- Email: [email protected]
Structural and optical characterization for vapor-phase proton exchanged lithium niobate waveguides
Materials Chemistry and Physics 78 (2002) 474–479
Structural and optical characterization for vapor-phase proton exchanged lithium niobate waveguides D.H. Tsou a , M.H. Chou b , P. Santhanaraghavan a , Y.H. Chen a , Y.C. Huang a,∗ a
Department of Electrical Engineering, National Tsing Hua University, 30043 Hsinchu, Taiwan, ROC b Tellium Inc., New Jersey, USA Received 21 November 2001; received in revised form 31 May 2002; accepted 21 June 2002
Abstract We have studied the crystalline phase transition for vapor-phase proton exchanged (VPE) lithium niobate waveguide by using pure benzoic acid. The VPE layers in the z-cut lithium niobate crystal have been characterized by X-ray rocking curves, OH− bond infrared absorption, and prism coupling measurements. The crystalline quality assessment by the X-ray rocking curve measurement shows desirable 2 phase formation for nonlinear optical waveguide applications. It has been observed that the crystalline phase makes a transition from the 2 to the  phase with the increase in the vapor-phase exchange time. The prism coupling measurement also showed a step-like index profile for our VPE samples. From the step index profile, we deduced an effective diffusion coefficient of 0.14 m2 h−1 , which is twice than that of the previously reported value. Shift in the OH− bond absorption wavelength with crystalline phase change and the increase in absorption with the exchange time were also observed from our spectrophotometer measurement. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Vapor-phase; Lithium niobate; Waveguide
1. Introduction Lithium niobate, LiNbO3 , has become a very attractive material for integrated optical applications because of its excellent electro-optical (EO), acousto-optical (AO), and nonlinear optical properties. The method of proton exchange (PE) for preparing optical waveguide on LiNbO3 and LiTaO3 crystals dates back to 1982 [1], and since then lot of works have been reported on the various ways of preparing proton exchanged lithium niobate waveguides. Today the PE process has proved to be a simple and effective method of fabricating low-loss optical waveguides in LiNbO3 crystals. More and more applications integrated on LiNbO3 substrates such as EO modulation, directional coupler, periodically poled LiNbO3 (PPLN) waveguides were realized, in particular for optical communications. In comparison to the common Ti in-diffusion method [2], the PE process has the advantage of having a much higher photorefractive damage threshold for guiding a visible laser [3]. In the liquid-phase PE process, LiNbO3 was treated in the molten benzoic acid bath for several hours. The hydrogen ions are diffused into LiNbO3 and exchanges with ∗ Corresponding author. E-mail address: [email protected] (Y.C. Huang).
lithium ions in the LiNbO3 . The out-diffusion of lithium ions occurs and finally the lithium niobate crystal forms a new chemical composition of Hx Li1−x NbO3 in certain surface depth, where x is the value of the H+ /Li+ ratio after exchange. After the liquid-phase PE process, a so-called -mixture crystalline phase can generally be obtained, corresponding to a chemical composition of x lying in the range 0.5 < x < 0.85 [4]. The -mixture crystalline phase is characterized as step-like (0.05 < ne < 0.12) index profile structure and this causes the extraordinary refractive index change ne ∼ 0.12. However, this -mixture crystalline phase is not the proper phase for nonlinear frequency conversion due to the severe reduction of the nonlinear coefficient. With time, annealed proton exchange (APE) waveguide process was reported [5,6], in which the liquid-phase proton exchanged LiNbO3 was further annealed at a high temperature (above 300 ) for several hours. The annealing process results a change of the crystalline phase into the graded-index structure, or the ␣ phase with 0.01 < x < 0.1 and ne 0.03. A low-loss waveguide is formed in this ␣ phase crystal structure. In 1999, Rams and Cabrera [7] have reported the vapor-phase proton exchanged (VPE) waveguide in a lithium niobate crystal. The achieved crystal phase is believed to have 40-time higher photorefractive damage threshold than
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 3 3 0 - 9
D.H. Tsou et al. / Materials Chemistry and Physics 78 (2002) 474–479
that from the APE process, while at the same time having the excellent SHG efficiency and very low propagation loss (below 0.35 dB cm−1 ). However, without any rocking curve measurement, the crystalline phase obtained from the VPE process in Ref. [7] was loosely called ␣ and  according to the length of the VPE time. Although Ref. [8] has reported the desirable crystalline phase 2 produced in the VPE lithium niobate waveguide, the main interest in Ref. [8] remained in the effect of adding lithium benzoate in the VPE process for a given PE time. In this paper, we show for the first time the transition of crystalline phase over the VPE time in pure benzoic acid with evidence from X-ray rocking curve measurement. Our infrared absorption and prism coupling measurement also indicated improved waveguide quality compared to the previously reported results.
