Plasmonic Metasurfaces for Specific SERS Detection of Shiga Toxins.

The interest in the development of nanoscale plasmonic technologies has dramatically increased in recent years. The photonic properties of plasmonic nanopatterns can be controlled and tuned via their size, shape, or the arrangement of their constituents. In this work, we propose a 2D hybrid metallic polymeric nanostructure based on the octupolar framework with enhanced sensing property. We analyze its plasmonic features both numerically and experimentally, demonstrating the higher values of their relevant figures of merit: we estimated a surface-enhanced Raman spectroscopy (SERS) enhancement factor of 9 × 107 and a SPR bulk sensitivity of 430 nm/RIU. In addition, our nanostructure exhibits a dual resonance in the visible and near-infrared region, enabling our system toward multispectral plasmonic analysis. Finally, we illustrate our design engineering strategy as enabled by electron beam lithography by the outstanding performance of a SERS-based biosensor that targets the Shiga toxin 2a, a clinically relevant bacterial toxin. To the best of our knowledge, this is the first time that a SERS fingerprint of this toxin has been evidenced.

Somatosensory evoked potentials in the assessment of peripheral neuropathies: Commented results of a survey among French-speaking practitioners and recommendations for practice.

BACKGROUND: Somatosensory evoked potentials (SSEPs) are increasingly performed for the assessment of peripheral neuropathies, but no practical guidelines have yet been established in this specific application. STUDY AIM: To determine the relevant indication criteria and optimal technical parameters for SSEP recording in peripheral neuropathy investigation. METHODS: A survey was conducted among the French-speaking practitioners with experience of SSEP recording in the context of peripheral neuropathies. The results of the survey were analyzed and discussed to provide recommendations for practice. RESULTS: SSEPs appear to be a second-line test when electroneuromyographic investigation is not sufficiently conclusive, providing complementary and valuable information on central and proximal peripheral conduction in the somatosensory pathways. CONCLUSIONS: Guidelines for a standardized recording protocol, including the various parameters to be measured, are proposed. CLINICAL RELEVANCE: We hope that these proposals will help to recognize the value of this technique in peripheral neuropathy assessment in clinical practice.

Localized lasing modes of triangular organic microlasers.

We investigated experimentally the ray-wave correspondence in organic microlasers of various triangular shapes. Triangular billiards are of interest since they are the simplest cases of polygonal billiards and the existence and properties of periodic orbits in triangles are not yet fully understood. The microlasers with symmetric shapes that were investigated exhibited states localized on simple periodic orbits, and their lasing characteristics like spectra and far-field distributions could be well explained by the properties of the periodic orbits. Furthermore, asymmetric triangles that do not feature simple periodic orbits were studied. Their lasing properties were found to be more complicated and could not be explained by periodic orbits.

On effective electric field nano-octupoling in two dimensions.

Conditions towards effective electric field poling in two dimensions (2D) of octupolar molecules which can be achieved are being addressed, based on a lattice model which mimics the basic features of poling. The model is studied using the complementary approaches of analytical methods in statistical mechanics and Monte Carlo simulations. The poling field is imparted by a system of adequately shaped cylindrical electrodes. A topologically rich structure of local and global inhomogeneous octupolar order, including octupolar vortices, is present in the system. The poling criteria are shown to vary strongly throughout the cell: in close proximity to the contact points of neighboring electrodes, a high quality local octupolar order appears at temperature T ≃ 0.1 K while octupoling in the center of the cell requires temperatures as low as 10(-4) K. The highly demanding octupoling criteria are ascribed to symmetry-driven effects which decrease the quality of the octupolar phase even in the ground state, as well as to thermal fluctuations and numerical factors at above zero temperatures. Based on our results and using plausible conjectures related to the generalization of the model, it is argued that a weak global octupolar order can be reached at liquid Helium temperatures (a few Kelvins), based on current advances in optical techniques and nanotechnologies.

Trace formula for dielectric cavities. II. Regular, pseudointegrable, and chaotic examples.

Dielectric resonators are open systems particularly interesting due to their wide range of applications in optics and photonics. In a recent paper [Phys. Rev. E 78, 056202 (2008)] the trace formula for both the smooth and the oscillating parts of the resonance density was proposed and checked for the circular cavity. The present paper deals with numerous shapes which would be integrable (square, rectangle, and ellipse), pseudointegrable (pentagon), and chaotic (stadium), if the cavities were closed (billiard case). A good agreement is found between the theoretical predictions, the numerical simulations, and experiments based on organic microlasers.

