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Three-dimensional magnetophotonic crystals (3D-MPCs) are being postulated as appropriate platforms to tailor the magneto-optical spectral response of magnetic materials and to incorporate this functionality in a new generation of optical devices. By infiltrating self-assembled inverse opal structures with monodisperse nickel nanoparticles we have fabricated 3D-MPCs that show a sizable enhancement of the magneto-optical signal at frequencies around the stop-band edges of the photonic crystals. We have established a proper methodology to disentangle the intrinsic magneto-optical spectra from the nonmagnetic optical activity of the 3D-MPCs. The results of the optical and magneto-optical characterization are consistent with a homogeneous magnetic infiltration of the opal structure that gives rise to both a red-shift of the optical bandgap and a modification of the magneto-optical spectral response due to photonic bandgap effects. The results of our investigation demonstrate the potential of 3D-MPCs fabricated following the approach outlined here and offer opportunities to adapt the magneto-optical spectral response at optical frequencies by appropriate design of the opal structure or magnetic field strength. © 2011 American Chemical Society.

We extensively investigate in-plane light diffusion in systems with thicknesses larger than but comparable with the transport mean free path. By exploiting amplified spontaneous emission from dye molecules placed in the same holder of the sample, we obtain a directional probe beam precisely aligned to the sample plane. By comparing spatial intensity distribution of laterally leaking photons with predictions from random walk simulations, we extract accurate values of transport mean free path, opening the way to the investigation of a previously inaccessible kind of sample. © 2011 Optical Society of America.

Physisorbed and structurally bound (surface and internal) water in silica opals are distinguished and quantified by thermogravimetry. By controlled dehydroxylation with thermal annealing, we correlate these forms of water with the silica chemistry. In particular, we find that the silica capability to physically adsorb water from ambient moisture exhibits three regimes, associated with the distinct condensation behavior of bonded and unbonded surface silanols. Features in both opal IR absorbance and photonic band gap reproduce the physisorbed water regimes. This allows direct assessment of the water content and its evolution just by routine optical spectroscopy, being a useful tool for local and nondestructive analysis of colloidal silica. Besides, this provides a simple recipe for accurate tuning of the opal photonic band gap (about 10% in position and width) by just selecting the annealing temperature. © 2011 American Chemical Society.

We report on an extremely fast and versatile synthetic approach, based on microwave assisted sol-gel chemistry, that allows a conformal nanometric coating of intricate three-dimensional structures. Using this methodology, we have achieved a conformal coverage of large areas of three-dimensional opals with a superparamagnetic manganese ferrite layer, yielding magneto-photonic crystals with excellent quality. The use of a ternary oxide for the ultrathin coating demonstrates the potential of this methodology to realize three-dimensional structures with complex materials that may find applications beyond photonics, such as energy, sensing or catalysis. © 2011 The Royal Society of Chemistry.

In this letter, we present both experimental and numerical studies of the magneto-optical (MO) properties of nickel infiltrated opals. Ni can show interesting MO properties that can be controlled by nanostructuration through colloidal crystals templating. Nanostructuration allows the coupling of light to surface plasmon modes of Ni, and a clear dependence of the MO response as a function of the structural parameters of the template has been observed. This dependence can be used in future tunable devices such as switchers or MO modulators. © 2011 American Institute of Physics.

Optical anisotropy of synthetic opals has been investigated by measuring the spectral dependence of polarized transmitted light at normal and oblique light incidence at various sample azimuth orientations. The optical anisotropy of thin films and bulk samples has been measured by the polarization modulation technique in the visible spectral range. The observed optical anisotropy of synthetic opals was caused by several contributions. (i) Internal strain induced during the growth process. The optical anisotropy axes along [1 1 0] and 1̄12 directions in opal lattice of fcc symmetry were indicated in the (1 1 1) plane for both thin films and bulk samples of synthetic opal. (ii) Particular features of Brillouin zone manifesting themselves as the 3rd and 6th order rotation axes in the azimuth-resolved dependence of anisotropy parameters in the (1 1 1) plane. (iii) Oblique light incidence leading to the anisotropy with respect to the incidence plane. © 2010 Elsevier B.V. All rights reserved.

