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We report on the fabrication of planar and ridge waveguides in lithium niobate by proton exchange combined with oxygen ion implantation. The implanted energy ranges from 600 to 1400 keV with a dose of 1×1015 ions/cm2. The modes in proton exchanged waveguide can be modulated by O ion implantation. There are different damage profiles in proton-exchanged and ion-implanted waveguides in Rutherford backscattering/channeling spectra. The refractive index profile in single-mode waveguide in lithium niobate has been obtained based on Intensity Calculation Method. Also ridge waveguide was fabricated on the basis of planar waveguide by Ar ion beam etching. The measured near-field intensity distributions of the ridge waveguide modes show a reasonable agreement with the simulated ones. The estimated propagation loss was ~2.2 dB/cm.
©2010 Optical Society of America
1. Introduction
Lithium niobate (LiNbO3) called “silicon of photonics” is widely used in advanced photonics and nonlinear optics due to its intriguing combination of excellent physical properties, electro-optic and nonlinear-optic characteristics [1]. Optical waveguides are the structures defined as the high-refractive-index regions surrounded by low-index regions. These guiding structures can confine light propagation within small volumes in one dimensional (1D) planar waveguide or in two dimensional (2D) channel or ridge waveguide [2]. Compared with the planar waveguides, the 2D guiding structures can carry higher optical density and are widely applied in electro-optical circuits as the interconnecting elements. As the basic components of integrated photonic system, optical waveguide structures have been used to realize many functional devices in LiNbO3, including electro-optic modulators [3,4], waveguide laser [5,6], photorefractive spatial solitons [7], photonic bandgap crystals [8] and so on. Many methods have been developed to fabricate waveguides in LiNbO3, such as metal ion indiffusion [9], proton exchange [10–12], direct femtosecond laser writing [13], ion implantation [14–17] or the combination of the above-mentioned methods [18–20], etc. The proton exchange process introduces a very high concentration of protons by the exchange of H+ and Li+ and leads to a large increase of extraordinary refractive index (more than 0.12 at 633 nm) [10] in the near-surface region in contact with the proton source of benzoic acid or pyrophosphoric acid. Ion implantation technique is a very promising candidate to form photonic guiding structures in various materials owing to its accurate control of both the depth and the concentration of dopants at low temperature [14]. The energetic ion implantation can cause a positive change of different degree for extraordinary refractive index (~+0.5% for light ions, i.e. H, He; ~+2% for medium-mass ions such as C, O, Si), probably due to electronic energy deposition, in the near-surface layer in LiNbO3 matrix [2]. At the end of the ion track, a negative-index-changed barrier of extraordinary refractive index can occur due to the nuclear energy deposition [14]. As far as we know, some investigations have supported that the changes of spontaneous polarization, molar volume, elasto-optic effect and molar polarization play an important role on the index change in ion-implanted waveguide [21,22]. Similarly, the changes of the four factors can also influence the index in proton-exchanged waveguide [23]. Especially, the decrease of spontaneous polarization can enhance the refractive index of guiding region and the volume swelling can decrease the refractive index [21–23]. The refractive index distribution is a very important parameter for investigating the optical properties of the waveguide and the application of photonics device based on the waveguide. The well-known methods of reconstructing the index profile include Reflectivity Calculation Method (RCM) [24], iWKB [25], Intensity Calculation Method (ICM) [26] and so on. iWKB is suitable for the index profile of gradual change with the depth, such as indiffused or exchanged waveguides. RCM is a promising method to simulate the refractive index profile of the multimode ion-implanted waveguide. The method of Intensity Calculation Method (ICM) can be adopted to reconstruct the index profile of the single-mode waveguide, which is based on the beam propagation method (BPM) and image processing. The combination of proton exchange and ion implantation can be used to produce optical waveguide to meet our need. Schrempel et al. reported the formation of buried channel waveguide by He ion implantation in Rb-exchanged KTiOPO4 [19]. E. Glavas et al. studied the refractive index changes in proton-exchanged LiNbO3 waveguide followed light ion (He) implantation [20].
