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Singlet‐to‐triplet intersystem crossing (ISC) in organic molecules is intimately connected with their geometries: by modifying the molecular shape, symmetry selection rules pertaining to spin‐orbit coupling can be partially relieved, leading to extra matrix elements for increased ISC. As an analog to this molecular design concept, the study finds that the lattice symmetry of supramolecular polymers also defines their triplet formation efficiencies. A supramolecular polymer self‐assembled from weakly interacting molecules is considered. Its 2D oblique unit cell effectively renders it as a coplanar array of 1D molecular columns weakly bound to each other. Using momentum‐resolved photoluminescence imaging in combination with Monte Carlo simulations, the study found that photogenerated charge carriers in the supramolecular polymer predominantly recombine as spin‐uncorrelated carrier pairs through inter‐column charge transfer states. This lattice‐defined recombination pathway leads to a substantial triplet formation efficiency (≈60%) in the supramolecular polymer. These findings suggest that lattice symmetry of micro‐/macroscopic structures relying on intermolecular interactions can be strategized for controlled triplet formation. [ABSTRACT FROM AUTHOR]

We study numerically and experimentally the second-harmonic generation (SHG) from rectangular metagratings of V-shaped gold nanoantennas. We show that by carefully engineering the array pitch to steer the diffraction orders toward the single antenna emission, the extracted signal is maximized. This enhancement is attributed to the angular overlap between the radiation pattern and array factor and is comparable to the improvement yielded by the coupling of surface lattice resonances (SLRs) with local modes. Moreover, we demonstrate a simple technique to experimentally reconstruct the emission diagram of an antenna from measurements of the collective grating response as a function of the excitation angle. Excellent agreement is obtained with simulations when the sample is immersed either in air or in water, which is crucial in view of future sensing application. Thanks to the high signal-to-noise ratio and low dependence on the statistical particle dispersity, this method constitutes an effective alternative to back-focal plane imaging when very weak signals such as SHG are involved. [ABSTRACT FROM AUTHOR]

Manipulating the optical landscape of single quantum dots (QDs) is essential to increase the emitted photon output, enhancing their performance as chemical sensors and single-photon sources. Micro-optical structures are typically used for this task, with the drawback of a large size compared to the embedded single emitters. Nanophotonic architectures hold the promise to modify dramatically the emission properties of QDs, boosting light-matter interactions at the nanoscale, in ultracompact devices. Here, we investigate the interplay between gallium arsenide (GaAs) single QDs and aluminum gallium arsenide (AlGaAs) nanostructures, capitalizing on the Kerker condition for precise control of the QD emission directivity. An extensive analysis of the photoluminescence spectra of several QDs embedded in nanodisks revealed a pronounced directivity enhancement due to the Kerker effect, confirmed by theoretical simulations, resulting in a 14-fold increase of emitted intensity. Angle-resolved spectroscopy experiments also proved that the integration of GaAs QDs within nanostructures determines a precise angled emission, offering a distinctive avenue for manipulating the spatial characteristics of emitted light by exploiting Mie resonances. This work contributes to the optimization of QD integration in nanostructures and suggests potential improvements for applications in optical communications.

Metasurfaces have recently gained extensive interest because of their extraordinary optical behavior as artificial material interfaces with ultrahigh compactness. In this framework, dielectric platforms have newly become very promising for nonlinear nanophotonics, providing opportunities, especially for ultrafast optical switching, and high harmonic generation, opening the research field of nonlinear metaoptics. Up to now, nonlinear metaoptics have been mostly explored using single metasurfaces. However, in a long-term vision, the stacking of optical metasurfaces, very challenging in terms of fabrication, is one key goal of this research field. Here, we demonstrate a three-layer metasurface in the AlGaAs-on-insulator platform, which improves the second harmonic generation efficiency by more than one order of magnitude with respect to its one-layer counterpart. Our achievement paves the way toward phase-shaping multilayer and multifunctional all-dielectric metasurfaces.

Near-infrared excited rare-earth (RE)-doped up-conversion (UC)-luminescent materials have attracted enormous attention because of their unique emission properties, such as narrow emission bands, long luminescence lifetimes, and multiple colors. However, current development of RE-doped luminescent material is hindered by weak and narrowband absorption problems and low photon-conversion quantum efficiencies. In addition to conventional approaches to enhance fluorescence intensity, controlling emission directivity to improve detection efficiency has become a promising approach to obtain higher luminescence brightnesses. In this paper, a self-suspended RE-doped UC luminescent waveguide is designed to realize directional emissions. Benefitting from the special morphology of the crown-like NaYF4:Yb3+/Er3+ microparticle, the points contact between the waveguide and substrate can be obtained to decrease energy loss. An attractive UC luminescent pattern accompanied by powerful and controllable directional emissions is observed, and the spatial emission angle and intensity distribution are explored and analyzed in detail by introducing Fourier imaging detection and simulation. This work provides a new method for achieving controllable directional fluorescence emissions and obtaining improved detection efficiency by narrowing emission directivity, which has potential applications in 3-dimensional displays and micro-optoelectronic devices, especially when fabricating self-fluorescence micron lasers.

