**1. Introduction**

Lithium niobate (LiNbO3), one of the most versatile ferroelectric materials, has been widely studied due to its excellent performance on electro-optic modulation [1,2], acousto-optic modulation [3] and nonlinear optics [4,5]. Recently, the technique of lithium niobate film on insulators (LNOI) [6–8] has attracted much attention for its potential applications in integrated devices. Numerous novel optical elements based on LNOI have been reported, including photonic crystals [9], high-Q microresonators [10], ridge waveguides [11], and hybrid lightwave circuits [12].

By applying a polarization reversal voltage, via an atomic force microscope (AFM)-tip, domain reversal and domain patterning can be realized in lithium niobate thin films. Based on this technique, Gainutdinov et al. [13] reported that the size and shape of domain patterns can be precisely controlled, enabling the realization of periodically poled lithium niobate (PPLN) with period of hundreds of nanometers. PPLN films can be used for quasi-phase-matching (QPM) devices, such as PPLN microcavities [14] and PPLN waveguides [15], to achieve frequency conversion. Obviously, the stability of written domains is very important for LNOI-based applications such as PPLN microcavities, PPLN waveguides, and nonvolatile ferroelectric domain memories [16–19].

Several groups have studied the thermal stability of domains in various ferroelectric materials, such as Rb-doped KTiOPO4 [20], LiTaO3 [21], Pb(Zr0.4Ti0.6)O3 [22], and LiNbO3 [23,24]. They reported that the ferroelectric domains would degrade, or even disappear, after heat treatment. Moreover, Shao et al. [25] reported that the domain structures fabricated on LNOI are unstable even at room temperature. Obviously, such instability would prevent the ferroelectric domains from applications where the device temperature will rise due to light absorption or due to high temperature environments.

In this paper, we propose a simple and effective method to improve the thermal stability of nano-domains in lithium niobate thin films. We confirmed that nano-domains written in LNOI by applying an AFM-tip voltage were unstable at high temperatures in the order of ∼100 ◦C. However, we found that the domain stability can be significantly improved if the LNOI sample experiences a pre-heat treatment before the nano-domain fabrication process. The underlying mechanism was also discussed.
