We report on the formation of KOTiPO4 optical ridge waveguide combined ion implantation and Ar-ion beam etching in this manuscript. We used 6 MeV silicon ion implanted into our samples with the fluence of 6 × 1014 ions/cm2, which is relative high values for both energy and fluence. The guided mode and light propagation properties were investigated by prism-coupling and end-face coupling method. Numerical simulation was performed based on the reconstructed 2D refractive index profile of waveguides cross section for comparison. We obtain non-leaky waveguide structure in nx direction after proper annealing treatment. The fabricated waveguide structures emerge as promising candidate for photonic design which will work at high temperature.
©2013 Optical Society of America
Potassium titanyl phosphate (KOTiPO4, KTP) has superior properties such as large effective second-order nonlinear coefficient, an excellent temperature window, a wide wavelength range for phase matching, and outstanding crystal stability . The combination of large electro-optic coefficients and low dielectric constants makes KTP prominent waveguide material. Waveguides are promising not only for they can increase light density and decrease size which allow for miniaturization and integration into more complex circuits, they can improve the efficiency of fluorescence  and laser  also. The current researches show that the KTP crystals waveguide doped with rare-earth ions are one of the most promising systems for practical implementations of quantum memory . Recently, the waveguide in periodically poled KTP (PPKTP) attracted much attention because its outstanding superiority in the aspect of SHG [5,6]. Based on KTP optical waveguide structure, electro-optic and nonlinear-optic devices have been fabricated which confirmed that KTP is a superior material for many optical waveguide applications.
By now, there are many methods for fabrication of KTP optical waveguide, such as ion exchange , film deposition , femtosecond laser direct writing (single or double line)  and ion implantation [9,10]. As a relatively mature technique for fabricating optical waveguides ion implantation has been used to fabricate optical waveguide structure successfully and reported in elsewhere [9–14]. The kinds of ion implantation were divided into three kinds according the species of ion. Firstly, light-ion (H or He) implantations have been used to form waveguides on KTP substrates [9,10], which requires relative low energy 0.4~0.6 MeV and high fluence to order of 1015~1017 ions/cm2. Secondly, the implantation of some medium-mass ions, such as C, O, or Si , has been utilized for waveguide formation in KTP crystals owing to the required fluence as low as 1013~1014 ions/cm2. Alternatively, swift heavy ion irradiation with high energy of several hundred MeV at ultralow fluences (~1012 ions/cm2) was used in KTP crystal also .
Compared to conventional guides, because of higher induced index contrast ridge waveguides reduce bending losses, improving the integration level of optical devices. Moreover, ridge guides reduce the size of optical modes, enhancing the efficiency of nonlinear effects . In a word, the research of ridge structure of KTP crystal is the requirement of the development on KTP integrated optical devices. The heat treatments process will remove the colour centres and to partially recover from crystal lattice damage produced by the implantation. A lot of research has been done on waveguides fabricated in KTP crystals by ion implantation. However, there has no research on ridge waveguide combined ion implantation with Ar ion beam etching and there has been little systematic research or deep discussion of some of the basic problems surrounding the annealing response of planar waveguides fabricated by Si ion implantation. In this work, we report on the fabrication of planar and ridge waveguide by using Si ion implantation combined with Ar ion beam etching and investigate the effects of thermal annealing on the structure of KTP planar waveguides to address these needs.
2. Experiment details
The z-cut KTP samples used in this work has sizes of 10(y) × 8(x) × 1.5(z) mm3 and obtained from the School of Chemistry and Chemical Engineering, Shandong University. Two pieces KTP substrates which are identical completely as our experimental samples for comparison conveniently. The largest plane (xy plane) was clean and optical polished before ion implantation. Firstly, we fabricated planar waveguides on KTP samples by ion implantation with 6MeV Si ion at the fluence of 6 × 1014 ions/cm2 by use of a 1.7 MV tandem accelerator in Peking University. After this step, we obtain two pieces of planar waveguides on KTP crystals. Secondly, we fabricated photoresist mask on one of the planar waveguide by use of standard lithography techniques. The BP218 thick positive photoresist (about 4.5 μm) was used in the fabrication of etching mask. The period of Cr mask plate is 50 μm with a series of Cr stripes of 10 μm (shielded) and separation of 40 μm (transparent) between the adjacent channels. The straight channels are parallel to the y direction. With this processing, the photoresist mask was developed. Thirdly, the ridge waveguide was formed on the planar waveguide which coated with mask by Ar ion beam etching technique. The specific process was introduced in Ref  clearly. In this step, Ar ion beam sputtering with energy of 500eV was used to etch the unshielded area of the planar waveguide for 5.5 hours. In the etching process, the ion beam, with an intensity of 17 mA/cm2 tilted 30° off the sample's normal direction and along the channels. Finally, the ridge waveguide was formed on the surface of KTP after remove the photoresist mask.
