1. Introduction

Lithium niobate (LiNbO₃) is one of the most attractive materials due to the intriguing combination of excellent electro-optic, nonlinear-optic characteristics [1]. Waveguide in LiNbO₃ is widely used in the telecommunication and optical system. Ion implantation is a very consolidated technique for waveguide fabrication. This technique has been widely used in manufacture about large scale integrated circuit. Ion implantation as the physical methods can modify many kinds of materials, including semiconductor, single crystal, glass, polycrystalline ceramic and organic material [2–5]. LiNbO₃ crystal lattice is a network of oxygen octahedron (BO₆) and most of the properties depend on the presence of BO₆ octahedron building blocks [6]. Ion implantation can induce damage and defects in the surface layer [7]. The damage in near-surface region is induced by electronic excitation. However, the damage at the end of the ion track is induced by the nuclear energy deposition [8]. Proton exchange is a famous method to form waveguide in LiNbO₃ by the replacements between lithium ions and protons when the LiNbO₃ crystal is immersed in molten acid. The high concentration of protons in the LiNbO₃ leads to a large increase of extraordinary refractive index (n_e) in the near-surface region.

In previous papers, some researchers have fabricated optical waveguides by the ion implantation in the proton-exchanged materials. Buried channel waveguide has been formed by He ion implantation in Rb-exchanged KTiOPO₄ [9]. The changes of refractive index have been studied in proton-exchanged LiNbO₃ waveguide followed by He ion implantation [10]. In our previous works, we also have studied the planar and ridge waveguides formed by O ion implantation in proton-exchanged LiNbO₃, the effects induced by swift argon-ion irradiation in proton-exchanged LiNbO₃ [11,12]. However, little information concerns the influence on proton exchange process by ion implantation in LiNbO₃. So, the first purpose of our work is to study this influence.

Proton exchange method has been widely used in many devices, and ion-implanted technique can control the experiment conditions accurately to make the refined structure. So, it is a meaningful work to combine the proton-exchanged devices with the ion-implanted ones. This study can realize two kinds of devices well matched in the application, which is the second purpose of our work.

2. Experimental details

x-cut LiNbO₃ wafers were implanted by low-energy (200 keV) He ions with a fluence of 3 × 10¹⁶ ions/cm² at liquid nitrogen temperature. In order to avoid the channelling effect, all wafers were titled by 7° off the incident beam direction. This process was carried out by the 500 kV implanter of the Institute of Semiconductors, Chinese Academy of Science. The implanted beam current was maintained below 1 μA/cm². Subsequently, some implanted samples were immersed in the molten benzoic acid for proton exchange at different conditions (listed in Table 1) when the proton source was heated to 230 °C in a sealed furnace. After proton exchange, the samples were pulled out from benzoic acid and hanged in furnace. For comparison, proton exchange was carried out in pure LiNbO₃ at the same conditions (listed in Table 1).

Table 1. Preparation conditions, measured effective refractive indices (N_eff) at 633 nm and 1539 nm for the seven samples

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In order to analyse the influence on proton exchange by degree and depth of the implanted damage in LiNbO₃ wafers with the different treatment processes, Rutherford backscattering/channelling (RBS/C) measurement was carried out by 2.1 MeV He ions at 1.7 MV tandem accelerator of Shandong University. The detection current was maintained at about 7-8 nA/cm². The dark mode spectra of the samples were measured by prism-coupling technique at wavelength 633 nm and 1539 nm, respectively. The near-field intensity profile was detected by the end-face coupling system at 633 nm.

