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Percolation is revealed by Fano resonance
Reflectance with Fano lineshape The understanding of how the arrangement of defects in photonic crystals impacts its photonic properties is cru- cial for the design of functional materials based thereon. By preparing photonic crystals with random missing scatterers we create crystals where disorder is embodied as vacancies in an otherwise perfect lattice rather than the usual positional or size disorder. We show that the amount of defects not only determines the intensity but also the nature of the light scattering. As the amount of defects varies, light scattering undergoes a transition whereby the usual signatures of photonic gaps (Bragg peak) suffer line-shape changes (Bragg dip) that can be readily described with the Fano resonance q parameter. When the amount of vacancies reaches the percolation threshold, q undergoes a sign change signaling the transition from a crystal to a mosaic of microcrystals through a state where scattering is maximum. Beyond that point the system reenters a state of low scattering that appears in the guise of normal Bragg diffraction. ( read more)
Mie resonances in the visible
Mie for several sizes A new method to fabricate silica photonic glasses from powder by using a uni-axial press in dry conditions. This novel procedure is much faster than the traditional ones and is capable to yield thick solid, free-standing photonic glasses that show resonant optical modes. This novel material lends itself useful in the field of granular media as well as in disorder photonics. The packing process and the flow of particles under pressure can be studied in pressed PGs. We have demonstrated by total transmittance and coherent back scattering measurements that the applied pressure does not affect the resonant optical response but has sizeable impact on the mechanical stability of the resultant PG. A simple but powerful phenomenological model of forward scattering has been developed that accounts for the ripples in the scattering cross section. (More soon)


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Replica symmetry breaking
SMP cycle Intensity fluctuations in lasers are commonly studied above threshold in some special configurations (especially when emission is fed back into the cavity or when two lasers are coupled) and related with their chaotic behaviour. Similar fluctuating instabilities are usually observed in random lasers, which are open systems with plenty of quasi-modes whose non orthogonality enables them to exchange energy and provides the sort of loss mechanism whose interplay with pumping leads to replica symmetry breaking. The latter however, had never been observed in plain cavity lasers where disorder is absent or not intentionally added. Here we show a fluctuating lasing behaviour at the lasing threshold both in solid and liquid dye lasers. Above and below a narrow range around the threshold the spectral line- shape is well correlated with the pump energy. At the threshold such correlation disappears, and the system enters a regime where emitted laser fluctuates between narrow, intense and broad, weak peaks. The immense number of modes and the reduced resonator quality favour the coupling of modes and prepares the system so that replica symmetry breaking occurs without added disorder. (DOI: 10.1038/srep32134). (read more)
Monodisperse carbon spheres
SMP cycle Highly monodisperse core-shell carbon spheres in a size range between 500 and 900 nm can be grown by hydrothermal reaction from glucose in the presence of polystyrene seeds. Careful control over temperature, time, glucose concentration and seed size yields excellent hybrid spheres with no aggregation and no additional spheres population. Pyrolysis transforms the hybrid spheres in hollow carbon spheres with preserved monodispersity. This approach provides a basis to synthesize functional hollow carbon spheres with potential in photonics and energy applications. (DOI: 10.1002/smll.201600902). (read more)
Water in colloidal crystals
SMP cycle Solid colloidal ensembles inherently containwater adsorbed fromthe ambientmoisture. This water, confined in the porous network formed by the building submicron spheres, greatly affects the ensemble properties. Inverse- ly, one can benefit from such influence on collective features to explore the water behavior in such nanoconfinements. Recently, novel approaches have been developed to investigate in-depth where and how water is placed in the nanometric pores of self-assembled colloidal crystals.Here,wesummarize these advances, alongwith newones, that are linked to general interfacialwater phenomena like adsorption, capillary forces, and flow. Water-dependent structural properties of the colloidal crystal give clues to the interplay between nanoconfined water and solid fine particles that determines the behavior of ensembles.We elaborate on how the knowledge gained onwater in colloidal crystals provides newopportunities formultidisciplinary study of in- terfacial and nanoconfined liquids and their essential role inthe physics ofutmost important systems suchaspar- ticulate media. (DOI: 10.1016/j.cis.2016.05.004). (read more)
RL emission regimes
SMP cycle We report the experimental results obtained with a novel architecture for random lasing, in which the active material, free of scatterers, is placed between two large scattering regions. Lasing emission is investigated as a function of the illuminated area of the scattering regions, obtaining typical “resonant” and “non-resonant” random lasing spectra, depending on the device geometry. We propose a theoretical approach for the understanding of the observed phenomena, modelling the scattering elements with arbitrary spectral profiles in amplitude and phase and considering strong coupling between lasing modes. Good agreement between experiments and simulation results is obtained. (DOI: 10.1364/OE.24.010912). (read more)
Organic−Inorganic Trihalide Perovskite Films
SMP cycle In this work we demonstrate that the different processes occurring during hybrid organic−inorganic lead iodide perovskite film formation can be identified and analyzed by a combined in situ analysis of their photophysical and structural properties. Our observations indicate that this approach permits unambiguously identifying the crystal nucleation and growth regimes that lead to the final material having a cubic crystallographic phase, which stabilizes to the well-known tetragonal phase upon cooling to room temperature. Strong correlation between the dynamic and static photoemission results and the temperature-dependent X-ray diffraction data allows us to provide a description and to establish an approximate time scale for each one of the stages and their evolution. The combined characterization approach herein explored yields key information about the kinetics of the process, such as the link between the evolution of the defect density during film formation, revealed by a fluctuating photoluminescence quantum yield, and the gradual changes observed in the PbI2- related precursor structure. (DOI: 10.1021/acs.jpcc.6b00398). (read more)
Engineering mean free path
SMP cycle A photonic glass composed of monodisperse silica spheres is stepwise infiltrated with additional conformal layers of silica, by using chemical vapor deposition. It is shown that for a small infiltration degree, the resonant features of the glass are preserved. The possibility of modifying the light transport mean free path in the sample is further discussed, with an approximately ten-fold increase being reported.