Collective Mie resonances in silicon (Si) nanoparticle arrays (NPAs) feature low absorption losses and strong field enhancement extending to a large area. They provide a high-efficient scheme to manipulate the emission properties of monolayer semiconductors. However, the poor quality factor of the current reported Si NPA limits the performance of light-emitting devices. It is mainly due to the constituent materials of nanoparticles being amorphous or polycrystalline Si, which have higher absorption coefficients in comparison with monocrystalline silicon (c-Si) among the visible band. This invited paper demonstrates a versatile technique to integrate the atomic layers onto the c-Si NPA. We show that our method can fully preserve the monolayer sample. We further investigate the directional emission tailored by the NPA with different diameters by combining back-focal-plane imaging and reciprocity simulations. The flexible tune of the geometry parameters of NPAs can offer many possibilities to control and manipulate the emission from monolayer semiconductors by engineering their photonic environments. [ABSTRACT FROM AUTHOR]

Abstract Disordered optical media are an emerging class of materials that can strongly scatter light. These materials are useful to investigate light transport phenomena and for applications in imaging, sensing and energy storage. While coherent light can be generated using such materials, its directional emission is typically hampered by their strong scattering nature. Here, the authors directly image Rayleigh scattering, photoluminescence and weakly localized Raman light from a random network of silicon nanowires via real‐space microscopy and Fourier imaging. Direct imaging enables us to gain insight on the light transport mechanisms in the random material, to visualize its weak localization length and to demonstrate out‐of‐plane beaming of the scattered coherent Raman light. The direct visualization of coherent light beaming in such random networks of silicon nanowires offers novel opportunities for fundamental studies of light propagation in disordered media. It also opens venues for the development of next generation optical devices based on disordered structures, such as sensors, light sources, and optical switches.

Disordered optical media are an emerging class of materials that can strongly scatter light. These materials are useful to investigate light transport phenomena and for applications in imaging, sensing and energy storage. While coherent light can be generated using such materials, its directional emission is typically hampered by their strong scattering nature. Here, the authors directly image Rayleigh scattering, photoluminescence and weakly localized Raman light from a random network of silicon nanowires via real‐space microscopy and Fourier imaging. Direct imaging enables us to gain insight on the light transport mechanisms in the random material, to visualize its weak localization length and to demonstrate out‐of‐plane beaming of the scattered coherent Raman light. The direct visualization of coherent light beaming in such random networks of silicon nanowires offers novel opportunities for fundamental studies of light propagation in disordered media. It also opens venues for the development of next generation optical devices based on disordered structures, such as sensors, light sources, and optical switches. [ABSTRACT FROM AUTHOR]

In this paper, we consider the use of total variation (TV) minimization for compressive imaging, that is, image reconstruction from subsampled measurements. Focusing on two important imaging modalities--namely, Fourier imaging and structured binary imaging via the Walsh--Hadamard transform--we derive uniform recovery guarantees asserting stable and robust recovery for arbitrary random sampling strategies. Using this, we then derive a class of sampling strategies which are theoretically near-optimal for recovery of approximately gradient-sparse images. For Fourier sampling, we show recovery of such an image from m ≳ s · log²(s) · log4(N) measurements, in d ≥ 1 dimensions. When d = 2, this improves the current state-of-the-art result by a factor of log(s) · log(N). It also extends it to arbitrary dimensions d ≥ 1. For Walsh sampling, we prove that m ≳ds · log²(s) · log²(N/s) · log³(N) measurements suffice in d ≥ 2 dimensions. To the best of our knowledge, this is the first recovery guarantee for structured binary sampling with TV minimization. [ABSTRACT FROM AUTHOR]

Fourier single-pixel imaging (FSI) produces images through the acquisition of Fourier domain information using a single-pixel detector and a sequence of light modulation patterns. Conventional sampling approach in FSI tends to perform poorly when it comes to capturing the intricate details present in high-frequency components of the image. The variable density sampling method follows a predefined mechanism where the power of image information decreases when frequency increases. To enhance the sampling efficiency, we propose a self-adaptive sampling method to dynamically determine the order of the illumination pattern based on the scene's spectrum distribution through the probability estimation of the low-frequency samples. The image is subsequently reconstructed through compressed sensing technique. Our results indicate improved image quality even at low sampling ratios. Unlike existing adaptive approaches, the proposed method does not require dataset training or redundant sampling. Instead, it exhibits remarkable versatility by adapting to various types of images. [ABSTRACT FROM AUTHOR]

