Showing papers by "Olivier J. F. Martin published in 2011"

Influence of electromagnetic interactions on the line shape of plasmonic Fano resonances.

Abstract: The optical properties of plasmonic nanostructures supporting Fano resonances are investigated with an electromagnetic theory. Contrary to the original work of Fano, this theory includes losses in the materials composing the system. As a result, a more general formula is obtained for the response of the system and general conclusions for the determination of the resonance parameters are drawn. These predictions are verified with surface integral numerical calculations in a broad variety of plasmonic nanostructures including dolmens, oligomers, and gratings. This work presents a robust and consistent analysis of plasmonic Fano resonances and enables the control of their line shape based on Maxwell’s equations. The insights into the physical understanding of Fano resonances gained this way will be of great interest for the design of plasmonic systems with specific spectral responses for applications such as sensing and optical metamaterials.

Ab initio theory of Fano resonances in plasmonic nanostructures and metamaterials

Abstract: An ab initio theory for Fano resonances in plasmonic nanostructures and metamaterials is de- veloped using Feshbach formalism. It reveals the role played by the electromagnetic modes and material losses in the system, and enables the engineering of Fano resonances in arbitrary geome- tries. A general formula for the asymmetric resonance in a non-conservative system is derived. The influence of the electromagnetic interactions on the resonance line shape is discussed and it is shown that intrinsic losses drive the resonance contrast, while its width is mostly determined by the coupling strength between the non-radiative mode and the continuum. The analytical model is in perfect agreement with numerical simulations.

Excitation and Reemission of Molecules near Realistic Plasmonic Nanostructures

Abstract: The enhancement of excitation and reemission of molecules in close proximity to plasmonic nanostructures is studied with special focus on the comparison between idealized and realistically shaped nanostructures. Numerical experiments show that for certain applications choosing a realistic geometry closely resembling the actual nanostructure is imperative, an idealized simulation geometry yielding significantly different results. Finally, a link between excitation and reemission processes is formed via the theory of optical reciprocity, allowing a transparent view of the electromagnetic processes involved in plasmon-enhanced fluorescence and Raman-scattering.

Relation between near–field and far–field properties of plasmonic Fano resonances

Abstract: The relation between the near–field and far–field properties of plasmonic nanostructures that exhibit Fano resonances is investigated in detail. We show that specific features visible in the asymmetric lineshape far–field response of such structures originate from particular polarization distributions in their near–field. In particular we extract the central frequency and width of plasmonic Fano resonances and show that they cannot be directly found from far–field spectra. We also address the effect of the modes coupling onto the frequency, width, asymmetry and modulation depth of the Fano resonance. The methodology described in this article should be useful to analyze and design a broad variety of Fano plasmonic systems with tailored near–field and far–field spectral properties.

Fabrication of sub-10 nm gap arrays over large areas for plasmonic sensors

Abstract: We report a high-throughput method for the fabrication of metallic nanogap arrays with high-accuracy over large areas. This method, based on shadow evaporation and interference lithography, achieves sub-10 nm gap sizes with a high accuracy of ±1.5 nm. Controlled fabrication is demonstrated over mm2 areas and for periods of 250 nm. Experiments complemented with numerical simulations indicate that the formation of nanogaps is a robust, self-limiting process that can be applied to wafer-scale substrates. Surface-enhanced Raman scattering (SERS) experiments illustrate the potential for plasmonic sensing with an exceptionally low standard-deviation of the SERS signal below 3% and average enhancement factors exceeding 1 × 106.

Plasmonic trapping with realistic dipole nanoantennas: Analysis of the detection limit

Abstract: We use numerical simulations based on the surface integral technique to study the detection limit of plasmonic trapping with realistic dipole antennas. The induced plasmon resonance shift due to the coupling between an antenna and a nanoparticle is studied for different antennas geometries, different positions, sizes, and materials for the trapped nanoparticle. The shift of the antenna resonance is found to be linear with the near-field intensity enhancement caused by the antenna and further dependents on the volume and refractive index of the trapped nanoparticle. Detection limit of 5 nm for plasmonic particles and 6.5 nm for high index dielectrics is reported.

Simulation of complex plasmonic circuits including bends

Abstract: Using a finite-element, full-wave modeling approach, we present a flexible method of analyzing and simulating dielectric and plasmonic waveguide structures as well as their mode coupling. This method is applied to an integrated plasmonic circuit where a straight dielectric waveguide couples through a straight hybrid long-range plasmon waveguide to a uniformly bent hybrid one. The hybrid waveguide comprises a thin metal core embedded in a two–dimensional dielectric waveguide. The performance of such plasmonic circuits in terms of insertion losses is discussed.

