S. Reynaud

Evidence of surface plasmon resonance in ultrafast laser-induced ripples.

The sensitivity of grating-coupled Surface Plasmon Polaritons (SPPs) on metallic surface has been exploited to investigate the correlation between ripples formation under ultrashort laser exposure and SPPs generation conditions. Systematic examination of coupling of single ultrashort laser pulse on gratings with appropriate periods ranging from 440 nm to 800 nm has been performed. Our approach reveals that a surface plasmon is excited only for an appropriate grating period, the nickel sample exhibits fine ripples pattern, evidencing the plasmonic nature of ripples generation. We propose a systematic investigation supported by a comprehensive study on the obtained modulation of such a coupling efficiency by means of a phenomenological Drude-Lorentz model which captures possible optical properties modification under femtosecond irradiation.

Reversible and Irreversible Laser Microinscription on Silver‐Containing Mesoporous Titania Films

Silver species adsorbed on colloidal titania have been known for a long time to exhibit photochromism. [ 1 ] They reversibly change color in response to light exposure. Under UV illumination, i.e. at photon energy greater than the band-gap of TiO 2 (3.2 eV), the photogenerated electrons of titania migrate towards the positively charged Ag(I) species and reduce them, leading to the formation of silver nanoparticles. [ 2 ] These particles exhibit a grey-brown color caused by an inhomogeneous broadening of their surface plasmon resonance, which depends on the nanoparticle size, shape and close environment. [ 3–5a ] Reversibly, under visible light the material can bleach or change color. This multicolour photochromism has been reported recently on nanoporous titania fi lms loaded with silver nanoparticles. [ 5a,b ]

99% efficiency measured in the -1st order of a resonant grating

A resonant diffraction grating comprising a mirror, a dielectric layer and a high index corrugation at the layer-air interface is shown to exhibit off-Littrow the record diffraction efficiency of 99% in the -1st reflected order at 1064 nm wavelength thanks to the excitation of a leaky mode of the layer. Such high figure is obtained by a grating 5 to 10 times shallower than in current attempts to realize high efficiency all-dielectric gratings.

Irradiance influence on the multicolor photochromism of mesoporous TiO2 films loaded with silver nanoparticles

The photochromism of heterogeneous silver nanoparticles grown thermally in mesoporous TiO2 films is investigated under visible laser illuminations with different irradiances. At low irradiance, a selective oxidation occurs that changes the film color in accordance with the illuminant wavelength. At higher irradiance, the oxidation of large nanoparticles is going with the formation of many small nanoparticles in their vicinity, which gives the films an orangy color whatever the wavelength. This color fades after long exposure times at a laser line. The laser polarization has also an influence on the dichroism of the colored films.

Self-organized growth of metallic nanoparticles in a thin film under homogeneous and continuous-wave light excitation

Using a monochromatic plane wave to generate periodic arrays of metallic nanoparticles with tunable features buried in thin films is the original work we report here. We focus on the way such waveguiding metallic photonic crystals can self-emerge from thin films homogeneously loaded with metallic precursors under continuous-wave and homogeneous laser excitation. This paper fully describes the conditions leading to the formation of periodic structures and highlights the role of several parameters in the underlying physical mechanisms. The laser exposure parameters, especially, fix the geometrical and optical properties of the generated structures. Grating lines are parallel to the laser polarization and the period is directly linked to the laser wavelength. Both electron resonances of metal nanoparticles and optical resonances of guided modes interact to form the periodic patterns under homogeneous exposure. A model, based on the coupled mode theory, can be proposed to predict the spontaneous generation of such periodic nanostructures. It concludes that the guided waves exponentially enhance during illumination due to a positive feedback loop with the ordered growth of particles. This process opens up new fabrication techniques for making optical devices and may find applications in various fields such as polarization imaging, displays, security or lighting.

Dichroic colored luster of laser-induced silver nanoparticle gratings buried in dense inorganic films

This paper deals with the colorimetric properties of silver nanoparticle gratings buried in a dense titania film that result from a continuous wave laser-induced self-organization process. The samples exhibit shining colors in the direction of the specular reflection, which are very sensitive to polarization. We show that a large color gamut and a tunable dichroism can be reached by varying the exposure conditions. We also discuss the physical meaning of the observed variations in the dichroism. This laser process produces a single pass marking with a micrometer resolution and could be useful for developing innovative solutions in fields like active color displays, security, polarization imaging, or design.

