The array waveguides formed in LiNbO3 crystal by oxygen-ion implantation

Nuclear Instruments and Methods in Physics Research B 268 (2010) 2923–2925

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The array waveguides formed in LiNbO3 crystal by oxygen-ion implantation Jin-Hua Zhao a, Xiu-Hong Liu a, Qing Huang a, Peng Liu a, Lei Wang a,b, Xue-Lin Wang a,b,* a b

School of Physics, Shandong University, Jinan 250100, PR China State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China

a r t i c l e

i n f o

Article history: Received 23 September 2009 Received in revised form 30 March 2010 Available online 7 May 2010 Keywords: Array channel waveguide Ion implantation LiNbO3

a b s t r a c t We have proposed and discussed an array channel waveguide with a period of 11 lm, which was fabricated using direct multiple-energy oxygen-ion implantation at a total dose of 6  1014 ions/cm2 in z-cut LiNbO3 crystal, for the first time, to our knowledge. The extraordinary refractive index profile experiences positive changes with respect to the bulk according to the dark mode spectra measurement results. The coupling effect of light propagating in the array was observed, and the numerically calculated modal distributions were in good agreement with the experimental results. This design may be useful for the formation of discrete spatial solitons. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Lithium niobate is a versatile material for its unique electro-optic, acousto-optic and nonlinear optic properties [1]. LiNbO3 (LN) crystal waveguides are widely used in a variety of integrated optics devices, including switches, amplifiers, modulators and communications [2–4]. Holographic storage is another important application of LiNbO3 crystal because of its excellent photorefractive properties [5]. The channel waveguide is the basic structure for many optoelectronics devices because the light is confined in two directions and propagation is allowed in only one direction. The channel waveguide array, which can be fabricated by modulating the distance between the adjacent channels, has potential application in the generation of optical solitons [6–8], so it has attracted much interest in recent years. Nonlinear optics has become an incredibly rich field because nonlinear effects are readily accessible with the powers available from lasers and because the samples and the conditions for experiments can be controlled precisely [9]. Nonlinear effects in waveguide arrays have been studied intensively in the past several years, both theoretically and experimentally [8,10]. The study of optical solitary phenomena has been a constantly growing field for almost 30 years, ever since Zakharov and Shabat formulated the basic theory for instantaneous Kerr solitons [11]. In this work, we fabricated an array of channel waveguides in LiNbO3 crystal with a period of 11 lm and measured its optical properties using the end-face coupling method. The period of the channel wave-

* Corresponding author at: School of Physics, Shandong University, Jinan 250100, PR China. Tel.: +86 531 88364655. E-mail address: [email protected] (X.-L. Wang). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.05.009

guide is short enough to obtain light coupling between channels, and it may be useful for the formation of discrete spatial solitons. Ion implantation is one of the most important techniques for modifying surface properties, and it has been widely used in many materials because it offers accurate control of both dopant composition and penetration depth through use of a particular species, as well as the energy of the ions [12,13]. In this paper, we select oxygen as the implantation ion to form a refractive index enhanced type waveguide. Standard lithography is the conventional method used for formatting the photoresist mask because this technique is relatively simple and mature. 2. Theory It is well known that SRIM [14] (stopping and range of ions in matter) is a very powerful instrument for calculating the process of ion implantation. In this paper the relative damage profile induced by the O ion implantation process were calculated by SRIM 2006. One of the fundamental aspects of integrated optics is the analysis and simulation of electromagnetic wave propagation in photonics devices based on waveguide geometries, including optical waveguides. The beam propagation method (BPM) is widely used for the light propagation properties of waveguide. The problem can be solved as follows: given an arbitrary distribution of refractive index n(x, y, z), BPM can give the spatial distribution of light at an arbitrary position in the propagation direction. BPM based on the finite difference method (FD-BPM) [15,16] is used in this paper in order to solve optical propagation in 2D waveguides (channel waveguides). This procedure gives very good results in general [17]. An additional advantage of FD-BPM, is the possibility of incorporating wide-angle propagation and full vector algorithms [18].