(DOI: 10.1002/ppsc.201500202). (read more)
Decoupling gain and feedback
SMP cycle We propose and demonstrate a coherent random laser in which the randomly distributed scattering centres are placed outside the active region. This architecture is implemented by enclosing a dye solution between two agglomerations of randomly positioned titanium dioxide nanoparticles. The same spectral signature, consisting of sharp spikes with random spectral positions, is detected emerging from both ensembles of titanium dioxide nanoparticles. We interpret this newly observed behaviour as due to the optical feedback given by back-scattered light from the scattering agglomerations, which also act as output couplers. A simple model is presented to simulate the observed behaviour, considering the amplitude and phase round trip conditions that must be satisfied to sustain lasing action. Numerical simulations reproduce the experimental reports, validating our simple model. The presented results suggest a new theoretical and experimental approach for studying the complex behavior of coherent random lasers and stimulate the realization of new devices based on the proposed architecture, with different active and scattering materials. (DOI: 10.1038/srep16848). (read more)
Large area DNA based random laser
SMP cycle We report resonant feedback random lasing from dye-doped biopolymer films, consisting of a deoxyribonucleic acid-cetyltrimethylammonium (DNA-CTMA) complex doped with DCM dye. In the proposed devices, the optical feedback for random lasing is given by scattering centers randomly positioned along the edges of the active area. Scattering elements are either titanium dioxide nanoparticles or random defects at the interface between active polymer and air. Different emission spectra are observed, depending on the geometry of the excited area. A single random resonator with dimensions of 2.6 mm x 0.65 mm is fabricated and random emission with resonant feedback is obtained by uniformly pumping the full device. (DOI: 10.1364/OE.23.029954). (read more)
Dye-doped truxene-based organogels
SMP cycle Organic systems comprising a truxene-based organogel doped with an organic dye have been fabricated and their photophysical properties examined in the search for an organic matrix with tunable luminescent properties. The addition of the organic dopant has been observed to introduce changes in the morphology of the gel which alters the ratio between monomer and excimer species. Further, the luminescent properties of the doped organogel have been studied and their evolution with dopant concentration explained in terms of resonant energy transfer between the excimer species (acting as a donor) and the organic dopant (acceptor). The interplay between blue, green and red emission bands associated with monomers, excimers and organic dopants allows tuning the luminescence of the system within the visible region reaching white light emission under certain conditions. The origin of the energy transfer is found to be the aggregation of the molecules upon solvent evaporation, the more stable xerogel phase being the extreme case which constitutes a technologically relevant approach where solvent evaporation is not an issue.
