Renaud A. L. Vallée

In situ tuning the optical properties of a cavity by wrinkling

In this letter we propose an original, in situ, approach to tune the optical properties of an optical cavity, based on the wrinkling of compressed metal/polymer multilayer thin films. This phenomenon is conceptually described, simulated, and experimentally confirmed. The main idea is to use wrinkling to modulate the effective refractive index of the upper interface. This modulation induces a spectral shift of the cavity modes. The work presented here constitutes a first step to the development of stretchable and curved photonics.

Fine tuning of emission through the engineering of colloidal crystals

We describe the preparation and characterization of photonic colloidal crystals from silica spheres with incorporated luminescent [Mo(6)Br(14)](2-) cluster units. These structures exhibit strong angle-dependent luminescent properties. The incorporation of one or several planar defects in the periodic structures gives rise to the creation of a passband in the stopband. In the energy range of this passband, an increase of the emission intensity has been found.

Probe molecules in polymer melts near the glass transition: A molecular dynamics study of chain length effects.

Molecular dynamics simulations of a dense melt of short bead-spring polymer chains containing N=5, 10, or 25 effective monomers are presented and analyzed. Parts of our simulations include also a single dumbbell (N=2) of the same type, which is interpreted to represent a coarse-grained model for a fluorescent probe molecule as used in corresponding experiments. We obtain the mean-square displacements of monomers and chains center of mass, and intermediate incoherent scattering functions of both monomers in the chains and particles in the dumbbells as function of time for a broad regime of temperatures above the critical temperature T(c) of mode-coupling theory. For both the chains and the dumbbell, also orientational autocorrelation functions are calculated and for the dumbbell time series for the time evolution of linear dichroism and its autocorrelation function are studied. From both sets of data we find that both the mode-coupling critical temperature T(c) (representing the cage effect) and the Vogel-Fulcher temperature T(0) (representing the caloric glass transition temperature) systematically increase with chain length. Furthermore, the dumbbell dynamics yields detailed information on the differences in the matrix dynamics that are caused by the chain length variation. Deviations from the Stokes-Einstein relation are discussed, and an outlook to related experiments is given.

Wavelength-dependent emission enhancement through the design of active plasmonic nanoantennas

Owing to the competition between the radiative and non-radiative decay channels occurring in plasmonic assemblies, we show here how to conceive a long pass emission filter and actually design it. We report the synthesis of gold@silica nanoparticles grafted with dye molecules. The control of the thickness of the silica shell allows us to tune the distance between the metal core and the dye molecules. Assemblies of small number (1 to 7) of these core-shell (CS) particles, considered as multimers, have also been produced for the first time. We show that the shaping of the emission spectra of the multimers is drastically enhanced by comparison with the corresponding monomers. We also show a strong enhancement of the decay rates at the LSP resonance, dominated by the non-radiative energy tranfer from the active medium to the metal. The decay rates decrease as the detuning between the long wavelength emission and the LSP resonance increases.

Far-field disentanglement of modes in hybrid plasmonic-photonic crystals by fluorescence nano-reporters

Abstract In this paper, the resonance modes exhibited by a hybrid nanostructure have been disentangled in the far-field owing to narrow-band fluorescence nano-reporters. Hybrid plasmonic-photonic crystals were fabricated using large (457 nm) monodisperse polystyrene spheres self-assembled into 2D photonic crystals and subsequently coated by a 30 nm thick silver layer. Such structures exhibit a complex resonance pattern, which has been elucidated owing to numerical simulations and electric near-field patterns obtained with a scattering type scanning near-field optical microscope (s-SNOM). For the sake of disentangling the resonance modes of the hybrid structure in the far-field, different types of semiconductor quantum dots (QDs), acting as nano-reporters of the local interactions, were dispersed on top of distinct structures. Depending on the relative overlap of the emission spectrum of a particular type of QDs with the resonance features of the hybrid structure, we affect their emission rate in a unique way, as a consequence of the complex interaction occurring between the plasmo-photonic modes and the excitons. Such plasmonic structures appear to be particularly relevant for fluorescence-based sensing devices.

DEVELOPMENT OF MAGNETIC MATERIALS FOR PHOTONIC APPLICATIONS

In this manuscript, the synthesis and characterization of superparamagnetic particles and their silica-coated counterparts as building blocks for magnetic photonic crystals is fully described. The advantages and disadvantages of the presented synthetic method are discussed. Preliminary results considering the presence of magnetic species within a photonic crystal are also presented. Suppression of emission of the quantum dots within photonic crystals is attributed to a decrease of the number of available photonic modes for radiative decay. The presence of materials with permanent magnetic moments within photonic crystals shows that suppression of their emission is scaled with the strength of the magnetic field.