Ultrafast imaging based on plane-wave (PW) insonification is an active area of research due to its capability of reaching high frame rates. Among PW imaging methods, Fourier-based approaches have demonstrated to be competitive compared with traditional delay and sum methods. Motivated by the success of compressed sensing techniques in other Fourier imaging modalities, like magnetic resonance imaging, we propose a new sparse regularization framework to reconstruct highquality ultrasound (US) images. The framework takes advantage of both the ability to formulate the imaging inverse problem in the Fourier domain and the sparsity of US images in a sparsifying domain. We show, by means of simulations, in vitro and in vivo data, that the proposed framework significantly reduces image artifacts, i.e., measurement noise and sidelobes, compared with classical methods, leading to an increase of the image quality. [ABSTRACT FROM AUTHOR]

An active terahertz wave imaging method is investigated for improved system simplicity, inexpensive implementation, and distance approximation. The proposed technique is composed of a single-pixel setup that allows acquiring the two-dimensional (2-D) Fourier transform intensity map of the imaged object and additional depth data using time of flight method. A raster scan is performed to achieve the object's 2-D Fourier transform intensity and depth information. Iterative phase retrieval methods are employed to accomplish good image reconstruction using only the measured object's Fourier transform magnitude. The proposed method uses a glow discharge detector (GDD) as its single millimeter wave pixel and offers a simple noncalibrating scheme for 2-D imaging and distance approximation. This work shows experimental results of using the GDD as a distance approximation detector and specifies the advantages, disadvantages, and constraints when using such a sensor. Basic aperture imaging (transmission imaging) experimental results are also shown, and complex aperture imaging simulations and their corresponding reconstructions are presented. Finally, both 2-D imaging and acquired depth data are fused into a single three-dimensional reconstructed image to reveal the potential of the proposed method. [ABSTRACT FROM AUTHOR]

Metal nanowires supporting propagating surface plasmons (SPs) can be used as nanowaveguides and nanoantennas for light manipulation beyond the diffraction limit. Here the control of the propagation and radiation of SPs on silver nanowires is investigated. By covering an Al2O3 layer onto a silver nanowire to change the local dielectric environment, the wave vector of the propagating SPs is increased. Thus, the radiation direction of SPs into the substrate is changed according to the phase matching condition, which is experimentally shown by Fourier imaging method. The radiation angle is sensitively dependent on the Al2O3 thickness. By depositing 1 nm Al2O3, the increase of the radiation angle can be close to 1 degree. These results show that dielectric-layer-coating provides a simple and effective method to control the propagation and radiation of SPs, which will be of great importance for designing plasmonic circuits, antennas and sensors based on silver nanowires. [ABSTRACT FROM AUTHOR]

Sidelobe artifacts are a common problem in image reconstruction from finite-extent Fourier data obtained by microwave imaging system. In order to enhance the image quality, sidelobe reduction must be implemented. Taking the sidelobe reduction in microwave imaging as the research subject, this paper presents Fourier imaging technique firstly. Then the point spread function is obtained through Fourier inversion of rectangular spectral support data and the changing effect of sidelobe direction due to reshaping spectral support is demonstrated. Spectrum reshaping makes that the sidelobes of two Fourier images do not overlap. Based on the idea of separating sidelobes from mainlobe, a nolve method that reduces sidelobe levels while maintaining the imaging resolution is proposed. The performance of this method is analyzed. Finally, the simulated and measured data of typical microwave imaging systems such as synthetic aperture radar (SAR), inverse synthetic aperture radar (ISAR) and real aperture radar are used to demonstrate the validity of the proposed method. [ABSTRACT FROM AUTHOR]

The Square Kilometre Array (SKA) will be an ultrasensitive radio telescope, built to further the understanding of the most important phenomena in the Universe, including some pertaining to the birth and eventual death of the Universe itself. Over the next few years, the SKA will make the transition from an early formative to a well-defined design. This paper outlines how the scientific challenges are translated into technical challenges, how the application of recent technology offers the potential of affordably meeting these challenges, and how the choices of technology will ultimately be made. The SKA will be an array of coherently connected antennas spread over an area about 3000 km in extent, with an aggregate antenna collecting area of up to 106 m² at centimeter and meter wavelengths. A key scientific requirement is the ability to carry out sensitive observations of the sky over large areas (surveys). The "survey speed" of the SKA will be enabled by the application of the most up-to-date signal-processing technology available. The SKA science impact will be widely felt in astroparticle physics and cosmology, fundamental physics, galactic and extragalactic astronomy, solar system science, and astrobiology. [ABSTRACT FROM AUTHOR]

