**Yuejian Jiao, Zhen Shao, Sanbing Li, Xiaojie Wang, Fang Bo, Jingjun Xu and Guoquan Zhang \***

The MOE Key Laboratory of Weak-Light Nonlinear Photonics, School of Physics and TEDA Applied Physics Institute, Nankai University, Tianjin 300457, China; jiaoyuejian@mail.nankai.edu.cn (Y.J.); 2120160179@mail.nankai.edu.cn (Z.S.); 2120180199@mail.nankai.edu.cn (S.L.); xjw@nankai.edu.cn (X.W.); bofang@nankai.edu.cn (F.B.); jjxu@nankai.edu.cn (J.X.)

**\*** Correspondence: zhanggq@nankai.edu.cn

Received: 23 December 2019; Accepted: 22 January 2020; Published: 30 January 2020

**Abstract:** We present a simple and effective way to improve the thermal stability of nano-domains written with an atomic force microscope (AFM)-tip voltage in a lithium niobate film on insulator (LNOI). We show that nano-domains in LNOI (whether in the form of stripe domains or dot domains) degraded, or even disappeared, after a post-poling thermal annealing treatment at a temperature on the order of ∼100 ◦C. We experimentally confirmed that the thermal stability of nano-domains in LNOI is greatly improved if a pre-heat treatment is carried out for LNOI before the nano-domains are written. This thermal stability improvement of nano-domains is mainly attributed to the generation of a compensating space charge field parallel to the spontaneous polarization of written nano-domains during the pre-heat treatment process.

**Keywords:** thermal stability; nano-domain; LNOI; pre-heat treatment
