Extended time-of-flight measurements down to 100 keV at the AMANDE facility with a stilbene scintillator.

The time-of-flight (ToF) method with scintillators is routinely used for determining neutron energy. However, a technical difficulty related to the loss of scintillator efficiency below 1 MeV makes this technique difficult to implement for the energy decade [100 keV-1 MeV]. New crystal production techniques provide stilbene scintillators efficient in this low neutron energy region, making it possible to extend the ToF technique below 1 MeV. In this manner, measurements of secondary reactions (d,n) on carbon or oxygen nuclei in this range become feasible, which should lead to improved reference calibration conditions in neutron fields produced by a deuterium ion beam.

Scattering matrix of arbitrarily shaped objects: combining finite elements and vector partial waves.

We demonstrate the interest of combining finite element calculations with the vector partial wave formulation (used in T-matrix and Mie theory) in order to characterize the electromagnetic scattering properties of isolated individual scatterers. This method consists of individually feeding the finite element problem with incident vector partial waves in order to numerically determine the T-matrix elements of the scatterer. For a sphere and a spheroid, we demonstrate that this method determines the scattering matrix to high accuracy. Recurrence relations for a fast determination of the vector partial waves are given explicitly, and an open-source code allowing the retrieval of the presented numerical results is provided.

Interplay between spontaneous decay rates and Lamb shifts in open photonic systems.

In this Letter, we describe the modified decay rate and photonic Lamb (frequency) shift of quantum emitters in terms of the resonant states of a neighboring photonic resonator. This description illustrates a fundamental distinction in the behaviors of closed (conservative) and open (dissipative) systems: the Lamb shift is bounded by the emission linewidth in closed systems while it overcomes this limit in open systems.

Multilevel fast multipole method based on a potential formulation for 3D electromagnetic scattering problems.

A combination of the multilevel fast multipole method (MLFMM) and boundary element method (BEM) can solve large scale photonics problems of arbitrary geometry. Here, MLFMM-BEM algorithm based on a scalar and vector potential formulation, instead of the more conventional electric and magnetic field formulations, is described. The method can deal with multiple lossy or lossless dielectric objects of arbitrary geometry, be they nested, in contact, or dispersed. Several examples are used to demonstrate that this method is able to efficiently handle 3D photonic scatterers involving large numbers of unknowns. Absorption, scattering, and extinction efficiencies of gold nanoparticle spheres, calculated by the MLFMM, are compared with Mie's theory. MLFMM calculations of the bistatic radar cross section (RCS) of a gold sphere near the plasmon resonance and of a silica coated gold sphere are also compared with Mie theory predictions. Finally, the bistatic RCS of a nanoparticle gold-silver heterodimer calculated with MLFMM is compared with unmodified BEM calculations.

Controllable emission of a dipolar source coupled with a magneto-dielectric resonant subwavelength scatterer.

We demonstrate experimentally and theoretically that a local excitation of a single scatterer of relative dielectric permittivity ε = 6 permits to excite broad dipolar and quadrupolar electric and magnetic resonances that shape the emission pattern in an unprecedented way. By suitably positioning the feed with respect to the sphere at a λ/3 distance, this compact antenna is able to spectrally sort the electromagnetic emission either in the forward or in the backward direction, together with a high gain in directivity. Materials with ε = 6 can be found in the whole spectrum of frequencies promising Mie antennas to become an enabling technology in numbers of applications, ranging from quantum single photon sources to telecommunications.

Boosting the directivity of optical antennas with magnetic and electric dipolar resonant particles.

Dielectric particles supporting both magnetic and electric Mie resonances are shown to be able to either reflect or collect the light emitted by a single photon source. An analytical model accurately predicts the scattering behavior of a single dielectric particle electromagnetically coupled to the electric dipole transition moment of a quantum emitter. We derive near field extensions of the Kerker conditions in order to determine the conditions that strongly reduce scattering in either the forward or backward directions. This concept is then employed to design a lossless dielectric collector element whose directivity is boosted by the coherent scattering of both electric and magnetic dipoles.

