Peer-reviewed Publications |
Barbry, M., Koval, P., Marchesin, F., Esteban, R., Borisov, A. G., Aizpurua, J., & Sanchez-Portal, D. (2015). Atomistic near-field nanoplasmonics: reaching atomic-scale resolution in nanooptics. Nano Lett., 15(5), 3410–3419.
Résumé: Electromagnetic field localization in nanoantennas is one of the leitmotivs that drives the development of plasmonics. The near-fields in these plasmonic nanoantennas are commonly addressed theoretically within classical frameworks that neglect atomic-scale features. This approach is often appropriate since the irregularities produced at the atomic scale are typically hidden in far-field optical spectroscopies. However, a variety of physical and chemical processes rely on the fine distribution of the local fields at this ultraconfined scale. We use time-dependent density functional theory and perform atomistic quantum mechanical calculations of the optical response of plasmonic nanoparticles, and their dimers, characterized by the presence of crystallographic planes, facets, vertices, and steps. Using sodium clusters as an example, we show that the atomistic details of the nanoparticles morphologies determine the presence of subnanometric near-field hot spots that are further enhanced by the action of the underlying nanometric plasmonic fields. This situation is analogue to a self-similar nanoantenna cascade effect, scaled down to atomic dimensions, and it provides new insights into the limits of field enhancement and confinement, with important implications in the optical resolution of field-enhanced spectroscopies and microscopies.
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Esteban, R., Aguirregabiria, G., Borisov, A. G., Wang, Y. M., Nordlander, P., Bryant, G. W., & Aizpurua, J. (2015). The Morphology of Narrow Gaps Modifies the Plasmonic Response. ACS Photonics, 2(2), 295–305.
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Esteban, R., Zugarramurdi, A., Zhang, P., Nordlander, P., Garcia-Vidal, F. J., Borisov, A. G., & Aizpurua, J. (2015). A classical treatment of optical tunneling in plasmonic gaps: extending the quantum corrected model to practical situations. Faraday Discuss., 178, 151–183.
Résumé: The optical response of plasmonic nanogaps is challenging to address when the separation between the two nanoparticles forming the gap is reduced to a few nanometers or even subnanometer distances. We have compared results of the plasmon response within different levels of approximation, and identified a classical local regime, a nonlocal regime and a quantum regime of interaction. For separations of a few Angstroms, in the quantum regime, optical tunneling can occur, strongly modifying the optics of the nanogap. We have considered a classical effective model, so called Quantum Corrected Model (QCM), that has been introduced to correctly describe the main features of optical transport in plasmonic nanogaps. The basics of this model are explained in detail, and its implementation is extended to include nonlocal effects and address practical situations involving different materials and temperatures of operation.
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Gauyacq, J. - P., & Lorente, N. (2015). Decoherence-governed magnetic-moment dynamics of supported atomic objects. J. Phys. Condens. Matter., 27(45), 455301.
Résumé: Due to the quantum evolution of molecular magnetic moments, the magnetic state of nanomagnets can suffer spontaneous changes. This process can be completely quenched by environment-induced decoherence. However, we show that for typical small supported atomic objects, the substrate-induced decoherence does change the magnetic-moment evolution but does not quell it. To be specific and to compare with experiment, we analyze the spontaneous switching between two equivalent magnetization states of atomic structures formed by Fe on Cu2N/Cu (1 0 0), measured by Loth et al (2012 Science 335 196-9). Due to the substrate-induced decoherence, the Rabi oscillations proper to quantum tunneling between magnetic states are replaced by an irreversible decay of long characteristic times leading to the observed stochastic magnetization switching. We show that the corresponding switching rates are small, rapidly decreasing with system's size, with a 1/T thermal behavior and in good agreement with experiments. Quantum tunneling is recovered as the switching mechanism at extremely low temperatures below the muK range for a six-Fe-atom system and exponentially lower for larger atomic systems. The unexpected conclusion of this work is that experiments could detect the switching of these supported atomic systems because their magnetization evolution is somewhere between complete decoherence-induced stability and unobservably fast quantum-tunneling switching.
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Lin, L., Zapata, M., Xiong, M., Liu, Z., Wang, S., Xu, H., Borisov, A. G., Gu, H., Nordlander, P., Aizpurua, J., & Ye, J. (2015). Nanooptics of Plasmonic Nanomatryoshkas: Shrinking the Size of a Core-Shell Junction to Subnanometer. Nano Lett., 15(10), 6419–6428.