After that we investigated the guided mode effective index (neff) and light propagation properties of the planar and ridge waveguide. We made a series of heat treatment for investigate the temperature stability of Si-implanted KTP planar waveguide. The specific annealing conditions are shown in Table 1 . The annealing properties were investigated by prism-coupling and end-face coupling method at the wavelength of 633 nm. The end faces (xz plane) of the sample were polished to achieve the requirements for direct end-face coupling of the light. The height of ridge was measured by a Stylus Profiler for the ridge waveguide. We measured the mode profile and near-field light intensity profile of the planar and ridge waveguides by end-face coupling method both before and after annealing treatment. SRIM 2010 is used to calculate the electronic and nuclear energy losses for obtain the formation mechanism of our planar waveguide. The reflectivity calculation method (RCM) is used to calculate the refractive index profile (RIP) of the planar waveguide. The finite difference beam propagation method (FD-BPM) is used to investigate the guided mode for comparison to the experimental results. The propagation loss was measured used the method introduced in Ref  by end-face coupling equipment and the error is about 0.01 dB/cm.
3. Results and discussion
Figure 1 shows the guided modes measurement results of the KTP planar waveguide (S1 and S10) via the prism-coupling equipment at the wavelength of 633 nm. Figure 1(a) is the relative intensity of the light reflected from a prism with TE polarized light of nx direction, Figs. 1(b) and 1(c) corresponding to ny, nz direction respectively. As seen from Fig. 1, the surface refractive index (nsur) of our KTP planar waveguide (S1) has almost no difference among nx, ny, and nz directions. Because of this, we can say that the surface of KTP crystal has reach isotropy at our implantation conditions. The single narrow and deep dip in the mode-profile indicates that there is a real guide mode in the planar waveguide. From Fig. 1, we can see that there is only one narrow and deep dip which located in solid line profile of Fig. 1(a) which indicates that the planar waveguide has a real guided mode in nx direction after S10 annealing treatment. In other words, the waveguide can propagate TE0 mode of nx only after S10 annealing treatment. It is found that a single-mode waveguide may be formed in this sample when the annealing temperature equal to 550°C and the neff of the guided mode is slightly higher than nx of the virgin crystal (nsub = 1.7618 at the wavelength of 633 nm).
The measured neff of guided modes (zero order mode) at nx, ny and nz directions for Si-implanted KTP waveguides at a wavelength of 633 nm after each annealing step are shown in Table 2 . We can see that, there are slightly differences of the neff of the zero order modes in nx, ny, and nz directions for the Si-implanted KTP waveguide when the annealing temperature is lower than or equal to 500°C, whereas the neff of the zero order mode in nx direction is higher than ny, and nz directions obviously. For clarity, the effective refractive indices of the TE0 mode (nx direction) 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 in Fig. 2 is the monotonic increase of the index as continue of the annealing. The ascending trend over sequential annealing treatments was found. The annealing treatments partially recover the crystal lattice damage produced by the implantation.