(DOI: 10.1039/c5tc00706b). (read more)
Shape-memory and optical gain
SMP cycle This makes it possible to efficiently embed titania particles, homogeneously dispersed, in the shape memory polymer PDDC-HD, an elastomeric matrix, and take advantage of their strong scattering efficiency for developing new diffusive media with gain embedded. (DOI: 10.1002/adom.201500128). (read more)
Shape memory optical polymer
SMP cycle This work addresses the fabrication of periodic nanofeatures with shape memory effect by using a replica molding approach for imprinting a 2D photonic nanostructure on the surface of a shape-memory elastomer. Structural and optical characterization of the as-produced system allowed to demonstrate its shape-memory functionality for configuring the lattice parameter or erasing the nanopattern. This property might be very attractive, for example, for the development of reusable or self-healing photonic elements with adaptive properties. (DOI: 10.1002/adom.201300532). (read more)
Silica protected QDs
Shelled QDs SiO2 encapsulation of alloyed CdSeZnS nanocrystals (NCs) shows differences in terms of optical properties and luminescence quantum yield, depending on the surface composition, size, and ligand content. In this work, emphasis has been placed on the fine control required to obtain luminescent SiO2 encapsulated NCs by studying the role of oleic acid (OA), stearic acid (SA), and dodecanethiol (DDT) ligands on the alloyed NCs. While the use of anchored DDT molecules is essential to preserve the optical properties, intercalated OA and SA play a critical role for SiO2 nucleation, as stated by 1H NMR (including DOSY and NOESY) spectroscopy. These results emphasize the importance of surface chemistry in NCs; it is crucial to control their reactivity, and therefore their impact, in different applications, from optics to biomedicine. DOI: 10.1021/acsami.5b00820 (read more)
Red FRET photonic crytals
RL by Coupled FRET dyes Red-luminescent PMMA spheres containing a Fo ¨rster resonance energy transfer (FRET) pair were
synthesized via a two-step polymerization method. Two reaction parameters, time and monomer volume, are scanned in order to tune the sphere diameter in the 250–500 nm range. Further the polydispersity of the spheres is kept low, at ca. 3%, regardless of sphere diameter or dye concentration. A thorough optical characterization via spectroscopy and time resolved measurements shows a FRET efficiency of over 40% before concentration quenching effects take place, allowing for a precise tuning of their emission in the red region of the visible spectrum. The high quality of these spheres makes them suitable to fabricate self-assembled 3D photonic crystals which act as photonic environment to modify the spectral properties of the FRET pair via Bragg diffraction. DOI: 10.1039/c5tc00528k (read more)
Where's water?
RL by Coupled FRET dyes Colloidal crystals may contain a significant amount of water, mainly from adsorption of surrounding moisture. Their unique features (photonic properties, well-defined configuration) make colloidal crystals a adequate environment to study water-interface nanoscale phenomena, relevent not only for colloidal science but also for more general matters like wetting, capillarity, micro/nanofluidics, nanotribology, etc. We gather and give perpspective to novel methods developed and results achieved recently by our laboratory. DOI: 10.1002/adma.201405008 (read more)
FRET enabled RL
RL by Coupled FRET dyes We report the fabrication and characterization of a novel type of optical gain material. A biopolymeric matrix (DNA−lipid complex) doped with two families of organic dyes is combined with a strongly scattering medium. While the optical gain of the biopolymer complex is controlled via the FRET efficiency between the incorporated dyes, multiple scattering provides the necessary feedback to achieve lasing. This introduces two mechanisms to control the lasing wavelength: optical gain (via resonant energy transfer) and resonant scattering. In this way, an organic laser with a spectral tunability range over 40 nm is demonstrated. (read more)
From Boch to RL
From Bloch mode lasing to RL Lasing is achieved in ZnO ordered and disordered nanostructures. In the former by exploiting very low group-velocity Bloch modes in the photonic crystals and in the latter, photonic glasses are explioted to realize random lasing. We determine the lasing threshold in both cases and its dependence on the structural parameters and observe the transition from Bloch to random lasing by deliberately doping a ZnO inverse photonic crystal with a controlled amount of lattice vacancies effectively converting it into a translationally correlated photonic glass. (read more)
Dynamics of phase.locking in RL
Spectra under nano or pico pumping Laser modes may coalesce into a mode-locked state that enables femtosecond pulse compression. The nature of the interaction and the interaction time play fundamental roles in the onset of this collective state, but the investigation of the transition dynamics is technically challenging because phases are not always experimentally accessible. This is even more difficult for random lasers. A dedicated experimental setup capable to pump individual modes with different pulse durations shows that the mode-locked regime builds only for quasicontinuous pumping, resulting in an emission linewidth dependent on the pump duration. Numerical simulation confirms experimental data. (read more)
FRET in dye-doped DNA-lipid complex
FRET The FRET efficiency of DNA-lipid complexes can be controlled by means of the donor/acceptor concentration ratio reaching efficiencies close to 100%. Time-resolved techniques reveal that in this kind of systems a distribution of donor-acceptor configurations leads to a distribution of FRET decay rates whose mean value and width strongly depend on the concentrantion ratio. Further we have monitored the evolution of ASE in our system as the FRET efficiency is changed and found that for efficiencies as low as 29% the energy transferred to the accepptor is enough to create ASE with a frequency for which the acceptor is transparent. The possibility of introducing a DNA FRET-based gain medium in resonators is beheld as a means to reduce the amount of gain medium needed to achieve lasing. (read more)
