Mie-excitons: understanding strong coupling in dielectric nanoparticles

We theoretically analyse the hybrid Mie-exciton optical modes arising from the strong coupling of excitons in organic dyes or transition-metal dichalcogenides with the Mie resonances of high-index dielectric nanoparticles. Detailed analytic calculations show that silicon--exciton core--shell nanoparticles are characterised by a richness of optical modes which can be tuned through nanoparticle dimensions to produce large anticrossings in the visible or near infrared, comparable to those obtained in plexcitonics. The complex magnetic-excitonic nature of these modes is understood through spectral decomposition into Mie-coefficient contributions, complemented by electric and magnetic near-field profiles. In the frequency range of interest, absorptive losses in silicon are sufficiently low to allow observation of several periods of Rabi oscillations in strongly coupled emitter-particle architectures, as confirmed here by discontinuous Galerkin time-domain calculations for the electromagnetic field beat patterns. These results suggest that Mie resonances in high-index dielectrics are promising alternatives for plasmons in strong-coupling applications in nanophotonics, while the coupling of magnetic and electric modes opens intriguing possibilities for external control

Fluctuations and noise-limited sensing near the exceptional point of PT symmetric resonator systems

We theoretically explore the role of mesoscopic fluctuations and noise on the spectral and temporal properties of systems of PT-symmetric coupled gain-loss resonators operating near the exceptional point, where eigenvalues and eigenvectors coalesce. We show that the inevitable detuning in the frequencies of the uncoupled resonators leads to an unavoidable modification of the conditions for reaching the exceptional point, while, as this point is approached in ensembles of resonator pairs, statistical averaging significantly smears the spectral features. We also discuss how these fluctuations affect the sensitivity of sensors based on coupled PT-symmetric resonators. Finally, we show that temporal fluctuations in the detuning and gain of these sensors lead to a quadratic growth of the optical power in time, thus implying that maintaining operation at the exceptional point over a long period can be rather challenging. Our theoretical analysis clarifies issues central to the realization of PT-symmetric devices, and should facilitate future experimental work in the field

Threading plasmonic nanoparticle strings with light

This work is licensed under a Creative Commons Attribution 4.0 International License.-- et al.Nanomaterials find increasing application in communications, renewable energies, electronics and sensing. Because of its unsurpassed speed and highly tuneable interaction with matter, using light to guide the self-assembly of nanomaterials can open up novel technological frontiers. However, large-scale light-induced assembly remains challenging. Here we demonstrate an efficient route to nano-assembly through plasmon-induced laser threading of gold nanoparticle strings, producing conducting threads 12±2nm wide. This precision is achieved because the nanoparticles are first chemically assembled into chains with rigidly controlled separations of 0.9nm primed for re-sculpting. Laser-induced threading occurs on a large scale in water, tracked via a new optical resonance in the near-infrared corresponding to a hybrid chain/rod-like charge transfer plasmon. The nano-thread width depends on the chain mode resonances, the nanoparticle size, the chain length and the peak laser power, enabling nanometre-scale tuning of the optical and conducting properties of such nanomaterials.We acknowledge financial support from EPSRC grants EP/G060649/1, EP/K028510/1 and EP/L027151/1, ERC grants LINASS 320503 and ASPiRe 240629, and project FIS2010-19609-C02-01 from the Spanish Ministry of Science and Innovation. J.S.B. acknowledges the School of Physical Science, University of Cambridge, for the funding of the transmission electron microscope. S.K. acknowledges funding from the Biochemical Society (Krebs Memorial Scholarship) and the Cambridge Commonwealth Trust.Peer Reviewe

Monitoring morphological changes in 2D monolayer semiconductors using atom-thick plasmonic nanocavities

This is an open access article published under a Creative Commons Attribution (CC-BY) License.-- et al.Nanometer-sized gaps between plasmonically coupled adjacent metal nanoparticles enclose extremely localized optical fields, which are strongly enhanced. This enables the dynamic investigation of nanoscopic amounts of material in the gap using optical interrogation. Here we use impinging light to directly tune the optical resonances inside the plasmonic nanocavity formed between single gold nanoparticles and a gold surface, filled with only yoctograms of semiconductor. The gold faces are separated by either monolayers of molybdenum disulfide (MoS2) or two-unit-cell thick cadmium selenide (CdSe) nanoplatelets. This extreme confinement produces modes with 100-fold compressed wavelength, which are exquisitely sensitive to morphology. Infrared scattering spectroscopy reveals how such nanoparticle-on-mirror modes directly trace atomic-scale changes in real time. Instabilities observed in the facets are crucial for applications such as heat-assisted magnetic recording that demand long-lifetime nanoscale plasmonic structures, but the spectral sensitivity also allows directly tracking photochemical reactions in these 2-dimensional solids.This work was supported by the UK EPSRC grants EP/G060649/1, EP/L027151/1, Defence Science and Technology Laboratory (DSTL), and ERC grant 320503 LINASS. C.T. and J.A. acknowledge financial support from Project FIS2013-41184-P from MINECO, ETORTEK 2014-15 of the Basque Department of Industry and IT756-13 from the Basque consolidated groups.Peer Reviewe

Active loaded plasmonic antennas at terahertz frequencies: Optical control of their capacitive-inductive coupling

