Imaging-based spectrometer-less optofluidic biosensors based on dielectric metasurfaces for detecting extracellular vesicles

Nanoplasmonic Sensing of Heterogeneous Extracellular Vesicles: From Bulk to Single Vesicles

Extracellular vesicles (EVs) are heterogeneous nanoscale membrane vesicles released by almost all cell types into the circulation. Depending on their biogenesis and cells of origin, EVs show considerable heterogeneity in their biophysical and biomolecular composition and can serve as reflective and dynamic blood biomarkers for personalized medicine. Conventional analytical technologies, however, often lack the compatibility to reveal nanoscale EV features and resolve vesicle heterogeneity. The past decade has since witnessed the development of various nanoplasmonic technologies to empower EV analysis, through bulk and single-vesicle characterization, at an unprecedented scale and resolution. These platforms achieve versatile measurements that are not only size-matched to EV dimensions but can also probe multiplexed biomolecular contents, thereby providing new insights into EV heterogeneity and enabling transformative clinical opportunities. In this review, key characteristics of EVs and their remarkable heterogeneity are introduced. The sensing principles of plasmonic platforms are also discussed, with recent technology developments highlighted to resolve EV heterogeneity, through bulk analyses of EV subpopulations as well as high-resolution single-EV measurements. An outlook is further provided on emerging opportunities, at the interface of biomarker discovery and technology innovation, to develop empowering nanoplasmonic EV platforms for personalized medicine. biosensing; bulk analysis; extracellular vesicles; nanoplasmonics; single-vesicle analysis.

Integrative plasmonics: optical multi-effects and acousto-electric-thermal fusion for biosensing, energy conversion, and photonic circuits

Surface plasmons, a unique optical phenomenon arising at the interface between metals and dielectrics, have garnered significant interest across fields such as biochemistry, materials science, energy, optics, and nanotechnology. Recently, plasmonics is evolving from a focus on "classical plasmonics," which emphasizes fundamental effects and applications, to "integrative plasmonics," which explores the integration of plasmonics with multidisciplinary technologies. This review explores this evolution, summarizing key developments in this technological shift and offering a timely discussion on the fusion mechanisms, strategies, and applications. First, we examine the integration mechanisms of plasmons within the realm of optics, detailing how fundamental plasmonic effects give rise to optical multi-effects, such as plasmon-phonon coupling, nonlinear optical effects, electromagnetically induced transparency, chirality, nanocavity resonance, and waveguides. Next, we highlight strategies for integrating plasmons with technologies beyond optics, analyzing the processes and benefits of combining plasmonics with acoustics, electronics, and thermonics, including comprehensive plasmonic-electric-acousto-thermal integration. We then review cutting-edge applications in biochemistry (molecular diagnostics), energy (harvesting and catalysis), and informatics (photonic integrated circuits). These applications involve surface-enhanced Raman scattering (SERS), surface-enhanced infrared absorption (SEIRA), surface-enhanced fluorescence (SEF), chirality, nanotweezers, photoacoustic imaging, perovskite solar cells, photocatalysis, photothermal therapy, and triboelectric nanogenerators (TENGs). Finally, we conclude with a forward-looking perspective on the challenges and future of integrative plasmonics, considering advances in mechanisms (quantum effects, spintronics, and topology), materials (Dirac semimetals and hydrogels), technologies (machine learning, edge computing, in-sensor computing, and neuroengineering), and emerging applications (5G, 6G, virtual reality, and point-of-care testing).

Nanophotonic sensing and label-free imaging of extracellular vesicles

This review examines imaging-based nanophotonic biosensing and interferometric label-free imaging, with a particular focus on vesicle detection. It specifically compares dielectric and plasmonic metasurfaces for label-free protein and extracellular vesicle detection, highlighting their respective advantages and limitations. Key topics include: (i) refractometric sensing principles using resonant dielectric and plasmonic surfaces; (ii) state-of-the-art developments in both plasmonic and dielectric nanostructured resonant surfaces; (iii) a detailed comparison of resonance characteristics, including amplitude, quality factor, and evanescent field enhancement; and (iv) the relationship between sensitivity, near-field enhancement, and analyte overlap in different sensing platforms. The review provides insights into the fundamental differences between plasmonic and dielectric platforms, discussing their fabrication, integration potential, and suitability for various analyte sizes. It aims to offer a unified, application-oriented perspective on the potential of these resonant surfaces for biosensing and imaging, aiming at addressing topics of interest for both photonics experts and potential users of these technologies.

Broadband Full Polarization Control Using a Staggered Arrangement of Metamolecules

Metasurfaces show an extraordinary ability for polarization control, despite the limitations imposed by their working bandwidth and complex meta-atom designs. Here, we present a strategy for generating arbitrary polarization in the broadband, relying solely on the Pancharatnam-Berry phase. The proposed metasurface design comprises two metamolecules arranged in a staggered configuration. Importantly, the orientation of the two metamolecules provides a new degree of freedom to control both the amplitude and phase of right- and left-circular polarization components. We experimentally demonstrated that the polarization of the scattered light can cover the entire Poincaré sphere by merely tuning the metamolecule orientation. These results provide a unique approach for fabricating ultracompact photonic devices and integrated quantum optical systems.

Nanofabrication for Nanophotonics

Nanofabrication, a pivotal technology at the intersection of nanoscale engineering and high-resolution patterning, has substantially advanced over recent decades. This technology enables the creation of nanopatterns on substrates crucial for developing nanophotonic devices and other applications in diverse fields including electronics and biosciences. Here, this mega-review comprehensively explores various facets of nanofabrication focusing on its application in nanophotonics. It delves into high-resolution techniques like focused ion beam and electron beam lithography, methods for 3D complex structure fabrication, scalable manufacturing approaches, and material compatibility considerations. Special attention is given to emerging trends such as the utilization of two-photon lithography for 3D structures and advanced materials like phase change substances and 2D materials with excitonic properties. By highlighting these advancements, the review aims to provide insights into the ongoing evolution of nanofabrication, encouraging further research and application in creating functional nanostructures. This work encapsulates critical developments and future perspectives, offering a detailed narrative on the state-of-the-art in nanofabrication tailored for both new researchers and seasoned experts in the field.

Surface-enhanced spectroscopy technology based on metamaterials

Surface-enhanced spectroscopy technology based on metamaterials has flourished in recent years, and the use of artificially designed subwavelength structures can effectively regulate light waves and electromagnetic fields, making it a valuable platform for sensing applications. With the continuous improvement of theory, several effective universal modes of metamaterials have gradually formed, including localized surface plasmon resonance (LSPR), Mie resonance, bound states in the continuum (BIC), and Fano resonance. This review begins by summarizing these core resonance mechanisms, followed by a comprehensive overview of six main surface-enhanced spectroscopy techniques across the electromagnetic spectrum: surface-enhanced fluorescence (SEF), surface-enhanced Raman scattering (SERS), surface-enhanced infrared absorption (SEIRA), terahertz (THz) sensing, refractive index (RI) sensing, and chiral sensing. These techniques cover a wide spectral range and address various optical characteristics, enabling the detection of molecular fingerprints, structural chirality, and refractive index changes. Additionally, this review summarized the combined use of different enhanced spectra, the integration with other advanced technologies, and the status of miniaturized metamaterial systems. Finally, we assess current challenges and future directions. Looking to the future, we anticipate that metamaterial-based surface-enhanced spectroscopy will play a transformative role in real-time, on-site detection across scientific, environmental, and biomedical fields.

