1. Introduction

Lithium niobate on insulator (LNOI) has emerged as a promising platform for integrated optoelectronic applications, offering superior piezoelectric, electro-optic, acousto-optic, and nonlinear optic properties [1–3]. The successful fabrication of periodically poled LNOI through applying an electric field [4–8] or a light field [9,10] has significantly enhanced the conversion efficiency of on-chip nonlinear/quantum optic processes, employing the quasi-phase matching (QPM) theory [11]. Recently, advanced applications including narrow-linewidth quantum light sources [12,13] and nonlinear optical neural network [14,15] mandate the reduction of domain sizes to submicron dimensions. Piezoelectric force microscopy (PFM) tip poling has demonstrated its effectiveness in engineering the ferroelectric domain at the nanoscale [16,17]. This technique involves applying a bias voltage to the sample surface using a conductive probe, facilitating domain formation as the localized electric field exceeding lithium niobate (LN) coercive field [18,19]. This technique has been applied to fabricate100-nm-period domain arrays in polar-cut (z-cut) LNOI [20].

However, the applications of PFM-tip poling in nonpolar-cut (for example, x-cut) LNOI has been focused predominantly on isolated wedge domains induced by local switching [21–23] and self-organized domain structures [24,25]. In comparison to z-cut LNOI, x-cut LNOI presents distinctive advantages in the realm of integrated optoelectronic devices. Specifically, it facilitates the realization of low-loss and ultra-wideband electro-optic modulators, eliminating the need for a bottom electrode [26–28]. Moreover, x-cut LNOI offers the possibility to control the growth of domain lines along a predesigned angle relative to the z-direction, which is useful in nanoelectronic [6,29,30] and acoustic devices [31,32]. It is critical to explore the nanodomain growth mechanism and realize controllable fabrication of periodic nanodomain array in x-cut LNOI for practical applications.

In this paper, we investigate the PFM-tip-induced domain formation mechanism in x-cut LNOI. The tip electric field presents a tail-to-tail (or head-to-head) distribution when applying a positive (or negative) bias voltage. According to the moving direction of the tip, the electric field can be divided into the preceding and sequential halves. Interestingly, the polarization direction of the generated domain is decided by the preceding component of the electric field rather than the sequential one as in previous reports [33]. The mechanism can be attributed to that the screening field formed by the preceding field counteracts the effect of the subsequent field. In experiment, we have successfully fabricated drop-shaped domain dots, domain lines, and periodic domain arrays. Our work paves a way to precisely fabricate nanoscale domain structures in x-cut LNOI for advanced applications in integrated optics and nanoelectronics.

2. Methods

In experiment, we use an x-cut LNOI wafer (Fig. 1) consisting of a 500-nm-thick LN thin film, a Cr/Au/Cr (30 nm/100 nm/10 nm) conductive layer, a SiO₂ buffer layer (2 µm) and an LN substrate (500 µm). The scanning probe microscope is a commercial system (MFP-3D, Asylum Research, USA) equipped with gold coated probes (Multi-75GB-G, Budget Sensors, USA). The radius of curvature of the tip (R_tip) is 25 nm. The fabrication of domain structures is conducted in Litho mode using the conductive probe as the top electrode. The metal interlayer of LNOI serves as the bottom electrode, grounded through silver glue. The probe can be moved along a designed path within the y-z plane. The visualization of domain structures employs a dual AC resonance tracking PFM in lateral mode working at an AC driving voltage of 0.5 V and a resonant frequency of 650 kHz. The experiments are performed in room temperature with a relative humidity of about 40%. The experimental data are recorded and analyzed using Igor software.

 

Fig. 1. Schematic diagram of PFM tip poling in x-cut LNOI.

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3. Experimental results and discussions

We numerically analyze the tip electric field as shown in Figs. 2(a) and 2(b). Considering that the spontaneous polarization of LN is along + z direction, the effective component E_z of the electric field is depicted, which presents a head-to-head (Fig. 2(a)) or tail–to-tail (Fig. 2(b)) distribution, corresponding to a negative or positive bias voltage, respectively. The symmetry of the electric field indicates that the domain poling depends on the moving direction of the tip.

 

Fig. 2. (a) and (b) depict the simulated spatial distributions of the effective component E_z of the tip-induced electric field by applying a bias voltage of −100 V and +100 V, respectively. The white and black arrows represent the directions of E_z and the LN spontaneous polarization P_s. (c) and (d) are the lateral PFM phase images of the drop-shaped domains by applying a negative and positive bias, respectively. (e) and (f) show the corresponding lateral PFM amplitude images. The pulse amplitude ranges from 50 V to 120 V and the durations is extended from 0.1 s to 30 s.

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In experiment, we first write individual domains by applying single pulses with various amplitude (50 V ∼ 120 V) and duration (0.1 s ∼ 30 s) to each point on the x-cut LN surface. Figures 2(c)–2(f) show the generated domains with their sizes increasing as the voltage is enhanced or the duration is extended. The domain shape is quite different from the previously reported wedged domains [21–23]. This deviation can be attributed to the presence of the metal conductive layer beneath the LN thin film. Generally, the domain significantly elongates along the polar axis in response to the depolarization field generated by unscreened charged kinks [23,34]. In contrast, the charge screening is more effective in our experiment because the grounded metal layer establishes an efficient channel for charges. As a result, the growth of domains is mainly confined within the vicinity of the tip.

