The supply of natural diamond can no longer meet the normal production and living needs of humans due to the increasing number of applications for diamond. High-quality and large-scale production of synthetic diamond has been a topic of research since Tennant discovered diamond was an allotrope of carbon in 1796. Diamond substrates used for optical devices can be classified as single-crystal diamond (SCD) or polycrystalline diamond (PCD). Diamond materials can be divided into four types according to their composition and nitrogen impurity content: I a, I b, Ⅱ a, and Ⅱ b, as listed in Table 1 with their specific parameters. Diamond manufacturing methods can be divided, depending on the characteristics of the technology used, into static, dynamic, and low pressure. Diamond formation can be divided into direct, melting, and epitaxial methods based on the characteristics of the diamond produced. Gem-grade diamonds are usually prepared by high-temperature and high-pressure (HPHT) and chemical vapor deposition (CVD).

A window material with high transmittance is an important guarantee of the high precision and stability of a photoelectric system. Diamond has high transmittance in the wavelength range from ultraviolet (0.225 μm) to microwave (~8000 μm) and has become one of the best choices for manufacturing wide-band window materials for photoelectric systems with the invent of increasingly sophisticated diamond preparation technology. Diamond is a third-generation ultra-wide bandgap semiconductor material and has a typical face-centered cubic structure (lattice constant is 0.35668 nm, bond length is 0.155 nm, bond angle is 109°28') with several characteristics such as high hardness (Mohs hardness 10), high thermal conductivity (2000 W/mK), and low thermal expansion coefficient (1.2×10^-6 K^-1).

The high refractive index of the visible and infrared bands of synthetic diamond results in a diamond transmittance of only 71%, which limits its in-depth application in high-precision photoelectric systems. At present, the most commonly used method is to construct a surface anti-reflection film to reduce the reflection loss of the diamond surface. The preparation principles and processes involved in this method are relatively simple. The anti-reflection effect is achieved using a film by selecting different materials so that the reflected light on the upper and lower surfaces of the film interfere and counteract the reflection. However, there are a limited selection of viable anti-reflection coating materials for some wavelengths, resulting in a narrow bandwidth and insufficient angular spectrum range. The damage threshold of anti-reflection coatings is lower than that of diamond, which significantly reduces the service life, stability, and output power of high-power lasers. Recent studies have suggested this problem can be solved by exploiting micro-nano structures designed by theoretical calculation to produce diamond anti-reflection micro-nano structures with sizes ranging from 1 nm to 100 μm by laser etching and ion etching.

Although research in this area has been widely reported, there is no systematic summary in the literature of the design and preparation of anti-reflection micro-nano structures on diamond surfaces. Therefore, this paper classifies and summarizes the design theory and preparation technology of diamond surface anti-reflection micro-nano structures, which provides a technical reference for preparing diamond micro-nano structures and expanding the anti-reflection applications of diamond micro-nano structures in the future.

In this paper, the research progress in diamond anti-reflection micro-nano structures in recent years is reviewed. First, the anti-reflection mechanism of the micro-nano structure (vector diffraction theory, EMT) is introduced and the viability of the two mechanisms are discussed before the anti-reflection mechanism of EMT is theoretically deduced (Fig. 1 and Fig. 2). Second, the basic principles and technological conditions of laser processing and ion etching are summarized. The different pulse widths determine the micro-nano structures on the diamond surface prepared by laser processing and that allows them to be divided into two categories: micro-nano structure etched by short-pulse lasers (ns) and periodic structure induced by ultrashort pulse (fs) lasers. Ion etching uses high-energy ions to bombard the diamond surface to obtain a smaller feature size on the diamond surface. Through different process conditions, a vertical or angled-side wall morphology can be obtained. The influence of the micro-nano structure on the transmittance of diamond prepared by the two methods is summarized (Table 2), and the advantages and disadvantages of various preparation technologies in the application of diamond anti-reflection are compared (Table 2). The application prospects of diamond are briefly described, with the aim of providing a technical reference for related fields.

This paper gives a systematic summary of the basic principle and processing method for laser machining and ion etching anti-reflection micro-nano structures on diamond surfaces. The influence of micro-nano structures with different periods and contours on transmittance properties is explored, and the advantages and disadvantages of various preparation technologies are clarified. This provides a systematic reference for researchers in related fields from which they can design and prepare anti-reflection micro-nano structures on diamond. This research shows that it is necessary to use simulation software to optimize the design of micro-nano structure parameters, such as size, period, duty cycle, and height, before the diamond surface anti-reflection micro-nano structure is prepared (using the technology presented in this paper), and to design the method for micro-nano processing based on theoretical calculation results. However, current preparation processes cause errors in the micro-nano structure size, period, and other parameters that may lead to deviations in the peak transmittance. Therefore, understanding how to match the preparation technology with the simulation results and achieve the ideal anti-reflection effect on a diamond surface is still an urgent problem requiring a solution.