Optical Properties of Bismuth Nanostructures Towards the Ultrathin Film Regime

Bulk bismuth presents outstanding optical properties, such as a giant infrared refractive index (n near 10) and a negative ultraviolet visible permittivity induced by giant interband electronic transitions. Although such properties are very appealing for applications in nanophotonics, the dielectric function of bismuth nanostructures has been scarcely studied. Here, we determine by spectroscopic ellipsometry the far infrared to ultraviolet dielectric function of pulsed laser deposited bismuth thin films with nominal thickness tBi varied from near 10 nm to several tens of nm. For tBi above 15 nm, the films display a continuous structure and their dielectric function is comparable with that of bulk bismuth. For tBi below 15 nm, the film structure is discontinuous, and the dielectric function differs markedly from that of bulk bismuth. It is proposed from FDTD simulations that this marked difference arises mainly from effective medium effects induced by the discontinuous film structure, where quantum electronic confinement does not play a dominant role. This suggests that ultrathin and continuous bismuth films should present the same outstanding optical properties as bulk bismuth for high performance nanophotonic devices

Critical coupling and extreme confinement in nanogap antennas

International audienceNanogap antennas are compelling structures for squeezing light into ultrasmall volumes. However, when gaps are shrunk to the nanometer scale, the mode losses dramatically increase. In this Letter, we report the conditions of critical coupling between the arrays of nanogap resonant metal-insulator-metal (MIM) antennas and free space. Adapting the antenna density, critical coupling is achievable for any thickness of insulator, from 100 down to 0.1 nm. The fundamental optical mode can be described as continuous transitions through three types of modes: a perfect MIM mode, coupling between the MIM mode and surface plasmon polariton, and a gap plasmon mode. We found that the space between adjacent antennas is an essential parameter to perform critical coupling for thinner gaps. These results pave the way towards understanding extreme confinement in nanogap antenna structures such as MIM or nanoparticle arrays

Mapping the Electromagnetic Near-Field Enhancements of Gold Nanocubes

International audienceWe imaged and quantitatively characterized electromagnetic hot spots near the surfaces of plasmon resonant gold nanocubes. The strongest fields are localized at the nanocube corners as compared to those on the sides. The near-field enhancement on the surface of the cube was imaged as a function of incident polarization, leading to information on the localization of fields on specific regions on the surface. We found that the field intensity drops dramatically when the nanocube corner is slightly tilted with respect to the incident laser polarization. This dramatic dependence on angle was verified by electrodynamics simulations. These results will enable the use of gold nanocubes in field enhancement applications and refractive-index sensing

Propriétés Optiques de Nanostructures Métalliques sondées par des molécules photosensibles

While past research has considered the interaction between metal nanoparticles and photosensitive molecules, especially the possibility of initiating nanoscale photopolymerization based on the localized surface plasmons of such particles, this PhD dissertation describes the in-depth characterization and optimization of such interactions that result in nanoscale photopolymerization. The present work demonstrates our ability to use the nanophotopolymerization process to quantitatively map with unprecedented resolution, better than 5 nm, both, the near-field of metallic nanoparticles associated with their localized surface plasmons, and the local electric fields resulting from surface charges density at metal/dielectric interfaces.We will emphasize that a precise characterization of the nanoscale molecular mold of the confined electromagnetic field of metal colloids enabled us to quantify the near-field depth and its enhancement factor. Moreover, a near-field spectrum corresponding to the response of localized surface plasmons of a single metal nanoparticle will be assessed. Additionally, we present nanoscale resolution maps of the spatial distribution of the surface charge density created by the electric field discontinuity at a non-resonant metal/dielectric interface. Furthermore, this work will prove that the nanoscale photopolymerization approach does not only map the near-field of metal nanoparticles, yet it constitutes, from a more fundamental point of view, a unique opportunity to investigate nanophotochemistry.Les premières études dans ce domaine ont examiné l'interaction entre les structures métalliques et les molécules photosensibles et ont prouvé la possibilité de déclencher une photopolymérisation à l'échelle nanométrique, par le biais des plasmons de surface de ces nanoparticules. Il a été également montré que la nanophotopolymérisation constitue une technique puissante pour l'imagerie du champ proche des nanostructures, évitant ainsi la perturbation de la physique de l'échantillon en apportant une sonde à proximité.Au cours de cette thèse, nous avons été beaucoup plus quantitatifs que nos prédécesseurs dans ce domaine. En irradiant les nanoparticules de métal à leur résonance, nous avons moulé le profil dipolaire du champ électromagnétique par un polymère photo-actif, avec une résolution inédite de 5 nm. Ensuite et par une caractérisation précise des moules polymères, des valeurs précises du facteur d'exaltation et de la profondeur du champ proche de colloïdes d'argent ont été extraites. En outre, nous avons montré notre capacité à avoir une signature spectrale de la résonance plasmon d'une nanoparticule métallique unique directement en champ proche.En outre, nous présentons des cartes de résolution nanométrique de la distribution spatiale de la densité surfacique de charge créée par la discontinuité du champ électrique au niveau d’une interface métal non-résonant/diélectrique. Enfin, ce travail a prouvé que l'approche de nanophotopolymérisation constitue, d’un point de vue fondamental, une opportunité pour étudier la nanophotochimie

