Metasurfaces, which are the two-dimensional counterparts of metamaterials, have demonstrated unprecedented capabilities to manipulate the wavefront of electromagnetic waves in a single flat device. Despite various advances in this field, the unique functionalities achieved by metasurfaces have come at the cost of the structural complexity, resulting in a time-consuming parameter sweep for the conventional metasurface design. Although artificial neural networks provide a flexible platform for significantly improving the design process, the current metasurface designs are restricted to generating qualitative field distributions. In this study, we demonstrate that by combining a tandem neural network and an iterative algorithm, the previous restriction of the design of metasurfaces can be overcome with quantitative field distributions. As proof-of-principle examples, metalenses predicted via the designed network architecture that possess multiple focal points with identical/orthogonal polarisation states, as well as accurate intensity ratios (quantitative field distributions), were numerically calculated and experimentally demonstrated. The unique and robust approach for the metasurface design will enable the acceleration of the development of devices with high-accuracy functionalities, which can be applied in imaging, detecting, and sensing.
Fluorescence microscopy is a powerful tool for scientists to observe the microscopic world, and the fluorescence excitation light source is one of the most critical components. To compensate for the short operation lifetime, integrated light sources, and low excitation efficiency of conventional light sources such as mercury, halogen, and xenon lamps, we designed an LED-integrated excitation cube (LEC) with a decentralized structure and high optical power density. Using a Fresnel lens, the light from the light-emitting diode (LED) was effectively focused within a 15 mm mounting distance to achieve high-efficiency illumination. LEC can be easily designed in the shape of fluorescence filter cubes for installation in commercial fluorescence microscopes. LECs’ optical efficiency is 1–2 orders of magnitude higher than that of mercury lamps; therefore, high-quality fluorescence imaging with spectral coverage from UV to red can be achieved. By replacing conventional fluorescence filter cubes, LEC can be easily installed on any commercial fluorescence microscope. A built-in LEC driver can identify the types of LEDs in different spectral bands to adopt the optimal operating current and frequency of pulses. Moreover, high-contrast images can be achieved in pulse mode by time-gated imaging of long-lifetime luminescence.
Wafer-level mass production of photonic integrated circuits (PIC) has become a technological mainstay in the field of optics and photonics, enabling many novel and disrupting a wide range of existing applications. However, scalable photonic packaging and system assembly still represents a major challenge that often hinders commercial adoption of PIC-based solutions. Specifically, chip-to-chip and fiber-to-chip connections often rely on so-called active alignment techniques, where the coupling efficiency is continuously measured and optimized during the assembly process. This unavoidably leads to technically complex assembly processes and high cost, thereby eliminating most of the inherent scalability advantages of PIC-based solutions. In this paper, we demonstrate that 3D-printed facet-attached microlenses (FaML) can overcome this problem by opening an attractive path towards highly scalable photonic system assembly, relying entirely on passive assembly techniques based on industry-standard machine vision and/or simple mechanical stops. FaML can be printed with high precision to the facets of optical components using multi-photon lithography, thereby offering the possibility to shape the emitted beams by freely designed refractive or reflective surfaces. Specifically, the emitted beams can be collimated to a comparatively large diameter that is independent of the device-specific mode fields, thereby relaxing both axial and lateral alignment tolerances. Moreover, the FaML concept allows to insert discrete optical elements such as optical isolators into the free-space beam paths between PIC facets. We show the viability and the versatility of the scheme in a series of selected experiments of high technical relevance, comprising pluggable fiber-chip interfaces, the combination of PIC with discrete micro-optical elements such as polarization beam splitters, as well as coupling with ultra-low back-reflection based on non-planar beam paths that only comprise tilted optical surfaces. Based on our results, we believe that the FaML concept opens an attractive path towards novel PIC-based system architectures that combine the distinct advantages of different photonic integration platforms.
