Grayscale-to-Color: Scalable Fabrication of Custom Multispectral Filter Arrays

Ultraviolet light blocking optically clear adhesives for foldable displays via highly efficient visible-light curing

In developing an organic light-emitting diode (OLED) panel for a foldable smartphone (specifically, a color filter on encapsulation) aimed at reducing power consumption, the use of a new optically clear adhesive (OCA) that blocks UV light was crucial. However, the incorporation of a UV-blocking agent within the OCA presented a challenge, as it restricted the traditional UV-curing methods commonly used in the manufacturing process. Although a visible-light curing technique for producing UV-blocking OCA was proposed, its slow curing speed posed a barrier to commercialization. Our study introduces a highly efficient photo-initiating system (PIS) for the rapid production of UV-blocking OCAs utilizing visible light. We have carefully selected the photocatalyst (PC) to minimize electron and energy transfer to UV-blocking agents and have chosen co-initiators that allow for faster electron transfer and more rapid PC regeneration compared to previously established amine-based co-initiators. This advancement enabled a tenfold increase in the production speed of UV-blocking OCAs, while maintaining their essential protective, transparent, and flexible properties. When applied to OLED devices, this OCA demonstrated UV protection, suggesting its potential for broader application in the safeguarding of various smart devices.

Large-aperture electromagnetically actuated MEMS Fabry-Perot filtering chips for visible spectral imaging

Benefitting from the inherent merits of tiny volume, customizable performance, good system compatibility and high-yield production, micro-electro-mechanical-system-based Fabry-Perot filtering chip (MEMS-FPFC) with a large aperture size gives a feasible way for the realization of miniaturized spectral imagers which can serve in many civilian and military scenarios. Although the aperture size of MEMS-FPFCs in mid-wave and long-wave infrared has reached to the centimeter scale, that of visible wavelength (VIS) MEMS-FPFC is always unsatisfied which is mainly limited by micromachining stress, especially in the thin films. In this work, we propose a large-aperture electromagnetically actuated MEMS-FPFC based on Si3N4 supporting membrane for VIS spectral imaging, which is designed with the assistance of multi-field coupling simulation model. A low-stress wafer-scale bulk micromachining process is developed to guarantee the high-quality and high-yield production for the aimed VIS MEMS-FPFCs. Finally, by the strictly controlling and rationally allocating the film stress of multi-layer film stack, VIS MEMS-FPFCs with 6 mm aperture size are thus developed, which can be tuned bidirectionally and continuously in 612-678 nm waveband with a good linear response of better than 95%. The achieved VIS MEMS-FPFCs can be utilized to construct miniaturized spectral imagers directly, aiming for such applications as intelligent agriculture, environmental protection and industrial inspection.

Flexible long-wave infrared snapshot multispectral imaging with a pixel-level spectral filter array

This paper proposes and demonstrates a flexible long-wave infrared snapshot multispectral imaging system consisting of a simple re-imaging system and a pixel-level spectral filter array. A six-band multispectral image in the spectral range of 8-12 µm with full width at half maximum of about 0.7 µm each band is acquired in the experiment. The pixel-level multispectral filter array is placed at the primary imaging plane of the re-imaging system instead of directly encapsulated on the detector chip, which diminishes the complexity of pixel-level chip packaging. Furthermore, the proposed method possesses the merit of flexible functions switching between multispectral imaging and intensity imaging by plugging and unplugging the pixel-level spectral filter array. Our approach could be viable for various practical long-wave infrared detection applications.

