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In this work, optical properties of epitaxial CaxBa1-xNb2O6, CBN (x = 0.28) thin film based waveguides are studied at 1550 nm optical communications wavelength. CBN thin films are deposited epitaxially on MgO substrates using Pulsed Laser Deposition and characterized by prism coupling to extract the refractive index and propagation loss. It is shown that the 2 µm-thick epitaxial CBN thin films have a refractive index close to the bulk form and the CBN planar waveguides have a propagation loss of 4.3 ± 0.5 dB/cm. 1 cm-long rib waveguide structures were fabricated using a high density plasma etching. Their propagation losses were measured by the cutback method at 8.4 ± 0.6 dB/cm.
© 2016 Optical Society of America
1. Introduction
Integrated electro-optic (EO) devices are getting more and more imperative to sustain the constant needs in higher data transfer for the telecommunications. If most of the work in this field has been conducted to develop EO devices based on lithium niobate (LiNbO3, LN) in the form of bulk crystals [1] or thin films [2], the research community has been interested in the use of ferroelectric materials due to their unique ability to tune efficiently some of their physical properties (polarization, refractive index, dielectric constant, etc.) using an external electric field. In particular, some ferroelectric materials have shown very high EO coefficients, that have been used to make high performance EO devices [3,4]. Amongst these materials, the Tetragonal Tungsten Bronze (TTB) family has shown the highest EO coefficient in both bulk (1340 pm / V) and thin film form (844 pm/V) for SrxBa1-xNb2O6 (SBN) [5]. This is more than 40 times higher than that of conventional LN, which reveals a potential to build more compact and less energy consuming EO devices. However, thermal stability of SBN is quite low with a Curie temperature below 80°C [6] which is unfavorable to be used as the active layer for electro-optic modulators operating at hundreds of GHz, where the temperature can rise up above 100°C due to the high density of optical data transmitted [7]. CaxBa1-xNb2O6 (CBN) is a TTB material where both high EO coefficient and thermal stability have been confirmed. Its Curie temperature is approximately 200°C higher than SBN [8], and the measured EO coefficient of 130 pm / V has been reported in the thin film form [9], which is more than 4 times higher than LN. In an effort to develop CBN thin film based EO devices, epitaxial nature of CBN thin films up to 0.6 µm thickness have been verified on MgO substrate using PLD technique [10].
CBN thin film (up to 2.5 µm thickness) based optical waveguides were demonstrated on SiO2 / Si substrates [11,12]. However, those thin films were polycrystalline with random crystalline orientation thus showed deteriorated EO properties. Moreover, the waveguides were fabricated without patterning the thin film, while direct etching of the CBN thin film improves the mode confinement in the structures. Finally, those previously fabricated waveguides were also just millimeter-sized long when conventional EO devices based on ferroelectrics are usually up to a few cm in length [13,14].
To overcome those problems, recently we reported the design, simulation and optimization of CBN thin film based rib waveguide structure [15]. In the present work, in order to overcome the limitations of the previous waveguides, the deposition of epitaxial CBN thin films up to 2 µm thickness on MgO substrates is conducted and fabrication of one inch-long CBN rib waveguide by plasma etching is detailed. Finally, optical characterizations are done for both planar and rib waveguides to resolve the origin of the loss mechanisms in CBN-based optical waveguides.
2. Material growth and characterization
2.1 Growth conditions
CBN thin films were grown by Pulsed Laser Deposition (PLD) using a stoichiometric ceramic target (3 inches in diameter). Ablation was done using a KrF excimer laser (λ = 248 nm, pulse duration = 20 ns) in a vacuum chamber. Repetition rate was 20 Hz and the fluence was set at 2 J / cm2. Deposition pressure is 1 mTorr O2. The chamber was pumped below 10−6 mTorr before inserting the O2 gas. The substrates were mounted on a rotating substrate holder heated at 800 °C. Detailed deposition conditions are presented in [10]. CBN thin films were deposited on (001) MgO substrates of dimensions 1 inch x 1 inch x 500 µm. In order to ensure thickness homogeneity on the 1 inch2 substrates, the substrates and target were rotating during deposition. Also, the laser beam was deflected dynamically during deposition in order to do a raster scanning on the target. Scanning speed was controlled using the manufacturer software and was optimized to get 8% thickness homogeneity on the substrates. The PLD system is also adjusted so that the ablation area stays the same during laser deflection, which is required to keep the same fluence. Following the deposition, the samples were annealed in situ at 800 °C in 300 mTorr O2 pressure to reduce the oxygen vacancies and thus improve the transparency of the CBN thin films.
2.2 Structural characterization
Evaluation of the crystalline structure of the CBN thin films was done using X-Ray Diffraction (XRD). Thickness of the deposited thin films was verified by prism coupling technique. Figure 1(a)-1(c) shows the out-of-plane XRD pattern of CBN thin films with thicknesses 2, 1.5 and 1 µm respectively deposited on MgO.
