Frédéric Bedu

All-Dielectric Colored Metasurfaces with Silicon Mie Resonators

The photonic resonances hosted by nanostructures provide vivid colors that can be used as color filters instead of organic colors and pigments in photodetectors and printing technology. Metallic nanostructures have been widely studied due to their ability to sustain surface plasmons that resonantly interact with light. Most of the metallic nanoparticles behave as point-like electric multipoles. However, the needs of an another degree of freedom to tune the color of the photonic nanostructure together with the use of a reliable and cost-effective material are growing. Here, we report a technique to imprint colored images based on silicon nanoparticles that host low-order electric and magnetic Mie resonances. The interplay between the electric and magnetic resonances leads to a large palette of colors. This all-dielectric fabrication technique offers the advantage to use cost-effective, reliable, and sustainable materials to provide vivid color spanning the whole visible spectrum. The interest and potential of this all-dielectric printing technique are highlighted by reproducing at a micrometer scale a Mondrian painting.

All-Dielectric Silicon Nanogap Antennas To Enhance the Fluorescence of Single Molecules

Plasmonic antennas have a profound impact on nanophotonics as they provide efficient means to manipulate light and enhance light-matter interactions at the nanoscale. However, the large absorption losses found in metals can severely limit the plasmonic applications in the visible spectral range. Here, we demonstrate the effectiveness of an alternative approach using all-dielectric nanoantennas based on silicon dimers to enhance the fluorescence detection of single molecules. The silicon antenna design is optimized to confine the near-field intensity in the 20 nm nanogap and reach a 270-fold fluorescence enhancement in a nanoscale volume of λ(3)/1800 with dielectric materials only. Our conclusions are assessed by combining polarization resolved optical spectroscopy of individual antennas, scanning electron microscopy, numerical simulations, fluorescence lifetime measurements, fluorescence burst analysis, and fluorescence correlation spectroscopy. This work demonstrates that all-silicon nanoantennas are a valid alternative to plasmonic devices for enhanced single molecule fluorescence sensing, with the additional key advantages of reduced nonradiative quenching, negligible heat generation, cost-efficiency, and complementary metal-oxide-semiconductor (CMOS) compatibility.

Optimized 2D array of thin silicon pillars for efficient antireflective coatings in the visible spectrum

Light reflection occuring at the surface of silicon wafers is drastically diminished by etching square pillars of height 110 nm and width 140 nm separated by a 100 nm gap distance in a square lattice. The design of the nanostructure is optimized to widen the spectral tolerance of the antireflective coatings over the visible spectrum for both fundamental polarizations. Angle and polarized resolved optical measurements report a light reflection remaining under 5% when averaged in the visible spectrum for both polarizations in a wide angular range. Light reflection remains almost insensitive to the light polarization even in oblique incidence.

A field effect transistor biosensor with a γ-pyrone derivative engineered lipid-sensing layer for ultrasensitive Fe3+ ion detection with low pH interference.

Field effect transistors have risen as one of the most promising techniques in the development of biomedical diagnosis and monitoring. In such devices, the sensitivity and specificity of the sensor rely on the properties of the active sensing layer (gate dielectric and probe layer). We propose here a new type of transistor developed for the detection of Fe(3+) ions in which this sensing layer is made of a monolayer of lipids, engineered in such a way that it is not sensitive to pH in the acidic range, therefore making the device perfectly suitable for biomedical diagnosis. Probes are γ-pyrone derivatives that have been grafted to the lipid headgroups. Affinity constants derived for the chelator/Fe(3+) complexation as well as for other ions demonstrate very high sensitivity and specificity towards ferric ions with values as high as 5.10(10) M and a detected concentration as low as 50 fM.

Transmission enhancement through square coaxial aperture arrays in metallic film: when leaky modes filter infrared light for multispectral imaging.

The diffractive behavior of arrays of square coaxial apertures in a gold layer is studied. These structures exhibit a resonant transmission enhancement that is used to design tunable bandpass filters for multispectral imaging in the 7-13 μm wavelength range. A modal analysis is used for this design and the study of their spectral features. Thus we show that the resonance peak is due to the excitation of leaky modes of the open photonic structure. Fourier transform infrared (FTIR) spectrophotometry transmission measurements of samples deposited on Si substrate show good agreement with numerical results and demonstrate angular tolerance of up to 30 degrees of the fabricated filters.

Indirect Nanoplasmonic Sensing to Probe with a High Sensitivity the Interaction of Water Vapor with Soot Aerosols

We demonstrate in this work that the indirect nanoplasmonic sensing lets us follow the adsorption/desorption of water molecules on soot particles that are a major contributor of the global warming. Increasing the relative humidity of the surrounding medium we measure a shift in wavelength of the localized surface plasmon resonance response of gold nanodisks on which soot particles are deposited. We show a singular and reversible blue shift with hydrophilic aircraft soot particles interpreted from a basic model as a reversible morphological change of the soot aggregates. This new method is highly sensitive and interesting to follow the change of optical properties of aerosols during their aging in the atmosphere, where they can adsorb and react with different gas molecules.

