Recent Progress in Heterogeneous III-V-on-Silicon Photonic Integration
Di Liang, John E. Bowers
Published Published online: 24 February 2021,  doi: 10.37188/lam.2021.005
Silicon (Si) photonics is a disruptive technology on the fast track to revolutionise integrated photonics. An indispensable branch thereof, heterogeneous Si integration, has also evolved from a science project 15 years ago to a growing business and compelling research field today. We focus on the scope of III-V compound semiconductors heterogeneously integrated on Si substrates. The commercial success of massively produced integrated optical transceivers based on first-generation innovation is discussed. Then, we review a number of technological breakthroughs at the component and platform levels. In addition to the numerous new device performance records, our emphasis is on the rationale behind and the design principles underlying specific examples of materials and device integration. Finally, we offer perspectives on development trends catering to the increasing demand in many existing and emerging applications.
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3D-printed miniature spectrometer for the visible range with a 100 × 100 μm2 footprint
Andrea Toulouse, Johannes Drozella, Simon Thiele, Harald Giessen, Alois Herkommer
Published Published online: 04 February 2021,  doi: 10.37188/lam.2021.002
The miniaturisation of spectroscopic measurement devices opens novel information channels for size critical applications such as endoscopy or consumer electronics. Computational spectrometers in the micrometre size range have been demonstrated, however, these are calibration sensitive and based on complex reconstruction algorithms. Herein we present an angle-insensitive 3D-printed miniature spectrometer with a direct separated spatial-spectral response. The spectrometer was fabricated via two-photon direct laser writing combined with a super-fine inkjet process. It has a volume of less than 100 × 100 × 300 μm3. Its tailored and chirped high-frequency grating enables strongly dispersive behaviour. The miniature spectrometer features a wavelength range of 200 nm in the visible range from 490 nm to 690 nm. It has a spectral resolution of 9.2 ± 1.1 nm at 532 nm and 17.8 ± 1.7 nm at a wavelength of 633 nm. Printing this spectrometer directly onto camera sensors is feasible and can be replicated for use as a macro-pixel of a snapshot hyperspectral camera.
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Holography, and the future of 3D display
Pierre-Alexandre Blanche
Published Published online: 20 December 2021,  doi: 10.37188/lam.2021.028
The pioneers of holography, Gabor, Leith, Upatnieks, and Denisyuk, predicted very early that the ultimate 3D display will be based on this technique. This conviction was rooted on the fact that holography is the only approach that can render all optical cues interpreted by the human visual system. Holographic 3D displays have been a dream chased after for many years, facing challenges on all fronts: computation, transmission, and rendering. With numbers such as 6.6 × 1015 flops required for calculations, 3 × 1015 b/s data rates, and 1.6 × 1012 phase pixels, the task has been daunting. This article is reviewing the recent accomplishments made in the field of holographic 3D display. Specifically, the new developments in machine learning and neural network algorithms demonstrating that computer-generated holograms approach real-time processing. A section also discuss the problem of data transmission that can arguably be solved using clever compression algorithms and optical fiber transmission lines. Finally, we introduce the last obstacle to holographic 3D display, which is is the rendering hardware. However, there is no further mystery. With larger and faster spatial light modulators (SLMs), holographic projection systems are constantly improving. The pixel count on liquid crystal on silicon (LCoS) as well as microelectromechanical systems (MEMS) phase displays is increasing by the millions, and new photonic integrated circuit phased arrays are achieving real progress. It is only a matter of time for these systems to leave the laboratory and enter the consumer world. The future of 3D displays is holographic, and it is happening now.
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Multi-material multi-photon 3D laser micro- and nanoprinting
Liang Yang, Frederik Mayer, Uwe H. F. Bunz, Eva Blasco, Martin Wegener
Published Published online: 21 June 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.
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Two-photon photopolymerization directly initiated by spiropyran photochromic molecules
Dandan Ge, Jean Aubard, Erell Bodinier, Safi Jradi, Stéphanie Lau-Truong, Nordin Felidj, Renaud Bachelot, Anne-Laure Baudrion
Accepted  doi: 10.37188/lam.2023.004