Polarized SERS on linear arrays of silver half-shells: SERS re-radiation modulated by local density of optical states

This work investigates polarization effects in surface-enhanced Raman scattering (SERS) on a particular kind of hybrid colloidal plasmonic-photonic crystal consisting in linear arrays of polystyrene colloids coated by a silver film forming caps (half-shells). The polarization of Raman scattering of adsorbed molecules is found to be imposed by the linear morphology of the plasmonic nanostructures, independent of the incident polarization. Specifically, it is demonstrated that the electric field component parallel to the linear plasmonic nanostructures brings the main contribution to the SERS enhancement for both parallel and perpendicular excitation. The stronger plasmon resonance, polarized along the chain of metal half-shells, is the one that determines the polarization of the SERS scattering, even if this resonance is not directly excited by the incident laser, but only re-radiated photons couple to it. The observed dependences indicate a strong contribution of the second part of the E 4 enhancement, related to the enhancement at the frequency of the Raman scattered photons. FDTD electromagnetic simulations, taking into account both the excitation enhancement at the location of the scatterer and the enhancement of the scattering (re-radiation) by local density of optical states (LDOS) effects, support the experimental results. The presented results emphasize the practical importance of knowing and controlling the excitation/detection conditions in SERS experiments with nanostructures possessing well-defined and oriented morphological features. The LDOS approach employed here for the theoretical analysis, despite not being common for this purpose, proves its potential for becoming a useful ingredient in the analysis of plasmon-enhanced spectroscopies.

Colocalized dark-field scattering, atomic force and surface-enhanced Raman scattering microscopic imaging of single gold nanoparticles

Colocalized dark-field scattering, atomic force and surface-enhanced Raman scattering microscopic (DFSM, AFM and SERS) imaging of single gold nanoparticles is presented using a home-made instrument. A high numerical aperture total internal reflection objective allows us to probe the polarization-dependence of DFSM spectra associated with individual nanoparticles, which reveal their degree of anisotropy and reflect the morphology and orientation observed on AFM images. Under convenient light excitation, the SERS enhancement efficiency reaches 106–107 for the investigated gold nanorod and ellipsoid, in good agreement with the 103–105 electromagnetic enhancement factor assessed from finite-difference-time-domain (FDTD) calculations and a 102–103 chemical contribution. FDTD calculations also confirm the possibility to reproduce nicely anisotropic behaviours disclosed by DFSM studies using SERS measurements.

Quasi-omnidirectional total light absorption in nanostructured gold surfaces

Theoretical calculations have predicted the possibility of omnidirectional absorption on a metallic surface with a closed packed layer of voids/spheres buried just beneath the surface. We have carried out a series of experiments to verify the existence of this theoretically predicted phenomenon. We report the observation of quasi omnidirectional total absorption of light on our fabricated surfaces and the tunability of the absorption wavelength by varying the size of the spheres/pores. The strongly enhanced absorption is observed for angles of incidence up to 65°.

Nonaqueous sol–gel chemistry applied to atomic layer deposition: tuning of photonic band gap properties of silica opals

Combining both electromagnetic simulations and experiments, it is shown that the photonic pseudo band gap (PPBG) exhibited by a silica opal can be fully controlled by Atomic Layer Deposition (ALD) of titania into the pores of the silica spheres constituting the opal. Different types of opals were assembled by the Langmuir-Blodgett technique: homogeneous closed packed structures set up of, respectively, 260 and 285 nm silica spheres, as well as opal heterostructures consisting of a monolayer of 430 nm silica spheres embedded within 10 layers of 280 nm silica spheres. For the stepwise infiltration of the opals with titania, titanium isopropoxide and acetic acid were used as metal and oxygen sources, in accordance with a recently published non-aqueous approach to ALD. A shift of the direct PPBG, its disappearance, and the subsequent appearance and shifting of the inverse PPBG are observed as the opal is progressively filled. The close agreement between simulated and experimental results is striking, and promising in terms of predicting the properties of advanced photonic materials. Moreover, this work demonstrates that the ALD process is rather robust and can be applied to the coating of complex nanostructures.