The purpose of this work is first, to explore the possibility of planar waveguide formed in LiNbO3 by proton exchange combined with O ion implantation; second, to use Rutherford backscattering (RBS) /channeling measurement for studying the damage in ion implanted and proton exchanged waveguides; third, to demonstrate the properties of the ridge waveguide fabricated by the Ar ion beam etching on the basis of the modified waveguide by ion implantation.
2. Experimental details
A z-cut LiNbO3 wafer was immersed in the molten pyrophosphoric acid for 3 hours when the proton source was heated to 200 °C in a sealed furnace. After proton exchange, the sample was pulled out from the pyrophosphoric acid and hanged in the furnace until the temperature gradually dropped to the room temperature. Subsequently, the proton-exchanged waveguides were implanted by O ion with a dose of 1×1015 ions/cm2 at room temperature. The implanted energy was from 600 to 1400 keV at the energy interval of 200 keV. In order to avoid channeling effect, the samples were tilted by 7° off the incident beam direction. The ion implantation was performed by a 1.7 MV tandem accelerator at Peking University. In order to observe the damage in ion implanted and proton exchanged waveguides, RBS/channeling measurement has been carried out in ion implanted and proton exchanged waveguides by 2.1 MeV He ions at 1.7 MV tandem accelerator of Shandong University. All the proton-exchanged samples modified by ion implantation were annealed in the ambient air at 200 °C for 30 min for removing the color centers and reducing absorption loss. The m-line technique and end-fire coupling method were used to determine the properties of waveguides. The refractive index profile of the optical waveguide after annealing was reconstructed by ICM.
The ridge waveguide was fabricated with the planar waveguide modified by 800 keV O ion implantation after annealing at 200 °C for 30 min. The standard lithographic technique was applied to form specially designed photoresist strips mask on one x-y face of the sample. The thick positive photoresist (~5 µm thick) acted as the ion etching mask consisted of open stripes with width 7 µm and a spacing of 43 µm between the adjacent stripes. After post baking, Ar ion beam etching was performed on the uncovered regions by 90 min. The energy of the Ar ion beam was about 500 eV. The beam current is about 2 mA/cm2. The sample was titled by 40° off the incident beam direction. Finally, the photoresist mask was removed and a series of ridge waveguides were formed. The propagation loss was measured by Fabry-Perot resonance method [27].
3. Results and discussion
3.1 Planar waveguide
Figure 1(a) shows the intensity spectrum of transverse magnetic (TM) polarized light reflected from prism (intensity versus effective refractive index) on the planar waveguide by proton exchange at 200 °C for 3 hours at 633 nm. Obviously, three sharp dips are formed, acting as three guiding modes, where effective refractive indices are larger than the substrate index (2.2020). Figure 1(b) shows the intensity spectrum (TM) on the waveguide by proton exchange combined with 800 keV O ion implantation at a dose of 1×1015 ions/cm2 after annealing at 633 nm. After ion implantation, one mode is formed and its effective extraordinary refractive index (~2.2379, 2.2105 before annealing) is larger than the substrate index. It is interesting to study the wavelength of 1539 nm due to its extensive application in modern photonic telecommunication systems, although we focus on the 633 nm wavelength in this work. There is also an index-raised TM mode observed at 1539 nm.
Fig. 1 Measured relative intensity of the TM polarized light reflected from the prism versus the effective refractive index of the waveguides by (a) proton exchange at 200 °C for 3 hours at 633 nm, (b) and (c) proton exchange combined with 800 keV O ion implantation at a dose of 1×1015 ions/cm2 after annealing at 633 nm and 1539 nm, respectively.
