Imaging-based intelligent spectrometer on a plasmonic rainbow chip

Controlling the wavefront aberration of a large-aperture and high-precision holographic diffraction grating

The scanning interference field exposure technique is an effective method to fabricate holographic diffraction grating with meter-level size and nano-level precision. The main problems of fabricating large-aperture and high-precision grating by this technique are the high-precision displacement measurement of the stage, the high-precision control of the interference fringe and the real time compensation of the grating phase error. In this paper, the influence of grating groove error on the wavefront aberration is analyzed. In order to improve the precision of the stage with displacement range more than one meter, an integrated displacement measurement combining grating sensing and laser interferometry is proposed, which suppresses the influence of environment on measurement precision under long displacement range. An interference fringe measurement method is proposed, which combines the diffraction characteristics of the measuring grating with the phase-shifting algorithm. By controlling the direction, period and phase nonlinear errors of the interference fringe, high quality interference fringe can be obtained. Further, a dynamic phase-locking model is established by using heterodyne interferometry to compensate grating phase error caused by stage motion error in real time. A grating with the aperture of 1500 mm × 420 mm is fabricated. The wavefront aberration reaches 0.327λ @ 632.8 nm and the wavefront gradient reaches 16.444 nm/cm. This research presents a novel technique for the fabrication of meter-level size and nano-level precision holographic grating, which would further promote the development of chirped pulse amplification systems, high-energy laser and ultra-high precision displacement measurement.

Plasmonic resonances of metallic moiré superlattices in the infrared range

The recent surge of interest in moiré photonics arises from the possibility of exploring many groundbreaking physical phenomena in photonics. These phenomena include photonic topological states and magic-angle lasing, which offer an attractive platform for manipulating the flow and confinement of light from remarkably simple device geometries. In this work, we fabricate a series of metallic moiré superlattices supporting moiré plasmon polaritons and explore the moiré-potential induced plasmonic resonances. We demonstrate that two-dimensional moiré plasmonic superlattices exhibit transmittance and polarization-dependent responses because of the localized plasmonic resonances in the infrared range, whose modes have a near-flat dispersion band. Our findings hold the potential for the understanding of localized plasmonic resonances within moiré superlattices.

Metasurface spectrometers beyond resolution-sensitivity constraints

Conventional spectrometer designs necessitate a compromise between their resolution and sensitivity, especially as device and detector dimensions are scaled down. Here, we report on a miniaturizable spectrometer platform where light throughput onto the detector is instead enhanced as the resolution is increased. This planar, CMOS-compatible platform is based around metasurface encoders designed to exhibit photonic bound states in the continuum, where operational range can be altered or extended simply through adjusting geometric parameters. This system can enhance photon collection efficiency by up to two orders of magnitude versus conventional designs; we demonstrate this sensitivity advantage through ultralow-intensity fluorescent and astrophotonic spectroscopy. This work represents a step forward for the practical utility of spectrometers, affording a route to integrated, chip-based devices that maintain high resolution and SNR without requiring prohibitively long integration times.

Advances in Miniaturized Computational Spectrometers

Miniaturized computational spectrometers have emerged as a promising strategy for miniaturized spectrometers, which breaks the compromise between footprint and performance in traditional miniaturized spectrometers by introducing computational resources. They have attracted widespread attention and a variety of materials, optical structures, and photodetectors are adopted to fabricate computational spectrometers with the cooperation of reconstruction algorithms. Here, a comprehensive review of miniaturized computational spectrometers, focusing on two crucial components: spectral encoding and reconstruction algorithms are provided. Principles, features, and recent progress of spectral encoding strategies are summarized in detail, including space-modulated, time-modulated, and light-source spectral encoding. The reconstruction algorithms are classified into traditional and deep learning algorithms, and they are carefully analyzed based on the mathematical models required for spectral reconstruction. Drawing from the analysis of the two components, cooperations between them are considered, figures of merits for miniaturized computational spectrometers are highlighted, optimization strategies for improving their performance are outlined, and considerations in operating these systems are provided. The application of miniaturized computational spectrometers to achieve hyperspectral imaging is also discussed. Finally, the insights into the potential future applications and developments of computational spectrometers are provided.

PbS-based SWIR micro-spectrometer with on-chip Fabry-Perot filter array

Miniaturized and portable on-chip spectrometers have been widely explored to facilitate many applications including chemical analysis, environmental monitoring, medical diagnostics, and astronomical observations. However, the optical spectra of micro-spectrometers are mostly within the visible range. Here, we develop high-performance short-wave infrared (SWIR) micro-spectrometers through the integration of wafer-scale uniform lead sulfide (PbS) thin films with an on-chip Fabry-Perot filter array. The optoelectronic performance of PbS-based detectors could be markedly improved through the optimization of chemical bath deposition (CBD) conditions. The high-sensitivity PbS detectors based on the Fabry-Perot filter array demonstrate chemical analysis application.