Photonic engineering of hybrid metal-organic chromophores.

An aureate dye: Confined electromagnetic fields in DNA-templated gold nanoparticle dimers were tuned to engineer the fluorescence properties of organic dyes in water (see picture). Purified suspensions of hybrid metal-organic chromophores featured unprecedented photophysical properties, such as a short lifetime and low quantum yield but high brightness.

Imaging the Gouy phase shift in photonic jets with a wavefront sensor.

A wavefront sensor is used as a direct observation tool to image the Gouy phase shift in photonic nanojets created by micrometer-sized dielectric spheres. The amplitude and phase distributions of light are found in good agreement with a rigorous electromagnetic computation. Interestingly the observed phase shift when travelling through the photonic jet is a combination of the awaited π Gouy shift and a phase shift induced by the bead refraction. Such direct spatial phase shift observation using wavefront sensors would find applications in microscopy, diffractive optics, optical trapping, and point spread function engineering.

Accelerated single photon emission from dye molecule-driven nanoantennas assembled on DNA.

A photon interacts efficiently with an atom when its frequency corresponds exactly to the energy between two eigenstates. But at the nanoscale, homogeneous and inhomogeneous broadenings strongly hinder the ability of solid-state systems to absorb, scatter or emit light. By compensating the impedance mismatch between visible wavelengths and nanometre-sized objects, optical antennas can enhance light-matter interactions over a broad frequency range. Here we use a DNA template to introduce a single dye molecule in gold particle dimers that act as antennas for light with spontaneous emission rates enhanced by up to two orders of magnitude and single photon emission statistics. Quantitative agreement between measured rate enhancements and theoretical calculations indicate a nanometre control over the emitter-particle position while 10 billion copies of the target geometry are synthesized in parallel. Optical antennas can thus tune efficiently the photo-physical properties of nano-objects by precisely engineering their electromagnetic environment.

Takotsubo cardiomyopathy in the setting of acute alcohol withdrawal.

Takotsubo Cardiomyopathy (TCM), also known as stress-induced cardiomyopathy, is a cardiomyopathy characterized by acute reversible apical ventricular dysfunction and apical akinesis in the absence of obstructive coronary artery disease. Although the disease may be precipitated by an acute emotional or physical stressor, the pathophysiology, postulated to involve excess catecholamine release, remains unproven. In contrast, the role of catecholamine excess and hyperadrenergic physiology in acute alcohol withdrawal (AAW) is more established. TCM in the context of acute alcohol withdrawal has been only rarely described. The authors present a new case of TCM in the setting of AAW, along with a review of other reported cases. Current theories on the etiology of TCM and a possible pathophysiologic linkage between TCM and AAW are discussed.

Optical and topological characterization of gold nanoparticle dimers linked by a single DNA double strand.

We demonstrate that symmetric or asymmetric gold nanoparticle dimers with substantial scattering cross sections and plasmon coupling can be produced with a perfectly controlled chemical environment and a high purity using a single DNA linker as short as 7 nm. A statistical analysis of the optical properties and morphology of single dimers is performed using darkfield and cryo-electron microscopies. These results, correlated to Mie theory calculations, indicate that the particle dimers are stretched in water by electrostatic interactions.

Crucial role of the emitter-particle distance on the directivity of optical antennas.

We demonstrate that the reflecting properties of a single particle nanoantenna can be extremely sensitive to its distance from a quantum emitter at frequencies lower than the plasmon resonance. The phenomenon is shown to arise from rapid phase variations of the emitter field at short distances associated with a phase of the antenna particle polarizability lower than π/4.

Compact metallo-dielectric optical antenna for ultra directional and enhanced radiative emission.

