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Focused ion beam (FIB) milling has been used for fast prototyping of lithium niobate (LiNbO 3 , LN) devices with feature size from sub-to hundreds of micrometers. However, a promising and challenging depth range of tens-of-nanometers or below is rarely attended. Moreover, the surface roughness, related closely with device performances, is particularly non-negligible for such an ultra-shallow etching. Here, the surface roughness evolution was studied on ultra-shallow FIB etched LN structures. It was found that the inhomogeneous etching of the metallic film, coated on LN surface to avoid charge accumulation, had a detrimental effect on the LN surface roughness control. By thinning the gold thickness to 7 nm, sub-nanometer surface roughness was reported for etching depth of several nanometers. This work paves the way towards a homogenous and ultra-shallow FIB milling of LN nano-structures. • Investigation of etching depth and surface roughness evolution on focused ion beam milled lithium niobate (LN) surface. • Revealing the mechanism of non-negligible LN surface roughness especially in the ultra-shallow etching range. • The inhomogeneous etching of the pre-metallized coating had a detrimental effect on LN surface roughness control. • Several-nanometer ultra-shallow steps with surface roughness below 1 nm was presented on LN with a 7-nm-thick Au coating. [ABSTRACT FROM AUTHOR]

• We develop a shallow-etched grating coupler with high efficiency for diffractive waveguides based on the lithium niobate on insulator platform. The key parameter of the etching depth of grating is precisely designed to balance the coupling efficiency and minimize the damage to the waveguide. By controlling the etching depth at the level of more than one hundred nanometers, a coupling efficiency of 33.9 % is achieved in experiment. • Such compact structures realized by micro-nano fabrication technologies offer a flexible solution for achieving efficiently optical coupling at selectable locations on the surfaces of waveguides. • The diffractive waveguide demonstrates excellent input/output functions and the angle-dependent spectral modulation effect. These characteristic allow for the development of visualization glasses customized for angle-resolved spectroscopy, thereby greatly amplifying the adaptability and utility of AR glasses. Diffractive waveguide integrates both the light transmission, optical modulation and the signal input/output functions based on a waveguide structure etched with gratings. Such compact structures realized by micro-nano fabrication technologies offer a flexible solution for achieving efficiently optical coupling at selectable locations on the surfaces of waveguides, while the grating etching process will inevitably affect the original waveguide efficiency. Deep-etched gratings can achieve high coupling efficiency but may cause nonnegligible damage to the waveguide device. In this study, we develop a shallow-etched grating coupler with high efficiency for diffractive waveguides based on the lithium niobate on insulator platform. The key parameter of the etching depth of grating is precisely designed to balance the coupling efficiency and minimize the damage to the waveguide. By controlling the etching depth at the level of hundred nanometers, a coupling efficiency of 33.9 % is achieved in experiment. Moreover, the diffractive waveguide demonstrates excellent input/output functions and the angle-dependent spectral modulation effect. This work provides valuable insights for the design and optimization of grating couplers for augmented reality glasses. [ABSTRACT FROM AUTHOR]