We report on the fabrication of planar waveguide in stoichiometric lithium niobate by 500 keV proton implantation with a dose of 1×1017 ions/cm2. The formation of ne enhancement planar waveguide in the crystal was disclosed by the dark mode spectra and the subsequent endface coupling measurement. The absorption spectra show that the postannealing treatments above 400°C temperature can remove the color centers induced by implantation efficiently. The propagation loss and near-field profiles of the planar waveguide were obtained with an end-face coupling system.
©2007 Optical Society of America
Lithium niobate (LiNbO3 or LN) crystals are of great importance for fabrication of integrated optical devices due to their excellent physical properties. Recently, ion implantation has attracted the interest of the scientific community for its excellent capability of fabrication of waveguide in crystals [1–3]. Optical waveguides in LiNbO3 crystal have been fabricated by ion implantation technique. In the early works MeV light ions were used to fabricate LiNbO3 waveguides, such as proton and He ions with doses of 1016–17ions/cm2. In waveguides of this type, an optical barrier is formed due to the end of range (EOR) defects so that the refractive index profile (RIP) essentially matches the profile of the energy deposited by nuclear process [4,5]. More recently, high energy implantation of medium-mass ion implantation such as C and O has been demonstrated to be another strategy for waveguide fabrication [6–9]. The waveguide formation mechanism of these waveguides is revealed that the electronic stopping energy dominates the ne enhancement scenario. Zhang et al. have found the “missing modes” of the LiNbO3 waveguides formed by He ion implantation [10,11]. The effective refractive indices of these “missing modes” are higher than ne of LiNbO3 substrate. The recent studies of He ions implanted LiNbO3 waveguides also indicated that ne increased waveguide structure could be constructed by MeV He ion implantation [12, 13].
The stoichiometric lithium niobate (SLN) crystal shows many improved performances compared to congruent lithium niobate (CLN) crystal because the mol ratio of Li/Nb is near to 1 in SLN crystals [14,15]. So far there have been no studies of waveguide with non-leaky guided modes formed by proton implantation. In the present work, we report the waveguide formation and characterization in SLN crystal implanted by 500 keV protons.
2. Experimental details
The z-cut SLN sample, with size of 5×7×1 mm3, was implanted by 500 keV protons with a dose of 1×1017 ions/cm2 at room temperature. The ion beam was electrically scanned on the sample in order to ensure a uniform implantation. To prevent channeling effects we tilted the sample 7° off the direction of the incident proton beam. After the ion implantation, the sample was annealed under a series of heat treatments in atmosphere (list in Table 1) to remove the color centers and scattering centers produced by implantation. The dark mode spectra of the waveguide were 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 reflectivity calculation method (RCM) has been carried out to reconstruct the refractive index profile of the present waveguide structure. The absorption spectra of the sample were measured by the Jasco U570 spectrophotometer after different annealing treatments. The near field profiles and the propagation loss of the fabricated planar waveguide were measured by an end-face coupling system.
3. Results and discussion
Figure 1 shows the TM dark mode spectra of as implanted sample and after S2 annealing treatment at wavelength of 633 nm. The extraordinary refractive index (ne) of SLN substrate is also signed in Fig. 1 for comparisons. During the measurement of dark mode spectra, the laser beam will be coupled into the waveguide region when the resonant condition is satisfied. Therefore these couplings will introduce lacks of reflected light. A lack of reflected light will result in a dip of the dark mode spectrum. As seen from Figs. 1(a) and 1(b), one of the main points to note here is that no resonant dip is detected in the dark mode spectrum of as implanted sample. But this is dramatically changed after the S2 annealing treatment. Obviously, we can find from Fig. 1(b) that several resonant dips have been detected in the TM (ne) dark mode spectra. This means that the waveguide structure has been formed in this sample. The most left two dips in Fig. 1(b) are very sharp. Their effective refractive indices are higher than ne of the virgin crystal (ne=2.1885). Thus these modes could be well confined in the waveguide region without the tunneling effect.
In order to obtain a farther understanding of formation of present waveguide formed by 500 keV proton implantation, we performed continuous annealing treatments in atmosphere (Table 1). Before the annealing treatments, we have measured the refractive index of the pure SLN crystal after annealing in 460°C for 10 hours in atmosphere and no refractive index increase due to the Li ions out-diffusion has been detected. This means that the SLN crystal is stable at such high temperature. We measured the dark mode spectra of the waveguide after each annealing step at wavelength of 633 nm and 1539 nm, respectively. It is found that mono-mode waveguide has been formed in this sample at wavelength of 1539 nm. Figure 2 indicates the effective refractive indices of TM0 mode of the sample at wavelength of 1539 nm after different annealing treatments. The feature to be remarked in Fig. 2 is the ascendingdescending behavior for the sequential annealing treatments. The effective refractive index increases after the first two annealing treatments and then decreases after the following two annealing treatments. We try to explain this phenomenon in the following section.