Self-assembly is a well established approach to fabricate photonic structures able to manipulate light propagation and emission at the nanoscale. Beyond its use as a means to fabricate photonic crystals recent reports have shown how self-assembly techniques can be used to fabricate hybrid photonic-plasmonic crystals that allow strong modification of the spontaneous emission of internal sources [2]. Further, the sensitivity of their optical response to their environment has been proposed as a means to use this kind of systems of optical sensors, and means to control their optical properties by modifying the dielectric components have been demonstrated. In these hybrid structures, where periodic arrays of dielectric colloids are deposited on metallic substrates, electromagnetic fields undergo a strong spatial redistribution and strong field enhancements can be achieved. Depending on the spectral range under consideration electromagnetic fields can be localized close to the metal substrate (plasmon-like modes) or within the dielectric array (waveguided-like modes). Several metallodielectric structures have been fabricated employing gold or silver substrates and their optical response has been compared to reference samples grown on dielectric substrates in order to better appreciate the effect of the metallic component. The dispersion relation of such structures has been probed by means of angle and polarization resolved reflectivity measurements.

Some characteristics of silica-based structuresa-like the photonic properties of artificial opals formed by silica spheresa-can be greatly affected by the presence of adsorbed water. The reversible modification of the water content of an opal is investigated here by moderate heating (below 300 °C) and measuring in situ the changes in the photonic bandgap. Due to reversible removal of interstitial water, large blueshifts of 30 nm and a bandgap narrowing of 7% are observed. The latter is particularly surprising, because water desorption increases the refractive index contrast, which should lead instead to bandgap broadening. A quantitative explanation of this experiment is provided using a simple model for water distribution in the opal that assumes a nonclose-packed fcc structure. This model further predicts that, at room temperature, about 50% of the interstitial water forms necks between nearest-neighbor spheres, which are separated by 5% of their diameter. Upon heating, dehydration predominantly occurs at the sphere surfaces (in the opal voids), so that above 65 °C the remaining water resides exclusively in the necks. A near-close-packed fcc arrangement is only achieved above 200 °C. The high sensitivity to water changes exhibited by silica opals, even under gentle heating of few degrees, must be taken into account for practical applications. Remarkably, accurate control of the distance between spheresa-from 16 to 1 nma-is obtained with temperature. In this study, novel use of the optical properties of the opal is made to infer quantitative information about water distribution within silica beads and dehydration phenomena from simple reflection spectra. Taking advantage of the well-defined opal morphology, this approach offers a simple tool for the straightforward investigation of generic adsorption-desorption phenomena, which might be extrapolated to many other fields involving capillary condensation. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Many modern systems are based on photoresponsive materials, in which properties such as the refractive index need to be effectively controlled. An extensively used means of achieving this is to photoinduce birefringence by alignment of anisotropic azochromophores via light-induced isomerization. However, the refractive index changes are typically small (<10-2), slow (seconds or minutes) and not spontaneously reversible, which excludes use of this approach in a variety of optical systems. The drawbacks are generally attributed to hindered photoalignment due to the molecular environment, which suggests that optimizing the arrangement of the functional moieties to minimize the mobility restrictions could decisively improve the photoresponse. Here, a simple solution-processing approach is reported for favorable distribution at the molecular level of neat azochromophore into a three-dimensionally nanostructured hybrid system exhibiting an extremely enhanced photoresponse. The standard azoderivative Disperse Red 1 is adsorbed on silica colloidal crystals that are chosen as 3D-templates because their photonic bandgap, which is sensitive to refractive index changes, provides a direct tool to study photostimulated processes in the chromophore. The system is thoroughly investigated with different techniques to identify molecular out-of-plane photoalignment as the main phenomenon responsible for the optical response, and to discern the key factors leading to improved performance. It is found that the dye molecules are spontaneously adsorbed on the silica spheres, building a highly photoreactive surface multilayer. A low amount of azochromophore allows for outstanding material response upon cw-irradiation, as a result of very large and fast refractive index changes in the chromophore ensemble (up to 0.36 - birefringence of 1.1 - in 15 ms at 0.09 J cm-2 ) that, in addition, is fully reversible by thermalization. Finally, as proof-of-principle for real applications, long-duty cyc

3D-nanostructured ensembles of silicon nanocrystals forming inverse opals, directly prepared by magnesioreduction, exhibited intense photoluminescence in a broadband range (visible and NIR). Light emission, which strongly diminished under photoexcitation, was preserved by engineering the nanocrystals interface with atomic layer deposition of alumina. The high accessibility of the nanocrystals throughout the self-supported structure was a key factor for efficient photoluminescence stabilization. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Se describe un dispositivo electroluminiscente conformado a partir de una monocapa de esferas que comprenden elementos emisores de luz tanto orgánicos como inorgánicos que se encuentra definida sobre un substrato que comprende una capa de oro depositada sobre una oblea.