Fig. 1 XRD out-of-plane θ-2θ pattern of CBN thin film deposited on MgO with (a) 2 µm, (b) 1.5 µm and (c) 1 µm thickness. (d) Typical in-plane Φ-scan of the oblique (311) plan of CBN / MgO, (e) Rocking Curve measurement of the (001) family of planes of CBN. The red line represents the fitting used to determine the FWHM.
Only the CBN (00l) family of planes is visible on the XRD patterns regardless of the deposited thickness. In order to further confirm the epitaxy, on-axis (Phi scan) measurements were conducted. For this purpose, the (311) planes of the CBN thin film, having an angle of 45.15° with the (001) planes were scanned. As can be observed on Fig. 1. (d), twenty peaks are found in the pattern. Four of them, that are denoted by stars, represent the position of the (110) plane of the MgO substrate and the other ones, denoted by squares, are for the (311) planes of the CBN thin film. Plain squares represent the −30.5° orientation around the MgO lattice and hollow squares are for the mirror + 30.5° orientation. These results are consistent with the previously reported epitaxial orientation of CBN on MgO substrate [10], thus confirming the epitaxial nature of our deposited samples with thicknesses up to 2 µm. The quality of the growth is further shown by Fig. 1(e) Rocking Curve measurement around the (001) peak. The FWHM is 0.15° which reflects a low level of mosaicity of the plans.
In order to confirm the quality of our thin films, a Selected Area Electron Diffraction (SAED) was performed on one of our samples in a Transmission Electron Microscope (TEM). The results are presented in Fig. 2. The experiment was realized as follows. A 15 x 10 micron thin section of a CBN specimen was prepared by focused ion beam (FIB) for TEM investigation. The microstructural observations were made with a JEOL JEM-2100F TEM operated at 200 kV. A double tilting specimen holder was used to tilt the specimen to acquire diffraction patterns of both the MgO substrate (Fig. 2(a)) and the CBN thin film along the MgO [001] zone axis (Fig. 2(b)). The diffraction pattern shows the MgO [001] zone axis and g vectors g200 and g020. The g vector g001 of CBN, which is the growth direction of the CBN thin film is parallel to the g vector g200 of the MgO substrate. There is a clear crystallographic relationship between the CBN thin film and the MgO substrate, which suggests an epitaxial orientation of CBN on MgO. The selected area aperture used to acquire this diffraction pattern is about 1 micron in diameter and spans many crystals of the CBN thin film. The bright field images show the columnar structure of the CBN thin film, but all the grains are oriented in the same direction. The streaks observed in the diffraction pattern parallel to the g020 MgO vector may be attributed to multiple diffraction.
Since we previously reported epitaxial CBN thin-film with high EO coefficient [9], the deposited epitaxial CBN samples on MgO substrate is also expected to achieve similar EO performance.
We have performed Scanning Electron Microscopy (SEM) to show the surface quality of the samples as shown in Fig. 3. It can be seen that the surface is smooth over a few tens of micrometers. We also can see the presence of a few particles with a small size (typically lower than 1 micron). These particles are typical of PLD and represent one of its drawbacks. However their density is very small, we can count only three particles over the surface of the image which is around 130 µm2. In order to confirm this good surface quality of our samples, the surface roughness was measured using Atomic Force Microscopy (AFM, not shown here) and found at 5.3 nm which is slightly lower than what was previously reported on epitaxial CBN growth by PLD [10].
3. Optical properties of CBN planar waveguides
Optical mode coupling to the epitaxial CBN / MgO thin film planar waveguide structure and propagation loss measurement were performed using a prism coupler (Metricon Model 2010) with laser wavelength (λ) of 1550 nm. In order to couple the laser beam to the guided modes of the planar waveguide, the sample was brought into contact with the base of a rutile prism (no = 2.861). By measuring the reflected intensity as a function of incident angle, the guided mode spectrum of the waveguide is mapped out. The fall-offs in the reflectivity curve at certain incident angles provides corresponding excitation of the guided modes. Further details about prism coupling technique and mode calculation can be found in [16]. The optical axis of the CBN / MgO epitaxial sample is parallel to the surface so the ordinary guided modes were studied by using TE polarized light. Figure 4 shows the measured guided TE mode spectrum in CBN / MgO thin film planar waveguides with (a) 2 µm, (b) 1.5 µm and (c) 1 µm film thicknesses.
Fig. 4 Reflected intensity from prism coupling measurement of a) 2 µm, b) 1.5µm and c) 1 µm thick epitaxial CBN thin films deposited on MgO, the arrows represent the substrate cut-off angle.
