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Due to their unique capabilities, hollow-core waveguides are playing an increasingly important role, especially in meeting the growing demand for integrated and low-cost photonic devices and sensors. Here, we present the antiresonant hollow-core microgap waveguide as a platform for the on-chip investigation of light-gas interaction over centimeter-long distances. The design consists of hollow-core segments separated by gaps that allow external access to the core region, while samples with lengths up to 5 cm were realized on silicon chips through 3D-nanoprinting using two-photon absorption based direct laser writing. The agreement of mathematical models, numerical simulations and experiments illustrates the importance of the antiresonance effect in that context. Our study shows the modal loss, the effect of gap size and the spectral tuning potential, with highlights including extremely broadband transmission windows (>200 nm), very high contrast resonance (>60 dB), exceptionally high structural openness factor (18%) and spectral control by nanoprinting (control over dimensions with step sizes (i.e., increments) of 60 nm). The application potential was demonstrated in the context of laser scanning absorption spectroscopy of ammonia, showing diffusion speeds comparable to bulk diffusion and a low detection limit. Due to these unique properties, application of this platform can be anticipated in a variety of spectroscopy-related fields, including bioanalytics, environmental sciences, and life sciences.

With their unique optoelectronic properties, perovskite nanocrystals are highly advantageous semiconductor materials for tailored light applications including an interaction with circularly polarized light. Although chiral perovskite nanocrystals have been obtained by the adsorption of chiral molecules, their chiroptical response is still intrinsically weak. Alternatively, perovskites have been combined with artificial chiral surfaces demonstrating enhanced chiroptical responses. However, bulk perovskite films of considerable thickness were required, mitigating the perovskite’s photoluminescence efficiency and processability. Here we developed a hybrid system of a dielectric chiral nanoantenna array that was coated with a monolayer of cubic all-inorganic lead halide perovskite nanocrystals. By tuning the thickness of the perovskite film down to one monolayer of nanocrystals, we restricted the interactions exclusively to the near-field regime. The chiral surface built of z-shaped Si nanoantennas features pronounced chiral resonances in the visible to IR region. We demonstrate that the two-photon excited photoluminescence emission of the nanocrystals can be enhanced by up to one order of magnitude in this configuration. This emission increase is controllable by the choice of the excitation wavelength and polarization with an asymmetry in emission of up to 25% upon left and right circularly polarized illumination. Altogether, our findings demonstrate a pathway to an all-optical control and modulation of perovskite light emission via strong polarization sensitive light–matter interactions in the near-field, rendering this hybrid system interesting for sensing and display technologies.

The terahertz (0.1–10 THz) range represents a fast-evolving research and industrial field. The great interest for this portion of the electromagnetic spectrum, which lies between the photonics and the electronics ranges, stems from the unique and disruptive sectors where this radiation finds applications in, such as spectroscopy, quantum electronics, sensing and wireless communications beyond 5G. Engineering the propagation of terahertz light has always proved to be an intrinsically difficult task and for a long time it has been the bottleneck hindering the full exploitation of the terahertz spectrum. Amongst the different approaches that have been proposed so far for terahertz signal manipulation, the implementation of metamaterials has proved to be the most successful one, owing to the relative ease of realisation, high efficiency and spectral versatility. In this review, we present the latest developments in terahertz modulators based on metamaterials, while highlighting a few selected key applications in sensing, wireless communications and quantum electronics, which have particularly benefitted from these developments.

We demonstrate a novel low power inductive wearable plethysmography system. This consists of ultra-sensitive 3D knitted helical coils integrated in a garment and an oscillator circuit with high quality factor. The low power oscillator is built using two cross coupled FET pairs with low capacitance drawing only 95 μA during operation and with a response time smaller than 10 μs . The sensor system is linear, with negligible hysteresis. The best compromise in sensitivity and power consumption is obtained with a 3D knitted helical coil using jersey knit with elastic yarn, a lower knitting needle size than recommended for the yarn and minimizing both the number of stitches per winding as well as the stitches containing metal. A sensitivity of 2.7 kHz per mm change in circumference with a power consumption of 6.85 mW per 30 ms measurement time is reported. This system can be used for long term breathing monitoring using a garment indistinguishable from everyday clothing.