Miniaturized on-chip spectrometer enabled by electrochromic modulation

Miniaturized on-chip spectrometers with small footprints, lightweight, and low cost are in great demand for portable optical sensing, lab-on-chip systems, and so on. Such miniaturized spectrometers are usually based on engineered spectral response units and then reconstruct unknown spectra with algorithms. However, due to the limited footprints of computational on-chip spectrometers, the recovered spectral resolution is limited by the number of integrated spectral response units/filters. Thus, it is challenging to improve the spectral resolution without increasing the number of used filters. Here we present a computational on-chip spectrometer using electrochromic filter-based computational spectral units that can be electrochemically modulated to increase the efficient sampling number for higher spectral resolution. These filters are directly integrated on top of the photodetector pixels, and the spectral modulation of the filters results from redox reactions during the dual injection of ions and electrons into the electrochromic material. We experimentally demonstrate that the spectral resolution of the proposed spectrometer can be effectively improved as the number of applied voltages increases. The average difference of the peak wavelengths between the reconstructed and the reference spectra decreases from 1.61 nm to 0.29 nm. We also demonstrate the proposed spectrometer can be worked with only four or two filter units, assisted by electrochromic modulation. In addition, we also demonstrate that the electrochromic filter can be easily adapted for hyperspectral imaging, due to its uniform transparency. This strategy suggests a new way to enhance the performance of miniaturized spectrometers with tunable spectral filters for high resolution, low-cost, and portable spectral sensing, and would also inspire the exploration of other stimulus responses such as photochromic and force-chromic, etc, on computational spectrometers.

Real-time machine learning-enhanced hyperspectro-polarimetric imaging via an encoding metasurface

Light fields carry a wealth of information, including intensity, spectrum, and polarization. However, standard cameras capture only the intensity, disregarding other valuable information. While hyperspectral and polarimetric imaging systems capture spectral and polarization information, respectively, in addition to intensity, they are often bulky, slow, and costly. Here, we have developed an encoding metasurface paired with a neural network enabling a normal camera to acquire hyperspectro-polarimetric images from a single snapshot. Our experimental results demonstrate that this metasurface-enhanced camera can accurately resolve full-Stokes polarization across a broad spectral range (700 to 1150 nanometer) from a single snapshot, achieving a spectral sensitivity as high as 0.23 nanometer. In addition, our system captures full-Stokes hyperspectro-polarimetric video in real time at a rate of 28 frames per second, primarily limited by the camera's readout rate. Our encoding metasurface offers a compact, fast, and cost-effective solution for multidimensional imaging that effectively uses information within light fields.

Tailoring resonant modes in dual cavities for transmissive structural colors with high brightness and high purity

We present quad-layered structural color filters producing transmissive red (R), green (G), and blue (B) colors with high brightness and high purity, where thicknesses of layers for the RGB colors are optimized by using a L-BFGS-B algorithm. To evaluate the performance of the proposed structural color filters, computer-based inverse designs based on meta-heuristic and reinforcement learning algorithms are employed, where the optical properties obtained from the inverse designs are comparable to those shown in our proposed design. A peak separation phenomenon in dual cavities is applied to make a spectral response rectangular, and also a resonance order is optimally tailored to maximize the transmittance at a resonant wavelength with the suppression of undesired higher-order resonances at the same time for achieving pure colors. Transmission efficiency over 75% and the full width at half-maximum (FWHM) less than 90 nm are achieved. Besides, selecting a cavity medium with a high refractive index allows the optical properties of the structural color filters to remain almost constant in wavelength over a broad range of incident angles up to 60°. Moreover, only a few deposition steps are necessary, thus leading to a much simple fabrication as compared to previous works that involve a series of complicated lithographic processes. The approach described in this study may provide new ways for achieving diverse applications, such as displays, imaging devices, decorations, and colored solar cells.

