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Abstract
Periodically poled lithium tantalate on insulator (PPLTOI) was successfully fabricated by applying a high-voltage electric field. The shape of the electrode, which determines the electric field distribution, as well as the poling time, and the strength of the electric field, are investigated in detail for the fabrication of periodically poled LTOI. By optimizing the poling parameters, the duty cycle of the inverted domain can be flexibly adjusted as well as be controlled to the optimal value of 50%. Moreover, the fabricated domain structure is uniform, and the standard deviation is less than 4.8%. The study presented in this work will pave the way for applications of LTOI in nonlinear integrated photonics.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, there has been a growing interest in thin-film ferroelectric materials for integrated photonics applications. One such material is lithium niobate on insulator (LNOI), which has garnered significant attention [1–5]. Due to the excellent material properties inherited from single-crystal lithium niobate (LiNbO3, LN), as well as the strong light confinement in the nanoscale waveguides, LNOI has found widespread applications in high-speed electro-optic modulators, acousto-optic devices and ultra-efficient nonlinear frequency converters [6–10]. As is well known, lithium tantalate (LiTaO3, LT), which belongs to the same family as LN, has also attracted considerable interest. Single-crystal lithium tantalate exhibits two distinct characteristics compared with LN: a higher optical damage threshold (240 MW/cm2) and a shorter wavelength ultra-violet (UV) absorption edge (280 nm) than that of LN (350 nm) [11–13]. More specifically, thin-film lithium tantalate possesses higher laser-induced surface damage threshold and photorefractive damage threshold [14]. These exceptional properties make lithium tantalate on insulator (LTOI) a promising material platform for high-power and short-wavelength integrated photonic devices.
Recently, LTOI microdisks have been fabricated for on-chip nonlinear frequency conversions, and own to the material properties of LTOI, these devices exhibit high optical damage threshold and short-wavelength operation down to UV [15–17]. In such microdisks, second harmonic generation (SHG) and third harmonic generation (THG) are realized based on the modal phase matching technique. To obtain more efficient and flexible nonlinear frequency conversions, quasi-phase matching based on ferroelectric domain engineering are preferred. Up to now, realization of bulk periodically poled lithium tantalate (PPLT) has already been thoroughly investigated [18–20]. However, the distinction in periodic poling between bulk and thin-film ferroelectric materials can be attributed to two key factors. Firstly, the thin-film LiTaO3 is bonded to the SiO2 layer, and the strain and stress within the thin film arising from the interfaces can lead to a higher coercive field compared to that of the bulk materials [21]. Secondly, the submicron thickness of the thin-film LT/LN will reduce the domain aspect ratio in comparison to bulk materials [22]. To the best of our knowledge, the fabrication of periodically poled LTOI by applying high-voltage electric field has not been reported.
In this study, we utilize the electric field poling technique to create periodic domain structures on an LTOI platform. Our aim is to produce high-quality inverted domains with a controlled duty cycle, a feature that finds extensive application in frequency conversion on the LTOI platform. We conducted simulations to understand the electric field distributions of two distinct poling electrode shapes. Subsequently, we analyzed the impact of these varied field distributions on domain inversion, referencing the Miller-Weinreich model. This analysis furnishes a crucial theoretical foundation for designing an optimal electrode shape to facilitate domain inversion experimentally. We explored the characteristics of domain inversion under various electric field strengths and durations using needle-shaped nickel electrodes on the LTOI in our poling experiment. The inverted domains were confirmed and characterized using confocal second-harmonic (SH) microscopy and piezoresponse-force microscopy (PFM). By selecting suitable poling parameters, we successfully fabricated domain structures in LTOI with diverse poling periods and controllable duty cycles.