Optical hotspots underpin a wide variety of photonic devices ranging from sensing, nonlinear generation to photocatalysis, taking advantage of the strong light-matter interaction at the vicinity of photonic nanostructures. While plasmonic nanostructures have been widely used for strongly localized electromagnetic energy on surfaces, they suffer from high loss and consequently a low quality factor. Resonance-based dielectric structures provide an alternative solution with a larger quality factor, but there is a mismatch between the maximum values of the light confinement (quality factor) and the leakage (intensity in the near-field). Here, we propose to apply the concept of topological photonics to the formation of hotspots, producing them in both topological edge states and topological corner states. The topology secures strong light localization at the surface of the nanostructures where the underlying topological invariant shows a jump, generating a field hotspot with simultaneous increment of quality factor and light intensity. Meanwhile, it leverages a good robustness to fabrication imperfection including fluctuation in shape and misalignment. After a systematic investigation and comparison of the robustness between 1D and 2D topological structures, we conclude that the hotspots from 1D topological edge states promise a fertile playground for emerging applications that require both enhanced light intensity and high spectral resolution.

The permeability of the intestinal barrier is altered in a multitude of gastrointestinal conditions such as Crohn’s and coeliac disease. However, the clinical utility of gut permeability is currently limited due to a lack of reliable diagnostic tests. To address this issue, we report a novel technique for rapid, non-invasive measurement of gut permeability based on transcutaneous (‘through-the-skin’) fluorescence spectroscopy. In this approach, participants drink an oral dose of a fluorescent dye (fluorescein) and a fibre-optic fluorescence spectrometer is attached to the finger to detect permeation of the dye from the gut into the blood stream in a non-invasive manner. To validate this technique, clinical trial measurements were performed in 11 healthy participants. First, after 6 h of fasting, participants ingested 500 mg of fluorescein dissolved in 100 ml of water and fluorescence measurements were recorded at the fingertip over the following 3 h. All participants were invited back for a repeat study, this time ingesting the same solution but with 60 g of sugar added (known to transiently increase intestinal permeability). Results from the two study datasets (without and with sugar respectively) were analysed and compared using a number of analysis procedures. This included both manual and automated calculation of a series of parameters designed for assessment of gut permeability. Calculated values were compared using Student’s T-tests, which demonstrated significant differences between the two datasets. Thus, transcutaneous fluorescence spectroscopy shows promise in non-invasively discriminating between two differing states of gut permeability, demonstrating potential for future clinical use.

Orbital angular momentum (OAM) of light represents an independent degree of freedom using orthogonal helical modes for optical and quantum multiplexing, offering great potential to transform future ultrahigh-bandwidth information systems. Practical OAM communication systems suffer from turbulence-induced phase distortions to the propagating beams, decreasing the orthogonality of OAM modes through introduced modal crosstalk. To date, optical systems used for measuring OAM orthogonality breakdown in different turbulence conditions are too bulky and slow (e.g., one OAM mode at a time) for any practical use. Here, we demonstrate the use of an ultrathin OAM mode-sorting metasurface for characterizing the OAM orthogonality breakdown under different turbulence conditions. Our approach allows the measurement of the whole OAM spectrum at the same time. This metasurface exhibits strong OAM selectivity with an average modal crosstalk below −42.4 dB for OAM modes with topological charges ranging from −15 to +15. Our results suggest that higher-order OAM modes are as robust as lower-order modes in particular turbulence environments, paving the way for future practical free-space OAM communications harnessing high-dimensional OAM multiplexing. We demonstrated that a flat optical device with a small form factor can be integrated with practical communication systems for compact, fast, and efficient generation and detection of twisted light.

Terahertz imaging is becoming a biological imaging modality in its own right, alongside the more mature infrared and X-ray techniques. Nevertheless, extraction of hyperspectral, biometric information of samples is limited by experimental challenges. Terahertz time domain spectroscopy reflection measurements demand highly precise alignment and suffer from limitations of the sample thickness. In this work, a novel hybrid Kramers-Kronig and Fabry-Pérot based algorithm has been developed to overcome these challenges. While its application is demonstrated through dielectric retrieval of glass-backed human bone slices for prospective characterisation of metastatic defects or osteoporosis, the generality of the algorithm offers itself to wider application towards biological materials.

We experimentally study the interaction between intense infrared few-cycle laser pulses and an ultrathin (∼2 µm) flat liquid sheet of isopropanol running in vacuum. We observe a rapid decline in transmission above a critical peak intensity of 50 TW/cm2 of the initially transparent liquid sheet, and the emission of a plume of material. We find both events are due to the creation of a surface plasma and are similar to processes observed in dielectric solids. After calculating the electron density for different laser peak intensities, we find an electron scattering rate of 0.3 fs-1 in liquid isopropanol to be consistent with our data. We study the dynamics of the plasma plume to find the expansion velocity of the plume front.

