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The solubilization of lipid membranes by Tween-20 is crucial for a number of biotechnological applications, but the mechanistic details remain elusive. Evidence from ensemble assays supports a solubilization model that encompasses surfactant association with the membrane and the release of mixed micelles to solution, but whether this process also involves intermediate transitions between regimes is unanswered. In search of mechanistic origins, increasing focus is placed on identifying Tween-20 interactions with controllable membrane mimetics. Here, we employed ultrasensitive biosensing approaches, including single-vesicle spectroscopy based on fluorescence and energy transfer from membrane-encapsulated molecules, to interrogate interactions between Tween-20 and submicrometer-sized vesicles below the optical diffraction limit. We discovered that Tween-20, even at concentrations below the critical micellar concentration, triggers stepwise and phase-dependent structural remodeling events, including permeabilization and swelling, in both freely diffusing and surface-tethered vesicles, highlighting the substantial impact the surfactant has on vesicle conformation and stability prior to lysis.

Semiconducting transition metal dichalcogenides (TMDs) have gained significant attention as a gain medium for nanolasers, owing to their unique ability to be easily placed and stacked on virtually any substrate. However, the atomically thin nature of the active material in existing TMD nanolasers presents a challenge, as their limited output power makes it difficult to distinguish between true laser operation and other "laser-like" phenomena. Here, we present comprehensive evidence of lasing from a CVD-grown tungsten disulphide (WS2) monolayer. The monolayer is placed on a dual-resonance dielectric metasurface with a rectangular lattice designed to enhance both absorption and emission; resulting in an ultralow threshold operation (threshold 2). We provide a thorough study of the laser performance at room temperature, paying special attention to directionality, output power, and spatial coherence. Notably, our lasers demonstrated a coherence length of over 30 μm, which is several times greater than what has been reported for 2D material lasers so far. Our realisation of a single-mode laser from a wafer-scale CVD-grown monolayer presents exciting opportunities for integration and the development of novel applications.

Transition metal dichalcogenides have emerged as promising materials for nanophotonic resonators because of their large refractive index, low absorption within a large portion of the visible spectrum, and compatibility with a wide range of substrates. Herein, we use these properties to fabricate WS2 double-pillar nanoantennas in a variety of geometries enabled by the anisotropy in the crystal structure. Using dark-field spectroscopy, we reveal multiple Mie resonances, to which we couple WSe2 monolayer photoluminescence and achieve Purcell enhancement and an increased fluorescence by factors up to 240 for dimer gaps of 150 nm. We introduce postfabrication atomic force microscope repositioning and rotation of dimer nanoantennas, achieving gaps as small as 10 ± 5 nm, which enables a host of potential applications, including strong Purcell enhancement of single-photon emitters and optical trapping, which we study in simulations. Our findings highlight the advantages of using transition metal dichalcogenides for nanophotonics by exploring applications enabled by their properties.

Vibrational spectroscopy is an important tool in chemical and biological analysis. A key issue when applying vibrational spectroscopy to dilute liquid samples is the inherently low sensitivity caused by short interaction lengths and small extinction coefficients, combined with low target molecule concentrations. Here, we introduce a novel type of surface-enhanced infrared absorption spectroscopy based on the resonance of a dielectric metasurface. We demonstrate that the method is suitable for probing vibrational bands of dilute analytes with a range of spectral linewidths. We observe that the absorption signal is enhanced by 1-2 orders of magnitude and show that this enhancement leads to a lower limit of detection compared to attenuated total reflection (ATR). Overall, the technique provides an important addition to the spectroscopist's toolkit especially for probing dilute samples.

