Alexis Devilez

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.

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.

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.

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.

Efficient excitation and collection of single- molecule fluorescence close to a dielectric microsphere

Dielectric microspheres illuminated by a tightly focused Gaussian beam can focus light on a tiny spot with subwavelength dimensions along the three directions of space. We report here a detailed experimental and theoretical study of the interaction between a single fluorescent molecule and this peculiar electromagnetic distribution. The microsphere increases the excitation intensity sensed by the molecule up to a factor of 2.2, while at the same time it allows for a collection efficiency of up to 60% by redirecting the light emitted at large incidences toward the optical axis. By combining these two effects, the number of collected fluorescence photons can be increased up to a factor of 5. We quantify the evolution of the excitation and collection contributions with the microsphere dimensions and compare our experimental findings with numerical simulations.

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.

Picosecond Lifetimes with High Quantum Yields from Single-Photon-Emitting Colloidal Nanostructures at Room Temperature

Minimizing the luminescence lifetime while maintaining a high emission quantum yield is paramount in optimizing the excitation cross-section, radiative decay rate, and brightness of quantum solid-state light sources, particularly at room temperature, where nonradiative processes can dominate. We demonstrate here that DNA-templated 60 and 80 nm diameter gold nanoparticle dimers, featuring one fluorescent molecule, provide single-photon emission with lifetimes that can fall below 10 ps and typical quantum yields in a 45-70% range. Since these colloidal nanostructures are obtained as a purified aqueous suspension, fluorescence spectroscopy can be performed on both fixed and freely diffusing nanostructures to quantitatively estimate the distributions of decay rate and fluorescence intensity enhancements. These data are in excellent agreement with theoretical calculations and demonstrate that millions of bright fluorescent nanostructures, with radiative lifetimes below 100 ps, can be produced in parallel.

Mode-balancing far-field control of light localization in nanoantennas

Light localization is controlled at a scale of /10 in the harmonic regime from the far eld domain in a plasmonic nanoantenna. The nanoantenna under study consists of 3 aligned spheres 50 nm in diameter separated by a distance of 5 nm. By simply tuning the orientation of an incident plane wave, symmetric and antisymmetric mode-balancing induces a strong enhancement of the near eld intensity in one cavity while nullifying the light intensity in the other cavity. Furthermore, it is demonstrated that the dipolar moment of a plasmonic particle can be fully extinguished when strongly coupled with a dimer of identical nanoparticles. Consequently, optical transparency can be achieved in an ultra-compact symmetric metallic structure.

Transverse and longitudinal confinement of photonic nanojets by compound dielectric microspheres

We discuss the compound set of two dielectric microspheres to confine light in a three dimensional region of dimensions on the order of the wavelength when the spheres are illuminated by a plane wave. This simple configuration enables the reduction of the longitudinal dimension of so called photonic jets, together with a strong focusing effect. The beam shaped in that way is suitable for applications requiring high longitudinal resolutions and/or strong peak intensities.

Optimal interactions of light with magnetic and electric resonant particles

This work studies the limits of far and near-field electromagnetic response of sub-wavelength scatterers, like the unitary limit of lossless scatterers and the ideal absorption limit of lossy particles. These limit behaviors are described in terms of analytic formulas that approximate finite size effects while rigorously including radiative corrections. This analysis predicts the electric and/or magnetic limit responses of both metallic and dielectric nanoparticles while quantitatively describing near-field enhancements. doi:10.1103/PhysRevB.00.005400