Metasurfaces for manipulating terahertz waves
Xiaofei Zang, Bingshuang Yao, Lin Chen, Jingya Xie, Xuguang Guo, et al.
Published Published online: 22 March 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.
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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: 26 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.
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Resolution enhancement of digital holographic microscopy via synthetic aperture: a review
Peng Gao, Caojin Yuan
Published Published online: 27 January 2022,  doi: 10.37188/lam.2022.006
Digital holographic microscopy (DHM), which combines digital holography with optical microscopy, is a wide field, minimally invasive quantitative phase microscopy (QPM) approach for measuring the 3D shape or the inner structure of transparent and translucent samples. However, limited by diffraction, the spatial resolution of conventional DHM is relatively low and incompatible with a wide field of view (FOV) owing to the spatial bandwidth product (SBP) limit of the imaging systems. During the past decades, many efforts have been made to enhance the spatial resolution of DHM while preserving a large FOV by trading with unused degrees of freedom. Illumination modulation techniques, such as oblique illumination, structured illumination, and speckle illumination, can enhance the resolution by adding more high-frequency information to the recording system. Resolution enhancement is also achieved by extrapolation of a hologram or by synthesizing a larger hologram by scanning the sample, the camera, or inserting a diffraction grating between the sample and the camera. For on-chip DHM, spatial resolution is achieved using pixel super-resolution techniques. In this paper, we review various resolution enhancement approaches in DHM and discuss the advantages and disadvantages of these approaches. It is our hope that this review will contribute to advancements in DHM and its practical applications in many fields.
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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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Effect of near-field optical angular momentum on molecular junctions
Jianchen Zi, Michaël Lobet, Luc Henrard, Zhiqiang Li, Chenhui Wang, Xiaohong Wu, Hai Bi
Accepted  doi: 10.37188/lam.2023.034

The role of molecular junctions in nanoelectronics is most often associated with electronic transport; however, their precise characterisation hinders their widespread development. The interaction of light with molecular junctions is a supplementary factor for the development of molecular switches, but it has rarely been addressed. The influence of light interaction with molecular junctions on the response of molecules in the near field was demonstrated by properly characterising the optical angular momentum at the junctions. Consequently, the molecular switching dynamics were observed in the Raman signatures of the conducting molecules. The illumination geometry and voltage applied to the junction were changed to demonstrate numerically and experimentally how the Raman intensity can be turned ON and OFF with a difference of nearly five orders of magnitude. These molecular-scale operations result from the combined interaction of a current-induced electronic rearrangement in the molecular junction and a plasmonically enhanced electromagnetic field near the tip of the junction. This study of the effect of optical angular momentum on the near field of the molecular junction shows significant potential for the development of molecular electronics.

Feature size below 100 nm realized by UV-LED-based microscope projection photolithography
L. Zheng, T. Birr, U. Zywietz, C. Reinhardt, B. Roth
Accepted  doi: 10.37188/lam.2023.033

The demand for miniaturization and integration of optical elements has fostered the development of various micro- and nanofabrication technologies. In this work, we developed a low-cost UV-LED-based microscope projection photolithography system for rapid and high-resolution fabrication. This system can be easily implemented using off-the-shelf components. It allows for micro- and nanostructuring within seconds. By optimizing the process, a minimum feature size down to approximately 85 nm was successfully realized. In addition, investigations on fabrication of the same structures using both costly and economic microscope objectives were performed. Feature sizes below 100 nm can be stably achieved. The demonstrated approach extends the technology capabilities and may find applications in fields such as nanophotonics, biophotonics sensing and material science.

Two-photon lithography for integrated photonic packaging
Shaoliang Yu, Qingyang Du, Cleber Renato Mendonca, Luigi Ranno, Tian Gu, Juejun Hu
Accepted  doi: 10.37188/lam.2023.032

Photonic integrated circuits (PICs) have long been considered as disruptive platforms that revolutionize optics. Building on the mature industrial foundry infrastructure for electronic integrated circuit fabrication, the manufacturing of PICs has made remarkable progress. However, the packaging of PICs has often become a major barrier impeding their scalable deployment owing to their tight optical alignment tolerance, and hence, the requirement for specialty packaging instruments. Two-photon lithography (TPL), a laser direct-write three-dimensional (3-D) patterning technique with deep subwavelength resolution, has emerged as a promising solution for integrated photonics packaging. This study provides an overview of the technology, emphasizing the latest advances in TPL-enabled packaging schemes and their prospects for adoption in the mainstream photonic industry.