The sharp dips in reflectivity of each guided mode indicate that the coupled light is well confined within the planar waveguide. At the beginning of the spectra, a sharp fall of the reflected intensity is observed (noted by vertical arrows), which is associated with the MgO substrate. Using the dispersion equation of the CBN / MgO structure together with the angular position of the observed modes, the ordinary refractive index of the material is calculated as 2.221, 2.219 and 2.224 for 1, 1.5 and 2 µm CBN film thicknesses, respectively. It is similar to previously reported values for CBN thin film (2.244 at 1550 nm) [10] as well as remains constant as a function of the thickness. Hence, the refractive index of CBN thin film at 1550 nm is significantly higher than the MgO (1.715). It can be also seen that the measured refractive index of CBN thin film is close to that of bulk (2.262 [17]) and remains constant for all the measured thicknesses suggesting a reliable and controlled deposition of this new material.
The propagation loss of planar waveguide is measured by scanning a fiber optic probe connected to a photodetector down the length of the waveguide to detect the light intensity scattered from the surface. The scanning fiber method is based on the assumption that the measured scattered light intensity is directly proportional to the intensity of the propagating modes in the waveguide. This assumption is reasonably acceptable in the absence of specific irregularities on the surface of the waveguides. Hence, a decrease in the measured light intensity as a function of waveguide position is representative of an attenuation of a corresponding propagating optical mode in the waveguide. Figure 5 shows the measured light intensity as a function of the scanning position in a 2 µm-thick CBN thin film planar waveguide. It can be seen that in the intensity curve there are two regions of roughly equal length. In the left region (marked as (1)) the intensity drops faster than in the right region (marked as (2)). We used region (2) because in region (1) we get lots of extra light scattered from the prism dying away quickly over the first couple of mm (region (1)). Using a logarithmic fitting on region (2) (as shown in the inset of Fig. 5(a)) the propagation loss of the planar waveguide is found to be 4.3 ± 0.5 dB / cm. An average of multiple measurements across the surface of the thin film provides a reliable estimation of waveguide propagation loss. Figure 5(b) shows the measured propagation loss in epitaxial CBN / MgO thin film planar waveguides as a function of CBN film thickness. The losses are decreasing slightly with increasing waveguide thickness. It may be due to a decrease in scattering loss as reported in the literature [18]. However, the overall loss in the planar waveguide is higher than that of some previously reported CBN thin film waveguides [11] and some other conventional materials such as implanted LiNbO3 planar waveguide [19]. It can be due to the lower dimension of our planar waveguides which can significantly increase the propagation loss as described in [18]. Moreover, PLD is a deposition technique that creates particles on the surface which in turn increase the surface roughness as it has been measured by atomic force microscopy to be below 6 nm rms in our samples. Therefore, another deposition technique such as radiofrequency sputtering [20] that minimizes surface roughness will improve the losses in CBN waveguides. In addition, the light collected at the output of the sample (shown by an arrow in Fig. 5(a) demonstrates the ability to guide light efficiently in over 1.4 cm epitaxial CBN thin film planar waveguides on MgO. The 2 µm-thick CBN thin films are used in the fabrication of rib waveguide structure as they show lower propagation loss, and also end-fire coupling in thicker films is more efficient.
Fig. 5 (a) Scattered light from the top of the CBN sample measured by fiber scanning, inset: zoom on the linear region, suitable for propagation loss measurement, (b) Measured propagation loss as a function of the CBN thickness.
4. CBN rib waveguides
4.1 Fabrication
Following the deposition of a 2 µm thick epitaxial CBN thin film on MgO, a 150 nm thick tungsten seed layer was sputtered on the sample. A 300 nm thick nickel hard mask was then electroplated through a UV-patterned photoresist (Shipley S1813) by using a nickel sulfamate solution. After removal of the photoresist and plasma etching of the tungsten seed layer, the CBN thin film was etched using a high density plasma etcher, PlasmaLab ICP 380 system 100 from Oxford Instruments. The sample was etched 200 nm-deep with a substrate temperature of 275°C using a Cl2 plasma operated at 1 mTorr, 1000W ICP power and 200 V bias. More details about plasma etching of CBN using the self-regenerating Ni mask can be found in [21,22]. The nickel mask was then removed by wet etching of the tungsten seed layer using the standard RCA-1 clean process. The width of the fabricated waveguides is 4 µm as it was well-suited for single mode propagation [15]. A schematic of the waveguide structure and cross-section Scanning Electron Microscope (SEM) micrograph of the entrance facet of a fabricated waveguide is demonstrated in Fig. 6 showing the high quality etching profiles with low sidewall roughness.
Fig. 6 (a) Typical cross-section SEM image of the fabricated waveguide, (b) Schematic of the fabricated multilayer structure.
