Publications by Year: 2011

Still T, Gantzounis G, Kiefer D, Hellmann G, Sainidou R, Fytas G, Stefanou N. Collective hypersonic excitations in strongly multiple scattering colloids. Physical Review Letters. 2011;106(17):175505 (4 pages).Abstract
Unprecedented low-dispersion high-frequency acoustic excitations are observed in dense suspensions of elastically hard colloids. The experimental phononic band structure for SiO 2   particles with different sizes and volume fractions is well represented by rigorous full-elastodynamic multiple-scattering calculations. The slow phonons, which do not relate to particle resonances, are localized in the surrounding liquid medium and stem from coherent multiple scattering that becomes strong in the close-packing regime. Such rich phonon-matter interactions in nanostructures, being still unexplored, can open new opportunities in phononics.
Papanikolaou N, Psarobas IE, Gantzounis G, Almpanis E, Stefanou N, Djafari-Rouhani B, Bonello B, Laude V, Martinez A. Phoxonic architectures for tailoring the acousto-optic interaction. Proceedings of SPIE. 2011;8071:80710Z (7 pages).Abstract
Periodic media offer impressive opportunities to manipulate the transport of classical waves namely light or sound. Elastic waves can scatter light through the so-called acousto-optic interaction which is widely used to control light in telecommunication systems and, additionally, the radiation pressure of light can generate elastic waves. Concurrent control of both light and sound through simultaneous photonic-phononic, often called phoxonic, bandgap structures is intended to advance both our understanding as well as our ability to manipulate light with sound and vise versa. In particular co-localization of light and sound in phoxonic cavities could trigger nonlinear absorption and emission processes and lead to enhanced acousto-optic effects. In the present communication, we present our efforts towards the design of different phoxonic crystal architectures such as three-dimensional metallodielectric structures, two-dimensional patterned silicon slabs and simple one-dimensional multilayers, and provide optimum parameters for operation at telecom light and GHz sound. These structures can be used to design phoxonic cavities and study the acousto-optic interaction of localized light and sound, or phoxonic waveguides for tailored slow light-slow sound transport. We also discuss the acousto-optic interaction in onedimensional multilayer structures and study the enhanced modulation of light by acoustic waves in a phoxonic cavity, where a consistent interpretation of the physics of the interaction can be deduced from the time evolution of the scattered optical field, under the influence of an acoustic wave.
Gantzounis G, Papanikolaou N, Stefanou N. Multiple-scattering calculations for layered phononic structures of nonspherical particles. Physical Review B. 2011;83(21):214301 (14 pages).Abstract
We present an extension of the layer-multiple-scattering method to phononic crystals of nonspherical particles in a homogeneous host medium by employing the extended-boundary-condition technique for the description of the individual scatterers. The efficiency of the method is demonstrated on specific examples of two- and three-dimensional periodic assemblies of spheroidal polymer particles in water and in silicon. We report a thorough analysis of the acoustic properties of these composites and emphasize aspects of the underlying physics that relate to the nonspherical shape of the particles.
Christofi A, Stefanou N, Gantzounis G. Photonic eigenmodes and light propagation in periodic structures of chiral nanoparticles. Physical Review B. 2011;83(24):245126 (7 pages).Abstract
We present a detailed analysis of the optical modes and light propagation in photonic crystals consisting of chiral spheres in a nonchiral isotropic medium, calculated by the full electrodynamic 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 diagrams exhibit remarkable features, peculiar to a system that possesses time-reversal but not space-inversion symmetry, which are analyzed in terms of group theory. In particular, we reveal the occurrence of strong band bending away from the Bragg points with consequent negative-slope dispersion inside the first Brillouin zone, slow-photon bands, and frequency gaps. The calculated band structure is discussed in conjunction with relevant reflection diagrams, providing a consistent interpretation of the underlying physics.
Christofi A, Stefanou N, Gantzounis G, Papanikolaou N. Spiral-staircase photonic structures of metallic nanorods. Physical Review B. 2011;84(12):125109 (7 pages).Abstract
By employing the layer-multiple-scattering method, properly extended to periodic assemblies of arbitrarily oriented axis-symmetric particles, we investigate the optical response of a three-dimensional spiral-staircase structure of metallic nanorods. We show that the combination of plasmonic modes and helical arrangement of the nanorods results in the formation of collective optical eigenmodes with a specific predominant circular polarization character, sizable polarization gaps, and negative group velocity bands that lead to negative refraction. Moreover, we demonstrate that multilayer slabs of the given crystal exhibit strong optical activity and circular dichroism combined with reduced dissipative losses, which make the proposed structure potentially useful for polarization control applications in miniaturized optoelectronic devices.
Tserkezis C, Stefanou N, Gantzounis G, Papanikolaou N. Photonic surface states in plasmonic crystals of metallic nanoshells. Physical Review B. 2011;84(11):115455 (5 pages).Abstract
We report on the occurrence and properties of photonic surface states in fcc crystals of metallic nanoshells, by means of full-electrodynamic calculations using the layer-multiple-scattering method, properly extended. Detailed dispersion diagrams of the surface states associated with the (001)  and (111)  surfaces are calculated for such semi-infinite crystals and corresponding finite slabs, and convergence by increasing the slab thickness is discussed. It is shown that these states can be tuned over a broad frequency range by varying the shell thickness and can be characterized, along high-symmetry directions, according to their symmetry. Absorption in the metallic material limits the propagation length which can, however, be as long as several tens of lattice constants for low-loss metals and relatively broad bands.
Gantzounis G, Papanikolaou N, Stefanou N. Nonlinear interactions between high-Q optical and acoustic modes in dielectric particles. Physical Review B. 2011;84(10):104303 (6 pages).Abstract
The interaction between acoustic breathing modes and optical Mie resonances in a spherical particle made of a chalcogenide glass material is investigated by means of rigorous calculations, correct to any order in the acousto-optic coupling parameter. Our results reveal the occurrence of strong effects beyond the linear-response approximation, which lead to enhanced modulation of light by acoustic waves through multiphonon exchange mechanisms when both photons and phonons have a very long lifetime inside the particle.
Tserkezis C, Stefanou N. Negative refraction in plasmonic crystals of metallic nanoshells. Metamaterials. 2011;5(4):189-177.Abstract
We report on the optical response and, in particular, on the refractive properties of an fcc crystal of metallic nanoshells by means of full-electrodynamic layer-multiple-scattering simulations. Exact numerical calculations of the isofrequency surfaces reveal the existence of two frequency regions where negative refraction occurs. A thorough analysis of the photonic band structure, in conjunction with corresponding transmission diagrams, attributes this behavior to the excitation of collective modes, which stem from dipole particle-plasmon resonances, and shows that only in one of the two frequency regions negative refraction without birefringence can be obtained. In addition, we discuss the effect of absorptive losses, and reveal the existence of narrow bands of slab modes in a finite slab of the crystal that can transfer the evanescent components of an incident wave field.