Miniaturized spectrometers based on graded photonic crystal films

Miniaturized spectrometers have become increasingly important in modern analytical and diagnostic applications due to their compact size, portability, and versatility. Despite the surge in innovative designs for miniaturized spectrometers, significant challenges persist, particularly concerning manufacturing cost and efficiency when devices become smaller. Here we introduce an ultracompact spectrometer design that is both cost-effective and highly efficient. The core dispersion element of this new design is a graded photonic crystal film, which is engineered by applying gradient stress during its fabrication. The film shows bandstop transmission spectral profiles, akin to a notch filter, enhancing light throughput compared to conventional narrowband filters. The spectral analysis, with a resolution of 5 nm and operating within the wavelength range of 450-650 nm, is conducted by reconstructing the spectrum from a series of such notch transmission profiles along the graded photonic crystal film, utilizing a sophisticated algorithm. This approach not only reduces manufacturing costs but also significantly improves the sensitivity (with a light throughput efficiency of 71.05%) and overall performance of the limitations of current technology, opening up new avenues for applications in diverse fields.

Multispectral imaging through metasurface with quasi-bound states in the continuum

By controlling light fields in subwavelength scales, metasurfaces enable novel ways for miniaturization and integration of spectral imaging system. Metasurfaces supporting quasi bound states in the continuum (quasi-BICs) can control the quality factor and spectral response by changing structural parameters. In this work, we present an ultra-compact multispectral imaging device, whereby spectral modulation is achieved by meta-atoms arrays supporting quasi-BICs. The designed meta-atom array can serve as filters over a wide range of wavelengths, which enables the device capable of a large operating range and high-fidelity spectral reconstruction with a fine spectral resolution. The microspectrometers composed of BIC metasurfaces also can work as imaging pixels to achieve computational imaging spectroscopy through periodic arrangement, which successfully resolves images with spatial aliasing in different channels. This spectrometer device can meet the market demand for miniaturization for rapidly object recognition and appropriate spatial spectral resolution at low cost.

Programmable, High-resolution Printing of Spatially Graded Perovskites for Multispectral Photodetectors

Micro/nanostructured perovskites with spatially graded compositions and bandgaps are promising in filter-free, chip-level multispectral, and hyperspectral detection. However, achieving high-resolution patterning of perovskites with controlled graded compositions is challenging. Here, a programmable mixed electrohydrodynamic printing (M-ePrinting) technique is presented to realize the one-step direct-printing of arbitrary spatially graded perovskite micro/nanopatterns for the first time. M-ePrinting enables in situ mixing and ejection of solutions with controlled composition/bandgap by programmatically varying driving voltage applied to a multichannel nozzle. Composition can be graded over a single dot, line or complex pattern, and the printed feature size is down to 1 µm, which is the highest printing resolution of graded patterns to the knowledge. Photodetectors based on micro/nanostructured perovskites with halide ions gradually varying from Br to I are constructed, which successfully achieve multispectral detection and full-color imaging, with a high detectivity and responsivity of 3.27 × 1015 Jones and 69.88 A W-1, respectively. The presented method provides a versatile and competitive approach for such miniaturized bandgap-tunable perovskite spectrometer platforms and artificial vision systems, and also opens new avenues for the digital fabrication of composition-programmable structures.

Dispersion-assisted high-dimensional photodetector

Intensity, polarization and wavelength are intrinsic characteristics of light. Characterizing light with arbitrarily mixed information on polarization and spectrum is in high demand1-4. Despite the extensive efforts in the design of polarimeters5-18 and spectrometers19-27, concurrently yielding high-dimensional signatures of intensity, polarization and spectrum of the light fields is challenging and typically requires complicated integration of polarization- and/or wavelength-sensitive elements in the space or time domains. Here we demonstrate that simple thin-film interfaces with spatial and frequency dispersion can project and tailor polarization and spectrum responses in the wavevector domain. By this means, high-dimensional light information can be encoded into single-shot imaging and deciphered with the assistance of a deep residual network. To the best of our knowledge, our work not only enables full characterization of light with arbitrarily mixed full-Stokes polarization states across a broadband spectrum with a single device and a single measurement but also presents comparable, if not better, performance than state-of-the-art single-purpose miniaturized polarimeters or spectrometers. Our approach can be readily used as an alignment-free retrofit for the existing imaging platforms, opening up new paths to ultra-compact and high-dimensional photodetection and imaging.

Miniature computational spectrometer with a plasmonic nanoparticles-in-cavity microfilter array

Optical spectrometers are essential tools for analysing light‒matter interactions, but conventional spectrometers can be complicated and bulky. Recently, efforts have been made to develop miniaturized spectrometers. However, it is challenging to overcome the trade-off between miniaturizing size and retaining performance. Here, we present a complementary metal oxide semiconductor image sensor-based miniature computational spectrometer using a plasmonic nanoparticles-in-cavity microfilter array. Size-controlled silver nanoparticles are directly printed into cavity-length-varying Fabry‒Pérot microcavities, which leverage strong coupling between the localized surface plasmon resonance of the silver nanoparticles and the Fabry‒Pérot microcavity to regulate the transmission spectra and realize large-scale arrayed spectrum-disparate microfilters. Supported by a machine learning-based training process, the miniature computational spectrometer uses artificial intelligence and was demonstrated to measure visible-light spectra at subnanometre resolution. The high scalability of the technological approaches shown here may facilitate the development of high-performance miniature optical spectrometers for extensive applications.