2. Material and method 2.1. VPE process Congruent 3 in. z-cut polished LiNbO3 wafers from Crystal Technology Industry (CTI) were diced into 10 mm × 20 mm × 0.5 mm pieces. After careful cleaning, each sample was put into a glass ampoule with two compartments connected by a neck. The benzoic acid powder was taken in the bottom compartment and the wafer is held in the top compartment. The method is similar to the one reported by Refs. [7,8], except that our ampoule diameter is 18 mm as compared to the 12 mm diameter used in Refs. [7,8]. The tube was evacuated and then sealed. The sealing ensures the benzoic acid to be vaporized under a constant volume. The benzoic acid saturated vapor pressure in the
475
Table 1 Extraordinary index change Sample
ne
a b c d e f
0.0266 0.1105 0.1081 0.1088 0.1122 0.1116
tube depends only on the temperature. The sealed tube was then put into the aluminum tube for better temperature uniformity and finally set into a furnace in a vertical configuration. The vertical configuration ensures that only vapor-phase exchange predominates and any liquid benzoic acid condensation on the substrate automatically trips to the bottom compartment. The experiment was carried out for different exchange timings (5, 10, 24, 30, 48, and 72 h) keeping the furnace temperature at 300 . For comparison, planar waveguides are also made with the conventional APE process (liquid-phase PE at 160 for 10 h and annealing at 330 for 24 h). 2.2. Prism coupling measurements A prism coupler was used to investigate the refractive index and the waveguide depth. By using our prism coupler (Metricon Model 2010), the refractive index in the waveguide was measured with an accuracy of ±0.001 and a thickness accuracy of ±(0.5% + 50 Å). Measurements were made with a 632.8 nm He–Ne laser source. Typically, we observed several guiding modes in the waveguide, based upon which a computer code calculated the effective
Fig. 1. Index profile curve: (a) APE waveguide; (b) VPE waveguide at 300 for 10 h; (c) 24 h; (d) 30 h; (e) 48 h; (f) 72 h.
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D.H. Tsou et al. / Materials Chemistry and Physics 78 (2002) 474–479
Fig. 2. Rocking curves of the waveguide samples with Bragg reflections from (00.12) planes parallel to the wafer: (a) APE waveguide, peak shift of 162 ; (b) liquid-phase PE waveguide, peak shift of 594 ; (c) 5 h exchange time VPE waveguide with peak shift of 486 ; (d) 10 h exchange time VPE waveguide with peak shift of 540 ; (e) 24 h exchange time VPE waveguide with peak shift of 540 ; (f) 30 h exchange time VPE waveguide with peak shift of 576 ; (g) 48 h exchange time VPE waveguide with peak shift of 576 ; (h) 72 h exchange time VPE waveguide with peak shift of 594 .
