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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Non-isoplanatic lens aberration correction in dark-field digital holographic microscopy for semiconductor metrology
Tamar van Gardingen-Cromwijk, Sander Konijnenberg, Wim Coene, Manashee Adhikary, Teus Tukker, Stefan Witte, Johannes F. de Boer, Arie den Boef
Accepted  doi: 10.37188/lam.2023.041

In the semiconductor industry, the demand for more precise and accurate overlay metrology tools has increased because of the continued shrinking of feature sizes in integrated circuits. To achieve the required sub-nanometre precision, the current technology for overlay metrology has become complex and is reaching its limits. Herein, we present a dark-field digital holographic microscope using a simple two-element imaging lens with a high numerical aperture capable of imaging from the visible to near-infrared regions. This combination of high resolution and wavelength coverage was achieved by combining a simple imaging lens with a fast and accurate correction of non-isoplanatic aberrations. We present experimental results for overlay targets that demonstrate the capability of our computational aberration correction in the visible and near-infrared wavelength regimes. This wide-ranged-wavelength imaging system can advance semiconductor metrology.

Integrated optical parametric amplifiers in silicon nitride waveguides incorporated with 2D graphene oxide films
Yang Qu, Jiayang Wu, Yuning Zhang, Yunyi Yang, Linnan Jia, Houssein El Dirani, Sébastien Kerdiles, Corrado Sciancalepore, Pierre Demongodin, Christian Grillet, Christelle Monat, Baohua Jia, David J. Moss
Accepted  doi: 10.37188/lam.2023.039

Optical parametric amplification (OPA) represents a powerful solution to achieve broadband amplification in wavelength ranges beyond the scope of conventional gain media, for generating high-power optical pulses, optical microcombs, entangled photon pairs and a wide range of other applications. Here, we demonstrate optical parametric amplifiers based on silicon nitride (Si3N4) waveguides integrated with two-dimensional (2D) layered graphene oxide (GO) films. We achieve precise control over the thickness, length, and position of the GO films using a transfer-free, layer-by-layer coating method combined with accurate window opening in the chip cladding using photolithography. Detailed OPA measurements with a pulsed pump for the fabricated devices with different GO film thicknesses and lengths show a maximum parametric gain of ~24.0 dB, representing a ~12.2 dB improvement relative to the device without GO. We perform a theoretical analysis of the device performance, achieving good agreement with experiment and showing that there is substantial room for further improvement. This work represents the first demonstration of integrating 2D materials on chips to enhance the OPA performance, providing a new way of achieving high performance photonic integrated OPA by incorporating 2D materials.

Facile integration of electro-optic SiO2/ITO heterointerfaces in MIS structures for CMOS-compatible plasmonic waveguide modulation
Nasir Alfaraj, Charles Chih-Chin Lin, Sherif Nasif, Swati Rajput, Amr S. Helmy
Accepted  doi: 10.37188/lam.2023.038

By taking advantage of the absence of diffraction limit restrictions in plasmonic structures, strong modal confinement is made possible, paving the way for improved optical processes and miniaturized photonic circuit integration. Indium tin oxide (ITO) has emerged as a promising plasmonic material that serves as a relatively low-carrier density Drude metal by its electro-optic tunability and versatility as an integrative oxide. We herein demonstrate the facile integration of SiO2/ITO heterointerfaces into metal–insulator–semiconductor (MIS) electro-optic structures. The first MIS device employs a SiO2/ITO heterostructure grown on thin polycrystalline titanium nitride (poly-TiN) and capped at the ITO side with thin aluminum (Al) film contact electrode. The TiN interlayer acts as a bottom electrode, forming a metal–insulator–semiconductor-metal (MISM) heterojunction device, and grows directly on (100)-oriented silicon (Si). This MISM device enables one to examine the electrical properties of semiconductive ITO layers. The second MIS device incorporates a semiconductive ITO layer with a SiO2 dielectric spacer implemented on a silicon-on-insulator (SOI) platform, forming a graded-index coupled hybrid plasmonic waveguide (CHPW) modulator. This device architecture represents a crucial step towards realizing plasmonic modulation using oxide materials. The CHPW device performance presented herein provides a proof-of-concept that demonstrates the advantages offered by such device topology to perform optical modulation via charge carrier dispersion. The graded-index CHPW can be dynamically reconfigured for amplitude, phase, or 4-quadrature amplitude modulation utilizing a triode-like biasing strategy. It exhibited extinction ratio (ER) and insertion loss (IL) levels of around 1 dB/μm and 0.128 dB/μm, respectively, for a 10 μm waveguide length.

