Ferroelectric domain inversion and its stability in lithium niobate thin film on insulator with different thicknesses

Ferroelectric domain inversion and its effect on the stability of lithium niobate thin films on insulator (LNOI) are experimentally characterized. Two sets of specimens with different thicknesses varying from submicron to microns are selected. For micron thick samples (~28 um), domain structures are achieved by pulsed electric field poling with electrodes patterned via photolithography. No domain structure deterioration has been observed for a month as inspected using polarizing optical microscopy and etching. As for submicron (540 nm) films, large-area domain inversion is realized by scanning a biased conductive tip in a piezoelectric force microscope. A graphic processing method is taken to evaluate the domain retention. A domain life time of 25.0 h is obtained and possible mechanisms are discussed. Our study gives a direct reference for domain structure-related applications of LNOI, including guiding wave nonlinear frequency conversion, nonlinear wavefront tailoring, electro-optic modulation, and piezoelectric devices.

indicate that the reversed domains in micron-thick LNOIs are as stable as those in LN bulk crystals.  Though 28-μm samples are not very commonly used for waveguide devices, it is still quite valuable in many other applications, including piezoelectric application, electro-optic modulation, and nonlinear wave front shaping. For example, if a Gaussian beam is injected to a fork-pattern poled LNOI sample, second-harmonic optical vortices could be generated and diffracted back. 18 More applications for complex wave front tailoring could be expected by using properly designed domain patterns.
To the best of our knowledge, 28 μm is the thinnest commercially available micron-scale LNOI so far. Although a 10-μm-thick free-standing LN thin film has been fabricated based on a similar process, it is quite difficult to obtain 10-μm-thick LNOI. According to our sample supplier, the fabricated processes are different for LNOIs with micron and submicron thicknesses. For some unknown reason, the metal electrode layer and the sliced LN thin film itself are quite fragile during micron thick LNOI fabrication if the thickness is less than 28 μm. However, LNOI with a submicron thickness (50 nm to 900 nm) can be achieved with a different fabrication pathway. Therefore, the study of domain inversion and domain stability in LNOI still have a thickness gap from 1 μm to 28 μm. We expect that advances in the LNOI fabrication technology would help to fill the gap soon.
As for sub-micron thick LNOI, we tried but in vain to pole the thin LNOI using patterned electrodes due to the difficulty in focus during photolithography. Domain inversion and domain stability are studied by PFM, instead. 19 Fig. 1(b) shows the schematic diagram of the thinner LNOI sample, provided by the same supplier. The thickness of the +z-cut LN thin film, Pt, SiO2 and LN substrate are 540 nm, 100 nm, 1.7 μm and 500 μm, respectively. The positive poling voltage is applied to the conductive tip scanning over an desired pattern on the sample surface. The optimized poling voltage is 30 V. A 3 μm  1.5 μm rectangular negative domain is patterned in 4 min.
