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Meta-device: advanced manufacturing
Borui Leng, Yao Zhang, Din Ping Tsai, Shumin Xiao
Published Published online: 07 March 2024,  doi: 10.37188/lam.2024.005
Metasurfaces are one of the most promising devices to break through the limitations of bulky optical components. By offering a new method of light manipulation based on the light-matter interaction in subwavelength nanostructures, metasurfaces enable the efficient manipulation of the amplitude, phase, polarization, and frequency of light and derive a series of possibilities for important applications. However, one key challenge for the realization of applications for meta-devices is how to fabricate large-scale, uniform nanostructures with high resolution. In this review, we review the state-of-the-art nanofabrication techniques compatible with the manufacture of meta-devices. Maskless lithography, masked lithography, and other nanofabrication techniques are highlighted in detail. We also delve into the constraints and limitations of the current fabrication methods while providing some insights on solutions to overcome these challenges for advanced nanophotonic applications.
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Adaptive multiscale microscope with fast zooming, extended working distance, and large field of view
Yi Zheng, Xin Wang, Zhao Jiang, Jinbo Xu, Rongying Yuan, et al.
Published Published online: 07 March 2024,  doi: 10.37188/lam.2024.008

The field-of-view (FOV), depth of field, and resolution of conventional microscopes are constrained by each other; therefore, a zoom function is required. Traditional zoom methods lose real-time performance and have limited information throughput, severely limiting their application, especially in three-dimensional dynamic imaging and large-amount or large-size sample scanning. Here, an adaptive multiscale (AMS) imaging mechanism combining the benefits of liquid lenses and multiscale imaging techniques is proposed to realize the functions of fast zooming, wide working distance (WD) range and large FOV on a self-developed AMS microscope. The design principles were revealed. Moreover, a nonuniform-distortion-correction algorithm and a composite patching algorithm were designed to improve image quality. The continuous tunable magnification range of the AMS microscope is from 9× to 18×, with the corresponding FOV diameters and resolution ranging from 2.31 to 0.98 mm and from 161 to 287 line-pairs/mm, respectively. The extended WD range is 0.8 mm and the zoom response time is 38 ms. Experiments demonstrated the advantages of the proposed microscope in pathological sample scanning, thick-sample imaging, microfluidic process monitoring, and the observation of living microorganisms. The proposed microscope is the first step towards zoom multiscale imaging technology and is expected to be applied in life sciences, medical diagnosis, and industrial detection.

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A multi-photon (7 × 7)-focus 3D laser printer based on a 3D-printed diffractive optical element and a 3D-printed multi-lens array
Pascal Kiefer, Vincent Hahn, Sebastian Kalt, Qing Sun, Yolita M. Eggeler, et al.
Published Published online: 06 March 2024,  doi: 10.37188/lam.2024.003

One of the challenges in the field of multi-photon 3D laser printing lies in further increasing the print speed in terms of voxels/s. Here, we present a setup based on a 7 × 7 focus array (rather than 3 × 3 in our previous work) and using a focus velocity of about 1 m/s (rather than 0.5 m/s in our previous work) at the diffraction limit (40×/NA1.4 microscope objective lens). Combined, this advance leads to a ten times increased print speed of about 108 voxels/s. We demonstrate polymer printing of a chiral metamaterial containing more than 1.7 × 1012 voxels as well as millions of printed microparticles for potential pharmaceutical applications. The critical high-quality micro-optical components of the setup, namely a diffractive optical element generating the 7 × 7 beamlets and a 7 × 7 lens array, are manufactured by using a commercial two-photon grayscale 3D laser printer.

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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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Article
Side-polished Silica-Fluoride Multimode Fibre Pump Combiner for Mid-IR Fibre Lasers and Amplifiers
Boris Perminov, Kirill Grebnev, Uwe Hübner, Maria Chernysheva
Accepted  doi: 10.37188/lam.2024.039
[PDF](154)

Side-pumping fibre combiners offer several advantages in fibre laser design, including distributed pump absorption, reduced heat load, and improved flexibility and reliability. These benefits are particularly important for all-fibre lasers and amplifiers operating in the mid-IR wavelength range and based on soft-glass optical fibres. However, conventional fabrication methods face limitations due to significant differences in the thermal properties of pump-delivering silica fibres and signal-guiding fluoride-based fibres. To address these challenges, this work introduces a design for a fuse-less side-polished (D-shaped) fibre-based pump combiner comprising multimode silica and double-clad fluoride-based fibres. The results demonstrate stable coupling efficiency exceeding 80% at a 980-nm wavelength over 8 hours of continuous operation under active thermal control. The developed pump combiner has also been successfully integrated into a linear Er-doped fibre laser cavity, showing continuous-wave generation at 2731 or 2781-nm central wavelength with an output power of 0.87~W. Overall, this innovative approach presents a simple, repeatable, and reproducible pump combiner design that opens up new possibilities for leveraging fibre-based component technology in soft glass matrices and other emerging fibres with unique compositions.