Imaging/nonimaging microoptical elements and stereoscopic systems based on femtosecond laser direct writing
Long Huang, Zhihan Hong, Qi-Dai Chen, Yong-Lai Zhang, Shaoqing Zhao, Yongjun Dong, Yu-Qing Liu, Hua Liu
Accepted  doi: 10.37188/lam.2023.037

The development of modern information technology has led to significant demand for microoptical elements with complex surface profiles and nanoscale surface roughness. Therefore, various micro- and nanoprocessing techniques are used to fabricate microoptical elements and systems. Femtosecond laser direct writing (FsLDW) uses ultrafast pulses and the ultraintense instantaneous energy of a femtosecond laser for micro-nano fabrication. FsLDW exhibits various excellent properties, including nonlinear multiphoton absorption, high-precision processing beyond the diffraction limit, and the universality of processable materials, demonstrating its unique advantages and potential applications in three-dimensional (3D) micro-nano manufacturing. FsLDW has demonstrated its value in the fabrication of various microoptical systems. This study details three typical principles of FsLDW, several design considerations to improve processing performance, processable materials, imaging/nonimaging microoptical elements, and their stereoscopic systems. Finally, a summary and perspective on the future research directions for FsLDW-enabled microoptical elements and stereoscopic systems are provided.

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.

Accuracy characterization of Shack–Hartmann sensor with residual error removal in spherical wavefront calibration
Yi He, Mingdi Bao, Yiwei Chen, Hong Ye, Jinyu Fan, et al.
Published Published online: 01 December 2023,  doi: 10.37188/lam.2023.036
The widely used Shack–Hartmann wavefront sensor (SHWFS) is a wavefront measurement system. Its measurement accuracy is limited by the reference wavefront used for calibration and also by various residual errors of the sensor itself. In this study, based on the principle of spherical wavefront calibration, a pinhole with a diameter of 1 µm was used to generate spherical wavefronts with extremely small wavefront errors, with residual aberrations of 1.0 × 10−4 λ RMS, providing a high-accuracy reference wavefront. In the first step of SHWFS calibration, we demonstrated a modified method to solve for three important parameters (f, the focal length of the microlens array (MLA), p, the sub-aperture size of the MLA, and s, the pixel size of the photodetector) to scale the measured SHWFS results. With only three iterations in the calculation, these parameters can be determined as exact values, with convergence to an acceptable accuracy. For a simple SHWFS with an MLA of 128 × 128 sub-apertures in a square configuration and a focal length of 2.8 mm, a measurement accuracy of 5.0 × 10−3 λ RMS was achieved across the full pupil diameter of 13.8 mm with the proposed spherical wavefront calibration. The accuracy was dependent on the residual errors induced in manufacturing and assembly of the SHWFS. After removing these residual errors in the measured wavefront results, the accuracy of the SHWFS increased to 1.0 × 10−3 λ RMS, with measured wavefronts in the range of λ/4. Mid-term stability of wavefront measurements was confirmed, with residual deviations of 8.04 × 10−5 λ PV and 7.94 × 10−5 λ RMS. This study demonstrates that the modified calibration method for a high-accuracy spherical wavefront generated from a micrometer-scale pinhole can effectively improve the accuracy of an SHWFS. Further accuracy improvement was verified with correction of residual errors, making the method suitable for challenging wavefront measurements such as in lithography lenses, astronomical telescope systems, and adaptive optics.
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Feature size below 100 nm realized by UV-LED-based microscope projection photolithography
Lei Zheng, Tobias Birr, Urs Zywietz, Carsten Reinhardt, Bernhard Roth
Published Published online: 16 November 2023,  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.
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Polarization-independent liquid-crystal phase modulator with multi-microdomain orthogonally twisted photoalignment
Mingyuan Tang, Kai Huang, Yumeng Zhang, Jiangang Lu
Published Published online: 15 November 2023,  doi: 10.37188/lam.2023.035

Polarization-independent liquid-crystal (LC) phase modulators can significantly improve the efficiency and reduce the complexity of optical systems. However, achieving good polarization independence for LC phase modulators with a simple structure is difficult. A light-controlled azimuth angle (LCAA) process based on the optical rotatory effect of cholesteric liquid crystals (CLC) was developed for fabricating single-layer, multi-microdomain, orthogonally twisted (MMOT) structures. The developed LC phase modulator with a single-layer MMOT structure may have a low polarization dependence with a large phase depth. This device shows good potential for applications in optical communications, wearable devices, and displays.

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Rapid fabrication of reconfigurable helical microswimmers with environmentally adaptive locomotion
Rui Li, Yuan Tao, Jiawen Li, Dongdong Jin, Chen Xin, et al.
Published Published online: 30 October 2023,  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.

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Effect of near-field optical angular momentum on molecular junctions
Jianchen Zi, Michaël Lobet, Luc Henrard, Zhiqiang Li, Chenhui Wang, et al.
Published Published online: 29 September 2023,  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.

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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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Two-photon lithography for integrated photonic packaging
Shaoliang Yu, Qingyang Du, Cleber Renato Mendonca, Luigi Ranno, Tian Gu, et al.
Published Published online: 01 November 2023,  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.

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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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ISSN 2689-9620    EISSN 2831-4093

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