High-permittivity dielectric particles with resonant magnetic properties are being explored as constitutive elements of new metamaterials and devices. Magnetic properties of low-loss dielectric nanoparticles in the visible or infrared are not expected due to intrinsic low refractive index of optical media in these regimes. Here we analyze the dipolar electric and magnetic response of lossless dielectric spheres made of moderate permittivity materials. For low material refractive index (. 3) there are no sharp resonances due to strong overlapping between different multipole contributions. However, we find that Silicon particles with index of refraction ∼ 3.5 and radius ∼ 200nm present strong electric and magnetic dipolar resonances in telecom and near-infrared frequencies, (i.e. at wavelengths ≈ 1.2−2mm) without spectral overlap with quadrupolar and higher order resonances. The light scattered by these Si particles can then be perfectly described by dipolar electric and magnetic fields.

The light confinement properties of hybrid metallodielectric systems have been studied employing numerical simulations to obtain the optical response as well as the total field intensity associated with the different modes of the structure. The effect of intrinsic losses, absorption, and out-of-plane leakage on the quality factors (Q) of different modes is discussed, and the results obtained are interpreted in terms of the optical constants of the metallic substrate. Large Q values (up to 600) can be attained in this kind of samples, much larger than in their purely dielectric counterpart, pointing to the ability of these systems to efficiently confine electromagnetic radiation.

Photonic crystals have proven their potential and are nowadays a familiar concept. They have been approached from many scientific and technological flanks. Among the many techniques devised to implement this technology self-assembly has always been one of great popularity surely due to its ease of access and the richness of results offered. Self-assembly is also probably the approach entailing more materials aspects owing to the fact that they lend themselves to be fabricated by a great many, very different methods on a vast variety of materials and to multiple purposes. To these well-known material systems a new sibling has been born (photonic glass) expanding the paradigm of optical materials inspired by solid state physics crystal concept. It is expected that they may become an important player in the near future not only because they complement the properties of photonic crystals but because they entice the researchers' curiosity. In this review a panorama is presented of the state of the art in this field with the view to serve a broad community concerned with materials aspects of photonic structures and more so those interested in self-assembly.

We report here on the fabrication and characterization of magnetophotonic crystals obtained by infiltrating magnetic nanoparticles (maghemite) into silica-based inverse opals. Good quality opals, with adjustable photonic and magnetic characteristics have been obtained by this method. Magnetic and magneto-optic measurements, performed by using SQUID magnetometry and Kerr and Faraday effects, have been used to monitor the magnetic filling of the opals. It is observed that the Kerr rotation is not proportional to the overall magnetization of the magnetophotonic crystal. Possible scenarios for this unexpected result are discussed. © 2009 Elsevier B.V. All rights reserved.

Monodisperse colloids have been prepared efficiently by (copolymerization of methyl methacrylate and fluorescent first- and second-generation poly(phenylenevinylene) dendrons under surfactant-free emulsion polymerization conditions. The copolymers were characterized by UV-vis and fluorescence spectroscopy and size exclusion chromatography. Transmission electron microscopy revealed that the copolymers were microspheres with smooth surfaces and narrow dispersity. The bead diameter could be varied by changing the monomer/water ratio. The materials could be crystallized to give polymer opal photonic crystals. The emission was not affected by the periodic structure because of the large spectral distance between the emission and the pseudogap position. © 2010 Wiley Periodicals, Inc.

A hybrid photonic-plasmonic crystal structure comprising a close-packed monolayer of dielectric spheres deposited on plasmon-supporting gold substrate is investigated. The spontaneous emission of organic dyes embedded in the beads experiences a strong modification in intensity, polarization, and directionality. These facts are accounted for considering the hybridization of modes arising from the photonic and plasmonic components of the structure. © 2010 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.

