Tuning the emission properties of electrically pumped semiconductor random lasers via controlled pulsed laser ablation.

Electrically pumped random lasers with distributed feedback can be obtained by introducing random defects into the device active layer, modifying the epitaxial growth process and losing the ease of fabrication potentially offered by disordered structures. We recently demonstrated an alternative and more practical approach in which random lasing emission is obtained from a modified Fabry-Perot laser diode after pulsed laser ablation of its output mirror. Here, we improve our fabrication technique by sweeping the ablating laser beam along the output mirror at different speeds and with different pulse energies, obtaining control over the total energy delivered at each point. We optimize the ablation parameters by evaluating the device performances in terms of lasing threshold and output power and we present the device emission characteristics. The proposed technique is tunable, fast and reliable, allowing the fabrication of devices with different properties by proper selection of the ablation parameters.

Assembly of Covalent Organic Frameworks into Colloidal Photonic Crystals.

Self-assembly of colloidal particles into ordered superstructures is an important strategy to discover new materials, such as catalysts, plasmonic sensing materials, storage systems, and photonic crystals (PhCs). Here we show that porous covalent organic frameworks (COFs) can be used as colloidal building particles to fabricate porous PhCs with an underlying face-centered cubic (fcc) arrangement. We demonstrate that the Bragg reflection of these can be tuned by controlling the size of the COF particles and that species can be adsorbed within the pores of the COF particles, which in turn alters the Bragg reflection. Given the vast number of existing COFs, with their rich properties and broad modularity, we expect that our discovery will enable the development of colloidal PhCs with unprecedented functionality.

Fano-Like Resonance from Disorder Correlation in Vacancy-Doped Photonic Crystals.

By preparing colloidal crystals with random missing scatterers, crystals are created where disorder is embodied as vacancies in an otherwise perfect lattice. In this special system, there is a critical defect concentration where light propagation undergoes a transition from an all but perfect reflector (for the spectral range defined by the Bragg condition), to a metamaterial exhibiting an enhanced transmission phenomenon. It is shown that this behavior can be phenomenologically described in terms of Fano-like resonances. The results show that the Fano's parameter q experiences a sign change signaling the transition from a perfect crystal exhibiting a reflectance Bragg peak, through a state where background scattering is maximum and Bragg reflectance reaches a minimum to a point where the system reenters a low scattering state recovering ordinary Bragg diffraction. A simple dipolar model considering the correlation between scatterers and vacancies is proposed and the reported evolution of the Fano-like scattering is explained in terms of the emerging covariance between the optical paths and polarizabilities and the effect of field enhancement in photonic crystal (PhC) defects.

Artificial Intelligence and Advanced Materials.

Artificial intelligence (AI) is gaining strength, and materials science can both contribute to and profit from it. In a simultaneous progress race, new materials, systems, and processes can be devised and optimized thanks to machine learning (ML) techniques, and such progress can be turned into innovative computing platforms. Future materials scientists will profit from understanding how ML can boost the conception of advanced materials. This review covers aspects of computation from the fundamentals to directions taken and repercussions produced by computation to account for the origins, procedures, and applications of AI. ML and its methods are reviewed to provide basic knowledge of its implementation and its potential. The materials and systems used to implement AI with electric charges are finding serious competition from other information-carrying and processing agents. The impact these techniques have on the inception of new advanced materials is so deep that a new paradigm is developing where implicit knowledge is being mined to conceive materials and systems for functions instead of finding applications to found materials. How far this trend can be carried is hard to fathom, as exemplified by the power to discover unheard of materials or physical laws buried in data.

Vacancies in Self-Assembled Crystals: An Archetype for Clusters Statistics at the Nanoscale.

