Publications by Year: 2012

Christofi A, Stefanou N, Thanos S. Optical modes of chiral photonic composites. Microelectronic Engineering. 2012;90:152-154.Abstract
We report on the eigenmodes of photonic crystals consisting of submicron homogeneous chiral spheres in a nonchiral isotropic medium, by means of full electrodynamic calculations using the layer-multiple-scattering method. It is shown that resonant modes of the individual spheres give rise to narrow bands that hybridize with the extended bands of the appropriate symmetry associated with light propagation in an underlying effective chiral medium. The resulting photonic dispersion diagram exhibits remarkable features, such as strong band bending away from the Bragg points with consequent negative-slope dispersion inside the first Brillouin zone and sizable frequency gaps specific to each polarization mode. We present a rigorous group-theory analysis to explain features of the calculated photonic band structure, peculiar to a system which possesses time-reversal but not space-inversion symmetry, and discuss some interesting aspects of the underlying physics.
Papanikolaou N, Psarobas IE, Stefanou N, Djafari-Rouhani B, Bonello B, Laude V. Light modulation in phoxonic nanocavities. Microelectronic Engineering. 2012;90:155-158.Abstract
We report on the occurrence of strong nonlinear acousto-optic interactions in phoxonic structures, that support, simultaneously, acoustic and optical localized resonant modes, under the influence of acoustic losses. Deploying a detailed theoretical investigation of the acousto-optic coupling in the specific case of a one-dimensional phoxonic cavity, realized by homogeneous SiO2 and Si layers, we demonstrate the possibility for an enhanced modulation of light with sound through multi-phonon exchange mechanisms. A full electrodynamic and elastodynamic multiple scattering approach is employed to describe the optical and acoustic modes, and to account for their mutual interaction and the underlying effects both in time and frequency domains. In particular, we discuss the influence of hypersonic attenuation on the acousto-optic interaction by considering typical acoustic losses in the GHz regime.
Tserkezis C, Stefanou N. Calculation of waveguide modes in linear chains of metallic nanorods. Journal of the Optical Society of America B. 2012;29(4):827-832.Abstract
We report on the calculation of the fundamental plasmon waveguide modes in linear periodic chains of finite silver nanorods, aligned perpendicular to the chain. The results of rigorous full-electrodynamic calculations by the layer-multiple-scattering method are discussed in conjunction with the results of the widely used coupled-dipole model and a critical evaluation of the latter is provided. More specifically, it is shown that both diameter and height of the nanorods must be much smaller than the interparticle distance; otherwise, for relatively long nanorods close to each other, the coupled-dipole model can fail completely to predict the waveguide dispersion diagram. Moreover, the model systematically underestimates the effect of dissipative losses and cannot describe the effect of a supporting substrate, which is always present in realistic cases and induces considerable changes in the waveguide dispersion diagram.
Christofi A, Stefanou N. Photonic structures of metal-coated chiral spheres. Journal of the Optical Society of America B. 2012;29(6):1165=1171.Abstract
A detailed analysis of the optical properties of photonic structures of metal-coated chiral spheres, calculated by the full electrodynamic layer-multiple-scattering method, is presented. Easily tunable narrow bands, originating from particle-like plasmon modes of the metallic shells, hybridize with the extended bands of the underlying effective chiral medium and give rise to sizable partial gaps and strong band bending with consequent negative-slope dispersion. The photonic band diagram is discussed in the light of group theory, in conjunction with relevant transmission spectra, and the occurrence of polarization-selective transmission and negative refraction for a short range of angles of incidence is demonstrated.
Christofi A, Stefanou N, Gantzounis G, Papanikolaou N. Helical assemblies of plasmonic nanorods as chiral metamaterials. Proceedings of SPIE. 2012;8423:84230A (7 pages).Abstract
We report on the optical properties of a layer-by-layer structure of silver nanorods, with their axes aligned perpendicular to the z direction and mutually twisted through an angle of 60° from layer to layer, by means of rigorous full electrodynamic calculations using the layer-multiple-scattering method, properly extended to describe axis-symmetric particles with arbitrary orientation. We analyze the complex photonic band structure of this crystal in conjunction with relevant polarization-resolved transmission spectra of finite slabs of it and explain the nature of the different eigenmodes of the electromagnetic field in the light of group theory. Our results reveal the existence of sizable polarization gaps and demonstrate the occurrence of strong optical activity and circular dichroism, combined with reduced dissipative losses, which make the proposed architecture potentially useful for practical applications as ultrathin circular polarizers and polarization rotators.