Rapid fabrication of reconfigurable helical microswimmers with environmentally adaptive locomotion
Rui Li, Yuan Tao, Jiawen Li, Dongdong Jin, Chen Xin, Shengyun Ji, Chaowei Wang, Yachao Zhang, Yanlei Hu, Dong Wu, Li Zhang, Jiaru Chu
Accepted  doi: 10.37188/lam.2023.029

Artificial helical microswimmers with shape-morphing capacities and adaptive locomotion have great potential for precision medicine and noninvasive surgery. However, current reconfigurable helical microswimmers are hampered by their low-throughput fabrication and limited adaptive locomotion. Here, a rotary holographic processing strategy (a helical femtosecond laser beam) is proposed to produce stimuli-responsive helical microswimmers (<100 μm) rapidly (<1 s). This method allows for the easy one-step fabrication of various microswimmers with controllable sizes and diverse bioinspired morphologies, including spirulina-, Escherichia-, sperm-, and Trypanosoma-like shapes. Owing to their shape-morphing capability, the helical microswimmers undergo a dynamic transition between tumbling and corkscrewing motions under a constant rotating magnetic field. By exploiting adaptive locomotion, helical microswimmers can navigate complex terrain and achieve targeted drug delivery. Hence, these microswimmers hold considerable promise for diverse precision treatments and biomedical applications.

Recent Progress on Micro-LEDs
Ayush Pandey, Maddaka Reddeppa, Zetian Mi
Accepted  doi: 10.37188/lam.2023.031

With the advent of technologies such as augmented/virtual reality (AR/VR) that are moving towards displays with high efficiency, small size, and ultrahigh resolution, the development of optoelectronic devices with scales on the order of a few microns or even smaller has attracted considerable interest. In this review article we provide an overview of some of the recent developments of visible micron-scale light emitting diodes (LEDs). The major challenges of higher surface recombination for smaller size devices, the difficulty in attaining longer emission wavelengths, and the complexity of integrating individual, full color devices into a display are discussed, along with techniques developed to address them. We then present recent work on bottom-up nanostructure-based sub-micron LEDs, highlighting their unique advantages, recent developments, and promising potential. Finally, we present perspectives for future development of micro-LEDs for higher efficiencies, better color output and more efficient integration.

Holographic laser fabrication of 3D artificial compound μ-eyes
Lei Wang, Wei Gong, Xiao-Wen Cao, Yan-Hao Yu, Saulius Juodkazis, et al.
Published Published online: 28 September 2023,  doi: 10.37188/lam.2023.026
The demand for fast optical image acquisition without movable optical elements (e.g., for self-driving car technology) can be met using bioinspired 3D compound eyes. 3D laser processing strategies enable designable 3D structuring but suffer from low fabrication efficiency, which significantly limits their applications in producing complex 3D optical devices. Herein, we demonstrate a versatile yet simple wet-etching-assisted holographic laser fabrication method for the development of 3D compound eyes. Artificial compound μ-eyes can be readily fabricated by programming a 3D spot array for the parallel ablation of a curved fused silica surface, followed by controllable etching in a hydrofluoric (HF) acid solution. A 3D-concave-lens array made on a curved surface over an area of 100 μm cross-section with each lenslet of 10 μm radius was fabricated with high fidelity and excellent imaging/focusing quality. The resultant 3D-concave-lens can serve as a hard template for the mass production of soft compound eyes through soft lithography. Additionally, using a generative adversarial network (GAN)-based deep learning algorithm, image restoration was conducted for each lenslet, which retained a large field of view and significantly improved image quality. This method provides a simple solution to the requirements of compound μ-eyes required by Industry 4.0.
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Dual-point noncoaxial rotational Doppler effect towards synthetic OAM light fields for real-time rotating axis detection
Yanxiang Zhang, Zijing Zhang, Han Lin, Zhongquan Nie, Rui Feng, et al.
Published Published online: 26 September 2023,  doi: 10.37188/lam.2023.027