Complex systems involving networks have attracted strong multidisciplinary attention since they are predicted to sustain fascinating phase transitions in the proximity of the percolation threshold. Developing stable and compact archetypes that allow one to experimentally study physical properties around the percolation threshold remains a major challenge. In nanoscale systems, this achievement is rare since it is tied to the ability to control the intentional disorder and perform a vast statistical analysis of cluster configurations. Here, a self-assembly method to fabricate perfectly ordered structures where random defects can be introduced is presented. Building binary crystals from two types of dielectric nanospheres and selectively removing one of them creates vacancies at random lattice positions that form a complex network of clusters. Vacancy content can be easily controlled and raised even beyond the percolation threshold. In these structures, the distribution of cluster sizes as a function of vacancy density is analyzed. For moderate concentrations, it is found to be homogeneous throughout the structure and in good agreement with the assumption of a random vacancy distribution.

Imbibition and dewetting of silica colloidal crystals: An NMR relaxometry study.

HYPOTHESIS: The wettability of the inner surfaces in a porous network is challenging to be accessed but essential to understand the complex performance of e.g. particulate systems. EXPERIMENTS: Here we investigate the water behavior in the macroporous (50-100 nm) voids of dense particle packings by performing low-field 1H NMR relaxometry in solid silica colloid crystals. The systems chosen guarantee a regular, known void size distribution with controllable water affinity (through thermal annealing), where the NMR experiment clearly discriminates the void water from micropore or bound water contributions, allowing separate monitoring. FINDINGS: Analysis of the saturated state indicates that the interparticle voids are completely filled after imbibition, even for less hydrophilic spheres. Due to the interaction with the silica surface, proton relaxation in void water is up to 100 times faster than that in bulk water, serving for assessment of the hydrophilicity within the sample. The relaxation time evolution upon dewetting provides an empirical measurement of the wettability inside the ensemble, revealing a progressively inhomogeneous wetting of increasingly hydrophobic surfaces. Our results provide insight into the imbibition state and dewetting performance in meso- and macroporous systems, with emphasis in the marked influence of the surface nature on the pore wetting distribution.

A Self-Assembled 2D Thermofunctional Material for Radiative Cooling.

The regulation of temperature is a major energy-consuming process of humankind. Today, around 15% of the global-energy consumption is dedicated to refrigeration and this figure is predicted to triple by 2050, thus linking global warming and cooling needs in a worrying negative feedback-loop. Here, an inexpensive solution is proposed to this challenge based on a single layer of silica microspheres self-assembled on a soda-lime glass. This 2D crystal acts as a visibly translucent thermal-blackbody for above-ambient radiative cooling and can be used to improve the thermal performance of devices that undergo critical heating during operation. The temperature of a silicon wafer is found to be 14 K lower during daytime when covered with the thermal emitter, reaching an average temperature difference of 19 K when the structure is backed with a silver layer. In comparison, the soda-lime glass reference used in the measurements lowers the temperature of the silicon by just 5 K. The cooling power of this simple radiative cooler under direct sunlight is found to be 350 W m-2 when applied to hot surfaces with relative temperatures of 50 K above the ambient. This is crucial to radiatively cool down devices, i.e., solar cells, where an increase in temperature has drastic effects on performance.

Template-Free, Surfactant-Mediated Orientation of Self-Assembled Supercrystals of Metal-Organic Framework Particles.

Mesoscale self-assembly of particles into supercrystals is important for the design of functional materials such as photonic and plasmonic crystals. However, while much progress has been made in self-assembling supercrystals adopting diverse lattices and using different types of particles, controlling their growth orientation on surfaces has received limited success. Most of the latter orientation control has been achieved via templating methods in which lithographic processes are used to form a patterned surface that acts as a template for particle assembly. Herein, a template-free method to self-assemble (111)-, (100)-, and (110)-oriented face-centered cubic supercrystals of the metal-organic framework ZIF-8 particles by adjusting the amount of surfactant (cetyltrimethylammonium bromide) used is described. It is shown that these supercrystals behave as photonic crystals whose properties depend on their growth orientation. This control on the orientation of the supercrystals dictates the orientation of the composing porous particles that might ultimately facilitate pore orientation on surfaces for designing membranes and sensors.

Random lasing emission tailored by femtosecond and picosecond pulsed polymer ablation.