2. Design and fabrication
The morphology of the inverted domains in LTOI using the EFP technique is highly influenced by the distribution of the applied external electric field. The shape of the electrode plays a crucial role in determining the electric field distribution. In this study, we simulate the electric field distributions for two type of commonly-used electrode shapes: comb-shaped and needle-shaped electrodes, which are shown in Figs. 1(a) and 1(b). For the simulations, we use an x-cut LTOI with a film thickness of 600 nm and the dielectric constant is set to be 40 [23]. The comb-shaped electrode has a thickness of 0.1 µm and a width of 0.6 µm respectively, while the spacing between the positive and negative electrodes is 8 µm. The needle-shaped electrode consists of a rectangular portion with a length of 20 µm and a triangular portion with a height of 5 µm. The total length of the needle-shaped electrode is 25 µm. Due to the fact that there is a considerable difference in periodic poling between bulk and thin-film ferroelectric materials, we set the electric field strength to 65 kV/mm to achieve domain inversion, which is approximately 3 times the coercive field of bulk LT. Figures 1(c) and 1(d) display the x-z and x-y cross-section profiles of the electric field (Ex) formed by the comb-shaped electrode, respectively. Figures 1(e) and 1(f) illustrate the electric field distributions formed by the needle-shaped electrodes. It is evident that the two types of electrode shapes generate distinct electric field distributions. These simulations provide important information on how different electrode shapes influence the electric field distribution, which will in turn affect the morphology of the inverted domains in LTOI.
Fig. 1. Simulation electric field distribution corresponding to different electrode shapes. (a) the comb-shaped electrode; (b) the needle-shaped electrode; (c) and (d) the x-z and x-y cross-section profiles of the electric field formed by the comb-shaped electrode; (e) and (f) the x-z and x-y cross-section profiles of the electric field formed by the needle-shaped electrode.
In order to further analyze the differences in the electric-field distributions generated by the comb-shaped and needle-shaped electrodes, numerical analysis is conducted on the profiles of the electric fields viewing along different directions. Figure 2(a) shows the normalized area occupied by the electric field strength of the needle-shaped electrode compared to that of the comb-shaped electrode along the z direction near the center region of a pair of positive and negative electrodes. It can be seen that the normalized area occupied by the electric field strength of the needle-shaped electrode is approximately 1/22 of that of the comb-shaped electrode. Figure 2(b) illustrates the electric field distribution along the y direction, in which the normalized area occupied by the electric field strength of the needle-shaped electrode is approximately 1/12 of that of the comb-shaped electrode. From the above, we can see that the average electric field intensity of the needle-shaped electrode is significantly lower than that of the comb-shaped electrode. The inset in Fig. 2(b) illustrates the domain wall motion is governed by the average electric field over the y-z cross-section of the domain according to the Miller-Weinreich model [24,25]. The velocity of the domain wall growth in the x-cut LTOI can be decomposed into vy and vz in the y-z plane. For LN/LT family ferroelectric crystals, vz is much higher than vy, and vy dominates the duty cycle of the inverted domains. According to the Ref. [26], domain duty cycle D(t) is a function of the poling time t, and vy increases exponentially with the change of the average electric field, which leads to an exponential increase in the duty cycle of the inverted domains. To precisely control the duty cycle of the inverted domain, it is crucial to choose a proper electrode shape. In this study, the needle-shaped electrode is chosen in order to achieve better control over the domain inversion process.
Fig. 2. Normalized electric field distribution of two electric field profiles along the different directions. (a) the comb- and needle-shaped electrode along the z direction; (b) the comb- and needle-shaped electrode along the y direction. The inset of (b) shows the decomposition of growth velocity in the y-z plane.