Condition of excitation and sensitivity of diffractively-coupled surface lattice resonances over plasmonic nanoparticle arrays in ATR geometry

We investigate conditions of excitation and properties of Plasmonic Surface Lattice Resonances (PSLR) over glass substrate-supported Au nanoparticle dimers (~100-200 nm) arranged in a periodic metamaterial lattice, in Attenuated Total Reflection (ATR) optical excitation geometry, and assess their sensitivities to variations of refractive index (RI) of the adjacent sample dielectric medium. We show that spectral sensitivity of PSLR to RI variations is determined by the lattice periodicity (~ 320 nm per RIU change in our case), while ultranarrow resonance lineshapes (down to a few nm full-widthat-half-maximum) provide very high figure-of-merit values evidencing the possibility of ultrasensitive biosensing measurements. Combining advantages of nanoscale architectures, including a strong concentration of electric field, the possibility of manipulation at the nanoscale etc, and high phase and spectral sensitivities, PSLRs promise a drastic advancement of current state-of-the-art plasmonic biosensing technology.

Water adsorption by a sensitive calibrated gold plasmonic nanosensor.

We demonstrate in this work that using nanoplasmonic sensing it is possible to follow the adsorption/desorption of water molecules on gold nanodisks nanofabricated by electron beam lithography. This quantitative method is highly sensitive allowing the detection of a few hundredths of adsorbed monolayer. Disk parameters (height, diameter, and interdisk distance) have been optimized after finite-difference time-domain (FDTD) simulations in order to obtain the best localized surface plasmon resonance (LSPR) signal-to-noise ratio. Finally, we have precisely measured the adsorption kinetics of water on gold as a function of the relative humidity of the surrounding medium.

Ultra-narrow surface lattice resonances in plasmonic metamaterial arrays for biosensing applications

When excited over a periodic metamaterial lattice of gold nanoparticles (~ 100nm), localized plasmon resonances (LPR) can be coupled by a diffraction wave propagating along the array plane, which leads to a drastic narrowing of plasmon resonance lineshapes (down to a few nm full-width-at-half-maximum) and the generation of singularities of phase of reflected light. These phenomena look very promising for the improvement of performance of plasmonic biosensors, but conditions of implementation of such diffractively coupled plasmonic resonances, also referred to as plasmonic surface lattice resonances (PSLR), are not always compatible with biosensing arrangement implying the placement of the nanoparticles between a glass substrate and a sample medium (air, water). Here, we consider conditions of excitation and properties of PSLR over arrays of glass substrate-supported single and double Au nanoparticles (~ 100-200nm), arranged in a periodic metamaterial lattice, in direct and Attenuated Total Reflection (ATR) geometries, and assess their sensitivities to variations of refractive index (RI) of the adjacent sample dielectric medium. First, we identify medium (PSLRair, PSLRwat for air and water, respectively) and substrate (PSLRsub) modes corresponding to the coupling of individual plasmon oscillations at medium- and substrate-related diffraction cut-off edges. We show that spectral sensitivity of medium modes to RI variations is determined by the lattice periodicity in both direct and ATR geometries (~ 320nm per RIU change in our case), while substrate mode demonstrates much lower sensitivity. We also show that phase sensitivity of PSLR can exceed 105 degrees of phase shift per RIU change and thus outperform the relevant parameter for all other plasmonic sensor counterparts. We finally demonstrate the applicability of surface lattice resonances in plasmonic metamaterial arrays to biosensing using standard streptavidin-biotin affinity model. Combining advantages of nanoscale architectures, including drastic concentration of electric field, possibility of manipulation at the nanoscale etc, and high phase and spectral sensitivities, PSLRs promise the advancement of current state-of-the-art plasmonic biosensing technology toward single molecule label-free detection.

Extrinsic spin- and orbital-Hall effect in cyclic group symmetric metamaterial(Conference Presentation)

We designed and fabricated cyclic group symmetric metamaterials (CGSMs), anisotropic media showing an extrinsic optical orbital Hall effect. An exchange of angular momentum between spin and orbital angular momenta takes place in an optical beam propagating through anisotropic media such as plasmonic nanoantennas of concentric ring and tapered arc (TA) shape. In case of TA antenna an cross-polarized circular polarization scattered beam exhibits an extrinsic orbital Hall effect. The CGSMs possess n-fold rotation symmetry and they are composed of plasmonic TA antennas. In case of circular polarization, the TA antennas effectively scatter incident light depending on the beam helicity. Both amplitude and phase gradients take place along the azimuthal direction for cross-polarized beams. We used electron beam lithography to fabricate 30nm thick gold metamaterials patterned on borosilicate glass substrates. Six types of CGSMs with the symmetry order n from 1 to 6 were fabricated and measured. Each CSGM is composed of multiple TA antennas with the width varying from 45nm to 150nm organized in 8*n azimuthal segments of concentric rings repeated with 600nm radial spacing. Measurements of orbital Hall transverse shifts of circularly polarized beams of right/left helicity were carried out at a wavelength of 1300nm. Because TA antennas are arranged in a metamaterial with a cyclic group n-fold rotation symmetry, the extrinsic orbital Hall transverse shifts from CGSM exhibit a geometrical pattern with the same symmetry. However, CGSMs with odd and even symmetry orders show a strongly contrasting difference in the character of transverse shifts. The observed geometrical patterns agree well with those obtained from FDTD theoretical simulation.