Showing papers by "Olivier J. F. Martin published in 2014"

First on-sky SCAO validation of full LQG control with vibration mitigation on the CANARY pathfinder.

Abstract: Adaptive optics provides real time correction of wavefront disturbances on ground based telescopes. Optimizing control and performance is a key issue for ever more demanding instruments on ever larger telescopes affected not only by atmospheric turbulence, but also by vibrations, windshake and tracking errors. Linear Quadratic Gaussian control achieves optimal correction when provided with a temporal model of the disturbance. We present in this paper the first on-sky results of a Kalman filter based LQG control with vibration mitigation on the CANARY instrument at the Nasmyth platform of the 4.2-m William Herschel Telescope. The results demonstrate a clear improvement of performance for full LQG compared with standard integrator control, and assess the additional improvement brought by vibration filtering with a tip-tilt model identified from on-sky data, thus validating the strategy retained on the instrument SPHERE at the VLT.

Nonlinear Plasmonic Nanorulers

Abstract: The evaluation of distances as small as few nanometers using optical waves Is a very challenging task that can pave the way for the development of new applications in biotechnology and nanotechnology. In this article, we propose a new measurement method based on the control of the nonlinear optical response of plasmonic nanostructures by means of Fano resonances. It is shown that Fano resonances resulting from the coupling between a bright mode and a dark mode at the fundamental wavelength enable unprecedented and direct manipulation of the nonlinear electromagnetic sources at the nanoscale. In the case of second harmonic generation from gold nanodolmens, the different nonlinear sources distributions induced by the different coupling regimes are clearly revealed in the far-field distribution. Hence, the configuration of the nanostructure can be accurately determined In 3-dimensions by recording the wave scattered at the second harmonic wavelength. Indeed, the conformation of the different elements building the system is encoded in the nonlinear far-field distribution, making second harmonic generation a promising tool for reading 3-dimension plasmonic nanorulers. Furthemore, it is shown that 3-dimension plasmonic nanorulers can be implemented with simpler geometries than in the linear regime while providing complete Information on the structure conformation, including the top nanobar position and orientation.

Fano resonances in the nonlinear optical response of coupled plasmonic nanostructures

Abstract: The coupling between metallic nanostructures is a common and easy way to control the optical properties of plasmonic systems. Even though the coupling between plasmonic oscillators has been widely studied in the linear regime, its influence on the nonlinear optical response of metallic nanostructures has been sparsely considered. Using a surface integral equation method, we investigate the second order nonlinear optical response of plasmonic metamolecules supporting Fano resonances revealing that the typical lineshape of Fano resonances is also clearly observable in the nonlinear regime. The physical mechanisms leading to nonlinear Fano resonances are revealed by the coupled oscillator model and the symmetry subgroup decomposition. It is found that the origin of the nonlinear scattered wave, i. e. the active plasmonic oscillator, can be selectively chosen. Furthermore, interferences between nonlinear emissions are clearly observed in specific configurations. The results presented in this article pave the way for the design of efficient nonlinear plasmonic metamolecules with controlled nonlinear radiation.

Surface second-harmonic generation from coupled spherical plasmonic nanoparticles: Eigenmode analysis and symmetry properties

Abstract: The surface second-harmonic generation from interacting spherical plasmonic nanoparticles building different clusters (symmetric and asymmetric dimers, trimers) is theoretically investigated. The plasmonic eigenmodes of the nanoparticle clusters are first determined using an ab initio approach based on the Green's functions method. This method provides the properties, such as the resonant wavelengths, of the modes sustained by a given cluster. The fundamental and second-harmonic responses of the corresponding clusters are then calculated using a surface integral method. The symmetry of both the linear and nonlinear responses is investigated, as well as their relationship. It is shown that the second-harmonic generation can be significantly enhanced when the fundamental field is such that its second harmonic matches modes with suitable symmetry. The role played by the nanogaps in second-harmonic generation is also underlined. The results presented in this article demonstrate that the properties of the second-harmonic generation from coupled metallic nanoparticles cannot be fully predicted from their linear response only, while, on the other hand, a detailed knowledge of the underlying modal structure can be used to optimize the generation of the second harmonic.