To design the QPM structures, one should refer to the dispersion of the thin-film LT, and we measured the refractive index curves of ordinary (no) and extraordinary light (ne) of LTOI using a dual rotating compensator ellipsometer, as shown in Fig. 3(a). The thin-film lithium tantalate is an anisotropic crystal with different refractive indices in different directions. It can be seen from the measurement result that LTOI exhibits a low birefringence characteristic. In the near-infrared region, the refractive index difference is ∼0.01. The parameters of x-cut LTOI are set as follows: a 600-nm thick film, a 3-µm-thick SiO2 buffer layer and a 500-µm-thick Si substrate. The top width of the ridge waveguide is set to 1.5 µm and the angle of the waveguide sidewall is 60°. Based on the measured dispersion relationship, we calculated the poling periods of the PPLTOI ridge waveguides with the etching depth ranging from 150 nm to 300 nm in steps of 50 nm, as shown in Fig. 3(b). For the calculation, the fundamental wavelength varies in the range from 0.6-1.6 µm. The commonly used tele-communication E-band and C-band are included. The near-infrared 1.06 µm which can be frequency doubled for green light generation is included as well.
Fig. 3. (a) The refractive index curves of ordinary (no) and extraordinary light (ne) of LTOI; (b) the poling periods of the PPLTOI ridge waveguides with the etching depths ranging from 150 nm to 300 nm in steps of 50 nm.
The fabrication process of the needle-shaped electrode for PPLTOI involves two main steps, which are illustrated in Figs. 4(a)-(c). Firstly, nickel is evaporated onto the LTOI using electron beam evaporation (EBE) after electron beam lithography (EBL) to create needle-shaped metal electrodes. Excess metal nickel is then removed by immersing the sample in N-methyl pyrrolidone solution (NMP). In the second step, the sample coated with AZnLOF photoresist is exposed using a UV lithography machine, followed by another round of NMP solution immersion to complete the preparation of the poling electrode. The x-cut congruent thin-film LT (NanoLN) has a thickness of 600 nm, and it is placed on a silicon substrate with a 3-µm SiO2 buffer layer. The thickness of the needle-shaped metal nickel electrode deposited by EBE is approximately 100 nm, and the spacing between the positive and negative electrodes in the y-z plane is 8 µm. The width of the needle-shaped electrode is 0.6 µm, as shown in Fig. 4(d).
Fig. 4. (a)-(c) The fabrication process of the needle-shaped electrodes of the PPLTOI; (d) the specific parameters of poling needle-shaped electrodes. (e) The schematic diagram of the poling experimental device. The inset of (e) shows the typical waveforms recorded by the digital oscilloscope.
The samples with different periodic electrodes are subjected to a high-voltage (HV) electric field for domain inversion. The experimental setup for poling is depicted in Fig. 4(e). Initially, electrical pulses are generated using an arbitrary waveform generator (AWG) and then amplified by a HV amplifier with a 2000× amplification factor. The sample is positioned between two probe stations, and the poling electrode is connected to the HV probe of the probe station. A digital oscilloscope is utilized to monitor the initial electrical pulses and the displacement current. It should be noted that we monitor the sampling current only to judge whether the domain inversion occurs in the experiment. In the periodic poling process, the duty cycle of the inverted-domain is controlled by the number of the applied high voltage pulses, and is monitored with the confocal SH microscopy which can non-destructively reveal the domain structure. A sampling resistor is connected to the HV amplifier to form a circuit and complete the poling process. The waveform of the applied high-voltage used in the experiment is similar to the one reported in the literature [27]. The high-voltage pulse consists of two main parts. The first part is the poling time required to complete the domain inversion, while the second part is used to prevent the de-polarization of the inverted domains.
3. Experiment and discussion
The samples with different periodic electrodes are subjected to HV electric field in a controlled manner. To overcome the coercive field strength of LT, multiple HV pulses are applied to the needle-shaped electrodes on the x-cut LTOI. The HV pulses have a peak voltage of 600 V, a poling time of 0.4 ms, and a pulse number of 30. However, initial observations using confocal second-harmonic microscopy do not reveal inverted domain structure even after applying multiple HV pulses. To induce domain inversion, the electric field strength is gradually increased. It is found that at an electric field strength of 80 kV/mm, partially inverted domain walls start to appear. Further increasing the field strength to 84 kV/mm with the same poling time and pulse number results in the formation of inverted domain structures with an average duty cycle of about 50%. Additionally, attempts are made to vary the poling time while keeping the field strength and pulse number unchanged. Domain coalescence or incomplete domain walls are observed in these domain-inverted structures, as shown in the insets in Figs. 5(a) and 5(b). This suggests that the poling time directly affects the duty cycle of the inverted domains [26]. If the poling time is too long, domain coalescence can occur, while insufficient poling time leads to incomplete domain inversion and only partial domain walls are observed. Further investigation is conducted to study the relationship between the average duty cycle of inverted domains with a 3-µm poling period and the poling field strength. The results of the controllable duty cycles are shown in Fig. 5(a), which indicate a correlation between the duty cycle and the field strength.
