Rapid fabrication of micro-nanometric tapered fiber lens and characterization by a novel scanning optical microscope with submicron resolution.

In numerous applications of optical scanning microscopy, a reference tapered fiber lens with high symmetry at sub-wavelength scale remains a challenge. Here, we demonstrate the ability to manufacture it with a wide range of geometry control, either for the length from several hundred nanometers to several hundred microns, or for the curvature radius from several tens of nanometers to several microns on the endface of a single mode fiber. On this basis, a scanning optical microscope has been developed, which allows for fast characterization of various sub-wavelength tapered fiber lenses. Focal position and depth of microlenses with different geometries have been determined to be ranged from several hundreds of nanometers to several microns. FDTD calculations are consistent with experimental results.

Integration of polymer microlens array at fiber bundle extremity by photopolymerization.

We present a novel route to directly integrate an array of microlenses at the extremity of an optical fiber bundle. The method is based on photopolymerization at the end of the fiber. The method is based on the control of exposure dose and volume of the deposited droplet of photopolymerizable formulation. Optical properties of the integrated microlenses are discussed on the basis of FDTD calculations.

Bidimensional near-field sampling spectrometry.

We report on a concept of compact optical Fourier-transform spectrometer based on bidimensional (2D) spatial sampling of a confined interferogram. The spectrometer consists of a nanostructured glass surface on which two light beams interfere in total internal reflection. Subwavelength spatial sampling of the interferogram near field is achieved by introducing a tilt angle between a 2D array of optical nanoantennas and the interferogram pattern. The intensity distribution of the scattered light is recorded on a 2D CCD camera, and a one-dimensional Fourier transform of the interferogram is used to recover the input light spectrum. Experimental results show a wide spectral bandwidth in the visible range, down to 380 nm, with spectral resolution of 1.6 nm around 780 nm.

Real-space observation of spectral degeneracy breaking in a waveguide-coupled disk microresonator.

We report on the real-space observation of resonant frequency splitting in a high-Q waveguide-coupled silicon-on-insulator microdisk resonator. Phase sensitive near-field analysis reveals the stationary nature of the two resonant states, and spectral investigations clearly show their orthogonality. These measurements emphasize the role of the coupling waveguide in this splitting phenomenon. The symmetry of the two stationary whispering gallery modes is clearly observed and is found to follow the axial symmetry of the waveguide-coupled microdisk as it has been reported by earlier theoretical predictions.

Tuning of an optical dimer nanoantenna by electrically controlling its load impedance.

Optical antennas are elementary units used to direct optical radiation to the nanoscale. Here we demonstrate an active control over individual antenna performances by an external electrical trigger. We find that by an in-plane command of an anisotropic load medium, the electromagnetic interaction between individual elements constituting an optical antenna can be controlled, resulting in a strong polarization and tuning response. An active command of the antenna is a prerequisite for directing light wave through the utilization of such a device.

High heterogeneity of plasma membrane microfluidity in multidrug-resistant cancer cells.

Diffusion-time distribution analysis (DDA) has been used to explore the plasma membrane fluidity of multidrug-resistant cancer cells (LR73 carcinoma cells) and also to characterize the influence of various membrane agents present in the extracellular medium. DDA is a recent single-molecule technique, based on fluorescence correlation spectroscopy (FCS), well suited to retrieve local organization of cell membrane. The method was conducted on a large number of living cells, which enabled us to get a detailed overview of plasma membrane microviscosity, and plasma membrane micro-organization, between the cells of the same line. Thus, we clearly reveal the higher heterogeneity of plasma membrane in multidrug-resistant cancer cells in comparison with the nonresistant ones (denoted sensitive cells). We also display distinct modifications related to a membrane fluidity modulator, benzyl alcohol, and two revertants of multidrug resistance, verapamil and cyclosporin-A. A relation between the distribution of the diffusion-time values and the modification of membrane lateral heterogeneities is proposed.

Plasmonic electromagnetic hot spots temporally addressed by photoinduced molecular displacement.

