Meta-devices, the conceptual expansion of metasurfaces composed of sub-wavelength artificial nanostructures, are the advanced optical devices that have drawn much attention in recent years. Compared with traditional bulky optical components, which can shape the electromagnetic fields via gradual phase accumulation through propagation inside the medium, meta-devices provide new degrees of freedom for manipulating the amplitude, phase, and polarization of the incident light at a two-dimensional flat interface. The light-matter interaction in the metasurfaces is achieved in sub-wavelength nanostructures, which endow the metasurfaces with inherent features of compact size. Besides their flat and compact dimension, the main distinctions between metasurfaces and their conventional counterparts are their multifunctionality, tenability, and easy-to-integration. The above unprecedented characteristics give the metasurfaces great potential. Versatile metasurface technologies have been proposed to fulfill the demands for various optical applications. Meta-optics opens a new era of flat optical components.

The design procedures or design flow for meta-devices are of importance to researchers in the field of flat optics. A general design flow facilitates the design, fabrication, and characterization of metasurfaces. By using commercial software to calculate the fundamental parameters for nanostructures with different dimensions and configurations, e.g., dispersion functions, phase, and efficiency, a data library for all the structure designs can be created. Metasurfaces with specified requirements can be built efficiently based on the data library, which thus significantly reduces the design load. Following the fabrication and characterization procedures indicated in the general design flow, one can achieve versatile designs for metasurfaces.

One great advantage of metasurfaces is their compatibility with the semiconductor microelectronics fabrication industry. Lithography, one of the most common nanofabrication technologies in the semiconductor industry, allows the fabrication of metasurfaces with high throughput, fidelity, and low cost. More nanofabrication technologies have been developed and applied to metasurface manufacturing to fulfill the growing demands and special requirements, such as laser-interference lithography, nano-imprint lithography, and micro-sphere projection lithography. Those state-of-art fabrication technologies contribute to academic research and real applications of metasurfaces.

Inspired by the promising features of metasurfaces, a variety of applications based on metasurfaces have been introduced, such as beam steering, meta-hologram, polarization control and analysis, imaging, nonlinear generation, focusing, biomedical applications, and high dimensional quantum entangle light source. Those applications further confirm the remarkable capability of metasurfaces. Still, plenty of possible applications based on metasurfaces have not been explored. In order to point out the possible development of metasurfaces, a summary of existing metasurface design and fabrication methods is required.

This review focus on the advances in meta-devices. The general design flow for meta-devices is introduced (Fig. 2). An example, the continuous broadband achromatic meta-lens, is demonstrated step by step to facilitate readers' understanding (Fig. 3). The nanofabrication technologies for optical metasurfaces are discussed. The fabrication methods for passive metasurfaces can be generally divided into three categories: direct-write lithography (Table 1), pattern-transfer lithography (Table 2), and hybrid patterning lithography (Table 3). Direct-write lithography is free from converging lens and photomask damage and has high resolution. High cost and time consumption are its main problems. Pattern-transfer lithography shows the merit of high throughput but only can produce limited patterns. Hybrid patterning lithography is capable of making large-area patterns, but it is difficult to make uniform patterns. Meta-devices for imaging have been well studied in recent years. The polarization generation and imaging based on metasurfaces are demonstrated. Inspired by the natural structure of compound eyes, an array of meta-lens, a lens based on metasurfaces, is proposed to achieve light-field imaging and detection. Meta-devices for bio-imaging are also discussed. Finally, a summary and the future prospects of meta-devices are provided.

Metasurfaces and meta-devices are optical components that have emerged in recent years. Their unprecedented ability to manipulate light on a sub-wavelength scale gains a lot of attention from the research community. Benefiting from the compatibility with semiconductor microelectronics fabrication technology, versatile meta-devices can be realized with high throughput and low cost. We believe that more advanced optical meta-devices will be raised by the research community and bring flat optics into our daily life in the future.