Wednesday 3 September Session 3: Metamaterials Theory (16:15 16:45, Huxley LT308)

Transcription

1 Session 3: Metamaterials Theory (16:15 16:45, Huxley LT308) (invited) TBC

2 Session 3: Metamaterials Theory (16:45 17:00, Huxley LT308) Light trapping states in media with longitudinal electric waves D McArthur, B Hourahine, and F Papoff University of Strathclyde, UK We show that it is possible to form light trapping states, in which radiative losses are completely suppressed, in media that support both internal longitudinal (irrotational) and transverse (divergence free) electric waves. Small metallic spheres and thin wires with non local dispersion are examples of such media, but this phenomenon is very general and happens in media where the photons are strongly coupled with material waves or excitations, such as phonon-polaritons or exciton-polaritons. The irrotational and divergence free waves can combine forming internal states which are decoupled from the scattering field and appear completely dark from the outside. This happens even for electric dipolar modes that would normally have strong radiative losses. The key feature is that the suppression of radiative losses depends on the relative values of the dielectric permittivity and magnetic permeability of internal and external media. This means that a variation of the permittivity of any of the two media will change the nature of the light trapping states, releasing light. The same mechanisms can be used with very small spheres to store light and monitor minute variations of the environment of the particles through the emission of radiation. This has potential applications to time resolved sensing of molecules or measures of temperature or field variations at the nanoscale. We can also propagate these modes over long ranges in waveguides with cross sections orders of magnitude smaller than the wavelength, which provide the essential connections in light processing nano scale devices such as, for instance, nano antenna phased arrays. Media that support light trapping states, furthermore, have a configuration of states that can lead to lasing without inversion.

3 Session 3: Metamaterials Theory (17:00 17:15, Huxley LT308) A time-dependent density functional theory study of quantum plasmons in graphene nano-flakes W Wu and N C Panoiu University College London, UK Recently graphene has attracted much experimental and theoretical interest [1,2]. In contrast, its zero-dimensional form, graphene nano-flake (GNF), also exists but has been studied much less extensively [3]. Especially, the surface plasmons in GNFs have received rapidly growing attention owing to their strong spatial confinement, increased tunability and long lifetimes of plasmons in GNFs. We have studied the plasmonic properties of single and coupled GNFs (the leftmost panels in Fig.1) using timedependent density functional theory. An analysis of these spectra suggests that the optical response of single GNFs is strongly dependant on shape and size. In the case of coupled GNFs (coplanar dimers), the small variations in the optical spectra demonstrate a weak interaction between the two components of the dimer. To illustrate the plasmon excitation in these GNFs, the time evolutions of the calculated charge densities, induced by a continuous-wave excitation electric field, are shown for increasing time values. For the small GNFs (a, c), the edge plasmons are dominant, whereas for larger ones (b, d) multipolar-type collective charge oscillations can be observed, which is a generic feature of localized surface plasmons. This study might be used to guide the experimental observation of quantum plasmons in graphene nano-structures. Fig 1: The optical spectra and charge densities are shown for GNFs. The electrical field direction is along X. T is the excitation period. Rainbow color scheme is chosen to plot the charge density that changes from positive to negative as the color turns from red to blue. [1] A. Castro Neto, et. al., Rev. Mod. Phys. 81, 109 (2009). [2] F. H. L. Koppens, D. E. Chang, and F. J. Garcia de Abajo, Nano Lett. 11, 3370 (2011). [3] Chapter 13, S. Mikhailov (Ed.), Physics and Applications of Graphene - Theory, InTech (2011).

4 Session 3: Metamaterials Theory (17:15 17:30, Huxley LT308) De-excitation of dipole emitters in finite ordered charge-sheet structures A M Hatta 1,2, A A Kamli 1 and M Babiker 3 1 Jazan University, Saudi Arabia, 2 Institut Teknologi Sepuluh Nopember, Indonesia, 3 University of York, UK Layered structures consisting of finite numbers of charge-sheets separating dielectric layers are considered as physically realisable media for the control of dipole de-excitation due to spontaneous emission. The dispersion relation and the corresponding field distributions of a general structure are determined using transfer matrix techniques. An excited dipole emitter localised in the vicinity of the truncation layer of the structure is coupled to the fields supported by such a structure and its de-excitation rate is evaluated for a number of scenarios characterised by different numbers of charge sheets, varying excitation frequency and varying emitter position. The analysis highlights significant enhancements of the de-excitation rate which can be readily controlled through the adjustable parameters of the structure.