Automated Electroosmotic Digital Optofluidics for Rapid and Label-Free Protein Detection

Rapid protein detection is crucial for medical diagnosis, clinical trials, and drug development but often faces challenges in balancing sensitivity with multiplex detection, low reagent consumption, and a short detection time. In this work, we developed an automated and sensitive electroosmotic digital optofluidics (e-DOF) platform for rapid and label-free protein biomarker quantification in microliter blood samples. The hyperspectral computation reveals nanoscale morphology changes caused by target protein capture, eliminating multifarious enzyme-linked labeling. Electroosmosis-driven molecular circulation accelerates the immuno-hybridization, enhancing sensitivity (with a detection limit of 0.21 nM) and reducing the detection time to 15 min, compared to 2-3 h for a traditional enzyme-linked immunosorbent assay. In multiplex detection of hepatitis A and E IgM in 17 clinical samples, the results were completely consistent with clinical trial outcomes. This e-DOF system presents an automated, rapid, and label-free platform for multiplex detection in microliter samples, highlighting potential applications in clinical diagnosis and immunoassay research.

Metasurface-enhanced biomedical spectroscopy

Enhancing the sensitivity of biomedical spectroscopy is crucial for advancing medical research and diagnostics. Metasurfaces have emerged as powerful platforms for enhancing the sensitivity of various biomedical spectral detection technologies. This capability arises from their unparalleled ability to improve interactions between light and matter through the localization and enhancement of light fields. In this article, we review representative approaches and recent advances in metasurface-enhanced biomedical spectroscopy. We provide a comprehensive discussion of various biomedical spectral detection technologies enhanced by metasurfaces, including infrared spectroscopy, Raman spectroscopy, fluorescence spectroscopy, and other spectral modalities. We demonstrate the advantages of metasurfaces in improving detection sensitivity, reducing detection limits, and achieving rapid biomolecule detection while discussing the challenges associated with the design, preparation, and stability of metasurfaces in biomedical detection procedures. Finally, we explore future development trends of metasurfaces for enhancing biological detection sensitivity and emphasize their wide-ranging applications.

Harnessing the Power of Plasmonics for in Vitro and in Vivo Biosensing

Plasmonic nanostructures exhibit localized surface plasmon resonances due to collective oscillation of conducting electrons that can be tuned by modulating the nanostructure size, shape, material composition, and local dielectric environment. The strong field confinement and enhancement provided by plasmonic nanostructures have been exploited over the years to enhance the sensitivity for analyte detection down to the single-molecule level, rendering these devices as potentially outstanding biosensors. Here, we summarize methods to detect biological analytes in vitro and in living cells, with a focus on plasmon-enhanced fluorescence, Raman scattering, infrared absorption, circular dichroism, and refractive index sensing. Given the tremendous advances in the field, we concentrate on a few recent examples toward biosensing under highly challenging detection conditions, including clinically relevant biomarkers in body fluids and nascent applications in living cells and in vivo. These emerging platforms serve as inspiration for exploring future directions of nanoplasmonics that can be further harnessed to advance real-world biosensing applications.

Quad-narrowband perfect absorption in near infrared for optical switching and sensing based on quasi-bound states in the continuum

The absorption of light based on quasi-BICs is a significant factor influencing the performance of solar cells and photodetectors. Nevertheless, the development of multiple narrowband perfect absorbers remains a significant challenge. In this study, three distinct types of BIC were first discovered to coexist within a metasurface structure. This paper proposes a dual grating metasurface (DGM) structure based on three classes of BICs supported by near-infrared spectroscopy. It achieves perfect absorption in four narrow bands dominated by quasi-BICs, with each of the peaks exceeding 99.5%. The physical mechanism of each resonance has been analysed using temporal coupled mode theory, which has revealed the existence of the Symmetry-Protected BIC, Friedrich-Wintgen BIC and Fabry-Pérot BIC. Moreover, the underlying mechanisms of the distinct resonance modes are revealed through the multipolar decomposition of these resonances. The metasurface has significant potential for utilisation as an optical switch, which is capable of achieving an optimal modulation depth, switching contrast, extinction ratio, and insertion loss of 99.9%, 127 932%, -31.1 dB, and 0.0007 dB, respectively. The DGM structure offers a superior quad-frequency synchronised optical switch in comparison to conventional optical switches. And it also exhibited a maximum sensitivity of 328.6 nm RIU-1 and a maximum FOM of 93.9 RIU-1 when used as a sensor. The work presented herein will facilitate the exploration of a novel avenue for the study of ultra-high performance multifunctional devices based on a multitude of types of BICs.

Hybrid Plasmonic Symmetry-Protected Bound state in the Continuum Entering the Zeptomolar Biodetection Range

Photonics bound states in the continuum (BICs) are peculiar localized states in the continuum of free-space waves, unaffected by far-field radiation loss. Although plasmonic nano-antennas squeeze the optical field to nanoscale volumes, engineering the emergence of quasi-BICs with plasmonic hotspots remains challenging. Here, the origin of symmetry-protected (SP) quasi-BICs in a 2D system of silver-filled dimers, quasi-embedded in a high-index dielectric waveguide, is investigated through the strong coupling between photonic and plasmonic modes. By tailoring the hybridizing plasmonic/photonic fractions, a trade-off is selected at which the quasi-BIC exhibits both high intrinsic Q-factor and strong near-field enhancement because of dimer-gap hotspot activation. Not only radiation loss is damped but in a configuration sustaining a lattice of plasmonic hotspots. This leads to an advantageous small modal volume for enhancing light-matter interaction. The layout of nearly embedded dimers is designed to maximize the spatial overlap between the optical field and the target molecules, enhancing reactive sensing efficiency. The architecture is evaluated for its ability to detect transactive response DNA-binding protein 43. The refractometric sensitivity outperforms current label-free biosensing platforms, reaching the zeptomolar range. The approach highlights the potential of combining plasmonic and dielectric nanomaterials for advanced sensing technologies.

Demystifying EV heterogeneity: emerging microfluidic technologies for isolation and multiplexed profiling of extracellular vesicles

Extracellular vesicles (EVs) are heterogeneous lipid containers carrying complex molecular cargoes, including proteins, nucleic acids, glycans, etc. These vesicles are closely associated with specific physiological characteristics, which makes them invaluable in the detection and monitoring of various diseases. However, traditional isolation methods are often labour-intensive, inefficient, and time-consuming. In addition, single biomarker analyses are no longer accurate enough to meet diagnostic needs. Routine isolation and molecular analysis of high-purity EVs in clinical applications is even more challenging. In this review, we discuss a promising solution, microfluidic-based techniques, that combine efficient isolation and multiplex detection of EVs, to further demystify EV heterogeneity. These microfluidic-based EV multiplexing platforms will hopefully facilitate development of liquid biopsies and offer promising opportunities for personalised therapy.

