1. Introduction

ZnO is a well-known II-VI semiconductor with a room temperature direct band gap of 3.37 eV, which is considered to be a promising material for the fabricating of optoelectronic devices working in the blue/UV range. It has been shown that many physical properties of ZnO crystal is similar with its major rival GaN, which currently dominates the market of short wavelength devices [1,2]. However ZnO has several advantages over GaN, such as larger high-quality bulk single crystals, higher radiation hardness, amenability to wet chemical etching, and relatively low materials costs [3,4]. In addition, the high exciton binding energy of ZnO crystal (60 MeV vs 26 MeV for GaN) can ensure efficient excitonic emission even at room temperature. In the past several decades, ZnO based optoelectronics, electro-optic and acousto-optic devices have attracted vast research interests, and UV luminescence, lasers (both Fabry-Perot type and random laser) at room temperature have been reported in nanostructures and thin films [5–9].

Optical waveguides are basic components of integrated photonics, which restrict light propagation in micro-scale regions, leading to enhancement of optical efficiency [10]. The achievement of waveguide structures in ZnO material makes it possible to extend its application in light emitter devices and can provide more choice in the design of ZnO micro-devices. There are a few technics to produce waveguides in optical materials, such as ion exchange [11,12], epitaxial film deposition [13,14], sol-gel [15], ion implantation/irradiation [16–19] and pulsed laser writing [20–23], etc. Among these, ion beam technique is an efficient and highly controllable method, and it does not rely on the chemical properties of the target materials [24]. Up to now, ion implantation/irradiation technique has been used successfully in the fabrication of waveguide structures in more than 100 optical materials [25].

Implantation of exotic ions into ZnO devices is often used to produce electrical isolation, however it is shown that in such a process, the modulation of its optical properties could also be achieved. A few attempts have been made in recent years to introduce the ion implantation method into the fabrication of waveguides in ZnO [26,27]. However serious mode leakage of the ion implanted ZnO waveguides existed, which limited its application in integrated systems. In addition, up to now only planar waveguides formed by ion beam technique were achieved. Urgent demand still exists for waveguide devices based on ZnO of both channel or ridge shape with better guiding properties.

In the past few years, precise mechanical cutting was proved to be a convenient and effective method in the fabrication of ridge waveguides [28,29]. Benefiting from the combination of cutting and surface polishing, smooth sidewalls and consequently low propagation losses can be achieved. In this work we take use of swift-heavy C⁵⁺ ion irradiation combined with a subsequent precise diamond blade dicing process to fabricate planar and ridge waveguides in ZnO crystal.

2. Experiments in details

The ZnO sample used in this work is a commercial z-cut single-crystal one, which was cut into wafers with dimensions of 8(x) × 2(y) × 8(z) mm³ and optically polished. Firstly we built planar waveguide on the surface of the ZnO wafer by swift-heavy ion irradiation. The ion irradiation process was carried on utilizing the 3 MV tandem accelerator at Hlmholtz-Zentrum Dresden-Rossendorf, Germany. C⁵⁺ ion beam with energy of 15 MeV and fluence of 2 × 10¹⁴ ions/cm² was irradiated on the x-z plane of the ZnO sample. The low ion beam fluence used here also could ensure the producing speed, which may be beneficial to commercial production. During the irradiation process, the sample was tilted with an angle of 7° to avoid channeling effect. The ion current density was kept less than 10 nA/cm² to avoid the heating and charging of the sample. Ridge waveguides were designed with different width ranging from 20 to 40μm, by utilizing a diamond rotating blade to cut carefully on the planar guide region to form ridge shapes.

As to characterize the propagation property of the formed waveguides, firstly we set up a conventional end-face coupling system to explore the near-field modal profiles. In such arrangement lasers with wavelength of 632.8 nm and 532nm were used as the light sources, and a 20 × objective lens was used to focus the light into the cross-section of the sample. Secondly a metalloscope (Carl Zeiss, Axio Imager) was used to focus directly on the cross-section of the waveguide sample to detect the morphology of guiding region. Propagation loss was also measured, using the back reflection method [30].

