Waveguides in single-crystal lithium niobate thin film by proton exchange

The proton exchanged (PE) planar and channel waveguides in a 500 nm thick single-crystal lithium niobate thin film (lithium niobate on insulator, LNOI) were studied. The mature PE technique and strong confinement of light in the LN single-crystal thin film were used. The single mode and cut-off conditions of the channel waveguides were obtained by finite difference simulation. The results showed that the single mode channel waveguide would form if the width of the PE region was between 0.75 μm and 2.1 μm in the β4 phase. The channel waveguide in LNOI had a much smaller mode size than that in the bulk material due to the high-refractive-index contrast. The mode size reached as small as 0.6 μm in simulation. In the experiment, the refractive index and phase transition after PE in LNOI were analyzed using the prism coupling method and X-ray diffraction. Three different width waveguides (5 μm, 7 μm and 11 μm) were optically characterized. Near-field intensity distribution showed that their mode sizes were 3.3 μm, 5 μm and 7 μm. The propagation losses were evaluated to be about 16 dB/cm, 12 dB/cm and 11 dB/cm, respectively. The results indicate that PE is a promising method for building more complicated photonic integrated circuits in single-crystal LN thin film. ©2015 Optical Society of America OCIS codes: (160.3730) Lithium niobate; (130.3120) Integrated optics devices; (310.0310) Thin


Introduction
In recent years, high-refractive-index contrast, single-crystal lithium niobate (LN) thin film on a low refractive index SiO 2 cladding layer (lithium niobate on insulator, LNOI) or other substrates, has been fabricated using crystal ion slicing and other wafer bonding technologies and is widely used [1][2][3][4][5]. The high-refractive-index contrast between LN and SiO 2 , resembling silicon on insulator (SOI), enables good confinement and strong guiding of light, thus a high-density photonic integrated circuit (PIC) is expected. Compared with SOI-based optical devices, LN has excellent electro-optic, nonlinear and acousto-optic properties, and can be easily doped with rare-earth ions to prepare laser material [6]. Various photonic devices have been reported in LN thin films, such as photonic crystals [7], electro-optical modulators [8,9], micro-ring resonators [10,11], microdisk resonators [12,13]  PE seems to be a suitable solution to build PIC on LNOI because the rough sidewall can be avoided and one can take advantage of the strong confinement of light in LN thin film. Therefore, it is necessary to study the PE process and the PE waveguides in LNOI. More complicated optical devices, such as electro-optical modulator and second harmonic generator, based on PE in LNOI, can then be explored.
In this paper, the PE planar and channel waveguides in LNOI were studied by simulation and experimentation. The single mode and cut-off conditions and the mode profile of the channel waveguide in LNOI were simulated using the commercial software "Lumerical: Mode Solution", based on a full-vectorial finite difference method [26,27]. In the β 4 phase, the channel waveguide was single mode when the PE width was between 0.75 μm and 2.1 μm. A much smaller mode size of 0.6 μm 2 were obtained compared with the conventional PE waveguide. The change in refractive index and the PE LNOI film thickness were measured via the prism coupling method. A crystalline phase change was found by the X-ray diffraction. The channel waveguides on LNOI (500 nm thick LN film) were formed by the photolithography process. Three channels with widths of 5 μm, 7 μm and 11 μm were characterized. The mode sizes of these channels were 3.3 μm 2 , 5 μm 2 and 7 μm 2 and the propagation losses were evaluated to be about 16 dB/cm, 12 dB/cm and 11 dB/cm, respectively by the Fabry-Perot resonator method. The realization of PE channel waveguide could lead to more advanced and complicated integrated optical devices and circuits based on LNOI.