D.H. Tsou et al. / Materials Chemistry and Physics 78 (2002) 474–479
index of the thin-film waveguide. From that information, the index profile can be deduced from the IWKB (inverse Wentzel–Kramers–Brillouin) method [9]. Fig. 1 and Table 1 show the index profile and extraordinary index change of different samples, respectively. From Fig. 1, one may compute the diffusion coefficient of hydrogen using the formulas according to the expression: Ee depth = 4tDeff , Deff = D0 exp − , (1) kTe where t is the time for the diffusion process, D0 the diffusion coefficient, Te the temperature during the VPE, k the Boltzmann constant, Ee = 0.974 eV for z-cut LiNbO3 , and Deff the effective diffusion coefficient. The prism coupling measurement provides the waveguide depth information in the calculation. 2.3. Crystalline phase characterization The most reliable method to define the crystalline phases is the peak shift in the X-ray rocking curve. The crystalline phase was investigated by X-ray rocking curve measurements using a Cu K␣ source diffracted from a double-crystal monochromator (Rigaku RU-H3R). Fig. 2 shows the measured results for APE and VPE samples under different fabrication conditions. Since the change of crystalline phase also affects the OH− bond absorption in the LiNbO3 , we also conducted the OH− absorption measurements by using a Shimadzu UV–IR
477
spectrophotometer. Fig. 3 shows the absorption peaks due to the OH− bond in various APE and VPE samples.
3. Experimental results: analysis and discussion In Fig. 1, sample a is obtained with the APE process, showing an ␣ phase structure with ne 0.03. The curves in Fig. 1(b)−(f) correspond to the waveguides obtained by the VPE process carried out at 300 for 10, 24, 30, 48, and 72 h, respectively. Table 1 summarizes the corresponding index change for the extraordinary wave, deduced from the prism coupling measurement. From the curves in Fig. 1 one can see the sharp index change as a function of time. For our vapor-phase experimental condition, the effective diffusion coefficient of hydrogen in the z-cut LiNbO3 was found to be 0.14 m2 h−1 , according to Eq. (1). This value is about twice than that reported in [8] at the same VPE temperature. The discrepancy could result from different sources of the lithium niobate wafers. However as shown in Ref. [8], adding lithium benzoate may significantly decrease the effective diffusion coefficient. The lithium out-diffusion can produce lithium benzoate in the ampoule, even though the VPE process is started with pure benzoic acid. As a result, our larger ampoule diameter, which helps to reduce the out-diffused lithium concentration, might have caused the larger effective diffusion coefficient in our measurement. In summary, from Fig. 1 and Table 1, it can be seen that the our process yields high index profiles as compared to that of APE waveguides, and this, for instance, allows
Fig. 3. OH− bond absorption: (a) APE waveguide with peak absorption at 2869 nm; (b) liquid-phase PE waveguide with peak absorption at 2854 nm; (c) 5 h exchange time VPE waveguide with peak absorption at 2854 nm; (d) 24 h exchange time VPE waveguide with peak absorption at 2852.6 nm; (e) 30 h exchange time VPE waveguide with peak absorption at 2851.8 nm; (f) 48 h exchange time VPE waveguide with peak absorption at 2852.2 nm; (g) 72 h exchange time VPE waveguide with peak absorption at 2853.6 nm.
478
D.H. Tsou et al. / Materials Chemistry and Physics 78 (2002) 474–479
the fabrication of a tightly confined optical waveguide for high-efficiency nonlinear frequency conversion. Since in the conventional liquid-phase PE process the PE is fast and often causes the formation of mixed crystal phases. On the other hand, the proton concentration in benzoic acid vapor is much lower when compared to that in liquid benzoic acid. This controlled PE in the vapor-phase process results in the formation of 2 phase. X-ray rocking curves are particularly useful for distinguishing the different crystalline phases in a proton exchanged waveguide. We performed the X-ray rocking curve measurement to investigate the influence of the exchange time on the crystalline phase at a given temperature. Our z-cut samples were probed by X-rays diffracted at the Bragg angle from (00.12) planes. In Fig. 2, the peak shifted from a pure lithium niobate substrate (right one) is the important information for identifying the crystalline phase change. Korkishko and Fedorov [10] have shown the correlation between the crystalline phase and the rocking