Fig. 5. Average duty cycle corresponding to different poling field strength for different poling periods. The insets of (a) and (b) show a typical result of domain coalescence and incomplete domain walls.
For fabrication of the samples with the poling period of 4.0-5.0 µm, the same electric field strength of 84 kV/mm and the same pulse number of 30 are used, while the poling time is varied from 0.3 ms to 0.6 ms in step of 0.1 ms. It can be seen that only when the poling time is 0.5 ms, domain coalescence or incomplete domain walls are not observed. In order to regulate the average duty cycle, the electric field strength are adjusted from 84 kV/mm to 90 kV/mm. The variation of the average duty cycle versus the poling field strength for different poling periods of 4.0, 4.5 and 5.0 µm are shown in Fig. 5(b). When the electric field strength is 88 kV/mm, 89 kV/mm and 89 kV/mm respectively, the average duty cycles can be close to 50%.
The fabricated PPLTOI samples with the poling periods 3.0, 3.5, 4.0, 4.5 and 5.0 µm are characterized using the confocal SH microscopy, as shown in Fig. 6. The black needle-shaped area is the electrodes. The upper part is connected to the positive electrodes, and the lower part is connected to the negative electrodes. The rectangle-shaped area surrounded by black edges represents the domain-inverted region between the positive and negative needle-shaped electrodes. The white arrows in the enlarged images indicate the orientation of the domain polarization (Ps). Through optimization of the poling parameters, the duty cycle of samples with different poling periods can be close to 50%, which is the optimal value to obtain efficient frequency conversions [28]. For confocal SH characterization, the inverted domains appeared transparent, with no observable domain boundaries (dark) within the inverted-domain area, indicating a complete domain inversion in depth [29].
Fig. 6. False-color images obtained from the confocal SH microscopy depicting the domain-inverted structures with varying poling periods. The pump wavelength of the confocal SH microscopy is 800 nm. The white arrows represent the orientation of the domain polarization Ps. Scale bar: 10 µm. (a) the poling period of 3.0 µm; (b) the poling period of 3.5 µm; (c) the poling period of 4.0 µm; (d) the poling period of 4.5 µm; (e) the poling period of 5.0 µm.
To confirm the domain inversion in the PPLTOI, we use PFM to analyze the domain-inverted structures with poling periods of 3.5 µm and 4.5 µm, as shown in Figs. 7(a) and 7(b). When an excitation voltage is applied to the samples, the domain inverted (in purple) and non-inverted regions (in orange) exhibit different behaviors (shrink or expand) due to the inverse piezoelectric effect, making it easy to distinguish between the inverted and non-inverted domains. Figures 7(c) and 7(d) present the corresponding phase distribution profiles along the dashed white line in Figs. 7(a) and 7(b), respectively. Clearly, a 180° PFM phase contrast is observed in two antiparallel domains, confirming the ferroelectric domain inversion in the lithium tantalite thin film. For the PFM characterization, the 180° phase contrast further confirms the complete domain inversion throughout the entire depth, as PFM can measure domain depths up to 1.7 µm and is sensitive to superimposed domains in the depth direction [30].
Fig. 7. PFM image of PPLTOI with sampling length of 18 µm. (a) and (b) are the poling periods of 3.5 and 4.5 µm. The white arrows represent the orientation of the domain polarization Ps. (c) The phase distribution profile along the dashed white line in (a). (d) The phase distribution profile along the dashed white line in (b).