We report the design of highly efficient optical antennas employing a judicious synthesis of metallic and dielectric materials. In the proposed scheme, a pair of metallic coupled nanoparticles permits large enhancements in both excitation strength and radiative decay rates, while a high refractive index dielectric microsphere is employed to efficiently collect light without spoiling the emitter quantum efficiency. Our simulations indicate potential fluorescence rate enhancements of 3 orders of magnitude over the entire optical frequency range.

Microwave measurements of the full amplitude scattering matrix of a complex aggregate: a database for the assessment of light scattering codes.

We present an extensive experimental study of microwave scattering by a fully characterized complex aggregate. We measured the full amplitude scattering matrix (amplitude and phase of the four elements) for a wide range of configurations. The presented results are of special interest to the light scattering community. Our experiments offer the possibility to validate numerical methods against experiments, since the geometrical and dielectric properties of the complex target are known to a high degree of precision, a situation difficult to attain in the optical regime. We analyze in detail the behaviour of amplitude and phase as a function of the scattering angle and target orientation. Furthermore, we compare different computational methods for a specific experimental configuration.

Three-dimensional subwavelength confinement of light with dielectric microspheres.

Dielectric microspheres are shown to be capable of confining light in a three-dimensional region of subwavelength dimensions when they are illuminated by tightly focused Gaussian beams. We show that a simple configuration, not involving resonances, permits one to reach an effective volume as small as 0.6 (lambda/n)(3). It is shown that this three-dimensional confinement arises from interferences between the field scattered by the sphere and the incident Gaussian beam containing high angular components.

Recursive T matrix algorithm for resonant multiple scattering: applications to localized plasmon excitations.

A matrix balanced version of the recursive centered T matrix algorithm applicable to systems possessing resonant interparticle couplings is presented. Possible domains of application include systems containing interacting localized plasmon resonances, surface resonances, and photonic jet phenomena. This method is of particular interest when considering modifications to complex systems. The numerical accuracy of this technique is demonstrated in a study of particles with strongly interacting localized plasmon resonances.

Strong electromagnetic confinement near dielectric microspheres to enhance single-molecule fluorescence.

Latex microspheres are used as a simple and low-cost means to achieve three axis electromagnetic confinement below the standard diffraction limit. We demonstrate their use to enhance the fluorescence fluctuation detection of single molecules. Compared to confocal microscopy with high numerical aperture, we monitor a detection volume reduction of one order of magnitude below the diffraction limit together with a 5-fold gain in the fluorescence rate per molecule. This offers new opportunities for a broad range of applications in biophotonics, plasmonics, optical data storage and ultramicroscopy.

Spectral analysis of three-dimensional photonic jets.

The spatial and spectral properties of three-dimensional photonic jets are studied in a framework employing rigorous Lorentz-Mie theory. The contributions to the field from each spectral component are studied quantitatively and highlight the distinctive features of photonic jets. In particular, the role of evanescent field in photonic jets generated by small spheres is investigated. Secondary lobes in the propagative frequency distribution are also singled out as a distinctive property of photonic jets. It is shown that these differences lead to angular openings of photonic jets at least twice as small as those in comparable 'Gaussian' beams.

Direct imaging of photonic nanojets.

We report the direct experimental observation of photonic nanojets created by single latex microspheres illuminated by a plane wave at a wavelength of 520 nm. Measurements are performed with a fast scanning confocal microscope in detection mode, where the detection pinhole defines a diffraction-limited observation volume that is scanned in three dimensions over the microsphere vicinity. From the collected stack of images, we reconstruct the full 3 dimensional photonic nanojet beam. Observations are conducted for polystyrene spheres of 1, 3 and 5 microm diameter deposited on a glass substrate, the upper medium being air or water. Experimental results are compared to calculations performed using the Mie theory. We measure nanojet sizes as small as 270 nm FWHM for a 3 microm sphere at a wavelength lambda of 520 nm. The beam keeps a subwavelength FWHM over a propagation distance of more than 3 lambda, displaying all the specificities of a photonic nanojet.