Surface plasmons in suspended graphene: launching with in-plane gold nanoantenna and propagation properties.

Graphene physics and plasmonics are two fields which, once combined, promise a variety of exciting applications. One of those applications is the integration of active nano-optoelectronic devices in electronic systems, using the fact that plasmons in graphene are tunable, highly confined and weakly damped. A crucial challenge remains before achieving these active devices: finding a platform enabling a high propagation of Graphene Plasmons Polaritons (GPPs). Suspended graphene presenting ultrahigh electron mobility has given rise to increasing interest. We numerically studied the plasmonic properties of suspended graphene. We propose a hybrid configuration and a set of conditions to launch graphene plasmons via an in-plane gold nanoantenna, for micrometric propagation of surface plasmons in suspended graphene. Finally, we propose a realistic optoelectronic device based on the use of suspended graphene.

Direct functionalization of an optical fiber by a plasmonic nanosensor.

We explore a rapid route for fabricating silver nanoparticles (NPs) at the end of an optical fiber. The size and number of silver NPs can be controlled by varying the exposure doses. The effect of the refractive index of different solvents on the extinction spectra have been studied as a proof of concept of a fiber integrated plasmon-based sensor.

Spatially controlled synthesis of silver nanoparticles and nanowires by photosensitized reduction.

The present paper reports on the spatially controlled synthesis of silver nanoparticles (NPs) and silver nanowires by photosensitized reduction. In a first approach, direct photogeneration of silver NPs at the end of an optical fiber was carried out. Control of both size and density of silver NPs was possible by changing the photonic conditions. In a further development, a photochemically assisted procedure allowing silver to be deposited at the surface of a polymer microtip was implemented. Finally, polymer tips terminated by silver nanowires were fabricated by simultaneous photopolymerization and silver photoreduction. The silver NPs were characterized by UV-visible spectroscopy and scanning electron microscopy.

Understanding near/far-field engineering of optical dimer antennas through geometry modification.

Numerical investigations based on the boundary element method (BEM) have been carried out to two-dimensional (2-D) silver dimer nano-antennas of various geometries. The near-field and far-field properties are mainly determined by the local geometry at the gap and the global shape of the antenna shafts respectively. A hybrid dimer antenna, which mixes the geometry ingredients of the rod dimer and the bowtie, benefits in both near and far field. Using a microcavity representation, the resonance in dimer nano-antennas is explained in a common and semi-analytical manner. The plasmonic enhancement and the wavelength mismatching in the optical dimer antenna are naturally embodied in this model. The quality factor of the resonance, which can be influenced by the wavelength and the geometry, is discussed intuitively. The understanding presented in this work could guide the future engineering of the optical dimer antenna.

Controlling the plasmon resonance of single metal nanoparticles by near-field anisotropic nanoscale photopolymerization.

We propose a new approach for tuning the Surface Plasmon (SP) resonance wavelength using hybrid nanoparticles. Our approach is based on nanoscale photopolymerization around metal nanoparticles. The enhanced optical near-field of silver nanoparticles triggers local photopolymerization. As a result, atomic force microscopy reveals two nanoscale polymerized lobes around nanoparticles, with a controlled effective index distribution. A spectral breaking degeneracy of surface plasmon resonance of the nanoparticles has been demonstrated by polarized extinction spectroscopy.

Near-field investigation of porous silicon photoluminescence modification after oxidation in water.

We report on local photo-induced oxidation of porous silicon in water at room temperature. Starting from a nonluminescent sample, the oxidation process induces luminescence which was found to first increase and then decrease as a function of the oxidation time. A clear blue shift is also observed. This effect is believed to be owing to size modification of silicon nanocrystallites and thus is explained in terms of quantum confinement. Optical near-field images and spectrum are used to monitor the photoluminescence modifications after oxidation. As the photoluminescence can be widely tuned in wavelength and intensity, this method offers a way to pattern the emission properties of the sample.

Field localization and enhanced Second-Harmonic Generation in silicon-based microcavities.

High-quality amorphous Silicon Nitride (a-Si(1-x)N(x):H) Fabry-Pérot microcavities can show resonant surface Second Harmonic Generation (SHG) effect. We consider two different layouts of planar microcavities with almost identical linear reflectance and show how the structure geometry can strongly affect SHG yield. In particular, a difference of more than one order of magnitude in the SHG intensity is observed when the fundamental beam is tuned at the cavity resonance frequency. We explain this finding on the basis of a theoretical model taking into account the spatial distribution of the electric fields of the pump and harmonic frequencies inside the structure. A satisfactory matching of experimental data with the theoretical model is obtained by considering the source of second-order nonlinearity as limited to surface contributions.

Spectral degeneracy breaking of the plasmon resonance of single metal nanoparticles by nanoscale near-field photopolymerization.

We report on controlled nanoscale photopolymerization triggered by enhanced near fields of silver nanoparticles excited close to their dipolar plasmon resonance. By anisotropic polymerization, symmetry of the refractive index of the surrounding medium was broken: C infinity v symmetry turned to C2v symmetry. This allowed for spectral degeneracy breaking in particles plasmon resonance whose apparent peak became continuously tunable with the incident polarization. From the spectral peak, we deduced the refractive-index ellipsoid fabricated around the particles. In addition to this control of optical properties of metal nanoparticles, this method opens new routes for nanoscale photochemistry and provides a new way of quantification of the magnitude of near fields of localized surface plasmons.