Controlling and manipulating individual quantum systems in solids underpins the growing interest in development of scalable quantum technologies1, 2. Recently, hexagonal boron nitride (hBN) has garnered significant attention in quantum photonic applications due to its ability to host optically stable quantum emitters3-7. However, the large band gap of hBN and the lack of efficient doping inhibits electrical triggering and limits opportunities to study electrical control of emitters. Here, we show an approach to electrically modulate quantum emitters in an hBN–graphene van der Waals heterostructure. We show that quantum emitters in hBN can be reversibly activated and modulated by applying a bias across the device. Notably, a significant number of quantum emitters are intrinsically dark, and become optically active at non-zero voltages. To explain the results, we provide a heuristic electrostatic model of this unique behaviour. Finally, employing these devices we demonstrate a nearly-coherent source with linewidths of ~ 160 MHz. Our results enhance the potential of hBN for tunable solid state quantum emitters for the growing field of quantum information science.

Structures which appear to move at or near the velocity of light contain singular points. Energy generated by the motion accumulates at these points into ever-narrowing peaks. In this paper we show that energy is generated by a curious process that conserves the number of photons, adding energy by forcing photons already present to climb a ladder of increasing frequency. We present both a classical proof based on conservation of lines of force, and a more formal QED proof demonstrating the absence of unpaired creation and annihilation operators. Exceptions to this rule are found when negative frequencies make an appearance. Finally we make a connection to laboratory-based models of black holes and Hawking radiation., 13 pages, 4 figures

The generation of structural color from wrinkled polydimethylsiloxane (PDMS) surfaces, fabricated by plasma exposure, subjected to uni- and multi-axial, and sequential strain fields is examined. The approach is based on the well-known, mechanically-induced, buckling instability of a supported bilayer, whereby the top glassy “skin” is formed by plasma oxidation. Surface periodicities 200 nm ≲ d ≲ 3 μm, encompassing the visible spectrum, are investigated in terms of the observed color, intensity spectrum, and color mixing from different diffraction orders, exhibiting good agreement with model predictions. By contrast with complex fabrication methods, color tunability and mechanochromic response are readily achieved by adjusting plasma and strain parameters, and by dynamically varying strain (ε ≲ 50%). Prescribed strain directionality, employing uniaxial, isotropic, gradient strain, and wave-sum wrinkling superposition, as well as skin thickness (and thus d) and amplitude gradients, using facile and scalable fabrication approaches, yield striking spatial color variation, homogeneity, and directionality.

A low-profile THz leaky-wave antenna (LWA) enabled by a Dielectric Image-Line (DIL) with wide beam steering capability is reported for the first time in this paper. The DIL as a low-loss dielectric waveguide synthesized directly on a substrate with high dielectric permittivity is an appropriate candidate for emerging directive compact antennas for future sub-THz wave communications. The dominant mode of DIL is disturbed by printing two periodic set of radiating elements on the top wall of DIL, including three barrier strips and two anti-reflection strips with an extensive tuning range for open stop-band (OSB) suppression. The operation bandwidth of the proposed antenna spans from 142 GHz to 225 GHz providing an impedance bandwidth of 48.8% in the fast wave region. It is observed from numerical simulations that the proposed 35-cell leaky wave antenna permits a beam steering angle of 128° (−68° to 60° (including the broadside)) in the operation frequency band with the gain varying from 14 to 19.3 dBi in the fast wave region. Detailed design guidelines along with simulation results are presented. The proposed antenna has the capability of being directly scaled to other frequency bands by considering its corresponding dispersion characteristic.

BACKGROUND: Visualization of diffuse myocardial fibrosis is challenging and mainly relies on histology. Cardiac magnetic resonance (CMR), which uses extracellular contrast agents, is a rapidly developing technique for measuring the extracellular volume (ECV). The objective of this study was to evaluate the feasibility of the synthetic myocardial ECV fraction based on 3.0 T CMR compared with the conventional ECV fraction. METHODS: This study was approved by the local animal care and ethics committee. Fifteen beagle models with diffuse myocardial fibrosis, including 12 experimental and three control subjects, were generated by injecting doxorubicin 30 mg/m(2) intravenously every three weeks for 24 weeks. Short-axis (SAX) and 4-chamber long-axis (LAX) T1 maps were acquired for both groups. The association between hematocrit (Hct) and native T1(blood) was derived from 9 non-contrast CMR T1 maps of 3 control beagles using regression analysis. Synthetic ECV was then calculated using the synthetic Hct and compared with conventional ECV at baseline and the 16(th) and 24(th) week after doxorubicin administration. The collagen volume fraction (CVF) value was measured on digital biopsy samples. Bland-Altman plots were used to analyze the agreement between conventional and synthetic ECV. Correlation analyses were performed to explore the association among conventional ECV, synthetic ECV, CVF, and left ventricular ejection fraction (LVEF). RESULTS: The regression model synthetic Hct = 816.46*R1(blood) − 0.01 (R(2)=0.617; P=0.012) was used to predict the Hct from native T1(blood) values. The conventional and synthetic ECV fractions of experimental animals at the 16(th) and 24(th) week after modeling were significantly higher than those measured at the baseline (31.4%±2.2% and 36.3%±2.1% vs. 22.9%±1.7%; 29.9%±2.4% and 36.1%±2.6% vs. 22.0%±2.4%; all with P