The optical properties of two-dimensional hybrid photonic-plasmonic crystals are fine-tuned by modifying the dielectric component of the system. The filling fraction of the dielectric component in monolayers of spheres deposited on gold substrates is controlled by means of oxygen-plasma etching. Doing so enables spectral tuning of the optical modes of the system. Experiments are performed on both optically passive and active samples showing the possibility for strong modification of the emission properties of samples containing emitters distributed within the spheres. The change in sphere diameter needed to substantially modify the sample's optical response points to a potential use of these samples as sensors or tunable emitting devices if appropriate polymeric components are employed. The optical properties of two-dimensional hybrid photonic-plasmonic crystals are fine-tuned by employing oxygen plasma to modify the filling fraction of the lattice. Simulations and experiments agree in showing a high degree of spectral tunability of the modes of the system in a continuous manner. When applied to active samples a strong modification of the emission of internal sources is obtained. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Self-assembly techniques are widely used to grow ordered structures such as, for example, opal-based photonic crystals. Here, we report on photonic glasses, new disordered materials obtained via a modified self-assembling technique. These random materials are solid thin films which exhibit rich novel light diffusion properties originating from the optical properties of their building blocks. This novel material inaugurated a wide range of nanophotonic materials with fascinating applications, such as resonant random lasers or Anderson localization. ©2010 WILEY-VCH Verlag GmbH & Co. KGaA.

Method of synthesis of photonic band gap (PBG) materials. The synthesis and characterization of high quality, very large scale, face centered cubic photonic band gap (PBG) materials consisting of pure silicon, exhibiting a complete three-dimensional PBG centered on a wavelength of 1.5 mu m. This is obtained by chemical vapor deposition and anchoring of disilane into a self-assembling silica opal template, wetting of a thick silicon layer on the interior surfaces of the template, and subsequent removal of the template. This achievement realizes a long standing goal in photonic materials and opens a new door for complete control of radiative emission from atoms and molecules, light localization and the integration of micron scale photonic devices into a three-dimensional all-optical micro-chip.

We describe a smooth transition from (fully ordered) photonic crystal to (fully disordered) photonic glass that enables us to make an accurate measurement of the scattering mean free path in nanostructured media and, in turn, establishes the dominant role of the density of states. We have found one order of magnitude chromatic variation in the scattering mean free path in photonic crystals for just $\sim 3%$ shift around the band-gap ($\sim 27$ nm in wavelength).

A study was conducted to demonstrate a process to fabricate silicon (Si) opals by magnesiothermic reduction of silica opals. The reduction of silica with gaseous Mg produced Si and magnesia keeping the structure unchanged, while the magnesia selective etching with HCl solutions allowed to obtain opals made of p-Si spheres. The porous nature of these p-Si spheres involved a lower dielectric constant than that of Si. X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and optical spectroscopy were performed to monitor the process and analyze the crystal quality and composition of the samples. The spheres changed during the magnesiothermic reduction and CVD process were analyzed by TEM. The silica spheres (575nm diameter) produced by the standard Stöber method were treated at 600°C to prevent the appearance of cracks during the opal magnesiothermic reduction.

A random laser is a system formed by a random assembly of elastic scatterers dispersed into an optical gain medium. The multiple light scattering replaces the standard optical cavity of traditional lasers and the interplay between gain and scattering determines the lasing properties. All random lasers studied to date have consisted of irregularly shaped or polydisperse scatterers, with a certain average scattering strength that was constant over the frequency window of the laser. In this letter we consider the case where the scattering is resonant. We demonstrate that randomly assembled monodisperse spheres can sustain scattering resonances over the gain frequency window, and that the lasing wavelength can therefore be controlled by means of the diameter and refractive index of the spheres. The system is therefore a random laser with an a priori designed lasing peak within the gain curve.

The realization of photonic glasses, solid random distributions of monodisperse spheres, which are a new photonic material was investigated. Thin disordered films was grown by combining a binary colloidal suspension composed by polymethyl metacrylate (PMMA) and polystyrene (PS) spheres. Few methods were tested to get a completely disordered arrangement of monodisperse spheres such as rapid sedimentation or modified vertical deposition. The two-body sphere-sphere potential of an aqueous colloidal suspension of charged spheres was expressed as a sum of two terms. It was observed that SEM images of sample 780 nm PS spheres was grown. It was also observed that the crystals grown by the colloidal charge interaction provides a very thick and completely random samples which can also be used as templates to grow higher refractive index materials.