Simulation
All of the LN films used in this study, in both simulations and experiments, were Z-cut. Since PE causes an n e increase, only the quasi-TM mode was studied. The schematic cross-section of the PE channel waveguide in LNOI is shown in the right inset of Fig. 1. A single-crystal LN thin film was bonded to a SiO 2 layer on a LN substrate. Then, a part of the LN thin film was proton exchanged and the waveguide was formed by the n e increase. The index-increased region, marked by "PE," was assumed to have a step-like profile [28]. This structure belonged to a type of waveguide called a "strip-loaded waveguide" [29], although there were some differences. In this study, the strip was the PE region of the film instead of on the surface of the film. In such a waveguide, the cut-off width of the strip was zero for the fundamental mode [28]. In the simulation of the PE channel waveguide in LNOI, a zero cut-off width (no matter how narrow the PE region was, there was always a guided mode) was indeed obtained. However, when the waveguide dimension was small enough and got smaller, more and more light energy of the mode distributed outside the waveguide core (PE region). Thus, in this paper, the cut-off width was defined as a minimum width at which 50% of the light energy was confined to the PE region. A full-vectorial finite difference method was used in the simulation and the result is shown in Fig. 1. The lines and symbols in black are the single mode and fundamental mode cut-off conditions with a refractive increase of 0.09 at 1539 nm, which was a measured value in this work. The X-axis is the thickness of the LN thin film and the Y-axis is the width of PE region. For example, for a 0.65 μm thick LN layer, the cut-off width was about 0.75 μm; when the waveguide width increased to above 2.1 μm, a second mode would appear. As a result, a single mode PE waveguide should have a width between 0.75 and 2.1 μm. The intensity distributions of the fundamental mode "a" and the second mode "b" were calculated and are shown in the left inset of Fig. 1 for the geometries labeled as "a" and "b" on the curves of Fig. 1. To enable the development of ultra-compact PIC and to strengthen the nonlinear and electro-optical effect, a small mode size is expected. Figure 2 shows the simulation results of the relationship between the mode size and the width of the PE region, using film thickness as the parameter. There was a minimal value for the mode size at specific widths. For example, if the LN thin film thickness was 0.6 μm, the minimum mode size was reached when the PE region width was around 0.8 μm. As the waveguide width increased, the PE region expanded, and so the mode size became larger. When the width decreased, the confinement of light would become weak, which also led to a larger mode size. In the inset of Fig. 2, a 0.6 μm thick and 0.8 μm wide channel waveguide had a mode size as small as 0.6 μm 2 (product of 1/e intensity in horizontal and vertical directions). This value was comparable to the calculated mode size of a 1 μm wide ridge waveguide (etched LN ridge on SiO 2 ) reported in [15] (~0.4 μm 2 ). The smallest mode sizes in etched LNOI waveguide were calculated to be as small as 0.15 μm 2 (0.35 μm wide ridge, TM polarization) and 0.16 μm 2 (0.45 μm wide ridge, TE polarization). The larger mode size in PE LNOI compared with ridge waveguide can be attributed to the weak confinement of light in the horizontal direction. However, the mode size we obtained in PE LNOI waveguide (0.6 μm 2 ) was more than one order of magnitude smaller than a previously reported single-mode PE waveguide in bulk LN [30]. Small mode size means tight confinement of light, which benefits a stronger non-linear optical process. For channel waveguides in LNOI, light was strongly confined due to the large refractive index contrast between the LN layer and the SiO 2 cladding, which resulted in a small mode size. In bulk material, the guided mode existed only when its effective refractive index was larger than the refractive index of bulk material (about 2.138). However, In LNOI, even without PE, the LN layer formed a planar waveguide. The channel waveguide mode could be excited with an effective refractive index larger than that of the LN planar waveguide (e.g. about 1.78 at the thickness of 0.5 μm, which is much smaller than 2.138). The effective index would decrease with the shrinking of the waveguide dimension. Therefore, the effective index and the dimension of channel waveguide in LNOI had more of a chance to decrease, which meant it reached a more compact size. Fig. 2. Relationship between the calculated mode size and width of the PE region, using film thickness as the parameter (∆n e = 0.09). Inset: A mode size as small as 0.6 μm 2 was obtained with a 0.6 μm thick film and a 0.8 μm wide PE region.

Experiment and results
Two identical Z-cut LNOI samples (#1 and #2) were prepared by crystal ion slicing and wafer bonding technology, in the research center of Nanoln, based on a previously described process [1]. The LN film was 500 nm thick and the structure of the sample is shown in Fig. 3. Sample #1 was used to investigate the refractive index change and phase transition in the LN thin film processed by the PE and sample #2 was used to fabricate channel waveguides.

