With the rapid advancements in optical technologies,there are increasing demands for precise light manipulation,e.g.,beam expansions and reductions. Traditional optical components are often limited by their bulky,heavy,and expensive nature,which poses significant challenges to the system efficiency and integration. Additionally,two-dimensional metamaterials,i.e.,the metasurfaces consisting of sub-wavelength functional structures allow to manipulate optical beam with unprecedented freedoms on micro-scale chip. Upon the development of metasurfaces,a plenty of optical components were miniaturized and witnessed success in applications ranging from lenses,holography,coloration,polarization control,spectroscopy,and many more. However,the existing metasurfaces typically rely on combinations of various dielectric materials on one single chip. This feature not only leads to the complicated fabrication process but also limits their application robustness. In order to address these limitations,this study introduces an alternative approach to conduct a metalens by designing a monolithic dielectric metasurface using lithium niobate (LiNbO₃,LN). LN was selected because of its extraordinary optical properties,including wide-band transparency for transmissive operations and high refractive index to localized optical field. Furthermore,beam expanders and reducers based on the monolithic dielectric metalenses were proposed and investigated numerically. The monolithic dielectric metalens was designed based on the theory of the Generalized Laws of Reflection and Refraction,in which way the phase delay of light through the LN metasurface was adjusted by altering the size of the cylindrical LN unit cell. Then,varied sizes of the LN nano-pillars were arranged according to the required phase profile to enable the metalens functionality. In detail,a series of Finite-Difference Time-Domain (FDTD) simulations were performed and the geometrical parameters of the unit cell providing required phase delay for light focusing were then optimized. The features of the proposed monolithic dielectric metalenses were evaluated in terms of focal length,spot size of the focused beam,intensity distribution,as well as the phase profiles of focused spots. The simulations confirmed the comparably excellent performances of the proposed monolithic dielectric metalens with the family of metalenses while the monolithic metasurfaces are unlike the majority of current ones with complex structures and various materials. For instance,one of the proposed monolithic dielectric metalenses has a diameter of 48 μm and an expected focal length of 96 μm. The simulated showed that a transmission efficiency of 56% and a focusing efficiency of 39% with the focused beam spot of 3 μm and the focal length of 89.71 μm were obtained. The focused beam size is smaller than the diffraction limit,while the difference between the expected and the actual focal length is only 6.6%. Furthermore,a Keplerian telescope structure was investigated based on the above monolithic dielectric metalens and beam expanders and reducers with 2-fold and 4-fold beam expansions and reductions were demonstrated. The results showed that the beam expander and reducer not only possess high efficiency (58%) beam manipulation but also maintain a high beam quality. Notably,the robustness of the beam expander and reducers were analyzed in detail. It was shown that the proposed beam expander and reducers exhibited high tolerances to 2% and 5% variations in operation wavelength and 0.5° and 1° angular drifts of the incident light,indicating the excellent robust performances under practical operation conditions. This research not only presents the new realizations of optical components based on metasurfaces but also paves the way for further integrations and optimizations of optical systems. In addition,the strategy to conduct optical components based on monolithic metasurfaces is generic and will benefit a broad range of integrated platforms with compact,efficient,and cost-effective advantages. The optical beam expanders and reducers based on dielectric metalens are promising in high-resolution imaging,laser communication,and microscopic measurements.