Probing the axis of a rotator is important in astrophysics, aerospace, manufacturing, machinery, automation, and virtual reality, etc. Existing optical solutions commonly require multiple sequential measurements via symmetry-broken light fields, which make them time-consuming, inefficient, and prone to accumulated errors. Herein, we propose the concept of a dual-point noncoaxial rotational Doppler effect (DNRDE) and demonstrate a one-shot detection technique to solve this problem. An on-demand synthetic orbital angular momentum (OAM) light beam impinges on a rotating scatterer surface, supporting dual-point rotational Doppler shifts, in which the information of the rotating axis is acquired by comparing these two frequency shifts with a prescribed threshold. The existence of arbitrary dual-point Doppler shifts enables the one-time direct identification of rotating axis orientations, which is fundamentally inaccessible in single-point detection. This robust detection technique is compatible with generalised synthetic OAM light fields by utilising optical modal filters. Compared with traditional approaches, our DNRDE-driven detection approach exhibits a four-fold enhancement in measurement speed, higher energy efficiency, and superior accuracy with a maximal absolute measurement error of 2.23°. The proposed dual-point detection method holds great promise for detecting rotating bodies in various applications, such as astronomical surveys and industrial manufacturing.

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Optically anisotropic, electrically tunable microlens arrays formed via single-step photopolymerization-induced phase separation in polymer/liquid-crystal composite materials
Wenfeng Cai, Delai Kong, Zongjun Ma, Mengjia Cen, Jiawei Wang, et al.
Published Published online: 26 September 2023,  doi: 10.37188/lam.2023.028

Microlenses or arrays are key elements in many applications. However, their construction methods involve multiple fabrication processes, thereby increasing the complexity and cost of fabrication. In this study, we demonstrate an optically anisotropic, electrically tunable liquid crystal (LC) microlens array using a simple, one-step fabrication method. The microlens array is formed via photopolymerization-induced phase separation inside a polymer/LC composite. It possesses both polarization-dependent and electrically tunable focusing and imaging properties. Without applying voltage, the microlens array has a natural focal length of 8 mm, which is a result of its inherent gradient refractive index profile. Upon applying voltage above the threshold, the LC molecules reorient along the electric field direction and the focal length of the microlens array gradually increases. Based on its superior properties, the microlens array is further used for integral imaging applications, demonstrating electrically tunable central depth plane. Such LC microlens arrays could find numerous potential applications owing to their advantageous features of being flat, ultra-thin, and tunable, including 3D displays, optical interconnects, and more.

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Ultra-wideband Waveguide-coupled Photodiodes Heterogeneously Integrated on a Thin-film Lithium Niobate Platform
Chao Wei, Youren Yu, Ziyun Wang, Lin Jiang, Zhongming Zeng, et al.
Published Published online: 12 September 2023,  doi: 10.37188/lam.2023.030

With the advantages of large electro-optical coefficient, wide transparency window, and strong optical confinement, thin-film lithium niobate (TFLN) technique has enabled the development of various high-performance optoelectronics devices, ranging from the ultra-wideband electro-optic modulators to the high-efficient quantum sources. However, the TFLN platform does not natively promise lasers and photodiodes. This study presents an InP/InGaAs modified uni-traveling carrier (MUTC) photodiodes heterogeneously integrated on the TFLN platform with a record-high 3-dB bandwidth of 110 GHz and a responsivity of 0.4 A/W at a 1,550-nm wavelength. It is implemented in a wafer-level TFLN-InP heterogeneous integration platform and is suitable for the large-scale, multi-function, and high-performance TFLN photonic integrated circuits.