Sample #1
Sample #1 was prepared under the following experimental conditions: Pyrophosphoric acid was applied as the proton source to exchange Li + in LN. The PE was performed in a sealed furnace at 200 °C for 80 min, which was sufficient to make protons penetrate the whole LN layer [28], so the thickness of the PE region was the same as LN layer (0.5 μm). When the proton exchange duration was reached, the sample was hung in the furnace until the temperature gradually dropped to room temperature. Finally, the sample was cleaned with de-ionized water. The dark modes of sample #1, before and after the PE process, were measured using a prism coupler (Model 2010 Metricon, USA) at 633 nm and 1539 nm, respectively. The measured effective refractive indices of the planner guiding modes at 633 nm are listed in Table 1. All of the effective extraordinary refractive indices of the LN planar waveguide modes increased after PE. The n e of the LN layer before and after PE, were 2.2018 and 2.3508 respectively as calculated from the effective refractive index of such modes. An increase of 0.149 was obtained at 633 nm and this value was 0.09 at 1539 nm. The film thickness, which was near the same before and after PE, was measured to be about 500 nm.  Figure 4 shows the X-ray diffraction (XRD) pattern around the LN (006) reflection for sample #1. The discrepancy between the LN substrate (39.22°) and LN film peak (39.32°) arose from the small deviation in the crystallographic Z-axis of the film from that of the substrate. After PE, a new phase occurred at the position of 39.1°. According to a previous report examining PE in bulk LN material [31,32], the new phase was β 4 phase (∆n e > 0.14).

Sample #2
Sample #2 was used to fabricate the channel waveguides. A 100 nm thick chromium (Cr) layer was deposited on the surface of the sample via the electron beam evaporation method. A photolithography process was used to define the PE mask, which had open strips of 1 to 7 μm. After the exposed Cr was wet etched and the photoresist was removed by acetone, the PE was performed under the same condition as sample #1, so the thickness of the PE region was 500 nm. Finally, the remaining Cr mask was removed by Cr etchant ((NH 4 ) 2 Ce(NO 3 ) 6 + C 2 H 4 O 2 + H 2 O) and the two facets of the fabricated waveguides were polished to facilitate the end face coupling. The final length of the sample was about 1.6 mm. Figure 5 shows the top view of the channel waveguide, which was defined by the 3 μm wide open mask. In this study we spent a long time to completely wet etch the exposed Cr. As a consequence, the width of the waveguide expanded to about 7 μm (increased by 4 μm) due to the lateral erosion of Cr in the wet etching process. This issue can be avoided in the future if another Cr deposited sample (deposited in the same condition) is used to calibrate the wet etching time to avoid excessive lateral erosion. In the following paragraphs, the width of the waveguides we mentioned are the value we observed by microscope rather than the opened mask width. A tunable semiconductor laser (Santec TSL-210) was used as the near-infrared light source (1260 nm-1630 nm). The linear polarized light emitted from this laser was transmitted through a polarization-maintaining fiber and then rotated to a TM polarized direction by a rotator. The light was coupled into the waveguide by the lensed tip of the fiber and the output light was collected by a 40 × / 0.65 objective lens. A knife-edge based beam profiler (BM-7, Coherent Inc.) and germanium photodiode were used to record the near-field intensity profile and the power of light, respectively. As shown in Fig. 6, the measured mode sizes (1/e intensity in horizontal and vertical directions) were about 3 μm × 1.1 μm (3.3 μm 2 ), 4.5 μm × 1.1 μm (5 μm 2 ) and 6.4 μm × 1.1 μm (7 μm 2 ) and the theoretical values were 3.6 μm × 0.38 μm (1.4 μm 2 ), 4.7 μm × 0.38 μm (1.8 μm 2 ) and 7 μm × 0.38 μm (2.7 μm 2 ), respectively. Considering that the vertical dimension of the waveguide (0.5 μm) is smaller than the resolution of the objective (~1.5 μm), the real profile and dimension in the vertical direction cannot be completely reflected, leading to the larger mode sizes (thrice the theoretical value in vertical direction). Other optical characterization techniques, such as Near-field Scanning Optical Microscopy, which is used to observe sub-micron scale photonic structures, is suitable if the local characteristics of guided modes in sub-micron scale photonic structures is needed [33]. The measured dimension in horizontal direction was a little smaller than the simulated one. This probably indicates a difference between the model we used in simulation and the actual structure of PE LNOI waveguide: the step-like index profile used in simulation is a approximate assumption; and in simulation, the refractive index change in channel waveguide was assumed to be 0.09, which was the measured value in planar waveguide. However, even under the same PE condition, the refractive index change in channel waveguide and in planar waveguide are not exactly the same. The Fabry-Perot resonator method was used to evaluate the propagation loss α of the channel waveguides. The polished end-faces of the waveguide were regarded as two Fabry-Perot resonant mirrors. The propagation loss was calculated through the transmission spectrum, which is shown in Fig. 7. The oscillating curve was due to Fabry-Perot interference and the mode propagation loss was determined by analyzing the contrast K of the cavity resonances [34]. The formulas to evaluate α and the related parameters were defined as follows:   . More efforts should be made to decrease the propagation loss. The loss came from several sources. First of all, a study has shown a degradation of the device performance (extinction ratio, V π L and insertion loss) due to the helium-ion-induced damage during the LN thin film preparation [36]. The degree of the overlap between the ion-damaged region and the guided optical modes has a significant effect on the degree of achievable modulation. Therefore, the helium-ion-induced damage in the PE region in our work is regarded to be a possible reason of the propagation loss. Secondly, LN layer lattice structure and constituent have changed from the PE process. The pure α phase waveguide has lower propagation loss than the β phase and the coexistence of two different crystal phase because the α phase has the least changes in lattice constant and the refractive index [37]. Next, the scattering loss might occur from the interface of the LN film and SiO 2 layer. Finally, for multi-mode waveguide, nearly always a combination of modes is excited by end-face coupling. This will lead to a reduction of the measured contrast K and an increased loss. Although the intensity distribution illustrated a fundamental mode profile, some higher order modes might be partly excited. In addition, imperfections induced by polishing would lower the reflectivity of the waveguide end face. So the loss number we obtained from formula (1) is the upper limit for the fundamental mode [33].
Therefore, to improve the quality of the PE waveguide, some parameters including implanted energy and dose and the following annealing condition in the process of LNOI fabrication, should be controlled and optimized to get the least helium-ion-induced damage in the LN thin film. The α phase waveguide should be fabricated since it changes the LN crystal structure the least among all of the PE LN phases. This could be done by performing PE in benzoic acid for a short time, followed by annealing or immersion in a benzoic acid melt, buffered with lithium benzoate for a long time [32,38]. To accurately evaluate the propagation loss, we need to fabricate single mode waveguide to avoid the reduction of the measured contrast K.