Ultrasensitive metasurface sensor based on quasi-bound states in the continuum

The quasi-bound state in the continuum (quasi-BIC) of dielectric metasurface provides a crucial platform for sensing, because its almost infinite Q-factor can greatly enhance the interactions between light waves and the analytes. In this work, we proposed an ultrasensitive all-dielectric metasurface sensor composed of periodic rectangular amorphous silicon pillars on a quartz substrate. By breaking symmetry of two pillars in unit cell, high Q quasi-BIC in the continuous near-infrared band can be excited. The magnetic toroidal dipole (MTD) is demonstrated to play a dominating role in the resonant modes by analyzing near-field distribution and multipole decomposition. The asymmetry degree has a significant impact on sensing performance of the proposed metasurface sensor, whose underlying physical mechanisms is analyzed by perturbation theory. The transmission spectrum and sensing performance of the fabricated metasurface sensor were measured. The experimental results show our designed metasurface sensor not only achieve a high sensitivity of 413/RIU, but also shows a high figure of merit (FOM) of 66 RIU-1. This work provides excellent prospects for the excitation of strong MTD resonance quasi-BIC in sensing applications.

Optical Metasurfaces for Biomedical Imaging and Sensing

Optical metasurfaces, arrays of nanostructures engineered to manipulate light, have emerged as a transformative technology in both research and industry due to their compact design and exceptional light control capabilities. Their strong light-matter interactions enable precise wavefront modulation, polarization control, and significant near-field enhancements. These unique properties have recently driven their application in biomedical fields. In particular, metasurfaces have led to breakthroughs in biomedical imaging technologies, such as achromatic imaging, phase imaging, and extended depth-of-focus imaging. They have also advanced cutting-edge biosensing technologies, featuring high-quality factor resonators and near-field enhancements. As the demand for device miniaturization and system integration increases, metasurfaces are expected to play a pivotal role in the development of next-generation biomedical devices. In this review, we explore the latest advancements in the use of metasurfaces for biomedical applications, with a particular focus on imaging and sensing. Additionally, we discuss future directions aimed at transforming the biomedical field by leveraging the full potential of metasurfaces to provide compact, high-performance solutions for a wide range of applications.

High Quality-Factor All-Dielectric Metacavity for Label-Free Biosensing

High sensitivity and high quality-factor are crucial for achieving outstanding sensing performance in photonic biosensors. However, strong optical field confinement and high light-biomolecule interactions on photonic surfaces are usually contradictory and challenging to satisfy simultaneously. Here, a distinctive configuration for addressing this issue is reported: embedding a nanophotonic metasurface inside a micro vertical cavity as a meta-channel (metacavity) biosensor. The analyte solution serves as the cavity medium, thereby maximizing the light-analyte interaction. Simulation validation is conducted to optimize the metacavity with high structural robustness and remarkable optical and sensing properties. Large-scale low-cost metacavity biosensors are realized by combining anodic aluminum oxide template technique and wafer bonding. Experimentally, the metacavity biosensor demonstrates a notable quality-factor (maximum 4140) and high bulk refractometric sensitivity (450 nm RIU-1), resulting in an unprecedented figure-of-merit (1670 RIU-1). Moreover, the metacavity biosensor achieves high surface sensitivity, together with a detection-limit of 119 viral copies mL-1 for label-free severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pseudovirus sensing, revealing remarkable performance in both bulk and surface sensing.

Advances in exosome plasmonic sensing: Device integration strategies and AI-aided diagnosis

Exosomes, as next-generation biomarkers, has great potential in tracking cancer progression. They face many detection limitations in cancer diagnosis. Plasmonic biosensors have attracted considerable attention at the forefront of exosome detection, due to their label-free, real-time, and high-sensitivity features. Their advantages in multiplex immunoassays of minimal liquid samples establish the leading position in various diagnostic studies. This review delineates the application principles of plasmonic sensing technologies, highlighting the importance of exosomes-based spectrum and image signals in disease diagnostics. It also introduces advancements in miniaturizing plasmonic biosensing platforms of exosomes, which can facilitate point-of-care testing for future healthcare. Nowadays, inspired by the surge of artificial intelligence (AI) for science and technology, more and more AI algorithms are being adopted to process the exosome spectrum and image data from plasmonic detection. Using representative algorithms of machine learning has become a mainstream trend in plasmonic biosensing research for exosome liquid biopsy. Typically, these algorithms process complex exosome datasets efficiently and establish powerful predictive models for precise diagnosis. This review further discusses critical strategies of AI algorithm selection in exosome-based diagnosis. Particularly, we categorize the AI algorithms into the interpretable and uninterpretable groups for exosome plasmonic detection applications. The interpretable AI enhances the transparency and reliability of diagnosis by elucidating the decision-making process, while the uninterpretable AI provides high diagnostic accuracy with robust data processing by a "black-box" working mode. We believe that AI will continue to promote significant progress of exosome plasmonic detection and mobile healthcare in the near future.

Continuous spectral and coupling-strength encoding with dual-gradient metasurfaces

To control and enhance light-matter interactions at the nanoscale, two parameters are central: the spectral overlap between an optical cavity mode and the material's spectral features (for example, excitonic or molecular absorption lines), and the quality factor of the cavity. Controlling both parameters simultaneously would enable the investigation of systems with complex spectral features, such as multicomponent molecular mixtures or heterogeneous solid-state materials. So far, it has been possible only to sample a limited set of data points within this two-dimensional parameter space. Here we introduce a nanophotonic approach that can simultaneously and continuously encode the spectral and quality-factor parameter space within a compact spatial area. We use a dual-gradient metasurface design composed of a two-dimensional array of smoothly varying subwavelength nanoresonators, each supporting a unique mode based on symmetry-protected bound states in the continuum. This results in 27,500 distinct modes and a mode density approaching the theoretical upper limit for metasurfaces. By applying our platform to surface-enhanced molecular spectroscopy, we find that the optimal quality factor for maximum sensitivity depends on the amount of analyte, enabling effective molecular detection regardless of analyte concentration within a single dual-gradient metasurface. Our design provides a method to analyse the complete spectral and coupling-strength parameter space of complex material systems for applications such as photocatalysis, chemical sensing and entangled photon generation.

High-throughput antibody screening with high-quality factor nanophotonics and bioprinting

Empirical investigation of the quintillion-scale, functionally diverse antibody repertoires that can be generated synthetically or naturally is critical for identifying potential biotherapeutic leads, yet remains burdensome. We present high-throughput nanophotonics- and bioprinter-enabled screening (HT-NaBS), a multiplexed assay for large-scale, sample-efficient, and rapid characterization of antibody libraries. Our platform is built upon independently addressable pixelated nanoantennas exhibiting wavelength-scale mode volumes, high-quality factors (high-Q) exceeding 5000, and pattern densities exceeding one million sensors per square centimeter. Our custom-built acoustic bioprinter enables individual sensor functionalization via the deposition of picoliter droplets from a library of capture antigens at rates up to 25,000 droplets per second. We detect subtle differentiation in the target binding signature through spatially-resolved spectral imaging of hundreds of resonators simultaneously, elucidating antigen-antibody binding kinetic rates, affinity constant, and specificity. We demonstrate HT-NaBS on a panel of antibodies targeting SARS-CoV-2, Influenza A, and Influenza B antigens, with a sub-picomolar limit of detection within 30 minutes. Furthermore, through epitope binning analysis, we demonstrate the competence and diversity of a library of native antibodies targeting functional epitopes on a priority pathogen (H5N1 bird flu) and on glycosylated therapeutic Cetuximab antibodies against epidermal growth factor receptor. With a roadmap to image tens of thousands of sensors simultaneously, this high-throughput, resource-efficient, and label-free platform can rapidly screen for high-affinity and broad epitope coverage, accelerating biotherapeutic discovery and de novo protein design.

