Electronic and geometrical structures of Co<sub>n</sub> (n=6, 8, 9, 10, 12, 14) particles have been studied using both the density
functional theory and Hartree-Fock calculations. Structural and electronic differences to the corresponding clusters are
presented. We have tried to recognize which structure (fcc or bcc) is more preferable for these particles. A four-fold and
higher coordination of the Co atoms was found to be the particularly preferable coordination environment in small Co<sub>n</sub>
species. The key element of the Co particle is alsosuggested.
We report on compact and simple refractive index sensors suitable for measuring indexes in the 1.320-1.432 range with
high resolution. The devices are based on modal interference and consist of a stub of large-mode area photonic crystal
fiber spliced to standard single mode fiber. In the splice regions the voids of the holey fiber are fully collapsed which
allows the coupling and recombination of core and cladding modes. The devices are robust and highly stable over time.
The interference patterns are observed in a broad wavelengths range. The devices operate in both reflection or in
We report an in-reflection photonic crystal fiber (PCF) interferometer which exhibits high sensitivity to different volatile
organic compounds (VOCs), without the need of any permeable material. The interferometer is compact, robust, and
consists of a stub of PCF spliced to standard optical fiber. In the splice the voids of the PCF are fully collapsed, thus
allowing the excitation and recombination of two core modes. The device reflection spectrum exhibits very regular
interference pattern which shifts differently when the voids of the PCF are infiltrated with VOC molecules. The volume
of voids responsible for the shift is around 500 picoliters whereas the detectable levels are in the nanomole range.
We report an all-optical fiber hydrogen sensor based on absorption changes of evanescent
fields caused by an annealed Pd/Au thin film. The sensor consists of a small piece of standard
single-mode fiber (SMF) coated with a Pd/Au thin film sandwiched between two multimode
fibers (MMFs). Due to core diameter mismatch the SMF cladding guides light. When the
device is exposed to hydrogen the layer refractive index diminishes and causes attenuation
changes of the evanescent fields. Adding gold to palladium allows the fabrication of fast,
durable, and reliable sensors suitable for the detection of hydrogen concentration below the
This work describes the infiltration of a polymeric solution into porous Si structures towards the fabrication of
tunable photonic crystals (PC) and microcavities for photonics applications. The tunability is achieved by infiltrating the
porous silicon based PCs by active organic materials, such as an emissive and nonlinear polymer called 2-methoxy-5-(2-
ethylhexyloxy)-p-phenylenevinylene (namely MEH-PPV). This preliminary work shows the infiltration of this polymeric
solution into PC based on macroporous Si structure as well as in microcavities based on multiple layers of microporous
Si. The solidification of the polymer was obtained by the evaporation of the solvent. Various techniques of infiltration
were studied to obtain an optimized filling of the pores. The infiltration was then characterized using photoluminescence
measurements. Finally, we will report on the study of third harmonic generation (THG) in samples of porous silicon
microcavity infiltrated with MEHPPV. The k-domain THG spectroscopy was applied and an increase of the THG
intensity up to an order of magnitude was achieved for the filled microcavity.
The fabrication of a compact all-fibre modal interferometer that can be used to sense different parameters is reported.
The device consists of a tapered large-mode-area microstructured optical fibre with collapsed air holes over a localized
region. The tapering is carried out by slowly elongating the fibre while it is heated with a high-temperature oscillating
flame torch. This non adiabatic method allows the collapsing of the air holes and transforms a section of the
microstructured fibre into a solid one. As a consequence the fundamental HE<sub>11</sub> mode is coupled to the HE<sub>1m</sub> cladding
modes which can beat or interfere. This makes the transmission of the device versus the wavelength to exhibit an
oscillatory pattern. Such a pattern shifts with strain, high temperature, or refractive index. The device is compact, can
operate in a broad range of wavelengths, and can be fabricated in a few minutes which makes it attractive for optical
We report on the electromagnetic interactions between a two-dimensional periodic arrangement of resonant gold nanoparticles and a flat gold metallic film. We observe multi-peaks in the extinction spectra attributed to resonant modes of the hybrid system, resulting from the coupling between the Localized Plasmon of the nanoparticles with the underlying Surface Plasmon Mode. Simulations based on the Fourier modal method give good agreement with the experimental measurements and allow for the identification of the respective contributions.
We investigate the local field spectroscopy of gold dimers by Two-Photon Photoluminescence (TPL) microscopy. A direct comparison with far-field scattering measurements shows that TPL provides additional data on the structure modes of major importance for their use in SERS, enhanced fluorescence and sensing.
We report here on the results of the characterization of a novel -OPhCN substituted thiophenic monomer, and of the obtained copolymers between the latter and the plastifying comonomer 3-hexylthiophene. The polymer evidences an excellent filmability from various organic solvents as well as an enhanced photoluminescence. The characteristics of the polymer were characterized by FTIR and XRD as well as photoluminescence. A bandgap of 2.0eV was obtained which corresponds to orange emission. Furthermore, a single layer organic device was fabricated and resulted in bright stable electroluminescence at room temperature. All of the results indicate that this polymer is a promising emissive material for application in light-emitting devices (LEDs).
Following the recent advances in nano-optics, optical manipulation by evanescent fields instead of conventional propagating fields has recently awaken an increasing interest. The main advantages of using low dimensionality fields are (i) the possibility of integrating on a chip applications involving optical forces but also (ii) the absence of limitation by the diffraction limit for the trapping volume. Previous works have investigated theoretically and experimentally the guiding of dielectric and metallic beads at an interface sustaining an extended surface wave. In this work, we study theoretically the radiation forces exerted on Rayleigh dielectric beads under local evanescent illumination. Especially, we consider the configuration where a three-dimensional Gaussian beam is totally reflected at the interface of a glass prism. The results point out the illumination parameters where the gradient forces exceed the scattering force and allow for a stable trapping. The effect of the Goos-Haenchen shift on the location of the trapping site is also discussed.