Resonant photonic sensors are enjoying much attention based on the worldwide drive toward personalized healthcare diagnostics and the need to better monitor the environment. Recent developments exploiting novel concepts such as metasurfaces, bound states in the continuum, and topological sensing have added to the interest in this topic. The drive toward increasingly higher quality (Q)-factors, combined with the requirement for low costs, makes it critical to understand the impact of realistic limitations such as losses on photonic sensors. Traditionally, it is assumed that the reduction in the Q-factor sufficiently accounts for the presence of loss. Here, we highlight that this assumption is overly simplistic, and we show that losses have a stronger impact on the resonance amplitude than on the Q-factor. We note that the effect of the resonance amplitude has been largely ignored in the literature, and there is no physical model clearly describing the relationship between the limit of detection (LOD), Q-factor, and resonance amplitude. We have, therefore, developed a novel, ab initio analytical model, where we derive the complete figure of merit for resonant photonic sensors and determine their LOD. In addition to highlighting the importance of the optical losses and the resonance amplitude, we show that, counter-intuitively, optimization of the LOD is not achieved by maximization of the Q-factor but by counterbalancing the Q-factor and amplitude. We validate the model experimentally, put it into context, and show that it is essential for applying novel sensing concepts in realistic scenarios.

Photonic biosensors are a major topic of research that continues to make exciting advances. Technology has now improved sufficiently for photonics to enter the realm of microbiology and to allow for the detection of individual bacteria. Here, we discuss the different nanophotonic modalities used in this context and highlight the opportunities they offer for studying bacteria. We critically review examples from the recent literature, starting with an overview of photonic devices for the detection of bacteria, followed by a specific analysis of photonic antimicrobial susceptibility tests. We show that the intrinsic advantage of matching the optical probed volume to that of a single, or a few, bacterial cell, affords improved sensitivity while providing additional insight into single-cell properties. We illustrate our argument by comparing traditional culture-based methods, which we term macroscopic, to microscopic free-space optics and nanoscopic guided-wave optics techniques. Particular attention is devoted to this last class by discussing structures such as photonic crystal cavities, plasmonic nanostructures and interferometric configurations. These structures and associated measurement modalities are assessed in terms of limit of detection, response time and ease of implementation. Existing challenges and issues yet to be addressed will be examined and critically discussed.

Resonant photonic sensors are enjoying much attention based on the worldwide drive toward personalized healthcare diagnostics and the need to better monitor the environment. Recent developments exploiting novel concepts such as metasurfaces, bound states in the continuum, and topological sensing have added to the interest in this topic. The drive toward increasingly higher quality (

We developed an analytical model to estimate the impacts of optical losses on the performance of photonic resonant sensors. We find that the resonance amplitude plays a significant, albeit underestimated, role on the performance.

We introduce a form-birefringent single layer metalens encoding two phase profiles on the same surface to correct spherical and off-axis aberrations allowing the switching from high resolution to wide field of view operation.

Near-field scanning optical microscopy is a powerful technique for imaging below the diffraction limit, which has been extensively used in biomedical imaging and nanophotonics. However, when the electromagnetic fields under measurement are strongly confined, they can be heavily perturbed by the presence of the near-field probe itself. Here, taking inspiration from scattering-cancellation invisibility cloaks, Huygens–Kerker scatterers, and cloaked sensors, we design and fabricate a cloaked near-field probe. We show that, by suitably nanostructuring the probe, its electric and magnetic polarizabilities can be controlled and balanced. As a result, probe-induced perturbations can be largely suppressed, effectively cloaking the near-field probe without preventing its ability to measure. We experimentally demonstrate the cloaking effect by comparing the interaction of conventional and nanostructured probes with a representative nanophotonic structure, namely, a 1D photonic-crystal cavity. Our results show that, by engineering the structure of the probe, one can systematically control its back action on the resonant fields of the sample and decrease the perturbation by > 70 % with most of our modified probes, and by up to 1 order of magnitude for the best probe, at probe-sample distances of 100 nm. Our work paves the way for non-invasive near-field optical microscopy of classical and quantum nanosystems.