Recent Advancements in Nanophotonics for Optofluidics

Optofluidics is dedicated to achieving integrated control of particle and fluid motion, particularly on the micrometer scale, by utilizing light to direct fluid flow and particle motion. The field has seen significant growth recently, driven by the concerted efforts of researchers across various scientific disciplines, notably for its successful applications in biomedical science. In this review, we explore a range of optofluidic architectures developed over the past decade, with a primary focus on mechanisms for precise control of micro and nanoscale biological objects and their applications in sensing. Regarding nanoparticle manipulation, we delve into mechanisms based on optical nanotweezers using nanolocalized light fields and light-based hybrid effects with dramatically improved performance and capabilities. In the context of sensing, we emphasize those works that used optofluidics to aggregate molecules or particles to promote sensing and detection. Additionally, we highlight emerging research directions, encompassing both fundamental principles and practical applications in the field.

LSPR-susceptible metasurface platform for spectrometer-less and AI-empowered diagnostic biomolecule detection

In response to the growing demand for biomolecular diagnostics, metasurface (MS) platforms based on high-Q resonators have demonstrated their capability to detect analytes with smart data processing and image analysis technologies. However, high-Q resonator meta-atom arrays are highly sensitive to the fabrication process and chemical surface functionalization. Thus, spectrum scanning systems are required to monitor the resonant wavelength changes at every step, from fabrication to practical sensing. In this study, we propose an innovative dielectric resonator-independent MS platform that enables spectrometer-less biomolecule detection using artificial intelligence (AI) at a visible wavelength. Functionalizing the focused vortex MS to capture gold nanoparticle (AuNP)-based sandwich immunoassays causes the resulting vortex beam profiles to be significantly affected by the localized surface plasmon resonance (LSPR) occurring between AuNPs and meta-atoms. The convolutional neural network algorithm was carefully trained to accurately classify the AuNP concentration-dependent focused vortex beam, facilitating the determination of the concentration of the targeted diagnostic biomolecule. Successful in situ identification of various biomolecule concentrations was achieved with over 99 % accuracy, indicating the potential of combining an LSPR-susceptible MS platform and AI for continuously tracking various chemical and biological compounds.

Shedding Light on Cellular Secrets: A Review of Advanced Optical Biosensing Techniques for Detecting Extracellular Vesicles with a Special Focus on Cancer Diagnosis

In the relentless pursuit of innovative diagnostic tools for cancer, this review illuminates the cutting-edge realm of extracellular vesicles (EVs) and their biomolecular cargo detection through advanced optical biosensing techniques with a primary emphasis on their significance in cancer diagnosis. From the sophisticated domain of nanomaterials to the precision of surface plasmon resonance, we herein examine the diverse universe of optical biosensors, emphasizing their specified applications in cancer diagnosis. Exploring and understanding the details of EVs, we present innovative applications of enhancing and blending signals, going beyond the limits to sharpen our ability to sense and distinguish with greater sensitivity and specificity. Our special focus on cancer diagnosis underscores the transformative potential of optical biosensors in early detection and personalized medicine. This review aims to help guide researchers, clinicians, and enthusiasts into the captivating domain where light meets cellular secrets, creating innovative opportunities in cancer diagnostics.

Explainable Encoder-Prediction-Reconstruction Framework for the Prediction of Metasurface Absorption Spectra

The correlation between metasurface structures and their corresponding absorption spectra is inherently complex due to intricate physical interactions. Additionally, the reliance on Maxwell's equations for simulating these relationships leads to extensive computational demands, significantly hindering rapid development in this area. Numerous researchers have employed artificial intelligence (AI) models to predict absorption spectra. However, these models often act as black boxes. Despite training high-performance models, it remains challenging to verify if they are fitting to rational patterns or merely guessing outcomes. To address these challenges, we introduce the Explainable Encoder-Prediction-Reconstruction (EEPR) framework, which separates the prediction process into feature extraction and spectra generation, facilitating a deeper understanding of the physical relationships between metasurface structures and spectra and unveiling the model's operations at the feature level. Our model achieves a 66.23% reduction in average Mean Square Error (MSE), with an MSE of 2.843 × 10-4 compared to the average MSE of 8.421×10-4 for mainstream networks. Additionally, our model operates approximately 500,000 times faster than traditional simulations based on Maxwell's equations, with a time of 3×10-3 seconds per sample, and demonstrates excellent generalization capabilities. By utilizing the EEPR framework, we achieve feature-level explainability and offer insights into the physical properties and their impact on metasurface structures, going beyond the pixel-level explanations provided by existing research. Additionally, we demonstrate the capability to adjust absorption by changing the metasurface at the feature level. These insights potentially empower designers to refine structures and enhance their trust in AI applications.

Snapshot computational spectroscopy enabled by deep learning

Spectroscopy is a technique that analyzes the interaction between matter and light as a function of wavelength. It is the most convenient method for obtaining qualitative and quantitative information about an unknown sample with reasonable accuracy. However, traditional spectroscopy is reliant on bulky and expensive spectrometers, while emerging applications of portable, low-cost and lightweight sensing and imaging necessitate the development of miniaturized spectrometers. In this study, we have developed a computational spectroscopy method that can provide single-shot operation, sub-nanometer spectral resolution, and direct materials characterization. This method is enabled by a metasurface integrated computational spectrometer and deep learning algorithms. The identification of critical parameters of optical cavities and chemical solutions is demonstrated through the application of the method, with an average spectral reconstruction accuracy of 0.4 nm and an actual measurement error of 0.32 nm. The mean square errors for the characterization of cavity length and solution concentration are 0.53 % and 1.21 %, respectively. Consequently, computational spectroscopy can achieve the same level of spectral accuracy as traditional spectroscopy while providing convenient, rapid material characterization in a variety of scenarios.

The rise of electrically tunable metasurfaces

Metasurfaces, which offer a diverse range of functionalities in a remarkably compact size, have captured the interest of both scientific and industrial sectors. However, their inherent static nature limits their adaptability for their further applications. Reconfigurable metasurfaces have emerged as a solution to this challenge, expanding the potential for diverse applications. Among the series of tunable devices, electrically controllable devices have garnered particular attention owing to their seamless integration with existing electronic equipment. This review presents recent progress reported with respect to electrically tunable devices, providing an overview of their technological development trajectory and current state of the art. In particular, we analyze the major tuning strategies and discuss the applications in spatial light modulators, tunable optical waveguides, and adaptable emissivity regulators. Furthermore, the challenges and opportunities associated with their implementation are explored, thereby highlighting their potential to bridge the gap between electronics and photonics to enable the development of groundbreaking optical systems.