Grayscale-patterned integrated multilayer-metal-dielectric microcavities for on-chip multi/hyperspectral imaging in the extended visible bandwidth

Pixelated filter arrays of Fabry-Perot (FP) cavities are widely integrated with photodetectors to achieve a WYSIWYG ("what you see is what you get") on-chip spectral measurements. However, FP-filter-based spectral sensors typically have a trade-off between their spectral resolution and working bandwidth due to design limitations of conventional metal or dielectric multilayer microcavities. Here, we propose a new idea of integrated color filter arrays (CFAs) consisting of multilayer metal-dielectric-mirror FP microcavities that, enable a hyperspectral resolution over an extended visible bandwidth (∼300 nm). By introducing another two dielectric layers on the metallic film, the broadband reflectance of the FP-cavity mirror was greatly enhanced, accompanied by as-flat-as-possible reflection-phase dispersion. This resulted in balanced spectral resolution (∼10 nm) and spectral bandwidth from 450 nm to 750 nm. In the experiment, we used a one-step rapid manufacturing process by using grayscale e-beam lithography. A 16-channel (4 × 4) CFA was fabricated and demonstrated on-chip spectral imaging with a CMOS sensor and an impressive identification capability. Our results provide an attractive method for developing high-performance spectral sensors and have potential commercial applications by extending the utility of low-cost manufacturing process.

Superposition Fabry-Perot filter array for a computational hyperspectral camera

Computational hyperspectral cameras with broadband encoded filter arrays enable high precision spectrum reconstruction with only a few filters. However, these types of hyperspectral cameras have limited application, because it is difficult for conventional encoded filter arrays to balance among the spectrum regulation capacity, angle insensitivity, and processibility. This Letter presents a new, to the best of our knowledge, encoded filter composed of superposition Fabry-Perot resonance cavity (SFP) that can simultaneously take all three aspects into consideration. By learning the parameters of an SFP encoder and a neural network decoder in an end-to-end manner, a computational hyperspectral camera based on an SFP filter array presents up to 2.24 times higher spectral reconstruction accuracy, 10 times wider working angle, and can be produced with a low-cost manufacturing process.

Modular snapshot multispectral-panchromatic imager (MSPI) with customized filter arrays

As one of the simplest methods to construct snapshot spectral imagers, multispectral filter array (MSFA) has been applied to commercial miniatured spectral imagers. While most of them have fixed configurations of spectral channels, lacking flexibility and replaceability. Moreover, conventional MSFA only comprises filtering channels but lacks the panchromatic channel which is essential in detecting dim and indistinct objects. Here, we propose a modular assembly method for snapshot imager which can simultaneously acquire the object's multispectral and panchromatic information based on a customized filter array. The multispectral-panchromatic filter array is batch fabricated and integrated with the imaging senor through a modular mode. Five-band spectral images and a broadband intensity image can be efficiently acquired in a single snapshot photographing. The efficacy and accuracy of the imager are experimentally verified in imaging and spectral measurements. Owing to the modular architecture, our proposed assembly method owns the advantages of compactness, simple assembling, rapid replacement, and customized designing, which overcomes the expensiveness and complexity of scientific-level snapshot spectral imaging systems.

Large-aperture, widely and linearly tunable, electromagnetically actuated MEMS Fabry-Perot filtering chips for longwave infrared spectral imaging

Longwave infrared spectral imaging (LWIR-SI) has potential in many important civilian and military fields. However, conventional LWIR-SI systems based on traditional dispersion elements always suffer the problems of high cost, large volume and complicated system structure. Micro-electro-mechanical systems Fabry-Perot filtering chips (MEMS-FPFC) give a feasible way for realizing miniaturized, low cost and customizable LWIR-SI systems. The LWIR MEMS-FPFC ever reported can't meet the demands of the next-generation LWIR-SI systems, due to the limitation of small aperture size and nonlinear actuation. In this work, we propose a large-aperture, widely and linearly tunable electromagnetically actuated MEMS-FPFC for LWIR-SI. A multi-field coupling simulation model is built and the wafer-scale bulk-micromachining process is applied to realize the design and fabrication of the proposed MEMS-FPFC. Finally, with the rational structural design and fabrication process, the filtering chip after packaging has an aperture size of 10 mm, which is the largest aperture size of LWIR MEMS-FPFC ever reported. The fabricated electromagnetically actuated MEMS-FPFC can be tuned continuously across the entire LWIR range of 8.39-12.95 µm under ±100 mA driving current with a pretty good linear response of better than 98%. The developed electromagnetically actuated MEMS-FPFC can be directly used for constructing miniaturized LWIR-SI systems, aiming for such applications as military surveillance, gas sensing, and industry monitoring.