Metasurface array for single-shot spectroscopic ellipsometry

Spectroscopic ellipsometry is a potent method that is widely adopted for the measurement of thin film thickness and refractive index. Most conventional ellipsometers utilize mechanically rotating polarizers and grating-based spectrometers for spectropolarimetric detection. Here, we demonstrated a compact metasurface array-based spectroscopic ellipsometry system that allows single-shot spectropolarimetric detection and accurate determination of thin film properties without any mechanical movement. The silicon-based metasurface array with a highly anisotropic and diverse spectral response is combined with iterative optimization to reconstruct the full Stokes polarization spectrum of the light reflected by the thin film with high fidelity. Subsequently, the film thickness and refractive index can be determined by fitting the measurement results to a proper material model with high accuracy. Our approach opens up a new pathway towards a compact and robust spectroscopic ellipsometry system for the high throughput measurement of thin film properties.

Meta-Attention Network Based Spectral Reconstruction with Snapshot Near-Infrared Metasurface

Near-infrared (NIR) spectral information is important for detecting and analyzing material compositions. However, snapshot NIR spectral imaging systems still pose significant challenges owing to the lack of high-performance NIR filters and bulky setups, preventing effective encoding and integration with mobile devices. This study introduces a snapshot spectral imaging system that employs a compact NIR metasurface featuring 25 distinct C4 symmetry structures. Benefitting from the sufficient spectral variety and low correlation coefficient among these structures, center-wavelength accuracy of 0.05 nm and full width at half maximum accuracy of 0.13 nm are realized. The system maintains good performance within an incident angle of 1°. A novel meta-attention network prior iterative denoising reconstruction (MAN-IDR) algorithm is developed to achieve high-quality NIR spectral imaging. By leveraging the designed metasurface and MAN-IDR, the NIR spectral images, exhibiting precise textures, minimal artifacts in the spatial dimension, and little crosstalk between spectral channels, are reconstructed from a single grayscale recording image. The proposed NIR metasurface and MAN-IDR hold great promise for further integration with smartphones and drones, guaranteeing the adoption of NIR spectral imaging in real-world scenarios such as aerospace, health diagnostics, and machine vision.

Polarimetric imaging combining optical parameters for classification of mice non-melanoma skin cancer tissue using machine learning

Polarimetric imaging systems combining machine learning is emerging as a promising tool for the support of diagnosis and intervention decision-making processes in cancer detection/staging. A present study proposes a novel method based on Mueller matrix imaging combining optical parameters and machine learning models for classifying the progression of skin cancer based on the identification of three different types of mice skin tissues: healthy, papilloma, and squamous cell carcinoma. Three different machine learning algorithms (K-Nearest Neighbors, Decision Tree, and Support Vector Machine (SVM)) are used to construct a classification model using a dataset consisting of Mueller matrix images and optical properties extracted from the tissue samples. The experimental results show that the SVM model is robust to discriminate among three classes in the training stage and achieves an accuracy of 94 % on the testing dataset. Overall, it is provided that polarimetric imaging systems and machine learning algorithms can dynamically combine for the reliable diagnosis of skin cancer.

Nanoplasmonic biosensors for precision medicine

Nanoplasmonic biosensors have a huge boost for precision medicine, which allows doctors to better understand diseases at the molecular level and to improve the earlier diagnosis and develop treatment programs. Unlike traditional biosensors, nanoplasmonic biosensors meet the global health industry's need for low-cost, rapid and portable aspects, while offering multiplexing, high sensitivity and real-time detection. In this review, we describe the common detection schemes used based on localized plasmon resonance (LSPR) and highlight three sensing classes based on LSPR. Then, we present the recent applications of nanoplasmonic in other sensing methods such as isothermal amplification, CRISPR/Cas systems, lab on a chip and enzyme-linked immunosorbent assay. The advantages of nanoplasmonic-based integrated sensing for multiple methods are discussed. Finally, we review the current applications of nanoplasmonic biosensors in precision medicine, such as DNA mutation, vaccine evaluation and drug delivery. The obstacles faced by nanoplasmonic biosensors and the current countermeasures are discussed.