Portable Aptasensor Based on Parallel Rolling Circle Amplification for Tumor-Derived Exosomes Liquid Biopsy

Here, a separation-free and label-free portable aptasensor is developed for rapid and sensitive analysis of tumor-derived exosomes (TEXs). It integrated a parallel rolling circle amplification (RCA) reaction, selective binding of metal ions or small molecules to nucleic acid-specific conformations, and a low-cost, highly sensitive handheld fluorometer. Lung cancer, for example, is targeted with two typical biomarkers (mucin 1 and programmed cell death ligand 1 (PD-L1)) on its exosomes. The affinity of aptamers to the targets modulated the amount of RCA products (T-Hg2+-T and cytosine (C)-rich single-stranded DNA), which in turn affected the fluorescence intensity of quantum dots (QDs) and methylene blue (MB). The results revealed that the limit of detection (LOD) of the handheld fluorometer for cell-derived exosomes can be as low as 30 particles mL-1. Moreover, its specificity, sensitivity, and area under the curve (AUC) are 93% (14/15), 92% (23/25), and 0.956, as determined by the analysis of 40 clinical samples. Retesting 16 of these samples with the handheld fluorometer yielded strong concordance between the fluorometer results and those acquired from clinical computed tomography (CT) and pathology.

Advancements and Perspectives in Optical Biosensors

Optical biosensors exhibit immense potential, offering extraordinary possibilities for biosensing due to their high sensitivity, reusability, and ultrafast sensing capabilities. This review provides a concise overview of optical biosensors, encompassing various platforms, operational mechanisms, and underlying physics, and it summarizes recent advancements in the field. Special attention is given to plasmonic biosensors and metasurface-based biosensors, emphasizing their significant performance in bioassays and, thus, their increasing attraction in biosensing research, positioning them as excellent candidates for lab-on-chip and point-of-care devices. For plasmonic biosensors, we emphasize surface plasmon resonance (SPR) and its subcategories, along with localized surface plasmon resonance (LSPR) devices and surface enhance Raman spectroscopy (SERS), highlighting their ability to perform diverse bioassays. Additionally, we discuss recently emerged metasurface-based biosensors. Toward the conclusion of this review, we address current challenges, opportunities, and prospects in optical biosensing. Considering the advancements and advantages presented by optical biosensors, it is foreseeable that they will become a robust and widespread platform for early disease diagnostics.

Surface plasmons-phonons for mid-infrared hyperspectral imaging

Surface plasmons have proven their ability to boost the sensitivity of mid-infrared hyperspectral imaging by enhancing light-matter interactions. Surface phonons, a counterpart technology to plasmons, present unclear contributions to hyperspectral imaging. Here, we investigate this by developing a plasmon-phonon hyperspectral imaging system that uses asymmetric cross-shaped nanoantennas composed of stacked plasmon-phonon materials. The phonon modes within this system, controlled by light polarization, capture molecular refractive index intensity and lineshape features, distinct from those observed with plasmons, enabling more precise and sensitive molecule identification. In a deep learning-assisted imaging demonstration of severe acute respiratory syndrome coronavirus (SARS-CoV), phonons exhibit enhanced identification capabilities (230,400 spectra/s), facilitating the de-overlapping and observation of the spatial distribution of two mixed SARS-CoV spike proteins. In addition, the plasmon-phonon system demonstrates increased identification accuracy (93%), heightened sensitivity, and enhanced detection limits (down to molecule monolayers). These findings extend phonon polaritonics to hyperspectral imaging, promising applications in imaging-guided molecule screening and pharmaceutical analysis.

Multifunctional Meta-Devices for Full-Polarization Rotation and Focusing in the Near-Infrared

The creation of multi-channel focused beams with arbitrary polarization states and their corresponding optical torques finds effective applications in the field of optical manipulation at the micro-nanoscale. The existing metasurface-based technologies for polarization rotation have made some progress, but they have been limited to single functions and have not yet achieved the generation of full polarization. In this work, we propose a multi-channel and spatial-multiplexing interference strategy for the generation of multi-channel focusing beams with arbitrary polarization rotation based on all-dielectric birefringent metasurfaces via simultaneously regulating the propagation phase and the geometric phase and independently controlling the wavefronts at different circular polarizations. For the proof of concept, we demonstrate highly efficient multi-channel polarization rotation meta-devices. The meta-devices demonstrate ultra-high polarization extinction ratios and high focusing efficiencies at each polarization channel. Our work provides a compact and versatile wavefront-shaping methodology for full-polarization control, paving a new path for planar multifunctional meta-optical devices in optical manipulation at micro-nano dimensions.

Pixelated High-Q Metasurfaces for in Situ Biospectroscopy and Artificial Intelligence-Enabled Classification of Lipid Membrane Photoswitching Dynamics

Nanophotonic devices excel at confining light into intense hot spots of electromagnetic near fields, creating exceptional opportunities for light-matter coupling and surface-enhanced sensing. Recently, all-dielectric metasurfaces with ultrasharp resonances enabled by photonic bound states in the continuum (BICs) have unlocked additional functionalities for surface-enhanced biospectroscopy by precisely targeting and reading out the molecular absorption signatures of diverse molecular systems. However, BIC-driven molecular spectroscopy has so far focused on end point measurements in dry conditions, neglecting the crucial interaction dynamics of biological systems. Here, we combine the advantages of pixelated all-dielectric metasurfaces with deep learning-enabled feature extraction and prediction to realize an integrated optofluidic platform for time-resolved in situ biospectroscopy. Our approach harnesses high-Q metasurfaces specifically designed for operation in a lossy aqueous environment together with advanced spectral sampling techniques to temporally resolve the dynamic behavior of photoswitchable lipid membranes. Enabled by a software convolutional neural network, we further demonstrate the real-time classification of the characteristic cis and trans membrane conformations with 98% accuracy. Our synergistic sensing platform incorporating metasurfaces, optofluidics, and deep learning reveals exciting possibilities for studying multimolecular biological systems, ranging from the behavior of transmembrane proteins to the dynamic processes associated with cellular communication.

Passive trapping of biomolecules in hotspots with all-dielectric terahertz metamaterials

Electromagnetic metamaterials feature the capability of squeezing photons into hotspot regions of high intensity near-field enhancement for strong light-matter interaction, underpinning the next generation of emerging biosensors. However, randomly dispersed biomolecules around the hotspots lead to weak interactions. Here, we demonstrate an all-silicon dielectric terahertz metamaterial sensor design capable of passively trapping biomoleculars into the resonant cavities confined with powerful electric field. Specifically, multiple controllable high-quality factor resonances driven by bound states in the continuum (BIC) are realized by employing longitudinal symmetry breaking. The dielectric metamaterial sensor with nearly 15.2 experimental figure-of-merit enabling qualitative and quantitative identification of different amino acids by delivering biomolecules to the hotspots for strong light-matter interactions. It is envisioned that the presented strategy will enlighten high-performance meta-sensors design from microwaves to visible frequencies, and serve as a potential platform for microfluidic sensing, biomolecular capture, and sorting devices.