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For investigation of light propagation in periodic optical media, such as waveguide arrays, a Floquet–Bloch analysis has often been used to characterize the modulated light behaviors because it offers better generality [6]. The propagation constant (b) spectrum of the array’s eigenmodes is divided into allowed bands, which are separated by forbidden band gaps. The linear modes of such structures are therefore extended Floquet–Bloch (FB) modes. In this work we launched a Gaussian beam at the wavelength of 633 nm to excite FB bands, and we obtained the simulation result by use of FD-BPM. 3. Experimental details The LiNbO3 crystal samples we used were z-cut, which is optical polished by precision polishing equipment with a size of 8(x)  6(y)  1.5(z) mm3. A thick positive photoresist mask (BP218) was spin-coated at 4000 rpm on the LiNbO3 crystal for 15 s to ensure uniform. We obtained a photoresist mask of open strips 6 lm in width with a spacing of 5 lm between the adjacent channels, and after a standard lithography process, we had a series of channel masks with a period of 11 lm. After post-baking for 30 min at 120 °C the samples were then implanted with oxygen ions on the polished face with multiple energies and low dose (2.2 + 1.8 + 1.6 MeV with a dose of (3 + 2 + 1)  1014 ions/cm2) to obtain the channel waveguide. The planar waveguide was fabricated by the same implantation conditions (without the lithography progress) for comparison. The implantation was performed in vacuum at room temperature using 2  1.7 MV tandem accelerators at Peking University. The ion beams were scanned to ensure a uniform implantation over the samples at a standard current density of 30 nA/cm2 to avoid the obvious heating of the sample. The samples were annealed in a dry oxygen atmosphere at 260 °C for 30 min to recover the color centers and decrease lattice damage. In the planar waveguide a prism coupler (Metricon 2010) was used to measure the dark mode spectra at 633 nm by m-line arrangement. The end face of the channel waveguide was optically polished, and a microscope was used to observe the end face of the channel waveguide. We know that coupling effects exist in array channel waveguides (see, for example, Ref. [19]), so we performed the end-face coupling measurement. The sample was mounted on a 6D optical stage to make it not only movable along the x, y, or z axes but also rotatable within the x–y, x–z, or y–z planes. A neutral density filter and a half-wave plate were used for controlling the optical power and light polarization. A cylindrical lens was used to change the light beam width and shape. A He–Ne laser with a wavelength of 633 nm was used as the light source. To investigate

the band-spectra of the channel waveguide array, we launched a Gaussian beam to excite the FB waves of the array by suitably tilting the incident angle of the light [19]. A 40 microscope lens injected the light into the channel waveguide, and a 20 microscope lens collected it from the other facet of the crystal. The crystal’s output facet was imaged onto a charge-coupled device (CCD). 4. Results and discussion In the planar case TM mode patterns were measured both before and after annealing at the reflected light at k = 633 nm. When the mode was excited with transverse magnetic (TM) polarization, a narrow and deep dip could be observed for both as-implanted and annealed samples. In the as-implanted case, the effective index of dark mode was 2.2099, and in the annealed case, it was 2.2071, consistent with a higher effective index than the substrate extraordinary refractive index (ne), which implied an enhanced index well into the near surface region for waveguide formation. It is worth noting that the enhanced, well-confined waveguide structure was constructed in LN crystal for ne by oxygen-ion implantation. After annealing at 260 °C for 30 min, the dip was sharper and narrower, demonstrating that annealing can recover the color centers and re-

Fig. 2. (a) Reconstructed refractive index profile of the array channel waveguides used in BPM. (b) A comparison of the refractive index profile of the 2D refractive index distribution of one channel waveguide and the planar waveguide at the regimes of its cross-section.

Fig. 1. (a) The cross-section of microscope photograph (500) of channel waveguide. (b) The magnification of (a) (1000).

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Fig. 3. (a) Near field image of TM modes from the end face of the sample captured by a CCD camera. (b) Modal intensity profile calculated by FD-BPM.

duce damage to decrease the propagation loss. Fig. 1 shows the cross-section (polished end face) of the channel waveguide as observed under a microscope that could provide reflected polarized light (Olympus BX51M, Japan). This was also important for determination of the 2D refractive index profile of the channel waveguides. The change of color in this figure represents the change in refractive index. The trapezoidal-shaped structures surrounded by closed barrier walls were constructed for the waveguide array structure. We can see that the change of color is periodic. This indicates that we successfully obtained an array of channel waveguides by photographic masking following direct multiple-energy oxygen-ion implantation. Further modal analysis shows that there were no real guide modes before annealing for either transverse electric (TE) or TM; a real guide mode did exist for TM after the anneal. The extraordinary refractive index profile was calculated by the reflectivity calculation method (RCM) [20], which has been proven to be particularly successful for ion implanted waveguides. SRIM was used to simulate the damage distribution. The implantation created a 0.8-lm-wide damage peak, and the position of maximum damage was about 1.45 lm. The maximum value of Dne was about 0.01. The enhanced well plus optical barrier creates the waveguide with an in-depth thickness of 1.5 lm. This value is in agreement with the calculated damage peak position of the multi-energy O ions based on the SRIM 2006 simulation. We performed the simulation using BPM. The effective refractive index of the guide mode was 2.2071; this result matches the experiment results (2.2071). Fig. 2 shows the 2D extraordinary refractive index profile (after annealing), which was used in the FD-BPM calculation of the channel waveguide. As one can see, we selected five channels [Fig. 2(a)] as the calculation domain. In Fig. 2(b), we give the 1D (for planar waveguide, obtained from the RCM reconstruction) and 2D refractive index profile (reconstructed by the refractive index profile of planar waveguide combined with the trapezoidal-shape of the cross-section shown in Fig. 1) of the single channel. Fig. 3(a) shows the intensity distribution of the band-spectra for the waveguide array, measured by the end-face coupling method. This proves that the coupling effect is apparent in an array channel waveguide with a period of 11 lm. It also proves that the array channel waveguide can be made with lithography techniques and ion implantation. Nonlinear effects in waveguide arrays have been studied intensively in the past several years, both theoretically and experimentally. In particular, numerical studies of vector solitons in waveguide arrays have been recently reported. A more general approach to waveguide arrays is the FB analysis. It predicts that the propagation-constant spectrum of the array’s eigenmodes (the FB waves) is divided into bands [21,22]. We can see that this array can carry three FB bands. The FD-BPM simulation of the light in the channel waveguide is based on the reconstructed 2D refractive index profile in the cross-section (shown in Fig. 2). Fig. 3(b)