Article
One-dimensional core/shell radial heterojunction with cascade type-II energy-band alignment for enhanced broadband photodetection
Yi Ma, Chunxiang Xu, Mengyang Wu, Fumeng Zhang, Xiaoxuan Wang, Jianqi Dong, Qiannan Cui, Zengliang Shi
Accepted  doi: 10.37188/lam.2024.040
[PDF](103)

Constructing a one-dimensional (1D) core/shell heterostructure is a rational and efficient way to integrate multiple functional materials into a single device, in which a distinct and reliable interface and suitable energy-band alignment play important roles in optoelectronic applications. Here, using a typical magnetron sputtering system, we constructed a 1D ZnO/CdS/CdTe core/shell nanorod arrays radial heterojunction with a well-designed cascade type-II energy band alignment and improved the broadband photodetector (PD) performance. The well-formed shell layers compensated for the defect states on the ZnO surface and extended the photoresponse range from ultraviolet to visible and near-infrared. Moreover, reliable and distinct heterointerfaces with a cascade type-II energy band alignment can guarantee more stable carrier migration and reduce energy loss, promoting effective photogenerated charge carrier separation and resulting in an enhanced photoresponse. The optimised 1D ZnO/CdS/CdTe core/shell heterojunction PD exhibited a fast photoresponse at 0 V bias with high responsivity and detectivity. These results provide an important reference for the rational design and controllable synthesis of multifunctional optoelectronic nanodevices.

Article
Generation of polarization-multiplexed terahertz orbital angular momentum combs via all-silicon metasurfaces
Ming-Zhe Chong, Yiwen Zhou, Zong-Kun Zhang, Jin Zhao, Yue-Yi Zhang, Chong-Qi Zhang, Xiaofei Zang, Chao-Hai Du, Pu-Kun Liu, Ming-Yao Xia
Accepted  doi: 10.37188/lam.2024.038
[PDF](117)

Electromagnetic waves carrying orbital angular momentum (OAM), namely OAM beams, are important in various fields including optics, communications, and quantum information. However, most current schemes can only generate single or several simple OAM modes. Multi-mode OAM beams are rarely seen. This paper proposes a scheme to design metasurfaces that can generate multiple polarization-multiplexed OAM modes with equal intervals and intensities (i.e., OAM combs) working in the terahertz (THz) range. As a proof of concept, we first design a metasurface to generate a pair of polarization-multiplexed OAM combs with arbitrary mode numbers. Furthermore, another metasurface is proposed to realize a pair of polarization-multiplexed OAM combs with arbitrary locations and intervals in the OAM spectrum. Experimental results agree well with full-wave simulations, verifying a great performance of OAM combs generation. Our method may provide a new solution to designing high-capacity THz devices used in multi-mode communication systems.

Article
Adaptive-optical 3D microscopy for microfluidic multiphase flows
Clemens Bilsing, Erik Nützenadel, Sebastian Burgmann, Jürgen Czarske, Lars Büttner
Accepted  doi: 10.37188/lam.2024.037
[PDF](103)

Measurements based on optical microscopy can be severely impaired if the access exhibits variations of the refractive index. In the case of fluctuating liquid-gas boundaries, refraction introduces dynamical aberrations that increase the measurement uncertainty. This is prevalent at multiphase flows (e. g. droplets, film flows) that occur in many technical applications as for example in coating and cleaning processes and the water management in fuel cells. In this paper, we present a novel approach based on adaptive optics for correcting the dynamical aberrations in real time and thus reducing the measurement uncertainty. The shape of the fluctuating water-air interface is sampled with a reflecting light beam (Fresnel Guide Star) and a Hartmann-Shack sensor which makes it possible to correct its influence with a deformable mirror in a closed loop. Three-dimensional flow measurements are achieved by using a double-helix point spread function. We measure the flow inside a sessile, oscillating 50-μl droplet on an opaque gas diffusion layer for fuel cells and show that the temporally varying refraction at the droplet surface causes a systematic underestimation of the flow field magnitude corresponding to the first droplet eigenmode which plays a major role in their detachment mechanism. We demonstrate that the adaptive optics correction is able to reduce this systematic error. Hence, the adaptive optics system can pave the way to a deeper understanding of water droplet formation and detachment which can help to improve the efficiency of fuels cells.

Article
Improving bonding strength of W/Cu dual metal interface through laser micro-structuring method
Xing Li, Quanjie Wang, Libing Lu, Yingchun Guan, Wei Zhou
Accepted  doi: 10.37188/lam.2024.033
[PDF](84)

Selective laser melting (SLM) is a promising technology for fabricating complex components with W/Cu dual metal. To enhance the interfacial bonding of W/Cu dual metal, we propose a novel approach using laser texturing to fabricate micro/nanostructures on the W surface. The micro/nanostructures promoted the spreading of Cu in the liquid, inhibited defects, and considerably increased the contact area between W and Cu by mechanical interlocking. To the best of our knowledge, a W/Cu dual metal was successfully prepared by SLM without a transition layer the first time. The bonding strength of the two materials reached 123 MPa, close to that of a W/Cu dual metal joint prepared by diffusion bonding.