curve peak shift. In Fig. 2(a), the shift from substrate peak is 162 , which characterizes the ␣ crystalline phase in an APE waveguide. In Fig. 2(b), one can see the diffraction peak corresponding to the -mixture phase structure with a characteristic shift of 594 in a liquid-phase PE waveguide. The pictures in Fig. 2(c)–(h) show the peak shift from 486 to 594 , which indicates the gradual transition from 2 phase towards the -mixture phase. The X-ray rocking curves in Fig. 2(c)–(h) correspond to the VPE samples with the exchange time of 5, 10, 24, 30, 48, and 72 h, respectively. It is obvious that a pure 2 structure can exist with a maximum depth of ∼5 m. For deeper depth, the crystal turns to be -mixture phase. Therefore the desirable waveguide depth with the 2 phase in Fig. 1(b)−(f) can be achieved by the optimized fabrication condition revealed in this paper. This flexibility allows one to design and fabricate high-quality waveguides for a variety of optical applications. Fig. 3 shows the shift in the OH− absorption peak (2854, 2852.6, 2851.8, 2852.2 and 2853.6 nm) for (a) APE, (b) liquid-phase PE, and (c)–(g) VPE samples with different exchange time (5, 24, 30, 48 and 72 h, respectively). It is seen that the ␣ phase has a characteristic absorption peak at 2869 nm, and the  and 2 phases have the same characteristic peak about 2853 nm. From Fig. 3, it can also be observed that with the increase in exchange time the absorption peak intensity also raises indicating more hydrogen diffuses into the substrate.
4. Conclusion We have fabricated and characterized high-quality VPE planar waveguides on z-cut LiNbO3 . The optical measurement with a prism coupler has shown step-like index profiles. From the index profile, we deduced an unusually large effective diffusion coefficient of 0.14 m2 h−1 , which
is twice that of the previously reported value. This result may be attributable to the reduction of the out-diffused lithium concentration in a larger-diameter ampoule. The index change of ne ∼ 0.11 and the step-like index profile with the 2 crystalline phase from our VPE process are desirable for high-efficiency nonlinear frequency conversion in such waveguides. The X-ray rocking curve measurements showed that long-duration PE in vapor-phase benzoic acid may result in crystalline phase change from the high-quality 2 phase structure (shift of 540 from the substrate peak) to the high-loss -mixture phase structure (peak shift of 594 from the substrate peak). A maximum depth of 5 m 2 phase waveguide in z-cut congruent lithium niobate was achieved in our experiment. This amount of waveguide depth will be very useful for near infrared guiding. We tried to obtain an even deeper waveguide depth by increasing the VPE time, but the proton exchanged crystalline phase started to change from the 2 phase to the -mixture phase. The OH− absorption peak in the VPE waveguide was found to be helpful in observing the phase transitions. In conclusion, we have characterized the diffusion coefficient, index profile, and crystal phase of the VPE planar waveguide. The results will provide guidance to the waveguide depth engineering and crystal phase optimization for a VPE optical waveguide in z-cut lithium niobate.
Acknowledgements The authors appreciate Bi-Chen Wong for polishing the waveguides. This work was supported in part by HC Photonics, Inc., under the NTHU project 0988-053J6. P. Santhanaraghavan and Y.H. Chen thank National Science Council for the support under contract NSC 89-3233-M007-049.
References [1] J.L. Jackel, C.E. Rice, J.J. Veselka, Proton exchange for high-index waveguide in LiNbO3 , Appl. Phys. Lett. 41 (1982) 607– 608. [2] D. Hofmann, G. Shreiber, C. Hasse, H. Herrmann, R. Ricken, W. Sohler, Continuous-wave mid-infrared optical parametric oscillators with periodically poled Ti:LiNbO3 channel waveguides, Opt. Lett. 24 (1999) 896–898. [3] T. Fujiwara, R. Srivastava, X. Cao, R.V. Ramasway, Comparison of photorefractive index change in proton-exchanged and Ti-diffused LiNbO3 waveguides, Opt. Lett. 18 (1993) 346–348. [4] W.Y. Hsu, C.S. Willand, V. Gopalan, M. Gupta, Appl. Phys. Lett. 61 (1992) 2263. [5] M.L. Bortz, Quasi-phasematched optical frequency conversion in lithium niobate waveguides, Ph.D. Dissertation, Department of Electrical Engineering, Stanford University, Stanford, CA, 1992. [6] M.-H. Chou, Optical frequency mixers using three-wave mixing for optical fiber communications, Ph.D. Dissertation, Department of Applied Physics, Stanford University, Stanford, CA, 1999.