Here, we report the ability of spiropyrans to initiate two-photon polymerization (TPP) for the first time in the literature. The comparison and synergies between the spiropyran photochromic molecule of interest, namely 6-nitro-BIPS, and well-known photoinitiators of radical photopolymerization have been studied. The spiropyran (SPy) molecule can initiate TPP in the presence of trifunctional acrylic monomers and create true 3D structures. The comparison with Irgacure 819, a well-known Type-I photoinitiator, shows that SPy has a comparable capability for TPP. In addition, the combination of SPy with methyl diethanolamine increased the reactivity of both one- and two-photon polymerizations. In the last section, we discuss which SPy isomer is the active photochromic species capable of generating radicals for initiating two-photon polymerization.

3D-printed facet-attached microlenses for advanced photonic system assembly
Yilin Xu, Pascal Maier, Mareike Trappen, Philipp-Immanuel Dietrich, Matthias Blaicher, Rokas Jutas, Achim Weber, Torben Kind, Colin Dankwart, Jens Stephan, Andreas Steffan, Amin Abbasi, Padraic Morrissey, Kamil Gradkowski, Brian Kelly, Peter O’Brien, Wolfgang Freude, Christian Koos
Accepted  doi: 10.37188/lam.2023.003

Wafer-level mass production of photonic integrated circuits (PIC) has become a technological mainstay in the field of optics and photonics, enabling many novel and disrupting a wide range of existing applications. However, scalable photonic packaging and system assembly still represents a major challenge that often hinders commercial adoption of PIC-based solutions. Specifically, chip-to-chip and fiber-to-chip connections often rely on so-called active alignment techniques, where the coupling efficiency is continuously measured and optimized during the assembly process. This unavoidably leads to technically complex assembly processes and high cost, thereby eliminating most of the inherent scalability advantages of PIC-based solutions. In this paper, we demonstrate that 3D-printed facet-attached microlenses (FaML) can overcome this problem by opening an attractive path towards highly scalable photonic system assembly, relying entirely on passive assembly techniques based on industry-standard machine vision and/or simple mechanical stops. FaML can be printed with high precision to the facets of optical components using multi-photon lithography, thereby offering the possibility to shape the emitted beams by freely designed refractive or reflective surfaces. Specifically, the emitted beams can be collimated to a comparatively large diameter that is independent of the device-specific mode fields, thereby relaxing both axial and lateral alignment tolerances. Moreover, the FaML concept allows to insert discrete optical elements such as optical isolators into the free-space beam paths between PIC facets. We show the viability and the versatility of the scheme in a series of selected experiments of high technical relevance, comprising pluggable fiber-chip interfaces, the combination of PIC with discrete micro-optical elements such as polarization beam splitters, as well as coupling with ultra-low back-reflection based on non-planar beam paths that only comprise tilted optical surfaces. Based on our results, we believe that the FaML concept opens an attractive path towards novel PIC-based system architectures that combine the distinct advantages of different photonic integration platforms.

Miniature viscometer incorporating GaN optical devices with an ultrawide measurement range
Yumeng Luo, Gaofei Lu, Qi Wang, Zhiqin Chu, Kwai Hei Li
Accepted  doi: 10.37188/lam.2023.002
Automated optical inspection of FAST’s reflector surface using drones and computer vision
Jianan Li, Shenwang Jiang, Liqiang Song, Peiran Peng, Feng Mu, et al.
Published Published online: 05 January 2023,  doi: 10.37188/lam.2023.001

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is the world ’ s largest single-dish radio telescope. Its large reflecting surface achieves unprecedented sensitivity but is prone to damage, such as dents and holes, caused by naturally-occurring falling objects. Hence, the timely and accurate detection of surface defects is crucial for FAST’s stable operation. Conventional manual inspection involves human inspectors climbing up and examining the large surface visually, a time-consuming and potentially unreliable process. To accelerate the inspection process and increase its accuracy, this work makes the first step towards automating the inspection of FAST by integrating deep-learning techniques with drone technology. First, a drone flies over the surface along a predetermined route. Since surface defects significantly vary in scale and show high inter-class similarity, directly applying existing deep detectors to detect defects on the drone imagery is highly prone to missing and misidentifying defects. As a remedy, we introduce cross-fusion, a dedicated plug-in operation for deep detectors that enables the adaptive fusion of multi-level features in a point-wise selective fashion, depending on local defect patterns. Consequently, strong semantics and fine-grained details are dynamically fused at different positions to support the accurate detection of defects of various scales and types. Our AI-powered drone-based automated inspection is time-efficient, reliable, and has good accessibility, which guarantees the long-term and stable operation of FAST.