The interest in high quality factor (high-Q) resonances in metasurfaces has been rekindled with the rise of the bound states in the continuum (BIC) paradigm, which describes resonances with apparently limitlessly high quality-factors (Q-factors). The application of BICs in realistic systems requires the consideration of the angular tolerance of resonances, however, which is an issue that has not yet been addressed. Here, we develop an ab-initio model, based on temporal coupled mode theory, to describe the angular tolerance of distributed resonances in metasurfaces that support both BICs and guided mode resonances (GMRs). We then discuss the idea of a metasurface with a perturbed unit cell, similar to a supercell, as an alternative approach for achieving high-Q resonances and we use the model to compare the two. We find that, while sharing the high-Q advantage of BIC resonances, perturbed structures feature higher angular tolerance due to band planarization. This observation suggests that such structures offer a route toward high-Q resonances that are more suitable for applications.

The authors G.B., F.T., and C.C acknowledge financial support by the POR of Apulia region, IT (FESR FSR 2014−2020, Action 10.4, “Research for Innovation” (REFIN) Initiative). The authors D.C., G.P., K.D., and T.F.K. acknowledge financial support by the EPSRC of the UK (Grant EP/P030017/1). Optical trapping has revolutionized our understanding of biology by manipulating cells and single molecules using optical forces. Moving to the near-field creates intense field gradients to trap very smaller particles, such as DNA fragments, viruses, and vesicles. The next frontier for such optical nanotweezers in biomedical applications is to trap multiple particles and to study their heterogeneity. To this end, we have studied dielectric metasurfaces that allow the parallel trapping of multiple particles. We have explored the requirements for such metasurfaces and introduce a structure that allows the trapping of a large number of nanoscale particles (>1000) with a very low total power P < 26 mW. We experimentally demonstrate the near-field enhancement provided by the metasurface and simulate its trapping performance. We have optimized the metasurface for the trapping of 100 nm diameter particles, which will open up opportunities for new biological studies on viruses and extracellular vesicles, such as studying heterogeneity, or to massively parallelize analyses for drug discovery. Publisher PDF

Dielectric metasurfaces support resonances that are widely explored both for far-field wavefront shaping and for near-field sensing and imaging. Their design explores the interplay between localised and extended resonances, with a typical trade-off between Q-factor and light localisation; high Q-factors are desirable for refractive index sensing while localisation is desirable for imaging resolution. Here, we show that a dielectric metasurface consisting of a nanohole array in amorphous silicon provides a favourable trade-off between these requirements. We have designed and realised the metasurface to support two optical modes both with sharp Fano resonances that exhibit relatively high Q-factors and strong spatial confinement, thereby concurrently optimizing the device for both imaging and biochemical sensing. For the sensing application, we demonstrate a limit of detection (LOD) as low as 1 pg/ml for Immunoglobulin G (IgG); for resonant imaging, we demonstrate a spatial resolution below 1 µm and clearly resolve individual E. coli bacteria. The combined low LOD and high spatial resolution opens new opportunities for extending cellular studies into the realm of microbiology, e.g. for studying antimicrobial susceptibility., Dielectric metasurfaces have different Q-factor and light localisation requirements for sensing and imaging. Here, the authors present a dielectric metasurface, supporting two optical modes with sharp Fano resonances for high Q-factors and strong spatial confinement, allowing both sensing and imaging.

The correction of multiple aberrations in an optical system requires different optical elements, which increases its cost and complexity. Metasurfaces hold great promise to providing new functionality for miniaturized and low-cost optical systems. A key advantage over their bulk counterparts is the metasurface's ability to respond to the polarization of light, which adds a new degree of freedom to the optical design. Here, we show that polarization control enables a form-birefringent metalens to correct for both spherical and off-axis aberrations using a single element only, which is not possible with bulk optics. The metalens encodes two phase profiles onto the same surface, thus allowing switching from high resolution to wide field of view operation. Such ability to obtain both high resolution and wide field of view in a single layer is an important step towards integration of miniaturized optical systems, which may find many applications, e.g., in microscopy and endoscopy.