Infrared metasurface absorber based on silicon-based CMOS process

Metasurface with metal-insulator-metal (MIM) structure has absorption properties for incident light at specific wavelengths. In this paper, we propose an infrared metasurface absorber based on silicon-based complementary metal oxide semiconductor (CMOS) process. By adding the prepared infrared metasurface absorber to the liquid crystal on silicon (LCoS) chip, it is used as the absorbing layer of LCoS configured between the pixel unit and the CMOS driver circuit. The effect of zero-order light caused by the gap between pixels in LCoS spatial light modulator (LCoS-SLM) on the light modulation function of the device is effectively reduced. Experiments show that the LCoS-SLM with infrared metasurface absorption structure can eliminate the zero-order light interference between the pixel gaps to a great extent and improve the modulation efficiency of the device. The proposed LCoS-SLM integrating infrared metasurface absorber structure based on silicon-based CMOS process has the advantages of low-cost and high modulation efficiency, which has high application value in the fields of holographic display, optical computing and optical communication.

Crosstalk elimination by rearranging thin-film filters

Patterned thin-film Fabry-Pérot filters are used to develop compact spectral cameras. Recent articles report on crosstalk in such devices, raising concerns regarding spectral and spatial resolution. It has been suggested that light entering a filter might spill over to neighboring filters but this has not yet been analyzed in detail. The proposed mechanism in this Letter is that the Fabry-Pérot filters act as coupled waveguides that can propagate crosstalk above the pixel array. The results show that the crosstalk can be asymmetric, enabling elimination by rearranging the filters on the sensor. Interestingly, the logical strategy where crosstalk to all neighbors is eliminated appears suboptimal due to additional resonances. These findings reveal untapped opportunities for developing better sensors and a corresponding need for further systematic investigations.

Noninvasive hemoglobin sensing and imaging: optical tools for disease diagnosis

SIGNIFICANCE

Measurement and imaging of hemoglobin oxygenation are used extensively in the detection and diagnosis of disease; however, the applied instruments vary widely in their depth of imaging, spatiotemporal resolution, sensitivity, accuracy, complexity, physical size, and cost. The wide variation in available instrumentation can make it challenging for end users to select the appropriate tools for their application and to understand the relative limitations of different methods.

AIM

We aim to provide a systematic overview of the field of hemoglobin imaging and sensing.

APPROACH

We reviewed the sensing and imaging methods used to analyze hemoglobin oxygenation, including pulse oximetry, spectral reflectance imaging, diffuse optical imaging, spectroscopic optical coherence tomography, photoacoustic imaging, and diffuse correlation spectroscopy.

RESULTS

We compared and contrasted the ability of different methods to determine hemoglobin biomarkers such as oxygenation while considering factors that influence their practical application.

CONCLUSIONS

We highlight key limitations in the current state-of-the-art and make suggestions for routes to advance the clinical use and interpretation of hemoglobin oxygenation information.

Optimized spectral filter design enables more accurate estimation of oxygen saturation in spectral imaging

Oxygen saturation (SO₂) in tissue is a crucially important physiological parameter with ubiquitous clinical utility in diagnosis, treatment, and monitoring, as well as widespread use as an invaluable preclinical research tool. Multispectral imaging can be used to visualize SO₂ non-invasively, non-destructively and without contact in real-time using narrow spectral filter sets, but typically, these spectral filter sets are poorly suited to a specific clinical task, application, or tissue type. In this work, we demonstrate the merit of optimizing spectral filter sets for more accurate estimation of SO₂. Using tissue modelling and simulated multispectral imaging, we demonstrate filter optimization reduces the root-mean-square-error (RMSE) in estimating SO₂ by up to 37% compared with evenly spaced filters. Moreover, we demonstrate up to a 79% decrease in RMSE for optimized filter sets compared with filter sets chosen to minimize mutual information. Wider adoption of this approach will result in more effective multispectral imaging systems that can address specific clinical needs and consequently, more widespread adoption of multispectral imaging technologies in disease diagnosis and treatment.

