Particle plasmons: Why shape matters

This is the author accepted manuscript. The final version is available from the American Association of Physics Teachers via the DOI in this record.Simple analytic expressions for the polarizability of metallic nanoparticles are in wide use in the field of plasmonics, but their origins are not obvious. In this article, expressions for the polarizability of a particle are derived in the quasistatic limit in a manner that allows the physical origin of the terms to be clearly seen. The discussion is tutorial in nature, with particular attention given to the role of particle shape since this is a controlling factor in particle plasmon resonances.The author is indebted to Baptiste Auguié, Euan Hendry, Chris Burrows, and Alastair Humphrey for many thought-provoking discussions. The assistance of Alastair Humphrey, Martin Gentile, and Ian Hooper with some of the figures is gratefully acknowledged. This work was sponsored both by the Leverhulme Trust and by the Royal Society. The many helpful and constructive comments from the reviewers are also appreciated

Plasmonic surface lattice resonances in arrays of metallic nanoparticle dimers

We investigate the optical response of square arrays of metallic nanoparticles where each lattice site is occupied by two particles, a dimer. In particular we examine the surface lattice resonances arising in these structures when the in-plane dipole moments associated with the plasmon modes of the nanoparticles couple together. The addition of a second particle to the basis leads to a more complex optical response, one that is anisotropic in the plane of the array. Extinction spectra are recorded at normal incidence for different orientations of the incident electric field. We compare our experimentally derived data with those from a coupled-dipole model. We show how the separation between the particles that comprise the dimer helps determine the overall response of the system.The support of The Leverhulme Trust and of the EPSRC (EP/K041150/1) is gratefully acknowledged

Hybridised exciton–polariton resonances in core–shell nanoparticles

This is the author accepted manuscript. The final version is available from IOP Publishing via the DOI in this record.The goal of nanophotonics is to control and manipulate light at length scales below the di↵raction limit. Typically nanostructured metals are used for this purpose, light being confined by exploiting the surface plasmon-polaritons such structures support. Recently excitonic (molecular) materials have been identified as an alternative candidate material for nanophotonics. Here we use theoretical modelling to explore how hybridisation of surface exciton-polaritons can be achieved through appropriate nanostructuring. We focus on the extent to which the frequency of the hybridised modes can be shifted with respect to the underlying material resonances.The work was supported in part by the UK Engineering and Physical Sciences Research Council ((EP/K041150/1), and in part by The Leverhulme Trust

Vibrational Strong Coupling with Surface Plasmons and the Presence of Surface Plasmon Stop Bands (article)

This is the final version. Available on open access from American Chemical Society via the DOI in this recordThe datasets associated with this article are available in ORE at https://doi.org/10.24378/exe.1604We demonstrate strong coupling between surface plasmon resonances and molecular vibrational resonances of poly(methyl methacrylate) (PMMA) molecules in the mid-infrared range through the use of grating coupling, complimenting earlier work using microcavities and localized plasmon resonances. We choose the period of the grating so that we may observe strong coupling between the surface plasmon mode associated with a patterned gold film and the C=O vibrational resonance in an overlying polymer film. We present results from experiments and numerical simulations to show that surface plasmon modes provide convenient open cavities for vibrational strong coupling experiments. In addition to providing momentum matching between surface plasmon modes and incident light, gratings may also produce a modification of the surface plasmon properties, notably their dispersion. We further show that for the parameters used in our experiment surface plasmon stop bands are formed, and we find that both stop-band edges undergo strong coupling.Engineering and Physical Sciences Research Council (EPSRC)European Research Council (ERC

Optimal position of an emitter in a wavelength-scale parabolic reflector (article)

This is the final version. Available on open access from the Optical Society of America via the DOI in this recordThe dataset associated with this article is available in ORE at https://doi.org/10.24378/exe.1883, with the title 'On the optimal position of an emitter in a wavelength-scale parabolic reflector'We investigate the optimum emitter position within reflecting parabolic antennas whose size is comparable to the emission wavelength. Using finite-element modeling we calculate the dependence of the amount of power directed into a 20° half-angle cone on the emitter’s position and compare with results obtained using geometrical optics. The spatially varying density of states within the wavelength-scale reflector is mapped and its impact discussed. In addition, it is demonstrated that changing the characteristic size of the reflector within the range from 0.5 to 1.5 times the emission wavelength has a strong bearing on the optimum emitter position, a position that does not in general coincide with the parabola’s focus. We calculate that the optimal antenna size and emitter position allow for the maximum directed power to exceed that obtained in the geometrical optics regime.DysonEngineering and Physical Sciences Research Council (EPSRC