SRIM calculation was used to simulate the C⁵⁺ ion irradiation process [31]. The refractive index profile of the waveguide was reconstructed, based on the damage distribution and N.A. measurement. Modal profile of the waveguide was simulated using finite-difference beam propagation method (FD-BPM), based on the reconstructed index and compared with the experimental result.

The fabrication of ridge waveguides was attempted by mechanical method. In such a process, the waveguide sample was mounted on a computer controlled motorized stage, being machined by a diamond rotating blade while moving in the direction parallel to the blade [32]. The cutting grooves were about 40μm deep. By controlling the distance of the adjacent two cutting grooves, ridge waveguides with different width were designed. In this work the ridge widths were designed to be 40, 30 and 20μm respectively.

3. Results and discussion

Microscope image of the formed planar waveguide captured by a Zeiss metalloscope was shown in Fig. 1(a), with the magnifying power adjusted to 500 × , transmission mode. From which the waveguide region can be observed directly. Also one can see that a region beneath the sample surface has shown a darker color compared to the unprocessed region, while no obvious change observed in the near surface region in the first 4μm. This color variation under microscope view can be attributed to scattering centers formed in the ion irradiation process.

 

Fig. 1 (a) Microscope image of the cross-section of the planar waveguide, with a magnifying power of 500 × . (b) and (c) are near-field modal profile of the waveguide, observed at TM polarization, at 633nm and 532nm respectively.

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Figure 1(b) and 1(c) present the near-field modal profile in 632.8nm and 532nm respectively, both captured in TM polarization. For both wavelengths, the waveguide could only support single-mode propagation. The modal profiles of TE polarization were in similar situations, not presented here for the purpose of clarity. The propagation loss of the planar waveguide was measured, using the back reflection method. The propagation losses at 632.8nm were estimated to be 0.7 dB/cm for TE mode and 1.3 dB/cm for TM mode respectively. And for 532nm, the loss values were determined to be 0.7 dB/cm (TE mode) and 1.1dB/cm (TM mode), which were in a similar level. The low propagation losses show good prospect for practical application.

The C⁵⁺ ion irradiation process was simulated by the computer code SRIM2010, as to provide a better understanding of the formation mechanism of waveguide. The results were shown in Fig. 2. As can be seen, the energy transfer by nuclear collision occurred mainly at the end of ion trajectory, bout 8μm beneath the surface. This process created a buried damage layer (Also in corresponds with dark region seen from the microscope image), causing volume expansion of the nuclear-damage dominated region, which is considered to be the main reason for the formation of optical barrier [24,33,34]. While the electronic energy loss caused only point defects or color centers in ion trajectory before the barrier region.

 

Fig. 2 SRIM calculation of 15MeV C⁵⁺ ion irradiation process. Red dashed line and black solid line represent the electronic energy loss and nuclear energy loss respectively.

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The refractive index modulation mechanism of ZnO crystal induced by ion irradiation was studied in reference [26], in which the effects of molar polarization and molar volume change by ion beam effect were determined to be the dominant factors for refractive index change, and Δn(M) (index change caused by molar polarization and molar volume modulation) shows a negative change and is basically proportional to the damage ratio caused by ion beam. Here we use the SRIM code to simulate the C⁵⁺ ion irradiation process and assume the normalized vacancy profile as the damage profile. In addition, we measured the N.A. of the waveguide in longitudinal direction to determine the maximum of refractive index change, by similar method described in reference [23]. Based on these results, we reconstructed the possible refractive index profile (633nm), as shown in Fig. 3(a), taking the n_e as example and for further analysis. From the profile one can clearly see the optical “barrier” beneath the sample surface. The maximum refractive index change was measured to be 1.6 × 10⁻³. By assuming a step-like index profile, the V factor of the waveguide was estimated to be ~0.64, showing only the fundamental mode could be supported, in keeping with the experiment observation. By using the FD-BPM method, we simulated the modal profile of the planar waveguide at 633nm, based on the reconstructed refractive index. The result was presented as Fig. 3(b). The result is in good accordance with the modal profile observed in experiments shown in Fig. 1(b). This indicates that the reconstructed refractive index is reasonable.

