Review

Design and manufacture AR head-mounted displays: A review and outlook
Dewen Cheng, Qiwei Wang, Yue Liu, Hailong Chen, Dongwei Ni, et al.
Published Published online: 25 September 2021 , doi: 10.37188/lam.2021.024
Augmented reality head-mounted displays (AR-HMDs) enable users to see real images of the outside world and visualize virtual information generated by a computer at any time and from any location, making them useful for various applications. The manufacture of AR-HMDs combines the fields of optical engineering, optical materials, optical coating, precision manufacturing, electronic science, computer science, physiology, ergonomics, etc. This paper primarily focuses on the optical engineering of AR-HMDs. Optical combiners and display devices are used to combine real-world and virtual-world objects that are visible to the human eye. In this review, existing AR-HMD optical solutions employed for optical combiners are divided into three categories: optical solutions based on macro-, micro-, and nanooptics. The physical principles, optical structure, performance parameters, and manufacturing process of different types of AR-HMD optical solutions are subsequently analyzed. Moreover, their advantages and disadvantages are investigated and evaluated. In addition, the bottlenecks and future development trends in the case of AR-HMD optical solutions are discussed.
A review of common-path off-axis digital holography: towards high stable optical instrument manufacturing
Jiwei Zhang, Siqing Dai, Chaojie Ma, Teli Xi, Jianglei Di, et al.
Published Published online: 01 August 2021 , doi: 10.37188/lam.2021.023

Digital holography possesses the advantages of wide-field, non-contact, precise, and dynamic measurements for the complex amplitude of object waves. Today, digital holography and its derivatives have been widely applied in interferometric measurements, three-dimensional imaging, and quantitative phase imaging, demonstrating significant potential in the material science, industry, and biomedical fields, among others. However, in conventional off-axis holographic experimental setups, the object and reference beams propagate in separated paths, resulting in low temporal stability and measurement sensitivity. By designing common-path configurations where the two interference beams share the same or similar paths, environmental disturbance to the two beams can be effectively compensated. Therefore, the temporal stability of the experimental setups for hologram recording can be significantly improved for time-lapsing measurements. In this review, we categorise the common-path models as lateral shearing, point diffraction, and other types based on the different approaches to generate the reference beam. Benefiting from compact features, common-path digital holography is extremely promising for the manufacture of highly stable optical measurement and imaging instruments in the future.

Review of laser powder bed fusion (LPBF) fabricated Ti-6Al-4V: process, post-process treatment, microstructure, and property
Sheng Cao, Yichao Zou, Chao Voon Samuel Lim, Xinhua Wu
Published Published online: 01 July 2021 , doi: 10.37188/lam.2021.020

Laser powder bed fusion (LPBF) is a timely important additive manufacturing technique that offers many opportunities for fabricating three-dimensional complex shaped components at a high resolution with short lead times. This technique has been extensively employed in manufacturing Ti-6Al-4V parts for aerospace and biomedical applications. However, many challenges, including poor surface quality, porosity, anisotropy in microstructure and property, and difficulty in tailoring microstructure, still exist. In this paper, we review the recent progress in post-process treatment and its influence on the microstructure evolution and material performance, including tensile, fatigue, fracture toughness, creep, and corrosion properties. The contradictions in simultaneously achieving high strength/ductility and strength/fracture toughness/creep resistance have been identified. Furthermore, research gaps in understanding the effects of the emerging bi-modal microstructure on fatigue properties and fracture toughness require further investigation.