Air-stable PbSe/PbS and PbSe/PbSexS1-x core-shell nanocrystal quantum dots and their applications.

The optical properties and functionality of air-stable PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell nanocrystal quantum dots (NQDs) are presented. These NQDs showed chemical robustness over months and years and band-gap tunability in the near infrared spectral regime, with a reliance on the NQD size and composition. Furthermore, these NQDs exhibit high emission quantum efficiencies of up to 65% and an exciton emission band that is narrower than that of the corresponding PbSe NQDs. In addition, the emission bands showed a peculiar energy shift with respect to the relevant absorption band, changing from a Stokes shift to an anti-Stokes shift, with an increase of the NQD diameter. The described core-shell structures and the corresponding PbSe core NQDs were used as passive Q-switches in eye-safe lasers of Er:glass, where they act as saturable absorbers. The absorber saturation investigations revealed a relatively large ground-state cross-section of absorption (sigma gs = 10(-16) - 10(-15) cm2) and a behavior of a "fast" absorber with an effective lifetime of tau eff approximately 4.0 ps is proposed. This lifetime is associated with the formation of multiple excitons at the measured pumping power. The product of sigma gs and tau eff enables sufficient Q-switching performance and tunability in the near infrared spectral regime. The amplified spontaneous emission properties of PbSe NQDs were examined under continuous illumination by a diode laser at room temperature, suitable for standard device conditions. The results revealed a relatively large gain parameter (g = 2.63 - 6.67 cm-1). The conductivity properties of PbSe NQD self-assembled solids, annealed at 200 degrees C, showed an Ohmic behavior at the measured voltages (up to 30 V), which is governed by a variable-range-hopping charge transport mechanism.

Deciphering output coupling mechanisms in spiral microcavities with femtosecond light bullets.

A pump-probe two-photon-excited fluorescence technique deciphers in space and time the propagation of ballistic wave packets sustained by whispering-gallery modes (WGMs) in a spiral-shaped microcavity. Diffraction on the spiral discontinuity does not prevent the WGMs from closing. The resultant average Q of the resonator is 3 x 10(4) +/- 50%. Experimental results are compared with numerical simulations, providing evidence of a new contribution to output coupling: Part of the WGM evanescent wave is reflected at the spiral notch and leads to a propagating wave at an angle that matches the previously observed laser emission direction in 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran-doped poly(methyl methacrylate) and InGaN spiral lasers.

Direct evidence of open ray orbits in a square two-dimensional resonator of dye-doped polymers.

Lasing has been observed in optically pumped 4-dicyanomethylene-2-methyl-6-( p-dimethylaminostyrl)-4H-pyran-doped poly(methyl methacrylate) square-shaped micropillars that allow four-bounce closed and open ray orbits with internal incident angle theta(inc) > theta(c) (the critical angle for total internal reflection) and with the associated surface waves that emit at the four corners. We also detect strongly TE-polarized and spatially varying emission from the square sidewalls that is due to leaky open ray orbits with theta(inc) near but less than theta(c) for two of the four bounces. By selectively pumping the square microcavity with a stripe-shaped beam, we excite different four-bounce ray orbits.

Spectral encoding of the polarization state of light in multiplexed polarization-sensitive lasing structures.

We report the experimental demonstration of spectral encoding of the polarization state of light by use of multiplexed distributed-feedback lasers in a dye-doped organic thin film by exploiting the pump polarization selectivity of the superimposed resonators. Measurement of the Stokes parameters of the pump light without a priori knowledge of the lasing properties of the structure is discussed.

All-optical tunability of holographically multiplexed organic distributed feedback lasers.

We report the fabrication and characterization of optically pumped multiple grating distributed feedback lasers in dye doped organic thin films. Each multiplexed laser structure is inscribed at a different angle in the sample plane and possesses a unique emission wavelength. The polarization sensitivity of these structures with respect to the pumping light is exploited to enable simple and high-speed switching of the device emission wavelength.

Local second-harmonic generation enhancement on gold nanostructures probed by two-photon microscopy.

The localization of surface second-harmonic generation (S-SHG) enhancements from granular gold structures that exhibit local plasmon resonance was investigated. A two-photon microscopy technique was used to perform high spatial resolution S-SHG imaging. The magnitude and the spatial density of S-SHG enhancement confined in submicroscopic regions are strongly dependent on the morphology of the gold's surface. Polarization measurements of local S-SHG responses reveal the local field anisotropy in enhancement regions and furthermore prove the incoherent and strongly depolarized nature of the emission, which is attributed to ultrafast fluctuations of the enhancement location in the focal volume.