 

Fig. 3 (a) Reconstructed extraordinary refractive index n_e (corresponding to the TM polarization) as the function of depth. (b) Near-field modal profile of the waveguide, simulated by FD-BPM method, in TM polarization. Dashed line indicates the interface of air and crystal.

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Ridge waveguides were produced by precise diamond rotating blade dicing, with different widths designed. Figure 4(a) presented the microscope image of the formed waveguides. Three ridge structures can be found in the image, captured by the same Zeiss metalloscope. G1-G3 represent waveguides with width of 40, 30 and 20μm respectively. In our experiment, corner breakages happened for several waveguides in the polish step, as can be seen in G3. This may be improved by optimizing the machining process. Nevertheless, restrictions of light with good propagation properties were detected, in a series of waveguides formed.

 

Fig. 4 (a) The microscope image of the ridge waveguides produced by diamond rotating blade cutting. G1-G3 represent waveguides with width of 40, 30 and 20μm respectively; (b)-(d) depicts the near-field modal images of G3-G1 under 633nm. (e)-(g) are modal images of G3-G1 under 532nm. All above captured in TE mode.

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We use a CCD camera to capture the near-field images of the formed ridge waveguides from the end-faces, as presented in Fig. 4. Here (b)-(d) represent modal profiles (TE mode) of waveguides G3, G2 and G1 respectively, captured in 632.8nm wavelength. While (e)-(g) were modal images at 532nm, in the same order and polarization. No apparent variation on shape and intensity detected with change of polarization, for waveguides with different width. Single mode propagations were achieved in G3, in both red and green range. As the increase of the width, modes with higher order appeared, especially for the 40μm ones.

We measured the propagation loss of a 20μm waveguide in 632.8nm, using Fabry-Perot method, which can be found in ref [16] for detail. The propagation losses were determined to be 4.1dB/cm and 3.7dB/cm for TE and TM modes, respectively. Measurements were also taken place for waveguides with different width. The results were in a similar level, of 4.3dB/cm for G2 and 4.1dB/cm for G1 (both in TM mode) respectively. The propagation losses of ridge waveguides are higher than the planar one, which may be attributed to unsmooth sidewalls and corner breakages.

Thermal treatments were taken place for the ZnO waveguide in a two-step process. First the sample was annealed at 200°C for 1 hour, then at 300 °C for 1 hour. The variation of the propagation loss with annealing temperature was studied, taken the 20μm ridge waveguide for example (Measured in TM mode). The result was shown in Table 1. As one can see, the propagation loss can be reduced to some extent by subsequent annealing processes. And the guiding properties were found to be stable in thermal treatments with temperature no more than 300°C, which can meet use requirements in most opto-electronic cases. The loss value may be further decreased by optimizing the machining parameters and providing proper annealing processes.

Table 1. Propagation losses of TM modes, measured before and after each annealing step, for waveguide G3 at two different wavelength.

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In conclusion, light restrictions are achieved, for ridge waveguides with width ranging from 20 to 40μm, with proper depth and good modal properties. Compared to previous reports on ZnO waveguides induced by ion implantation [26,27], here the energy leakage can be suppressed effectively, benefiting from proper guiding depth induced by 15 MeV C⁵⁺ ion irradiation, showing good prospects in future applications.

4. Summary

In this work, we have successfully fabricated planar and ridge waveguides in ZnO crystal, using C⁵⁺ ion irradiation and subsequent precise diamond blade dicing, which was to the best of our knowledge the first time for achievement of 2-dimensional waveguide structure in ZnO, by utilizing the energetic ion beam. The refractive index distribution of the formed waveguide was reconstructed, showing an optical “barrier” mainly in the region where nuclear collision took place. Propagation losses of the planar waveguide were measured to be 0.7 dB/cm for the TE mode and 1.3 dB/cm for the TM mode respectively, showing good propagation property. Light restrictions were achieved in ridge waveguides with width ranging from 20 to 40μm, with similar modal profiles in both polarizations. The swift-heavy ion irradiation method used here is proved to be effective in building high-quality waveguides in ZnO crystal, which provide a good choice for the design of ZnO micro- photoelectric devices.