The arrangement of atoms determines the properties of materials. The RBS/channeling technique is extensively used in the detection of the crystal damage (defect) [28,29]. The RBS/channeling spectra of the proton-exchanged waveguide at 200 °C for 3h after annealing and the implanted waveguide by 800 keV O ion at a dose of 1×1015 ions/cm2 are indicated in Figure. 2 obtained by 2.1 MeV He ion. The virgin and random spectra are also measured from the pure LiNbO3 wafer for comparison. The minimum yield is around 2% for pure LiNbO3. It means that LiNbO3 is a very good quality crystal. V and R represent the virgin and random spectra, respectively. 1 and 2 represent the damage spectra of the annealed proton-exchanged and the ion-implanted waveguides. There is a broad peak in ion implanted LiNbO3 at the channel 232, which can be regarded as the He ion scattering from the disorder profile of Nb atoms in the damage region at the end of the ion track. In the proton-exchanged waveguide, the damaged spectrum is a little different from the virgin spectrum in LiNbO3. It suggests that there are some Nb atom displacements from lattice sites and the atomic arrangement is not perfect crystal structure after proton exchange.
Figure 3 shows the damage depth distribution in LiNbO3 crystal induced by 800 keV O ion implantation with a dose of 1×1015 ions/cm2 at room temperature. Triangles represent the experimental data and the line is the damage profile simulated by SRIM2008 [30]. The damage peak is located at around 0.9 µm. The comparison indicates that the experimental damage distribution is found in good agreement with theoretical predication by SRIM2008, although there are some differences in the positions of the peaks.
Fig. 3 The triangles and line represent the measured damage profile of O ion implanted LiNbO3 with the energy 800 keV at a dose of 1×1015 ions/cm2 and the simulated value by the SRIM2008 in arbitrary units, respectively.
The refractive index profile of the photonic guiding structure is an important parameter for investigating the waveguide properties and its application in integrated optics. The information on refractive index profile of the LiNbO3 waveguide by proton exchange combined with ion implantation is needed. However, RCM and iWKB have some difficulties to accurately reconstruct the refractive index profile due to only single TM mode found in the post-implanted waveguide. In this work, we adopt the ICM to reconstruct the extraordinary refractive index profile. One of the advantages of ICM is that it can simulate the refractive index profile of the single-mode waveguide. We combine a set of refractive indices, then analyze and compare the near field intensity profile measured by the end-fire coupling as calculated by BPM. The refractive index profile of the annealed waveguide was reconstructed by ICM, displayed in Fig. 4 , by the black line. The refractive index profile of proton-exchanged waveguide (the dash dot line) and the substrate index (the dashed line) are also showed for comparison. The so small difference (0.0003) between the measured effective index (2.2379) and the calculated index (2.2382) of TM0 means that they match each other very well and the final index profile is assumed to be the optimum shape for the planar waveguide. The precision and the calculation errors of ICM have been discussed elsewhere [31]. In the modified region by O ion implantation, the refractive index is smaller than that in the proton-exchanged structure but larger than that in ion-implanted pure LiNbO3 crystal. The shape of the index profile in annealed waveguide is the index-raised well plus the low-index barrier type, just as the shape of He ion implantation in proton-exchanged waveguide [20]. The peak position of the barrier is in good agreement with the position of the damage peak calculated from the RBS/channeling spectra. As indicated in figure, the refractive index of the proton exchanged waveguide reduces drastically after ion implantation. Further, the effective index (2.2026) of the waveguide by 800 keV O ion implantation in pure LiNbO3 crystal is slightly larger than the substrate index (the figure is not showed here and the same phenomena of the enhanced index in guiding layer are reported elsewhere [2]). It suggests that the index is increased in guiding region. So, the refractive index changes in the proton exchanged waveguide combined with O ion implantation are not simply the sum of the two refractive index changes and the reason is very complicated. The similar index-reduced phenomena in guiding layer are also referred to in ref [20].
Fig. 4 (color online) Extraordinary refractive index profile for proton-exchanged waveguide combined with ion implantation reconstructed by ICM (the black line) and proton exchanged waveguide (the dash dot line) at 633 nm; the substrate refractive index (the dashed line); the star and the open circle represent the effective refractive indices of the measured and calculated modes, respectively.