Here, we report the ability of spiropyrans to initiate two-photon polymerization (TPP) for the first time in the literature. The comparison and synergies between the spiropyran photochromic molecule of interest, namely 6-nitro-BIPS, and well-known photoinitiators of radical photopolymerization have been studied. The spiropyran (SPy) molecule can initiate TPP in the presence of trifunctional acrylic monomers and create true 3D structures. The comparison with Irgacure 819, a well-known Type-I photoinitiator, shows that SPy has a comparable capability for TPP. In addition, the combination of SPy with methyl diethanolamine increased the reactivity of both one- and two-photon polymerizations. In the last section, we discuss which SPy isomer is the active photochromic species capable of generating radicals for initiating two-photon polymerization.
Optical fibres with diameters at micro- or sub-micrometre scale are widely adopted as a convenient tool for studying light–matter interactions. To prepare such devices, two elements are indispensable: a heat source and a pulling force. In this paper, we report a novel fibre-tapering technique in which micro-sized plasmonic heaters and elaborately deformed optical fibres are compactly combined, free of flame and bulky pulling elements. Using this technique, micro-nano fibres with abrupt taper and ultra-short transition regions were successfully fabricated, which would otherwise be a challenge for traditional techniques. The compactness of the proposed system enabled it to be further transferred to a scanning electron microscope for in-situ monitoring of the tapering process. The essential dynamics of “heat and pull” was directly visualised with nanometre precision in real time and theoretically interpreted, thereby establishing an example for future in-situ observations of micro and nanoscale light-matter interactions.
The advantage of spatial phase-shifting shearography is its ability to extract the phase from a single speckle pattern; however, it often faces spectrum overlapping, which seriously affects phase quality. In this paper, we propose a shearography phase-extraction method based on windowed Fourier ridges, which can effectively extract phase information even in the presence of severe spectrum overlapping. A simple and efficient method was applied to determine the parameters of the windowed Fourier ridges, and a linear variation window was used to match the phase-extraction requirements for different frequency coordinates. A numerical simulation was quantitatively conducted to compare the phase-extraction results of the proposed method with those of the conventional method for various cases, and a shearography system was built with two types of objects to demonstrate the feasibility of the proposed method.
Holography provides access to the optical phase. The emerging compressive phase retrieval approach can achieve in-line holographic imaging beyond the information-theoretic limit or even from a single shot by exploring the signal priors. However, iterative projection methods based on physical knowledge of the wavefield suffer from poor imaging quality, whereas the regularization techniques sacrifice robustness for fidelity. In this work, we present a unified compressive phase retrieval framework for in-line holography that encapsulates the unique advantages of both physical constraints and sparsity priors. In particular, a constrained complex total variation (CCTV) regularizer is introduced that explores the well-known absorption and support constraints together with sparsity in the gradient domain, enabling practical high-quality in-line holographic imaging from a single intensity image. We developed efficient solvers based on the proximal gradient method for the non-smooth regularized inverse problem and the corresponding denoising subproblem. Theoretical analyses further guarantee the convergence of the algorithms with prespecified parameters, obviating the need for manual parameter tuning. As both simulated and optical experiments demonstrate, the proposed CCTV model can characterize complex natural scenes while utilizing physically tractable constraints for quality enhancement. This new compressive phase retrieval approach can be extended, with minor adjustments, to various imaging configurations, sparsifying operators, and physical knowledge. It may cast new light on both theoretical and empirical studies.
The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is the world ’ s largest single-dish radio telescope. Its large reflecting surface achieves unprecedented sensitivity but is prone to damage, such as dents and holes, caused by naturally-occurring falling objects. Hence, the timely and accurate detection of surface defects is crucial for FAST’s stable operation. Conventional manual inspection involves human inspectors climbing up and examining the large surface visually, a time-consuming and potentially unreliable process. To accelerate the inspection process and increase its accuracy, this work makes the first step towards automating the inspection of FAST by integrating deep-learning techniques with drone technology. First, a drone flies over the surface along a predetermined route. Since surface defects significantly vary in scale and show high inter-class similarity, directly applying existing deep detectors to detect defects on the drone imagery is highly prone to missing and misidentifying defects. As a remedy, we introduce cross-fusion, a dedicated plug-in operation for deep detectors that enables the adaptive fusion of multi-level features in a point-wise selective fashion, depending on local defect patterns. Consequently, strong semantics and fine-grained details are dynamically fused at different positions to support the accurate detection of defects of various scales and types. Our AI-powered drone-based automated inspection is time-efficient, reliable, and has good accessibility, which guarantees the long-term and stable operation of FAST.