Papanikolaou N, Almpanis E, Gantzounis G, Stefanou N. Acousto-optic interaction enhancement in dual photonic-phononic cavities. Proceedings of SPIE. 2012;8425:84250M (7 pages).Abstract
Light control through elastic waves is a well established and mature technology. The underlying mechanism is the scattering of light due to the dynamic modulation of the refractive index and the material interfaces caused by an elastic wave, the so-called acousto-optic interaction. This interaction can be enhanced in appropriately designed structures that simultaneously localize light and elastic waves in the same region of space and operate as dual optical-elastic cavities, often called phoxonic or optomechanical cavities. Typical examples of phoxonic cavities are multilayer films with a dielectric sandwiched between two Bragg mirrors or, in general, defects in macroscopically periodic structures that exhibit dual band gaps for light and elastic waves. In the present work we consider dielectric particles as phoxonic cavities and study the influence of elastic eigenmode vibrations on the optical Mie resonances. An important issue is the excitation of elastic waves in such submicron particles and, in this respect, we analyze the excitation of high-frequency vibrations following thermal expansion induced by the absorption of a femtosecond laser pulse. For spherical particles, homogeneous thermalization leads to excitation of the particle breathing modes. We report a thorough study of the acousto-optic interaction, correct to all orders in the acousto-optic coupling parameter, by means of rigorous full electrodynamic and elastodynamic calculations, in both time and frequency domains. Our results show that, under double elastic-optical resonance conditions, strong acousto-optic interaction takes place and results in large dynamical shifts of the high-Q optical Mie resonances, manifested through multiphonon exchange mechanisms.
A.Christofi, Stefanou N, Gantzounis G, Papanikolaou N. Giant optical activity of helical architectures of plasmonic nanorods. Journal of Physical Chemistry C. 2012;116(31):16674-16679.Abstract
A systematic study, by means of full electrodynamic calculations, of the optical activity of layer-by-layer chiral crystals of finite silver nanorods is presented. The nature of the eigenmodes of the electromagnetic field and the formation of partial gaps for a specific circular polarization in these crystals are analyzed by reference to the hybrid plasmon modes of the structural basis of twisted nanorods. It is shown that collective plasmon modes of the helical assembly give rise to giant optical activity effects, which persist for any angle of incidence and polarization direction. The effects, which are robust against the twisting angle and become more pronounced with increasing particle concentration, can be tuned within a broad range of frequencies in the infrared and visible spectrum by appropriately choosing the rod length. Potential applications of these structures for polarization control in subwavelength optical components are anticipated.
Almpanis E, Papanikolaou N, Gantzounis G, Stefanou N. Tuning the spontaneous light emission in phoxonic cavities. Journal of the Optical Society of America B. 2012;29(9):2567-2574.Abstract
The modulation of spontaneous light emission of active centers through elastic waves in Si/SiO2 multilayer phoxonic structures that support dual photonic-phononic localized modes, in the bulk or at the surface, is studied by means of rigorous full electrodynamic and elastodynamic calculations. Our results show that strong dynamic modulation of the spontaneous emission can be achieved through an enhanced acousto-optic interaction when light and elastic energy are simultaneously localized in the same region.
Almpanis E, Papanikolaou N, Auguié B, Tserkezis C, Stefanou N. Diffractive chains of plasmonic nanolenses: Combining near-field focusing and collective enhancement mechanisms. Optics Letters. 2012;37(22):4624-4626.Abstract
We study, by means of full-electrodynamic calculations using the layer-multiple-scattering method, the effect of diffractive coupling on the enhancement of the local electromagnetic field in periodic arrays of nanolenses consisting of three silver spheres with progressively decreasing sizes and separations. The interaction between the hot-spot modes of an isolated nanolens with the Rayleigh–Wood anomalies of the periodic lattice leads to a further enhancement of the local field intensity, which can be controlled by an appropriate choice of the geometrical parameters involved.