Conclusions
In conclusion, PE waveguides in LNOI were studied. Simulations, including single mode conditions and mode size calculations, were performed. Compared with previously reported PE channel waveguides in bulk LN, a much smaller (more than one order of magnitude) mode size in LNOI, of about 0.6 μm 2 , was obtained. The small mode would enhance the nonlinear and electro-optical effect, leading to high efficiency all-optical signal processing and E-O modulation. The simulation also showed that, in the β 4 phase, the channel waveguide was single mode when the PE width was between 0.75 μm and 2.1 μm. The dark-modes before and after PE were measured, for the planar waveguide, and an increase in n e of 0.149 at 633 nm, was obtained. X-ray diffraction indicated a new phase (β 4 ) emerging in the PE LNOI. The fabrication and optical characterization of the PE channel waveguide on LNOI were presented. A standard photolithography followed by a PE process was performed on a LNOI sample with a 0.5 μm thick LN layer. For the 5 μm, 7 μm and 11 μm wide channel waveguides, the near-field intensity distribution showed 3.3 μm 2 , 5 μm 2 and 7 μm 2 mode sizes, and the propagation losses of them, which were obtained using the low-finesse Fabry-Perot resonator method, were 16 dB/cm, 12 dB/cm and 11 dB/cm, respectively. The fabrication of a low loss, single mode, α phase PE waveguide and electro-optical modulator in LNOI are now in progress. The simulation and successful fabrication of a PE LNOI waveguide confirmed that these two mature techniques (PE and LNOI fabrication) are compatible. This could lead to the realization and use of more advanced and complicated photonic integrated devices and circuits, based on PE LNOI, in the future.