We report on the optical properties of ZnWO4 planar waveguides created by ion implantation, and the effect annealing has on these structures. Planar optical waveguides in ZnWO4 crystals are fabricated by 5.0 MeV carbon ion implantation with a fluence of 1 × 1015 ions/cm2 or 500 keV helium ion implantation with the a fluence of 1 × 1016 ions/cm2. The thermal stability was investigated by 60 minute annealing cycles at different temperatures ranging from 260°C to 550°C in air. The guided modes were measured by a model 2010 prism coupler at wavelengths of 633 nm and 1539 nm. The reflectivity calculation method (RCM) was applied to simulate the refractive index profile in these waveguides. The near-field light intensity profiles were measured using the end-face coupling method. The absorption spectra show that the implantation processes have almost no influence on the visible band absorption.
©2010 Optical Society of America
ZnWO4 crystal is a kind of biaxial crystal that has a monoclinic wolframite structure and is a member of the P2/c space group with C2h point group symmetry . ZnWO4 is part of the families of metal tungstates that have a high potential for application in various fields such as scintillators [2, 3], laser hosts , acoustics , and photocatalysts . It is attractive to researchers [7–9] because of its unique combination of physical and chemical properties, including molecular and electronic versatility, reactivity, and stability. The luminescence properties of ZnWO4 were first reported by Kroger . Some investigations concerning the material’s light yield, emission spectrum and afterglow have also been made [2, 4]. The results indicate that ZnWO4 is a possible scintillation material for γ-spectroscopy. It is a promising material for x-ray scintillators because its luminescence output and afterglow are comparable to or better than those materials currently in use [2, 3]. In addition, zinc tungstate has the advantage of not being hygroscopic, and it is much cheaper than materials that are currently widely used, such as Bi4Ge3O12 (BGO).
In the light of these results, ZnWO4 has potential value as a laser waveguide. There are several techniques for fabricating waveguides in optical materials, which mainly include diffusion, ion exchange, sol-gel, ion implantation and deposition of epitaxial layers . Among these techniques, ion implantation possesses one of the most advantageous characteristics, that is, the wide applicability of materials . Moreover it allows accurate control of both dopant composition and penetration depth through the choice of the species and the energy of the ions [12–14]. Light ions, such as H and He ions, have been extensively used to form waveguides on crystals [14–16]; a fluence of the order of 1016~1017 ions/cm2 was used to form the waveguides. Another way to fabricate an optical waveguide is to use MeV low-dose medium-mass ion implantation [17–19] in crystals with C, O, or Si ions; in this form, the required dose can be as low as 1013~1015 ions/cm2. Planar and channel waveguides have been realized by using implantation of various medium-mass ions. In this work, we fabricated planar waveguides on ZnWO4 by using He (a light ion) and C (a medium-mass ion) ion implantation. Positive changes in the refractive index, nα, occurred in the guide region. We investigate the optical and annealing properties of the waveguides and analysed the causes of the enhanced refractive index.
2. Experiment and details
The samples of x-cut ZnWO4 crystals were grown by the Czochralski method, and they were obtained from the State Key Laboratory of Crystal Materials, Shandong University. The x-cut ZnWO4 samples, with a size of 10 × 8 × 2 mm3, were implanted with 500 keV helium ions at a dose of 1×1016 ions/cm2 using the implanter at the Institute of Semiconductors, Chinese Academy of Science or 5 MeV carbon ions at a dose of 1×1016 ions/cm2 using a tandem accelerator at Peking University. The implantation processes were performed at room temperature and at an angle tilted by 7° from the direction of the incident ion beam to avoid channelling effects.
