Recently, a team of researchers led by Prof. Xiaocong Yuan at Shenzhen University and Prof. Liangcai Cao at Tsinghua University jointly contribute a comprehensive review paper entitled "Diffractive optical elements 75 years on: from micro-optics to metasurfaces", which was published in the fourth issue of Photonics Insights, 2023. (Qiang Zhang, Zehao He, Zhenwei Xie, Qiaofeng Tan, Yunlong Sheng, Guofan Jin, Liangcai Cao, Xiaocong Yuan. Diffractive optical elements 75 years on: from micro-optics to metasurfaces[J]. Photonics Insights, 2023, 2(4): R09)
In this high-quality invited review paper, they first give a comprehensive overview of the development of diffractive optical elements (DOEs) from binary forms to metasurfaces, and then introduce the fundamental physics of light manipulation using DOEs. They also review the applications of DOEs in the fields of imaging, information transmission, computing, storage and display, and finally provide perspectives on future development directions and challenges for DOEs.
DOEs are intricately designed devices with the purpose of manipulating light fields by precisely modifying their wavefronts using diffraction, and their 75-year development is shown in Fig. 1. The concept of DOEs has its origins dating back to 1948 when Dennis Gabor first introduced holography. Later in the 1960s, with the progress of computer science and manufacturing technology, other forms of DOE emerged, such as the computer-generated hologram (CGH) including gray-scale holograms, kinoforms and Lohmann holograms. And in the 1980s, binary optical element (BOE) emerged due to the advancement of optical fabrication technologies, including binary blazed gratings, Talbot gratings and Dammann gratings. These DOEs have pixel sizes larger than the illumination wavelength, so they are often called micro-DOEs. This was the first revolution in optical devices.
In the 21st century, with further development of advanced fabrication technologies such as electron beam lithography, focused ion beam milling, direct laser writing and nano-impriting, the pixels of DOEs were pushed into subwavelength scale. The concepts of metamaterials and metasurfaces emerged. DOEs stepped into the realm of nanoscales. Early works in this realm include Sir John Pendry's and R. A. Shelby et al.'s investigations on negatively refractive metamaterials. In 2011, Federico Capasso et al. proposed the generalized Snell's law, which deepened our understanding of precisely controlling the wavefronts using metasurfaces. Compared to 3D metamaterials, 2D metasurfaces have the advantage of much simpler fabrication process and low-cost.
Fig. 1 75 years of development of DOEs
A micro-DOE typically modulates the incident beam by altering the optical paths in different regions with an encoded pattern, which brings about alterations in the complex amplitude of the incident beam. Based on the distribution of amplitude and phase of the encoded patterns, DOEs can be classified into binary amplitude-only form, gray-scale amplitude-only form, and phase-only form. After stepping into the realm of nanoscale, DOEs possessed the ability to manipulate light in more degrees of freedom with much smaller structures. As shown in Fig. 2, through intricately designed geometric parameters and spatial orientations of unit cells and versatile material choices, nano-DOEs including metasurfaces can perform arbitrary wavefront shaping, polarization multiplexing, nonlinear effects, color filters, achromatic superlens, toroidal spatial-time light pulses, encompassing every aspect of light's degree of freedoms, including space, time, phase, amplitude, polarization and frequency (or wavelength).
Fig. 2 Modulating multi degrees of freedom of light using DOEs
Besides the above-mentioned static abilities in manipulating light fields, dynamic DOEs have received increasing attentions recently. As demonstrated in Fig. 3, mechanisms for active tuning can be classified into two categories: 1) structure tuning and 2) material tuning. Structure tuning DOEs can reconfigurably changing the relative spatial positions of structural units of the DOEs, typical such devices including stretchable metasurfaces, MEMs metasurfaces, digital micro-mirror device (DMD), piezoelectric varifocal metalens, etc. Material tuning DOEs can reconfigurably changing the material properties of the materials composing the DOEs, such as refractive index and anisotropy, typical such devices including liquid crystal based DOEs that can change refractive index upon external applied voltage, phase change materials such as GST that can perform phase-change from dielectric to metal upon change of external temperatures, chemical metasurfaces relying on ambient gas compositions and metasurfaces integrated with 2D materials having excellent optoelectronic properties.
Fig. 3 Dynamic DOEs. (a) Liquid crystal micro-DOEs , (b) Liquid crystal SLMs, (c) Terahertz piezoelectric varifocal metalens (d) Phase-change metasurface, (e) Chemical reaction metasurface, (d) 2D material-integrated metasurface
Due to their powerful abilities in light manipulation, DOEs have great application potentials in many fields in information sciences, and have been successfully applied to the information acquisition, information transmission, information computing, information storage and information display. As shown in Fig. 4, in the field of information acquisition, DOEs can be used as sensors for refractive index, picometer displacement, and bio-chemical detection. In the field of information transmission, DOEs can be used as diffractive optical couplers, multi-channel OAM multiplexing/ de-multiplexing, and digital information processing spatial-time encoded metasurfaces. In the field of information computing, DOEs based on all-optical diffractive neural networks using machine-learning provide possibilities for super-high-speed computing. In the field of information storage, DOEs can be used to realize the requirement for super-high density information storage in multi-dimensional multiplexing. In the field of information display, DOEs can be used as broadband achromatic full-color imaging lens. In the field of optical computing, digital and intelligent.
Fig. 4 Applications of DOEs. (a) Free-space optical communications (b) Achromatic metalens imaging (c) Drug containment sensing of human sweat (d) Optical computing based on deep neural networks
In summary, the authors comprehensively reviewed the history and significant breakthroughs of DOEs. DOEs encompass intricately designed patterns capable of modulating light through the exploitation of its wave nature and diffraction phenomena. Overviewing the development history of DOEs, we can briefly outline the course of their evolution: initially designed by employing the interference effect, optical holograms, often referred to as HOEs, represent one of the earliest manifestations of DOEs. Subsequently, propelled by advances in computer science and manufacturing technologies, additional DOE variants emerged, including CGHs and BOEs. This is a key revolution in the history of optical devices. These DOEs are often categorized as micro-DOEs, given that their pixel dimensions exceed the illumination wavelength. After stepping into the 21st century, the device feature sizes continue to shrink, DOEs make significant inroads into the realm of nano-optics. The birth of metasurfaces catalyzed another revolution in optical devces. Micro-DOEs have made great progress in 75 years, although their performance still cannot meet the requirements of some practical applications.
The ultimate aspirations of scientists and engineers in the field of optics lie in the creation of versatile metasurfaces that excel in functional multiplexing, offer broad bandwidth, exhibit robust performance across different angles of incidence, and provide rapid tunability with high efficiency. Introducing new physics, new materials, new design methods and new fabrication technologies is an effective solution to these challenges. We remain optimistic that these hurdles can be surmounted with the continued progress of technologies.