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Crystal plane engineering of MAPbI3 in epoxy-based materials for superior gamma-ray shielding performance
Kai Cui, Yang Li, Wenjing Wei, Qianqian Teng, Tianyu Zhang, et al.
Published Published online: 08 December 2022,  doi: 10.37188/lam.2022.051

The rapid development of the aerospace and nuclear industries is accompanied by increased exposure to high-energy ionising radiation. Thus, the performance of radiation shielding materials needs to be improved to extend the service life of detectors and ensure the safety of personnel. The development of novel lightweight materials with high electron density has therefore become urgent to alleviate radiation risks. In this work, new MAPbI3/epoxy (CH3NH3PbI3/epoxy) composites were prepared via a crystal plane engineering strategy. These composites delivered excellent radiation shielding performance against 59.5 keV gamma rays. A high linear attenuation coefficient (1.887 cm−1) and mass attenuation coefficient (1.352 cm2 g−1) were achieved for a representative MAPbI3/epoxy composite, which was 10 times higher than that of the epoxy. Theoretical calculations indicate that the electron density of MAPbI3/epoxy composites significantly increases when the content ratio of the (110) plane in MAPbI3 increases. As a result, the chances of collision between the incident gamma rays and electrons in the MAPbI3/epoxy composites were enhanced. The present work provides a novel strategy for designing and developing high-efficiency radiation shielding materials.

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Tip-induced bond weakening, tilting, and hopping of a single CO molecule on Cu(100)
Xiaoru Dong, Ben Yang, Rui Zhu, Ruipu Wang, Yang Zhang, et al.
Published Published online: 01 December 2022,  doi: 10.37188/lam.2022.052

The interaction between a probing tip and an adsorbed molecule can significantly impact the molecular chemical structure and even induce its motion on the surface. In this study, the tip-induced bond weakening, tilting, and hopping processes of a single molecule were investigated by sub-nanometre resolved tip-enhanced Raman spectroscopy (TERS). We used single carbon monoxide (CO) molecules adsorbed on the Cu (100) surface as a model system for the investigation. The vibrational frequency of the C−O stretching mode is always redshifted as the tip approaches, revealing the weakening of the C−O bond owing to tip−molecule interactions. Further analyses of both the vibrational Stark effect and TERS imaging patterns suggest a delicate tilting phenomenon of the adsorbed CO molecule on Cu(100), which eventually leads to lateral hopping of the molecule. While a tilting orientation is found toward the hollow site along the [110] direction of the Cu(100) surface, the hopping event is more likely to proceed via the bridge site to the nearest Cu neighbour along the [100] or [010] direction. Our results provide deep insights into the microscopic mechanisms of tip−molecule interactions and tip-induced molecular motions on surfaces at the single-bond level.

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Mid-infrared all-optical modulators based on an acetylene-filled hollow-core fiber
Kaiyuan Zheng, Shoulin Jiang, Feifan Chen, Yan Zhao, Shoufei Gao, et al.
Published Published online: 17 November 2022,  doi: 10.37188/lam.2022.050

We report all-optical mid-infrared phase and intensity modulators based on the photo-thermal effect in an acetylene-filled anti-resonant hollow-core fiber. Optical absorption of the control beam promotes the gas molecules to a higher energy level, which induces localized heating through non-radiative relaxation and modulates the refractive index of the gas material and hence the accumulated phase of the signal beam propagating through the hollow-core fiber. By modulating the intensity of the control beam, the phase of the signal beam is modulated accordingly. By use of a 1.53 µm near-infrared control beam, all-optical phase modulation up to 2.2π rad is experimentally demonstrated at the signal wavelength of 3.35 µm. With the phase modulator placed in one arm of a Mach-Zehnder interferometer, intensity modulation with on-off ratio of 25 dB is achieved. The gas-filled hollow-core-fiber modulators could operate over an ultra-broad wavelength band from near- to mid-infrared and have promising application in mid-infrared photonic systems.