5 Session 3: Metamaterials Theory (17:30 17:45, Huxley LT308) Analysis of the large and small signal direct current modulation response of metal-clad nano-lasers Z A Sattar and K A Shore Bangor University,UK In recent year s semiconductor nano-lasers have attracted considerable attention due to their potential applications in photonic integrated circuits, optical information processing and system-on-a-chip technologies. For such applications the salient requirement is the miniaturization of laser structures. Such nano-lasers are anticipated to exhibit enhanced dynamical performance which may arise from a combination of physical factors including Purcell spontaneous emission enhancement factor F, and spontaneous emission coupling factor, β. In recent work, the impact of Purcell enhanced spontaneous emission on the modulation performance of nano-leds and nano-lasers have been examined. Enhancement of nano-laser dynamics have been studied based on the Purcell effect leading to a proposal of modulation bandwidths in excess of 100GHz.However, in complementary work on the dynamical performance of metal-clad nano-lasers it was shown that the direct-current modulation bandwidth of such lasers may suffer deleterious effects due to increased Purcell spontaneous emission enhancement factor and spontaneous emission coupling factor. In the present work, the response of metal clad nano-lasers to direct current modulation has been analysed in both the small signal and large signal regimes. Calculations have been performed using rate equations which include the Purcell cavity-enhanced spontaneous emission factor and the spontaneous emission coupling factor. Taking these effects into account it is observed that the maximum modulation response for large signal response is lower than that of the small signal due to the increased nonlinearity in the large signal regime. Nevertheless for both small and large signal regimes modulation bandwidth of approximately 60GHz can be achieved. Conditions are also identified to achieve peak modulation response occurs at resonant frequencies of 40GHz and 30GHz.

6 Session 3: Metamaterials Theory (17:45 18:00, Huxley LT308) The plasmonic modes of nanoparticle clusters F Papoff, D McArthur and B Hourahine University of Strathclyde, UK We have extended the approach of constructing the principal optical modes [1] of nanoparticles to study groups of particles. The resulting optical super-modes provide a clear picture of the interaction and hybridisation between the modes of the component nanostructures, both bright and dark. Focussing on metallic particles, we accurately solve the multiple scattering of light for dimer structures, investigating the nature of the new dipole resonance which appears as the inter-particle gap closes, and briefly comment on the effects on the modes of non-locality. Using this system as a simple model for the light scattering in surface enhanced spectroscopies, we then consider the coupling to an additional dielectric particle in the region of the dimer, demonstrating that hybridisation of the plasmonic modes with the optical modes of the perturbing extra species provides characteristic signals from the whole system. [1] F. Papoff and B. Hourahine, Geometrical Mie theory for resonances in nanoparticles of any shape, Opt. Express 19, (2011).

7 Session 3: Metamaterials Theory (18:00 18:15, Huxley LT308) Plasmon induced Casimir forces between graphene sheets V Giannini and S A Maier Imperial College London, UK Graphene is a two-dimensional crystal that has singular electronic, mechanical and optical properties. Being graphene a one atom thick structure makes particularly interesting for a fundamental point of view to study Casimir forces between graphene sheets. The Casimir effect is a fundamental quantum-mechanical relativistic phenomenon which originates from the vacuum fluctuations of the electromagnetic field. It interconnects electrically neutral objects with or without permanent electric and/or magnetic moments. In this work the graphene sheets are considered as infinitesimal thin, local and isotropic conductive surfaces and furthermore, the influence of surface plasmons is explored. We will show a detailed study of the Casimir force and we will highlight the surprising major contribution of surface plasmons.