Non-local and collective behaviour in random laser
individual and collective behaviour Since the optical cavities in random lasers are open, the modes are coupled and energy may pour from one state to another provided they are spatially overlapping. Although the electromagnetic modes are spatially localized, our system may be actively switched to a collective state, presenting a novel form of nonlocality revealed by high degree of spectral correlation between light emission collected at distant positions. In a nutshell light may be stored in a disordered nonlinear structure in different fashions strongly differing in their spatial properties. (read more)
Switching and amplification in random lasers
Switching and amplifying light in a random laser Energy can be spatially and spectrally transferred inside a disordered active medium by the coupling between individual random lasing modes. Thus it is possible to transmit an optical resonance to a remote point by employing specific control over optical excitations obtaining a random lasing system which acts both as a switch and as an amplifier. (read more)
Direct imaging water inside porous wet materials
mapping water on opal A simple method to visualize the morphology of water adsorbed within the pore network of colloidal crystals made of submicrometer silica spheres. Water is replicated making it visible to standard electronic microscopy and thus allowing one to discern the original water distribution. Different distribution patterns are identified depending on the water content, surface condition, and spheres arrangement. The dimension and shape of wetting layers (covering the submicrometer spheres) and capillary bridges (joining them) are measurable at the nanoscale. (Read more)
Photoinduced local heating
fast photoheating Fast and reversible tunability of the pseudo bandgap in silica artifi cial opals by simple cw photoirradiation. We proposed dyeing the opal with a chromophore that absorbs in the UV–Vis range to permit the use of inexpensive, low-intensity lasers as the light source. (read more)
Mode selection in RL
spectral cleaning We demonstrate an experimental technique that allows to achieve a robust control on the emission spectrum of a micro random laser and to select individual modes with sub-nanometer resolution. The presented approach relies on an optimization protocol of the spatial profile of the pump beam. Here we demonstrate not only the possibility to increase the emission at a wavelength but also that we can “isolate” an individual peak suppressing unwanted contributions form other modes This is the tunable dye-based source with the smallest resonator producing a subnanometer emission ever developed. (read more)
Governing threshold in RL
gain length In a random laser (RL), a system possessing in itself both resonator and amplifying medium while lacking a macroscopic cavity, the feedback is provided by the scattering, which forces light to travel very long random paths. Here we introduce a novel paradigm in the improvement of lasing efficiency, demonstrating that RL properties may be tuned by the topology of the scattering system retaining unchanged scattering strength and gain efficiency. This is possible in a system based on sparse clusters, possessing two relevant structural lengths: the macroscopic inter cluster separation and the mesoscopic intra-cluster mean free path. (read more)
Nanoindetation of wet opals
Nanomechanical proerties Nanoindentation is suitable to study in detail the micromechanical response of silica colloidal crystals. The sensitivity to displacements smaller than the submicrometer spheres size, even resolving discrete events and superficial features, reveales particulate features with analogies to atomic crystals. Significant robustness, long range structural deformation, and large energy dissipation are found. Temperature/ratedependent nanoindentation quantifies the paramount role of adsorbed water endowing silica opals with properties of wet granular materials like viscoplasticity. A novel nongranular material was fabricated by substituting capillary bridges with silica necks to directly test water-independent mechanical response. These materials may be useful model systems for granular science and capillary cohesion at the nanoscale. (read more)
How water adsorbes in opals
in situ optical characterization of water adsorption Thermally annealed (partially dehydroxylated) silica colloidal crystals are used as test systems for the sensitivity of their photonic properties to water. The location of physisorbed water on the crystal spheres is inferred in situ during water desorption by simple optical spectroscopy. Physisorbed water in hydrophilic (fully hydroxylated) compacts tends to accumulate between adjacent spheres forming large necks, whereas it distributs more uniformly (small necks) in hydrophobic (dehydroxylated) ones. Counterintuitively, water films on the spheres surface are released faster upon desorption in the case of hydrophilic crystals. With this exception, water desorption is identical irrespective of silica hydroxylation or water content. Remarkably, the separation between spheres in the nonclose-packed crystals exclusively depends on water content and not on hydrophilicity. This method reveals extreme accurate allowing to measure nanometer-scale changes like thin surface water films (from 5 to 0 nm) or slight sphere shrinkage upon annealing of less than 2% (4 to 7 nm), which are hardly discernible with other techniques like dynamic light scattering or electron microscopy. (read more)
Controlling localization length in Random Lasers
We show that the degree of localization for the modes of a random laser (RL) is affected by the inter mode interaction that is controlled by shaping the spot of the pump laser. By experimentally investigating the spatial properties of the lasing emission  we infer that strongly localized modes are activated in the low interacting regime while in the strongly interacting one extended modes are found lasing. Thus we demonstrate that the degree o localization may be finely tuned at the micrometer level. Read more!