Article
Dynamic 3D shape reconstruction under complex reflection and transmission conditions using multi-scale parallel single-pixel imaging
Zhoujie Wu, Haoran Wang, Feifei Chen, Xunren Li, Zhengdong Chen, Qican Zhang
Accepted  doi: 10.37188/lam.2024.034
[PDF](274)

Depth measurement and three-dimensional (3D) imaging under complex reflection and transmission conditions are challenging and even impossible for traditional structured light techniques, owing to the precondition of point-to-point triangulation. Despite recent progress in addressing this problem, there is still no efficient and general solution. Herein, a Fourier dual-slice projection with depth-constrained localization is presented to separate and utilize different illumination and reflection components efficiently, which can significantly decrease the number of projection patterns in each sequence from thousands to fifteen. Subsequently, multi-scale parallel single-pixel imaging (MS-PSI) is proposed based on the established and proven position-invariant theorem, which breaks the local regional assumption and enables dynamic 3D reconstruction. Our methodology successfully unveils unseen-before capabilities such as (1) accurate depth measurement under interreflection and subsurface scattering conditions, (2) dynamic measurement of the time-varying high-dynamic-range scene and through thin volumetric scattering media at a rate of 333 frames per second; (3) two-layer 3D imaging of the semitransparent surface and the object hidden behind it. The experimental results confirm that the proposed method paves the way for dynamic 3D reconstruction under complex optical field reflection and transmission conditions, benefiting imaging and sensing applications in advanced manufacturing, autonomous driving, and biomedical imaging.

Article
Laser material processing optimization using bayesian optimization: a generic tool
Tobias Menold, Volkher Onuseit, Matthias Buser, Michael Haas, Nico Bär, Andreas Michalowski
Accepted  doi: 10.37188/lam.2024.032
[PDF](307)

Optimizing laser processes is historically challenging, requiring extensive and costly experimentation. To solve this issue, we apply Bayesian optimization for process parameter optimization to laser cutting, welding, and polishing. We demonstrate how readily available Bayesian optimization frameworks enable efficient optimization of laser processes with only modest expert knowledge. Case studies on laser cutting, welding, and polishing highlight its adaptability to real-world manufacturing scenarios. Moreover, the examples emphasize that with suitable cost functions and boundaries an acceptable optimization result can be achieved after a reasonable number of experiments.

Article
Towards high quality transferred barium titanate ferroelectric hybrid integrated modulator on silicon
Mengxue Tao, Butong Zhang, Tianxiang Zhao, Xiaoxuan Wu, Ming Liu, Guohua Dong, Junjia Wang
Accepted  doi: 10.37188/lam.2024.031
[PDF](412)

Silicon photonics is currently the leading technology for the development of compact and low-cost photonic integrated circuits. However, despite its enormous potential, certain limitations, such as the absence of a linear electro-optical (EO) effect because of the symmetric crystal structure of silicon remain. In contrast, barium titanate (BTO) exhibits a strong Pockels effect. In this study, we demonstrated a high-quality transferred barium titanate ferroelectric hybrid integrated modulator on a silicon-on-insulator platform. The proposed hybrid integration of BTO on silicon Mach-Zehnder interferometers exhibited EO modulation with a VπL as low as 1.67 V·cm, thereby facilitating the realisation of compact EO modulators. The hybrid integration of BTO with SOI waveguides is expected to pave the way for the development of high-speed and high efficiency EO modulators.

Article
Magnetic field-assisted batch polishing method for the mass production of precision optical glass components
Yee Man Loh, Chunjin Wang, Rui Gao, Lai Ting Ho, Chi Fai Cheung
Accepted  doi: 10.37188/lam.2024.028
[PDF](520)

The demand for optical glass has been rapidly increasing in various industries, where an ultra-smooth surface and form accuracy are critical for the functional elements of the applications. To meet the high surface-quality requirements, a polishing process is usually adopted to finish the optical glass surface to ensure an ultra-smooth surface and eliminate sub-surface damage. However, current ultra-precision polishing processes normally polish workpieces individually, leading to a low production efficiency and high polishing costs. Current mass-finishing methods cannot be used for optical glasses. Therefore, magnetic-field-assisted batch polishing (MABP) was proposed in this study to overcome this research gap and provide an efficient and cost-effective method for industrial use. A series of polishing experiments were conducted on typical optical components under different polishing parameters to evaluate the polishing performance of MABP on optical glasses. The results demonstrated that MABP is an efficient method to simultaneously polish multiple lenses while achieving a surface roughness, indicated by the arithmetic mean height (Sa), of 0.7 nm and maintained a sub-micrometer surface form for all the workpieces. In addition, no apparent sub-surface damage was observed, indicating the significant potential for the high-quality rapid polishing of optical glasses. The proposed method is highly competitive compared to the current optical polishing methods, which has the potential to revolutionize the polishing process for small optics.