Plasmon delocalization onset in finite sized nanostructures

Abstract: The transition from localized to delocalized plasmons (i.e. the transition from a situation where the decay length of a travelling surface plasma wave is greater than its propagation distance to a situation where it is smaller) and hence the onset of plasmon delocalization is studied in a single 2D silver nanoparticle of increasing length. A fourier analysis in the near-field of the nanoparticle is used as the main tool for analysis. This method, along with far-field scattering spectra simulations and the near-field profile directly above and along the length of the nanoparticle are used to investigate and clearly show the transition from localized to delocalized modes. In particular, it is found that for a finite sized rectangular nanoparticle, both the emerging odd and even delocalized modes are nothing but a superposition of many standing wave plasmon modes. As a consequence, even very short metal films can support delocalized plasmons that bounce back and forth along the film.

Addendum: “A broadband and high-gain metamaterial microstrip antenna” [Appl. Phys. Lett. 96, 164101 (2010)]

Abstract: Reference EPFL-ARTICLE-173291doi:10.1063/1.3651481View record in Web of Science Record created on 2011-12-27, modified on 2016-08-09

Strongly coupled bio-plasmonic system: Application to oxygen sensing

Abstract: We investigate theoretically the strong coupling between surface plasmon resonances (SPRs) and absorption bands of hemoglobin. When the surface plasmon resonance spectrally overlaps the absorption bands of hemoglobin, the system is strongly coupled and its dispersion diagram exhibits an anti-crossing. Working in the conditions of strong coupling enhances the sensitivity of a SPR sensor up to a factor of 10. A model for the permittivity of hemoglobin, both in oxygenated and deoxygenated states, is presented and the study is carried out for both angle and wavelength modulated SPR sensors. Finally, a differential measurement is shown to increase the sensitivity further.

Analytical Description of Fano Resonances in Plasmonic Nanostructures

Abstract: We report on the derivation of analytical formulas for the lineshape of Fano resonances in plasmonic nanostructures as a function of their electromagnetic response. Contrary to the original work of Fano, the formalism proposed here includes losses in the materials composing the system. As a result, a more general formula is obtained for the response of the system and general conclusions for the determination of the resonance parameters are drawn, in particular on its width and asymmetry. The insights into the physical understanding of Fano resonances gained this way will be of great interest for the design of plasmonic sensing platforms and metamaterials.

Controlling and utilizing optical forces at the nanoscale with plasmonic antennas

Abstract: Plasmonic dipole antennas are powerful optical devices for many applications since they combine a high field enhancement with outstanding tunability of their resonance frequency. The field enhancement, which is mainly localized inside the nanogap between both arms, is strong enough to generate attractive forces for trapping extremely small objects flowing nearby. Furthermore it dramatically enhances their Raman scattering cross-section generating SERS emission. In this publication, we demonstrate how plasmonic antennas provide unique means for bringing analyte directly into hotspots by merely controlling the optical force generated by the plasmon resonance. This technique is very suitable for immobilizing objects smaller that the diffraction limit and requires a very little power density. In this work, 20nm gold nanoparticles functionalized with Rhodamine 6G are trapped in the gap of nanoantennas fabricated with e-beam lithography on glass substrate. The entire system is integrated into a microfluidic chip with valves and pumps for driving the analyte. The field enhancement is generated by a near-IR laser (λ=808nm) that provides the trapping energy. It is focused on the sample through a total internal reflection (TIRF) objective in dark field configuration with a white light source. The scattered light is collected through the same objective and the spectrum of one single antenna spectrum is recorded and analyzed every second. A trapping event is characterized by a sudden red-shift of the antenna resonance. This way, it is possible to detect the trapping of extremely small objects. The SERS signal produced by a trapped analyte can then be studied by switching from the white light source to a second laser for Raman spectroscopy, while keeping the trapping laser on. The trapping and detection limit of this approach will be discussed in detail.

Ab initio engineering of Fano resonances

Abstract: In this work, we pave the route towards the engineering of strong and spectrally sharp Fano resonances in plasmonic nanostructures and derive analytical formulas for their line shape as a function of their electromagnetic response. Contrary to the original work of Fano, the formalism proposed here includes losses in the materials composing the system. As a result, a more general formula is obtained for the response of the system and general conclusions for the determination of the resonance parameters are drawn, in particular on its width and asymmetry. Using a surface integral simulation technique for electromagnetic scattering on three-dimensional individual and periodic nanostructures, we numerically validate our model for structures that are currently under extensive investigation in the plasmonic and metamaterial communities. The insights into the physical comprehension of Fano resonances gained this way will be of great interest for the design of plasmonic sensing platforms and metamaterials.

Optical Forces in Plasmonic Nanostructures: New Functionalities for Nanophotonic Circuits

Abstract: We study in detail the modeling requirements for realistic plasmonic nanostructures and show that strong field gradients created at their vicinity can be used to trap nanostructures; this plasmonic trapping is also demonstrated experimentally.

Optical trapping at the ultimate nanoscale in the near-field of plasmonic antennas

Abstract: We study the trapping of nanoscopic objects in the near-field of plasmonic nanostructures and demonstrate experimentally that 10nm particles can be trapped in the 15nm gap of a dipole antenna. Applications for biosensing are discussed.