Jean-Paul Hugonin

Theory of surface plasmon generation at nanoslit apertures

In this Letter, we study the scattering of light by a single subwavelength slit in a metal screen. In contrast with previous theoretical works, we provide a microscopic description of the scattering process by emphasizing the generation of surface plasmons at the slit apertures. The analysis is supported by a rigorous formalism based on a normal-mode-decomposition technique and by a semianalytical model that provides accurate formulas for the plasmonic generation strengths. The generation is shown to be fairly efficient for metals with a low conductivity, such as gold in the visible regime. Verification of the theory is also shown by comparison with recent experimental data [H. F. Schouten, Phys. Rev. Lett. 94, 053901 (2005)].

Use of grating theories in integrated optics

Recently [Opt. Lett. 25, 1092 (2000)], two of the present authors proposed extending the domain of applicability of grating theories to aperiodic structures, especially the diffraction structures that are encountered in integrated optics. This extension was achieved by introduction of virtual periodicity and incorporation of artificial absorbers at the boundaries of the elementary cells of periodic structures. Refinements and extensions of that previous research are presented. Included is a thorough discussion of the effect of the absorber quality on the accuracy of the computational results, with highly accurate computational results being achieved with perfectly matched layer absorbers. The extensions are concerned with the diversity of diffraction waveguide problems to which the method is applied. These problems include two-dimensional classical problems such as those involving Bragg mirrors and grating couplers that may be difficult to model because of the length of the components and three-dimensional problems such as those involving integrated diffraction gratings, photonic crystal waveguides, and waveguide airbridge microcavities. Rigorous coupled-wave analysis (also called the Fourier modal method) is used to support the analysis, but we believe that the approach is applicable to other grating theories. The method is tested both against available numerical data obtained with finite-difference techniques and against experimental data. Excellent agreement is obtained. A comparison in terms of convergence speed with the finite-difference modal method that is widely used in waveguide theory confirms the relevancy of the approach. Consequently, a simple, efficient, and stable method that may also be applied to waveguide and grating diffraction problems is proposed.

Compact antenna for efficient and unidirectional launching and decoupling of surface plasmons.

Controlling the launching efficiencies and the directionality of surface plasmon polaritons (SPPs) and their decoupling to freely propagating light is a major goal for the development of plasmonic devices and systems. Here, we report on the design and experimental observation of a highly efficient unidirectional surface plasmon launcher composed of eleven subwavelength grooves, each with a distinct depth and width. Our observations show that, under normal illumination by a focused Gaussian beam, unidirectional SPP launching with an efficiency of at least 52% is achieved experimentally with a compact device of total length smaller than 8 μm. Reciprocally, we report that the same device can efficiently convert SPPs into a highly directive light beam emanating perpendicularly to the sample.

Controlling spontaneous emission with plasmonic optical patch antennas.

We experimentally demonstrate the control of the spontaneous emission rate and the radiation pattern of colloidal quantum dots deterministically positioned in a plasmonic patch antenna. The antenna consists of a thin gold microdisk separated from a planar gold layer by a few tens of nanometers thick dielectric layer. The emitters are shown to radiate through the entire patch antenna in a highly directional and vertical radiation pattern. Strong acceleration of spontaneous emission is observed, depending on the antenna geometry. Considering the double dipole structure of the emitters, this corresponds to a Purcell factor up to 80 for dipoles perpendicular to the disk.

Solid-state single photon sources: the nanowire antenna

We design several single-photon-sources based on the emission of a quantum dot embedded in a semiconductor (GaAs) nanowire. Through various taper designs, we engineer the nanowire ends to realize efficient metallic-dielectric mirrors and to reduce the divergence of the far-field radiation diagram. Using fully-vectorial calculations and a comprehensive Fabry-Perot model, we show that various realistic nanowire geometries may act as nanoantennas (volume of approximately 0.05 lambda(3)) that assist funnelling the emitted photons into a single monomode channel. Typically, very high extraction efficiencies above 90% are predicted for a collection optics with a numerical aperture NA=0.85. In addition, since no frequency-selective effect is used in our design, this large efficiency is achieved over a remarkably broad spectral range, Deltalambda=70 nm at lambda=950 nm.