Review
A comprehensive review of dwell time optimization methods in computer-controlled optical surfacing
Tianyi Wang, Xiaolong Ke, Lei Huang, Qingqing Cui, Zili Zhang, Chunjin Wang, Hyukmo Kang, Weslin Pullen, Heejoo Choi, Daewook Kim, Vipender Negi, Qian Kemao, Yi Zhu, Stefano Giorgio, Philip Boccabella, Nathalie Bouet, Corey Austin, Mourad Idir
Accepted  doi: 10.37188/lam.2024.021
[PDF](770)

Dwell time plays a vital role in determining the accuracy and convergence of the computer-controlled optical surfacing process. However, optimizing dwell time presents a challenge due to its ill-posed nature, resulting in non-unique solutions. To address this issue, several well-known methods have emerged, including the iterative, Bayesian, Fourier transform, and matrix-form methods. Despite their independent development, these methods share common objectives, such as minimizing residual errors, ensuring dwell time's positivity and smoothness, minimizing total processing time, and enabling flexible dwell positions. This paper aims to comprehensively review the existing dwell time optimization methods, explore their interrelationships, provide insights for their effective implementations, evaluate their performances, and ultimately propose a unified dwell time optimization methodology.

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In-situ real-time monitoring of ultrafast laser processing using wide-field high-resolution snapshot compressive microscopy
Xiaodong Wang, Miao Cao, Ziyang Chen, Jiao Geng, Ting Luo, et al.
Published Published online: 15 July 2024,  doi: 10.37188/lam.2024.029
Over the last few decades, ultrafast laser processing has become a widely used tool for manufacturing microstructures and nanostructures. The real-time monitoring of laser material processing provides opportunities to inspect processes and provide feedback. To date, in-situ and real-time monitoring of laser material processing has rarely been performed. To this end, we propose dual-path snapshot compressive microscopy (DP-SCM) for high-speed, large field-of-view, and high-resolution imaging for in-situ and real-time ultrafast laser processing. In the evaluation of DP-SCM, the field of view, lateral resolution, and imaging speed were measured to be 2 mm, 775 nm, and 500 fps, respectively. In ultrafast laser processing, the laser scanning process is observed using a DP-SCM system when translating the sample stage and scanning the focused femtosecond laser. Finally, we monitored the development of a self-organized nanograting structure to validate the potential of our system for unveiling new material mechanisms. The proposed method serves as an add-up (plug-and-play) module for any imaging setup and has vast potential for opening new avenues for high-throughput imaging in laser material processing.
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Flexible orbital angular momentum mode switching in multimode fibre using an optical neural network chip
Zhengsen Ruan, Yuanjian Wan, Lulu Wang, Wei Zhou, Jian Wang
Published Published online: 12 July 2024,  doi: 10.37188/lam.2024.023

Mode-division multiplexing technology has been proposed as a crucial technique for enhancing communication capacity and alleviating growing communication demands. Optical switching, which is an essential component of optical communication systems, enables information exchange between channels. However, existing optical switching solutions are inadequate for addressing flexible information exchange among the mode channels. In this study, we introduced a flexible mode switching system in a multimode fibre based on an optical neural network chip. This system utilised the flexibility of on-chip optical neural networks along with an all-fibre orbital angular momentum (OAM) mode multiplexer-demultiplexer to achieve mode switching among the three OAM modes within a multimode fibre. The system adopted an improved gradient descent algorithm to achieve training for arbitrary 3 × 3 exchange matrices and ensured maximum crosstalk of less than −18.7 dB, thus enabling arbitrary inter-mode channel information exchange. The proposed optical-neural-network-based mode-switching system was experimentally validated by successfully transmitting different modulation formats across various modes. This innovative solution holds promise for providing effective optical switching in practical multimode communication networks.

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Ultrafast laser processing of silk films by bulging and ablation for optical functional devices
Ming Qiao, Huimin Wang, Heng Guo, Ma Luo, Yuzhi Zhao, et al.
Published Published online: 12 July 2024,  doi: 10.37188/lam.2024.024

Organic proteins are attractive owing to their unique optical properties, remarkable mechanical characteristics, and biocompatibility. Manufacturing multifunctional structures on organic protein films is essential for practical applications; however, the controllable fabrication of specific structures remains challenging. Herein, we propose a strategy for creating specific structures on silk film surfaces by modulating the bulging and ablation of organic materials. Unique surface morphologies such as bulges and craters with continuously varying diameters were generated based on the controlled ultrafast laser-induced crystal-form transition and plasma ablation of the silk protein. Owing to the anisotropic optical properties of the bulge/crater structures with different periods, the fabricated organic films can be used for large-scale inkless color printing. By simultaneously engineering bulge/crater structures, we designed and demonstrated organic film-based optical functional devices that achieves holographic imaging and optical focusing. This study provides a promising strategy for the fabrication of multifunctional micro/nanostructures that can broaden the potential applications of organic materials.