Recent theory of compressed sensing informs us that near-exact recovery of an unknown sparse signal is possible from a very limited number of Fourier samples by solving a convex L1 optimization problem. The main contribution of the present letter is a compressed sensing-based novel nonparametric shape estimation framework and a computational algorithm for binary star shape objects, whose radius functions belong to the space of bounded-variation functions. Specifically, in contrast with standard compressed sensing, the present approach involves directly reconstructing the shape boundary under sparsity constraint. This is done by converting the standard pixel-based reconstruction approach into estimation of a nonparametric shape boundary on a wavelet basis. This results in an L1 minimization under a nonlinear constraint, which makes the optimization problem nonconvex. We solve the problem by successive linearization and application of one-dimensional Lx minimization, which significantly reduces the number of sampling requirements as well as the computational burden. Fourier imaging simulation results demonstrate that high quality reconstruction can be quickly obtained from a very limited number of samples. Furthermore, the algorithm outperforms the standard compressed sensing reconstruction approach using the total variation norm. [ABSTRACT FROM AUTHOR]

Abstract: Image reconstruction from partial k-space is an important matter in magnetic resonance imaging (MRI). The previously proposed methods are unable to cope with truncation artifacts without degrading image quality. We present a method that is particularly suitable for reconstructing magnetic resonance (MR) images from partial k-space by reducing substantially truncation artifacts while maintaining the spatial resolution of the image. The proposed method is based on the use of a so-called singularity function representation determined by singular points and singularity degrees. With this model, our strategy consists in restricting the singularity degrees within some dynamic range, beyond which the corresponding singular points are set to zero. The proposed method was evaluated on both simulated and human brain MR data. The results showed that this new strategy has significantly improved the reconstruction quality of partial k-space data. [Copyright &y& Elsevier]

Abstract: In analogy with Nuclear MRI, the ESR signal phase shift of conduction electrons moving in electrical currents along controlled magnetic field gradients can be used to generate spatial electronic current density maps. First two-dimensional images of the current density distribution in quasi-one-dimensional organic conductors are presented. [Copyright &y& Elsevier]

Abstract: Static gradient electron spin echo projection reconstruction imaging is favourable for X-band material science applications requiring temperature variation with a metal cryostat. To prevent imaging artefacts due to the high conduction electron diffusion coefficient in the preferred conduction direction of quasi-one-dimensional conductors, only pulsed gradient phase encoding for that direction can be tolerated. We present results of an appropriate cylindrical imaging scheme combining both methods. Conduction electron spin density images with 13×13×17μm3 volume element size or spin–lattice relaxation time images with inversion recovery sequence and 13×13×68μm3 volume element size are presented for fluoranthene radical cation salt single crystals of typical sizes of 0.4×0.4×1mm3. [Copyright &y& Elsevier]

Pulsed ESR is used for microscopic imaging of the conduction electron spin dynamics in the organic crystal (FA)2PF6. Previous conductivity and X-ray measurements of this crystal show a Peierls transition at 180 K. The `local'' diffusion measurements reveal that the spins diffuse freely far from the crystal boundaries. This enables the extraction of the free diffusion coefficient by a one-parametric fit. The temperature dependence of the diffusion coefficient was found to correlate with a previously reported structural transition occurring well above the Peierls transition. [Copyright &y& Elsevier]

Pulsed ESR is used for microscopic three-dimensional imaging of the conduction electron spin properties in the organic crystal (FA)2PF6 (FA: fluoranthene). The measurements enable a study of the local properties of the electron spins on the scale of ∼30 μm. The ‘local’ aspect of the information gained is found to be important, as the images generally exhibit pronounced inhomogeneity. The properties investigated so far are spin relaxation times and mobility. The relaxation times, T1 and T2, were found each to vary by a factor of ∼2 within the samples, and their variations are highly correlated with one another. The spin diffusion might be varying too, in negative correlation with the relaxation times. Viewing the relaxation maps, surface effects are also discernible in some of the samples. [Copyright &y& Elsevier]

Optical sensing is one of the key enablers of modern diagnostics. Especially label-free imaging modalities hold great promise as they eliminate labeling procedures prior to analysis. However, scattering signals of nanometric particles scale with their volume square. This unfavorable scaling makes it extremely difficult to quantitatively characterize intrinsically heterogeneous clinical samples, such as extracellular vesicles, as their signal variation easily exceeds the dynamic range of currently available cameras. Here, we introduce off-axis k-space holography that circumvents this limitation. By imaging the back-focal plane of our microscope, we project the scattering signal of all particles onto all camera pixels, thus dramatically boosting the achievable dynamic range to up to 110 dB. We validate our platform by detecting and quantitatively sizing metallic and dielectric particles over a 200 × 200 μm field of view and demonstrate that independently performed signal calibrations allow correctly sizing particles made from different materials. Finally, we present quantitative size distributions of extracellular vesicle samples.