Surface plasmon characteristics of tunable photoluminescence in single gold nanorods.

Light emission resulting from two-photon excited gold nanoparticles has been proposed to originate from the radiative decay of surface plasmon resonances. In this vein, we investigated luminescence from individual gold nanorods and found that their emission characteristics closely resemble surface plasmon behavior. In particular, we observed spectral similarities between the scattering spectra of individual nanorods and their photoluminescence emission. We also measured a blueshift of the photoluminescence peak wavelength with decreasing aspect ratio of the nanorods as well as an optically tunable shape-dependent spectrum of the photoluminescence. The emission yield of single nanorods strongly depends on the orientation of the incident polarization consistent with the properties of surface plasmons.

Reflection-mode scanning near-field optical microscopy using an apertureless metallic tip.

Recently, a reflection-mode near-field optical microscope with an apertureless tungsten tip has been introduced and 100-nm resolution has been achieved [R. Bachelot, P. Gleyzes, and A. C. Boccara, Microsc. Microanal. Microstruc. 5, 389-397 (1994)]. The optical signal is recorded in parallel with a tapping-mode atomic force microscope signal. By showing several images here, we confirm the capabilities of this device and clearly demonstrate a 20-nm (~lambda/35) resolution that has been achieved with smaller tips. A study of these images shows that both the topography and the near electromagnetic field of the sample can be independently probed by this device. Additionally, we discuss the principle of our approach, notably on the basis of interference phenomena between a Rayleigh scatterer and its image through the reflecting surface, and some of the setup's experimental characteristics are presented.

Infrared-reflection-mode near-field microscopy using an apertureless probe with a resolution of lambda/600.

We report a near-field optical microscopy experiment at lambda = 10.6 microm, using an apertureless metallic tip functioning simultaneously in the atomic force microscopy tapping mode. The 17-nm optical resolution (lambda/600) that we achieved confirms the validity and the potential of this concept for numerous applications.

Nano-patterning photosensitive polymers using local field enhancement at the end of apertureless SNOM tips.

We show experimentally that local optical field enhancement can occur at the end of an apertureless SNOM tip illuminated by an external light source. Our approach consists in the use of a photosensitive polymer, placed in the tip near-field, to record intensity distribution in the vicinity of the tip end. The excited nanometre-size light source permits us to produce nano-patterns on the polymer surface which are then characterized by atomic force microscopy. Experimental images show the influence, on the field enhancement, of three important experimental parameters: the polarization state of the incident light, the geometry of the external illumination and the radius of curvature of the tip apex. These results are shown to be in good agreement with two-dimensional numerical calculations based on the finite-difference time-domain method. We show preliminary nanometre-size patterns created by this nano-source excited at a metallic tip extremity and discuss the potential of this approach for near-field optical lithography.

Integration of micrometer-sized polymer elements at the end of optical fibers by free-radical photopolymerization.

A simple method of manufacturing micrometer-sized polymer elements at the extremity of both single-mode and multimode optical fibers is reported. The procedure consists of depositing a drop of a liquid photopolymerizable formulation on a cleaved fiber and using the light that emerges from the fiber to induce the polymerization process. After exposure and rinsing a polymer tip is firmly attached to the fiber as an extension of the fiber core. It is shown that the tip geometry can be adjusted by the variation of basic parameters such as the geometry of the deposited drop and the conditions of drop illumination. When this process is applied to a multimode fiber three-dimensional molds of the fiber's linearly polarized modes can be obtained. The process of polymer-tip formation was simulated by a numerical calculation that consisted of an iterative beam-propagation method in a medium whose refractive index is time varying. It is shown that this process is based on the gradual growth, just above the fiber core, of an optical waveguide in the liquid formulation. Experimental data concerning two potential uses of the tipped fibers are presented.

Apertureless scanning near-field optical microscopy for ion exchange channel waveguide characterization.

We report the characterization of an integrated Ag+/Na+ ion exchange waveguide realized in a silicate glass substrate using apertureless scanning near-field optical microscopy. Our experimental set-up is based on the combination of a commercial atomic force microscope with an optical confocal detection system. Thanks to this system, the topography and evanescent optical field at the waveguide top surface are mapped simultaneously. Also, the process of apertureless scanning near-field optical microscopy image formation is analysed. In particular, fringe patterns appearing in the image reveal the intrinsic interferometric nature of the collected signal, due to interference between the field scattered by the tip end and background fields related to guide losses. We give a quantitative interpretation of these fringes. Evanescent intensity mapping on the sample surface allowed us to extract physical waveguide parameters. In particular, it shows an unambiguous multimode beat along the waveguide propagation axis. Furthermore, we show that analysis of this intensity profile reveals back-reflection effects from the waveguide exit facet. The resulting standing waves pattern allows us to evaluate the eigenmode propagation constants.