Optical system design of a femi-class ultra-high-resolution spectrometer based on the quaternary dispersion of a middle echelon grating

In order to meet the needs of the femi-ultra-high spectral resolution test, an optical system of the femi-ultra-high spectral resolution spectrometer in the spectral range of 190-800 nm is designed based on the quaternary dispersion of the middle echelon grating under the condition that the volume and weight of the spectrometer do not increase sharply. After the optimization design, the spectral resolution can reach 51.149 fm in the full field of view of 0.5 mm; at the wavelength of 191 nm, the spectral resolution in the whole spectral range is better than 150 fm, and the maximum distortion of the system is 0.2288%, which can provide a feasible reference for the subsequent design of the spectrometer optical system to realize the simultaneous detection of a wide band and high spectral resolution.

A Panorama of Extracellular Vesicle Applications: From Biomarker Detection to Therapeutics

Extracellular vesicles (EVs) secreted by all cell types are involved in the cell-to-cell transfer of regulatory factors that influence cell and tissue phenotypes in normal and diseased tissues. EVs are thus a rich source of biomarker targets for assays that analyze blood and urinary EVs for disease diagnosis. Sensitive biomarker detection in EVs derived from specific cell populations is a key major hurdle when analyzing complex biological samples, but innovative approaches surveyed in this Perspective can streamline EV isolation and enhance the sensitivity of EV detection procedures required for clinical application of EV-based diagnostics and therapeutics, including nanotechnology and microfluidics, to achieve EV characterizations. Finally, this Perspective also outlines opportunities and challenges remaining for clinical translation of EV-based assays.

Advanced Terahertz Refractive Sensing And Fingerprint Recognition Through Metasurface-Excited Surface Waves

High-sensitive metasurface-based sensors are essential for effective substance detection and insightful bio-interaction studies, which compress light in subwavelength volumes to enhance light-matter interactions. However, current methods to improve sensing performance always focus on optimizing near-field response of individual meta-atom, and fingerprint recognition for bio-substances necessitates several pixelated metasurfaces to establish a quasi-continuous spectrum. Here, a novel sensing strategy is proposed to achieve Terahertz (THz) refractive sensing, and fingerprint recognition based on surface waves (SWs). Leveraging the long-range transmission, strong confinement, and interface sensitivity of SWs, a metasurface-supporting SWs excitation and propagation is experimentally verified to achieve sensing integrations. Through wide-band information collection of SWs, the proposed sensor not only facilitates refractive sensing up to 215.5°/RIU, but also enables the simultaneous resolution of multiple fingerprint information within a continuous spectrum. By covering 5 µm thickness of polyimide, quartz and silicon nitride layers, the maximum phase change of 91.1°, 101.8°, and 126.4° is experimentally obtained within THz band, respectively. Thus, this strategy broadens the research scope of metasurface-excited SWs and introduces a novel paradigm for ultrasensitive sensing functions.

Merging toroidal dipole bound states in the continuum without up-down symmetry in Lieb lattice metasurfaces

The significance of bound states in the continuum (BICs) lies in their potential for theoretically infinite quality factors. However, their actual quality factors are limited by imperfections in fabrication, which lead to coupling with the radiation continuum. In this study, we present a novel approach to address this issue by introducing a merging BIC regime based on a Lieb lattice. By utilizing this approach, we effectively suppress the out-of-plane scattering loss, thereby enhancing the robustness of the structure against fabrication artifacts. Notably, unlike previous merging systems, our design does not rely on the up-down symmetry of metasurfaces. This characteristic grants more flexibility in applications that involve substrates and superstrates with different optical properties, such as microfluidic devices. Furthermore, we incorporate a lateral band gap mirror into the design to encapsulate the BIC structure. This mirror serves to suppress the in-plane radiation resulting from finite-size effects, leading to a remarkable ten-fold improvement in the quality factor. Consequently, our merged BIC metasurface, enclosed by the Lieb lattice photonic crystal mirror, achieves an exceptionally high-quality factor of 105 while maintaining a small footprint of 26.6 × 26.6 μm. Our findings establish an appealing platform that capitalizes on the topological nature of BICs within compact structures. This platform holds great promise for various applications, including optical trapping, optofluidics, and high-sensitivity biodetection, opening up new possibilities in these fields.

Fabrication of Mie-resonant silicon nanoparticles using laser annealing for surface-enhanced fluorescence spectroscopy

Silicon nanostructures with unique Mie resonances have garnered considerable attention in the field of nanophotonics. Here, we present a simple and efficient method for the fabrication of silicon (Si) nanoparticle substrates using continuous-wave (CW) laser annealing. The resulting silicon nanoparticles exhibit Mie resonances in the visible region, and their resonant wavelengths can be precisely controlled. Notably, laser-annealed silicon nanoparticle substrates show a 60-fold enhancement in fluorescence. This tunable and fluorescence-enhancing silicon nanoparticle platform has tremendous potential for highly sensitive fluorescence sensing and biomedical imaging applications.

High-Sensitivity Optical Sensors Empowered by Quasi-Bound States in the Continuum in a Hybrid Metal-Dielectric Metasurface

Enhancing light-matter interaction is a key requisite in the realm of optical sensors. Bound states in the continuum (BICs), possessing high quality factors (Q factors), have shown great advantages in sensing applications. Recent theories elucidate the ability of BICs with hybrid metal-dielectric architectures to achieve high Q factors and high sensitivities. However, the experimental validation of the sensing performance in such hybrid systems remains equivocal. In this study, we propose two symmetry-protected quasi-BIC modes in a metal-dielectric metasurface. Our results demonstrate that, under the normal incidence of light, the quasi-BIC mode dominated by dielectric can achieve a high Q factor of 412 and a sensing performance with a high bulk sensitivity of 492.7 nm/RIU (refractive index unit) and a figure of merit (FOM) of 266.3 RIU-1, while the quasi-BIC mode dominated by metal exhibits a stronger surface affinity in the biotin-streptavidin bioassay. These findings offer a promising approach for implementing metasurface-based sensors, representing a paradigm for high-sensitivity biosensing platforms.

Coupling-Based Multiple Bound States in the Continuum and Grating-Assisted Permittivity Retrieval in the Terahertz Metasurface

The terahertz (THz) metasurfaces that support bound states in the continuum (BICs) provide a promising platform for various applications due to their high Q-factor resonance. In this study, we experimentally demonstrate multiple BICs with different resonance symmetries in the THz metasurface based on mode coupling. The proposed metasurface is composed of 2 × 2 split ring resonators (SRRs) metamolecules. The SRRs of two different gap angles in the metamolecule lattice provide intrinsic resonance with different frequencies, and the coupling between them exhibits high transmission quasi-BIC resonance, which can be tuned by varying the gap angle. The arrangement of SRRs in the 2 × 2 metamolecule lattice determines the types of coupling that govern the resonance symmetry of quasi-BIC. More interestingly, the multiple quasi-BICs enabled by different couplings can be simultaneously achieved in a metasurface. Apart from tuning the gap angles, the permittivity in the vicinity of SRRs also changes the resonance frequency. Consequently, quasi-BIC can be artificially formed by deliberately constructing the permittivity difference of the dielectric environment on the SRRs. In view of this, we introduce the scheme of permittivity retrieval for the dispersive analyte, assisted by the fixed-permittivity gratings. In addition, we experimentally demonstrate the metasurface in combination with the microfluidic chip for the sensing of the glucose solution concentration. Our findings offer a possible strategy for the existing manufactured metasurface to achieve quasi-BIC resonance and provide a promising candidate for approaching the spectral analysis of the biochemical.