Electromagnetic interactions in a pair of coupled split-ring resonators

This is the author accepted manuscript. The final version is available from American Physical Society via the DOI in this record.Split-ring Resonators (SRRs) are a fundamental building block of many electromagnetic metamaterials. Typically the response of a metamaterial is assumed to be independent of inter-element interactions in the material. We show that SRRs in close proximity to each other exhibit a rich coupling that involves both electric and magnetic interactions. We study experimentally and computationally the strength and nature of the coupling between two identical SRRs as a function of their separation and relative orientation. We characterise the electric and magnetic couplings and find that, when SRRs are close enough to be in each other's near-field, the electric and magnetic couplings may either reinforce each other or act in opposition. At larger separations retardation effects become important.We acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1)

Hybridization of multiple vibrational modes via strong coupling using confined light fields (article)

This is the final version. Available on open access from Wiley via the DOI in this record.Data associated with this paper can be found at https://doi.org/10.24378/exe.1403Strong coupling of molecules placed in an optical microcavity may lead to the formation of hybrid states called polaritons; states that inherit characteristics of both the optical cavity modes and the molecular resonance. This is possible for both excitonic and vibrational molecular resonances. Previous work has shown that strong coupling may be used to hybridize two different excitonic resonances, this can be achieved when more than one molecular species is included in the cavity. Here we show that under suitable conditions three different molecular vibrational resonances of the same molecular unit may also be coupled together, the resulting polariton having characteristics of all three vibrational resonances. Our results lead us to suggest that strong coupling might be used to manipulate vibrational resonances in a richer and subtler way than previously considered, opening a path to greater control of molecular systems and molecular processes via vibrational strong coupling.Engineering and Physical Sciences Research Council (EPSRC)European Research Counci

Classical antennas, quantum emitters, and densities of optical states

This is the final version. Available on open access from IOP Publishing via the DOI in this recordWe provide a pedagogical introduction to the concept of the local density of optical states (LDOS), illustrating its application to both the classical and quantum theory of radiation. We show that the LDOS governs the efficiency of a macroscopic classical antenna, determining how the antenna's emission depends on its environment. The LDOS is shown to similarly modify the spontaneous emission rate of a quantum emitter, such as an excited atom, molecule, ion, or quantum dot that is embedded in a nanostructured optical environment. The difference between the number density of optical states, the LDOS, and the partial LDOS is elaborated and examples are provided for each density of states to illustrate where these are required. We illustrate the universal effect of the LDOS on emission by comparing systems with emission wavelengths that differ by more than 5 orders of magnitude, and systems whose decay rates differ by more than 5 orders of magnitude. To conclude we discuss and resolve an apparent difference between the classical and quantum expressions for the spontaneous emission rate that often seems to be overlooked, and discuss the experimental determination of the LDOS.Royal SocietyLeverhulme Trus

Absence of Anderson localization in certain random lattices

This is the final version of the article. Available from American Physical Society via the DOI in this record.We report on the transition between an Anderson localized regime and a conductive regime in a one-dimensional microwave scattering system with correlated disorder. We show experimentally that when long-range correlations are introduced, in the form of a power-law spectral density with power larger than 2, the localization length becomes much bigger than the sample size and the transmission peaks typical of an Anderson localized system merge into a pass band. As other forms of long-range correlations are known to have the opposite effect, i.e., to enhance localization, our results show that care is needed when discussing the effects of correlations, as different kinds of long-range correlations can give rise to very different behavior.J.B. acknowledge support from the Leverhulme Trust's Philip Leverhulme Prize. I.R.H. acknowledges financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1)

Molecular Monolayer Strong Coupling in Dielectric Soft Microcavities (article)

This is the final version. Available on open access from the American Chemical Society via the DOI in this recordThe dataset associated with this article is available in ORE: https://doi.org/10.24378/exe.2823We report strong coupling of a monolayer of J-aggregated dye molecules to the whispering gallery modes of a dielectric microsphere at room temperature. We systematically studied the evolution of strong coupling as the number of layers of dye molecules was increased and found the Rabi splitting to rise from 56 meV for a single layer to 94 meV for four layers of dye molecules. We compare our experimental results with two-dimensional (2D) numerical simulations and a simple coupled oscillator model, finding good agreement. We anticipate that these results will act as a stepping stone for integrating molecule-cavity strong coupling in a microfluidic environment since microspheres can be easily trapped and manipulated in such an environment and provide open access cavities.European Research Council (ERC