To undertake THz spectroscopy and imaging, and accurately design and predict the performance of quasi-optical components, knowledge of the parameters of the beam (ideally Gaussian) emitted from a THz source is paramount. Despite its proliferation, relatively little work has been done on this in the frame of broadband THz photoconductive antennas. Using primarily pinhole scanning methods, along with stepwise angular spectrum simulations, we investigate the profile and polarization characteristics of the beam emitted by a commercial silicon-lens-integrated THz photoconductive antenna and collimated by a TPX (polymethylpentene) lens. Our study flags the limitations of the different beam profiling methods and their impact on the beam Gaussianity estimation. A non-Gaussian asymmetric beam is observed, with main lobe beam waists along x and y varying from 8.4 $\,\pm\,$ 0.7 mm and 7.7 $\,\pm\,$ 0.7 mm at 0.25 THz, to 1.4 $\,\pm\,$ 0.7 mm and 1.4 $\,\pm\,$ 0.7 mm at 1 THz, respectively. Additionally, we report a maximum cross-polar component relative to the on -axis co-polar component of $-$ 11.6 dB and $-$ 21.2 dB, at 0.25 THz and 1 THz, respectively.

A method is proposed for assessing the temporal resolution of Structured Illumination Microscopy (SIM), by tracking the amplitude of different spatial frequency components over time, and comparing them to a temporally-oscillating ground-truth. This method is used to gain insight into the performance limits of SIM, along with alternative reconstruction techniques (termed 'rolling SIM') that claim to improve temporal resolution. Results show that the temporal resolution of SIM varies considerably between low and high spatial frequencies, and that, despite being used in several high profile papers and commercial microscope software, rolling SIM provides no increase in temporal resolution over conventional SIM., Comment: 8 pages, 6 figures and 3 supplemental figures

Funder: Helmholtz Young Investigator Group Frontrunner, Over the past few years, lead‐halide perovskites (LHPs), both in the form of bulk thin films and colloidal nanocrystals (NCs), have revolutionized the field of optoelectronics, emerging at the forefront of next‐generation optoelectronics. The power conversion efficiency (PCE) of halide perovskite solar cells has increased from 3.8% to over 25.7% over a short period of time and is very close to the theoretical limit (33.7%). At the same time, the external quantum efficiency (EQE) of perovskite LEDs has surpassed 23% and 20% for green and red emitters, respectively. Despite great progress in device efficiencies, the photoactive phase instability of perovskites is one of the major concerns for the long‐term stability of the devices and is limiting their transition to commercialization. In this regard, researchers have found that the phase stability of LHPs and the reproducibility of the device performance can be improved by A‐site cation alloying with two or more species, these are named mixed cation (double, triple, or quadruple) perovskites. This review provides a state‐of‐the‐art overview of different types of mixed A‐site cation bulk perovskite thin films and colloidal NCs reported in the literature, along with a discussion of their synthesis, properties, and progress in solar cells and LEDs.

For over 25 years, lithium iron phosphate (LiFePO4) has been a material of interest for Li-ion batteries as it is environmentally benign, low cost, and structurally stable. Here, we employed density functional theory calculations to examine the formation of LiFePO4 via different reaction routes, intrinsic defect processes, solution of dopants, and impact of doping on its electronic structure. The most thermodynamically favorable process to synthesize LiFePO4 is predicted to be from its constitute elements in their standard states. The Li–Fe anti-site defect is the lowest defect energy process inferring the presence of a small amount of cation intermixing. The most promising isovalent dopants on the Li, Fe, P, and O are the Na, Ca, As, and S, respectively. The substitution of Ru for Fe is energetically favorable. The doping of Ge on the P site is a possible strategy to generate both Li interstitials and holes in this material. The stability of this material upon Li incorporation (up to four atoms per 112-atom supercell) was investigated. Although incorporation is slightly unfavorable, there is a clear enhancement in the incorporation with volume expansion. The insulating nature of this material is affected by the doping and incorporation of Li, which leads to the reduction of the bandgap.