2D confinement by hybrid system

Close-packed 2D arrays of submicron dielectric spheres can efficiently confine electromagnetic radiation.We have analyzed the evolution of such confinement from the ideal case of the free-standing structure to the more realistic scenarios of arrays resting on dielectric and metallic substrates, finding how the latter is the most favorable case. The dispersion relation of photonic-plasmonic hybrid systems are successfully reproduced with an effective medium analytical model. (read more)

Regimes of sorption of water on silica
Physisorbed and structurally bound (surface and internal) water in silica opals are distinguished and quantified by thermogravimetry. 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. (read more)
One-step inverse silica sphere monolayer
We have studied how colloidal suspensions of PS and SiO2 spheres of approximately 1 micrometre in diameter self assemble in wedge cells. Our comparative experiment permits to identify the factors that determine the quality of 2D hexagonal arrays built. For this purpose, crystallographic defects have been analyzed qualitatively and quantitatively. Monolayers disorder has been analyzed by FT's of SEM pictures, and corelated with optical quality. We have found two main limiting factors for obtaining single domain 2D hexagonal arrays. First, the lack of size uniformity, especially if there are great amounts of smaller spheres with diameter around 0.5 to 0.9 times the diameter of the average. Second, the possibility of great amounts of non-sphererical particles and linked spheres produced during the synthesis. Both types of extrinsic defects are the cause of intrinsic defects, such as dislocations and rotated domains, the latter producing the main effect regarding crystal disorder at medium and long range. Thus, the constraints offered by the growth method we used are reflected in short-range order and long-range disorder. (read more)
Nikel magnetophotonic crystals
We have prepared MPCs from selfassembled polymeric monolayers replicated on nickel. These structures exhibit an increase of the magneto-optical response with features at spectral regions controllable through structural parameters of the template. These features are due to the excitation of Ni Surface Plamons modes. We have found that the effect of disorder on the PKR is to decrease the amount of that enhancement. In the case of TMOKE, since it is largely affected by the crystal orientation probed, the effect of long- range disorder significantly alters
the signal. (all the details)
Microwave synthesis of magnetoopical material
Magnetooptically active by microwave synthesis An extremely fast and versatile synthetic approach, based on microwave assisted solgel chemistry allows a conformal nanometric coating of intricate threedimensional structures. Using this methodology it was possible to achieve a conformal coverage of large areas of threedimensional opals with a superparamagnetic manganese ferrite layer, yielding magnetophotonic crystals with excellent quality. The use of a ternary oxide for the ultrathin coating demonstrates the potential of this methodology to realize threedimensional structures with complex materials that may find applications beyond photonics, such as energy harvesting etc.
Enhancing Azomolecule photoalignment
Photoalignment upon irradiation This is a new, simple solution-processed approach to prepare reproducible, extremely efficient photoisomerizable systems. Neat azochromophore is directly infiltrated in conventional silica colloidal crystals by wetting where very high and fast photoalignment is achieved upon cw-irradiation, allowing to photoinduce huge birefringence even at low light doses (1.1 in 15 ms at 0.07 J cm-2). Such an efficient photoalignment is explained in terms of favourable 3D-distribution of the azomolecules, weakly anchored to the silanol groups in the silica, which greatly reduces aggregation and maximizes mobility. The non-necessity of a polymeric matrix avoides steric hindrances and allows complete, spontaneous reversibility of the photoalignment. Excellent reliability for high-rate photoswitching is also proved, which makes this system promising for applications like switches and modulators.
(read more)
Passivation of Si nanocrystals
Luminescence preservation after passivation We have prepared nc-Si inverse opals by a straightforward magnesiothermic process. Si nano crystal opals exhibit light emission, observable with the naked eye, in a broadband ranging from 500 to 1050 nm. Unless protected this photoluminescence strongly diminishes under photoexcitation. A nanometre thick Al2O3 layer succeeds to ppassivate Silicon nanocrystals surface so that emission is preserved to a large extent by engineering the interface of the NCs with direct deposition of alumina, accurately controlled by ALD. (read more)
The Random Laser mode-locking transition
The mode-locking transition in random lasers

Random Lasers may be prepared in two distinct regimes by controlling the shape of the pump. When pumping is nearly unidirectional, few (barely interacting) modes are turned on and appear as sharp, uncorrelated peaks in the spectrum. By increasing the angular span of the pump spot, many resonances contribute, generating a smooth emission spectrum with a high degree of correlation, and shorter-lived pulses. (read more) DOI: 10.1038/NPHOTON.2011.217

Watch a brief explanation (in Spanish) of how random lasers work

Disorder in Photonic crystals
optics of disorder Our measurements show that it is possible to control and fine tune the amount of multiple scattering in a photonic crystal by adding vacancies and hence without altering the crystal structure until it becomes effectively a glass. In the highly vacancy doped photonic glass we observe a resonant behavior, analogous to that observed due to Mie scattering in photonic glasses. Our materials might therefore be useful to explore Fano-like interactions between the extended Bloch mode of the photonic crystal and spatially confined Mie modes. (read more)