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Ultra-broadband polarisation beam splitters and rotators based on 3D-printed waveguides
Aleksandar Nesic, Matthias Blaicher, Pablo Marin-Palomo, Christoph Füllner, Sebastian Randel, et al.
Published Published online: 05 September 2023,  doi: 10.37188/lam.2023.022

Multi-photon lithography has emerged as a powerful tool for photonic integration, allowing to complement planar photonic circuits by 3D-printed freeform structures such as waveguides or micro-optical elements. These structures can be fabricated with a high precision on the facets of optical devices and enable highly efficient package-level chip–chip connections in photonic assemblies. However, plain light transport and efficient coupling is far from exploiting the full geometrical design freedom offered by 3D laser lithography. Here, we extended the functionality of 3D-printed optical structures to manipulation of optical polarisation states. We demonstrate compact ultra-broadband polarisation beam splitters (PBSs) that can be combined with polarisation rotators and mode-field adapters into a monolithic 3D-printed structure, fabricated directly on the facets of optical devices. In a proof-of-concept experiment, we demonstrate measured polarisation extinction ratios beyond 11 dB over a bandwidth of 350 nm at near-infrared telecommunication wavelengths around 1550 nm. We demonstrate the viability of the device by receiving a 640 Gbit/s dual-polarisation data signal using 16-state quadrature amplitude modulation (16QAM), without any measurable optical-signal-to-noise-ratio penalty compared to a commercial PBS.

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Fabrication of opaque and transparent 3D structures using a single material via two-photon polymerisation lithography
Parvathi Nair Suseela Nair, Chengfeng Pan, Hao Wang, Deepshikha Arora, Qing Yang Steve Wu, et al.
Published Published online: 31 August 2023,  doi: 10.37188/lam.2023.025

Two-photon polymerisation lithography enables the three-dimensional (3D)-printing of high-resolution micron- and nano-scale structures. Structures that are 3D-printed using proprietary resins are transparent and are suitable as optical components. However, achieving a mix of opaque and transparent structures in a single optical component is challenging and requires multiple material systems or the manual introduction of ink after fabrication. In this study, we investigated an overexposure printing process for laser decomposition, which typically produces uncontrollable and random ‘burnt’ structures. Specifically, we present a printing strategy to control this decomposition process, realising the on-demand printing of opaque or transparent structures in a single lithographic step using a single resin. Using this method, opaque structures can be printed with a minimum feature size of approximately 10 µm, which exhibit<15% transmittance at a thickness of approximately 30 µm. We applied this process to print an opaque aperture integrated with a transparent lens to demonstrate an improved imaging contrast.

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Fabry–Perot-based phase demodulation of heterodyne light-induced thermoelastic spectroscopy
Ziting Lang, Shunda Qiao, Yufei Ma
Published Published online: 21 August 2023,  doi: 10.37188/lam.2023.023

Fabry–Perot (F–P)-based phase demodulation of heterodyne light-induced thermoelastic spectroscopy (H-LITES) was demonstrated for the first time in this study. The vibration of a quartz tuning fork (QTF) was detected using the F–P interference principle instead of an electrical signal through the piezoelectric effect of the QTF in traditional LITES to avoid thermal noise. Given that an Fabry–Perot interferometer (FPI) is vulnerable to disturbances, a phase demodulation method that has been demonstrated theoretically and experimentally to be an effective solution for instability was used in H-LITES. The sensitivity of the F–P phase demodulation method based on the H-LITES sensor was not associated with the wavelength or power of the probe laser. Thus, stabilising the quadrature working point (Q-point) was no longer necessary. This new method of phase demodulation is structurally simple and was found to be resistant to interference from light sources and the surroundings using the LITES technique.

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High-fidelity mode scaling via topological-optimized on-chip metalens for compact photonic interconnection
Yingli Ha, Lijun Wang, Yinghui Guo, Mingbo Pu, Fang Zou, et al.
Published Published online: 18 August 2023,  doi: 10.37188/lam.2023.020

Photonic integrated circuits (PICs) have attracted significant interest in communication, computation, and biomedical applications. However, most rely on highly integrated PICs devices, which require a low-loss and high-integration guided wave path. Owing to the various dimensions of different integrated photonic devices, their interconnections typically require waveguide tapers. Although a waveguide taper can overcome the width mismatch of different devices, its inherent tapering width typically results in a long length, which fundamentally limits the efficient interconnection between devices with a high scaling ratio over a short distance. Herein, we proposed a highly integrated on-chip metalens that enables optical interconnections between devices with high width-scaling ratios by embedding a free-form metasurface in a silicon-on-insulator film. The special geometric features endow the designed metalens with high coupling efficiency and high integration. The device has a footprint of only 2.35 μm in the longitudinal direction and numerical aperture of 2.03, enabling beam focusing and collimation of less than 10 μm between devices with width-scaling ratio of 11. For the fundamental transverse electric field (TE0) mode, the relative transmittance is as high as 96% for forward incidence (from wide to narrow waveguides), whereas the metalens can realize wavefront shaping for backward incidence, which can be used in optical phase arrays. This study provides new ideas for optical interconnect design and wavefront shaping in high-integration PICs. Our design approach has potential applications in directional radiators, LiDAR, on-chip optical information processing, analogue computing, and imaging.