SARS-CoV-2 is one of the greatest threats to global human health. Point-of-care diagnostic tools for SARS-CoV-2 could facilitate rapid therapeutic intervention and mitigate transmission. In this work, we report CRISPR-Cas13a cascade-based viral RNA (Cas13C) assay for label-free and isothermal determination of SARS-CoV-2 and its mutations in clinical samples. Cas13a/crRNA was utilized to directly recognize the target of SARS-CoV-2 RNA, and the recognition events sequentially initiate the transcription amplification to produce light-up RNA aptamers for output fluorescence signal. The recognition of viral RNA via Cas13a-guide RNA ensures a high specificity to distinguish SARS-CoV-2 from MERS-CoV and SARS-CoV, as well as viral mutations. A post transcription amplification strategy was triggered after CRISPR-Cas13a recognition contributes to an amplification cascade that achieves high sensitivity for detecting SARS-CoV-2 RNA, with a limit of detection of 0.216 fM. In addition, the Cas13C assay could be able to discriminate single-nucleotide mutation, which was proven with N501Y in SARS-Cov-2 variant. This method was validated by a 100% agreement with RT-qPCR results from 12 clinical throat swab specimens. The Cas13C assay has the potential to be used as a routine nucleic acid test of SARS-CoV-2 virus in resource-limited regions.

While nitrogen doping has been investigated extensively in silicon, there is only limited information on its interaction with vacancies in germanium, despite most point defect processes in germanium being vacancy controlled. Thus, spin polarized density functional theory calculations are used to examine the association of nitrogen with lattice vacancies in germanium and for comparison in silicon. The results demonstrate significant charge transfer to nitrogen from the nearest neighbor Ge and strong N–Ge bond formation. The presence of vacancies results in a change in nitrogen coordination (from tetrahedral to trigonal planar) though the total charge transfer to N is maintained. A variety of nitrogen vacancy clusters are considered, all of which demonstrated strong binding energies. Substitutional nitrogen remains an effective trap for vacancies even if it has already trapped one vacancy.

Thin, flexible, and invisible solar cells will be a ubiquitous technology in the near future. Ultrathin crystalline silicon (c-Si) cells capitalize on the success of bulk silicon cells while being lightweight and mechanically flexible, but suffer from poor absorption and efficiency. Here we present a new family of surface texturing, based on correlated disordered hyperuniform patterns, capable of efficiently coupling the incident spectrum into the silicon slab optical modes. We experimentally demonstrate 66.5% solar light absorption in free-standing 1 μm c-Si layers by hyperuniform nanostructuring for the spectral range of 400 to 1050 nm. The absorption equivalent photocurrent derived from our measurements is 26.3 mA/cm2, which is far above the highest found in literature for Si of similar thickness. Considering state-of-the-art Si PV technologies, we estimate that the enhanced light trapping can result in a cell efficiency above 15%. The light absorption can potentially be increased up to 33.8 mA/cm2 by incorporating a back-reflector and improved antireflection, for which we estimate a photovoltaic efficiency above 21% for 1 μm thick Si cells.

The deployment of transducers to perform in situ inspections of industrial components can be complicated, and in many cases is still performed manually by a team of operators, which involves significant costs and can be dangerous. Robots capable of deploying probes in difficult to access locations are becoming available. Electromagnetic acoustic transducers (EMAT) are well suited to be used with robots since they are non-contact transducers that do not require a coupling medium, and can easily perform scans. However, existing acquisition systems for EMATs are generally not suitable to be directly mounted on robots. In this paper, a new wireless acquisition system for EMATs is presented. The system is standalone, it transmits the inspection data over WiFi, and is compatible with the robotics operating system (ROS). In addition, it is designed to be modular, small and lightweight so that it can be easily mounted on robots. The system design in terms of hardware and software is described in this paper. The resulting performance of the system is also reported.

Optical microrobots have a wide range of applications in biomedical research for both in vitro and in vivo studies. In most microrobotic systems, the video captured by a monocular camera is the only way for visualizing the movements of microrobots, and only planar motion, in general, can be captured by a monocular camera system. Accurate depth estimation is essential for 3D reconstruction or autofocusing of microplatforms, while the pose and depth estimation are necessary to enhance the 3D perception of the microrobotic systems to enable dexterous micromanipulation and other tasks. In this paper, we propose a data-driven method for pose and depth estimation in an optically manipulated microrobotic system. Focus measurement is used to obtain features for Gaussian Process Regression (GPR), which enables precise depth estimation. For mobile microrobots with varying poses, a novel method is developed based on a deep residual neural network with the incorporation of prior domain knowledge about the optical microrobots encoded via GPR. The method can simultaneously track microrobots with complex shapes and estimate the pose and depth values of the optical microrobots. Cross-validation has been conducted to demonstrate the submicron accuracy of the proposed method and precise pose and depth perception for microrobots. We further demonstrate the generalizability of the method by adapting it to microrobots of different shapes using transfer learning with few-shot calibration. Intuitive visualization is provided to facilitate effective human-robot interaction during micromanipulation based on pose and depth estimation results.

