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Herein, we have discussed three major methods which have been generally employed for the generation of optical beams with orbital angular momentum (OAM). These methods include the practice of diffractive optics elements (DOEs), metasurfaces (MSs), and photonic integrated circuits (PICs) for the production of in-plane and out-of-plane OAM. This topic has been significantly evolved as a result; these three methods have been further implemented efficiently by different novel approaches which are discussed as well. Furthermore, development in the OAM detection techniques has also been presented. We have tried our best to bring novel and up-to-date information to the readers on this interesting and widely investigated topic.

Over the last 20 years, thin and lightweight optical elements have become very desirable, especially for the terahertz (THz) range. Reduction of the volume of optical elements alongside an increase in their effective efficiency has begun a new direction of research leading to many practical applications. On top of that, diffractive optical elements can not only focus the incident beam, but also can shape the incoming wavefront into a desirable distribution or can redirect the energy. Starting from theoretical calculations of Fourier optics, diffractive elements have been transformed and nowadays form complicated structures that do not resemble a typical Fresnel lens. The precise control over a phase shift introduced by the designed element creates an opportunity to almost freely transform an incident wavefront. Moreover, the vast diversity of computer-generated holograms (also called synthetic) contributes substantially to this topic. Diffractive elements have a great impact on THz optical systems because their manufacturing is very simple in comparison with any other range of radiation (infrared, visible, ultraviolet, etc.). This review paper underlines developments in evolution of diffractive optics and highlights main principles and technological approaches for fabrication of diffraction optics within the terahertz range, thus serving as a guide to design and production considerations.

The angular response of thin diffractive optical elements is highly correlated. For example, the angles of incidence and diffraction of a grating are locked through the grating momentum determined by the grating period. Other diffractive devices, including conventional metasurfaces, have a similar angular behavior due to the fixed locations of the Fresnel zone boundaries and the weak angular sensitivity of the meta-atoms. To alter this fundamental property, we introduce angle-multiplexed metasurfaces, composed of reflective high-contrast dielectric U-shaped meta-atoms, whose response under illumination from different angles can be controlled independently. This enables flat optical devices that impose different and independent optical transformations when illuminated from different directions, a capability not previously available in diffractive optics.

Liquid crystal Pacharatnam-Berry phase optical elements (PBOEs) have found promising applications in augmented reality and virtual reality because of their slim formfactor, lightweight, and high optical efficiency. However, chromatic aberration remains a serious longstanding problem for diffractive optics, hindering their broader adoption. To overcome the chromatic aberrations for red, green and blue (RGB) light sources, in this paper, we propose a counterintuitive multi-twist structure to achieve narrowband PBOEs without crosstalk, which plays a vital role to eliminate the chromatic aberration. The performance of our designed and fabricated narrowband Pacharatnam-Berry lenses (PBLs) aligns well with our simulation results. Furthermore, in a feasibility demonstration experiment using a laser projector, our proposed PBL system indeed exhibits a diminished chromatic aberration as compared to a broadband PBL. Additionally, polarization raytracing is implemented to demonstrate the versatility of the multi-twist structure for designing any RGB wavelengths with high contrast ratios. This analysis explores the feasibility of using RGB laser lines and quantum dot light-emitting diodes. Overall, our approach enables high optical efficiency, low fabrication complexity, and high degree of design freedom to accommodate any liquid crystal material and RGB light sources, holding immense potential for widespread applications of achromatic PBOEs.

Optical-resolution photoacoustic microscopy can visualize wavelength-dependent optical absorption at the cellular level. However, this technique suffers from a limited depth of field due to the tight focus of the optical excitation beam, making it challenging to acquire high-resolution images of samples with uneven surfaces or high-quality volumetric images without z scanning. To overcome this limitation, we propose needle-shaped beam photoacoustic microscopy, which can extend the depth of field to around a 28-fold Rayleigh length via customized diffractive optical elements. These diffractive optical elements generate a needle-shaped beam with a well-maintained beam diameter, a uniform axial intensity distribution and negligible sidelobes. The advantage of using needle-shaped beam photoacoustic microscopy is demonstrated via both histology-like imaging of fresh slide-free organs using a 266 nm laser and in vivo mouse-brain vasculature imaging using a 532 nm laser. This approach provides new perspectives for slide-free intraoperative pathological imaging and in vivo organ-level imaging.The use of a needle-shaped optical beam improves the depth of field for photoacoustic imaging.