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Ultra-high-aspect-ratio structures through silicon using infrared laser pulses focused with axicon-lens doublets
Niladri Ganguly, Pol Sopeña, David Grojo
Published Published online: 10 July 2024,  doi: 10.37188/lam.2024.022

We describe how a direct combination of an axicon and a lens can represent a simple and efficient beam-shaping solution for laser material processing applications. We produce high-angle pseudo-Bessel micro-beams at 1550 nm, which would be difficult to produce by other methods. Combined with appropriate stretching of femtosecond pulses, we access optimized conditions inside semiconductors allowing us to develop high-aspect-ratio refractive-index writing methods. Using ultrafast microscopy techniques, we characterize the delivered local intensities and the triggered ionization dynamics inside silicon with 200-fs and 50-ps pulses. While similar plasma densities are produced in both cases, we show that repeated picosecond irradiation induces permanent modifications spontaneously growing shot-after-shot in the direction of the laser beam from front-surface damage to the back side of irradiated silicon wafers. The conditions for direct microexplosion and microchannel drilling similar to those today demonstrated for dielectrics still remain inaccessible. Nonetheless, this work evidences higher energy densities than those previously achieved in semiconductors and a novel percussion writing modality to create structures in silicon with aspect ratios exceeding ~700 without any motion of the beam. The estimated transient change of conductivity and measured ionization fronts at near luminal speed along the observed microplasma channels support the vision of vertical electrical connections optically controllable at GHz repetition rates. The permanent silicon modifications obtained by percussion writing are light-guiding structures according to a measured positive refractive index change exceeding 10−2. These findings open the door to unique monolithic solutions for electrical and optical through-silicon-vias which are key elements for vertical interconnections in 3D chip stacks.

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Fine 3D control of THz emission in air with dual femtosecond laser pre-pulses at tunnelling ionisation regime
Hsin-Hui Huang, Takeshi Nagashima, Kota Kumagai, Yoshio Hayasaki, Saulius Juodkazis, et al.
Published Published online: 10 July 2024,  doi: 10.37188/lam.2024.030

Emission of THz radiation from air breakdown at focused ultra-short fs-laser pulses (800 nm/35 fs) was investigated for the 3D spatio-temporal control where two pre-pulses are used before the main-pulse. The laser pulse induced air breakdown forms a ~ 120 μm-long focal volume generate shockwaves which deliver a denser air into the focal region of the main pulse for enhanced generation of THz radiation at 0.1–2.5 THz spectral window. The intensity of pre- and main-pulses was at the tunnelling ionisation intensities (1–3) × 1016 W/cm2 and corresponded to sub-critical (transparent) plasma formation in air. Polarisation analysis of THz radiation revealed that orientation of the air density gradients generated by pre-pulses and their time-position locations defined the ellipticity of the generated THz electrical field. The rotational component of electric current is the origin of THz radiation.

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A shoe-box-sized 3D laser nanoprinter based on two-step absorption
Tobias Messer, Michael Hippe, Jingya (Lilyn) Gao, Martin Wegener
Published Published online: 09 July 2024,  doi: 10.37188/lam.2024.027

State-of-the-art commercially available 3D laser micro- and nanoprinters using polymeric photoresists based on two- or multi-photon absorption rely on high-power pico- or femtosecond lasers, leading to fairly large and expensive instruments. Lately, we have introduced photoresists based on two-step absorption instead of two-photon absorption, allowing for the use of small and inexpensive continuous-wave 405 nm wavelength GaN semiconductor laser diodes with light-output powers below 1 mW. Here, using the identical photoresist system and similar laser diodes, we report on the design, construction, and characterization of a 3D laser nanoprinter that fits into a shoe box. This shoe box contains all optical components, namely the mounted laser, the collimation- and beam-shaping optics, a miniature MEMS xy-scanner, a tube lens, the focusing microscope objective lens (NA=1.4, 100× magnification), a piezo slip-stick z-stage, the sample holder, a camera monitoring system, LED sample illumination, as well as the miniaturized control electronics employing a microcontroller. We present a gallery of example 3D structures printed with this instrument. We achieve about 100 nm lateral spatial resolution and focus scan speeds of about 1 mm/s. Potentially, our shoe-box-sized system can be made orders of magnitude less expensive than today’ s commercial systems.

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Parallel fabrication of silica optical microfibers and nanofibers
Hubiao Fang, Yu Xie, Zipei Yuan, Dawei Cai, Jianbin Zhang, et al.
Published Published online: 08 July 2024,  doi: 10.37188/lam.2024.020

Optical micro/nanofibers (MNFs) taper-drawn from silica fibers possess intriguing optical and mechanical properties. Recently, MNF array or MNFs with identical geometries have been attracting more and more attention, however, current fabrication technique can draw only one MNF at a time, with a low drawing speed (typically 0.1 mm/s) and a complicated process for high-precision control, making it inefficient in fabricating multiple MNFs. Here, we propose a parallel-fabrication approach to simultaneously drawing multiple (up to 20) MNFs with almost identical geometries. For fiber diameter larger than 500 nm, measured optical transmittances of all as-drawn MNFs exceed 96.7% at 1550-nm wavelength, with a diameter deviation within 5%. Our results pave a way towards high-yield fabrication of MNFs that may find applications from MNF-based optical sensors, optical manipulation to fiber-to-chip interconnection.