Surface plasmons at an interface between dielectric and metal regions can in theory be made arbitrarily compact normal to the interface by introducing extreme anisotropy in the material parameters. We propose a metamaterial structure comprising a square array of gold cylinders and tune the filling factor to achieve the material parameters we seek. Theory is compared to a simulation wherein the unit cell dimensions of the metamaterial are shown to be the limiting factor in the degree of localisation achieved.

Mammals like rats, who live in dark burrows, heavily depend on tactile perception obtained through the vibrissal system to move through gaps and to discriminate textures. The organization of a mammalian whisker follicle contains multiple sensory receptors and glands strategically organized to capture tactile sensory stimuli of different frequencies. In this paper, we used a controllable stiffness soft robotic follicle to test the hypothesis that the multimodal sensory receptors together with the controllable stiffness tissues in the whisker follicle form a physical structure to maximize tactile information. In our design, the ring sinus and ringwulst of a biological follicle are represented by a linear actuator connected to a stiffness controllable mechanism in-between two different frequency-dependent data capturing modules. In this paper, we show for the first time the effect of the interplay between the stiffness and the speed of whisking on maximizing a difference metric for texture classification.

The coupled light-matter modes supported by plasmonic metasurfaces can be combined with topological principles to yield subwavelength topological valley states of light. We give a systematic presentation of the topological valley states available for lattices of metallic nanoparticles: All possible lattices with hexagonal symmetry are considered, as well as valley states emerging on a square lattice. Several unique effects which have yet to be explored in plasmonics are identified, such as robust guiding, filtering and splitting of modes, as well as dual-band effects. We demonstrate these by means of scattering computations based on the coupled dipole method that encompass the full electromagnetic interactions between nanoparticles., Comment: 10 pages, 9 figures

Plasmonic nanoantennas have revolutionized the way we study and modulate light–matter interaction. Due to nanofabrication limitations, dimer-type nanoantennas always exhibit some degree of asymmetry, which is desirable in some cases. For instance, in sensing applications, asymmetry is sometimes induced by design in plasmonic nanoantennas to favor higher order nonradiative modes with sharp Fano line shapes. Regardless of the actual origin of the asymmetry, unintentional or intentional, an analytical frame that can deal with it in a seamless manner would be beneficial. We resort to conformal mapping for this task and we track the influence of the degree of asymmetry of the circular sectors composing gold bowtie nanoantennas on the nonradiative Purcell enhancement of a nearby nanoemitter. This manuscript reviews the contributions of conformal mapping to plasmonic nanoantennas and illustrates the advantages of the elegant analytical solution provided by conformal mapping to grasp physical insights, which can serve as a springboard for new plasmonic asymmetric nanoantenna designs.

We show that by modifying the chemical interface of silver nanoparticles (AgNPs) with halide ions, it is possible to tune the total decay rate of adsorbed excited molecules and the plasmon damping rate. Through single-molecule surface-enhanced Raman scattering and surface-enhanced fluorescence enhancement factors of crystal violet (CV) and rhodamine 6G (R6G), we show that I–-modified AgNPs (AgNPs@I) and Br–-modified AgNPs (AgNPs@Br) lead to an increase in the total decay rate of excited CV and R6G by a factor between ∼1.6–2.6, compared to Cl–-modified AgNPs (AgNPs@Cl). In addition, we found that the chemical interface damping, which characterizes the plasmon resonance decay into surface states, is stronger on AgNPs@I and AgNPs@Br when compared to AgNPs@Cl. These results point toward the formation of metal–halide surface complexes. These new interfacial states can accept electrons from both excited molecular orbitals and surface plasmon excitations, completely altering the electronic dynamics and reactivity of plasmonic interfaces.

Physiologic chemoattractant gradients are shaped by diffusion, advection, binding to an extracellular matrix, and removal by cells. Previous in vitro tools for studying these gradients and the cellular migratory response have required cells to be constrained to a 2D substrate or embedded in a gel devoid of fluid flow. Cell migration in fluid flow has been quantified in the absence of chemoattractant gradients and shown to be responsive to them, but there is a need for tools to investigate the synergistic, or antagonistic, effects of gradients and flow. We present a microfluidic chip in which we generated precisely controlled gradients of the chemokine CCL19 under advective-diffusive conditions. Using torque-actuated membranes situated between a gel region and the chip outlet, the resistance of fluid channels adjacent to the gel region could be modified, creating a controllable pressure difference across the gel at a resolution inferior to 10 Pa. Constant supply and removal of chemokine on either side of the chip facilitated the formation of stable gradients at Péclet numbers between −10 and +10 in a collagen type I hydrogel. The resulting interstitial flow was steady within 0.05 μm s−1 for at least 8 h and varied by less than 0.05 μm s−1 along the gel region. This method advances the physiologic relevance of the study of the formation and maintenance of molecular gradients and cell migration, which will improve the understanding of in vivo observations.