D.H. Tsou et al. / Materials Chemistry and Physics 78 (2002) 474–479 [7] J. Rams, J.M. Cabrera, Preparation of proton-exchange LiNbO3 waveguide in benzoic acid vapor, J. Opt. Soc. Am. B 16 (3) (1999) 401–406. [8] J. Rams, J.M. Cabrera, Nonlinear optical efficient LiNbO3 waveguides proton exchanged in benzoic acid vapor: effect of the vapor pressure, J. Appl. Phys. 85 (1999) 1322.
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[9] K.S. Chiang, Construction of refractive-index profiles of planner dielectric waveguides from the distribution of effective indexes, J. Lightwave Technol. LT-3 (1985) 385–391. [10] Yu.N. Korkishko, V.A. Fedorov, Optical and X-ray characterization of Hx Li1−x NbO3 optical waveguides, J. Appl. Phys. 84 (5) (1998) pp. 2411–2419.
Structural and optical characterization for vapor-phase proton exchanged lithium niobate waveguides D.H. Tsou a , M.H. Chou b , P. Santhanaraghavan a , Y.H. Chen a , Y.C. Huang a,∗ a
Department of Electrical Engineering, National Tsing Hua University, 30043 Hsinchu, Taiwan, ROC b Tellium Inc., New Jersey, USA Received 21 November 2001; received in revised form 31 May 2002; accepted 21 June 2002
Abstract We have studied the crystalline phase transition for vapor-phase proton exchanged (VPE) lithium niobate waveguide by using pure benzoic acid. The VPE layers in the z-cut lithium niobate crystal have been characterized by X-ray rocking curves, OH− bond infrared absorption, and prism coupling measurements. The crystalline quality assessment by the X-ray rocking curve measurement shows desirable 2 phase formation for nonlinear optical waveguide applications. It has been observed that the crystalline phase makes a transition from the 2 to the  phase with the increase in the vapor-phase exchange time. The prism coupling measurement also showed a step-like index profile for our VPE samples. From the step index profile, we deduced an effective diffusion coefficient of 0.14 m2 h−1 , which is twice than that of the previously reported value. Shift in the OH− bond absorption wavelength with crystalline phase change and the increase in absorption with the exchange time were also observed from our spectrophotometer measurement. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Vapor-phase; Lithium niobate; Waveguide
1. Introduction Lithium niobate, LiNbO3 , has become a very attractive material for integrated optical applications because of its excellent electro-optical (EO), acousto-optical (AO), and nonlinear optical properties. The method of proton exchange (PE) for preparing optical waveguide on LiNbO3 and LiTaO3 crystals dates back to 1982 [1], and since then lot of works have been reported on the various ways of preparing proton exchanged lithium niobate waveguides. Today the PE process has proved to be a simple and effective method of fabricating low-loss optical waveguides in LiNbO3 crystals. More and more applications integrated on LiNbO3 substrates such as EO modulation, directional coupler, periodically poled LiNbO3 (PPLN) waveguides were realized, in particular for optical communications. In comparison to the common Ti in-diffusion method [2], the PE process has the advantage of having a much higher photorefractive damage threshold for guiding a visible laser [3]. In the liquid-phase PE process, LiNbO3 was treated in the molten benzoic acid bath for several hours. The hydrogen ions are diffused into LiNbO3 and exchanges with ∗ Corresponding author. E-mail address: [email protected] (Y.C. Huang).