Modal-reflectivity enhancement by geometry tuning in Photonic Crystal microcavities.

When a guided wave is impinging onto a Photonic Crystal (PC) mirror, a fraction of the light is not reflected back and is radiated into the claddings. We present a theoretical and numerical study of this radiation problem for several three-dimensional mirror geometries which are important for light confinement in micropillars, air-bridge microcavities and two-dimensional PC microcavities. The cause of the radiation is shown to be a mode-profile mismatch. Additionally, design tools for reducing this mismatch by tuning the mirror geometry are derived. These tools are validated by numerical results performed with a three-dimensional Fourier modal method. Several engineered mirror geometries which lower the radiation loss by several orders of magnitude are designed.

High-order effective-medium theory of subwavelength gratings in classical mounting: application to volume holograms

We derive closed-form expressions for the effective index of subwavelength gratings up to the fourth and the second order for TE and TM polarization, respectively. These expressions are valid for arbitrary grating structures and are a generalization of previous results obtained for lamellar gratings with one groove per period (a structure often called a two-component layered medium). The effective-medium-theory predictions are carefully validated with exact electromagnetic theories for slanted and unslanted sinusoidally modulated volume gratings and for classical mounting. It is shown that, even for large period-to-wavelength ratios near the cutoff value, the form birefringence is accurately predicted at any angle of incidence.

Approximate model for surface-plasmon generation at slit apertures

We present a semianalytical model that quantitatively predicts the scattering of light by a single subwavelength slit in a thick metal screen. In contrast to previous theoretical works related to the transmission properties of the slit, the analysis emphasizes the generation of surface plasmons at the slit apertures. The model relies on a two-stage scattering mechanism, a purely geometric diffraction problem in the immediate vicinity of the slit aperture followed by the launching of a bounded surface-plasmon wave on the flat interfaces surrounding the aperture. By comparison with a full electromagnetic treatment, the model is shown to provide accurate formulas for the plasmonic generation strength coefficients, even for metals with a low conductivity. Limitations are outlined for large slit widths (>lambda) or oblique incidence (>30 degrees ) when the slit is illuminated by a plane wave.

Combined MEG and EEG source imaging by minimization of mutual information

Though very frequently assumed, the necessity to operate a joint processing of simultaneous magnetoencephalography (MEG) and electroencephalography (EEG) recordings for functional brain imaging has never been clearly demonstrated. However, the very last generation of MEG instruments allows the simultaneous recording of brain magnetic fields and electrical potentials on the scalp. But the general fear regarding the fusion between MEG and EEG data is that the drawbacks from one modality will systematically spoil the performances of the other one without any consequent improvement. This is the case for instance for the estimation of deeper or radial sources with MEG. In this paper, the authors propose a method for a cooperative processing of MEG and EEG in a distributed source model. First, the evaluation of the respective performances of each modality for the estimation of every dipole in the source pattern is made using a conditional entropy criterion. Then, the algorithm operates a preprocessing of the MEG and EEG gain matrices which minimizes the mutual information between these two transfer functions, by a selective weighting of the MEG and EEG lead fields. This new combined EEG/MEG modality brings major improvements to the localization of active sources, together with reduced sensitivity to perturbations on data.

Non-blinking quantum dot with a plasmonic nanoshell resonator

Colloidal semiconductor quantum dots are fluorescent nanocrystals exhibiting exceptional optical properties, but their emission intensity strongly depends on their charging state and local environment. This leads to blinking at the single-particle level or even complete fluorescence quenching, and limits the applications of quantum dots as fluorescent particles. Here, we show that a single quantum dot encapsulated in a silica shell coated with a continuous gold nanoshell provides a system with a stable and Poissonian emission at room temperature that is preserved regardless of drastic changes in the local environment. This novel hybrid quantum dot/silica/gold structure behaves as a plasmonic resonator with a strong Purcell factor, in very good agreement with simulations. The gold nanoshell also acts as a shield that protects the quantum dot fluorescence and enhances its resistance to high-power photoexcitation or high-energy electron beams. This plasmonic fluorescent resonator opens the way to a new family of plasmonic nanoemitters with robust optical properties.