To evaluate the quality of the periodically-poled samples with the poling periods ranging from 3.0 µm to 5.0 µm, a reliable method using MATLAB for numerical analysis of periodic domain structures is proposed. Figure 8(a) displays a partial image of the domain structures with a poling period of 5.0 µm after grayscale processing using MATLAB. In Fig. 8(b), the result after binarization of Fig. 8(a) is shown. The white rectangle-shaped area surrounded by black edges represents the domain-inverted region between the positive and negative needle-shaped electrodes. The black edge indicates domain walls. By detecting the black edges in the central region of the inverted domain, we distinguish between domain inverted and non-inverted regions. The analyzed results, represented by the red dashed lines in Fig. 8(b), are shown in Fig. 8(c). The ratio of l1 to l2 is the duty cycle of the inverted domain in single poling period. Having a uniformly poled area and the desired duty cycle throughout the analysis window is an important criterion for evaluating poling results. Therefore, the quality of the inverted domains is characterized by calculating the average value and standard deviation of the duty cycle for all inverted domains with the same poling period. As shown in Fig. 8(d), calculations and analyses are performed for several samples with poling periods of 3.0, 3.5, 4.0, 4.5, and 5.0 µm. The results indicate that both the average duty cycle and its standard deviation decrease as the poling period increases. The average duty cycles for the inverted domains with the same poling period are all less than 55%, and the standard deviation of the duty cycles is less than 4.8%. Clearly, these periodically poled domain structures with the poling periods of 3-5 µm not only exhibit the desired average duty cycle of approximately 50% but also demonstrate good uniformity. Moreover, the proposed method for analyzing domain inversion is not limited to the fabricated 3-5 µm periodically poled lithium tantalate (LTOI) domain structures in this experiment. It can also be applied to evaluate the quality of periodic domain-inverted structures in other materials, such as LNOI. This demonstrates the versatility and effectiveness of the numerical analysis approach in assessing the poling quality of various periodic domain structures. In this work, we fabricated samples with simple poling periods and uniform duty cycles. For some particular applications, such as squeezed light generation, varying poling period or duty cycle is required [31], to obtain such inverted-domain structures, the duty cycle of the electrodes should be properly adjusted. In cases where the poling period varies significantly, it may be necessary to divide the poling area into sub-regions for individual poling.
Fig. 8. Numerical domain structure with the poling period of 5.0 µm obtained by MATLAB. (a) Image after grayscale processing; (b) the result after binarization; (c) The analyzed results represented by the red dashed lines in (b); (d) average duty cycles and standard deviation corresponds to the poling periods of 3-5 µm.
4. Conclusion
To sum up, we have accomplished the fabrication of high-quality periodically poled lithium tantalate-on-insulator (PPLTOI) structures with a controllable duty cycle, featuring poling periods between 3.0 and 5.0 µm. Through the application of the Miller-Weinreich model and numerical simulations, we have identified the notable influence of the poling electrode shape on the domain inversion process. Consequently, we adjusted the electric field strength to regulate the average duty cycle of the inverted domain experimentally using needle-shaped nickel electrodes. We also discovered the critical role of the duration of the poling electric pulse in the poling process. Our experimental fabrication resulted in high-quality PPLTOI structures, characterized by confocal SH microscopy and PFM. The average duty cycle of the fabricated samples was measured to be approximately 50% with a minimal standard deviation of less than 4.8%. Building on the present results, we will incorporate the periodically poled structure into LTOI waveguides, constructing high-power and UV-band nonlinear integrated devices.
Funding
National Key Research and Development Program of China (2022YFA1205100, 2019YFA0705000); National Natural Science Foundation of China (12174185, 12304432, 91950206, 92150302, 92163216); Leading-edge Technology Program of Natural Science Foundation of Jiangsu Province (BK20192001); China Postdoctoral Science Foundation (2021M702968).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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