A hyperpolar, multichromophoric cyclodextrin derivative: synthesis, and linear and nonlinear optical properties.

A chiral, highly polar, multichromophoric supermolecule has been designed by gathering seven push-pull chromophores onto a beta-cyclodextrin assembling unit through covalent flexible linkers. The photophysical and nonlinear optical properties of this mutichromophoric conical bundle were investigated and compared with those of the monomeric chromophore. The strongly absorbing multichromophoric system combines interesting features: it has a high molecular first-order hyperpolarisability and a very large dipolar moment (u = 38 D) which reveal a self-arrangement of the dipolar chromophores within the supermolecule. The confinement of the push-pull chromophores within the nanoscopic bundle affects their optical properties and promotes interactions: the multichromophoric supermolecule is hypochromically and hypsochromically shifted with respect to its monomeric analogue. In addition, the close proximity promotes excitonic coupling, as well as excimer formation phenomena.

Dipolar donor-acceptor-substituted schiff base complexes with large off-diagonal second-order nonlinear optical tensor components.

The synthesis, characterization, and two-dimensional second-order nonlinear optical (NLO) response of a dipolar NiII donor- acceptor Schiff base complex and the related ligand are reported. Electric-field-induced second-harmonic generation and harmonic light (hyper-Rayleigh) scattering techniques, in combination with INDO/SCI-SOS theoretical calculations, were used to investigate the vector part of the hyperpolarizability tensor and the two-dimensional character of the molecular nonlinearity, respectively. Off-diagonal hyperpolarizability tensors can be related to charge-transfer transitions that are polarized perpendicular to the molecular dipolar axis, while parallel transitions account for the diagonal hyperpolarizability tensor. The role of the metal center in enhancing the two-dimensional NLO response of such molecules is twofold since it acts both as the donor and the bridging moiety of the planar donor-(pi-conjugate-bridge)-acceptor system. These dipolar two-dimensional molecules are interesting candidates from the perspective of polarization-independent NLO materials.

Synthesis of a highly thermally stable octupolar polyimide for nonlinear optics.

A new polyimide with covalently incorporated 4,4'-bis-(dialkylaminostyryl)-2,2'-bipyridine chromophores is described which allows the synthesis of the corresponding NLO octupolar metallo-polymer.

Polymer-based microcavity with photoencoded quadratic nonlinearity.

Functional electro-optic polymer thin films embedded in microcavity structures have been poled by an all-optical procedure based on the interference of multiphoton absorption processes. The photoinduced X((2)) tensor was then further addressed at modal resonance for the fundamental wavelength, leading to significant enhancement of the second-harmonic-generation efficiency. An order-of-magnitude enhancement, which is due to electric field resonant conditions inside the microcavity, has been probed by an optical parametric oscillator, in comparison with a single-path thin-film configuration. This configuration opens new perspectives in the realm of nonlinear photonic device processing.

Control of the polarization dependence of optically poled nonlinear polymer films.

We demonstrate the possibility of controlling the symmetry properties of photoinduced chi((2)) macroscopic susceptibility in polymer films. Ellipsometric adjustment of the write beams allows one to monitor the macroscopic chi((2)) symmetry from a dipolar to an octupolar configuration. Experimental results are in agreement with an irreducible spherical tensor-based model jointly applied to the molecular beta hyperpolarizability and field tensors. We found a purely octupolar polarization-independent photoinduced second-harmonic-generation response in a Dispersed Red 1-methyl methacrylate thin film. Such a configuration, as generated by an octupolar tensor write field tensor made up of counterclockwise circularly polarized omega and 2omega beams, is not within the reach of the classical electric-field poling technique.

Saturation of cubic optical nonlinearity in long-chain polyene oligomers.

The scaling of the cubic nonlinearity gamma with chain length in polyenic molecules has received considerable theoretical attention. Earlier experimental investigations have been restricted to oligomers with fewer than 20 double bonds because of problems associated with the synthesis and solubility of conjugated molecules. These synthetic difficulties have been overcome in the present study by the use of modern living polymerization techniques. Solution measurements of gamma as a function of chain length in long-chain (up to 240 double bonds) model polyene oligomers are reported. A saturation of the increase of gamma with chain length is observed, and the onset of this saturation occurs for chain lengths considerably longer than predicted from theory.