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).We demonstrate the photogeneration of loaded dipole plasmonic antennas resonating at THz frequencies. This is achieved by the patterned optical illumination of a semiconductor surface using a spatial light modulator. Our experimental results indicate the existence of capacitive and inductive coupling of localized surface plasmon polaritons. By varying the load in the antenna gap we are able to switch between both coupling regimes. Furthermore, we determine experimentally the effective impedance of the antenna load and verify that this load can be effectively expressed as a LC resonance formed by a THz inductor and capacitor connected in a parallel circuit configuration. These findings are theoretically supported by full electrodynamic calculations and by simple concepts of lumped circuit theory. Our results open new possibilities for the design of active THz circuits for optoelectronic devices.This work has been supported by the ERC through Grant No. 259727 THZ-PLASMON and by the Netherlands Foundation for Fundamental Research on Matter (FOM) and the Netherlands Organization for Scientific Research (NWO). J.A. and C.T. acknowledge support from Project FIS2013-41184-P from the Spanish MINECO and Project IT756-13 of the Department of Education of the Basque Government.Peer Reviewe

On the applicability of quantum-optical concepts in strong-coupling nanophotonics

Rooted in quantum optics and benefiting from its well-established foundations, strong coupling in nanophotonics has experienced increasing popularity in recent years. With nanophotonics being an experiment-driven field, the absence of appropriate theoretical methods to describe ground-breaking advances has often emerged as an important issue. To address this problem, the temptation to directly transfer and extend concepts already available from quantum optics is strong, even if a rigorous justification is not always available. In this Review we discuss situations where, in our view, this strategy has indeed overstepped its bounds. We focus on exciton--plasmon interactions, and particularly on the idea of calculating the number of excitons involved in the coupling. We analyse how, starting from an unfounded interpretation of the term N/V that appears in theoretical descriptions at different levels of complexity, one might be tempted to make independent assumptions for what the number N and the volume V are, and attempt to calculate them separately. Such an approach can lead to different, often contradictory results, depending on the initial assumptions (e.g. through different treatments of $V$ as the -- ambiguous in plasmonics -- mode volume). We argue that the source of such contradictions is the question itself -- How many excitons are coupled?, which disregards the true nature of the coupled components of the system, has no meaning and often not even any practical importance. If one is interested in validating the quantum nature of the system -- which appears to be the motivation driving the pursuit of strong coupling with small N -- one could instead focus on quantities such as the photon emission rate or the second-order correlation function

Plasmonic response and SERS modulation in electrochemical applied potentials

We study the optical response of individual nm-wide plasmonic nanocavities using a nanoparticle-on-mirror design utilised as an electrode in an electrochemical cell. In this geometry Au nanoparticles are separated from a bulk Au film by an ultrathin molecular spacer, giving intense and stable Raman amplification of 100 molecules. Modulation of the plasmonic spectra and the SERS response is observed with an applied voltage under a variety of electrolytes. Different scenarios are discussed to untangle the various mechanisms that can be involved in the electronic interaction between NPs and electrode surfaces.We acknowledge financial support from EPSRC grant EP/G060649/1, EP/L027151/1, EP/G037221/1, EPSRC NanoDTC, and ERC grant LINASS 320503. C. T. was supported by funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007–2013) under REA grant agreement number 609405 (COFUNDPostdocDTU)

Tracking Optical Welding through Groove Modes in Plasmonic Nanocavities.

We report the light-induced formation of conductive links across nanometer-wide insulating gaps. These are realized by incorporating spacers of molecules or 2D monolayers inside a gold plasmonic nanoparticle-on-mirror (NPoM) geometry. Laser irradiation of individual NPoMs controllably reshapes and tunes the plasmonic system, in some cases forming conductive bridges between particle and substrate, which shorts the nanometer-wide plasmonic gaps geometrically and electronically. Dark-field spectroscopy monitors the bridge formation in situ, revealing strong plasmonic mode mixing dominated by clear anticrossings. Finite difference time domain simulations confirm this spectral evolution, which gives insights into the metal filament formation. A simple analytic cavity model describes the observed plasmonic mode hybridization between tightly confined plasmonic cavity modes and a radiative antenna mode sustained in the NPoM. Our results show how optics can reveal the properties of electrical transport across well-defined metallic nanogaps to study and develop technologies such as resistive memory devices (memristors).Engineering and Physical Sciences Research Council (Grant IDs: EP/ G060649/1, EP/L027151/1, EP/G037221/1, EPSRC NanoDTC EP/L015978/1), European Research Council (Grant ID: LINASS 320503), Winton Programme of the Physics of Sustainability, Project FIS2013- 41184-P from MINECO and IT756-13 from the Basque Government consolidated groupsThis is the author accepted manuscript. The final version is available from the American Chemical Society via http://dx.doi.org/10.1021/acs.nanolett.6b0216

Interactions of nanorod particles in the strong coupling regime

The plasmon coupling in a nanorod dimer obeys the exponential size dependence according to the Universal Plasmon Ruler Equation. However, it was shown recently that such a model does not hold at short nanorod distance (Nano Lett. 2009, 9, 1651). Here we study the nanorod coupling in various cases, including nanorod dimer with the asymmetrical lengths and symmetrical dimer with the varying gap width. The asymmetrical nanorod dimer causes two plasmon modes: one is the attractive lower- energy mode and the other the repulsive high-energy mode. Using a simple coupled LC-resonator model, the position of dimer resonance has been determined analytically. Moreover, we found that the plasmon coupling of symmetrical cylindrical (or rectangular) nanorod dimer is governed uniquely by gap width scaled for the (effective) rod radius rather than for the rod length. A new Plasmon Ruler Equation without using the fitting parameters has been proposed, which agrees well with the FDTD calculations. The method has also been extended to study the plasmonic wave-guiding in a linear chain of gold nanorod particles. A field decay length up to 2700nm with the lateral mode size about 50nm (~wavelength/28) has been suggested.Comment: 31 pages, 6 figures, 58 reference