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Functional plastic films: nano-engineered composite based flexible microwave antennas with near-unity relative visible transmittance
Cheng Zhang, Liang Zhu, Chengang Ji, Zhilu Ye, Nabeel Alsaab, et al.
Published Published online: 30 June 2024,  doi: 10.37188/lam.2024.036

Microwave antennas are essential elements for various applications, such as telecommunication, radar, sensing, and wireless power transport. These antennas are conventionally manufactured on rigid substrates using opaque materials, such as metal strips, metallic tapes, or epoxy pastes; thus, prohibiting their use in flexible and wearable devices, and simultaneously limiting their integration into existing optoelectronic systems. Here, we demonstrate that mechanically flexible and optically transparent microwave antennas with high operational efficiencies can be readily fabricated using composite nanolayers deposited on common plastic substrates. The composite nanolayer structure consists of an ultra-thin copper-doped silver film sandwiched between two dielectric films of tantalum pentoxide and aluminum oxide. The material and thickness of each constituent layer are judiciously selected such that the whole structure exhibits an experimentally measured averaged visible transmittance as high as 98.94% compared to a bare plastic substrate, and simultaneously, a sheet resistance as low as 12.5 Ω/sq. Four representative types of microwave antennas are implemented: an omnidirectional dipole antenna, unidirectional Yagi-Uda antenna, low-profile patch antenna, and Fabry-Pérot cavity antenna. These devices exhibit great mechanical flexibility with bending angle over 70°, high gain of up to 13.6 dBi, and large radiation efficiency of up to 84.5%. The proposed nano-engineered composites can be easily prepared over large areas on various types of substrates and simultaneously overcome the limitations of poor mechanical flexibility, low electrical conductivity, and reduced optical transparency usually faced by other constituent materials for flexible transparent microwave antennas. The demonstrated flexible microwave antennas have various applications ranging from fifth-generation and vehicular communication systems to bio-signal monitors and wearable electronics.

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Ultrafast laser-induced decomposition for selective activation of GaAs
Ke-Mi Xu, Chao Liu, Lei Wang, Feng-Chun Pang, Xin-Jing Zhao, et al.
Published Published online: 29 June 2024,  doi: 10.37188/lam.2024.026

The manipulation of micro/nanostructures to customise their inherent material characteristics has garnered considerable attention. In this study, we present the selective activation of gallium arsenide (GaAs) via ultrafast laser-induced decomposition, which correlates with the emergence of ripples on the surface. This instigated a pronounced enrichment in the arsenic (As) concentration around the surface while inducing a depletion of gallium (Ga) at the structural depth. Theoretical simulations based on first principles exhibited a robust inclination towards the phase separation of GaAs, with the gasification of As–As pairs proving more facile than that of Ga–Ga pairs, particularly above the melting point of GaAs. As an illustrative application, a metal-semiconductor hybrid surface was confirmed, showing surface chemical bonding and surface-enhanced Raman scattering (SERS) through the reduction of silver ions on the laser-activated pattern. This laser-induced selective activation holds promise for broader applications, including the controlled growth of Pd and the development of Au/Ag alloy-based platforms, and thereby opens innovative avenues for advancements in semiconductors, solar cell technologies, precision sensing, and detection methodologies.

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Laser speckle grayscale lithography: a new tool for fabricating highly sensitive flexible capacitive pressure sensors
Yong Zhou, Kun Wang, Junkun Mao, Yifei Ma, Mei Wang, et al.
Published Published online: 05 June 2024,  doi: 10.37188/lam.2024.016

Achieving a high sensitivity for practical applications has always been one of the main developmental directions for wearable flexible pressure sensors. This paper introduces a laser speckle grayscale lithography system and a novel method for fabricating random conical array microstructures using grainy laser speckle patterns. Its feasibility is attributed to the autocorrelation function of the laser speckle intensity, which adheres to a first-order Bessel function of the first kind. Through objective speckle size and exposure dose manipulations, we developed a microstructured photoresist with various micromorphologies. These microstructures were used to form polydimethylsiloxane microstructured electrodes that were used in flexible capacitive pressure sensors. These sensors exhibited an ultra-high sensitivity: 19.76 kPa−1 for the low-pressure range of 0–100 Pa. Their minimum detection threshold was 1.9 Pa, and they maintained stability and resilience over 10,000 test cycles. These sensors proved to be adept at capturing physiological signals and providing tactile feedback, thereby emphasizing their practical value.

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Single-shot 3D incoherent imaging with diffuser endoscopy
Julian Lich, Tom Glosemeyer, Jürgen Czarske, Robert Kuschmierz
Published Published online: 30 May 2024,  doi: 10.37188/lam.2024.015

Minimally invasive endoscopy offers a high potential for biomedical imaging applications. However, conventional fiberoptic endoscopes require lens systems which are not suitable for real-time 3D imaging. Instead, a diffuser is utilized for passively encoding incoherent 3D objects into 2D speckle patterns. Neural networks are employed for fast computational image reconstruction beyond the optical memory effect. In this paper, we demonstrate single-shot 3D incoherent fiber imaging with keyhole access at video rate. Applying the diffuser fiber endoscope for fluorescence imaging is promising for in vivo deep brain diagnostics with cellular resolution.