Losses in plasmonic-photonic hybrid
Quality and index The chromatic dispersion of the Q factor of the different types of modes present in this kind of sample has been observed to be strongly dependent on the optical constants of the metal substrate. Q factors up to 600, much larger than those attainable in similar systems with dielectric substrates and free-standing configurations, can be obtained by choosing the appropriate sphere diameter. The combination of such optical performances with a straightforward fabrication process could make the use of these systems interesting in a number of applications such as photovoltaic cells as recently proposed. (read more)
Mean free path in thin flims
Light diffusion 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. (read more)
Moisture distribution probed optically
The reversible modification of the water content of an opal by moderate heating (below 300° C) is measured in situ by the changes in the photonic bandgap. Due to reversible removal of interstitial water, large blueshifts of 30 nm and 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-neighbour 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 spheres—from 16 to 1 nm—is obtained with temperature. (read more)
Magneto Photonic Crystals
Magnetic circular dichroism
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. (read more)
Magnetic resonance in Silicon spheres
We show that Silicon particles with index of refraction around 3.5 and radius of 200nm present strong electric and magnetic dipolar resonances in telecom and near-infrared frequencies, (i.e. at wavelengths ≈ 1.2−2μm) 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. (Read more)
Optical amplification and DOS in Photonic Crystals
Improving and controlling the efficiency of a gain medium is one of the most challenging problems of laser research. By measuring the gain length in an opal-based photonic crystal doped with laser dye, we demonstrate that optical amplification is more than twenty-fold enhanced along the Γ-K symmetry directions of the facecentered-cubic photonic crystal. These results are theoretically explained by directional variations of the density of states, providing a quantitative connection between density of the states and light amplification. (read more)
Photonic-plasmonic resonace controlled by plasma etching
The optical properties of two-dimensional hybrid photonic-plasmonic crystals are fine tuned by modifying the filling fraction of the lattice employing plasma oxygen. Simulations and experiments agree in showing an ample 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.(see more)
Photonic-plasmoniccontrols emision polarization, extraction, direction
Monolayers of dielectric spheres deposited on metallic substrates can strongly modify the emission of organic dyes contained in the spheres through coupling to hybrid plasmonic–photonic modes of the structure. Emission enhancement due to strong field confinement inside the spheres has been demonstrated together with its polarization dependence. Evidence for the control of emission directionality and polarization is presented too.(see more)
Increased gain in dye doped DNA
Gain as a function of concentration and power Gain length is here measured in an active medium that is obtained by doping DNA strands with DCM dye molecules. The superior thermal stability of the composite and its low quenching permit to obtain an optical gain coefficient larger than 300 cm−1. We also demonstrate that such an active material is feasible for the infiltration into photonic nano-structures, allowing one to obtain fluorescent photonic crystals and promising lasing properties. We demosntrate random lasing in photonic glass (more here)
Strong dispersive effects in the scattering mean free path
Lambert-Beer law

We have performed measurements of the scattering mean free path (ℓs) in photonic crystals with different and controlled amounts of disorder. In the most perfect crystals, 1 order of magnitude chromatic variation in ℓs for just 3% shift around the band gap (27 nm in wavelength) is obtained. It is argued that the ℓs dispersion is governed by both the total density of states and the group index in the incident direction, with the latter quantity being responsible for the large dispersion of ℓs. (read more)

Silicon monodisperse colloidal spheres
SiO2--Si-MgO--p-Si--Si To date, it had been a challenge to synthesise monodisperse high refractive index spheres of micrometre size. The use two stages of magnesiothermic reduction and CVD has enabled us to obtain Si spheres in large quantities starting from ordinary state of the art colloidal silica particles. The porous structures of the Si spheres after the former process and the infilling of their pores after the later one is demonstrated. The optical properties of opals formed with these spheres are used to assess the power of the method and the quality of the final product. (More here)
Thermochromic Inverse opals made of VO2

We report a controlled process to fabricate large-area high quality VO2-SiO2 opals with fine control over filling volume. The method comprises two stages. The first stage is a Chemical Vapour Deposition synthesis whereby the vanadium pentoxide is grown. The second one is a thermal annealing that allows reducing the vanadium pentoxide to vanadium dioxide. We have carried out a steady-state study of the semiconductor-metal transition, for opals with the Bragg peak around the 1.55 μm spectral region, by means of reflectance spectroscopy. As the temperature increases approaching the phase transition the intensity of the reflectance peak decrease and a small blue shift can be observed at 65ºC. When the phase transitions is achieved at 68ºC the intensity of the reflectance peak
decrease drastically and the Fabry-Perot oscillations disappear.