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Tilted Wave Fizeau Interferometer for flexible and robust asphere and freeform testing
Christian Schober, Rolf Beisswanger, Antonia Gronle, Christof Pruss, Wolfgang Osten
Published Published online: 28 October 2022,  doi: 10.37188/lam.2022.048

Tilted Wave Interferometry (TWI) is a measurement technique for fast and flexible interferometric testing of aspheres and freeform surfaces. The first version of the tilted wave principle was implemented in a Twyman-Green type setup with separate reference arm, which is intrinsically susceptible to environmentally induced phase disturbances. In this contribution we present the TWI in a new robust common-path (Fizeau) configuration. The implementation of the Tilted Wave Fizeau Interferometer requires a new approach in illumination, calibration and evaluation. Measurements of two aspheres and a freeform surface show the flexibility and also the increased stability in both phase raw data and surface measurements, which leads to a reduced repeatability up to a factor of three. The novel configuration significantly relaxes the tolerances of the imaging optics used in the interferometer. We demonstrate this using simulations on calibration measurements, where we see an improvement of one order of magnitude compared to the classical Twyman-Green TWI approach and the capability to compensate higher order error contributions on the used optics.

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FEM-based modeling of microsphere-enhanced interferometry
Tobias Pahl, Lucie Hüser, Sebastian Hagemeier, Peter Lehmann
Published Published online: 28 October 2022,  doi: 10.37188/lam.2022.049

To improve the lateral resolution in microscopic imaging, microspheres are placed close to the object’ s surface in order to support the imaging process by optical near-field information. Although microsphere-assisted measurements are part of various recent studies, no generally accepted explanation for the effect of microspheres exists. Photonic nanojets, enhancement of the numerical aperture, whispering-gallery modes and evanescent waves are usually named reasons in context with microspheres, though none of these effects is proven to be decisive for the resolution enhancement. We present a simulation model of the complete microscopic imaging process of microsphere-enhanced interference microscopy including a rigorous treatment of the light scattering process at the surface of the specimen. The model consideres objective lenses of high numerical aperture providing 3D conical illumination and imaging. The enhanced resolution and magnification by the microsphere is analyzed with respect to the numerical aperture of the objective lenses. Further, we give a criterion for the achievable resolution and demonstrate that a local enhancement of the numerical aperture is the most likely reason for the resolution enhancement.

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High-speed, large-area and high-precision fabrication of aspheric micro-lens array based on 12-bit direct laser writing lithography
Shiyi Luan, Fei Peng, Guoxing Zheng, Chengqun Gui, Yi Song, et al.
Published Published online: 19 September 2022,  doi: 10.37188/lam.2022.047

Aspheric micro-lens array (AMLA), featured with low dispersion and diffraction-limited imaging quality, plays an important role in advanced optical imaging. Ideally, the fabrication of commercially applicable AMLAs should feature low cost, high precision, large area and high speed. However, these criteria have been achieved only partially with conventional fabrication process. Herein, we demonstrate the fabrication and characterization of AMLAs based on 12-bit direct laser writing lithography, which exhibits a high fabrication speed, large area, perfect lens shape control via a three-dimensional optical proximity correction and average surface roughness lower than 6 nm. In particular, the AMLAs can be flexibly designed with customized filling factor and arbitrary off-axis operation for each single micro-lens, and the proposed pattern transfer approach with polydimethylsiloxane (PDMS) suggests a low-cost way for mass manufacturing. An auto-stereoscopic-display flexible thin film with excellent display effect has been prepared by using above technology, which exhibits a new way to provide flexible auto-stereoscopic-display at low cost. In brief, the demonstrated fabrication of AMLAs based on direct laser writing lithography reduce the complexity of AMLA fabrication while significantly increasing their performance, suggesting a new route for high-quality three-dimentional optical manufacturing towards simplified fabrication process, high precision and large scale.