3. Results and discussion

The properties of materials are determined by Atoms arrangement. RBS/C technique is an important method to detect the crystal damage or defects. Figure 1(a) shows the RBS/C spectra of He-ion-implanted sample at liquid nitrogen temperature and samples formed by proton exchange combined with ion implantation. The virgin and random spectra of pure LiNbO₃ crystal are also showed in figure for comparison, respectively. More defects can be retained at low-implanted temperature than that at room temperature. So, in Fig. 1(a), there are broad peaks around the channel 270 both in He-ion-implanted sample and the ion-implanted combined with proton-exchanged ones. The peaks can be regarded as He ion scattering yield from the disorder profile of Nb atoms at the end of 200 keV He ion track. It can be found that there are heavily damage and a lot of Nb atom displacements from lattice sites in S0, because the implantation temperature is very low and many defects are retained. It suggests that the sample with this atomic arrangement experienced the formation of a damage after He ion implantation at liquid nitrogen temperature. Figure 1(b) shows the RBS/C spectra of proton-exchanged LiNbO₃ for 6 min (P1) and 10 min (P2) in pure LiNbO₃, respectively. There are many differences between the He-ion-implanted spectra in Fig. 1(a) and those of the proton-exchanged ones in LiNbO₃ in Fig. 1(b).

 

Fig. 1 (a) RBS/C spectra of samples S0, S1, S2, and S3. (b) RBS/C spectra of samples P1 and P2.

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The damage depth distributions were deduced from the RBS/C spectra in Fig. 2(a), and 2(b) [13]. Figure 2(c) shows the profile of nuclear energy deposition of 200 keV He ion in LiNbO₃ simulated by SRIM2008, which is accordance with the experiment data of sample S0 in Fig. 2(a) [14]. From the data contrast, nuclear energy deposition is the important factor for the damage formed in the ion-implanted process. For 200 keV He ion implanted in LiNbO₃, the electron energy is so small that there is no obvious contribution to damage. We make multiple comparisons among the samples with different treatments, as follows. We can see that the damage shapes of samples S1 and S2 are similar with that of the ion-implanted LiNbO₃ (S0) in the first half of the profiles in Fig. 2(a). It means that there are not larger changes of the Nb atoms displacements after proton exchange than before in the ion-implanted region. However, the damage is formed by proton exchange from 0.8 to 1.0 μm in the damage depth profile of the sample S2. From damage contrast, there are different shapes of damage between S1 with P1, S2 with P2 due to the influence of ion implantation. The proton exchanged depth of sample S2 in Fig. 2(a) is deeper than the depth of sample P2 in Fig. 2(b) about 0.3 μm at the same exchanged condition. These results indicate that the proton-exchanged rate is faster in the ion-implanted region than that in the pure LiNbO₃. After He ion implantation in LiNbO₃ at liquid nitrogen temperature, the LiNbO₃ crystal is not perfect crystal and there are many defects and heavy damage showed for S0 from Fig. 2(a). It suggests that the defects and damage induced by He ion implantation can enhance the proton-exchanged rate and contribute to the proton exchange process.

 

Fig. 2 (a) Damage depth profiles of S0, S1, and S2 samples deduced from RBS/C spectra. (b) Damage depth profiles of proton-exchanged LiNbO₃ for samples P1 and P2. (c) Nuclear energy deposition profile of 200 keV He ion in LiNbO₃ simulated by SRIM2008.

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There is no guiding mode at 633 nm in LiNbO₃ after 200 keV He ion implantation at liquid nitrogen temperature due to the small implanted depth. However, in Table 1, we can see that there are guiding modes after proton exchange at 633 nm and 1539 nm in the ion-implanted region, even if the exchanged duration is only 6 min. The reason for forming waveguides in LiNbO₃ by proton exchange combined with ion implantation is important to investigate the properties of waveguide and its applications in integrated optics. Compared the damage depth profile of the sample S0 with that of S1, we can see that the profile shapes are similar with each other and the lattice disorder of S1 is only a bit heavier than that of S0. However, at the same depth, there are guiding modes formed in sample S1. To sample S0, the zero order mode is cut-off. In these waveguides, effective refractive indices of guiding modes depend on height and extraordinary refractive index of the guiding layer [8]. When height and refractive index are raised in guiding layer, effective refractive index of the same order mode can increase. So, it can be regarded that n_e of the guiding layer has been raised in sample S1. In general, the change of refractive index mainly depends on the spontaneous polarization and the molar volume, which can affect the changes both in proton-exchanged and ion-implanted waveguides for LiNbO₃ [15–17]. In this paper, we suppose that the influence of elasto-optic effect and molar polarization could be ignored, due to the small influence on the changes of the refractive index by them. The decrease of spontaneous polarization can induce the positive change of n_e. However, molar volume swelling can pull down the n_e [15–17]. In this case, due to the domination of n_e change by a large molar volume expansion after ion implantation, there is no waveguide formed in sample S0. This phenomenon appears in another study [2] and is also stated in literatures [8,15,18]. During proton exchange, the spontaneous polarization in ion-implanted sample is sharply reduced. It means that there is a large positive change of n_e. At the same time, the molar volume is not swelling again or small change by proton-exchange process in sample S1, which is accordance with the damage profiles in Fig. 2(a). Taken together, positive change is larger than negative change of n_e. So, the waveguides can be structured in sample S1, even at the same depth in sample S0 and S1.