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3D micro-devices for enhancing the lateral resolution in optical microscopy
Gordon Zyla, Göran Maconi, Anton Nolvi, Jan Marx, Dimitra Ladika, et al.
Published Published online: 28 May 2024,  doi: 10.37188/lam.2024.019

Although optical microscopy is a widely used technique across various multidisciplinary fields for inspecting small-scale objects, surfaces or organisms, it faces a significant limitation: the lateral resolution of optical microscopes is fundamentally constrained by light diffraction. Dielectric micro-spheres, however, offer a promising solution to this issue as they are capable of significantly enhancing lateral resolution through extraordinary phenomena, such as a photonic nanojet.Building upon the potential of dielectric micro-spheres, this paper introduces a novel approach for fabricating 3D micro-devices designed to enhance lateral resolution in optical microscopy. The proposed 3D micro-device comprises a modified coverslip and a micro-sphere, facilitating easy handling and integration into any existing optical microscope. To manufacture the device, two advanced femtosecond laser techniques are employed: femtosecond laser ablation and multi-photon lithography. Femtosecond laser ablation was employed to create a micro-hole in the coverslip, which allows light to be focused through this aperture. Multi-photon lithography was used to fabricate a micro-sphere with a diameter of 20 µm, along with a cantilever that positions the above the processed micro-hole and connect it with the coverslip. In this context, advanced processing strategies for multi-photon lithography to produce a micro-sphere with superior surface roughness and almost perfect geometry (λ/8) from a Zr-based hybrid photoresist are demonstrated. The performance of the micro-device was evaluated using Mirau-type coherence scanning interferometry in conjunction with white light illumination at a central wavelength of 600 nm and a calibration grid (Λ = 0.28 µm, h > 50 nm). Here, the 3D micro-device proved to be capable of enhancing lateral resolution beyond the limits achievable with conventional lenses or microscope objectives when used in air. Simultaneously, it maintained the high axial resolution characteristic of Mirau-type coherence scanning interferometry. The results and optical properties of the micro-sphere were analyzed and further discussed through simulations.

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Accuracy verification methodology for computer-generated hologram used for testing a 3.5-meter mirror based on an equivalent element
Kai Xu, Haixiang Hu, Xin Zhang, Hongda Wei, Zhiyu Zhang, et al.
Published Published online: 15 May 2024,  doi: 10.37188/lam.2024.025

Interferometry with computer-generated holograms (CGHs) is a unique solution for the highly accurate testing of large-aperture aspheric mirrors. However, no direct testing method for quantifying the measurement accuracy of CGHs has been developed. In this study, we developed a methodology for verifying CGH accuracy based on an element that is functionally equivalent to a large-aperture mirror in terms of accuracy verification. The equivalent element decreased the aperture by one or higher orders of magnitude, implying that the mirror could be replaced by a non-CGH technology in a comparison test. In this study, a 281 mm diamond-turned mirror was fabricated as the equivalent element of a 3.5 m aspheric mirror and measured using CGH and LUPHOScan profilometers. Surface error composition and root-mean-square (RMS) density analyses were performed. The methodology verification accuracy of the CGH was 4 nm (RMS) in the low- to mid-frequency bands, with a measured surface accuracy of approximately 10 nm (RMS). This methodology provides a feasible solution for CGH accuracy verification, ensuring high-accuracy and reliable testing of large-aperture aspheric mirrors.

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Breaking the speed limitation of wavemeter through spectra-space-time mapping
Zheng Gao, Ting Jiang, Mingming Zhang, Yuxuan Xiong, Hao Wu, et al.
Published Published online: 10 May 2024,  doi: 10.37188/lam.2024.013

Speckle patterns generated by the intermodal interference of multimode fibers enable accurate broadband wavelength measurements. However, the measurement speed is limited by the frame rate of the camera that captures the patterns. We propose a compact and cost-effective ultrafast wavemeter based on multimode and multicore fibers, which employs spectral–spatial–temporal mapping. The speckle patterns generated by multimode fibers enable spectral-to-spatial mapping, which is then sampled by a multicore fiber into a pulse sequence to implement spatial-to-temporal mapping. A high-speed single-pixel photodetector is employed to capture the pulse sequence, which is analysed using a multilayer perceptron to estimate the wavelength. The feasibility of the proposed wavelength estimation method is experimentally verified, achieving a measurement rate of 100 MHz with a resolution of 2.7 pm in a 1 nm operation bandwidth.