Towards rapid colorimetric detection of extracellular vesicles using optofluidics-enhanced color-changing optical metasurface

Efficient transportation and delivery of analytes to the surface of optical sensors are crucial for overcoming limitations in diffusion-limited transport and analyte sensing. In this study, we propose a novel approach that combines metasurface optics with optofluidics-enabled active transport of extracellular vesicles (EVs). By leveraging this combination, we show that we can rapidly capture EVs and detect their adsorption through a color change generated by a specially designed optical metasurface that produces structural colors. Our results demonstrate that the integration of optofluidics and metasurface optics enables spectrometer-less and label-free colorimetric read-out for EV concentrations as low as 107 EVs/ml, achieved within a short incubation time of two minutes.

Color Routing of the Emission from Magnetic and Electric Dipole Transitions of Eu3+ by Broken-Symmetry TiO2 Metasurfaces

Manipulation of magnetic dipole emission with resonant photonic nanostructures is of great interest for both fundamental research and applications. However, obtaining selective control over the emission properties of magnetic dipole transitions is challenging, as they usually occur within a manifold of spectrally close emission lines associated with different spin states of the involved electronic levels. Here we demonstrate spectrally selective directional tailoring of magnetic dipole emission using designed photonic nanostructures featuring a high quality factor. Specifically, we employ a hybrid nanoscale optical system consisting of a Eu3+ compound coupled to a designed broken-symmetry TiO2 metasurface to demonstrate directional color routing of the compound's emission through its distinct electric and magnetic-dominated electronic transition channels. Using low numerical aperture collection optics, we achieve a fluorescence signal enhancement of up to 33.13 for the magnetic-dominated dipole transition at 590 nm when it spectrally overlaps with a spectrally narrow resonance of the metasurface. This makes the, usually weak, magnetic dipole transition the most intense spectral line in our recorded fluorescence spectra. By studying the directional emission properties for the coupled system using Fourier imaging and time-resolved fluorescence measurements, we demonstrate that the high-quality-factor modes in the metasurface enable free-space light routing, where forward-directed emission is established for the magnetic-dominated dipole transition, whereas the light emitted via the electric dipole transition is mainly directed sideways. Our results underpin the importance of magnetic light-matter interactions as an additional degree of freedom in photonic and optoelectronic systems. Moreover, they facilitate the development of spectrometer-free and highly integrated nanophotonic imaging, sensing, and probing devices.

Printed Divisional Optical Biochip for Multiplex Visualizable Exosome Analysis at Point-of-Care

Rapid detection of various exosomes is of great significance in early diagnosis and postoperative monitoring of cancers. Here, a divisional optical biochip is reported for multiplex exosome analysis via combining the self-assembly of nanochains and precise surface patterning. Arising from resonance-induced near-field enhancement, the nanochains show distinct color changes after capturing target exosomes for direct visual detection. Then, a series of divisional nanochain-based biochips conjugated with several specific antibodies are fabricated through designed hydrophilic and hydrophobic patterns. Because of the significant wettability difference, one sample droplet is precisely self-splitting into several microdroplets enabling simultaneous identification of multiple target exosomes in 30 min with a sensitivity of 6 × 107 particles mL-1 , which is about two orders lower than enzyme-linked immunosorbent assay. Apart from the trace amount detection, excellent semiquantitative capability is demonstrated to distinguish clinical exosomes from glioblastoma patients and healthy people. This method is simple, versatile, and highly efficient that can be extended as a diagnostic tool for many diseases, promoting the development of liquid biopsy.

High-Q Fano resonances in all-dielectric metastructures for enhanced optical biosensing applications

Fano resonance with high Q-factor is considered to play an important role in the field of refractive index sensing. In this paper, we theoretically and experimentally investigate a refractive index sensor with high performance, realizing a new approach to excite multiple Fano resonances of high Q-factor by introducing an asymmetric parameter to generate a quasi-bound state in the continuum (BIC). Combined with the electromagnetic properties, the formation mechanism of Fano resonances in multiple different excitation modes is analyzed and the resonant modes of the three resonant peaks are analyzed as toroidal dipole (TD), magnetic quadrupole (MQ), and magnetic dipole (MD), respectively. The simulation results show that the proposed metastructure has excellent sensing properties with a Q-factor of 3668, sensitivity of 350 nm/RIU, and figure of merit (FOM) of 1000. Furthermore, the metastructure has been fabricated and investigated experimentally, and the result shows that its maximum Q-factor, sensitivity and FOM can reach 634, 233 nm/RIU and 115, respectively. The proposed metastructure is believed to further contribute to the development of biosensors, nonlinear optics, and lasers.

Quasi-bound states in the continuum in a metal nanograting metasurface for a high figure-of-merit refractive index sensor

In this work, we investigate the bound states in the continuum (BICs) in a gold nanograting metal-insulator-metal metasurface structure at oblique angles of incidence. The nanograting metasurface consists of a gold nanograting patterned on a silicon dioxide dielectric film deposited on a thick gold film supported by a substrate. With rigorous full-wave finite difference time domain simulations, two bound states in the continuum are revealed upon transverse magnetic wave angular incidence. One BIC is formed by the interference between the surface plasmon polariton mode of the gold nanograting and the FP cavity mode. Another BIC mode is formed by the interference between the metal-dielectric hybrid structure guided mode resonance mode and the FP cavity mode. While true BIC modes cannot be observed, quasi-BIC modes are investigated at angles of incidence slightly off from the corresponding true BIC angles. It is shown that quasi-BIC modes can suppress radiation loss, resulting in narrow resonance spectral linewidths and high quality-factors. The quasi-BIC mode associated with the surface plasmon polariton mode is investigated for refractive index sensing. As a result, a high sensitivity refractive index sensor with a large figure-of-merit of 364 has been obtained.

Terahertz Metamaterials for Biosensing Applications: A Review

In recent decades, THz metamaterials have emerged as a promising technology for biosensing by extracting useful information (composition, structure and dynamics) of biological samples from the interaction between the THz wave and the biological samples. Advantages of biosensing with THz metamaterials include label-free and non-invasive detection with high sensitivity. In this review, we first summarize different THz sensing principles modulated by the metamaterial for bio-analyte detection. Then, we compare various resonance modes induced in the THz range for biosensing enhancement. In addition, non-conventional materials used in the THz metamaterial to improve the biosensing performance are evaluated. We categorize and review different types of bio-analyte detection using THz metamaterials. Finally, we discuss the future perspective of THz metamaterial in biosensing.

