Polarisation charges and scattering behaviour of realistically rounded plasmonic nanostructures

We study the effect of realistically rounding nanorod antennae and gap antennae on their far field and near field properties. The simulations show that both scattering behaviour and polarisation charge distribution depend significantly on rounding. Rounding is also seen to have a major effect on coupling between nanostructures. The results suggest that it is important to incorporate the effect of rounding to be able to design plasmonic nanostructures with desired properties. (C) 2013 Optical Society of Americ

Enhanced light emission by magnetic and electric resonances in dielectric metasurfaces

We demonstrate an enhanced emission of high quantum yield molecules coupled to dielectric metasurfaces formed by periodic arrays of polycrystalline silicon nanoparticles. Radiative coupling of the nanoparticles, mediated by in-plane diffraction, leads to the formation of collective Mie scattering resonances or Mie surface lattice resonances (M-SLRs), with remarkable narrow line widths. These narrow line widths and the intrinsic electric and magnetic dipole moments of the individual Si nanoparticles allow to resolve electric and magnetic M-SLRs. Incidence angle- and polarization-dependent extinction measurements and high-accuracy surface integral simulations show unambiguously that magnetic M-SLRs arise from in- and out-of-plane magnetic dipoles, while electric M-SLRs are due to in-plane electric dipoles. Pronounced changes in the emission spectrum of the molecules are observed, with almost a 20-fold enhancement of the emission in defined directions of molecules coupled to electric M-SLRs, and a 5-fold enhancement of the emission of molecules coupled to magnetic M-SLRs. These measurements demonstrate the potential of dielectric metasurfaces for emission control and enhancement, and open new opportunities to induce asymmetric scattering and emission using collective electric and magnetic resonances.Comment: 27 pages with 9 figure

Contrast in spin-valley polarization due to competing indirect transitions in few-layer WS2_2 and WSe2_2

Controlling the momentum of carriers in semiconductors, known as valley polarization, is a new resource for optoelectronics and information technologies. Materials exhibiting high polarization are needed for valley-based devices. Few-layer WS$_2$ shows a remarkable spin-valley polarization above 90%, even at room temperature. In stark contrast, polarization is absent for few-layer WSe$_2$ despite the expected material similarities. Here, we explain the origin of valley polarization in both materials due to the interplay between two indirect optical transitions. We show that the relative energy minima at the $\Lambda$- and K-valleys in the conduction band determine the spin-valley polarization of the K-K transition. Polarization appears as the energy of the K-valley rises above the $\Lambda$-valley as a function of temperature and number of layers. Our results advance the understanding of the high spin-valley polarization in WS$_2$. This insight will impact the design of both passive and tunable valleytronic devices operating at room temperature.Comment: 22 pages, 6 figures, 2 table

Controlling lasing around Exceptional Points in Coupled Nanolasers

Coupled nanolasers are of growing interest for on-chip optical computation and data transmission, which requires an understanding of how lasers interact to form complex systems. The non-Hermitian interaction between two coupled resonators, when excited selectively, can lead to parity-time symmetry, the formation of exceptional points, and subsequently spectral control and increased sensitivity. These investigations have been limited to pump energies close to the lasing threshold, and large or narrow-line lasers. Here, by programmable optical excitation we study two coupled nanolasers significantly above threshold, where mode instability plays an important role. We map the mode evolution around two exceptional points, and observe lasing gaps due to reversed pump dependence which compare well with nonlinear theory. Finally, the coupling can be exploited to control the lasing threshold and wavelength, and for frequency switching around the lasing gap. Controlled and integrated nanolasers constitutes a promising platform for future highly sensitive and programmable on-chip laser sources.Comment: 8 pages, 4 figure

Origin of enhancement in Raman scattering from Ag-dressed carbon-nanotube antennas: experiment and modelling

The D- and G-band Raman signals from random arrays of vertically aligned, multi-walled carbon nanotubes are significantly enhanced (up to ∼14×) while the signal from the underlying Si substrate is simultaneously attenuated (up to ∼6×) when the nanotubes are dressed, either capped or coated, with Ag. These Ag-induced counter-changes originate with the difference in geometry of the nanotubes and planar Si substrate and contrast in the Ag depositions on the substrate (essentially thin film) and the nanotube (nano-particulate). The surface integral equation technique is used to perform detailed modelling of the electromagnetic response of the system in a computationally efficient manner. Within the modelling the overall antenna response of the Ag-dressed nanotubes is shown to underpin the main contribution to enhancement of the nanotube Raman signal with hot-spots between the Ag nanoparticles making a subsidiary contribution on account of their relatively weak penetration into the nanotube walls. Although additional hot-spot activity likely accounts for a shortfall in modelling relative to experiment it is nonetheless the case that the significant antenna-driven enhancement stands in marked contrast to the hot-spot dominated enhancement of the Raman spectra from molecules adsorbed on the same Ag-dressed structures. The Ag-dressing procedure for amplifying the nanotube Raman output not only allows for ready characterisation of individual nanotubes, but also evidences a small peak at ∼1150 cm−1 (not visible for the bare, undressed nanotube) which is suggested to be due to the presence of trans-polyacetylene in the structures

Orientation Dependence of Plasmonically Enhanced Spontaneous Emission

We computationally explore how the orientation of dipolar emitters placed near plasmonic nanostructures affects their radiative enhancement and spontaneous emission rate. We demonstrate that the expressions for these quantities show a subtle dependence on the molecular orientation, and this information is lost when typical calculations assume a random orientation and perform an average over;all directions. This orientation dependence is strongly affected by the location of the emitter, the emission wavelength, and the symmetry of the system. While the plasmonic nanostructure can significantly modify the far-field from a molecule in its vicinity, this modification is heavily dependent on both the wavelength and the orientation of the emitter. We show that if a fluorescent molecule can be constrained to emit in a specific direction, we are able to obtain far superior; control over its spontaneous emission and decay rate than otherwise and discuss implications for single molecule experiments

Internal optical forces in plasmonic nanostructures

\u3cp\u3eWe present a computational study of the internal optical forces arising in plasmonic gap antennas, dolmen structures and split rings. We find that very strong internal forces perpendicular to the propagation direction appear in these systems. These internal forces show a rich behaviour with varying wavelength, incident polarisation and geometrical parameters, which we explain in terms of the polarisation charges induced on the structures. Various interesting and anomalous features arise such as lateral force reversal, optical pulling force, and circular polarisation-induced forces and torques along directions symmetry-forbidden for orthogonal linear polarisations. Understanding these effects and mastering internal forces in plasmonic nanostructures will be instrumental in implementing new functionalities in these nanophotonic systems.\u3c/p\u3

Boosting Fluorescence-Based Chiral Sensing with Nanophotonics

The handedness of chiral molecules can be detected in their circularly polarized fluorescence, which is typically very weak. Here, we propose dielectric nanophotonics to increase both the fluorescence intensity and polarization contrast. (C) 2021 The Author(s