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A quasi-3D Fano resonance cavity on optical fiber end-facet for high signal-to-noise ratio dip-and-read surface plasmon sensing
Xiaqing Sun, Zeyu Lei, Hao Zhong, Chenjia He, Sihang Liu, et al.
Published Published online: 18 September 2022,  doi: 10.37188/lam.2022.046

Surface plasmon devices mounted at the end-facets of optical fibers are appealing candidates for rapid and point-of-care sensing applications, by offering a special dip-and-read operation mode. At present, these devices’ noise-equivalent limits-of-detection lag far behind the free-space counterparts, leaving them incapable of most biosensing applications. Here we report a quasi-3D Fano resonance cavity and its fabrication method to fundamentally improve the quality factor and coupling efficiency for fiber-coupled surface plasmon resonance. In this device, the Fano resonance combines the high coupling efficiency of a Fabry-Pérot etalon and the high quality factor resonance of a plasmonic crystal cavity. The quasi-3D device was fabricated on a planar substrate and transferred to a single-mode fiber end-facet, which requires a low-adhesion yet surface-plasmon-tunneling interface between the device and the planar substrate. Such an interface was realized with a nanocap-slit unit structure, of which the plasmonic crystal was consisted. A noise-equivalent limit of detection of ~ 10-7 RIU was experimentally obtained, allowing bovine serum albumin physical adsorption to be distinguished at ng mL-1 level concentrations. Therefore, breaking through the long-standing signal-to-noise ratio bottleneck, this work makes fiber end-facet surface plasmon devices into one of high sensitivity label-free sensing technologies. At the same time, it provides an enabling top-down fabrication technology for making 3D plasmonic structures on fiber end-facets at the nanometer scale.