We report the observation of temporally varying electromagnetic hot spots in plasmonic nanostructures. Changes in the field amplitude, position, and spatial features are induced by embedding plasmonic silver nanorods in the photoresponsive azo-polymer. This polymer undergoes cis-trans isomerization and wormlike transport within resonant optical fields, producing a time-varying local dielectric environment that alters the locations where electromagnetic hot spots are produced. Finite-difference time-domain and Monte Carlo simulations that model the induced field and corresponding material response are presented to aid in the interpretation of the experimental results. Evidence for propagating plasmons induced at the ends of the rods is also presented.

Phase sensitive optical near-field mapping using frequency-shifted laser optical feedback interferometry.

The use of laser optical feedback Imaging (LOFI) for scattering-type scanning near-field optical microscopy (sSNOM) is proposed and investigated. We implement this sensitive imaging method by combining a sSNOM with optical heterodyne interferometry and the dynamic properties of a B class laser source which is here used both as source and detector. Compared with previous near field optical heterodyne experiments, this detection scheme provides an optical amplification that is several orders of magnitude higher, while keeping a low noise phase-sensitive detection. Successful demonstration of this complex field imaging technique is done on Silicon on Insulator (SOI) optical waveguides revealing phase singularities and directional leakage.

Polarization-sensitive printing of surface plasmon interferences.

Surface plasmon assisted lithography is currently a matter of growing interest since it allows nanopatterning in photosensitive films without being restricted by the diffraction limit. Using specially designed metallic nanostructures coated with a photosensitive azobenzene-dye polymer, we have generated a plasmon interference field in the polymer layer. The atomic force microscopy observation of the azo-dye polymer surface after exposure exhibits complex topographies which are found to be well explained by an analytically computed surface plasmon interference model that highlights the polarization influence on the pattern shape. The results presented here are believed to be a first step towards a new approach of high resolution plasmonic nanolithography based on the use of longitudinal field components.

Heterodyne apertureless near-field scanning optical microscopy on periodic gold nanowells.

Heterodyne detection for apertureless near-field scanning optical microscopy was used to study periodic gold nanowell arrays. Optical near-field amplitude and phase signals were obtained simultaneously with the topography of the gold nanowells and with different polarizations. Theoretical calculations of the near-fields were consistent with the experiments; in particular, the calculated amplitudes were in especially good agreement. The heterodyne method is shown to be particularly effective for these types of periodic photonic structures and other highly scattering media, which can overwhelm the near-field scattered signal when conventional apertureless near-field scanning optical microscopy is used.

Photoresponsive polymers for topographic simulation of the optical near-field of a nanometer sized gold tip in a highly focused laser beam.

The local perturbation of a diffraction-limited spot by a nanometer sized gold tip in a popular apertureless scanning near-field optical microscopy (ASNOM) configuration is reproduced through topography changes in a photoresponsive polymer. Our method relies on the observation of the photochemical migration of azobenzene molecules grafted to a polymer placed beneath the tip. A local molecular displacement has been shown to be activated by a gold tip as a consequence of the lateral surface charge density present at the edges of the tip's end, resulting from a strong near-field depolarization predicted by theory.

Heterodyne detection of guided waves using a scattering-type Scanning Near-Field Optical Microscope.

An inherent problem to the study of waveguides with strong propagation losses by Scattering-type Scanning Near field Optical Microscopy is the coherent optical background field which disrupts strongly the weak detected near-field signal. We present a technique of heterodyne detection allowing us to overcome this difficulty while amplifying the near field signal. As illustrated in the case of a highly confined SOI structure, this technique, besides the amplitude, provides the local phase variation of the guided field. The knowledge of the complex field cartography leads to the modal analysis of the propagating radiation.

Electromagnetic interactions in plasmonic nanoparticle arrays.

Single two-dimensional planar silver arrays and one-dimensional linear gold chains of nanoparticles were investigated by dark-field surface plasmon spectroscopy and studied as a function of interparticle distance, particle size, and number of particles. In agreement with recent theoretical predictions, a red shift of the surface plasmon resonance occurring in two-dimensional arrays was found for lattice spacings below 200 nm. This red shift is associated with a significant broadening of the resonance and is attributed to the onset of near-field interactions. We found that the relative contributions of the long-range and short-range interactions in two-dimensional arrays of particles are fundamentally different to those occurring in individual linear chains.