In general, the refractive index profile mainly depends on the spontaneous polarization and the molar volume, which can affect the change of refractive index both in proton-exchanged and ion-implanted waveguides for LiNbO3 [22,23]. We suppose that the influence of elasto-optic effect and molar polarization could be ignored, which is reasonable due to the small change of refractive index affected by them. The Δne,P (the change of extraordinary refractive index) induced by the decrease of spontaneous polarization can be written as [22,23]
where ne is the extraordinary refractive index of LiNbO3 crystal (2.2020); g 33=0.09 m4/C2 is the quadratic electro-optic coefficient; Ps=0.71 C/m2 and Ps´ are the spontaneous polarization of LiNbO3 crystal and in the modified region, respectively. In addition, from the Clausius-Mossotti equation, the Δne,V induced by the molar volume swelling can be expressed as [22,23]where n, VM, VM´ are the average refractive index (no2ne)1/3, the molar volume in LiNbO3 and in the modified region, respectively. It is noted that the positive change of the refractive index can be induced by the decrease of spontaneous polarization as Eq. (1) and the decrease of index can be affected by the swelling of molar volume as Eq. (2). In proton-exchanged structure, the spontaneous polarization has sharply reduced and the molar volume has slightly swelled, according to the data in the reference [23], therefore there is a large positive change of the extraordinary refractive index. In present case, during O ion implantation, the change of the spontaneous polarization is very small due to much large drop in the proton-exchanged structure. Therefore, the positive Δne,P component is zero or near zero for implanted proton-exchanged LiNbO3 waveguide. It seems reasonable that the molar volume swelling dominates the change of the refractive index and so, the index can decrease after implantation, just as in the figure, relative to the proton-exchanged waveguide. The extraordinary index change is mainly dependent on the negative Δne,V component. The similar explanation of this phenomena was also found in He ion implantation in proton-exchanged LiNbO3 [21]. However, the changes of spontaneous polarization and molar volume are both caused by the damage during O ion implantation in proton-exchanged LiNbO3. The energetic ions implantation can generate the damage through electronic energy deposition and nuclear energy deposition [2, 6]. In Fig. 2 , we can see that LiNbO3 is not perfect crystal structure after proton exchange and the crystal lattices have been disordered a certain extent. So, when the energetic ion implantation can cause intensive damage in proton-exchanged LiNbO3 more easily than in pure crystal in the region of electronic energy deposition. Meanwhile, there is also large damage at the end of the ion track due to the nuclear energy deposition. Then, the damage leads to the molar volume swelling which reduces the refractive index. In other word, we could also believe that the changes of refractive index are caused by the damage [32].Fig. 2 (color online) RBS/channeling spectra for ion-exchanged LiNbO3 waveguide after annealing (1) and implanted LiNbO3 waveguide by 800 keV O ion at a dose of 1 × 1015 ions/cm2 (2); V and R represent the virgin and random spectra of LiNbO3 crystal for comparison, respectively.
In this work, all the photonic guiding structures, modified by O ion implantation ranging from 600 to 1400 keV in proton-exchanged LiNbO3, support the single mode measured by prism coupling at 633 nm. As indicated in Fig. 5 , the effective refractive indices of TM0 decrease gradually with the increase of implanted energy for proton-exchanged waveguides modified by the ion implantation at the same dose. The indices become larger after annealing at 200 °C for 30 min. It is possible that, in medium-mass ion-implanted waveguide, the spontaneous polarization decrease dominates in the near-surface region. Therefore, there are index-raised modes in ion-implanted LiNbO3. However, in the proton-exchanged waveguide modified by ion implantation, the indices reduce because of the molar volume swelling induced by the damage, relative to the proton-exchanged waveguide. After annealing, the indices become larger due to the recovery of the lattices in the near-surface region to a certain extent.
Fig. 5 (color online) The squares and the circles represent effective refractive indices of the modes in waveguide by proton exchange combined with ion implantation before and after annealing, respectively.