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Quantitative phase imaging (QPI) through random diffusers using a diffractive optical network
Yuhang Li, Yi Luo, Deniz Mengu, Bijie Bai, Aydogan Ozcan
Published Published online: 22 July 2023,  doi: 10.37188/lam.2023.017
Quantitative phase imaging (QPI) is a label-free computational imaging technique used in various fields, including biology and medical research. Modern QPI systems typically rely on digital processing using iterative algorithms for phase retrieval and image reconstruction. Here, we report a diffractive optical network trained to convert the phase information of input objects positioned behind random diffusers into intensity variations at the output plane, all-optically performing phase recovery and quantitative imaging of phase objects completely hidden by unknown, random phase diffusers. This QPI diffractive network is composed of successive diffractive layers, axially spanning in total ~70\begin{document}$ \lambda $\end{document}, where \begin{document}$ \lambda $\end{document} is the illumination wavelength; unlike existing digital image reconstruction and phase retrieval methods, it forms an all-optical processor that does not require external power beyond the illumination beam to complete its QPI reconstruction at the speed of light propagation. This all-optical diffractive processor can provide a low-power, high frame rate and compact alternative for quantitative imaging of phase objects through random, unknown diffusers and can operate at different parts of the electromagnetic spectrum for various applications in biomedical imaging and sensing. The presented QPI diffractive designs can be integrated onto the active area of standard CCD/CMOS-based image sensors to convert an existing optical microscope into a diffractive QPI microscope, performing phase recovery and image reconstruction on a chip through light diffraction within passive structured layers.
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Large viewing angle holographic 3D display system based on maximum diffraction modulation
Di Wang, Nan-Nan Li, Yi-Long Li, Yi-Wei Zheng, Zhong-Quan Nie, et al.
Published Published online: 20 July 2023,  doi: 10.37188/lam.2023.018

An ideal holographic 3D display should have the characteristics of large viewing angle, full color, and low speckle noise. However, the viewing angle of the holographic 3D display is usually limited by existing strategies, which vastly hinders its extensive application. In this paper, a large viewing angle holographic 3D display system based on maximum diffraction modulation is proposed. The core of the proposed system comprises the spatial light modulators (SLMs) and liquid crystal grating. We also present a new feasible scheme for the realization of large viewing angle holographic 3D display. This is achieved by considering the maximum diffraction angle of SLM as the limited diffraction modulation range of each image point. By doing so, we could not only give access to the maximum hologram size of the object, but also tune the reconstructed image of secondary diffraction by using a self-engineered liquid crystal grating. More importantly, the proposed maximum diffraction modulation scheme enables the viewing angle of the proposed system to be enlarged to 73.4°. The proposed system has huge application potential in the fields such as education, culture, and entertainment.

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Laser-based defect characterization and removal process for manufacturing fused silica optic with high ultraviolet laser damage threshold
Xiaocong Peng, Xin Cheng, Chaoyang Wei, Songlin Wan, Kaizao Ni, et al.
Published Published online: 20 July 2023,  doi: 10.37188/lam.2023.021

Residual processing defects during the contact processing processes greatly reduce the anti-ultraviolet (UV) laser damage performance of fused silica optics, which significantly limited development of high-energy laser systems. In this study, we demonstrate the manufacturing of fused silica optics with a high damage threshold using a CO2 laser process chain. Based on theoretical and experimental studies, the proposed uniform layer-by-layer laser ablation technique can be used to characterize the subsurface mechanical damage in three-dimensional full aperture. Longitudinal ablation resolutions ranging from nanometers to micrometers can be realized; the minimum longitudinal resolution is < 5 nm. This technique can also be used as a crack-free grinding tool to completely remove subsurface mechanical damage, and as a cleaning tool to effectively clean surface/subsurface contamination. Through effective control of defects in the entire chain, the laser-induced damage thresholds of samples fabricated by the CO2 laser process chain were 41% (0% probability) and 65.7% (100% probability) higher than those of samples fabricated using the conventional process chain. This laser-based defect characterization and removal process provides a new tool to guide optimization of the conventional finishing process and represents a new direction for fabrication of highly damage-resistant fused silica optics for high-energy laser applications.