Résumé: Quantum effects in plasmonic systems play an important role in defining the optical response of structures with subnanometer gaps. Electron tunneling across the gaps can occur, altering both the far-field optical response and the near-field confinement and enhancement. In this study, we experimentally and theoretically investigate plasmon coupling in gold “nanomatryoshka” (NM) nanoparticles with different core-shell separations. Plasmon coupling effects between the core and the shell become significant when their separation decreases to 15 nm. When their separation decreases to below 1 nm, the near- and far-field properties can no longer be described by classical approaches but require the inclusion of quantum mechanical effects such as electron transport through the self-assembled monolayer of molecular junction. In addition, surface-enhanced Raman scattering measurements indicate strong electron-transport induced charge transfer across the molecular junction. Our quantum modeling provides an estimate for the AC conductances of molecules in the junction. The insights acquired from this work pave the way for the development of novel quantum plasmonic devices and substrates for surface-enhanced Raman scattering.
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Marinica, D. C., Zapata, M., Nordlander, P., Kazansky, A. K., M Echenique, P., Aizpurua, J., & Borisov, A. G. (2015). Active quantum plasmonics. Sci. Adv., 1(11), e1501095.
Résumé: The ability of localized surface plasmons to squeeze light and engineer nanoscale electromagnetic fields through electron-photon coupling at dimensions below the wavelength has turned plasmonics into a driving tool in a variety of technological applications, targeting novel and more efficient optoelectronic processes. In this context, the development of active control of plasmon excitations is a major fundamental and practical challenge. We propose a mechanism for fast and active control of the optical response of metallic nanostructures based on exploiting quantum effects in subnanometric plasmonic gaps. By applying an external dc bias across a narrow gap, a substantial change in the tunneling conductance across the junction can be induced at optical frequencies, which modifies the plasmonic resonances of the system in a reversible manner. We demonstrate the feasibility of the concept using time-dependent density functional theory calculations. Thus, along with two-dimensional structures, metal nanoparticle plasmonics can benefit from the reversibility, fast response time, and versatility of an active control strategy based on applied bias. The proposed electrical manipulation of light using quantum plasmonics establishes a new platform for many practical applications in optoelectronics.
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Rogez, B., Horeis, R., Le Moal, E., Christoffers, J., Al-Shamery, K., Dujardin, G., & Boer-Duchemin, E. (2015). Optical and electrical excitation of hybrid guided modes in an organic nanofiber-gold film system. J. Phys. Chem. C, 119, 22217.
Résumé: We report on the optical and electrical excitation of the modes of a “hybrid” waveguide consisting of a single organic nano fiber on a thin gold film. In the first set of experiments, light is used to excite the photoluminescence of an organic nano fiber on a thin gold film and the resulting emission is analyzed using Fourier-space leakage radiation microscopy. Two guided modes and the dispersion relations of this hybrid waveguide are thus determined. From numerical calculations, both a fundamental and excited mode of mixed photonic − plasmonic character are identified. In a second experiment, a local electrical nanosource of surface plasmon polaritons (SPPs) is coupled to the hybrid waveguide. The SPP nanosource consists of the inelastic electron tunnel current between the tip of a scanning tunneling microscope (STM) and the gold film. We show that the electrically excited SPPs couple to the fundamental mode and that the coupling efficiency is highest when the SPP nanosource is aligned with the nano fiber axis. Moreover, the electrically excited SPPs strongly scatter into out-of-plane light at the nano fiber end. This light from scattered SPPs measured in the substrate is phase shifted by about π with respect to the direct light emission from beneath the STM tip. These experiments lead to a better understanding of the processes that must be optimized in order to exploit such hybrid waveguide structures.
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Wang, T., Rogez, B., Comtet, G., Le Moal, E., Abidi, W., Remita, H., Dujardin, G., & Boer-Duchemin, E. (2015). Scattering of electrically excited surface plasmon polaritons by gold nanoparticles studied by optical interferometry with a scanning tunneling microscope. Phys. Rev. B, 92(4), 045438.
