Diffractive lens arrays are important optical components in various applications, such as integral imaging, concentrator photovoltaics and LED lighting [1–4]. The diffractive lens arrays are usually composed of a same diffractive lens feature in multiform arrangements. The focusing and imaging performance of diffractive lens depends on the form accuracy and surface finish of its feature. Many methods for the manufacture of diffractive lens arrays have been extensively investigated, including laser direct writing [5–7], focused ion beam , and photolithography . By increasing the number of orders, the optical properties of the multiorder diffractive optical elements fabricated by lithography can be obviously improved. However, due to the influence of the straight edge diffraction of the binary structure, the light cannot be concentrated in the far field. Thus, in theory, the focusing ability obtained by discrete-step profile cannot be compared with that of the continuous profile. The continuous profile can be machined by laser direct writing and focused ion beam technology , but the pursuit of surface quality while maintaining a high machining efficiency in those processes has proven to be challenging. In general, most of these techniques require expensive equipment and involve complicated process steps, in addition, these processes are usually time consuming as well. Accordingly, they are not the perfect solutions for manufacturing large-area diffractive array optics with intricate shapes, particularly continuous profiles.
Precision glass molding is a high-efficiency and low-cost manufacturing process, which can easily cope with the manufacture of optical elements in macroscopic and micro/nano scale [10–11]. It is becoming a preferred alternative method for high volume production of plastic and glass optics. In precision glass molding, the geometrical accuracy and surface quality of the workpieces are basically determined by the mold. Therefore, it is the critical step to fabricate the precision master mold with inverse microstructure. At present, the material removal methods, such as precision grinding, polishing and diamond turning still play a dominant role in mold fabrication. Hard materials, such as metals (e.g. Ni), SiC and WC, are usually the popular choices for precision mold, due to the excellent physical properties at high temperature and inert chemistry. However, higher the hardness a material has, more difficult the machining process is. Especially for the large area micro/nano-scale structure arrays, the uniformity of the whole array is difficult to maintain due to tool wear. In addition, fabrication of intricate microstructures with sharp angles on hard materials, such as the diffractive lens array with continuous profile in this research, is also a technical challenge using existing manufacturing processes [12–13]. In addition, initial cost for machining such features should also be considered. These issues limit the wide applications of precision compression molding in replicating high resolution and complex surfaces.
These limitations of the hard material mold in machining, in principle, can be overcome by using soft material molds. Recently, polydimethylsiloxane (PDMS), one of the soft material molds, has been widely adopted as the mold for precision molding, due to its simple, mature and low-cost manufacturing process, as well as the flexible mechanical properties [14–15]. Compared with the hard material, PDMS has the following advantages as a mold: (1) PDMS can be easily obtained over large areas with good reproducibility and high accuracy, by casting a liquid onto a master substrate followed by thermal curing. (2) PDMS has low surface energy, which is convenient to separate from workpiece, and does not damage the surfaces of workpiece during separation stage. (3) PDMS mold is less sensitive to surface defects and contamination. However, the PDMS, as a precision mold, also encounters some problems. The first one is the difficulty of fabricating master substrate with large-area intricate features. Inkjet printing technologies is a popular flexible fabrication method of microlens array, but the structures of the microlens are limited to spherical and aspheric shapes . Other sophisticated technologies, including photolithography, e-beam lithography and wet-etching process [17–18], are good at creating complex features with high precision. But their process is extremely complex and costly. The second one is the poor rigidity of PDMS. The rigidity of PDMS is lower than that of chalcogenide glass at room temperature, and decrease with the increase of working temperature . Although the rigidity can be improved by changing the components of pre-curing PDMS  and adjusting the manufacturing parameters of PDMS, such as mixing ratio , curing time, and thermal ageing temperature [22–24], but these enhanced methods have limited performance improvements. In addition, by increasing the molding temperature , the workpiece can be softened to the point that it can slip into the mold cavities without much pressure. As expected, the deformation of the PDMS mold was almost avoided in the molding stage under this condition. However, high temperature leads to the loss of sulfur elements in the chalcogenide glass , which directly affects the optical properties of the lens. And the higher the working temperature, the shorter the life of mold. Therefore, although the existing manufacturing methods can replicate the features of PDMS mold to chalcogenide glass, there is an urgent demand for introducing a new manufacturing process with better energy efficient, stable and longer life of the mold.
Focusing on the above problems, in this paper, a fast and low-cost process chain of inscribing large-area diffractive lens arrays on chalcogenide glass by combining step-and-repeat hot imprinting and non-isothermal compression molding is proposed and investigated. Three major process steps are needed: the first step is the fabrication of a diffractive lens array on plastic master substrate employing hot imprinting. A single diffractive feature with continuous profile is machined on a brass mold by diamond turning. The continuous profile diffractive feature is copied completely to the polymer substrate by hot imprinting consecutively until the entire array is obtained. The direct hot imprinting ensures that the continuous features of the mold is replicated with high fidelity, and the accuracy of the entire array can be guaranteed by the stability of imprinting temperature and the high motion accuracy of the servo platform. At the end of this process a precise PDMS mold is generated by pouring a well-prepared PDMS liquid into the master substrate followed with curing. Lastly, the diffractive lens array is fully replicated on chalcogenide glass using non-isothermal glass molding. The non-isothermal molding process allows the PDMS mold to remain rigidity at lower temperature, to press into the softened chalcogenide glass at relatively high temperature. To demonstrate the feasibility of the proposed process chain, a 30×30 square diffractive lens array was fabricated on As2S3 glass. Each fabrication step and the resulting accuracy are carefully described throughout the whole process chain. The results showed that the proposed method proved has potentials for manufacturing optics elements with better optical performance and a much lower cost and higher efficiency.