The work was financially supported by the Engineering and Physical Sciences Research Council of the UK through grants EP/P02324X/1 (“MAPS”) and EP/ P030017/1 (“Resonant Photonics”). T.F.K. also acknowledges support through a Royal Society Wolfson fellowship. Optical biosensors have experienced a rapid growth over the past decade because of their high sensitivity and the fact that they are label-free. Many optical biosensors rely on tracking the change in a resonance signal or an interference pattern caused by the change in refractive index that occurs upon binding to a target biomarker. The most commonly used method for tracking such a signal is based on fitting the data with an appropriate mathematical function, such as a harmonic function or a Fano, Gaussian, or Lorentz function. However, these functions have limited fitting efficiency because of the deformation of data from noise. Here, we introduce an extended Kalman filter projection (EKFP) method to address the problem of resonance tracking and demonstrate that it improves the tolerance to noise, reduces the 3σ noise value, and lowers the limit of detection (LOD). We utilize the method to process the data of experiments for detecting the binding of C-reactive protein in a urine matrix with a chirped guided mode resonance sensor and are able to improve the LOD from 10 to 1 pg/mL. Our method reduces the 3σ noise value of this measurement compared to a simple Fano fit from 1.303 to 0.015 pixels. These results demonstrate the significant advantage of the EKFP method to resolving noisy data of optical biosensors. Publisher PDF

Metalenses are nanostructured surfaces that mimic the functionality of optical elements. Many exciting demonstrations have already been made, for example, focusing into diffraction-limited spots or achromatic operation over a wide wavelength range. The key functionality that is yet missing, however, and that is most important for applications such as smartphones or virtual reality, is the ability to perform the imaging function with a single element over a wide field of view. Here, by relaxing the constraint on diffraction-limited resolution, we demonstrate the ability of single-layer metalenses to perform wide field of view (WFOV) imaging while maintaining high resolution suitable for most applications. We also discuss the WFOV physical properties and, in particular, we show that such a WFOV metalens mimics a spherical lens in the limit of infinite radius and infinite refractive index. Finally, we use Fourier analysis to explain the dependence of the FOV on the numerical aperture.

Research toward photonic biosensors for point-of-care applications and personalized medicine is driven by the need for high-sensitivity, low-cost, and reliable technology. Among the most sensitive modalities, interferometry offers particularly high performance, but typically lacks the required operational simplicity and robustness. Here, we introduce a common-path interferometric sensor based on guided-mode resonances to combine high performance with inherent stability. The sensor exploits the simultaneous excitation of two orthogonally polarized modes, and detects the relative phase change caused by biomolecular binding on the sensor surface. The wide dynamic range of the sensor, which is essential for fabrication and angle tolerance, as well as versatility, is controlled by integrating multiple, tuned structures in the field of view. This approach circumvents the trade-off between sensitivity and dynamic range, typical of other phase-sensitive modalities, without increasing complexity. Our sensor enables the challenging label-free detection of procalcitonin, a small protein (13 kDa) and biomarker for infection, at the clinically relevant concentration of 1 pg mL−1, with a signal-to-noise ratio of 35. This result indicates the utility for an exemplary application in antibiotic guidance, and opens possibilities for detecting further clinically or environmentally relevant small molecules with an intrinsically simple and robust sensing modality., Biosensors: Nanostructures team up to turn down the noise Portable devices that spot biomarkers in a patient’s bloodstream stand to benefit from a photonic microchip that can achieve ultra-low detection limits. Isabel Barth from the University of York in the United Kingdom and colleagues designed a biosensor based on guided-mode resonances, a phenomenon that uses laser light to switch on strong resonant signals inside nanoscale gratings. The researchers improved this technology by simultaneously exciting two resonant modes with a low-cost laser diode. This approach enables sensing of protein-antibody binding induced refractive index changes using relative phase differences between the two modes, minimizing the impact of noise due to outside sources including mechanical vibrations. The team demonstrated label-free detection of procalcitonin, a key biomarker for bacterial infections, at picogram/milliliter scales, which could, for example, be relevant in the context of bacterial co-infections of COVID-19 patients.

