1. Introduction

Potassium tantalate-niobate (KTa_1-xNb_xO₃ with 0 ≤ x ≤1, KTN) is a highly promising material for photonics because of the exceptionally large values for its relative permittivity (larger than 50000) [1] as well as the Pockels (up to 10⁴ pm/V) [2] and Kerr electro-optic (EO) coefficients (> 10⁻¹⁴ m²/V²) [3], occurring in proximately to a ferroelectric-paraelectric (tetragonal-cubic) phase transition, making the KTN crystal family highly promising for electrically tunable optical devices such as high-speed beam scanners and EO modulators [1–9]. Furthermore, KTN is also very attractive for photorefractive photonics, electrocalorics and pyroelectrics [10,11]. The KTN crystals are compatible solid solutions of potassium tantalate (KTaO₃) and potassium niobate (KNbO₃). Since these crystals have almost identical cubic-phase unit cell sizes but very different Curie temperatures, the phase-transition temperatures of the solid solution and also its main properties at a given temperature can be controlled by adjusting the Ta/Nb ratios of a KTN crystal [3–5]. For example, temperature-dependent EO coefficients have been demonstrated with abrupt changes (Kerr coefficient s₁₁ increases > 20 times) appearing around the Curie point of KTN [3]. Fortunately, the Curie temperature can be adjusted to be close to room temperature, which makes it easier to access the beneficial optical material properties on a device level [1]. Despite the outstanding material properties, several of them known since decades, KTN is still rarely employed in particular in its ferroelectric phase. This stems from the fact that high-quality and sizable KTN single crystals are difficult to produce because the material does not melt congruently, i.e. the composition of the crystals grown is different from that of the molten ingredients [3]. However, replacing single crystals by KTN thin films would very well address the bulk-homogeneity problem and offer plenty of opportunities for integrated nonlinear optics.

Pulsed laser deposition (PLD) is a simple and versatile technique for fabricating multifunctional thin film devices. One of the most important and enabling characteristics in PLD is the ability to realize stoichiometric transfer of ablated material from targets for many materials [12]. This feature may enable the growth of high-quality KTN films with variable and precisely controlled composition. In fact, PLD-grown paraelectric and ferroelectric KTN films have been realized on different substrates since pioneering work in 1991 [13–16]. However, to obtain better nonlinear optical performance in ferroelectric films and to use such films as efficient optical waveguides for integrated optics, single-domain films and low-loss waveguiding are mandatory. Yet, so far, reports on such high-quality KTN optical films are still lacking. The only reported waveguide loss of such films was estimated to be as large as −33 dB/cm at 632.8 nm [14]. This high optical propagation loss might be attributed to an imperfect target and insufficient optimization of the deposition parameters and finishing process, resulting in high scattering losses caused by a large quantity of micron-size particulates present on the film surfaces and embedded in the guiding layers [12].

In this work, ferroelectric domain structures and crystalline properties of PLD-grown KTa_0.5Nb_0.5O₃ thin films on MgO(001) and KTaO₃(001) substrates are analyzed and compared, demonstrating, to the best of our knowledge, the first PLD-grown epitaxial KTN films with preferred orientation and consistent polarization. Furthermore, the linear optical and waveguiding properties of KTa_0.5Nb_0.5O₃/MgO films are also investigated. The waveguide loss is determined to be as low as −3.5 ± 0.5 dB/cm, which is, compared to the previously reported results, a huge step forward in pursuing of practical applications of KTN films in nonlinear integrated optics.