In proton-exchanged LiNbO₃, the extraordinary refractive index increases while the ordinary refractive index (n_o) decreases [19,20]. The ordinary polarization is not guided in these samples.

The effective refractive indices of the guiding modes measured are listed in Table 1 at different conditions, respectively. Compared the only proton-exchanged LiNbO₃ with the samples formed by two processes, the later are much easier to form waveguide and to produce more guiding modes in the ion-implanted region than the former. There are two reasons for explaining these phenomena. Firstly, the disorder of lattice and the damage of ionic bond by low-temperature ion implantation can speed up proton exchange. Secondly, the refractive index of n_e is sharply increased due to proton exchange in ion-implanted region. It indicates that ion implantation can help the formation of waveguide easily by following proton exchange. Taking proton exchange for 10 min for example, the intensity spectra of transverse electric (TE) polarized light reflected from prism (intensity versus effective refractive index) are showed in Fig. 3. Figure 3(a) shows the dark mode spectrum of sample P2 at 633 nm, but no guiding mode formed at 1539 nm. Figures 3(b) and 3(c) show the dark mode spectra of the sample S2 at 633 nm and 1539 nm, respectively. The refractive indices of LiNbO₃ substrate (n_e) are 2.2020 at 633 nm and 2.1370 at 1539 nm, respectively.

 

Fig. 3 Relative intensity of light reflected from the prism versus the effective refractive index for samples P2 at 633 nm (a), for sample S2 at 633 nm (b), and for sample S2 at 1539 nm (c).

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The function of optical waveguide is to confine the light propagation in a small space and to enhance the light intensity density. So, the near-field intensity profile for fundamental mode (TE₀) in planar waveguide S2 was collected by the end-face coupling arrangement at 633 nm, showed in Fig. 4. The near-field intensity profile is homogeneous and there is little light leaking into the substrate, which implies that a good planar waveguide is formed. The propagation loss is about 2.0 dB/cm at the 633 nm, measured by the back-reflection method in the same end-face coupling arrangement [21].

 

Fig. 4 The near-field intensity distribution of the waveguide’s TE mode at a wavelength of 633 nm measured at the output facet of sample S2. The chromatic scale represents the relative light intensity.

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4. Summary

The damage and defects induced by He ion implantation at liquid nitrogen temperature can enhance the proton-exchanged rate and contribute to the proton exchange process in LiNbO₃. The planar waveguides are formed by proton exchange in the ion-implanted samples and there are guiding modes at 633 nm and 1539 nm, respectively, even if the proton exchange in a very short time. The near-field intensity profile of the fundamental mode in planar waveguide formed by proton exchange for 10 min combined with ion implantation is homogenous and the light is well confined in the waveguide. The propagation loss of such waveguide is about 2.0 dB/cm at 633 nm. It indicates that a good planar waveguide is formed by combining two methods in LiNbO₃. Also, waveguide can be formed by ion implantation in the proton-exchanged LiNbO₃, which has been done in another work [11,12]. We think ion-implanted structure can join with ion-exchanged one well in LiNbO₃.