The rapid development of the aerospace and nuclear industries is accompanied by increased exposure to high-energy ionising radiation. Thus, the performance of radiation shielding materials needs to be improved to extend the service life of detectors and ensure the safety of personnel. The development of novel lightweight materials with high electron density has therefore become urgent to alleviate radiation risks. In this work, new MAPbI3/epoxy (CH3NH3PbI3/epoxy) composites were prepared via a crystal plane engineering strategy. These composites delivered excellent radiation shielding performance against 59.5 keV gamma rays. A high linear attenuation coefficient (1.887 cm−1) and mass attenuation coefficient (1.352 cm2 g−1) were achieved for a representative MAPbI3/epoxy composite, which was 10 times higher than that of the epoxy. Theoretical calculations indicate that the electron density of MAPbI3/epoxy composites significantly increases when the content ratio of the (110) plane in MAPbI3 increases. As a result, the chances of collision between the incident gamma rays and electrons in the MAPbI3/epoxy composites were enhanced. The present work provides a novel strategy for designing and developing high-efficiency radiation shielding materials.
The interaction between a probing tip and an adsorbed molecule can significantly impact the molecular chemical structure and even induce its motion on the surface. In this study, the tip-induced bond weakening, tilting, and hopping processes of a single molecule were investigated by sub-nanometre resolved tip-enhanced Raman spectroscopy (TERS). We used single carbon monoxide (CO) molecules adsorbed on the Cu (100) surface as a model system for the investigation. The vibrational frequency of the C−O stretching mode is always redshifted as the tip approaches, revealing the weakening of the C−O bond owing to tip−molecule interactions. Further analyses of both the vibrational Stark effect and TERS imaging patterns suggest a delicate tilting phenomenon of the adsorbed CO molecule on Cu(100), which eventually leads to lateral hopping of the molecule. While a tilting orientation is found toward the hollow site along the [110] direction of the Cu(100) surface, the hopping event is more likely to proceed via the bridge site to the nearest Cu neighbour along the [100] or [010] direction. Our results provide deep insights into the microscopic mechanisms of tip−molecule interactions and tip-induced molecular motions on surfaces at the single-bond level.
We report all-optical mid-infrared phase and intensity modulators based on the photo-thermal effect in an acetylene-filled anti-resonant hollow-core fiber. Optical absorption of the control beam promotes the gas molecules to a higher energy level, which induces localized heating through non-radiative relaxation and modulates the refractive index of the gas material and hence the accumulated phase of the signal beam propagating through the hollow-core fiber. By modulating the intensity of the control beam, the phase of the signal beam is modulated accordingly. By use of a 1.53 µm near-infrared control beam, all-optical phase modulation up to 2.2π rad is experimentally demonstrated at the signal wavelength of 3.35 µm. With the phase modulator placed in one arm of a Mach-Zehnder interferometer, intensity modulation with on-off ratio of 25 dB is achieved. The gas-filled hollow-core-fiber modulators could operate over an ultra-broad wavelength band from near- to mid-infrared and have promising application in mid-infrared photonic systems.
Tilted Wave Interferometry (TWI) is a measurement technique for fast and flexible interferometric testing of aspheres and freeform surfaces. The first version of the tilted wave principle was implemented in a Twyman-Green type setup with separate reference arm, which is intrinsically susceptible to environmentally induced phase disturbances. In this contribution we present the TWI in a new robust common-path (Fizeau) configuration. The implementation of the Tilted Wave Fizeau Interferometer requires a new approach in illumination, calibration and evaluation. Measurements of two aspheres and a freeform surface show the flexibility and also the increased stability in both phase raw data and surface measurements, which leads to a reduced repeatability up to a factor of three. The novel configuration significantly relaxes the tolerances of the imaging optics used in the interferometer. We demonstrate this using simulations on calibration measurements, where we see an improvement of one order of magnitude compared to the classical Twyman-Green TWI approach and the capability to compensate higher order error contributions on the used optics.