Random lasing photonic glasses
Resonance-driven Random Lasing Random lasers are generally difficult to control: they emit in every direction at once and in many different colors. Now, exploiting a fundamental physical phenomenon, it is possible to choose their color.
A focus story at See also the full article or Nature Photonics highlight or interview.
Energy velocity resonant in photonic glasses
Enregy velocity resonance In this experiment we demonstrate Mie resonances mediated transport of light in randomly arranged, monodisperse dielectric spheres packed at high filling fractions. By means of both static and dynamic optical experiments we show resonant behavior in the key transport parameters and, in particular, we find that the energy transport velocity, which is lower than the group velocity, also displays a resonant behavior.
Exploring the Brilouin zone Spectroscopy of (100) oriented opals
reciprocal space The optical response of artificial opals in the surroundings of the [100] crystallographic direction has been measured by means of micro-reflectance and transmittance spectroscopy. The results indicate that for such sample orientation, the optical properties are determined entirely by low-dispersion bands responsible for out of plane diffraction. This is corroborated by phase-sensitive spectroscopy which shows strong anomalies in the measured phase above the onset of diffraction. Such anomalies translate into group velocity values of ±c/20, evidencing slow and superluminal light propagation. These findings could be relevant in developing new routes for enhanced light-matter interaction.
Ultrathin opals
It is well known that colours of photonic crystals are deternmined by their periodicity and in oplas this is set by the spheres size. For thin opals however this also depends on the actual number of layers (thickness and orientation).
What is less well known is that for ultrathin opals colour depends also on the stacking order. We study the optical properties of ultra thin self-assembled photonic crystals. We found that the visible colour unambiguously reveals the stacking order. FDTD calculations give satisfactory account of the spectra taken without any adjustable parameters. (read more)
Photonic glass
A new photonic material is presented: “photonic glass”. This is a three dimensional completely disordered solid arrangement of monodisperse dielectric spheres. Monodispersity will ensure single sphere optical resonances to be observed. Very thick (from tenths to few millimetres uniform samples can be grown by attenuating the inter-sphere interaction. Thin (correlated) disordered films can be grown by vertical convective deposition and selective etching of binary colloidal suspensions. (Read more)
Opal-templated QD photonic crystal
This is a novel structured composite that integrates two very distinct materials such as ZnO and CdTe QDs in a photonic crystal. The fabrication is performed by an accurate infiltration in two hierarchical steps of colloidal self-assembly. We observe the effect that the photonic matrix has on the PL in the high energy range of the PC. We tune the QDs emission to this regime of the ZnO template to profit from the wide range of infiltration allowed by the large size spheres. The effect of a high energy pseudogap on the spontaneous emission is experimentally viewed as an inhibition for frequencies contained in the gap and enhancement of those outside. Careful and progressive infiltration with more ZnO is useful to tune this photonic effect through the PL emission band. (Read more)
Velocity of energy is resonant in photonic glasses
Silicon Onion-layer opals
In this investigation we found a method to infiltrate silicon in polymeric self-assembled microstructures. By means of combined silica and silicon CVD it is possible to build up multi-layered structures arranged in 3D with photonic crystal optical response. These structures are very open and can host other materials with additional functionalities. Bearing in mind the undeniable advantages brought about by polymer opals (and other polymeric structures) in terms of quality, ease of production and available range of sizes, the method demonstrated constitutes a great progress in the fabrication of photonic crystals. Novel topologies as silicon concentric layers surrounding spherical cavities are created with this fabrication process as an example. (Read
Extra gaps in ZnO inverse opals

We can open two new pseudogaps in the high energy region of the L direction in ZnO inverted opals. The appearance of these extra pseudogaps is brought about by the increase of refractive index contrast. The calculated band diagram in the Γ-L direction, which corresponds to propagation along (111) crystallographic direction, accounts for the spectral width and position dependence of the new pseudogaps on the ZnO fraction present in the opal. To prove this we have performed an optical study by means of reflection and transmission spectroscopy of ZnO inverted opals with different degrees of infiltration. We also see the pseudogap build up as a function of sample thickness. The band engineering developed here allows to isolate a non-dispersive band from the others. (Read
Absolute optical phase in opals
From the measured absolute phase and zero order transmission in opals the real and imaginary parts of the complex effective refractive index were derived. The real part of the refractive index shows a region of anomalous dispersion across the pseudogap. The imaginary part provides information on Bragg diffraction as well as on scattering by structural disorder. Finally, from the derivative of the measured phase, the group velocity has been extracted. The evolution of these physical quantities as the thickness of the samples tends towards the infinite crystal behavior agrees with previous optical spectroscopy characterization . Near the low energy edge of the pseudogap the effect of scattering by structural defects can be detected. (Read more)
ΓX pseudogap
in ZnO opals