Extracellular vesicles (EVs) are very promising biomarkers for the diagnosis of various diseases, including cardiovascular, infectious and neurodegenerative disorders. Of particular relevance is their importance in cancer liquid biopsy, where they play a key role in the early detection and monitoring of the tumour. A number of technologies have recently been developed to improve the performance of current EV detection methods, but a technique that can provide high resolution, high accuracy and a multiplexing capability for the detection of several biomarkers in parallel remains a challenge in this field. Here, we demonstrate the detection of large unilamellar vesicles, which are excellent models of EVs, down to a concentration 3 EV ml−1 (Q-factor of our metasurface. We also verify the selectivity of the approach by detecting vesicles that have been surface-functionalised with a CD9 protein. The ease of integration of our method into a point-of-care instrument offers a path towards personalised cancer medicine.

According to the World Health Organization forecasts, AntiMicrobial Resistance (AMR) is expected to become one of the leading causes of death worldwide in the following decades. The rising danger of AMR is caused by the overuse of antibiotics, which are becoming ineffective against many pathogens, particularly in the presence of bacterial biofilms. In this context, non-destructive label-free techniques for the real-time study of the biofilm generation and maturation, together with the analysis of the efficiency of antibiotics, are in high demand. Here, we propose the design of a novel optoelectronic device based on a dual array of interdigitated micro- and nanoelectrodes in parallel, aiming at monitoring the bacterial biofilm evolution by using optical and electrical measurements. The optical response given by the nanostructure, based on the Guided Mode Resonance effect with a Q-factor of about 400 and normalized resonance amplitude of about 0.8, allows high spatial resolution for the analysis of the interaction between planktonic bacteria distributed in small colonies and their role in the biofilm generation, calculating a resonance wavelength shift variation of 0.9 nm in the presence of bacteria on the surface, while the electrical response with both micro- and nanoelectrodes is necessary for the study of the metabolic state of the bacteria to reveal the efficacy of antibiotics for the destruction of the biofilm, measuring a current change of 330 nA when a 15 µm thick biofilm is destroyed with respect to the absence of biofilm.

Subwavelength periodic structures have been intensively investigated for sensing applications due to the exquisit photon control they offer [1]. In particular, plasmonic nanoapertures, or "nanohole arrays" have been widely studied due to their high sensitivity to refractive index changes, e.g. allowing the detection of human proteins down to 10 pg/mL [2]. However, the optical transmission through a plasmonic nanohole array is limited by metal losses, resulting in a typical extinction ratio (ER) < 0.1, together with low Q-factors that make further improvements difficult.

A multi-analyte biosensing platform for the selective label-free detection of protein biomarkers has been designed through a three-dimensional model based on the finite element method. The sensing element of the platform is a planar plasmonic nanocavity, consisting of a one-dimensional periodic structure (Bragg grating) in gold, with a defect, placed on the buried oxide of a silicon-on-insulator substrate. The footprint of this sensing element, which has a good chemical stability, is only 1.57 μm2. The sensor has a detection limit of 128 pg/mm2 and a surface sensitivity of 1.8 nm/nm.

The study critically reviews the silicon-based microphotonic biosensors for biomedical applications. In particular, integrated optical devices and their advantages, in terms of high performance and compactness, and also high reliability and long life term, are reported. These features, together with the complementary metal oxide semiconductor compatibility of the silicon technology, since the last few years, have been allowing to realise high-efficiency biosensing platforms with on-chip integration of several biosensors for a multi-analyte detection. Many lab-on-chip systems integrated into portable medical instruments have been proposed in the literature and some of them already commercialised in the worldwide market, so attaining extraordinary improvements in the early detection and monitoring of several diseases. Therefore, fast and accurate self-tests achievable with silicon photonic biosensors are remarkably opening new possibilities and applications in the healthcare industry.