Propriétés optiques de nanostructures métalliques sondées par des molécules photosensibles

Les premières études dans ce domaine ont examiné l'interaction entre les structures métalliques et les molécules photosensibles et ont prouvé la possibilité de déclencher une photo-polymérisation à l'échelle nanométrique, par le biais des plasmons de surface de ces nanoparticules. Il a été également montré que la nanophotopolymérisation constitue une technique puissante pour l'imagerie du champ proche des nanostructures, évitant ainsi la perturbation de la physique de l'échantillon en apportant une sonde à proximité. Au cours de cette thèse, nous avons été beaucoup plus quantitatifs que nos prédécesseurs dans ce domaine. En irradiant les nanoparticules de métal à leur résonance, nous avons moulé le profil dipolaire du champ électromagnétique par un polymère photo-actif, avec une résolution inédite de 5 nm. Ensuite et par une caractérisation précise des moules polymères, des valeurs précises du facteur d'exaltation et de la profondeur du champ proche de colloïdes d'argent ont été extraites. En outre, nous avons montré notre capacité à avoir une signature spectrale de la résonance plasmon d'une nanoparticule métallique unique directement en champ proche. De plus, nous présentons des cartes de résolution nanométrique de la distribution spatiale de la densité surfacique de charge créée par la discontinuité du champ électrique au niveau d une interface métal non-résonant/diélectrique. Enfin, ce travail a prouvé que l'approche de nanophotopolymérisation constitue, d un point de vue fondamental, une opportunité pour étudier la nanophotochimieWhile past research has considered the interaction between metal nanoparticles and photo-sensitive molecules, especially the possibility of initiating nanoscale photopolymerization based on the localized surface plasmons of such particles, this PhD dissertation describes the in-depth characterization and optimization of such interactions that result in nanoscale photopolymerization. The present work demonstrates our ability to use the nanophotopolymerization process to quantitatively map with unprecedented resolution, better than 5 nm, both, the near-field of metallic nanoparticles associated with their localized surface plasmons, and the local electric fields resulting from surface charges density at metal/dielectric interfaces. We will emphasize that a precise characterization of the nanoscale molecular mold of the confined electromagnetic field of metal colloids enabled us to quantify the near-field depth and its enhancement factor. Moreover, a near-field spectrum corresponding to the response of localized surface plasmons of a single metal nanoparticle will be assessed. Additionally, we present nanoscale resolution maps of the spatial distribution of the surface charge density created by the electric field dis-continuity at a non-resonant metal/dielectric interface. Furthermore, this work will prove that the nanoscale photopolymerization approach does not only map the near-field of metal nanoparticles, yet it constitutes, from a more fundamental point of view, a unique opportunity to investigate nanophotochemistryTROYES-SCD-UTT (103872102) / SudocSudocFranceF

Electrically driven nanogap antennas and quantum tunneling regime

The optical and electrical characteristics of electrically-driven nanogap antennas are extremely sensitive to the nanogap region where the fields are tightly confined and electrons and photons can interplay. Upon injecting electrons in the nanogap, a conductance channel opens between the metal surfaces modifying the plasmon charge distribution and therefore inducing an electrical tuning of the gap plasmon resonance. Electron tunneling across the nanogap can be harnessed to induce broadband photon emission with boosted quantum efficiency. Under certain conditions, the energy of the emitted photons exceeds the energy of electrons, and this overbias light emission is due to spontaneous emission of the hot electron distribution in the electrode. We conclude with the potential of electrically controlled nanogap antennas for faster on-chip communication

Influence of Cracks on the Optical Properties of Silver Nanocrystals Supracrystal Films

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Cartographie du champ proche de dimères de nanoparticules d'or par SNOM sans ouverture en mode exaltation

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Nanolithography by plasmon-based free-radical photopolymerization

International audienceAn approach recently proposed for controlling photolithography at nanoscale consists of using plasmon resonances in metal nanostructures to confine and enhance optical fields. Plasmonic confinement and enhancement were successfully used to induce a local photopolymerization reaction. Photoinduced processes are of considerable interest for harnessing and controlling polymerization reactions at the nanoscale beyond chemical methods based on surface-confined living polymerization. The approach can be effectively employed for designing hybrid nanostructures with polarization-dependent optical properties, for quantitatively determining local electromagnetic field magnitude, and for developing advanced subwavelength optical lithography techniques. We focus here on the mechanisms responsible for nanoscale photopolymerization induced by confined and enhanced electromagnetic fields. Surface plasmon dipolar resonance of individual Ag nanoparticles was used as an optical near-field source to locally trigger the reaction of a photopolymerizable formulation. We found that the diffusion of the dye is the main process limiting the polymerization reaction, as opposed to what is observed at the microscale with an equivalent chemical system. This approach demonstrates that plasmon-based polymerization can achieve true nanometer scale resolution and also provides a unique opportunity to investigate photochemistry at this length scale

Plasmon-based free-radical photopolymerization : effect of diffusion on nanolithography processes

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