Nitrogen-vacancy defects are important for the material properties of silicon and for the performance of silicon-based devices. Here, we employ spin polarized density functional theory to calculate the minimum energy structures of the vacancy-nitrogen substitutional, vacancy-dinitrogen substitutionals, and divacancy-dinitrogen substitutionals. The present simulation technique enabled us to gain insight into the defect structures and charge distribution around the doped N atom and the nearest neighboring Si atoms. Using the dipole–dipole interaction method, we predict the local vibration mode frequencies of the defects and discuss the results with the available experimental data.

Significance: Diffuse reflectance spectroscopy (DRS) allows discrimination of tissue type. Its application is limited by the inability to mark the scanned tissue and the lack of real-time measurements. Aim: This study aimed to develop a real-time tracking system to enable localization of a DRS probe to aid the classification of tumor and non-tumor tissue. Approach: A green-colored marker attached to the DRS probe was detected using hue-saturation-value (HSV) segmentation. A live, augmented view of tracked optical biopsy sites was recorded in real time. Supervised classifiers were evaluated in terms of sensitivity, specificity, and overall accuracy. A developed software was used for data collection, processing, and statistical analysis. Results: The measured root mean square error (RMSE) of DRS probe tip tracking was 1.18±0.58 mm and 1.05±0.28 mm for the x and y dimensions, respectively. The diagnostic accuracy of the system to classify tumor and non-tumor tissue in real time was 94% for stomach and 96% for the esophagus. Conclusions: We have successfully developed a real-time tracking and classification system for a DRS probe. When used on stomach and esophageal tissue for tumor detection, the accuracy derived demonstrates the strength and clinical value of the technique to aid margin assessment in cancer resection surgery.

Multiphoton contributions pose a significant challenge for the realisation of heralded single-photon sources (HSPS) based on nonlinear processes. In this work, we improve the quality of single photons generated in this way by harnessing the photon-number resolving (PNR) capabilities of commercial superconducting nanowire single-photon detectors (SNSPDs). We report a $13 \pm 0.4 \%$ reduction in the intensity correlation function $g^{(2)}(0)$ even with a collection efficiency in the photon source of only $29.6\%$. Our work demonstrates the first application of the PNR capabilities of SNSPDs and shows improvement in the quality of an HSPS with widely available technology., Comment: 10 pages, 4 figures

The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence, and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, we present the results from the first megahertz repetition rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL. We illustrate the experimental capabilities that the SCS instrument offers, resulting from the operation at MHz repetition rates and the availability of the novel DSSC 2D imaging detector. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative, providing an ideal test-bed for operation at megahertz rates. Our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range., Comment: 13 pages, 5 figures. Supplementary Information as ancillary file

Disordered photonic nanostructures have attracted tremendous interest in the past three decades, not only due to the fascinating and complex physics of light transport in random media, but also for peculiar functionalities in a wealth of interesting applications. Recently, the interest in dielectric disordered systems has received new inputs by exploiting the role of long-range correlation within scatterer configurations. Hyperuniform photonic materials, that share features of photonic crystals and random systems, constitute the archetype of systems where light transport can be tailored from diffusive transport to a regime dominated by light localization due to the presence of photonic band gap. Here, advantage is taken of the combination of the hyperuniform disordered (HuD) design in slab photonics, the use of embedded quantum dots for feeding the HuD resonances, and near-field hyperspectral imaging with sub-wavelength resolution in the optical range to explore the transition from localization to diffusive transport. It is shown, theoretically and experimentally, that photonic HuD systems support resonances ranging from strongly localized modes to extended modes. It is demonstrated that Anderson-like modes with high Q/V are created, with small footprint, intrinsically reproducible and resilient to fabrication-induced disorder, paving the way for a novel photonic platform for quantum applications.

Sensing based on a passive transducer that is wirelessly linked to a nearby data collection node can offer an attractive solution for use in remote, inaccessible, or harsh environments. Here we report a pipe flow sensor based on this principle. A transducer mounted inside the pipe generates an acoustic signal that is picked up by an external microphone. The passive transducer comprises a cavity with a trapped ball that can oscillate in response to flow. Its collisions generate an acoustic signal correlated to the flow speed. The transducer is implemented on a 6 mm diameter probe and characterized as a water flow meter. The time - average microphone voltage output is calculated by an analogue circuit, without any further signal processing. With the microphone mounted on the probe, and for flow rates in the range 0.35 m/s to 6.5 m/s, correlation between the sensor voltage output and flow rate data from a commercial flow meter is demonstrated with a worst-case accuracy of 2%. This was achieved by simple averaging of the acoustic pulse train over a 5-second time interval. Consistent correlation with the microphone mounted on the pipe wall at distances up to 150 mm from the probe location is also reported. These results demonstrate the viability of remote acoustic flow sensing using passive structures and offer a simple and minimally invasive flow monitoring method.