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Single-end hybrid Rayleigh Brillouin and Raman distributed fibre-optic sensing system
Linjing Huang, Xinyu Fan, Haijun He, Lianshan Yan, Zuyuan He
Published Published online: 13 July 2023,  doi: 10.37188/lam.2023.016

Backscattered lightwaves from an optical fibre are used to realise distributed fibre optic sensing (DFOS) systems for measuring various parameters. Rayleigh, Brillouin, and Raman backscattering provide different sensitivities to different measurands and have garnered the attention of researchers. A system combining the three principles above can effectively separate the measured strain and temperature completely as well as provide measurements of both dynamic and static parameters. However, the combined system is extremely complicated if the three systems are independent of each other. Hence, we propose a single-end hybrid DFOS system that uses two successive pulses to realise the Brillouin amplification of Rayleigh backscattering lightwaves for combining Rayleigh and Brillouin systems. A 3-bit pulse-coding method is employed to demodulate the Raman scattering of the two pulses to integrate Raman optical time-domain reflectometry into the hybrid system. Using this hybrid scheme, a simultaneous measurement of multiple parameters is realised, and a favourable measurement accuracy is achieved.

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Volumetric helical additive manufacturing
Antoine Boniface, Florian Maître, Jorge Madrid-Wolff, Christophe Moser
Published Published online: 16 June 2023,  doi: 10.37188/lam.2023.012

3D printing has revolutionized the manufacturing of volumetric components and structures for various fields. Thanks to the advent of photocurable resins, several fully volumetric light-based techniques have been recently developed to push further the current limitations of 3D printing. Although fast, this new generation of printers cannot fabricate objects whose typical size exceeds the centimeter without severely affecting the final resolution. Based on tomographic volumetric additive manufacturing, we propose a method for volumetric helical additive manufacturing (VHAM) multi-cm scale structures without magnifying the projected patterns. It consists of illuminating the photoresist while the latter follows a helical motion. This movement allows to increase the printable object’ s height. Additionally, we off-center the modulator used for projecting the light patterns to double the object’ s lateral size. We demonstrate experimentally the interest of using these two tricks for printing larger objects (up to 3 cm × 3 cm × 5 cm)  with fine details (650 μm)  and short print time (< 10 min).

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Ultra-compact lithium niobate photonic chip for high-capacity and energy-efficient wavelength-division-multiplexing transmitters
Hongxuan Liu, Bingcheng Pan, Yishu Huang, Jianghao He, Ming Zhang, et al.
Published Published online: 29 May 2023,  doi: 10.37188/lam.2023.013
Recently, high-performance thin-film lithium niobate optical modulators have emerged that, together with advanced multiplexing technologies, are highly expected to satisfy the ever-growing demand for high-capacity optical interconnects utilizing multiple channels. Accordingly, in this study, a compact lithium-niobate-on-insulator (LNOI) photonic chip was adopted to establish four-channel wavelength-division-multiplexing (WDM) transmitters, comprising four optical modulators based on ultracompact 2 × 2 Fabry-Perot cavities and a four-channel WDM filter based on multimode waveguide gratings. The fabricated chip with four wavelength channels has a total footprint as compact as 0.3 × 2.8 mm2, and exhibits an excess loss of ~0.8 dB as well as low inter-channel crosstalk of < –22 dB. Using this LNOI photonic chip, high-capacity data transmissions of 320 Gbps (4 × 80 Gbps) on-off-keying signals and 400 Gbps (4 × 100 Gbps) four-level pulse amplitude signals were successfully realized with the ultra-low power consumption of 11.9 fJ/bit.
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Fast and deterministic optical phased array calibration via pointwise optimisation
Jingwen Shan, Hanyi Zhang, Lijing Li, Weijie Deng, Mingjie Sun
Published Published online: 19 April 2023,  doi: 10.37188/lam.2023.010
Owing to the structural errors in the optical phased array, an initial random phase reduces the quality of the deflection beam. The most commonly applied approach to phase calibration is based on adaptive optics. However, adaptive optimisation approaches have slow convergence and low diffraction efficiency. We proposed a pointwise optimisation approach to achieve fast and accurate beam deflection. This approach conducts phase calibration, combining global traversal and local searches individually for each array element. We built a phase-calibration optical system containing a one-dimensional optical waveguide phase array for further verification and designed the relevant mechanics. The simulation and experimental results demonstrate that the pointwise optimisation approach accelerates the calibration process and improves the diffraction efficiency.
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3D-printed facet-attached microlenses for advanced photonic system assembly
Yilin Xu, Pascal Maier, Mareike Trappen, Philipp-Immanuel Dietrich, Matthias Blaicher, et al.
Published Published online: 17 April 2023,  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.
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Contrast-enhanced fluorescence microscope by LED integrated excitation cubes
Yuanhua Liu, Xiang Zhang, Fei Su, Zhiyong Guo, Dayong Jin
Published Published online: 31 March 2023,  doi: 10.37188/lam.2023.008