Opti-MSFA: a toolbox for generalized design and optimization of multispectral filter arrays

Multispectral imaging captures spatial information across a set of discrete spectral channels and is widely utilized across diverse applications such as remote sensing, industrial inspection, and biomedical imaging. Multispectral filter arrays (MSFAs) are filter mosaics integrated atop image sensors that facilitate cost-effective, compact, snapshot multispectral imaging. MSFAs are pre-configured based on application-where filter channels are selected corresponding to targeted absorption spectra-making the design of optimal MSFAs vital for a given application. Despite the availability of many design and optimization approaches for spectral channel selection and spatial arrangement, major limitations remain. There are few robust approaches for joint spectral-spatial optimization, techniques are typically only applicable to limited datasets and most critically, are not available for general use and improvement by the wider community. Here, we reconcile current MSFA design techniques and present Opti-MSFA: a Python-based open-access toolbox for the centralized design and optimization of MSFAs. Opti-MSFA incorporates established spectral-spatial optimization algorithms, such as gradient descent and simulated annealing, multispectral-RGB image reconstruction, and is applicable to user-defined input of spatial-spectral datasets or imagery. We demonstrate the utility of the toolbox by comparing against other published MSFAs using the standard hyperspectral datasets Samson and Jasper Ridge, and further show application on experimentally acquired fluorescence imaging data. In conjunction with end-user input and collaboration, we foresee the continued development of Opti-MSFA for the benefit of the wider research community.

Opti-MSFA: a toolbox for generalized design and optimization of multispectral filter arrays

Multispectral imaging captures spatial information across a set of discrete spectral channels and is widely utilized across diverse applications such as remote sensing, industrial inspection, and biomedical imaging. Multispectral filter arrays (MSFAs) are filter mosaics integrated atop image sensors that facilitate cost-effective, compact, snapshot multispectral imaging. MSFAs are pre-configured based on application-where filter channels are selected corresponding to targeted absorption spectra-making the design of optimal MSFAs vital for a given application. Despite the availability of many design and optimization approaches for spectral channel selection and spatial arrangement, major limitations remain. There are few robust approaches for joint spectral-spatial optimization, techniques are typically only applicable to limited datasets and most critically, are not available for general use and improvement by the wider community. Here, we reconcile current MSFA design techniques and present Opti-MSFA: a Python-based open-access toolbox for the centralized design and optimization of MSFAs. Opti-MSFA incorporates established spectral-spatial optimization algorithms, such as gradient descent and simulated annealing, multispectral-RGB image reconstruction, and is applicable to user-defined input of spatial-spectral datasets or imagery. We demonstrate the utility of the toolbox by comparing against other published MSFAs using the standard hyperspectral datasets Samson and Jasper Ridge, and further show application on experimentally acquired fluorescence imaging data. In conjunction with end-user input and collaboration, we foresee the continued development of Opti-MSFA for the benefit of the wider research community.

Integrated near-infrared spectral sensing

Spectral sensing is increasingly used in applications ranging from industrial process monitoring to agriculture. Sensing is usually performed by measuring reflected or transmitted light with a spectrometer and processing the resulting spectra. However, realizing compact and mass-manufacturable spectrometers is a major challenge, particularly in the infrared spectral region where chemical information is most prominent. Here we propose a different approach to spectral sensing which dramatically simplifies the requirements on the hardware and allows the monolithic integration of the sensors. We use an array of resonant-cavity-enhanced photodetectors, each featuring a distinct spectral response in the 850-1700 nm wavelength range. We show that prediction models can be built directly using the responses of the photodetectors, despite the presence of multiple broad peaks, releasing the need for spectral reconstruction. The large etendue and responsivity allow us to demonstrate the application of an integrated near-infrared spectral sensor in relevant problems, namely milk and plastic sensing. Our results open the way to spectral sensors with minimal size, cost and complexity for industrial and consumer applications.

What are the minimal hardware requirements for a given class of sensing problems? Here, authors investigate this while proposing a miniaturized near-infrared spectral sensor, based on an array of resonant-cavity enhanced photodetectors, and capable of operating without the need for spectral reconstruction.