Domain stability is investigated by measuring the pizeoresponses to 1.0 V AC stimulus applied on the tip at certain spans of time after poling. [20][21][22][23] In positive domain areas, the positive voltage compresses the domain, while the negative voltage stretches it. The piezoelectric deformation is opposite in negative domain areas because the piezoelectric coefficient changes sign. Therefore, a 180° PFM phase contrast should be observed in two antiparallel domains. Indeed, in Fig. 3(a), the 180° phase contrast is observed as expected, as measured immediately after poling. Since the original LN thin film is +z cut (in yellow), the poled area (in purple) must be negative, which is similar to micron-thick LNOI and bulk LN. However, the negative domain is not stable, as evidenced in the PFM phase image measured 25 h after poling. As shown in Fig. 3(b), the patterned negative domain is partially switched back, leaving separate nanometersized negative domains in the positive matrix. Gainutdinov et al. 17 studied the creation and stability of nanodomains in LNOI. However, the domain size is too small for practical applications. In most applications, larger domains with long retention are desired. The negative domain is checked from time to time to study the domain structure evolution. As shown in Fig. 3 Since these nano-sized negative domains remain even after 42 h, the domains might have penetrated the top LN film. Otherwise, the domain may decay in a short time. 17 However, the negative domain is obviously not very stable, as compared with the thick LNOI. To quantitatively evaluate the domain evolution, the negative domain area in each snapshots shown in Fig. 3(c)-(h) is estimated by counting the purple pixels. Figure 4 shows the negative domain area as a function of retention time. It is observed that ~14.4% of the negative domain written into LNOI switches back to positive polarization within 5 h. Only ~54.6% of the negative domain remains after 25 h. We fit the data with a reduced exponential equation and obtain the relation ≈ 194378 × −t 12.475 ⁄ + 192766 . The residual negative domain area saturates to about ~50% of its original value. If we define a critical domain lifetime at which 1/e 2 of the original negative domain remains, the obtained lifetime is ~25.0 h. Moreover, our sample is always in the chamber during the measurement under constant environmental conditions. As domain stability is very sensitive to the environment, it may deteriorate as temperature or humidity changes unfavorably. In this work, we successfully obtained stable PPLNOIs in a sample with a LN layer 28 μm in thickness. For a thinner LNOI sample, 540 nm in thickness, the lifetime of negative domains is estimated at about 25.0 h. This, however, is much longer than the 0.5 min as reported by Gainutdinov et al. 17 The obtained lifetime might be useful for short-time research and applications employing unique advantages in LNOI. Because of the complexity in LNOI fabrication, many factors may affect the domain stability at different sample thickness. Earlier works have shown that the weak stability in thin LN samples may result from domain electrostatic repulsion. 24,25 The electrostatic field inside the LN thin film may drive the negative domain to switch back to its original positive state. Besides, according to the sample supplier, the bonding methods in the fabrication processes are different for submicron-and micron-thick LNOI, which may also cause different stability behaviors.
Although retention issues of negative domains have been observed in submicron-thick LNOI, this problem is still solvable as achieved in free standing PPLN thin films. 26 In addition to the fabrication processes, some postprocessing techniques can also be used to stabilize ferroelectric domains in 10-μm or thinner LNOIs. In contrast to the conventional proton exchange or Ti diffusion processes in bulk LN, LNOI may support both TE and TM modes without any extra propagation loss. Therefore, more multifunctional integrated photonic circuits could be expected based on a single LNOI chip containing a nonlinear light source, high speed electro-optic modulators, and other units. Even long-range surface plasmon polariton could be experimentally realized, as the structure of PPLNOI is similar to that designed by Wu, et al.. 27 Besides, an optical frequency comb generator could be expected in a poled LNOI microring resonator through the QPM quadratic frequency conversion. 13,28 If needed, the conductive tip could supply high enough voltages to pole micron-scale LNOI or even bulk LN. Though taking longer time, the tip-poling process is precise positioning, providing a possible way to selectively pole a particularly small area with designed pattern. Thus it could be an effective method to fabricate artificial defect induced structures within as-fabricated domain patterns for intriguing applications. 29,30 In conclusion, two different large-area domain poling techniques were demonstrated for micronand submicron-thick LNOI samples. Micron-thick PPLNOI was successfully fabricated by electric field application and examined by polarizing optical microscopy and etching. No domain back-switching has been observed in patterned negative domains after 35 days. For submicron-thick LNOI, negative domains can be written by scanning a biased conductive tip over the sample surface. However, the negative domain gradually switches back to the positive domain after poling. The domain lifetime is only 25.0 h. Eventually, about 50% of the negative domain area could be retained. Our study may give some direct guidance for domain-related LNOI applications such as guiding wave QPM nonlinear frequency conversion, nonlinear wavefront tailoring, electro-optic modulation, and piezoelectric devices. Fig. 1. Schematic diagrams of LNOIs and their corresponding poling techniques for 28-μm (a) and 540-nm (b) thick samples. Blue, red, light green, gray, orange, and pink stand for LN, Pt/Au, SiO2, silver paste, stainless steel plate, and Cr electrodes, respectively. The stainless steel plate is connected to the ground. Positive DC poling voltages are applied to the Cr electrodes in (a) or a conductive tip in (b).