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.
|
|
Bergeron, H., Czuchry, E., Gazeau, J. - P., Malkiewicz, P., & Piechocki, W. (2015). Singularity avoidance in a quantum model of the Mixmaster universe. Physical Review D, 92(12), 124018.
Résumé: We present a quantum model of the vacuum Bianchi-IX dynamics. It is based on four main elements. First, we use a compound quantization procedure: an affine coherent state quantization for isotropic variables and a Weyl quantization for anisotropic ones. Second, inspired by standard approaches in molecular physics, we make an adiabatic approximation (Born-Oppenheimer-like approximation). Third, we expand the anisotropy potential about its minimum in order to deal with its harmonic approximation. Fourth, we develop an analytical treatment on the semiclassical level. The resolution of the classical singularity occurs due to a repulsive potential generated by the affine quantization. This procedure shows that during contraction the quantum energy of anisotropic degrees of freedom grows much slower than the classical one. Furthermore, far from the quantum bounce, the classical recollapse is reproduced. Our treatment is put in the general context of methods of molecular physics, which can include both adiabatic and nonadiabatic approximations.
|
|
Bergeron, H., Czuchry, E., Gazeau, J. - P., Malkiewicz, P., & Piechocki, W. (2015). Smooth quantum dynamics of the mixmaster universe. Physical Review D, 92(6), 061302.
Résumé: We present a new approach to the vacuum Bianchi IX model by combining affine coherent state quantization with Born-Oppenheimer-type adiabatic approximation in the analogy with quantum molecular physics. The analytical treatment is carried out on both quantum and semiclassical levels. Our quantization method by itself generates a specific repulsive potential that resolves the classical singularity. The quantized oscillatory degrees of freedom behave as radiation energy density. The Friedmann-like lowest-energy eigenstates of the system are found to be dynamically stable against small anisotropy perturbations, in contrast to the classical case.
|
|
Bergeron, H., Dapor, A., Gazeau, J. P., & Małkiewicz, P. (2015). Smooth bounce in the affine quantization of a Bianchi I model. Phys. Rev. D, 91(12), 124002.
Résumé: We present the affine coherent state quantization of the Bianchi I model. As in our previous paper on quantum theory of Friedmann models, we employ a variable associated with a perfect fluid to play a role of clock. Then we deparametrize the model. A distinctive feature, absent in isotropic models, is an extra nonholonomic constraint, which survives the deparametrization and constrains the range of physical variables. The appearance of the constraint reflects the “amplification” of singularity due to anisotropy. The quantization smoothes the extra constraint and allows quantum contracting trajectories to be smoothly transformed into expanding ones. Making use of an affine coherent state we develop a semiclassical description. Figures are included to illustrate our result.
|
|
|
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.
|
|
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.
|
|
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.
|
|
Houplin, J., Amiaud, L., Sedzik, T., Dablemont, C., Teillet-Billy, D., Rougeau, N., & Lafosse, A. (2015). A combined DFT/HREELS study of the vibrational modes of terphenylthiol SAMs. Eur. Phys. J. D, 69(9), 9 pp.
Résumé: Self-assembled monolayers of p-terphenylthiol (TPT, HS-(C6H4)(2)-C6H5) deposited onto gold can serve as model systems for aromatic lithography resists. Such thin molecular films are suitably probed using high resolution electron energy loss spectroscopy, due to its high surface sensitivity. Extended energy loss spectra were measured at different probing energies. The TPT monolayer overlapping.(CH) stretching modes could be modelled by a single effective anharmonic oscillator sustained by a Morse potential energy curve, thanks to the resonant excitation of the associated overtone series at 6 eV. A remarkably good agreement was obtained between the TPT monolayer energy loss spectrum and the computer-simulated infrared vibrational spectrum of the isolated TPT molecule. Density Functional Theory calculations for TPT, fully deuterated TPT and benzenethiol isolated molecules were performed with the exchange correlation functional B3LYP and a dispersion correction, using a triple zeta+ polarisation basis set. By comparing the vibrational patterns obtained for these parent systems, (re-) assignments of all the features observed in the TPT self-assembled monolayer energy loss spectrum are discussed. The obtained vibrational assignments can be confidently transposed to other related systems, such as benzenethiol and biphenyl self-assembled monolayers.
|
|
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.
|
|
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.
|
|
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.
|
|
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° .
|
|