Résumé: We study the scattering of electrically excited surface plasmon polaritons (SPP) from individual nanostructures. The tunneling electrons from a scanning tunneling microscope (STM) are used to excite an out-going, circular SPP wave on a thin (50-nm) gold film on which isolated gold nanoparticles (NPs) have been deposited. Interaction of the excited SPPs with theNPs leads to both in-plane (SPP-to-SPP) and out-of-plane (SPP-to-photon) scattering. We use SPP leakage radiation microscopy to monitor the interference between the incident and in-plane scattered SPP waves in the image plane. By changing the location of the STM tip, the distance of the pointlike SPP source to the scatterers can be varied at will, which constitutes a key advantage over other existing techniques. As well, the out-of-plane scattered radiation interferes with the direct light emission from the STM tip in the back focal plane (Fourier plane). This confirms the mutual coherence of the light and SPP emission resulting from the inelastic tunneling of an electron in the STM junction. We use this effect to demonstrate that SPP-to-photon scattering at NPs is highly directional.
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Wardé, M., Herinx, M., Ledieu, J., Serkovic Loli, L. N., Fournée, V., Gille, P., Le Moal, S., & Barthés-Labrousse, M. - G. (2015). Adsorption of O2 and C2Hn (n=2, 4, 6) on the Al9Co2(001) and o-Al13Co4(100) complex metallic alloy surfaces. Appl. Surf. Sci., 357, 1666–1676.
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Zapata, M., Camacho Beltran, A. S., Borisov, A. G., & Aizpurua, J. (2015). Quantum effects in the optical response of extended plasmonic gaps: validation of the quantum corrected model in core-shell nanomatryushkas. Opt. Express, 23(6), 8134–8149.
Résumé: Electron tunneling through narrow gaps between metal nanoparticles can strongly affect the plasmonic response of the hybrid nanostructure. Although quantum mechanical in nature, this effect can be properly taken into account within a classical framework of Maxwell equations using the so-called Quantum Corrected Model (QCM). We extend previous studies on spherical cluster and cylindrical nanowire dimers where the tunneling current occurs in the extremely localized gap regions, and perform quantum mechanical time dependent density functional theory (TDDFT) calculations of the plasmonic response of cylindrical core-shell nanoparticles (nanomatryushkas). In this axially symmetric situation, the tunneling region extends over the entire gap between the metal core and the metallic shell. For core-shell separations below 0.5 nm, the standard classical calculations fail to describe the plasmonic response of the cylindrical nanomatryushka, while the QCM can reproduce the quantum results. Using the QCM we also retrieve the quantum results for the absorption cross section of the spherical nanomatryushka calculated by V. Kulkarni et al. [Nano Lett. 13, 5873 (2013)]. The comparison between the model and the full quantum calculations establishes the applicability of the QCM for a wider range of geometries that hold tunneling gaps.
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Zugarramurdi, A., Momeni, A., Debiossac, M., Lunca-Popa, P., Mayne, A.J., Borisov, A.G., Mu, Z., Roncin, P. & Khemliche, H. (2015). Determination of the geometric corrugation of graphene on SiC(0001) by grazing incidence fast atom diffraction. Appl. Phys. Lett., 106(10), 101902.
Résumé: We present a grazing incidence fast atom diffraction (GIFAD) study of monolayer graphene on 6H-SiC(0001). This system shows a Moiré-like 13x13 superlattice above the reconstructed carbon buffer layer. The averaging property of GIFAD results in electronic and geometric corrugations that are well decoupled; the graphene honeycomb corrugation is only observed with the incident beam parallel to the zigzag direction while the geometric corrugation arising from the superlattice is revealed along the armchair direction. Full-quantum calculations of the diffraction patterns show the very high GIFAD sensitivity to the amplitude of the surface corrugation. The best agreement between the calculated and measured diffraction intensities yields a corrugation height of 0.27 +- 0.3A° .
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Actes de Conférences |
Boer-Duchemin, E., Wang, T., Le Moal, E., Dujardin, G. (2015). Electrically-driven surface plasmon nanosources. In PROCEEDINGS OF SPIE (Vol. 9361, 93610R).
Résumé: Electrical nanosources of surface plasmons will be an integral part of any future plasmonic circuits. Three different types of such nanosources (based on inelastic electron tunneling, high energy electron bombardment, and the electrical injection of a semiconductor device) are briefly described here. An example of a fundamental experiment using an electrical nanosource consisting of the tunnel junction formed between a scanning tunneling microscope (STM) and a metallic sample is given. In this experiment, the temporal coherence of the broadband STM-plasmon source is probed using a variant of Young's double slit experiment, and the coherence time of the broadband source is estimated to be about 5-10 fs.
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