2. Material growth and structural characterizations

2.1 Thin film preparation

About 1-μm-thick KTN films were grown by employing a KrF excimer laser (λ = 248 nm, pulse duration of 20 ns) operating at a repetition rate of 2 Hz with a fluence of 2 J/cm². The laser beam for ablation, incident at 45°, was focused on a rotating target located in a standard vacuum chamber (background pressure of 2 × 10⁻⁶ mbar). The home-made KTa_0.5Nb_0.5O₃ ceramic targets were prepared according to the recipe from literature [15]. The main characteristic of this procedure is that KTaO₃ and KNbO₃ are prepared separately by a solid-state reaction and then mixed to provide the desired composition of KTN. A potassium excess of 50 atomic% was added in the form of KNO₃ to compensate for the well-known deficit of potassium in deposited films [13]. The composition x = 0.5 was chosen because its corresponding Curie temperature (T_c ≈100 °C according to the formula T_c[K] = 676x + 32, for x ≥ 4.5%) [3–5] ensures that the crystalline structure of KTN (tetragonal) stays in the ferroelectric phase at room temperature, which could be used to achieve χ⁽²⁾-nonlinear-optical effects. During the deposition, the KTN thin films were grown at a molecular oxygen atmosphere with a pressure of 0.15 mbar on MgO(001) and KTaO₃(001) single crystals (dimensions of 10 × 10 × 0.5 mm³), respectively, with a substrate rotation speed of 10 rpm. The substrates were placed opposite to the target at a distance of 60 mm and heated from the backside up to a temperature of 700 °C. The film growth rate was around 6.7 nm/min. In order to further smoothen the as-deposited KTN films, i.e. to lower the scattering losses and enhance the optical properties, in situ thermal post-annealing treatment at 700 °C was carried out at an oxygen pressure of 0.3 mbar for one hour following the deposition. The KTN films were then slowly cooled down to room temperature at a rate of 2 °C/min to prevent cracking. A subsequent chemo-mechanical fine-polishing procedure employing a Logitech PM2 polishing machine and alkaline polishing suspension with 60 nm and 20 nm SiO₂ particles (MasterMet1 and MasterMet2 from Buehler) was also carried out before the optical characterization. As a result, with the help of the optimization and finishing treatments, the RMS surface roughness of the KTN films prepared in this work was reduced from around 10 nm to optically smooth grade of only 1 nm, as measured by atomic force microscopy (AFM), which is the lowest value reported so far for PLD-grown KTN thin films [17]. The thickness values (1 ± 0.1 μm) of the polished KTN thin films were determined by a prism coupler (Metricon Model 2010) and confirmed via SEM analysis of the film end face.

2.2 Structural analysis

The crystallographic orientation and phase composition of KTN films were evaluated through high-resolution X-ray diffractometry (HRXRD) using Cu Kα₁ radiation. The as-deposited KTN films on both substrates MgO(001) and KTaO₃(001) are completely (001)-oriented, as shown by the θ-2θ XRD patterns in Fig. 1(a), exhibiting only 00l peaks of KTN, referring to the pseudocubic subcell. Rotationally-ordered growth of the films could be confirmed by a φ-scan XRD analysis performed on the 111 reflection of KTN [17]. These results reveal high crystalline qualities and epitaxial growth. While the KTN layer is relatively relaxed in case of the MgO substrate, it is pseudomorphically strained when using the KTaO₃ substrate, as indicated by the HRXRD reciprocal space maps (RSM) in Fig. 1(b). This difference in crystalline quality can be related to the difference in lattice mismatches between KTN and the substrates (about 5% for MgO to be compared to less than 0.3% for KTaO₃, as also indicated by the separations between KTN 00l reflexes and that of substrates), resulting in some large-island-growth features with larger grain boundaries in the KTN/MgO case, as shown in Fig. 2(a), with respect to KTN/KTaO₃ (Fig. 2(b)). The crystal growth of KTN/MgO corresponds to a typical Stranski-Krastanov growth mode. The peak corresponding to KTN on MgO has a noticably smaller peak intensity and is also broader, generally indicating lower crystal quality. This corresponds to crystal defects (e.g. dislocations) that are associated with pronounced island formation due to the large lattice mismatch of KTN and MgO. In spite of this mismatch, KTN/MgO thin films are still very interesting, because the low refractive index of MgO allows KTN films to be used as efficient optical waveguides and integrated optical devices, as it will be discussed in the following section. In contrast, this would be very difficult when using the high-refractive-index KTaO₃ substrate.