Graphene-based textured surface by pulsed laser deposition as a robust platform for surface enhanced Raman scattering applications

We have developed a surface enhanced Raman scattering (SERS)-active substrate based on gold nanoparticles-decorated few-layer (fl) graphene grown by pulsed laser deposition. Diamond-Like Carbon film has been converted to fl-graphene after thermal annealing at low temperature. The formation of fl-graphene was confirmed by Raman spectroscopy, and surface morphology was highlighted by scanning electron microscopy. We found that textured fl-graphene film with nanoscale roughness was highly beneficial for SERS detection. Rhodamine 6G and p-aminothiophenol proposed as test molecules were detected with high sensitivity. The detection at low concentration of deltamethrin, an active molecule of a commercial pesticide was further demonstrated.

Subwavelength cylindrical grating by holistic phase-mask coordinate transform

A periodic grating with an integer number of periods is fabricated at the resist-coated wall of a cylinder by exposing a circularly symmetrical planar high index phase mask to a cylindrical wave. This extends the spatial coherence features easily achievable in a planar 2D space to the 3D space of cylindrical waves and elements.

Permanent dichroic coloring of surfaces by laser-induced formation of chain-like self-organized silver nanoparticles within crystalline titania films

Laser-induced periodic surface structures, also known as ripples, have been investigated for a long time on various materials. They are self-organized grating-like structures that form spontaneously upon irradiation with a single laser beam; their formation requiring a high temperature rise, it generally results from interactions with ultrashort laser pulses. Recently, similar phenomena leading to periodic changes in the morphology of metallic nanoparticles embedded in polymer films have been reported under femtosecond laser exposure. Here, we demonstrate that chain-like self-organized silver nanoparticles can be grown, from an ionic silver precursor, within titania films under continuous wave visible laser beam. The nanoparticle chains are periodically spaced with a period that depends on the wavelength and are oriented parallel to the laser polarization. The samples present no significant surface modulation since all of the grown nanoparticles are located at the substrate-film interface, protected by a crystallized TiO2 film. SEM, TEM, HRTEM and EDX characterizations of the film nanostructuring are shown. Due to the coupling of incident light to a guided mode of the TiO2 film by the first diffraction order of the NP grating, such samples exhibit a strong dichroism whose characteristics depend on the laser exposure conditions. Color changes and spectral variations with polarization are measured and interpreted. The high stability of such colored films under high temperature rises or high intensity UV or visible exposures is demonstrated; it makes them good candidates for colored data storage.

Polarization-driven self-organization of silver nanoparticles in 1D and 2D subwavelength gratings for plasmonic photocatalysis.

One of the main challenges in plasmonics is to conceive large-scale, low-cost techniques suitable for the fabrication of metal nanoparticle patterns showing precise spatial organization. Here, we introduce a simple method based on continuous-wave laser illumination to induce the self-organization of silver nanoparticles within high-index thin films. We show that highly regular and homogeneous nanoparticle gratings can be produced on large areas using laser-controlled self-organization processes. This very versatile technique can provide 1D and 2D patterns at a subwavelength scale with tunable features. It does not need any stabilization or expensive devices, such as those required by optical or electron lithography, and is rapid to implement. Accurate in-plane and in-depth characterizations provide valuable information to explain the mechanisms that lead to pattern formation and especially how 2D self-organization can fall into place with successive laser scans. The regular and homogeneous 2D self-organization of metallic NPs with a single laser scan is also reported for the first time in this article. As the reported nanostructures are embedded in porous TiO2, we also theoretically explore the interesting potential of organization on the photocatalytic activity of Ag-NP-containing TiO2 porous films, which is one of the most promising materials for self-cleaning or remediation applications. Realistic electromagnetic simulations demonstrate that the periodic organization of silver nanoparticles can increase the light intensity within the film more than ten times that produced with randomly distributed nanoparticles, leading as expected to enhanced photocatalytic efficiency.