We report quantitative measurements of the radiation forces exerted on a micrometer dielectric sphere by a Surface Plasmon Polariton (SPP) excited at a gold/water interface. We separate the contributions of the two constituents of the plasmon wave - the electromagnetic field and the charge-density oscillations - to the total radiation force. Measurements performed with a Photonic Force Microscope (PFM) show an enhanced attraction to the surface compared to a conventional evanescent wave at the dielectric interface (10<sup>2</sup> enhancement factor).
The trapping of micro-objects by optical radiation forces, so-called optical tweezers, has become widely used in physical, chemical and biological experiments where accurate and non-invasive manipulation is required. Recent advances in beam shaping render it possible for instance to rotate or to dynamically manipulate independently several elements. Today, one of the remaining challenges of conventional optical tweezers is the direct manipulation of systems with sizes belonging to the sub-wavelength or Rayleigh regime. Indeed, the diffraction limit prevents in that case from achieving a commensurable trapping volume and thus does not allow for minimizing the fluctuations in position of the trapped object due to its strong Brownian motion. In order to overcome this limitation, it has been proposed to use evanescent fields instead of the usual propagating fields. Recent advances in optics of noble metal nano-structures have recently provided new configurations to achieve nano-optical tweezers. Especially, tightly localized modes resulting from the coupling between resonant noble metal nanostructures may offer the gradient forces able to trap and manipulate Rayleigh objects. In this work, we calculate the radiation forces exerted on a nanometric dielectric sphere when exposed to a patterned optical near-field landscape at an interface decorated with resonant gold nanostructures. By comparing their magnitude with other forces that affect the movement of the particle, we discuss the practical ability of our configuration for multiple parallel optical manipulation.
In the ongoing general trend for miniaturization, there is an
increasing interest in the manipulation of electromagnetic fields
at the nanometer scale. A main obstacle to this goal is the
diffraction limit that prevents from focussing light down to
volumes much smaller than the incident wavelength. In order to
overcome this limitation, it has been proposed to deal with
evanescent fields instead of the conventional propagating beams.
Especially, plasmon fields bound at a noble metal interface or
around metal nanostructures have shown to be very suitable to
control the light confinement down to the nanometer scale. In this
work we investigate the near-field coupling in finite metal
particle chain geometry. The Green Dyadic method is used to
demonstrate that high enhancement factors can be achieved by
exploiting the in-plane forward scattering of the particles, with
no need for cumbersome geometries with nanometer features.
Macroporous silicon structures have been fabricated by electrochemical etching. Such fabrication process is known to result in the presence of a thin microporous Si layer at the walls of the macropores and at the surface. Photoluminescence measurements conducted in plan-view and cross-section exhibit a wide emission peak around 650nm which can be attributed to the microporous Si. The combination of a photonic crystal and a light emitter in one structure represents a potential for applications that has not been studied previously. This preliminary study shows the influence of the main fabrication parameters, namely the current density and the etchant solution, on the emission properties of the microporous Si layer.
Resonant noble metal nanoparticles with dimensions of a few tens of nanometers and sustaining Localized Surface Plasmon (LSP) modes have been recently proposed as good candidates for increasing both integration and sensitivity compared to conventional extended thin metal films. Very recently several groups have reported results of sensing with a single nano-particle.
The study we present contains two main parts. First, using randomly distributed colloidal gold spheres, we demonstrate the ability of LSP sensors for monitoring quantitatively and without the use of any label, the binding between small organic molecules and antibodies with real-life applications. In a second part, the Fourier Modal Method (FMM) is used to model controlled geometries of particles that allow for optimizing the sensor properties. In particular, we show that the electromagnetic coupling within a periodic 2D particle array can be optimized to increase the field localization and thus the sensitivity of the sensor.
We demonstrate an ultra sensitive method for Two Photon Fluorescence (TPF) excitation using resonant Grating Waveguide Structures (GWS). In its basic configuration, a GWS consists of a substrate, a waveguide layer and an additional grating layer. When illuminated with laser light under resonant conditions, the GWS reflects all light and leads to very high local surface intensities. This field enhancement can be exploited for TPF spectroscopy, without the need for a highly intense, focused laser light. We present the enhanced TPF signal obtained from a 23 nM drop of tetramethylrhodamine (TMR) on the top of high-finesse resonant polymeric GWS. The resonant behaviour of the GWS was tested for normal incidence with TE polarization illumination. As expected, the transmission spectral profile has a dip at resonant wavelength. The TPF spectra of TMR molecules were observed for different excitation wavelengths. Close to resonance, TPF intensity increases and the maximum signal is obtained when the excitation wavelength coincides with the resonance wavelength of the GWS. These results clearly indicate that the huge field localization at grating surface is responsible for the TPF excitation. We obtained a detection limit down to picomolar concentration of the dye molecules, offering the possibility of a highly sensitive, compact and non-destructive tool for widespread biochemical applications.
We report the sub-wavelength patterning of the optical near-field
by total internal reflection illumination of a regular array of
resonant gold nano-particles. Under appropriate conditions, the
in-plane coupling between localized surface plasmon fields
gives rise to sub-wavelength light spots between the structures.
Measurements performed with an Apertureless Scanning Near-Field
Optical Microscope (ASNOM) show good agreement with theoretical
predictions based on the Green dyadic method. This concept might
offer a convenient way to elaborate extended optical trap
landscapes for manipulation of sub-micrometer systems.