 

Fig. 1 (a) θ-2θ scan XRD patterns of (001)-oriented KTa_0.5Nb_0.5O₃ thin films. (b) HRXRD reciprocal space maps of (420) reflex of KTN films grown on MgO(001) and KTaO₃(001) substrates.

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Fig. 2 SEM images of as-deposited KTN thin films on (a) MgO and (b) KTaO₃ substrates. Piezoresponse force microscopy (PFM) images of (c), (d) piezoelectric amplitude and (e), (f) piezoelectric phase for polished thin films.

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In Figs. 2(a) and 2(b), apart from the differences in grain sizes and boundaries, both KTN/MgO and KTN/KTaO₃ films exhibit crack-free, dense and well-organized in-plane microstructures, suggesting epitaxial-like growth. The surface of as-deposited films on KTaO₃ is much smoother than that of KTN on MgO substrates. The RMS surface roughness is about 8 nm for KTN/KTaO₃, to be compared to >10 nm for KTN/MgO before polishing, as measured by AFM. Moreover, energy dispersive X-ray spectroscopy (EDX) analysis performed on films confirms the K/(Nb + Ta) ratio is 0.95 ± 0.05 and that the Nb/Ta ratio in films is the same as that in the starting targets, indicating good stoichiometry transfer between the target and films.

The piezoelectric phase polarity distribution and piezoelectric activity of KTN thin films on nano-scale was evaluated using piezoresponse force microscopy (PFM) with applied ac voltage of 10 V at 12 kHz drive frequency through a conductive tip in AFM operating in contact mode, as shown in Figs. 2(c)-(f). Square-shaped grains with no large variation in surface flatness were detected in the piezoelectric amplitude measurement due to very low piezoelectric activity at the grain boundaries (Fig. 2(c)). The same small grains showed instability in piezoelectric phase signal that could be interpreted as inversion domains (bright areas in Fig. 2(e)). In contrast, the piezoelectric properties of KTN/KTaO₃ films are very homogeneous as evidenced by the uniform piezoelectric amplitude signal (Fig. 2(d)) and piezoelectric phase distribution (Fig. 2(f)). The latter is very well aligned, indicating homogeneous polarization phase. And this, to the best of our knowledge, is the first demonstration of PLD-grown epitaxial KTN films with such high quality. Furthermore, very little activity in lateral PFM results confirms the KTN films are preferentially c-axis oriented. The difference in the PFM results of these two KTN films are most likely again related to the different lattice mismatches, as discussed above for the XRD results. Regardless of the grain features in KTN/MgO films, the piezoelectric polarity in most areas is almost in the same direction as that of KTN/KTaO₃, i.e. c-axis orientation is dominating. Thus KTN/MgO films are also attractive for second-order nonlinear optical devices, despite the fact that they are much more likely to possess additional scattering losses from grain boundaries.

3. Optical characterization

The KTaO₃ substrate has a high refractive index and an optical band gap very close to that of KTN, therefore it is reasonable to focus optical characterization on KTN/MgO films. A KTa_0.57Nb_0.43O₃ single crystal chip (4.0 × 3.2 × 1.2 mm³), available from NTT-AT, was used as a bulk reference sample. The linear optical transmission spectra of the KTN/MgO films and KTN bulk material were studied by using an UV-Vis-NIR spectrophotometer (Cary 5000), as shown in Fig. 3(a). The transmittance of PLD-grown KTN films and bulk KTN at 400-3000 nm wavelength region are almost the same, if one considers that the transmission oscillations are obviously a result of interference between the air-film and film-substrate interfaces, which is also another evidence of the good homogeneity and surface smoothness of the polished KTN films. From this interference effect, the refractive index dispersion (n_o) of KTN in the wavelength range 400-2000 nm was calculated using the Swanepoel envelope method [18]. The result is shown in Fig. 3(b). This dispersion curve is in fair agreement with that of KTN bulk crystals, as calculated based on Cauchy’s equation of KTN given in Ref [19]. Employing a prism coupler at 543.5, 594, and 632.8 nm, we got additional experimental data of the refractive indices of the KTN/MgO films, being in good agreement with the other data. Furthermore, the extraordinary refractive indices (n_e) were also measured with the prism coupler, giving a birefringence of ∆n = −0.045. This also directly confirms the tetragonal crystal structure (ferroelectric) of the KTN grown on MgO.