Hyperspectral Imaging for Clinical Applications

Measuring morphological and biochemical features of tissue is crucial for disease diagnosis and surgical guidance, providing clinically significant information related to pathophysiology. Hyperspectral imaging (HSI) techniques obtain both spatial and spectral features of tissue without labeling molecules such as fluorescent dyes, which provides rich information for improved disease diagnosis and treatment. Recent advances in HSI systems have demonstrated its potential for clinical applications, especially in disease diagnosis and image-guided surgery. This review summarizes the basic principle of HSI and optical systems, deep-learning-based image analysis, and clinical applications of HSI to provide insight into this rapidly growing field of research. In addition, the challenges facing the clinical implementation of HSI techniques are discussed.

Filter width affects the transmittance of patterned all-dielectric Fabry-Perot filters

Thin-film all-dielectric Fabry-Perot filters can nowadays be patterned onto pixels of commercial imaging sensors used for spectral imaging. For these patterned filters, standard transfer-matrix thin-film calculations fail to predict their angular dependency. This Letter attributes the discrepancy to the finite filter size and is also, to my knowledge, the first study to analyze this for patterned all-dielectric Fabry-Perot filters. An angular spectrum approach that enables prediction without full knowledge of the filter design is introduced. In addition, the contribution of diffraction at normal incidence is characterized by a single dimensionless parameter. Knowing that patterned filter size matters and having a method to efficiently simulate its effect can guide ongoing miniaturization efforts and filter design.

First-in-human pilot study of snapshot multispectral endoscopy for early detection of Barrett's-related neoplasia

SIGNIFICANCE

The early detection of dysplasia in patients with Barrett's esophagus could improve outcomes by enabling curative intervention; however, dysplasia is often inconspicuous using conventional white-light endoscopy.

AIM

We sought to determine whether multispectral imaging (MSI) could be applied in endoscopy to improve detection of dysplasia in the upper gastrointestinal (GI) tract.

APPROACH

We used a commercial fiberscope to relay imaging data from within the upper GI tract to a snapshot MSI camera capable of collecting data from nine spectral bands. The system was deployed in a pilot clinical study of 20 patients (ClinicalTrials.gov NCT03388047) to capture 727 in vivo image cubes matched with gold-standard diagnosis from histopathology. We compared the performance of seven learning-based methods for data classification, including linear discriminant analysis, k-nearest neighbor classification, and a neural network.

RESULTS

Validation of our approach using a Macbeth color chart achieved an image-based classification accuracy of 96.5%. Although our patient cohort showed significant intra- and interpatient variance, we were able to resolve disease-specific contributions to the recorded MSI data. In classification, a combined principal component analysis and k-nearest-neighbor approach performed best, achieving accuracies of 95.8%, 90.7%, and 76.1%, respectively, for squamous, non-dysplastic Barrett's esophagus and neoplasia based on majority decisions per-image.

CONCLUSIONS

MSI shows promise for disease classification in Barrett's esophagus and merits further investigation as a tool in high-definition "chip-on-tip" endoscopes.

Batch fabrication and compact integration of customized multispectral filter arrays towards snapshot imaging

Snapshot multispectral imaging (MSI) has been widely employed in the rapid visual inspection by virtues of the non-invasive detection mode and short integration time. As the critical functional elements of snapshot MSI, narrowband, customizable, and pixel-level multispectral filter arrays (MSFAs) that are compatible with imaging sensors are difficult to be efficiently manufactured. Meanwhile, monolithically integrating MSFAs into snapshot multispectral imagers still remains challenging considering the strict alignment precision. Here, we propose a cost-efficient, wafer-level, and customized approach for fabricating transmissive MSFAs based on Fabry-Perot structures, both in the pixel-level and window-tiled configuration, by utilizing the conventional lithography combined with the deposition method. The MSFA chips own a total dimension covering the area of 4.8 mm × 3.6 mm with 4 × 4 bands, possessing the capability to maintain narrow line widths (∼25 nm) across the whole visible frequencies. After the compact integration with the imaging sensor, the MSFAs are validated to be effective in filtering and target identification. Our proposed fabrication method and imaging mode show great potentials to be an alternative to MSFAs production and MSI, by reducing both complexity and cost of manufacturing, while increasing flexibility and customization of imaging system.