Refractive index sensing with Fano resonant plasmonic nanostructures: a symmetry based nonlinear approach

Abstract: Sensing using surface plasmon resonances is one of the most promising practical applications of plasmonic nanostructures and Fano resonances allow achieving a lower detection limit thanks to their narrow spectral features. However, a narrow spectral width of the subradiant mode in a plasmonic system, as observed in the weak coupling regime, is in general associated with a low modulation of the complete spectral response. In this article, we show that this limitation can be overcome by a nonlinear approach based on second harmonic generation and its dependence on symmetry at the nanoscale. The Fano resonant systems considered in this work are gold nanodolmens. Their linear and nonlinear responses are evaluated using a surface integral equation method. The numerical results demonstrate that a variation of the refractive index of the surrounding medium modifies the coupling between the dark and bright modes, resulting in a modification of the electromagnetic wave scattered at the second harmonic wavelength, especially the symmetry of the nonlinear emission. Reciprocally, we show that evaluating the asymmetry of the nonlinear emission provides a direct measurement of the gold nanodolmens dielectric environment. Interestingly, the influence of the refractive index of the surrounding medium on the nonlinear asymmetry parameter is approximately 10 times stronger than on the spectral position of the surface plasmon resonance: hence, smaller refractive index changes can be detected with this new approach. Practical details for an experimental realization of this sensing scheme are discussed and the resolution is estimated to be as low as Δn = 1.5 × 10(-3), respectively 1.5 × 10(-5), for an acquisition time of 60 s for an isolated gold nanodolmen, respectively an array of 10 × 10 nanodolmens.

Periodicity-induced symmetry breaking in a Fano lattice: hybridization and tight-binding regimes.

Abstract: We investigate experimentally and theoretically the role of periodicity on the optical response of dolmen plasmonic arrays that exhibit a Fano line shape. Contrary to previous works on single nanostructures, this study deals with the in-plane near-field coupling between adjacent unit cells. By making an analogy to the electronic properties of atoms in the tight-binding model, specific behaviors of photonic states are investigated numerically as a function of the structural asymmetry for different coupling directions. These predictions are verified experimentally with dark-field measurements on nanostructure arrays which exhibit high tunability and fine control of their spectral features as a function of the lattice constants. These effects, originated from symmetry-breaking and selective excitation of the subradiant mode, provide additional degree of freedom for tuning the spectral response and can be used for the sensitive detection of local perturbations. This study provides a general understanding of t...

Optical forces and torques on realistic plasmonic nanostructures: a surface integral approach

Abstract: We develop a novel formalism to calculate the optical forces and torques on complex and realistic nanostructures by combining the surface integral equation (SIE) technique with Maxwell’s stress tensor. The optical force is calculated directly on the scatterer surface from the currents obtained from the SIE, which does not require an additional surface to evaluate Maxwell’s stress tensor; this is especially useful for intricate geometries such as plasmonic antennas. SIE enables direct evaluation of forces from the surface currents very efficiently and accurately for complex systems. As a proof of concept, we establish the accuracy of the model by comparing the results with the calculations from the Mie theory. The flexibility of the method is demonstrated by simulating a realistic plasmonic system with intricate geometry.

Metal double layers with sub-10 nm channels.

Abstract: Double-layer plasmonic nanostructures are fabricated by depositing metal at normal incidence onto various resist masks, forming an antenna layer on top of the resist post and a hole layer on the substrate. Antenna plasmon resonances are found to couple to the hole layer, inducing image charges which enhance the near-field for small layer spacings. For continued evaporation above the resist height, a sub-10 nm gap channel develops due to a self-aligned process and a minimal undercut of the resist sidewall. For such double layers with nanogap channels, the average surface-enhanced Raman scattering intensity is improved by a factor in excess of 60 in comparison to a single-layer antenna with the same dimensions. The proposed design principle is compatible with low-cost fabrication, straightforward to implement, and applicable over large areas. Moreover, it can be applied for any particular antenna shape to improve the signals in surface-enhanced spectroscopy applications.