Probing photonic and optoelectronic structures by apertureless scanning near-field optical microscopy.

This report presents the Apertureless Scanning Optical Near-Field Microscope as a powerful tool for the characterization of modern optoelectronic and photonic components with sub-wavelength resolution. We present an overview of the results we obtained in our laboratory over the past few years. By significant examples, it is shown that this specific probe microscopy allows for in situ local quantitative study of semiconductor lasers in operation, integrated optical waveguides produced by ion exchange (single channel or Y junction), and photonic structures.

Biopolymers phase separation monitored by a plasmonic sensor.

We report here a real-time study of interactions induced phase separation between ß-lactoglobulin (BLG) and Acacia gum (AG) by analyzing the localized surface plasmon resonance of silver nanoparticles. We showed that the binding of BLG to AG is accompanied by refractive index changes, in relation with optical properties and structural changes of the complexes formed.

Off-Resonant Optical Excitation of Gold Nanorods: Nanoscale Imprint of Polarization Surface Charge Distribution.

We report on the nanoscale optical characterization of gold nanorods irradiated out of their plasmonic resonance. Our approach is based on the reticulation of a photopolymerizable formulation locally triggered by enhanced electromagnetic fields. The tiny local field enhancement stems from the surface polarization charges associated with the electric field discontinuity at the metal/dielectric interface. This allows us to get a nanoscale signature of the spatial distribution of the surface charge density in metallic nanoparticles irradiated off-resonance.

Enhancement and quenching regimes in metal-semiconductor hybrid optical nanosources.

We report on the emission of hybrid nanosources composed of gold nanoparticles coupled with quantum dots. The emission relies on energy transfer from the quantum dots to gold nanoparticles which could be de-excited through radiative plasmon relaxation. The dependence of the emission efficiency is studied systematically as a function of the size of gold nanoparticles and interdistance between gold nanoparticles and quantum dots. We demonstrate a size-dependent transition between quenching and enhancement and a nonradiative energy transfer from the quantum dots to the gold nanoparticles.

Quantitative analysis of localized surface plasmons based on molecular probing.

We report on the quantitative characterization of the plasmonic optical near-field of a single silver nanoparticle. Our approach relies on nanoscale molecular molding of the confined electromagnetic field by photoactivated molecules. We were able to directly image the dipolar profile of the near-field distribution with a resolution better than 10 nm and to quantify the near-field depth and its enhancement factor. A single nanoparticle spectral signature was also assessed. This quantitative characterization constitutes a prerequisite for developing nanophotonic applications.

Design of a compact static Fourier transform spectrometer in integrated optics based on a leaky loop structure.

A compact static Fourier transform spectrometer for integrated optics is proposed. It is based on a plane leaky loop structure combined with a plane waveguide. The interference pattern produced in the loop structure leaks outside of it and is guided in the plane waveguide to the photodetector array. This configuration allows one to control the shape of the field pattern at the end of the plane waveguide. A large fringe pattern with a high interference fringe contrast is obtained. A two-dimensional model based on an aperiodic Fourier modal method is used to modelize the coupling between the bent and the plane waveguides, completed with the Helmholtz-Kirchhoff propagation. This concept gives access to plan and compact spectrometers requiring only a single low-cost realization process step. The simulation has been done to realize a spectrometer in glass integrated optics (Deltalambda=6.1 nm at 1500 nm).

Multiscale model for photoinduced molecular motion in azo polymers.

Light-induced isomerization processes in azobenzene-containing polymers produce mass transport that is of much interest for nanoscale imaging and lithography. Yet, despite the development of numerous models to simulate the mass transport mechanism, no model precisely describes all the experimental observations. We develop a new statistical approach that correctly reproduces light-driven mass motion in azobenzene-containing polymers with a high degree of accuracy. Comparisons with experiments show that our model predicts the nanoscale topographic modifications for many different incident field configurations, including optical near-fields produced by plasmonic structures with complex polarization states. In particular, the model allows the detailed molecular motions that lead to these topographic modifications to be identified.