In this work we have performed an optical study of the X and ΓL pseudogap, profiting for this purpose from naturally occurring (100) and (111) oriented growth in artificial opals that show facets with square and hexagonal symmetry arrangements respectively. Its dependence on dielectric contrast is demonstrated. We have thus successfully shown an application of bandgap engineering to open can otherwise closed pseudogap. The ability to control the growth of such square layers would redound to the benefit of photonic engineering as it would eliminate a major constrain in artificial opal growth. (More details)
ZnO inverse opals grown by CVD
Here, we report a controlled process to fabricate large-area high quality ZnO/Polystyrene (PS) composites and ZnO inverted opals with extremely fine control over volume filling fraction (shells nanometric in thickness can be grown) by a modified metalorganic chemical vapour deposition (MOCVD) method. Furthermore, once the ZnO structures are obtained, a sulfidation process is shown capable to produce ZnS inverted opals maintaining the crystal quality of the original opal structures.. (More details)
Diffraction by photonic crystals
A band structure based interpretation of the diffraction phenomena observed in three-dimensional photonic crystals is plausible. Not only qualitative but quantitative information about the diffraction patterns produced can be obtained in a simple manner from the band structure. Simple diffraction experiments explain phenomena occurring at frequencies above the first stop band that were not previously understood. Optical features observed in transmission spectra from opaline photonic crystals are now clarified by relating them to diffraction phenomena. We also observe an intriguing change in the diffraction pattern symmetry when the photonic crystal refractive index contrast is modified. More details
Zinc inverted opals
Zinc inverted opals have been fabricated by means of linear cyclic voltametry and square wave pulsating potential on polystyrene artificial opals grown on semiconductor substrates, namely, ITO and n-doped Silicon. Several differences in the infiltration were observed in samples obtained by means of cyclic voltametry or square wave pulsating potential approaches. This is the first time than Zn inverse opals are fabricated and this work proves the possibility of growing macroporous structures by electrochemistry on semiconductor substrates.More details
Patterning thin film opals by EBL
Here is a method that allows the introduction of micrometer-size three-dimensional extrinsic defects in PMMA-SiO2 composites. PMMA opals are first infiltrated with a thin layer of silica. This layer is transparent to the electrons and permit the exposure to the e-beam. These structures have good mechanical stability to withstand e-beam lithography process and lead to inverted structures. The optical properties of the composites reveal the morphology because the average refractive index depends on whether the PMMA has been exposed and dissolved or not. Selectively inverted structures allow further growth and processing leading to 3D buried structures.
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Optics in the high energy bands
Reflectance above the first pseudogap (L) spectral range presents a rich structure. The nature of these peaks is clearly different from that of the L pseudogap since in the latter, no states are available at the corresponding frequency. This behaviour is caused by the coupling of light to bands with no forward propagation. A peak in the region of the band structure where the ГL bands split at the centre of zone (second order Bragg diffraction) is found. As opposed to first order Bragg diffraction, here the gap is filled with photonic states. Other features can be associated with anticrossings occurring in forward propagating bands. A third order Bragg peak can also be observed.
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Engineering planar defects in opals.
An innovative method is presented which allows the fabrication of opal slab heterostructures including engineered planar defects. The method is based on a multi-step process which combines the ability to grow high quality, thickness controlled opals by convective colloidal self-assembly and oxide infiltration by Chemical Vapour Deposition. The method allows a precise control of the parameters that determine the optical properties of these systems permitting their tuning. The engineered defects act as microcavities whose resonance can be easily tuned.
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Si/Ge shell structures in opal
Apart from mastering single semiconductor infiltration, this work shows that it is feasible to combine Si and Ge to obtain multilayer structures. Furthermore, some of the components can be selectively modified or removed. This allows us to create layers of air between semiconductors which, optically, means high refractive index contrast and, as a result, important variations in the optical features of the crystals.
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First diamond lattice opal
Robot built diamond opal Diamond-like structures with micron size periodicity were fabricated by means of nanorobotic manipulation of microspheres. The method takes advantage of concepts and disciplines such as colloids, robotics, epitaxial growth, plasma etching and photolithography. Results demonstrate the viability of this method to prepare macroporous lattices, open the way to controllable formation of a wide variety of microstructures and provide a new route to the study of novel lattices with photonic properties.
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Highest energy full photonic gap
Large Sb2S3 inverse opals have been prepared by hydrolysis of an ethoxy precursor and sulfidation followed by chemical etching. The optical characterization confirms the extraordinary photonic behavior of this material. A wide and well-defined peak in the reflectance spectrum in the range 700-800 nm defines the existence of the most energetic full PBG described in the literature. The homogeneity and degree of infiltration of the samples improve on the antimony trisulfide inverse opals
prepared by CBD methods.
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