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‘Plug-and-play’ plasmonic metafibers for ultrafast fibre lasers
Lei Zhang, Huiru Zhang, Ni Tang, Xiren Chen, Fengjiang Liu, et al.
Published Published online: 02 September 2022,  doi: 10.37188/lam.2022.045
Metafibers expand the functionalities of conventional optical fibres to unprecedented nanoscale light manipulations by integrating metasurfaces on the fibre tips, becoming an emerging light-coupling platform for both the nanoscience and fibre optics communities. Current metafibers remain proof-of-concept demonstrations that mostly explore isolated bare fibres owing to the lack of standard interfaces with universal fibre networks. Here, we develop methodologies for fabricating well-defined plasmonic metasurfaces directly on the end facets of commercial single -mode fibre jumpers using standard planar technologies and provide the first demonstration of their practical applications in the nonlinear plasmonic regime. Featuring plug-and-play connections with fibre circuitry and arbitrary metasurface landscapes, the metafibers with tunable plasmonic resonances are implemented into fibre laser cavities, yielding all-fibre sub-picosecond (minimum 513 fs) soliton mode locked lasers at optical wavelengths of 1.5 μm and 2 μm, demonstrating their unusual polarimetric nonlinear transfer functions and superior saturation absorption responses. The nanofabrication process flow is compatible with existing cleanroom technologies, offering metafibers an avenue to become a regular member of functionalised fibre components. This work paves the way toward the next generation of ultrafast lasers, optical frequency combs, and ultracompact ‘all-in-fibre’ optical systems.
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Design of broadband terahertz vector and vortex beams: II. Holographic assessment
Nikolay V. Petrov, Bogdan Sokolenko, Maksim S. Kulya, Andrei Gorodetsky, Aleksey V. Chernykh
Published Published online: 02 August 2022,  doi: 10.37188/lam.2022.044
In this paper, we demonstrate the capabilities of the terahertz pulse time-domain holography in visualisation, simulation, and assessment of broadband THz vortex beam formation dynamics upon its shaping by elements of beam converter, and further propagation and manipulation. By adding Jones matrix formalism to describe broadband optical elements, we highlight the differences in the spatio-spectral and spatio-temporal structure of the formed vortex and vector beams dependence on the modulator used and visualise their modal features. The influence of diffraction field structure from each element in the broadband vortex modulator is revealed in numerical simulation and the formed beams are analysed against the simplified Laguerre-Gaussian beam model.
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Material removal model of magnetorheological finishing based on dense granular flow theory
Yang Bai, Xuejun Zhang, Chao Yang, Longxiang Li, Xiao Luo
Published Published online: 06 July 2022,  doi: 10.37188/lam.2022.041
Magnetorheological finishing (MRF) technology is widely used in the fabrication of high-precision optical elements. The material removal mechanism of MRF has not been fully understood because MRF technology involves the integration of electromagnetics, contact mechanics, and materials science. In this study, the rheological properties of the MR polishing fluid in oscillation model have been investigated. We propose that the shear-thinned MR polishing fluid over the polishing area should be considered a dense granular flow, based on which a new contact model of MRF over the polishing area has been constructed. Removal function and processing force test experiments were conducted under different working gaps. The normal pressure and effective friction equations over the polishing area were built based on the continuous medium and dense granular flow theories. Then, a novel MRF material removal model was established. A comparison of the results of the theoretical model with actual polishing results demonstrated the accuracy of the established model. The novel model proposed herein reveals the generation mechanism of shear force over a polished workpiece and realizes effective decoupling of the main processing parameters that influence the material removal of MRF. The results of this study will provide new and effective theoretical guidance for the process optimization and technology improvement of MRF.
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Design of broadband terahertz vector and vortex beams: I. Review of materials and components
Nikolay V. Petrov, Bogdan Sokolenko, Maksim S. Kulya, Andrei Gorodetsky, Aleksey V. Chernykh
Published Published online: 02 August 2022,  doi: 10.37188/lam.2022.043
In this paper, we review the existing approaches for vortex and vector beam shaping and generation in the terahertz frequency range. The particular focus of this review is on the possibility of homogeneous topological charge formation in the ultra-wide spectral interval inherent to ultrashort terahertz pulses. We review the available materials and components, analyse proposed and potentially possible solutions for broadband terahertz vortex and vector beam shaping, compare all developed approaches, and put forward a unified concept for constructing passive shapers of such beams from the existing component base.
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Holographic optics in planar optical systems for next generation small form factor mixed reality headsets
Bernard C. Kress, Maria Pace
Published Published online: 02 August 2022,  doi: 10.37188/lam.2022.042
Helmet Mounted Displays (HMDs), such as in Virtual Reality (VR), Augmented Reality (AR), Mixed reality (MR), and Smart Glasses have the potential to revolutionize the way we live our private and professional lives, as in communicating, working, teaching and learning, shopping and getting entertained. Such HMD devices have to satisfy draconian requirements in weight, size, form factor, power, compute, wireless communication and of course display, imaging and sensing performances. We review in this paper the various optical technologies and architectures that have been developed in the past 10 years to provide adequate solutions for the drastic requirements of consumer HMDs, a market that has yet to become mature in the next years, unlike the existing enterprise and defense markets that have already adopted VR and AR headsets as practical tools to improve greatly effectiveness and productivity. We focus specifically our attention on the optical combiner element, a crucial element in Optical See-Through (OST) HMDs that combines the see-through scene with a world locked digital image. As for the technological platform, we chose optical waveguide combiners, although there is also a considerable effort today dedicated to free-space combiners. Flat and thin optics as in micro-optics, holographics, diffractives, metasurfaces and other nanostructured optical elements are key building blocks to achieve the target form factor.
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Review of engineering techniques in chaotic coded aperture imagers
Vijayakumar Anand, Joseph Rosen, Saulius Juodkazis
Published Published online: 01 May 2022,  doi: 10.37188/lam.2022.024
Coded aperture imaging (CAI) is a technique to image three-dimensional scenes with special controlled abilities. In this review, we survey several recently proposed techniques to control the parameters of CAI by engineering the aperture of the system. The prime architectures of these indirect methods of imaging are reviewed. For each design, we mention the relevant application of the CAI recorders and summarize this overview with a general perspective on this research topic.
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