Quantitative Extraction of Equivalent Lumped Circuit Elements for Complex Plasmonic Nanostructures

Abstract: We establish a general method to bridge the gap between full-field electromagnetic calculations and equivalent lumped circuit elements to describe the optical response of plasmonic nanostructures. The exact value of each lumped element is extracted from one single full-field calculation using the Poynting vector and considerations on the energy flow in the system. The equivalent circuit obtained this way describes the complete response of the system at any frequency and can be used to optimize it for specific applications or perform parametric studies. This powerful approach can accurately reproduce the behavior of complex plasmonic nanostructures, such as Fano resonances, retardation effects, and polarization coupling. Furthermore, the influence of coupling parameters within the different modes supported by a given plasmonic structure can be investigated, thus providing new physical insights into its functioning mechanisms.

Large-scale sub-100 nm compound plasmonic grating arrays to control the interaction between localized and propagating plasmons

Abstract: Compound plasmonic resonances arise due to the interaction between discrete and continuous metallic nanostructures. Such combined nanostructures provide a versatility and tunability beyond that of most other metallic nanostructures. In order to observe such resonances and their tunability, multiple nanostructure arrays composed of periodic metallic gratings of varying width and an underlying metallic film should be studied. Large-area compound plasmonic structures composed of various Au grating arrays with sub-100 nm features spaced nanometers above an Au film were fabricated using extreme ultraviolet interference lithography. Reflection spectra, via both numerical simulations and experimental measurements over a wide range of incidence angles and excitation wavelengths, show the existence of not only the usual propagating and localized plasmon resonances, but also compound plasmonic resonances. These resonances exhibit not only propagative features, but also a spectral evolution with varying grating width. Additionally, a reduction of the width of the grating elements results in coupling with the localized dipolar resonance of the grating elements and thus plasmon hybridization. This newly acquired perspective on the various interactions present in such a plasmonic system will aid in an increased understanding of the mechanisms at play when designing plasmonic structures composed of both discrete and continuous elements. c 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

Up-scalable method to amplify the diffraction efficiency of simple gratings

Abstract: An innovative class of coupling gratings for efficiently redirecting light beams is described. They are based on symmetric binary gratings equipped with an asymmetric high refractive index coating. This kind of structure exhibits a grating blaze effect and produces high diffraction efficiencies. Coupling gratings based on the described principle have been fabricated by UV casting and subsequent oblique ZnS evaporation. The gratings were characterized by measuring their diffraction efficiencies. For unpolarized light, efficiencies of around 70% have been measured at 510 nm wavelength. Simulations revealed that diffraction efficiencies of up to 90% can be obtained for polarized light.

Spectral tunability of realistic plasmonic nanoantennas

Abstract: Single nanoantenna spectroscopy was carried out on realistic dipole nanoantennas with various arm lengths and gap sizes fabricated by electron-beam lithography. A significant difference in resonance wavelength between realistic and ideal nanoantennas was found by comparing their spectral response. Consequently, the spectral tunability (96 nm) of the structures was significantly lower than that of simulated ideal nanoantennas. These observations, attributed to the nanofabrication process, are related to imperfections in the geometry, added metal adhesion layer, and shape modifications, which are analyzed in this work. Our results provide important information for the design of dipole nanoantennas clarifying the role of the structural modifications on the resonance spectra, as supported by calculations.

A New Fabrication Method for Aluminum and Silver Plasmonic Nanostructures

Abstract: We develop a new recipe for fabrication of nanostructures in silver and aluminum. A thin layer of silver oxide has been deposited, which acts as seeding layer for the material growth.

A new fabrication method for aluminium and silver plasmonic nanostructures

Abstract: Keywords: Plasmonics ; Nanophotonics Reference EPFL-CONF-207118 Record created on 2015-04-13, modified on 2017-05-10

A New Fabrication Method for Aluminum and Silver

Abstract: Keywords: Plasmonics ; Nanophotonics Reference EPFL-CONF-207411 Record created on 2015-04-13, modified on 2017-05-10

Up-Scalable Method to Amplify the 1st Order Transmittance of Simple Gratings

Abstract: Shadow evaporation is used to asymmetrically coat binary gratings and enhance the diffraction efficiency into a single order. First order transmittances of close to 70% are achieved experimentally for perpendicularly incident, non-polarized light.