The RCM code was used to reconstruct the ne RIP of the sample . We have chosen a RIP that consists of two half Gaussians based on our experience. If the calculated values match the experimental ones within a satisfactory error, the RIP we assumed is considered to be a reasonable one. Figure 3 shows the graphical representation of RIP of the sample after S2 treatment at wavelength of 633 nm. The experimental and calculated values of effective refractive indices are also marked. We find that a positive index enhancement (~0.4%) occurred in the guiding region while a negative index change (~2%, usually called an optical barrier) is formed at the interface between the waveguide region and the substrate.
The result that the mode of the ion implanted LiNbO3 waveguide is confined in a raised refractive index region has also been confirmed by some other researchers whereas these waveguides are fabricated by implantation of heavier ions compared to proton, such as Cu, Ni etc [17,18]. In contrast, the refractive index enhanced region in the present waveguide is formed by the lightest ion-proton implantation at single energy. Therefore it is attractive to investigate the reason of generation of enhanced refractive index region in the present waveguide. The inset of Fig. 3 shows the electronic energy deposition of the incident protons as a function of the penetration depth in the SLN crystal simulated by SRIM2006 . During their penetration into the SLN crystal, the implanted protons slow down because of energy loss from interactions with the electrons and nuclei of the SLN crystal. As a result, these interactions will decrease the spontaneous polarization and electro-optic effect of SLN crystal; also they can induce the volume expansion effect mostly at the end of the ion range. We can find from the inset that the electronic excitation domain of the crystal ions has a great superposition with the refractive index enhanced region in the waveguide. Therefore we suggested that the reduction of spontaneous polarization in the region of electronic excitation mainly governed the change of ne in the waveguide .
Below, we focus to the experimental results and explain them. Based on the information mentioned in the previous studies, we can concluded that appropriate reduction of spontaneous polarization in LiNbO3 crystals will lift the extraordinary refractive index, however, severe reduction of it will decrease extraordinary refractive index instead [18, 20]. In our work, the dose used here (1×1017ions/cm2) is too high so that the surface of the as-implanted sample is damaged at a high degree. The spontaneous polarization of surface region is severely reduced and ne of the as-implanted sample decreases. Thus there is no waveguide structure formed and we can not detect the dark mode, as seen in Fig. 1(a). On the other hand, the lattice damage can be reduced by using the heat annealing treatment so that the spontaneous polarization in the surface region will be restored partially and ne of surface region increases sequentially. As a result, the ne enhanced waveguide structure is formed in the surface region and the dark modes can be detected (Fig. 1(b)). More sequential annealing will recover the lattice structure farther. When the lattice damage reduces to a certain degree, ne of surface region will reach at its maximum and then decrease. So we can find from Fig. 2 that the effective refractive index increases first and then decreases upon the refractive index of the substrate. For the optical barrier inside the waveguide, we suggest that the end of the range defects induced by the nuclear energy loss are responsible its formation due to the volume expansion [1, 4, 5].
After implantation, the SLN sample has become gray. This indicates that color centers have been created in the waveguide due to the implantation. The absorption spectra of the waveguide after different annealing treatments were measured by Jasco U570 spectrophotometer. The upper and bottom faces of the sample are optically polished before the absorption measurement to ensure accurate results. Our data were depicted in Fig. 4. As one can see, there is strong visible-infrared absorption in the absorption spectrum of the as implanted waveguide (curve A). This scenario coincides with the observation from our naked eyes. The absorption is reduced after the S1 annealing treatment (curve C). Curve D is the measured absorption spectrum of sample after S4 treatment. The measured absorption curve D has a great superposition with the virgin SLN spectrum (curve B). This indicates that the color centers can be reduced efficiently after post-annealing treatments.
The near field profiles of the sample after S4 annealing treatment were collected by the end-face coupling method at wavelength of 633 nm. The TM (ne) mode profiles we collected by the photo screen (a) and CCD camera (b) are shown in Fig. 5. The propagation loss of this waveguide was also measured by this end-face coupling system. From the maximum transmitted power, and taking into account the Fresnel reflections (0.64 dB) at the crystal-air boundaries as well as a laser beam-mode coupling efficiency of ~70% (1.5 dB), we obtain a wave propagation loss of ~0.54 dB/cm. Moreover, we cannot obtain the intensity profile of the TE polarized (no) mode by end-face coupling method since no transmitted light was observed. This result indicates that the present z-cut SLN waveguide can only propagate TM mode.
In conclusion, we have demonstrated the fabrication and characterization of planar waveguide in z-cut SLN by 500 keV proton implantation with a dose of 1×1017 ions/cm2. The dark mode spectra were measured by the prism coupling method. The reconstructed RIP includes a non-leaky guiding region which can confine the light efficiently. The near field profiles and the propagation loss of TM mode of the sample after S4 annealing treatment were also obtained. Our data show that this waveguide fabrication technique could be of particular interest for the quick integration of waveguide polarizer on a SLN substrate.
We would like to thank Sun-Qian Sun and Dong-Gang Ran for assistance in measuring of the absorption spectra of our sample. This work was supported by the National Natural Science Foundation of China (Grant No. 10735070, 10575067 and 10505013).
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