Nanosensor Chemical Cytometry: Advances and Opportunities in Cellular Therapy and Precision Medicine

With the definition of therapeutics now encompassing transplanted or engineered cells and their molecular products, there is a growing scientific necessity for analytics to understand this new category of drugs. This Perspective highlights the recent development of new measurement science on label-free single cell analysis, nanosensor chemical cytometry (NCC), and their potential for cellular therapeutics and precision medicine. NCC is based on microfluidics integrated with fluorescent nanosensor arrays utilizing the optical lensing effect of a single cell to real-time extract molecular properties and correlate them with physical attributes of single cells. This new class of cytometry can quantify the heterogeneity of the multivariate physicochemical attributes of the cell populations in a completely label-free and nondestructive way and, thus, suggest the vein-to-vein conditions for the safe therapeutic applications. After the introduction of the NCC technology, we suggest the technological development roadmap for the maturation of the new field: from the sensor/chip design perspective to the system/software development level based on hardware automation and deep learning data analytics. The advancement of this new single cell sensing technology is anticipated to aid rich and multivariate single cell data setting and support safe and reliable cellular therapeutics. This new measurement science can lead to data-driven personalized precision medicine.

Dielectric Metasurface Enabled Compact, Single-Shot Digital Holography for Quantitative Phase Imaging

Quantitative phase imaging (QPI) enables nondestructive, real-time, label-free imaging of transparent specimens and can reveal information about their fundamental properties such as cell size and morphology, mass density, particle dynamics, and cellular fluctuations. Development of high-performance and low-cost quantitative phase imaging systems is thus required in many fields, including on-site biomedical imaging and industrial inspection. Here, we propose an ultracompact, highly stable interferometer based on a single-layer dielectric metasurface for common path off-axis digital holography and experimentally demonstrate quantitative phase imaging. The interferometric imaging system leveraging an ultrathin multifunctional metasurface captures image plane holograms in a single shot and provides quantitative phase information on the test samples for extraction of its physical properties. With the benefits of planar engineering and high integrability, the proposed metasurface-based method establishes a stable miniaturized QPI system for reliable and cost-effective point-of-care devices, live cell imaging, 3D topography, and edge detection for optical computing.

Photonic Bound States in the Continuum in Nanostructures

Bound states in the continuum (BIC) have garnered considerable attention recently for their unique capacity to confine electromagnetic waves within an open or non-Hermitian system. Utilizing a variety of light confinement mechanisms, nanostructures can achieve ultra-high quality factors and intense field localization with BIC, offering advantages such as long-living resonance modes, adaptable light control, and enhanced light-matter interactions, paving the way for innovative developments in photonics. This review outlines novel functionality and performance enhancements by synergizing optical BIC with diverse nanostructures, delivering an in-depth analysis of BIC designs in gratings, photonic crystals, waveguides, and metasurfaces. Additionally, we showcase the latest advancements of BIC in 2D material platforms and suggest potential trajectories for future research.

Cost-Effective Nanophotonic Metasurfaces with Spatially Gradient Structures for Ultrasensitive Imaging-Based Refractometric Sensing

Nanophotonic metasurfaces are widely utilized in various domains, such as biomedical, healthcare, and environmental monitoring, benefiting from their unique advantages of label-free, noninvasive, and real-time response. However, nanophotonic metasurfaces usually rely on sophisticated instruments, and expensive and time-consuming fabrication processes, which severely restricts their practical applications. Herein, a spatially gradient metasurface is integrated with an imaging-based sensing scheme, waiving the requirement of spectrometers and achieving an ultrahigh imaging-based sensitivity of 3321 pixels/refractive index unit superior to that characterized using conventional compact spectrometers. The metasurface is fabricated by nanoimprint lithography using a reusable cyclic olefin copolymer template featuring millions of unique nanostructures. Under the illumination of monochromatic light, the transmittance of different nanostructures on the metasurface differs, resulting in grayscale images with varied intensity distributions. Analyzing the intensity change of the metasurface's recorded image can obtain the covering medium's refractive index. Furthermore, through theory and experimentation, the high reliability of the proposed reusable and flexible template has been verified for nanophotonic metasurface fabrication which further reduces the fabrication cost of core sensing elements. Finally, with proper optimization of the metasurface structure and imaging system, this setup is expected to be applied to many emerging areas of point-of-care, real-time, and on-site biosensing.

Unlocking the out-of-plane dimension for photonic bound states in the continuum to achieve maximum optical chirality

The realization of lossless metasurfaces with true chirality crucially requires the fabrication of three-dimensional structures, constraining experimental feasibility and hampering practical implementations. Even though the three-dimensional assembly of metallic nanostructures has been demonstrated previously, the resulting plasmonic resonances suffer from high intrinsic and radiative losses. The concept of photonic bound states in the continuum (BICs) is instrumental for tailoring radiative losses in diverse geometries, especially when implemented using lossless dielectrics, but applications have so far been limited to planar structures. Here, we introduce a novel nanofabrication approach to unlock the height of individual resonators within all-dielectric metasurfaces as an accessible parameter for the efficient control of resonance features and nanophotonic functionalities. In particular, we realize out-of-plane symmetry breaking in quasi-BIC metasurfaces and leverage this design degree of freedom to demonstrate an optical all-dielectric quasi-BIC metasurface with maximum intrinsic chirality that responds selectively to light of a particular circular polarization depending on the structural handedness. Our experimental results not only open a new paradigm for all-dielectric BICs and chiral nanophotonics, but also promise advances in the realization of efficient generation of optical angular momentum, holographic metasurfaces, and parity-time symmetry-broken optical systems.

Dual optofluidic distributed feedback dye lasers for multiplexed biosensing applications

Integrated optofluidic devices have become subjects of high interest for rapid biosensor devices due to their unique ability to combine the fluidic processing of small volumes of microfluidics with the analysis capabilities of photonic structures. By integrating dynamically reconfigurable optofluidic lasers on-chip, complex coupling can be eliminated while further increasing the capabilities of sensors to detect an increasing number of target biomarkers. Here, we report a polydimethylsiloxane-based device with two on-chip fluidic distributed feedback (DFB) laser cavities that are integrated with an orthogonal analyte channel for multiplexed fluorescence excitation. One DFB grating is filled with 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran dissolved in dimethyl sulfoxide. The second grating is filled with rhodamine 6G dissolved in a diluted ethylene glycol solution. We present characterization of both lasers through analysis of the lasing spectra for spectral narrowing along with a power series to observe threshold behavior. We then demonstrate simultaneous detection of two different fluorescent microbeads as a proof of concept for scalable, single biomarker analysis using on-chip optofluidic lasers.

Label-free optical biosensing: going beyond the limits

Label-free optical biosensing holds great promise for a variety of applications in biomedical diagnostics, environmental and food safety, and security. It is already used as a key tool in the investigation of biomolecular binding events and reaction constants in real time and offers further potential additional functionalities and low-cost designs. However, the sensitivity of this technology does not match the routinely used but expensive and slow labelling methods. Therefore, label-free optical biosensing remains predominantly a research tool. Here we discuss how one can go beyond the limits of detection provided by standard optical biosensing platforms and achieve a sensitivity of label-free biosensing that is superior to labelling methods. To this end we review newly emerging optical implementations that overcome current sensitivity barriers by employing novel structural architectures, artificial materials (metamaterials and hetero-metastructures) and using phase of light as a sensing parameter. Furthermore, we elucidate the mechanism of plasmonic phase biosensing and review hyper-sensitive transducers, which can achieve detection limits at the single molecule level (less than 1 fg mm-2) and make it possible to detect analytes at several orders of magnitude lower concentrations than so far reported in literature. We finally discuss newly emerging layouts based on dielectric nanomaterials, bound states in continuum, and exceptional points.