After the implantation process, the samples containing implanted C ions or He ions were annealed in a series of 60 minutes heat treatments at temperatures ranging from 260° to 550° in an air atmosphere to remove the colour centres and to partially recover from crystal lattice damage produced by the implantation. The specific annealing conditions are shown in Table 1 . The optical properties of the planar waveguides created with implanted C or He ions and the effects of annealing the structure were investigated. We were only concerned with the TM guided mode (nα) for convenience and to avoid confusion. The neff of the guided mode was measured by the prism-coupling technique at wavelengths of 633 nm (He-Ne laser) and 1539 nm (diode laser) with a resolution better than 0.0002. The end-face coupling method was used to investigate the near-field light intensity profile. The end faces of the sample were polished to achieve the requirements for direct end-fire coupling of the light. A linearly polarized He-Ne laser with a wavelength of 633 nm was used as the light source. SRIM 2006  is used to calculate the electronic and nuclear energy losses. The refractive index profile (RIP) of the planar waveguide is reconstructed by the reflectivity calculation method (RCM) . The finite difference beam propagation method (FD-BPM)  is used to investigate the guided mode of the planar waveguide for comparison to the experimental results. The absorption spectra of the waveguide after different annealing treatments were measured by a Jasco U570 spectrophotometer. The sequence of events in a spectrophotometer is as follows. Firstly, the light source shines through a monochromator; secondly, an output wavelength is selected and beamed at the sample surface; finally, a fraction of the monochromatic light is transmitted through the x axis direction of the sample and to the photodetector.
3. Results and discussion
Figures 1(a) and 1(b) show the measured relative intensity of the TM polarized light at a wavelength of 633 nm reflected from the prism formed by the ZnWO4 planar waveguides after the S3 annealing treatment and implantation by He or C ions, respectively. The substrate refractive index (nα) of the ZnWO4 crystal is also indicated in Fig. 1 for comparison. During the prism-coupling measurement, a lack of reflected light will result in a dip of the dark mode spectrum when the laser beam is coupled into the waveguide region. We can see that two guided modes are observed at a wavelength of 633 nm in the helium implanted waveguide, while only one mode is observed for the waveguide formed by carbon ion implantation. Obviously, we see from Fig. 1(a) that a single resonant dip has been detected in the TM (nα) dark mode spectra. This means that a waveguide structure containing a single mode may be formed in this sample. The effective refractive index is higher than nα of the virgin crystal (nα=2.1651). Thus, this mode could be well-confined in the waveguide region without the tunnelling effect. On the other hand, there are three resonant dips in Fig. 1(b). The two left most dips in Fig. 1(b) are very sharp, and their effective refractive indices are both higher than the nα of the virgin crystal. Thus, there are two modes that could be well-confined in the waveguide region.
In order to better understand the formation of the present waveguide by implantation of 500 keV He ions or 5 MeV C ions, we performed continuous annealing treatments in an air atmosphere (Table 1). We performed prism-coupling measurements of both the He and C implanted waveguides after each annealing step at a wavelength of 633 nm. The measured neff of the TM guided modes for He and C implanted waveguides at a wavelength of 633 nm after different annealing treatments are shown in Table 2 . We can see that there are two real guided modes (index greater than the substrate) in the C implanted waveguide when the annealing temperature is lower than or equal to 450°C. For clarity, the effective refractive indices of the TM0 mode of the sample at a wavelength of 633 nm after different annealing treatments are also pictured in Fig. 2 . The main feature to be remarked on in Fig. 2 is the monotonic decrease of the index for the He implanted waveguide as annealing temperature increases, whereas the C implanted waveguide shows ascending-descending behaviour through sequential annealing treatments. Table 3 shows the measured neff of the TM guided modes of a C implanted waveguide at a wavelength of 1539 nm after different annealing treatments. It is found that a single-mode waveguide has been formed in this sample when the annealing temperature is less than or equal to 450°C and the neff of the guided mode is higher than nα of the virgin crystal (nα=2.1124 at the wavelength of 1539 nm). The ascending-descending trend over sequential annealing treatments was also found. We will explain this phenomenon in the following section.