Interestingly, the domain poling exhibits distinct behaviors by applying positive or negative bias. Under the same pulse amplitude and duration, the length of the domain induced by a negative bias pulse (Fig. 2(c)) is slighter longer than that induced by a positive one (Fig. 2(d)). This morphological distinction can be attributed to the lower mobility of positive carriers (for charge screening in Fig. 2(c)) compared to negative ones (for charge screening in Fig. 2(d)) [35]. Additionally, domain backswitching is observed in Fig. 2(c) rather than in Fig. 2(d) for the same reason [21,36].

In the fabrication of domain lines, we employ a DC-biased tip moving along a predetermined path. As shown in Figs. 3(a) and 3(b), the tip electrical field can be divided into the preceding half and the subsequent half, each with opposite directions. Notably, the domain poling is dependent on the moving direction of the tip. One might anticipate that the final domain polarization direction aligns with the subsequent electric field direction since it interacts with the LN material last (Fig. 3(a)) [33]. However, experimental results reveal the opposite (Fig. 3(b)). For example, when applying a positive voltage to the tip, the electric field E_z exhibits a tail-to-tail distribution. Under our experimental conditions, the domain lines can only be written by moving the positively biased tip along the -z direction antiparallel to the original LN polarization. In this case, the final LN domain polarization aligns with the preceding half of E_z. This can be explained by the mechanism in which the influence of the preceding field hinders the impact of the subsequent one. Following the creation of an LN domain by the preceding field, an electric field generated by the screening charges emerges and counteracts the effects of the subsequent field. Figures 3(c) and 3(d) present the lateral PFM phase images of the produced domain lines. One can observe that the domain linewidth increases as reducing the scanning speed or enhancing the voltage. At a voltage of 40 V, the domain linewidth continuously decreases from 155 nm to 85 nm as increasing scanning velocity from 0.1 µm/s to 20 µm/s (Fig. 3(c)). When keeping the scanning velocity at 20 µm/s, the domain linewidth increases from 88 nm to 564 nm as increasing the bias voltage from 40 V to 120 V (Fig. 3(d)). The dependencies of the linewidths on the scanning velocity and bias voltage fit well with negatively logarithmic (Fig. 3(e)) and linear (Fig. 3(f)) functions, respectively. This can be understood by the domain growth mechanism based on the sideways movement of the domain wall [37,38].

 

Fig. 3. (a) The schematic of tip-induced domain poling without considering charge screening. The final domain polarization direction is consistent with the subsequent electric field direction. (b) shows the scheme with an effective charge screening, in which the final LN domain polarization is decided by the preceding half of E_z. (c) and (d) show the lateral PFM phase images of the nanodomain lines by moving a positively-biased tip along the -z direction by using various scanning speeds and bias voltages, respectively. The dependencies of domain linewidths on the scanning velocity and bias voltage satisfy a negatively logarithmic (e) and linear (f) functions, respectively.

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In our experiment, the domain line writing by using a negatively biased tip is also dominant by the preceding electric field. Since the electric field in this case features a head-to-head distribution, the domain lines can be written by moving the tip in the same direction (+z) as the original LN polarization. Figure 4 presents the lateral PFM phase images of domain lines. By adjusting the bias voltage and scanning velocity, the domain linewidth varies from ∼100 nm to ∼1 µm in our experiment. The dependencies of the linewidths on the scanning velocity and bias voltage are shown in Fig. 4(c) and Fig. 4(f), respectively.

 

Fig. 4. (a)-(b) and (d)-(e) show the nanodomain lines fabricated by scanning a negatively-biased tip in the + z direction. The dependencies of domain linewidths on the scanning velocity and bias voltage satisfy a negatively logarithmic (c) and linear (f) functions, respectively.

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Finally, we investigate the fabrication of periodic domain structures in x-cut LNOI. Figure 5(a) shows an 800-nm-period domain array written by using a + 95 V bias voltage and a 15 µm/s scanning speed. The tip moves in the -z direction antiparallel to the original LN polarization. In addition, our technique is capable of writing domain lines along a specific angle θ relative to the + z direction. Figure 5(b) shows two examples with θ being 20° and 10°, respectively. The periods are 600 nm and 850 nm, respectively. The details in the enlarged patterns indicate high-quality fabrication of periodic domain array through tip poling.

 

Fig. 5. The fabrication of periodic domain structures. (a) The domain array is fabricated at a 95 V voltage and a 15 µm/s scanning speed. The period is 800 nm. (b) The periodic domain structures are arranged along an angle of 20° (left) and 10° (right) relative to the + z direction.

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4. Conclusions

In conclusion, we have investigated the nanodomain formation in x-cut LNOI and achieved the successful fabrication of domain dots, lines, and periodic arrays. The charge screening is critical to LN domain poling. In the formation of individual domains, effective charge screening confines the domain growth within the vicinity of the tip. During the domain writing, the formation of the screening field counteracts the influence of the subsequent component of the tip electric field. In addition, we demonstrate the fabrication of nanodomain arrays along a designed angle relative to the z direction. Considering the dependence of domain-wall conductivity on this angle, it has potential applications in nanoelectronic devices. Notably, the metal interlayer could cause extra loss in optical applications. One may use thin-film transfer or chemical etching techniques to avoid this problem [39–41]. Our results reveal the intrinsic mechanism of nanodomain engineering in x-cut LNOI and pave a way for future applications in high-performance integrated optoelectronic devices.