lithium ions in the LiNbO3 . The out-diffusion of lithium ions occurs and finally the lithium niobate crystal forms a new chemical composition of Hx Li1−x NbO3 in certain surface depth, where x is the value of the H+ /Li+ ratio after exchange. After the liquid-phase PE process, a so-called -mixture crystalline phase can generally be obtained, corresponding to a chemical composition of x lying in the range 0.5 < x < 0.85 [4]. The -mixture crystalline phase is characterized as step-like (0.05 < ne < 0.12) index profile structure and this causes the extraordinary refractive index change ne ∼ 0.12. However, this -mixture crystalline phase is not the proper phase for nonlinear frequency conversion due to the severe reduction of the nonlinear coefficient. With time, annealed proton exchange (APE) waveguide process was reported [5,6], in which the liquid-phase proton exchanged LiNbO3 was further annealed at a high temperature (above 300 ) for several hours. The annealing process results a change of the crystalline phase into the graded-index structure, or the ␣ phase with 0.01 < x < 0.1 and ne 0.03. A low-loss waveguide is formed in this ␣ phase crystal structure. In 1999, Rams and Cabrera [7] have reported the vapor-phase proton exchanged (VPE) waveguide in a lithium niobate crystal. The achieved crystal phase is believed to have 40-time higher photorefractive damage threshold than
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 3 3 0 - 9
D.H. Tsou et al. / Materials Chemistry and Physics 78 (2002) 474–479
that from the APE process, while at the same time having the excellent SHG efficiency and very low propagation loss (below 0.35 dB cm−1 ). However, without any rocking curve measurement, the crystalline phase obtained from the VPE process in Ref. [7] was loosely called ␣ and  according to the length of the VPE time. Although Ref. [8] has reported the desirable crystalline phase 2 produced in the VPE lithium niobate waveguide, the main interest in Ref. [8] remained in the effect of adding lithium benzoate in the VPE process for a given PE time. In this paper, we show for the first time the transition of crystalline phase over the VPE time in pure benzoic acid with evidence from X-ray rocking curve measurement. Our infrared absorption and prism coupling measurement also indicated improved waveguide quality compared to the previously reported results.
2. Material and method 2.1. VPE process Congruent 3 in. z-cut polished LiNbO3 wafers from Crystal Technology Industry (CTI) were diced into 10 mm × 20 mm × 0.5 mm pieces. After careful cleaning, each sample was put into a glass ampoule with two compartments connected by a neck. The benzoic acid powder was taken in the bottom compartment and the wafer is held in the top compartment. The method is similar to the one reported by Refs. [7,8], except that our ampoule diameter is 18 mm as compared to the 12 mm diameter used in Refs. [7,8]. The tube was evacuated and then sealed. The sealing ensures the benzoic acid to be vaporized under a constant volume. The benzoic acid saturated vapor pressure in the
475
Table 1 Extraordinary index change Sample
ne
a b c d e f
0.0266 0.1105 0.1081 0.1088 0.1122 0.1116
tube depends only on the temperature. The sealed tube was then put into the aluminum tube for better temperature uniformity and finally set into a furnace in a vertical configuration. The vertical configuration ensures that only vapor-phase exchange predominates and any liquid benzoic acid condensation on the substrate automatically trips to the bottom compartment. The experiment was carried out for different exchange timings (5, 10, 24, 30, 48, and 72 h) keeping the furnace temperature at 300 . For comparison, planar waveguides are also made with the conventional APE process (liquid-phase PE at 160 for 10 h and annealing at 330 for 24 h). 2.2. Prism coupling measurements A prism coupler was used to investigate the refractive index and the waveguide depth. By using our prism coupler (Metricon Model 2010), the refractive index in the waveguide was measured with an accuracy of ±0.001 and a thickness accuracy of ±(0.5% + 50 Å). Measurements were made with a 632.8 nm He–Ne laser source. Typically, we observed several guiding modes in the waveguide, based upon which a computer code calculated the effective
Fig. 1. Index profile curve: (a) APE waveguide; (b) VPE waveguide at 300 for 10 h; (c) 24 h; (d) 30 h; (e) 48 h; (f) 72 h.