To improve the lateral resolution in microscopic imaging, microspheres are placed close to the object’ s surface in order to support the imaging process by optical near-field information. Although microsphere-assisted measurements are part of various recent studies, no generally accepted explanation for the effect of microspheres exists. Photonic nanojets, enhancement of the numerical aperture, whispering-gallery modes and evanescent waves are usually named reasons in context with microspheres, though none of these effects is proven to be decisive for the resolution enhancement. We present a simulation model of the complete microscopic imaging process of microsphere-enhanced interference microscopy including a rigorous treatment of the light scattering process at the surface of the specimen. The model consideres objective lenses of high numerical aperture providing 3D conical illumination and imaging. The enhanced resolution and magnification by the microsphere is analyzed with respect to the numerical aperture of the objective lenses. Further, we give a criterion for the achievable resolution and demonstrate that a local enhancement of the numerical aperture is the most likely reason for the resolution enhancement.
Aspheric micro-lens array (AMLA), featured with low dispersion and diffraction-limited imaging quality, plays an important role in advanced optical imaging. Ideally, the fabrication of commercially applicable AMLAs should feature low cost, high precision, large area and high speed. However, these criteria have been achieved only partially with conventional fabrication process. Herein, we demonstrate the fabrication and characterization of AMLAs based on 12-bit direct laser writing lithography, which exhibits a high fabrication speed, large area, perfect lens shape control via a three-dimensional optical proximity correction and average surface roughness lower than 6 nm. In particular, the AMLAs can be flexibly designed with customized filling factor and arbitrary off-axis operation for each single micro-lens, and the proposed pattern transfer approach with polydimethylsiloxane (PDMS) suggests a low-cost way for mass manufacturing. An auto-stereoscopic-display flexible thin film with excellent display effect has been prepared by using above technology, which exhibits a new way to provide flexible auto-stereoscopic-display at low cost. In brief, the demonstrated fabrication of AMLAs based on direct laser writing lithography reduce the complexity of AMLA fabrication while significantly increasing their performance, suggesting a new route for high-quality three-dimentional optical manufacturing towards simplified fabrication process, high precision and large scale.
Surface plasmon devices mounted at the end-facets of optical fibers are appealing candidates for rapid and point-of-care sensing applications, by offering a special dip-and-read operation mode. At present, these devices’ noise-equivalent limits-of-detection lag far behind the free-space counterparts, leaving them incapable of most biosensing applications. Here we report a quasi-3D Fano resonance cavity and its fabrication method to fundamentally improve the quality factor and coupling efficiency for fiber-coupled surface plasmon resonance. In this device, the Fano resonance combines the high coupling efficiency of a Fabry-Pérot etalon and the high quality factor resonance of a plasmonic crystal cavity. The quasi-3D device was fabricated on a planar substrate and transferred to a single-mode fiber end-facet, which requires a low-adhesion yet surface-plasmon-tunneling interface between the device and the planar substrate. Such an interface was realized with a nanocap-slit unit structure, of which the plasmonic crystal was consisted. A noise-equivalent limit of detection of ~ 10-7 RIU was experimentally obtained, allowing bovine serum albumin physical adsorption to be distinguished at ng mL-1 level concentrations. Therefore, breaking through the long-standing signal-to-noise ratio bottleneck, this work makes fiber end-facet surface plasmon devices into one of high sensitivity label-free sensing technologies. At the same time, it provides an enabling top-down fabrication technology for making 3D plasmonic structures on fiber end-facets at the nanometer scale.
ISSN 2689-9620 EISSN 2831-4093
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2021, 2(3): 350-369. doi: 10.37188/lam.2021.024
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2021, 2(1): 59-83. doi: 10.37188/lam.2021.005
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2021, 2(3): 313-332. doi: 10.37188/lam.2021.020
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2022, 3(1): 137-150. doi: 10.37188/lam.2022.009
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2021, 2(4): 446-459. doi: 10.37188/lam.2021.028