To reconstruct the RIP in the waveguide, we use the SRIM and RCM methods. Although many methods can be used to reconstruct the RIP [23, 24], we chose RCM because it has proven to be more suitable for analysis of the waveguides formed by ion implantation techniques . Figure 3 shows the RIP of He and C implanted waveguides after the S5 annealing treatment. SRIM 2006 simulations of the electronic energy loss and nuclear energy loss profiles by He and C implantation are shown in Fig. 4 . As shown, the He and C ions lose most of their energy to electronic ionizations along their paths inside the target sample of ZnWO4 crystal, which results in the formation of colour centres and possibly damage in certain regions. At the end of the ions’ tracks, nuclear collisions result in lattice disorder and a decrease in physical density, which may cause a reduction in the refractive index in this region. The positions of the optical barriers by He and C implantation are confirmed to be located at 1.19 μm and 2.83 μm (shown in Fig. 4), respectively, according to the peak positions of the nuclear energy loss. As one can see, the index distribution is a typical “well” + “barrier” type. Under the conditions of He ion implantation, an enhanced-index well with Δn w = +0.0084 is built up in the near-surface regions and an optical barrier with Δn b =−0.007 is created near the end of the incident ion’s track. For C ion implantation, the numbers are Δn w = +0.011 and Δn b =−0.0015.
The near-field light intensity profile of planar waveguides was obtained by the end-face coupling method. Figure 5(a) and Fig. 6(a) show the mode profiles of He and C implanted waveguides, respectively, collected by a CCD after the S5 annealing treatment. They show that the light can be confined to the waveguide area (between the surface and the optical barrier). Our present data show that planar waveguides could be produced by He or C ion implantation under our experimental conditions. Based on the RIP (shown in Fig. 3), we used the commercial software “BeamPROP”, which is a part of Rsoft Photonics Suite, to simulate the light propagation in our planar waveguides. Figure 5(b) is the TM0 mode profile of a He implanted planar waveguide; the simulation result for a C implanted waveguide is shown in Fig. 6(b). By comparing (a) to (b) in Figs. 5 and 6, we may conclude that there is good agreement between the experiment and the simulation results. Therefore, the RIP that we selected is reasonable.
Compare the results obtained for the Carbon and Helium implantation, we can see that the light leakage in the substrate of Carbon implanted waveguide is obviously less than the helium implanted waveguide. This result may be attributed to the unsuitable implantation fluence and the other more likely explanation is the pretty small thickness of the waveguide layer (1.19 μm for Helium implanted and 2.83 μm for Carbon implanted) which is most likely insufficient for good restrict waveguide structure. From our work, we can see that the conditions of “5.0 MeV carbon ion implantation with a fluence of 1×1015 ions/cm2” are relatively appropriate to fabricate waveguides. This will provide good guidance for waveguide devices on ZnWO4 crystal. Annealing treatments affect the light absorption characteristics of ZnWO4 waveguides created by C and He implantation as shown in Fig. 7 . All optical absorption spectral shapes were almost the same. ZnWO4 presented an optical absorption threshold at 330 nm. The absorption spectra of the annealed ZnWO4 samples show a lower intensity in the UV–vis range, which is due to a decrease in absorption by defects. In the visible band, there were almost no changes between the ZnWO4 substrate and the waveguides, and we can say that the implantation processes had almost no influence on the visible band absorption.
In the following part, we will focus on the relationship between the RIP and the ion implantation process. First, we will discuss the reason the optical barrier forms. We suggest that the end-of-the-range defects induced by the nuclear energy loss are responsible for the optical barrier due to volume expansion [11, 12]. The nuclear energy loss is mainly focused at the end of the ions’ track. At the end of the ions’ track, severe disordering of the crystal lattice is created by elastic energy deposition, which results in a decrease of the refractive index. The region of decreased refractive index acts as an optical barrier. The waveguide layer is thus surrounded by the low indices of air and this optical barrier. Such a barrier confines the light in a narrow layer containing relatively high refractive index, which forms an optical waveguide between itself and the crystal surface [16, 17]. These statements about the optical barrier have been accepted by more and more researchers [11, 12, 16–18].