shows the simulation results of the channel waveguide array with a launch field of a Gaussian beam at the location of X = 0, Z = 1 lm, with a wavelength of 633 nm and a period of 11 lm, with 5 lm between every waveguide region. Compared with the experiment results indicated in Fig. 3, we can conclude that there is reasonable agreement of the mode profile calculation with the experiment results. 5. Summary A 1D channel waveguide array was fabricated in z-cut LN crystal by triple O ion implantation and lithography processes. The results show that a well-confined waveguide array can be fabricated. The band-spectra of the light propagating in the array were observed by the end-face coupling method and the results match the simulation results from FD-BPM. The results also imply that this design may be useful for discrete soliton-managed devices. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. 10975094 and 10735070), National Basic Research Program of China (Grant No. 2010CB832906), NCET and FANEDD of China. References [1] R.S. Weis, T.K. Gaylord, Appl. Phys. A 37 (1985) 191. [2] R.V. Schmidt, I.R. Kaminow, Appl. Phys. Lett. 25 (1974) 458. [3] F. Wehrmann, C. Harizi, H. Hermann, U. Rust, W. Sohler, S. Westenhofer, IEEE J. Sel. Top. Quantum 2 (1996) 263. [4] I. Nee, O. Beyer, M. Müller, K. Buse, J. Opt. Soc. Am. B 20 (2003) 1593. [5] Y. Nie, R. Wang, B. Wang, Mater. Chem. Phys. 102 (2007) 281. [6] D. Mandelik, H.S. Eisenberg, Y. Silberberg, R. Morandotti, J.S. Aitchison, Phys. Rev. Lett. 90 (2003) 253902. [7] D. Mandelik, H.S. Eisenberg, Y. Silberberg, R. Morandotti, J.S. Aitchison, Phys. Rev. Lett. 90 (2003) 053902. [8] H.S. Eisenberg, Y. Silberberg, R. Morandotti, A.R. Boyd, J.S. Aitchison, Phys. Rev. Lett. 81 (1998) 3383. [9] R.W. Boyd, Nonlinear Optics, Academic Press, Boston, 1992. [10] D.N. Christodoulides, R.I. Joseph, Opt. Lett. 13 (1988) 794. [11] V.E. Zakharov, A.B. Shabat, Zh. Eksp. Teor. Fiz. 61 (1971) 118. [12] P.D. Townsend, P.J. Chandler, L. Zang, Optical Effects of Ion Implantation, Cambridge University Press, 1994. [13] I. Banyasz, M. Fried, Cs. Ducso, Z. Vertesy, Appl. Phys. Lett. 79 (2001) 3755. [14] J.F. Ziegler, Computer Code SRIM, . [15] F. Caccavale, F. Segato, I. Mansour, M. Gianesin, J. Lightwave Technol. 16 (1998) 1348. [16] G.L. Yip, P.C. Noutsios, L. Chen, Appl. Opt. 35 (1996) 2060. [17] W. Huang, C. Xu, S. Chu, K. Chaudhuri, J. Lightwave Technol. 10 (1992) 295. [18] G.R. Hadley, Opt. Lett. 17 (1992) 1743. [19] F. Chen, L. Wang, X.L. Wang, K.M. Wang, Q.M. Lu, Appl. Phys. Lett. 89 (2006) 191102. [20] P.J. Chandler, F.L. Lama, Opt. Acta 33 (1986) 127. [21] P. Yeh, A. Yariv, C.S. Hong, J. Opt. Soc. Am. 67 (1977) 423. [22] P.St.J. Russell, Appl. Phys. B 39 (1986) 231.