Tunable Structural Color Images by UV-Patterned Conducting Polymer Nanofilms on Metal Surfaces

Precise manipulation of light-matter interactions has enabled a wide variety of approaches to create bright and vivid structural colors. Techniques utilizing photonic crystals, Fabry-Pérot cavities, plasmonics, or high-refractive-index dielectric metasurfaces have been studied for applications ranging from optical coatings to reflective displays. However, complicated fabrication procedures for sub-wavelength nanostructures, limited active areas, and inherent absence of tunability of these approaches impede their further development toward flexible, large-scale, and switchable devices compatible with facile and cost-effective production. Here, a novel method is presented to generate structural color images based on monochromic conducting polymer films prepared on metallic surfaces via vapor phase polymerization and ultraviolet (UV) light patterning. Varying the UV dose enables synergistic control of both nanoscale film thickness and polymer permittivity, which generates controllable structural colors from violet to red. Together with grayscale photomasks this enables facile fabrication of high-resolution structural color images. Dynamic tuning of colored surfaces and images via electrochemical modulation of the polymer redox state is further demonstrated. The simple structure, facile fabrication, wide color gamut, and dynamic color tuning make this concept competitive for applications like multifunctional displays.

Spectral Endoscopy Enhances Contrast for Neoplasia in Surveillance of Barrett's Esophagus

Early detection of esophageal neoplasia enables curative endoscopic therapy, but the current diagnostic standard of care has low sensitivity because early neoplasia is often inconspicuous with conventional white-light endoscopy. Here, we hypothesized that spectral endoscopy could enhance contrast for neoplasia in surveillance of patients with Barrett's esophagus. A custom spectral endoscope was deployed in a pilot clinical study of 20 patients to capture 715 in vivo tissue spectra matched with gold standard diagnosis from histopathology. Spectral endoscopy was sensitive to changes in neovascularization during the progression of disease; both non-dysplastic and neoplastic Barrett's esophagus showed higher blood volume relative to healthy squamous tissue (P = 0.001 and 0.02, respectively), and vessel radius appeared larger in neoplasia relative to non-dysplastic Barrett's esophagus (P = 0.06). We further developed a deep learning algorithm capable of classifying spectra of neoplasia versus non-dysplastic Barrett's esophagus with high accuracy (84.8% accuracy, 83.7% sensitivity, 85.5% specificity, 78.3% positive predictive value, and 89.4% negative predictive value). Exploiting the newly acquired library of labeled spectra to model custom color filter sets identified a potential 12-fold enhancement in contrast between neoplasia and non-dysplastic Barrett's esophagus using application-specific color filters compared with standard-of-care white-light imaging (perceptible color difference = 32.4 and 2.7, respectively). This work demonstrates the potential of endoscopic spectral imaging to extract vascular properties in Barrett's esophagus, to classify disease stages using deep learning, and to enable high-contrast endoscopy. SIGNIFICANCE: The results of this pilot first-in-human clinical trial demonstrate the potential of spectral endoscopy to reveal disease-associated vascular changes and to provide high-contrast delineation of neoplasia in the esophagus. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/12/3415/F1.large.jpg.

Periodic planar Fabry-Perot nanocavities with tunable interference colors based on filling density effects

Structural colors of high performance and economically feasible fabrication are desired in various applications. Herein, we demonstrate that reflective full-color filters based on the interference effect can be realized in periodic Fabry-Perot (F-P) nanocavity arrays of the same thickness. Enabled by simply adjusting the nanocavity size and array period, the resonant wavelengths can be successively tuned in the whole visible light range, which is mainly attributed to the varied effective refractive index introduced by the different filling density of the F-P nanocavity. Compared to the plasmonic colors utilizing the similar nanostructures, the proposed interference colors offer unique advantages of higher color contrast, wider gamut, and lower fabrication requirements. Besides, these color filters do not involve modulating the vertical dimensions of the F-P nanocavities, which is conducive to the monolithic integration of multicolor optical cavities and their large-area applications in consumable products combined with replica patterning techniques, such as nanoimprinting and soft lithography.

