Spatial wave intensity correlations in quasi-one-dimensional wires

Spatial intensity correlations between waves transmitted through random media are analyzed within the framework of the random matrix theory of transport. Assuming that the statistical distribution of transfer matrices is isotropic, we found that the spatial correlation function can be expressed as the sum of three terms, with distinctive spatial dependences. This result coincides with the one obtained in the diffusive regime from perturbative calculations, but holds all the way from quasi-ballistic transport to localization. While correlations are positive in the diffusive regime, we predict a transition to negative correlations as the length of the system decreases.Comment: 10 pages, 3 figures. Submitted to Physical Review Letter

High-quality photonic crystals with a nearly complete band gap obtained by direct inversion of woodpile templates with titanium dioxide

Photonic crystal materials are based on a periodic modulation of the dielectric constant on length scales comparable to the wavelength of light. These materials can exhibit photonic band gaps; frequency regions for which the propagation of electromagnetic radiation is forbidden due to the depletion of the density of states. In order to exhibit a full band gap, 3D PCs must present a threshold refractive index contrast that depends on the crystal structure. In the case of the so-called woodpile photonic crystals this threshold is comparably low, approximately 1.9 for the direct structure. Therefore direct or inverted woodpiles made of high refractive index materials like silicon, germanium or titanium dioxide are sought after. Here we show that, by combining multiphoton lithography and atomic layer deposition, we can achieve a direct inversion of polymer templates into TiO₂ based photonic crystals. The obtained structures show remarkable optical properties in the near-infrared region with almost perfect specular reflectance, a transmission dip close to the detection limit and a Bragg length comparable to the lattice constant

Controlling dispersion forces between small particles with artificially created random light fields

Appropriate combinations of laser beams can be used to trap and manipulate small particles with optical tweezers as well as to induce significant optical binding forces between particles. These interaction forces are usually strongly anisotropic depending on the interference landscape of the external fields. This is in contrast with the familiar isotropic, translationally invariant, van der Waals and, in general, Casimir-Lifshitz interactions between neutral bodies arising from random electromagnetic waves generated by equilibrium quantum and thermal fluctuations. Here we show, both theoretically and experimentally, that dispersion forces between small colloidal particles can also be induced and controlled using artificially created fluctuating light fields. Using optical tweezers as a gauge, we present experimental evidence for the predicted isotropic attractive interactions between dielectric microspheres induced by laser-generated, random light fields. These light-induced interactions open a path towards the control of translationally invariant interactions with tuneable strength and range in colloidal systemsThis work was supported by the Swiss National Science Foundation through project numbers 132736 and 149867, and through the National Centre of Competence in Research Bio-Inspired Materials. J.J.S. acknowledges financial support by the Spanish MEC (grant number FIS2012-36113), Comunidad de Madrid (grant number S2009/TIC-1476-Microseres Program-) and by an IKERBASQUE Visiting Fellowship (J.J.S.

Speckle fluctuations resolve the interdistance between incoherent point sources in complex media

We study the fluctuations of the light emitted by two identical incoherent point sources in a disordered environment. The intensity-intensity correlation function and the speckle contrast, obtained after proper temporal and configurational averaging, encode the relative distance between the two sources. This suggests the intriguing possibility that intensity measurements at only one point in a speckle pattern produced by two incoherent sources can provide information about the relative distance between the sources, with a precision that is not limited by diffraction. The theory also suggests an alternative approach to the Green's-function retrieval technique, where the correlations of the isotropic ambient noise detected by two receivers are replaced by a measurement at a single point of the noise due to two fluctuating incoherent sources

Band gap formation and Anderson localization in disordered photonic materials with structural correlations