Smoke generated during surgery affects tissue visibility and degrades image quality, affecting surgical decisions and limiting further image processing and analysis. Polarization is a fundamental property of light and polarization-resolved imaging has been studied and applied to general visibility restoration scenarios such as for smog or mist removal or in underwater environments. However, there is no related research or application for surgical smoke removal. Due to differences between surgical smoke and general haze scenarios, we propose an alternative imaging degradation model by redefining the form of the transmission parameters. The analysis of the propagation of polarized light interacting with the mixed medium of smoke and tissue is proposed to realize polarization-based smoke removal (visibility restoration). Theoretical analysis and observation of experimental data shows that the cross-polarized channel data generated by multiple scattering is less affected by smoke compared to the co-polarized channel. The polarization difference calculation for different color channels can estimate the model transmission parameters and reconstruct the image with restored visibility. Qualitative and quantitative comparison with alternative methods show that the polarization-based image smoke-removal method can effectively reduce the degradation of biomedical images caused by surgical smoke and partially restore the original degree of polarization of the samples.

We ask the question 'what happens to Bloch waves in gratings synthetically moving at near the speed of light?'. First we define a constant refractive index (CRI) model in which Bloch waves remain well defined as they break the light barrier, then show their dispersion rotating through 360 degrees from negative to positive and back again. Next we introduce the effective medium approximation (EMA) then refine it into a 4-wave model which proves to be highly accurate. Finally using the Bloch waves to expand a pulse of light we demonstrate sudden inflation of pulse amplitude combined with reversal of propagation direction as a luminal grating is turned on., Comment: 11 pages, 5 figures

Analytical technologies that can improve disease diagnosis are highly sought after. Current screening/diagnostic tests for several diseases are limited by their moderate diagnostic performance, invasiveness, costly and laborious methodologies or the need for multiple tests before a definitive diagnosis. Spectroscopic techniques, including infrared (IR) and Raman, have attracted great interest in the medical field, with applications expanding from early disease detection to monitoring and real-time diagnosis.\ud This review highlights applications of IR and Raman spectroscopy, with a focus on cancer and infectious diseases since 2015, and underscores the diverse sample types that can be analyzed, such as biofluids, cells and tissues. Studies involving more than 25 participants per group (disease and control group; if no control group >25 in disease group) were considered eligible, to retain the clinical focus of the paper. Following literature searches, we identified 94 spectroscopic studies on different cancers and 30 studies on infectious diseases. The review suggests that such technologies have the potential to develop into an objective, inexpensive, point-of-care test or facilitate disease diagnosis and monitoring. Up-to-date considerations for the implementation of spectroscopic techniques into a clinical setting, health economics and successful applications of vibrational spectroscopic tests in the clinical arena are also discussed.

Scattering-type scanning near-field optical microscopy (s-SNOM) is one of the predominant techniques for the nanoscale characterization of optical properties. The optical response of nanoantennas in s-SNOM is highly sensitive to their environment, including influences of the probing tip or neighboring resonators. Dielectric tips are commonly employed to minimize tip-related perturbations, although they provide a comparatively weak scattering signal. Here we show that when using metallic tips, it is possible to select between distinct weak and strong tip–antenna coupling regimes by careful tailoring of the illumination conditions and resonator orientation. This enables the use of highly scattering metallic instead of dielectric tips for mapping plasmonic modes with comparatively higher signal strengths. This is a particular advantage for the retrieval of near-field spectra, which simultaneously require high near-field signals and unperturbed field patterns. We leverage our approach to analyze the collective effects of nanoantenna arrays, phenomena that are well understood in the optical far-field but have not been extensively studied in the near-field. Probing the dependence of the optical response on the array field size, we identify three regimes: the single rod regime, the intermediate regime, and the array-like regime. We show that these array effects give rise to characteristic spectral features originating from a complex interplay of radiative coupling and plasmon hybridization. These results provide evidence that long-range interactions of antennas also influence the local optical response that is probed in s-SNOM and demonstrate how collective resonances emerge from single building blocks, providing guidelines for optimized array designs for near- and far-field applications.

We study the polarisation properties of random N × N scattering matrices distributed according to the circular orthogonal ensemble. We interpret 2 × 2 sub-blocks of the scattering matrix as Jones matrices and study their statistical properties. Using the polar decomposition, we derive probability density functions for retardance and diattenuation from scattering matrices of arbitrary size and in the limit N → ∞ .