3.2 Ridge waveguide
The ridge waveguide by proton exchange combined with 800 keV O ion implantation at a dose of 1×1015 ions/cm2 has been fabricated by Ar ion beam etching. The etching depth of the ridge waveguide is about 0.75 µm. In Fig. 6 , the top view (a) and the transverse cross section image (b) of the ridge waveguide are shown by a microscope with reflected polarized light (Olympus BX51M, Japan). The position of ridge waveguide is marked in the figure. We can see that there is acceptable quality for the ridge waveguides.
Fig. 6 (color online) Microscope images of the ridge waveguide by Ar ion beam etching: (a) the cross section image of the ridge waveguide and (b) the top view of the ridge waveguide surface.
For practical applications, the ridge waveguide is useful. It is necessary to investigate the propagation properties of the ridge waveguide configuration formed on the basis of the planar waveguide by ion implantation after annealing. To investigate the propagation property, the end-fire coupling was performed with He-Ne laser at wavelength 633 nm. The measured 3D end-fire intensity distributions of TM00 and TM10 are depicted in Fig. 7 (a) and (b) . It can be seen that the light is guided well in the index-raised well. In the figure, two transverse propagation modes can be supported in the ridge waveguides because of the large index difference and the broad width of each ridge waveguide (7 µm). The light propagation in ridge waveguide is simulated by means of the finite-difference beam propagation method (FD-BPM) using a commercial software BeamPROP [33]. Figure 7 (c) and (d) are the simulated modal distributions of TM00 and TM10 modes with the refractive index profile in Fig. 4. By comparing Fig. 7 (a), (b) and (c), (d), we can see that the measured modes are reasonable agreement with the numerical modes in the ridge waveguide.
Fig. 7 (color online) Measured near-field intensity distributions of TM00 (a) and TM10 (b) from the ridge waveguide; the distributions of simulated modes TM00 (c) and TM10 (d).
The propagation loss is measured about 2.2 dB/cm at wavelength 633 nm by Fabry-Perot resonance method. The large propagation loss attributes to two aspects. Firstly, many defects are induced in the guiding structure during ion implantation. The defects can cause intensive absorption and scattering loss. As far as we know, the moderate annealing can remove them. However, to avoid proton fast diffusing at high temperature, the low annealing temperature is adopted in this work and the defects cannot be removed entirely. Secondly, the rough sidewalls of ridge waveguides are induced by Ar ion beam etching because of the physical process for Ar ion dry etching and the redeposition effect.
3. Summary
In summary, the single-mode planar waveguides in LiNbO3 have been fabricated by proton exchange combined with O ion implantation at the energy ranging from 600 to 1400 keV with the dose of 1 × 1015 ions/cm2. It is suggested that the modes in proton exchanged waveguide can be modulated by O ion implantation. Rutherford backscattering/channeling measurement has been carried out in 800 keV O ion implanted and proton exchanged waveguides in order to observe the atomic displacement from lattice site. The results show that there are different damage profiles. The refractive index profile in single-mode waveguide in lithium niobate with index-raised well and low-index barrier type has been obtained based on Intensity Calculation Method. The decrease of the refractive index after implantation is governed by the molar volume swelling when the spontaneous polarization sharply drops in proton exchanged structure. The molar volume swelling is induced by the damage during ion implantation in proton-exchanged waveguide. Also we have fabricated the ridge waveguides by Ar ion beam etching. We have displayed the near-field intensity profiles of TM00 and TM10 modes of the ridge waveguide which are reasonable consistent to the calculated ones. The propagation loss is ~2.2 dB/cm. The results demonstrate that the planar and ridge waveguides with acceptable quality can be obtained by proton exchange combined with O ion implantation.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (Grant No. 10735070), 973 program (Grant No. 2010CB832906), and the State Key Laboratory of Nuclear Physics and Technology, Peking University, China. We desire to express our thanks to professors F. Chen and X.-L. Wang for their discussions and suggestions on this manuscript. We also wish to thank L. Wang, H.- J. Ma and M. Chen for their help.
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