Structured light has proven instrumental in three-dimensional imaging, LiDAR and holographic light projection. Metasurfaces, comprising subwavelength-sized nanostructures, facilitate 180°-field-of-view structured light, circumventing the restricted field of view inherent in traditional optics like diffractive optical elements. However, extant-metasurface-facilitated structured light exhibits sub-optimal performance in downstream tasks, due to heuristic design patterns such as periodic dots that do not consider the objectives of the end application. Here we present 360° structured light, driven by learned metasurfaces. We propose a differentiable framework that encompasses a computationally efficient 180° wave propagation model and a task-specific reconstructor, and exploits both transmission and reflection channels of the metasurface. Leveraging a first-order optimizer within our differentiable framework, we optimize the metasurface design, thereby realizing 360° structured light. We have utilized 360° structured light for holographic light projection and three-dimensional imaging. Specifically, we demonstrate the first 360° light projection of complex patterns, enabled by our propagation model that can be computationally evaluated 50,000× faster than the Rayleigh–Sommerfeld propagation. For three-dimensional imaging, we improve the depth-estimation accuracy by 5.09× in root-mean-square error compared with heuristically designed structured light. Such 360° structured light promises robust 360° imaging and display for robotics, extended-reality systems and human–computer interactions. A single-metasurface-based holographic light projection covering the whole 360° field of view is realized by optimizing the metasurface design through a neural network and applying 360° structured light for holographic light projection and three-dimensional imaging.

Augmented reality (AR) display, which superimposes virtual images on ambient scene, can visually blend the physical world and the digital world and thus opens a new vista for human–machine interaction. AR display is considered as one of the next-generation display technologies and has been drawing huge attention from both academia and industry. Current AR display systems operate based on a combination of various refractive, reflective, and diffractive optical elements, such as lenses, prisms, mirrors, and gratings. Constrained by the underlying physical mechanisms, these conventional elements only provide limited light-field modulation capability and suffer from issues such as bulky volume and considerable dispersion, resulting in large size, severe chromatic aberration, and narrow field of view of the composed AR display system. Recent years have witnessed the emerging of a new type of optical elements—metasurfaces, which are planar arrays of subwavelength electromagnetic structures that feature an ultracompact footprint and flexible light-field modulation capability, and are widely believed to be an enabling tool for overcoming the limitations faced by current AR displays. Here, we aim to provide a comprehensive review on the recent development of metasurface-enabled AR display technology. We first familiarize readers with the fundamentals of AR display, covering its basic working principle, existing conventional-optics-based solutions, as well as the associated pros and cons. We then introduce the concept of optical metasurfaces, emphasizing typical operating mechanisms, and representative phase modulation methods. We elaborate on three kinds of metasurface devices, namely, metalenses, metacouplers, and metaholograms, which have empowered different forms of AR displays. Their physical principles, device designs, and the performance improvement of the associated AR displays are explained in details. In the end, we discuss the existing challenges of metasurface optics for AR display applications and provide our perspective on future research endeavors.

Deep optical optimization has recently emerged as a new paradigm for designing computational imaging systems using only the output image as the objective. However, it has been limited to either simple optical systems consisting of a single element such as a diffractive optical element or metalens, or the fine-tuning of compound lenses from good initial designs. Here we present a DeepLens design method based on curriculum learning, which is able to learn optical designs of compound lenses ab initio from randomly initialized surfaces without human intervention, therefore overcoming the need for a good initial design. We demonstrate the effectiveness of our approach by fully automatically designing both classical imaging lenses and a large field-of-view extended depth-of-field computational lens in a cellphone-style form factor, with highly aspheric surfaces and a short back focal length. The authors present a design method based on curriculum learning, able to learn optical designs of compound lenses from randomly initialized surfaces without human intervention, demonstrating fully automated design of both classical imaging lenses and extended depth-of-field computational lenses.

Recently, diffractive optics systems have garnered increasing attention due to their myriad benefits in various applications, such as creating vortex beams, Bessel beams, or optical traps, while refractive optics systems still exhibit some disadvantages related to materials, substrates, and intensity shapes. The manufacturing of diffractive optical elements has become easier due to the development of lithography techniques such as direct laser writing, photo lithography, and electron beam lithography. In this paper, we improve the results from previous research and propose a new methodology to design and fabricate advanced binary diffractive optical elements that achieve a square focal spot independently, reducing reliance on additional components. By integrating a binary square zone plate with an axicon zone plate of the same scale, we employ machine learning for laser path optimization and direct laser lithography for manufacturing. This streamlined approach enhances simplicity, accuracy, efficiency, and cost effectiveness. Our upgraded binary diffractive optical elements are ready for real-world applications, marking a significant improvement in optical capabilities.

This review focuses on recent developments in additive manufacturing (AM) of precision optical devices, particularly devices consisting of components with critical features at the micro- and nanoscale. These include, but are not limited to, microlenses, diffractive optical elements, and photonic devices. However, optical devices with large-size lenses and mirrors are not specifically included as this technology has not demonstrated feasibilities in that category. The review is roughly divided into two slightly separated topics, the first on meso- and microoptics and the second on optics with nanoscale features. Although AM of precision optics is still in its infancy with many unanswered questions, the references cited on this exciting topic demonstrate an enabling technology with almost unlimited possibilities. There are many high quality reviews of AM processes of non-optical components, hence they are not the focus of this review. The main purpose of this review is to start a conversion on optical fabrication based on information about 3D AM methods that has been made available to date, with an ultimate long-term goal of establishing new optical manufacturing methods that are low cost and highly precise with extreme flexibility.