4.2 Properties of the CBN rib waveguides
The CBN rib waveguides have been characterized by end-fire coupling from an optical fiber to the sample end using the beam generated by a CW laser diode at 1550 nm. The guided light from the waveguide end-facet is extracted using a long working distance (LWD) 10x microscope objective lens. The input-output facets of the waveguides are prepared by cleaving, and the quality of the cleaved facets is verified by SEM. In order to efficiently inject light within the thin waveguide (2 μm thick CBN core), a lensed fiber (Lase Optics SMF-28 Conical 1 μm SS) with ~2 μm spot diameter have been installed at the output of the laser source to focus the laser light at the waveguide input facet. The input fiber, output objective, and the waveguide sample are separately mounted on high precision micro-positioners and stages (THOR LABS NanoMax-TS) to achieve accurate and stable alignment, where both top and front views are controlled to achieve the efficient coupling. Top-view observation is performed using an optical microscope attached to a camera mounted on top of the measuring system. The camera is computer-controlled by an image processing software. The front view is captured by focalizing the light coming from the objective to a high performance computer-controlled charge coupled device (CCD) camera (Hamamatsu C2741). The image of the propagating optical modes from the output facet of CBN / MgO rib waveguide structures is captured by an in-house developed LAB-VIEW program, and optical mode profiles are further analyzed by Matlab. Figure 7 (inset) shows the measured optical mode profiles of a 1 inch long and 4 μm wide CBN / MgO (2 μm / 500 μm thick) rib waveguide. We previously reported that the 4 μm wide and 2 μm thick CBN/MgO rib waveguides are mono-mode [15]. The high contrast between the waveguide modes and substrate modes confirm that the light is well confined within the rib and extend through the core.
Fig. 7 Total loss as a function of waveguide length to estimate the facet loss and propagation loss. (inset) Typical mode profile at the output of the waveguide.
Propagation loss of the rib waveguides is measured by using the cut-back technique. The waveguides are cleaved at different lengths and the intensity of the transmitted light is measured. To measure the intensity of the light, CCD is replaced by an InGaAs optical detector (Newport 818-IS-1). The sample was cleaved several times to obtain various lengths from 0.2 to 1.2 cm. The propagation loss value is obtained by calculating the decrease in the measured intensity versus waveguide length. An average over multiple measurements for each length was performed for more accuracy.
Figure 7 shows the measured total waveguide loss as a function of waveguide length. An extrapolation of total waveguide loss curve to zero waveguide length provides insertion/facet loss. It is measured at 12.3 dB, which is high, but consistent with the end-fire coupling technique. Hence, an alternative approach such as prism or grating coupling to improve the light coupling in CBN waveguides needs to be investigated in the future. A linear fit of the total loss curve (regression coefficient of 0.99) in Fig. 7 provides a propagation loss value of 8.4 ± 0.6 dB/cm, which is higher than that of planar waveguide. This additional loss can be due to some process-related loss (roughness) or the design of the structure. The measured loss of the waveguides in this work is also higher than the previously fabricated CBN thin film waveguides. Our group has reported between 3 dB / cm [11] and 6.5 dB / cm [12] in the past using strip loaded or stripe CBN waveguide that were fabricated without any etching of the thin film itself. However, if the optical losses were lower than that of this work, the thin films were polycristalline, thus with low electro-optic properties (between 12 and 15 pm / V). Epitaxial CBN has an electro-optic coefficient of about 130 pm / V [9] and has shown recently promising non-linear properties [23]. As the deposition technique is the same as what we have published previously and the quality of the thin films identical, we expect our waveguide to show both high EO coefficient and interesting non-linear properties despite higher propagation loss. We also note that this loss is also significantly higher than that of conventional EO channel LiNbO3 based waveguides [24]. The higher loss of CBN/MgO thin film waveguides can be improved further by using other deposition techniques, by employing other waveguide design structures and by reducing the process related losses, which are subject to further study. However, the propagation of light through the 1 cm long epitaxial CBN/MgO rib waveguide structure with moderate propagation loss is an important step towards the fabrication of EO devices based on this material.
5. Conclusion
In summary, we have studied the optical properties of epitaxial CBN thin films and waveguides. In particular, we have shown bulk-like refractive index in CBN thin films for thicknesses up to 2 µm and propagation loss of 4.3 ± 0.5 dB / cm in planar waveguides. There is no evidence of degradation of optical material quality in CBN thin film regardless of the thickness. We also demonstrated the fabrication of 1 cm-long rib waveguides with moderate propagation loss. These results represent an important step towards the fabrication of high performance and high speed EO or non-linear optical devices based on epitaxial CBN thin film material for future telecommunication systems.
Funding
Natural Sciences and Engineering Research Council of Canada (NSERC) Strategic projects program.
Acknowledgements
The authors wish to thank M. Masse for the TEM measurement.
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