Infrared light, which is also known as infrared radiation, is located between the visible and microwave bands, with a wavelength range of 0.76?1000.00 μm. Unlike visible light, which can be directly perceived by the human eye, infrared radiation is located outside the range of human visual perception, with a wavelength range that is approximately 2500 times wider than that of visible light. The special physical properties of infrared radiation give it a wide range of applications in the aviation, aerospace, biomedical, industrial, military, and scientific research fields, along with others. The infrared-radiation propagation process includes strong absorption and scattering, along with a wide range of wavelengths. General optical devices cannot detect and utilize infrared radiation, and there are different application requirements for different infrared wavelengths. It is also necessary to choose appropriate infrared materials when preparing infrared optical devices. With the development of theoretical research, and science and technology, future opto-mechanical systems need to integrate as many functions as possible in as small a range as possible, which leads to new requirements for the miniaturization and integration of micro-optical components. Compared with traditional infrared optical devices, infrared micro-optical devices have advantages that include a small size, light weight, good stability, flexible manufacturing methods, low cost, and easy integration. Therefore, they have very bright application prospects in fields that include infrared sensing and imaging, infrared detectors, and infrared window penetration enhancement. At the same time, hard and brittle materials can withstand high temperature and high pressure in various extreme environments because of their good optical properties and physicochemical stability, and have irreplaceable roles in certain demanding military fields. However, their excellent physicochemical stability also produces greater challenges when preparing infrared micro-optical devices made of various hard and brittle materials.

In response to this demand, various high-precision preparation methods have been proposed, such as diamond turning, photolithography, and nanoimprinting. All of these methods have their own advantages, but they are not well suited for the preparation of complex three-dimensional micro-optical devices on the surfaces of hard and brittle materials. Femtosecond-laser processing is also an emerging high-precision micro-nano manufacturing technology. Because of the extremely short pulse width and high peak power of a femtosecond laser, high-precision true three-dimensional micro-nano structures can be prepared on the surfaces of various hard and brittle materials, as well as inside of them, which leads to a new method for solving the problem of preparing infrared micro-optical devices using hard and brittle materials. Femtosecond-laser processing technology has been rapidly developing in recent years, and new femtosecond-laser composite processing technologies have also been continuously derived. Therefore, this paper summarizes the preparation and application of infrared micro-optical devices using hard and brittle materials based on femtosecond-laser processing in recent years, with the goal of promoting the development and application of the technology in this field.

Starting with femtosecond-laser processing technology and commonly used hard and brittle infrared materials, this paper surveys the developments in this field in recent years. First, femtosecond-laser-based direct writing and various composite processing techniques such as etch-assisted femtosecond-laser processing and optical-field-modulated femtosecond-laser processing are briefly introduced. Next, some hard and brittle materials commonly used in the infrared region are introduced, such as diamond, which has the best overall performance but is also the most difficult to process with high precision, and sapphire, which is widely used in a variety of infrared window devices and has extreme hardness, excellent physicochemical stability, etc. (Table 1). Next, the paper focuses on infrared micro-optical devices such as refractive and diffractive devices. These infrared micro-optical components, as the basic units of an infrared integrated optical system, are characterized by a small size, light weight, and low cost. They can realize new functions such as miniaturization, arrays, and integration, which are difficult to realize with common optical components, and have very important application value in the infrared imaging, detection, national defense, and civil fields. Finally, the paper introduces related applications based on various types of infrared micro-optical devices, which are categorized according to their different application requirements. These mainly include sensing and imaging applications based on various kinds of lenses and their array structures, various kinds of infrared detectors based on the absorption of infrared light, and various kinds of window materials based on the enhancement of infrared-light permeability. Finally, the future development trend for the femtosecond-laser processing of infrared micro-optical devices using hard and brittle materials is envisioned.

The rapid development of femtosecond-laser processing technology for the high-precision preparation of infrared micro-optical devices using hard and brittle materials leads to new ideas. Scholars at home and abroad also perform much research in this field, from the principle to the application. This leads to many achievements and greatly promotes the development of this field. Of course, at present, there are still some problems in the preparation of infrared micro-optical devices using femtosecond-laser processing. For example, because of the unique stability of the diamond material, it is difficult to process with high precision and high efficiency. In addition, there is still much room for improvement in the high-precision preparation technology used for other hard and brittle infrared materials. It is believed that through the efforts of so many researchers, the use of femtosecond-laser processing technology in the preparation of micro-optical devices using hard and brittle infrared materials will gradually improve and will bring extensive social and economic benefits.