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Differential mode-gain equalization via femtosecond laser micromachining-induced refractive index tailoring
Cong Zhang, Senyu Zhang, Yan Zeng, Yue Wang, Meng Xiang, et al.
Published Published online: 30 April 2024,  doi: 10.37188/lam.2024.014

The mode-division multiplexing technique combined with a few-mode erbium-doped fiber amplifier (FM-EDFA) demonstrates significant potential for solving the capacity limitation of standard single-mode fiber (SSMF) transmission systems. However, the differential mode gain (DMG) arising in the FM-EDFA fundamentally limits its transmission capacity and length. Herein, an innovative DMG equalization strategy using femtosecond laser micromachining to adjust the refractive index (RI) is presented. Variable mode-dependent attenuations can be achieved according to the DMG profile of the FM-EDFA, enabling DMG equalization. To validate the proposed strategy, DMG equalization of the commonly used FM-EDFA configuration was investigated. Simulation results revealed that by optimizing both the length and RI modulation depth of the femtosecond laser-tailoring area, the maximum DMG (DMGmax) among the 3 linear-polarized (LP) mode-group was mitigated from 10 dB to 1.52 dB, whereas the average DMG (DMGave) over the C-band was reduced from 8.95 dB to 0.78 dB. Finally, a 2-LP mode-group DMG equalizer was experimentally demonstrated, resulting in a reduction of the DMGmax from 2.09 dB to 0.46 dB, and a reduction of DMGave over the C band from 1.64 dB to 0.26 dB, with only a 1.8 dB insertion loss. Moreover, a maximum range of variable DMG equalization was achieved with 5.4 dB, satisfying the requirements of the most commonly used 2-LP mode-group amplification scenarios.

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All-silicon low-loss THz temporal differentiator based on microring waveguide resonator platform
Yunjie Rui, Shuyu Zhou, Xuecou Tu, Xu Yan, Bingnan Yan, et al.
Published Published online: 24 April 2024,  doi: 10.37188/lam.2024.017

Microring resonators have been widely used in passive optical devices such as wavelength division multiplexers, differentiators, and integrators. Research on terahertz (THz) components has been accelerated by these photonics technologies. Compact and integrated time-domain differentiators that enable low-loss, high-speed THz signal processing are necessary for THz applications. In this study, an on-chip THz temporal differentiator based on all-silicon photonic technology was developed. This device primarily consisted of a microring waveguide resonator and was packaged with standard waveguide compatibility. It performed time-domain differentiation on input signals at a frequency of 405.45 GHz with an insertion loss of 2.5 dB and a working bandwidth of 0.36 GHz. Various periodic waveforms could be handled by this differentiator. This device could work as an edge detector, which detected step-like edges in high-speed input signals through differential effects. This development holds significant promise for future THz data processing technologies and THz communication systems.

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Motion-copying method with symbol sequence-based phase switch control for intelligent optical manufacturing
Yutang Wang, Dapeng Tian, Haixiang Hu, Yan Li, Shiquan Ni
Published Published online: 08 April 2024,  doi: 10.37188/lam.2024.012

Implementation of robot-based motion control in optical machining demonstrably enhances the machining quality. The introduction of motion-copying method enables learning and replicating manipulation from experienced technicians. Nevertheless, the location uncertainties of objects and frequent switching of manipulated spaces in practical applications impose constraints on their further advancement. To address this issue, a motion-copying system with a symbol-sequence-based phase switch control (SSPSC) scheme was developed by transferring the operating skills and intelligence of technicians to mechanisms. The manipulation process is decomposed, symbolised, rearranged, and reproduced according to the manufacturing characteristics regardless of the change in object location. A force-sensorless adaptive sliding-mode-assisted reaction force observer (ASMARFOB), wherein a novel dual-layer adaptive law was designed for high-performance fine force sensing, was established. The uniformly ultimate boundedness (UUB) of the ASMARFOB is guaranteed based on the Lyapunov stability theory, and the switching stability of the SSPSC was examined. Validation simulations and experiments demonstrated that the proposed method enables better motion reproduction with high consistency and adaptability. The findings of this study can provide effective theoretical and practical guidance for high-precision intelligent optical manufacturing.

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Review
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Advances in femtosecond laser synthesis and micromachining of halide perovskites
Shijie Du, Fangteng Zhang, Lin Ma
Published Published online: 15 July 2024,  doi: 10.37188/lam.2024.035
Perovskite materials have become a popular research topic because of their unique optical and electrical properties, that enable extensive applications in information storage, lasers, anti-counterfeiting, and planar lenses. However, the success of the application depends on accomplishing high-precision and high-quality perovskite patterning technology. Numerous methods have been proposed for perovskite production, including, a femtosecond laser with an ultrashort pulse width and ultrahigh peak power with unique advantages such as high precision and efficiency, nonlinearity, and excellent material adaptability in perovskite material processing. Furthermore, femtosecond lasers can induce precipitation of perovskite inside glass/crystals, which markedly enhances the stability of perovskite materials and promotes their application and development in various fields. This review introduces perovskite precipitation and processing via femtosecond lasers. The methods involved and advantages of femtosecond-laser-induced perovskite precipitation and patterning are systematically summarized. The review also provides an outlook for further optimization and improvement of femtosecond laser preparation and processing methods for perovskites, which may offer significant support for future research and applications of perovskite materials.
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ISSN 2689-9620    EISSN 2831-4093

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