The electronic energy loss (Se) and nuclear energy loss (Sn) profiles of 6 MeV Si ions implantation simulated by SRIM 2010 are shown in Fig. 3(a) . As shown, the Si ions lose most of their energy to electronic ionizations along their paths inside the target sample of KTP 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. Based on the simulation results shown in Fig. 3(a) and the dark mode spectrum shown in Fig. 1, we reconstructed the RIPs of the Si ions implanted KTP planar waveguide after S10 annealing treatment by RCM calculation and shown in Fig. 3(b). The position of the optical barrier is confirmed to be located at 2.8 μm according to the peak positions of the nuclear energy loss profile. As one can see, the RIP of nx is a typical “well” + “barrier” type when the RIPs of ny and nz are “barrier” type. The positive-index well for nx is most important, in which the waveguide could confine the extra-polarized light propagation in a non-leaky way. For “barrier” type distribution, the negative barrier is the only factor that confines the light in the waveguide. For this reason, the RIP of nx with “well” + “barrier” type could confine the light propagation in waveguide region more effectively and it was reported elsewhere also [11,12]. The differences among RIPs of nx, ny, and nz directions could explain the phenomenon of the KTP waveguide could carry TE0 mode of nx only after S10 annealing treatment. Under the conditions of Si ion implantation, an enhanced-index well with Δnw = + 0.004 is built up in the near-surface regions and an optical barrier with Δnb = −0.09 is created near the end of the incident ion’s track for the RIP of nx after S10 annealing treatment. 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 . The electron energy loss originate from interactions between ions and crystal will decrease the spontaneous polarization of the KTP crystal, and this can explain the formation of “well” in crystal surface .
Figure 4(a) shows the near-field light intensity profile for TE0 mode of the planar waveguide by end-face coupling method after S10 annealing treatment. As one can see, the light can be confined to the waveguide area (between the surface and the optical barrier). Our present data provides direct evidence for the single mode planar waveguide in nx direction could be produced by Si ion implantation under our experimental conditions. Based on the RIP [the solid line shown in Fig. 3(b)], we used the commercial software “BeamPROP” (BPM), which is a part of Rsoft Photonics Suite, to simulate the light propagation in our planar waveguides. Figure 4(b) is the simulation result of TE0 mode profile for our planar waveguide. It can compare with Fig. 4(a) very well and we may conclude that there is good agreement between the experiment and the simulation results. Therefore, the RIP that we selected is reasonable. The propagation loss is about 0.8 dB/cm measured by end-face coupling method.
By combining lithography with Ar ion beam etching technique, we fabricated ridge structure on the KTP planar waveguide. The etching depth and surface shape were investigated by a stylus profiler. As shown in Fig. 5(a) , the height of the rib is about 1.8 μm, which demonstrates that the etch rate is approximately 5.5 nm/min under our etching conditions. The shape of the rib is trapezoid, and the width of the rib varies between 5 μm at the top and 10 μm at the bottom, which is some deviation from the process design. The side-etching and trapezoid cross section of photoresist may be the reasons for this shape. For the reconstructed 2D refractive index profile of the xz plane of the ridge waveguide after thermal annealing, which is shown in Fig. 5(b), we carefully consider the morphology of ridge structure and the RIP of planar waveguide [shown in Fig. 3(b)]. Figure 6(a) shows the intensity profiles of the excited mode (quasi-TE00) in the ridge waveguide detected on the outer facet of the sample by use of the end-face coupling method. The process of light propagation in the ridge waveguide is simulated by BPM, the mode intensity profile of quasi-TE00 is shown in Fig. 6(b). We can conclude that they show good agreement with each other, and it can provide an efficient method for practical device design in ion implanted KTP ridge waveguide. The loss of the TE00 mode about ridge waveguide is 3 dB/cm measured by end-face coupling method. The larger etching depth and rib height may result in even better light confinement, and the smoothness of the side walls potentially leads to good guiding properties and low scattering losses.
It is notable that we selected relative heavier ions, higher energy and higher fluence compared with other researchers , and we obtain the waveguide structure after annealing treatment with relative higher temperature. Our experimental results indicate that our KTP waveguide could keep stabilization when apply at high temperature (equal to or lower than 550°C).
We successfully fabricated planar and ridge waveguides on a z-cut KTP crystal using an ion implantation method combined with Ar ion beam etching technique. The results of prism-coupling measurement show that the surface of the KTP crystal has reach isotropy with energy of 6MeV and fluence of 6 × 1014 ions/cm2. The end-face coupling tests demonstrated that the waveguide can propagate light very well at 633 nm when the annealing temperature equal to 550°C which provide possibility in high temperature application, and the experiments agreed with the BPM simulations. The results also imply that potential photonic applications, such as optical interconnections, waveguide switches, and modulators, may be realized by using this design.
This work is supported by the National Science Foundation of China (Grant Nos. 11205096 and 11005070), the Natural Science Foundation of Shandong Province (Grant No. ZR2012AQ019), and the Science and Technology Development Program of Jinan City (Grant Nos. 201202092 and out-02440).
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