 

Fig. 3 (a) Transmission spectra and (b) refractive index profiles of KTa_0.5Nb_0.5O₃/MgO film and KTa_0.57Nb_0.43O₃ bulk single crystal (NTT-AT, Japan). The dispersion of the refractive indices was fitted with an one-pole Sellmeier equation including a quadratic IR correction term in the form of (n_o)² = A + B/(λ² – C) – Dλ².

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Furthermore, the waveguiding properties of KTN/MgO films including the waveguide losses and the propagation mode profiles were characterized by using a standard end-face coupling system, which is composed of a microscope objective to couple a polarized HeNe laser beam at 632.8 nm into the thin films, KTN samples (after end-face polishing) mounted on a three-dimensional optical stage, and another microscope objective for collecting the output light. A CCD camera was employed to image the waveguide modes. Assuming a graded refractive index profile of the KTN layer and taking account the refractive index values measured by the prism coupler, the refractive index profile could be reconstructed [20], as illustrated in Fig. 4(a). Then, based on this refractive index profile, the simulated modal distribution (Fig. 4(b)) of the waveguide was obtained by utilizing the finite-difference beam propagation (FD-BPM) method (Rsoft BeamProp 8.0) [21], which is in fair agreement with the experimental results. The propagation losses were determined to be −3.5 ± 0.5 and −6.5 ± 0.5 dB/cm at TM (guided wave) and TE (unguided wave) polarizations, respectively, through comparing the measured optical powers at the waveguide input and output facets (insertion/total losses are −10 ± 0.5 and −13 ± 0.5 dB for TM and TE polarizations, respectively) and cancelling out the input coupling loss (around −4.5 ± 0.5 dB) and Fresnel reflection losses (around −1.2 dB at each waveguide-air interface). The coupling loss was estimated based on the FD-BPM simulation by taking into account the modal mismatch between the launched Gaussian laser mode and the propagation modal profile in the waveguide [22]. Additionally, we also investigated the guiding properties at 1.5 μm by replacing the He-Ne laser with a fiber laser. However, no valid guiding modes were found at such wavelength, which could be attributed to the longer wavelength resulting in much lower quantity of guiding modes. Even though the waveguide loss values in this work are still higher than that of KTN bulk as a result of the high scattering losses originated from grain boundaries, the data obtained is an order-of-magnitude improvement of the optical quality of KTN layers reported in earlier studies [14]. This reveals the great potential of PLD for the growth of high-quality ferroelectric KTN films suitable for various applications, in particular in integrated nonlinear optics.

 

Fig. 4 (a) Reconstructed refractive index profile of PLD-grown KTN/MgO thin films. (b) Experimental and simulated results of near-field intensity distributions of transmitted guided waves at 632.8 nm.

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4. Summary

High-optical-quality KTa_0.5Nb_0.5O₃ thin films in the ferroelectric phase were grown by using pulsed laser deposition (PLD) with optimized deposition parameters and subsequent fine polishing. The successful formation of preferred orientation and consistency polarization are supported by piezoresponse force microscopy (PFM) analysis, and we believe this is the first report of a PLD-grown KTN thin film achieving this. Furthermore, we have shown bulk-like optical transmittance and refractive indices in 1-μm-thick KTN/MgO layers, which exhibit a propagation loss as low as −3.5 ± 0.5 dB/cm. The present investigation suggests that PLD-grown KTN thin films can be employed in integrated nonlinear optics, allowing KTN waveguide-based electro-optics modulators, nonlinear-optical frequency converters, and on-chip micro-resonators. Moreover, KTN thin films with even lower waveguide losses could be realized by employing a low-refractive-index substrate material with smaller lattice mismatch, as suggested by the results of KTN/KTaO₃ films in this work, or by material modification techniques such as ion implantation to construct a low-refractive-index boundary within the KTN layer.