476
D.H. Tsou et al. / Materials Chemistry and Physics 78 (2002) 474–479
Fig. 2. Rocking curves of the waveguide samples with Bragg reflections from (00.12) planes parallel to the wafer: (a) APE waveguide, peak shift of 162 ; (b) liquid-phase PE waveguide, peak shift of 594 ; (c) 5 h exchange time VPE waveguide with peak shift of 486 ; (d) 10 h exchange time VPE waveguide with peak shift of 540 ; (e) 24 h exchange time VPE waveguide with peak shift of 540 ; (f) 30 h exchange time VPE waveguide with peak shift of 576 ; (g) 48 h exchange time VPE waveguide with peak shift of 576 ; (h) 72 h exchange time VPE waveguide with peak shift of 594 .
D.H. Tsou et al. / Materials Chemistry and Physics 78 (2002) 474–479
index of the thin-film waveguide. From that information, the index profile can be deduced from the IWKB (inverse Wentzel–Kramers–Brillouin) method [9]. Fig. 1 and Table 1 show the index profile and extraordinary index change of different samples, respectively. From Fig. 1, one may compute the diffusion coefficient of hydrogen using the formulas according to the expression: Ee depth = 4tDeff , Deff = D0 exp − , (1) kTe where t is the time for the diffusion process, D0 the diffusion coefficient, Te the temperature during the VPE, k the Boltzmann constant, Ee = 0.974 eV for z-cut LiNbO3 , and Deff the effective diffusion coefficient. The prism coupling measurement provides the waveguide depth information in the calculation. 2.3. Crystalline phase characterization The most reliable method to define the crystalline phases is the peak shift in the X-ray rocking curve. The crystalline phase was investigated by X-ray rocking curve measurements using a Cu K␣ source diffracted from a double-crystal monochromator (Rigaku RU-H3R). Fig. 2 shows the measured results for APE and VPE samples under different fabrication conditions. Since the change of crystalline phase also affects the OH− bond absorption in the LiNbO3 , we also conducted the OH− absorption measurements by using a Shimadzu UV–IR
477
spectrophotometer. Fig. 3 shows the absorption peaks due to the OH− bond in various APE and VPE samples.
3. Experimental results: analysis and discussion In Fig. 1, sample a is obtained with the APE process, showing an ␣ phase structure with ne 0.03. The curves in Fig. 1(b)−(f) correspond to the waveguides obtained by the VPE process carried out at 300 for 10, 24, 30, 48, and 72 h, respectively. Table 1 summarizes the corresponding index change for the extraordinary wave, deduced from the prism coupling measurement. From the curves in Fig. 1 one can see the sharp index change as a function of time. For our vapor-phase experimental condition, the effective diffusion coefficient of hydrogen in the z-cut LiNbO3 was found to be 0.14 m2 h−1 , according to Eq. (1). This value is about twice than that reported in [8] at the same VPE temperature. The discrepancy could result from different sources of the lithium niobate wafers. However as shown in Ref. [8], adding lithium benzoate may significantly decrease the effective diffusion coefficient. The lithium out-diffusion can produce lithium benzoate in the ampoule, even though the VPE process is started with pure benzoic acid. As a result, our larger ampoule diameter, which helps to reduce the out-diffused lithium concentration, might have caused the larger effective diffusion coefficient in our measurement. In summary, from Fig. 1 and Table 1, it can be seen that the our process yields high index profiles as compared to that of APE waveguides, and this, for instance, allows
Fig. 3. OH− bond absorption: (a) APE waveguide with peak absorption at 2869 nm; (b) liquid-phase PE waveguide with peak absorption at 2854 nm; (c) 5 h exchange time VPE waveguide with peak absorption at 2854 nm; (d) 24 h exchange time VPE waveguide with peak absorption at 2852.6 nm; (e) 30 h exchange time VPE waveguide with peak absorption at 2851.8 nm; (f) 48 h exchange time VPE waveguide with peak absorption at 2852.2 nm; (g) 72 h exchange time VPE waveguide with peak absorption at 2853.6 nm.