Disordered dielectric materials with structural correlations show unconventional optical behavior: They can be transparent to long-wavelength radiation, while at the same time have isotropic band gaps in another frequency range. This phenomenon raises fundamental questions concerning photon transport through disordered media. While optical transparency in these materials is robust against recurrent multiple scattering, little is known about other transport regimes like diffusive multiple scattering or Anderson localization. Here, we investigate band gaps, and we report Anderson localization in 2D disordered dielectric structures using numerical simulations of the density of states and optical transport statistics. The disordered structures are designed with different levels of positional correlation encoded by the degree of stealthiness χχ. To establish a unified view, we propose a correlation-frequency (χχ– νν) transport phase diagram. Our results show that, depending only on χχ, a dielectric material can transition from localization behavior to a band gap crossing an intermediate regime dominated by tunneling between weakly coupled states

Light emission statistics in correlated random photonic nanostructures

Comunicación presentada en la Conferencia Española de Nanofotónica (CEN2012), celebrada en Carmona (Sevilla) del 1 al 4 de octubre de 2012.The statistical properties of light transport and emission in disordered media has been a matter of intense research during the last century. Being the basis of coherent multiple scattering of wave s well known, the phenomenon itself is not yet fully explored and understood. These multiple wave scattering effects are at the heart of emerging behaviors like Anderson localization of light and electrons, band structure in crystalline solids or photonic crystals (PhC), among many others.Peer Reviewe

Magneto-Optical Activity in High Index Dielectric Nanoantennas

The magneto-optical activity, namely the polarization conversion capabilities of high- index, non-absorbing, core-shell dielectric nanospheres is theoretically analyzed. We show that, in analogy with their plasmonic counterparts, the polarization conversion in resonant dielectric particles is linked to the amount of electromagnetic field probing the magneto-optical material in the system. However, in strong contrast with plasmon nanoparticles, due to the peculiar distribution of the internal fields in resonant dielectric spheres, the magneto-optical response is fully governed by the magnetic (dipolar and quadrupolar) resonances with little effect of the electric ones

Localized magnetic plasmons in all-dielectric μ<0 metastructures

6 pags.; 4 figs.; PACS number(s): 41.20.Jb, 42.70.−a, 52.40.Db, 78.67.−n© 2015 American Physical Society. Metamaterials are known to exhibit a variety of electromagnetic properties nonexisting in nature. We show that an all-dielectric (nonmagnetic) system consisting of deep-subwavelength, high-permittivity resonant spheres possesses effective negative magnetic permeability (dielectric permittivity being positive and small). Due to the symmetry of the electromagnetic wave equations in classical electrodynamics, localized >magnetic> plasmon resonances can be excited in a metasphere made of such metamaterial. This is theoretically demonstrated by the coupled-dipole approximation and numerically for real spheres, in full agreement with the exact analytical solution for the scattering process by the same metasphere with effective material properties predicted by effective medium theory. The emergence of this phenomenon as a function of structural order within the metastructures is also studied. Universal conditions enabling effective negative magnetic permeability relate subwavelength sphere permittivity and size with critical filling fraction. Our proposal paves the way towards (all-dielectric) magnetic plasmonics, with a wealth of fascinating applications.This work was supported by the Spanish MINECO (FIS2012- 31070 and FIS2012-36113) and Consolider-Ingenio EMET (CSD2008-00066).Peer Reviewe

Resonant metal-semiconductor nanostructures as building blocks of low-loss negative- and zero-index metamaterials

5th International Conference on Metamaterials, Photonic Crystals and Plasmonics; Conferencia invitada.Here we propose a 2D isotropic metamaterial with negative electric and magnetic responses in the optical regime, based on hybrid metallo-dielectric core-shell nanowires. The magnetic response stems from the lowest magnetic resonance of the dielectric shell with high refractive index (i.e., lossless semiconductor), and can be tuned to coincide with the plasmon resonance of the metal core, responsible for the electric response. Also, the same metamaterial design is shown to yield zero refractive index for a different spectral regime (in connection with overlapping resonances), exhibiting in turn an impedance close to that of vacuum.The authors acknowledge financial support from the Spanish “Ministerio de Economía y Competitividad” (CSD2008-00066 and FIS2012-31070), and European Social Fund and CSIC (JAE-Pre and JAE-Doc grants).Peer Reviewe