Integrated multispectral real-time imaging system based on metasurfaces

In this paper, a highly integrated array-based imaging system, composed of a lens array and a metasurface array, is proposed to achieve multispectral real-time imaging within a wide range of 400-1100 nm numerically. Each channel has an achromatic bandwidth of 50 nm and an aperture of about 5 mm, with the system average efficiency reaching over 91%. It breaks the restrictions of cumbersome volumes and limited materials that deteriorate the performance of conventional systems, facilitating miniaturization and integration. Moreover, the design method is also suitable for other spectral bands, widening applications of metasurfaces in multispectral imaging.

Snapshot multispectral imaging using a pixel-wise polarization color image sensor

This study proposes a new imaging technique for snapshot multispectral imaging in which a multispectral image was captured using an imaging lens that combines a set of multiple spectral filters and polarization filters, as well as a pixel-wise color polarization image sensor. The author produced a prototype nine-band multispectral camera system that covered from visible to near-infrared regions and was very compact. The camera's spectral performance was evaluated using experiments; moreover, the camera was used to detect the freshness of food and the activity of wild plants and was mounted on a vehicle to obtain a multispectral video while driving.

Robustness to misalignment of low-cost, compact quantitative phase imaging architectures

Non-interferometric approaches to quantitative phase imaging could enable its application in low-cost, miniaturised settings such as capsule endoscopy. We present two possible architectures and both analyse and mitigate the effect of sensor misalignment on phase imaging performance. This is a crucial step towards determining the feasibility of implementing phase imaging in a capsule device. First, we investigate a design based on a folded 4f correlator, both in simulation and experimentally. We demonstrate a novel technique for identifying and compensating for axial misalignment and explore the limits of the approach. Next, we explore the implications of axial and transverse misalignment, and of manufacturing variations on the performance of a phase plate-based architecture, identifying a clear trade-off between phase plate resolution and algorithm convergence time. We conclude that while the phase plate architecture is more robust to misalignment, both architectures merit further development with the goal of realising a low-cost, compact system for applying phase imaging in capsule endoscopy.

Microfabrication of a color filter array utilizing colored SU-8 photoresists

Patterned color filter arrays are important components in digital cameras, camcorders, scanners, and multispectral detection and imaging instruments. In addition to the rapid and continuous progress to improve camera resolution and the efficiency of imaging sensors, research into the design of color filter arrays is important to extend the imaging capability beyond conventional applications. This paper reports the use of colored SU-8 photoresists as a material to fabricate color filter arrays. Optical properties, fabrication parameters, and pattern spatial resolution are systematically studied for five color photoresists: violet, blue, green, yellow, and red. An end-to-end fabrication process is developed to realize a five-color filter array designed for a wide angle multiband artificial compound eye camera system for pentachromatic and polarization imaging. Colored SU-8 photoresists present notable advantages, including patternability, color tunability, low-temperature compatibility, and process simplicity. The results regarding the optical properties and the fabrication process for a colored SU-8 photoresist provide significant insight into its usage as an optical material to investigate nonconventional color filter designs.

Scalable and High-Throughput Top-Down Manufacturing of Optical Metasurfaces

Metasurfaces have shown promising potential to miniaturize existing bulk optical components thanks to their extraordinary optical properties and ultra-thin, small, and lightweight footprints. However, the absence of proper manufacturing methods has been one of the main obstacles preventing the practical application of metasurfaces and commercialization. Although a variety of fabrication techniques have been used to produce optical metasurfaces, there are still no universal scalable and high-throughput manufacturing methods that meet the criteria for large-scale metasurfaces for device/product-level applications. The fundamentals and recent progress of the large area and high-throughput manufacturing methods are discussed with practical device applications. We systematically classify various top-down scalable patterning techniques for optical metasurfaces: firstly, optical and printing methods are categorized and then their conventional and unconventional (emerging/new) techniques are discussed in detail, respectively. In the end of each section, we also introduce the recent developments of metasurfaces realized by the corresponding fabrication methods.

