; Optical waveguide; Neutron depth pro®ling
1. Introduction The proton exchange (PE) technique has become a widely used method of fabrication of optical waveguides in single crystalline wafers of lithium niobate. The process consists of two steps: reaction of the substrate with a molten proton source, typically an acid (such as adipic acid), which involves the out-diusion of lithium ions (Li ) and the diusion of protons (H ) into a bulk substrate. This one-for-one exchange gives in a large increase of the extraordinary refractive index ne and a small decrease of the ordinary refractive * Corresponding author. Fax: +420 2 311 2206; e-mail: jarmila.hradilova@(ovscht.cz.
index no of the altered layer. The increase of the ne is due to the waveguiding properties of proton exchanged lithium niobate. For the waveguide formation only partial proton exchange is necessary, so that the resulting composition of the exchanged layer is Lil-x Hx NbO3 , where x stands for a portion of the exchanged ions. A second fabrication step, in the form of thermal annealing, is typically required to eliminate the ne instabilities and to restore desirable optical properties. The annealed proton exchanged (APE) waveguides are widely used in sophisticated optoelectronic structures. The relationship between the ne and the actual composition of the waveguiding layer is of particular interest for determining the optimum fabrication procedure. This paper is primarily concerned
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 1 6 0 - 8
P. Kol arova et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 498±500
499
with correlations between the depth pro®le of the extraordinary refractive index ne and the distribution of lithium in the waveguiding layers, obtained using the neutron depth pro®ling (NDP) analytical method. The preliminary results presented here illustrate an application of the NDP method in the characterization of lithium niobate optical waveguides. 2. Experimental Waveguides were fabricated by immersing Xand Z-cuts of lithium niobate into molten adipic acid [1] (either pure or doped one with a small amount of Li-adipate) for 4 h at 213°C. The as-exchanged waveguides were then annealed in air for typically 1 ± 1.5 h at 350°C. The optical properties of the fabricated samples were measured by the mode spectroscopy (prism coupling method) at 0.633 lm and their refractive index pro®les were reconstructed from the measured eective-index spectra using the inverse WKB method [2]. Lithium depth pro®les were determined using the neutron depth pro®ling (NDP) method based on the 6 Li(n,a)3 H nuclear reaction induced by thermal neutrons [3].
Fig. 1. NDP Li-depletion depth pro®les of as-exchanged Z-cuts fabricated from (a) pure and (b) Li-doped (containing 1.5 mol% of Li-adipate) adipic acid for 4 h at 213°C. The samples were measured 45 min after the proton exchange. Both waveguides supported three modes.
and at the same time the refractive index gain Dne 0.12 for the PE waveguide is reduced to Dne 0.05 at the surface. The number of supported modes has increased from 4 PE to 6 APE. Similar specimen eects are observed on the Li depth pro®les measured by the NDP method (Fig. 3). The annealing results in a massive Li transport towards the surface and a corresponding increase of the waveguiding layer depth from 1.2 lm PE to 7.3 lm APE (see Fig. 2). The rather complicated form of the Li depth pro®le in the 1 lm thick sur-
3. Results NDP depth pro®les of the as-exchanged waveguides in the Z-cuts produced using pure and Lidoped adipic acid are presented in Fig. 1. One can see that with pure adipic acid practically all Li atoms are leached from the surface layer about 1 lm thick. Treatment in Li-doped adipic acid leads only to a partial proton exchange, which is indicated by the higher Li concentration in the surface layer. The approximately constant Li concentration to a depth of about 0.5 lm may be due to the diusion of atoms from the Li-rich reaction mixture into the specimen. The eects of the post-exchange annealing on the ne depth pro®le and on the lithium distribution in the waveguiding layers, are shown in Figs. 2 and 3 for the X-cut waveguide. After annealing, the original step-like depth pro®le of ne is smeared
Fig. 2. Depth pro®les of the extraordinary refractive index ne for as-exchanged PE and post-exchange annealed APE waveguides in a X-cut crystal. The PE specimen was prepared by proton exchange in adipic acid +0.5% Li-adipate mixture for 4 h at 213°C and the APE specimen by subsequent annealing for 1. 5 h at 350°C.
500
P. Kol arova et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 498±500
Fig. 3. NDP Li-depth pro®les of the (a) as-exchanged PE and (b) post-exchange annealed APE waveguides of Fig. 2.
Fig. 4. Relations between the extraordinary refractive index ne and lithium depletion (measured in the depths of particular modes) for as-exchanged PE and post-exchange annealed APE waveguides in the Z-cut. The PE specimen was prepared by proton exchange in pure adipic acid for 4 h at 213°C and the A-PE specimen by subsequent annealing for 1.5 h at 350°C. The numbers stand for the depth of the particular modes (in lm).
face layer of APE (Fig. 3) may be due to Li exchange between the specimen's surface and the Li-rich reaction mixture. To demonstrate more clearly the strong correlation between ne and the Li content in the specimen surface layer, the dependences of the ne versus Li depletion for PE and APE specimens are shown in Fig. 4. It was demonstrated in several experiments that the particular form of the ne vs. the Li depletion curve depends strongly on the specimen preparation (e.g. composition of reaction mixture and temperatures of the proton exchanging and annealing).
It appears that three factors are responsible for observed changes of ne namely: (a) the actual concentration of Li, (b) the PE induced stress and (c) the concentration of the incorporated hydrogen. The discussion of the last two factors is out of the scope of this short note. More complete data and their interpretation will be published elsewhere.
4. Conclusions
Acknowledgements
Using mode spectroscopy and a neutron depth pro®ling method, a strong correlation between ne and Li depletion was found in lithium niobate based waveguides, prepared by proton exchange and post-exchange annealing. The post-exchange annealing results in a Li redistribution, a corresponding reduction of the surface ne . and an increase in the waveguiding layer depth. The optical properties and the structure of the waveguides examined depend strongly on the fabrication procedure but, for the specimens prepared under the same conditions, they are reproducible.
This research has been supported by the Grant Agency of the Czech Republic under the contract no. 102/96/1610. References [1] P. Kol arova, J. Hradilov a, P. Nov ak, J. Schr ofel, J. Vacõk, J. Ctyrok y, PHOTONICS '95, Eur. Opt. Soc. Ann. Meet. Dig. Ser. 2A, Prague, 13±26 August 1995, p. 151. [2] J.M. White, P.F. Heinrich, Appl. Opt. 15 (1976) 151. [3] J. Vacõk, J. Cerven a, V. Hnatowicz, V. Havr anek, D. Fink, Acta Phys. Hungar. 75 (1994) 369±372.