Fluorescence microscopy is a powerful tool for scientists to observe the microscopic world, and the fluorescence excitation light source is one of the most critical components. To compensate for the short operation lifetime, integrated light sources, and low excitation efficiency of conventional light sources such as mercury, halogen, and xenon lamps, we designed an LED-integrated excitation cube (LEC) with a decentralized structure and high optical power density. Using a Fresnel lens, the light from the light-emitting diode (LED) was effectively focused within a 15 mm mounting distance to achieve high-efficiency illumination. LEC can be easily designed in the shape of fluorescence filter cubes for installation in commercial fluorescence microscopes. LECs’ optical efficiency is 1–2 orders of magnitude higher than that of mercury lamps; therefore, high-quality fluorescence imaging with spectral coverage from UV to red can be achieved. By replacing conventional fluorescence filter cubes, LEC can be easily installed on any commercial fluorescence microscope. A built-in LEC driver can identify the types of LEDs in different spectral bands to adopt the optimal operating current and frequency of pulses. Moreover, high-contrast images can be achieved in pulse mode by time-gated imaging of long-lifetime luminescence.

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Metasurfaces designed by a bidirectional deep neural network and iterative algorithm for generating quantitative field distributions
Yang Zhu, Xiaofei Zang, Haoxiang Chi, Yiwen Zhou, Yiming Zhu, et al.
Published Published online: 28 March 2023,  doi: 10.37188/lam.2023.009

Metasurfaces, which are the two-dimensional counterparts of metamaterials, have demonstrated unprecedented capabilities to manipulate the wavefront of electromagnetic waves in a single flat device. Despite various advances in this field, the unique functionalities achieved by metasurfaces have come at the cost of the structural complexity, resulting in a time-consuming parameter sweep for the conventional metasurface design. Although artificial neural networks provide a flexible platform for significantly improving the design process, the current metasurface designs are restricted to generating qualitative field distributions. In this study, we demonstrate that by combining a tandem neural network and an iterative algorithm, the previous restriction of the design of metasurfaces can be overcome with quantitative field distributions. As proof-of-principle examples, metalenses predicted via the designed network architecture that possess multiple focal points with identical/orthogonal polarisation states, as well as accurate intensity ratios (quantitative field distributions), were numerically calculated and experimentally demonstrated. The unique and robust approach for the metasurface design will enable the acceleration of the development of devices with high-accuracy functionalities, which can be applied in imaging, detecting, and sensing.

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Hybrid-integrated chalcogenide photonics
Bin Zhang, Di Xia, Xin Zhao, Lei Wan, Zhaohui Li
Published Published online: 22 September 2023,  doi: 10.37188/lam.2023.024

High-quality photonic materials are critical for promoting integrated photonic devices with broad bandwidths, high efficiencies, and flexibilities for high-volume chip-scale fabrication. Recently, we designed a home-developed chalcogenide glass (ChG)-Ge25Sb10S65 (GeSbS) for optical information processing chips and systems, which featured an ultrabroad transmission window, a high Kerr nonlinearity and photoelastic coefficient, and compatibility with the photonic hybrid integration technology of silicon photonics. Chip-integrated GeSbS microresonators and microresonator arrays with high quality factors and lithographically controlled fine structures were fabricated using a modified nanofabrication process. Moreover, considering the high Kerr nonlinearity and photoelastic effect of ChGs, we realised a novel ChG hybrid integrated chip, inspired by recent advances in integrated soliton microcombs and acousto-optic (AO) modulators.