Next, we will analyse the formation of an enhanced-index well in our waveguides. Creation of a mode of an ion implanted waveguide confined in a raised refractive index region has been confirmed by some other researchers. In 2001, a single-mode extraordinary index (ne) raised waveguide in LiNbO3 formed by heavier ion (Cu+) implantation with low fluence (~1014 ions/cm2) was reported by Hu et al . Figure 4 shows the electronic energy deposition of the incident ions as a function of the penetration depth in the ZnWO4 crystal simulated by SRIM 2006. As they penetrate into the ZnWO4 crystal, the implanted ions slow down because of energy losses from interactions with the electrons and nuclei of the ZnWO4 crystal. As a result, these interactions will decrease the spontaneous polarization of the ZnWO4 crystal and can also induce a volume expansion effect (mostly at the end of the ion range). We find from Fig. 4 that the electronic excitation domain of the crystal ions has a strong overlap with the enhanced refractive index region in the waveguide. Therefore, we suggest that the reduction of spontaneous polarization in the region of electronic excitation mainly governs the change of nα in the waveguide .
Based on the information mentioned in the previous studies, we can conclude that appropriate reduction of spontaneous polarization in LiNbO3 crystals will raise the extraordinary refractive index. It will reach a maximum value (Δn=0.0132 for a wavelength of 633 nm) when the implantation dose reaches a critical value; however, severe reduction of spontaneous polarization will decrease the extraordinary refractive index instead [25, 26]. We think a similar phenomenon occurs in the ZnWO4 crystal. In other words, appropriate reduction of spontaneous polarization in ZnWO4 crystals will raise the nα and if the spontaneous polarization is severely reduced and nα of the as-implanted sample decreases. On the other hand, the lattice damage can be reduced by using a heat annealing treatment to partially restore the spontaneous polarization in the surface region and increase the nα of the surface region, sequentially. When the lattice damage is reduced to a certain degree, the nα in the surface region will reach its maximum point and then begin to decrease. In our work, the dose of He ions (1×1016 ions/cm2) is below the critical value and the dose of C ions used here (1×1015 ions/cm2) is higher than that value thus, we can see from Fig. 2 that the neff of the guided modes have the gradually descending trend of a He implanted waveguide while we see ascending-descending behaviour for the C implanted waveguide after sequential annealing treatments. The variation of surface refractive index (Δnα) reach a maximum value (Δnα=0.0128 for a wavelength of 633 nm) after S3 annealing treatment when C ion implantation.
We have demonstrated that two guided modes were observed in the helium implanted waveguide, while only one mode was observed for the waveguide formed by carbon ion implantation at a wavelength of 633 nm after proper annealing treatment. We obtained a single-mode ZnWO4 waveguide with a raised index at a wavelength of 1539 nm for the carbon implanted waveguide. The reconstructed RIP includes a non-leaky guiding region which can confine the light efficiently. The near-field profiles of the TM mode for the samples given the S5 annealing treatment were obtained, and they show good agreement between experimental and theoretical results. The annealing treatment results show that the planar waveguide formed by C ion implantation has a high thermal stability. The maximum value of Δnα (Δnα=0.0128 for a wavelength of 633 nm) was obtained by use of carbon ion implantation and proper annealing treatment. Our data show that this waveguide fabrication technique could be of particular interest for optical waveguide devices on ZnWO4 crystals.
This work was supported by the National Natural Science Foundation of China under Grant 10975094 and Grant 10735070, the National Basic Research Program of China under Grant 2010CB832906, and the NCET and in FANEDD of China. The authors thank Prof. Huai-Jin Zhang at Shandong University for providing experimental samples.
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