A large-area, high-quality, new composite materials obtained by structuring a ZnO inverted opal with colloidal CdTe nanocrystals in two steps of hierarchical colloidal self-assembly was reported. The fabrication is performed by an accurate infiltration in two hierarchical steps of colloidal self-assembly and the method of infiltrating a ZnO matrix with CdTe nanocrystals provides fine control over the pore-filling fraction. The emission properties of the composites prepared were measured and compared with those of the same sources without a pseudogap (pG) environment. The high ZnO refractive index provides enough dielectric contrast in the inverted structure to open additional pGs in the high-energy regime which have been used for enabling laser emission.

A prominent type of amorphous material, nanoribbons, is developed in the field of nanoscience that grow as extremely long wires whose cross section is rectangular with a large aspect ratio. It was found that such materials present good mechanical properties that allow them to be micromanipulated. Crystallinity and covalent bonding provides oxides with a structure and mechanical properties that prevent fatigue. The extreme aspect ratio allows these nanoribbons to be bent into very tight loops, although these materials are brittle. One of the main application for such nanoribbons is waveguiding, typically in an optical fiber where the fibers are usually a few micrometers in diameter to ensure single mode operation. The guided luminescence was analyzed both spectroscopically and as a function of the length traveled by detecting the light exiting the ribbon at one end. The results show the nanoribbons can be used for integrating sub-wavelength photonic components.

The growth of high-quality stoichiometric indium phosphide by atmospheric pressure metal-organic chemical vapor deposition within thin-film artificial opals has been optimized. The optical properties of these systems have been studied as a function of the filling fraction. A consistent behavior has been found which substantially differs from that of other common infilled opals. The evolution of the InP morphology seems to be strongly correlated with this anomalous photonic response. At the first infiltration stages, the growth leads to the formation of nanocrystallites that eventually, as the infiltration increases, coalesce in single-crystalline grains of increasing size. The possibility to oxidize the material adds to its potential as a useful optical material. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Zinc oxide (ZnO) was grown in synthetic opals by using chemical vapor deposition (CVD) method. The CVD method yielded large area, high quality ZnO-polystyrene (PS) and ZnO inverse opals, with very precise control of degree of infiltration. ZnO was grown in thin PS opal films on silicon substrates, using a diluted solution of DMZn in hexane as zinc precursor. Double-distilled water (DDW) was used as a oxygen precursor. Inverse opals were obtained upon calcination of PS-ZnO composites at 450 degree C for 3 hours in air.

An optical and morphological study has been carried out to understand the role of intrinsic defects in the optical properties of opal-based photonic crystals. By doping poly(methylmethacrylate) (PMMA) thin-film opals with larger polystyrene (PS) spheres, structural disorder has being generated perturbing the PMMA matrix periodicity. It is shown that this disorder dramatically affects the optical response of the system worsening its photonic properties. It has been found that the effect of doping is highly dependent not only on the concentration but also on the relative size of the dopant with reference to the matrix. Through a detailed scanning electron microscopy inspection, the sort of structural defects involved, derived from the different particle size used, has been characterized. A direct relationship between the observed optical response with the different perturbations generated in the lattice has been found. In addition, from this study it can be concluded that it is possible to grow high quality alloyed photonic crystals, exhibiting intermediate photonic properties between pure PMMA and pure PS opals by simple sphere size matching and variation of the relative concentration of both components.

A method that allows the introduction of micrometer-sized, three-dimensional extrinsic defects in poly(methyl methacrylate) (PMMA)/SiO 2 composites was discussed. Patterning of the PMMA/SiO2 composite was performed by electron beam lithography (EBL). The PMMA/SiO 2 composites provide better mechanical stability of the colloidal structure to withstand EBL and selective removal of PMMA leads to inverted structures. The results show that the method is very convenient to introduce controlled, three-dimensional defects into 3D photonic colloidal crystals.

A method that allows the fabrication of opal slab heterostructures incorporating planar defects was discussed. The method is based on a multi-step process combining chemical vapor deposition and convective colloidal self assembly techniques. It allows precise control over the parameters that determine the optical properties of these systems, permitting their tuning. The optical properties of the fabricated structures were also discussed.