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Hybrid integration of 2D materials for on-chip nonlinear photonics
Vincent Pelgrin, Hoon Hahn Yoon, Eric Cassan, Zhipei Sun
Published Published online: 01 August 2023,  doi: 10.37188/lam.2023.014

Interests surrounding the development of on-chip nonlinear optical devices have been consistently growing in the past decades due to the tremendous applications, such as quantum photonics, all-optical communications, optical computing, on-chip metrology, and sensing. Developing efficient on-chip nonlinear optical devices to meet the requirements of those applications brings the need for new directions to improve the existing photonic approaches. Recent research has directed the field of on-chip nonlinear optics toward the hybrid integration of two-dimensional layered materials (such as graphene, transition metal dichalcogenides, and black phosphorous) with various integrated platforms. The combination of well-known photonic chip design platforms (e.g., silicon, silicon nitride) and different two-dimensional layered materials has opened the road for more versatile and efficient structures and devices, which has the great potential to unlock numerous new possibilities. This review discusses the modeling and characterization of different hybrid photonic integration structures with two-dimensional materials, highlights the current state of the art examples, and presents an outlook for future prospects.

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Measurement techniques for aspheric surface parameters
Qun Hao, Yiming Liu, Yao Hu, Xin Tao
Published Published online: 28 July 2023,  doi: 10.37188/lam.2023.019

Aspheric surfaces are widely used in advanced optical instruments. Measuring the aspheric surface parameters (ASPs) with high accuracy is vital for manufacturing and aligning optical aspheric surfaces. This paper provides a review of various techniques for measuring ASPs and discusses the advantages/disadvantages of these approaches. The aim of this review is to contribute to advancements in the fabrication and testing of aspheric optical elements and their practical applications in diverse fields.

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Perovskite light-emitting diodes toward commercial full-colour displays: progress and key technical obstacles
Changjiu Sun, Yuanzhi Jiang, Keyu Wei, Mingjian Yuan
Published Published online: 13 July 2023,  doi: 10.37188/lam.2023.015

Metal-halide perovskite light-emitting diodes (PeLEDs) possess wide colour gamut, high luminescence efficiency, and low-cost synthesis, making them a promising photonic source for next-generation display applications. Since the first room-temperature emission PeLED was demonstrated in 2014, their performance has improved rapidly within a few years, leading to considerable attention from academia and industry. In this review, we discuss the primary technical bottlenecks of PeLEDs for commercial display applications, including large-area PeLED preparation, patterning strategies, and flexible PeLED devices. We review the technical approaches for achieving these targets and highlight the current challenges while providing an outlook for these perovskite materials and PeLED devices to meet the requirements of the next-generation high-colour-purity full-colour display market.

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Laser-scribed graphene for sensors: preparation, modification, applications, and future prospects
Xing Liu, Fangyi Zhang, Qiwen Zhang, Zhengfen Wan, Xi Chen
Published Published online: 05 June 2023,  doi: 10.37188/lam.2023.011
Sensors are widely used to acquire biological and environmental information for medical diagnosis, and health and environmental monitoring. Graphene is a promising new sensor material that has been widely used in sensor fabrication in recent years. Compared with many other existing graphene preparation methods, laser-scribed graphene (LSG) is simple, low-cost, environmentally friendly, and has good conductivity and high thermal stability, making it widely used in the sensor field. This paper summarizes existing LSG methods for sensor fabrication. Primary LSG preparation methods and their variants are introduced first, followed by a summary of LSG modification methods designed explicitly for sensor fabrication. Subsequently, the applications of LSG in stress, bio, gas, temperature, and humidity sensors are summarized with a particular focus on multifunctional integrated sensors. Finally, the current challenges and prospects of LSG-based sensors are discussed.
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ISSN 2689-9620    EISSN 2831-4093

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