Multilevel nanoimprint lithography with a binary mould for plasmonic colour printing

Pigment-free colouration based on plasmonic resonances has recently attracted considerable attention for potential in manufacturing and other applications. For plasmonic colour utilizing the metal-insulator-metal (MIM) configuration, the generated colour is not only dependent on the geometry and transverse dimensions, but also to the size of the vertical gap between the metal nanoparticles and the continuous metal film. The complexity of conventional fabrication methods such as electron beam lithography (EBL), however, limits the capacity to control this critical parameter. Here we demonstrate the straightforward production of plasmonic colour via UV-assisted nanoimprint lithography (NIL) with a simple binary mould and demonstrate the ability to control this gap distance in a single print by harnessing the nanofluidic behaviour of the polymer resist through strategic mould design. We show that this provides a further avenue for controlling the colour reflected by the resulting plasmonic pixels as an adjunct to the conventional approach of tailoring the transverse dimensions of the nanostructures. Our experimental results exhibit wide colour coverage of the CIE 1931 XY colour space through careful control of both the length and periodicity and the resulting vertical gap size of the structure during the nanoimprinting process. Furthermore, to show full control over the vertical dimension, we show that a fixed gap size can be produced by introducing complementary microcavities in the vicinity of the nanostructures on the original mould. This demonstrates a simple method for obtaining an additional degree of freedom in NIL not only for structural colouration but also for other industrial applications such as high-density memory, biosensors and manufacturing.

Tunable mid-wave infrared Fabry-Perot bandpass filters using phase-change GeSbTe

We demonstrate spectrally-tunable Fabry-Perot bandpass filters operating across the MWIR by utilizing the phase-change material GeSbTe (GST) as a tunable cavity medium between two (Ge:Si) distributed Bragg reflectors. The induced refractive index modulation of GST increases the cavity's optical path length, red-shifting the passband. Our filters have spectral-tunability of ∼300 nm, transmission efficiencies of 60-75% and narrowband FWHMs of 50-65 nm (Q-factor ∼70-90). We further show multispectral thermal imaging and gas sensing. By matching the filter's initial passband to a CO2 vibrational-absorption mode (∼4.25 µm), tunable atmospheric CO2 sensing and dynamic plume visualization of added CO2 is realized.

Optical vortex rotation and propagation from a spiral phase plate resonator with surface reflective coating

A spiral phase plate resonator (SPPR) is created by depositing a reflective coating on the surfaces of a single conventional spiral phase plate (SPP) for the first time to the best of our knowledge. Optical transmission through the SPPR on the output plane of the device is measured to give sharp Fabry-Perot resonances as a function of beam roll angle. Similar measurements are performed for the reflected light emerging from the input plane of the SPPR device. Varying the light frequency going into the SPPR changes the orientation of the angular pattern (Fabry-Perot resonances) to give the rotational constant of the device, in agreement with theory. The optical mode profile is measured after the beam has propagated beyond the plane of the SPPR device while remaining in the diffraction near field, thus revealing new features in the transmitted optical beam. These new results have important implications for developing the SPPR for microscopy, imaging, angle measurement, rotational scanning, and LiDAR.

Engineering Microneedles for Therapy and Diagnosis: A Survey

Microneedle (MN) technology is a rising star in the point-of-care (POC) field, which has gained increasing attention from scientists and clinics. MN-based POC devices show great potential for detecting various analytes of clinical interests and transdermal drug delivery in a minimally invasive manner owing to MNs' micro-size sharp tips and ease of use. This review aims to go through the recent achievements in MN-based devices by investigating the selection of materials, fabrication techniques, classification, and application, respectively. We further highlight critical aspects of MN platforms for transdermal biofluids extraction, diagnosis, and drug delivery assisted disease therapy. Moreover, multifunctional MNs for stimulus-responsive drug delivery systems were discussed, which show incredible potential for accurate and efficient disease treatment in dynamic environments for a long period of time. In addition, we also discuss the remaining challenges and emerging trend of MN-based POC devices from the bench to the bedside.