Multi-material multi-photon 3D laser micro- and nanoprinting
Liang Yang, Frederik Mayer, Uwe H. F. Bunz, Eva Blasco, Martin Wegener
Published Published online: 01 May 2021 , doi: 10.37188/lam.2021.017
Three-dimensional (3D) laser micro- and nanoprinting based upon multi-photon absorption has made its way from early scientific discovery to industrial manufacturing processes, e.g., for advanced microoptical components. However, so far, most realized 3D architectures are composed of only a single polymeric material. Here, we review 3D printing of multi-materials on the nano- and microscale. We start with material properties that have been realized, using multi-photon photoresists. Printed materials include bulk polymers, conductive polymers, metals, nanoporous polymers, silica glass, chalcogenide glasses, inorganic single crystals, natural polymers, stimuli-responsive materials, and polymer composites. Next, we review manual and automated processes achieving dissimilar material properties in a single 3D structure by sequentially photo-exposing multiple photoresists as 3D analogs of 2D multicolor printing. Instructive examples from biology, optics, mechanics, and electronics are discussed. An emerging approach – without counterpart in 2D graphical printing – prints 3D structures combining dissimilar material properties in one 3D structure by using only a single photoresist. A controlled stimulus applied during the 3D printing process defines and determines material properties on the voxel level. Change of laser power and/or wavelength, or application of quasi-static electric fields allow for the seamless manipulation of desired materials properties.
Digital holography in production: an overview
Markus Fratz, Tobias Seyler, Alexander Bertz, Daniel Carl
Published Published online: 01 April 2021 , doi: 10.37188/lam.2021.015
Many challenging measurement tasks in production simultaneously have high requirements for accuracy, measurement field size, lateral sampling, and measurement time. In this paper, we provide an overview of the current state of the art in digital holography for surface topography measurements and present three applications from completely different productions with no alternative to digital holography; we describe the HoloTop sensor family, which has been designed specifically for industrial use, and present the most recent results achieved in real-life industrial applications. All applications address measurement tasks that could not be solved until now, either by optical or tactile means. We start with a description of the first-ever inline integration of a digital holographic measurement system that inspects precision turned parts for the automotive industry. We proceed by presenting measurements performed with a compact sensor that can be placed inside a tooling machine and operated fully wirelessly. In this case, the tool holder was used to position the sensor directly. Integration into a tooling machine places high demands on both robustness and reliability. Finally, the quality control of electronic interconnectors such as microbumps with the highest demand for accuracy and repeatability is demonstrated. All of these applications illustrate the major advantages of digital holographic systems: it is possible to measure a relatively large field of view with interferometric precision and very short acquisition times. Additionally, both reflective and matt surfaces can be measured simultaneously. We end this publication with an assessment of the future potential of this technology and the necessary development steps involved.
Recent Advances in the Fabrication of Highly Sensitive Surface-Enhanced Raman Scattering Substrates: Nanomolar to Attomolar Level Sensing
Shi Bai, Koji Sugioka
Published Published online: 30 June 2021 , doi: 10.37188/lam.2021.013
Surface-enhanced Raman scattering (SERS) techniques have rapidly advanced over the last two decades, permitting multidisciplinary trace analyses and the potential detection of single molecules. This paper provides a comprehensive review of recent progress in strategies for the fabrication of highly sensitive SERS substrates, as a means of achieving sensing on the attomolar scale. The review examines widely used performance criteria, such as enhancement factors. In addition, femtosecond laser-based techniques are discussed as a versatile tool for the fabrication of SERS substrates. Several approaches for enhancing the performance of SERS sensing devices are also introduced, including photo-induced, transient, and liquid-interface assisted strategies. Finally, substrates for real-time sensing and biological applications are also reviewed to demonstrate the powerful analytical capabilities of these methods and the significant progress in SERS research.
Metasurfaces for manipulating terahertz waves
Xiaofei Zang, Bingshuang Yao, Lin Chen, Jingya Xie, Xuguang Guo, et al.
Published Published online: 30 June 2021 , doi: 10.37188/lam.2021.010
Terahertz (THz) science and technology have attracted significant attention based on their unique applications in non-destructive imaging, communications, spectroscopic detection, and sensing. However, traditional THz devices must be sufficiently thick to realise the desired wave-manipulating functions, which has hindered the development of THz integrated systems and applications. Metasurfaces, which are two-dimensional metamaterials consisting of predesigned meta-atoms, can accurately tailor the amplitudes, phases, and polarisations of electromagnetic waves at subwavelength resolutions, meaning they can provide a flexible platform for designing ultra-compact and high-performance THz components. This review focuses on recent advancements in metasurfaces for the wavefront manipulation of THz waves, including the planar metalens, holograms, arbitrary polarisation control, special beam generation, and active metasurface devices. Such ultra-compact devices with unique functionality make metasurface devices very attractive for applications such as imaging, encryption, information modulation, and THz communications. This progress report aims to highlight some novel approaches for designing ultra-compact THz devices and broaden the applications of metasurfaces in THz science.