478
D.H. Tsou et al. / Materials Chemistry and Physics 78 (2002) 474–479
the fabrication of a tightly confined optical waveguide for high-efficiency nonlinear frequency conversion. Since in the conventional liquid-phase PE process the PE is fast and often causes the formation of mixed crystal phases. On the other hand, the proton concentration in benzoic acid vapor is much lower when compared to that in liquid benzoic acid. This controlled PE in the vapor-phase process results in the formation of 2 phase. X-ray rocking curves are particularly useful for distinguishing the different crystalline phases in a proton exchanged waveguide. We performed the X-ray rocking curve measurement to investigate the influence of the exchange time on the crystalline phase at a given temperature. Our z-cut samples were probed by X-rays diffracted at the Bragg angle from (00.12) planes. In Fig. 2, the peak shifted from a pure lithium niobate substrate (right one) is the important information for identifying the crystalline phase change. Korkishko and Fedorov [10] have shown the correlation between the crystalline phase and the rocking curve peak shift. In Fig. 2(a), the shift from substrate peak is 162 , which characterizes the ␣ crystalline phase in an APE waveguide. In Fig. 2(b), one can see the diffraction peak corresponding to the -mixture phase structure with a characteristic shift of 594 in a liquid-phase PE waveguide. The pictures in Fig. 2(c)–(h) show the peak shift from 486 to 594 , which indicates the gradual transition from 2 phase towards the -mixture phase. The X-ray rocking curves in Fig. 2(c)–(h) correspond to the VPE samples with the exchange time of 5, 10, 24, 30, 48, and 72 h, respectively. It is obvious that a pure 2 structure can exist with a maximum depth of ∼5 m. For deeper depth, the crystal turns to be -mixture phase. Therefore the desirable waveguide depth with the 2 phase in Fig. 1(b)−(f) can be achieved by the optimized fabrication condition revealed in this paper. This flexibility allows one to design and fabricate high-quality waveguides for a variety of optical applications. Fig. 3 shows the shift in the OH− absorption peak (2854, 2852.6, 2851.8, 2852.2 and 2853.6 nm) for (a) APE, (b) liquid-phase PE, and (c)–(g) VPE samples with different exchange time (5, 24, 30, 48 and 72 h, respectively). It is seen that the ␣ phase has a characteristic absorption peak at 2869 nm, and the  and 2 phases have the same characteristic peak about 2853 nm. From Fig. 3, it can also be observed that with the increase in exchange time the absorption peak intensity also raises indicating more hydrogen diffuses into the substrate.
4. Conclusion We have fabricated and characterized high-quality VPE planar waveguides on z-cut LiNbO3 . The optical measurement with a prism coupler has shown step-like index profiles. From the index profile, we deduced an unusually large effective diffusion coefficient of 0.14 m2 h−1 , which
is twice that of the previously reported value. This result may be attributable to the reduction of the out-diffused lithium concentration in a larger-diameter ampoule. The index change of ne ∼ 0.11 and the step-like index profile with the 2 crystalline phase from our VPE process are desirable for high-efficiency nonlinear frequency conversion in such waveguides. The X-ray rocking curve measurements showed that long-duration PE in vapor-phase benzoic acid may result in crystalline phase change from the high-quality 2 phase structure (shift of 540 from the substrate peak) to the high-loss -mixture phase structure (peak shift of 594 from the substrate peak). A maximum depth of 5 m 2 phase waveguide in z-cut congruent lithium niobate was achieved in our experiment. This amount of waveguide depth will be very useful for near infrared guiding. We tried to obtain an even deeper waveguide depth by increasing the VPE time, but the proton exchanged crystalline phase started to change from the 2 phase to the -mixture phase. The OH− absorption peak in the VPE waveguide was found to be helpful in observing the phase transitions. In conclusion, we have characterized the diffusion coefficient, index profile, and crystal phase of the VPE planar waveguide. The results will provide guidance to the waveguide depth engineering and crystal phase optimization for a VPE optical waveguide in z-cut lithium niobate.
Acknowledgements The authors appreciate Bi-Chen Wong for polishing the waveguides. This work was supported in part by HC Photonics, Inc., under the NTHU project 0988-053J6. P. Santhanaraghavan and Y.H. Chen thank National Science Council for the support under contract NSC 89-3233-M007-049.
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