Engineering the wavefront of light in random media allows the control of wave propagation in space and time by exploiting the spatial and spectral degrees of freedom introduced by multiple scattering (M. Mounaix et al, Phys. Rev. Lett. 116, 253901 (2016)). To apply this far-field control strategy and focus electromagnetic energy at the nanoscale, it is necessary to introduce scatterers that feature strongly enhanced and confined optical fields such as plasmonic nanoantennas. In particular, semi-continuous gold films close to the percolation threshold feature high local field enhancements (S. Gresillon et al, Phys. Rev. Lett. 82, 4520 (1999)) but also propagating surface plasmon waves that can be controlled using a spatial light modulator (P. Bondareff et al, ACS Photonics 2, 1658 (2015)). In this presentation, we demonstrate how controlling the phase of an incoming pulsed laser on a chosen 10 µm x 10 µm area of a random plasmonic metasurface allows us to optimize the two-photon luminescence (TPL) of gold at a given position of the sample. The optimized TPL intensities, that are associated with strong local field enhancements, are increased by a factor of 50 for semi-continuous films that are close to percolation compared to samples far from it, demonstrating that the morphology and randomness of the plasmonic film play an essential role in the control of nonlinear luminescence. Furthermore, we show that TPL intensities can be enhanced at any position of a percolated film, opening exciting perspectives for the wavefront engineering of local field enhancements in random plasmonic metasurfaces.
Single biomolecule behaviour can reveal crucial information about processes not accessible by ensemble
measurements. It thus represents a real biotechnological challenge. Common optical microscopy approaches
require pico- to nano-molar concentrations in order to isolate an individual molecule in the observation
volume. However, biologically relevant conditions often involve micromolar concentrations, which impose a
drastic reduction of the conventional observation volume by at least three orders of magnitude. This
confinement is also crucial for mapping sub-wavelength heterogeneities in cells, which play an important role
in many biological processes. We propose an original approach, which couples Fluorescence Correlation
Spectroscopy (FCS), a powerful tool to retrieve essential information on single molecular behaviour, and
nano-fakir substrates with strong field enhancements and confinements at their surface. These
electromagnetic singularities at nanometer scale, called "hotspots", are the result of the unique optical
properties of surface plasmons. They provide an elegant means for studying single-molecule dynamics at
high concentrations by reducing dramatically the excitation volume and enhancing the fluorophore signal by
several orders of magnitude. The nano-fakir substrates used are obtained from etching optical fiber bundles
followed by sputtering of a gold thin-film. It allows one to design reproducible arrays of nanotips.
Active plasmonic devices are much promising for optical devices and circuits at the nanoscale. We show that
single nanoparticles coupled to metallic surfaces are good candidates for integrated components with
nanometric dimensions. The localized plasmon of the nanoparticle launches propagating surface plasmons in
the metallic thin film. Direct particle observation using leaky wave microscope geometry permits easy
detection through the interference of the direct transmitted excitation light and the surface plasmon leaky
mode. Investigations of the optical response of a nanoparticle deposited on metallic thin metal films reveals
unexpectedly high transmission of light associated to contrast inversion in the images.
In our attempt to reveal highly localized field enhancements on random metallic films using near-field
scattering probe microscopy we experimentally demonstrated the existence of narrow peaks when using a
monochromatic illumination. In order to get a better understanding of the second harmonic generation taking
place on such films we have undertaken the same kind of near-field experiments using femtosecond lasers
sources with high peak power able to induce the non linear response. These lasers have a spectral bandwidth
associated with the pulse duration, which is in the femtosecond range. With such spectral broadening we have
observed, as expected, a spatial broadening of the peaks at ω, which spread over distances in the 100-500 nm
range. The behavior of the peaks is quite different at 2 ω: they are found to be always very <i>well localized</i> (~10
nm) despite of the polychromatic nature of the light; moreover there is no clear correlation between the peaks
position at ω and those at 2 ω. This observation indicates, as often underlined in non linear processes, that
<i>coherent interactions</i> involving a distribution of available frequencies in the lasers spectra take place. These
frequencies ωn, coherently induce second harmonic generation as long as ω<sub>n</sub> + ω<sub>m</sub> = 2 ω.
We have developed a new detection scheme based on a scanning near-field optical microscope to image both the linear and the non-linear (e.g. second harmonic generation (SHG)) on surfaces with sub-wavelength resolution. The microscope we used here scatters the evanescent waves that contain sub-wavelength information with the apex of a metallic tip. The resolution of this microscope is directly given by the size of the radius of curvature of the metal tip end
(about 5 nm). Our set-up is applied to the optical study of crystalline films and random metal surfaces. Using thin SBN films
(Strontium Barium Niobate) we demonstrate that near-field optics is a surface sensitive measurement. The ability to perform high quality and highly resolved images is transposed to increase the resolution of imaging in the THz domain. It is also used in the visible domain on random metal films. Several studies have demonstrated that random metal surfaces show localization of the electric field on small area called "hot spots" where the electric field can exceed the applied field by several orders of magnitude. Position of the hot spots depends on film structure, on the polarization and wavelength of the illuminating laser beam. In addition, these random metal films are known to be the source of nonlinear optical effects. We are currently working to precisely locate the respective position of the linear and non-linear hot spots on silver.
Scanning near-field optical microscopy (SNOM) has proven to be very powerful in terms of both resolution and
efficiency. We report on new advances of this technique using metallic tips to scatter the optical field and induce
dramatic field enhancements. We also present a new technique under development using multiple nanometric beads as
scattering probes dispersed in the volume of the sample, rather than using a single tip. The bead positions are
determined in three dimensions (3-D) with a precision better than the diffraction limit, making possible high-resolution
3-D imaging of hollow structures in relatively transparent materials.
It is now known that plasmon oscillations supported by nanostructured metal thin films of fractal morphology, can result in large local fields and strong enhancement of optical phenomena, for example Raman scattering. The localized plasmons, acting like nano-antennas, can concentrate very large electromagnetic energy in nanometer- sized areas, hot spots, and provide particularly strong enhancement of optical responses, in a very broad spectral range. Our new experimental results show up position dependence of the hot spots on the polarization state of the light. Moreover as expected from recent theoretical predictions, on this kind of thin percolating films, there is a dramatic enhancement of the second harmonic generation (2(omega) ) out of the specular directions. This unusual diffuse SHG could be connected to possible chirality of the percolating metallic films, which is expected to manifest itself as change in the hot-spot distribution for the left and right circularly polarized incident light.
Proc. SPIE. 3749, 18th Congress of the International Commission for Optics
KEYWORDS: Metals, Dielectrics, Optical microscopy, Near field scanning optical microscopy, Near field, Spatial resolution, Electromagnetism, Dielectric polarization, Resolution enhancement technologies, Near field optics
Enhanced electromagnetic fields are investigated, both theoretically and experimentally, on two model systems using high spatial resolution. Strong field enhancements at the apex of a tungsten tip illuminated by an external light source are studied as a function of the incident polarization. The surface of percolating random metal- dielectric films consist of several spectral resonances, which have been calculated and are observed here in near field with 10 nm lateral resolution.