We report the realization of random lasers with spatially localized feedback in which the average number of lasing modes is tuned via the fabrication process. The scattering elements required for optical feedback are obtained by short-pulsed laser ablation. By varying the pulse parameters, we control the scattering properties of the induced defects and, thus, the emission spectra. We demonstrate a large variety of spectral signatures typical of resonant random lasing with sub-nanometer linewidths, low thresholds (about 40 pJ/µm2), and single-to-multimode emission. Our simple approach allows us to obtain optical resonators with sharp linewidths at frequencies covering the entire gain window for multiple applications.

Unexpected Optical Blue Shift in Large Colloidal Quantum Dots by Anionic Migration and Exchange.

Compositional changes taking place during the synthesis of alloyed CdSeZnS nanocrystals (NCs) allow shifting of the optical features to higher energy as the NCs grow. Under certain synthetic conditions, the effect of those changes on the surface/interface chemistry competes with and dominates over the conventional quantum confinement effect in growing NCs. These changes, identified by means of complementary advanced spectroscopic techniques such as XPS (X-ray photoelectron spectroscopy) and XAS (X-ray absorption spectroscopy), are understood in the frame of an ion migration and exchange mechanism taking place during the synthesis. Control over the synthetic routes during NC growth represents an alternative tool to tune the optical properties of colloidal quantum dots, broadening the versatility of the wet chemical methods.

Self-assembly of polyhedral metal-organic framework particles into three-dimensional ordered superstructures.

Self-assembly of particles into long-range, three-dimensional, ordered superstructures is crucial for the design of a variety of materials, including plasmonic sensing materials, energy or gas storage systems, catalysts and photonic crystals. Here, we have combined experimental and simulation data to show that truncated rhombic dodecahedral particles of the metal-organic framework (MOF) ZIF-8 can self-assemble into millimetre-sized superstructures with an underlying three-dimensional rhombohedral lattice that behave as photonic crystals. Those superstructures feature a photonic bandgap that can be tuned by controlling the size of the ZIF-8 particles and is also responsive to the adsorption of guest substances in the micropores of the ZIF-8 particles. In addition, superstructures with different lattices can also be assembled by tuning the truncation of ZIF-8 particles, or by using octahedral UiO-66 MOF particles instead. These well-ordered, sub-micrometre-sized superstructures might ultimately facilitate the design of three-dimensional photonic materials for applications in sensing.

Lasing optical cavities based on macroscopic scattering elements.

Two major elements are required in a laser device: light confinement and light amplification. Light confinement is obtained in optical cavities by employing a pair of mirrors or by periodic spatial modulation of the refractive index as in photonic crystals and Bragg gratings. In random lasers, randomly placed nanoparticles embedded in the active material provide distributed optical feedback for lasing action. Recently, we demonstrated a novel architecture in which scattering nanoparticles and active element are spatially separated and random lasing is observed. Here we show that this approach can be extended to scattering media with macroscopic size, namely, a pair of sand grains, which act as feedback elements and output couplers, resulting in lasing emission. We demonstrate that the number of lasing modes depends on the surface roughness of the sand grains in use which affect the coherent feedback and thus the emission spectrum. Our findings offer a new perspective of material science and photonic structures, facilitating a novel and simple approach for the realization of new photonics devices based on natural scattering materials.

Random Lasing at Localization Transition in a Colloidal Suspension (TiO2@Silica).

Anderson localization of light and random lasing in this critical regime is an open research frontier, which besides being a basic research topic could also lead to important applications. This article investigates the random laser action at the localization transition in a strongly disordered scattering medium composed of a colloidal suspension of core-shell nanoparticles (TiO2@Silica) in ethanol solution of Rhodamine 6G. The classical superfluorescence band of the random laser was measured separately by collecting the emission at the back of the samples, showing a linear dependence with pumping fluence without gain depletion. However, frontal collection showed saturation of the absorption and emission. Narrow peaks of approximately equal intensity are observed on top of the classical superfluorescence band, indicating suppression of the interaction between the peaks modes. The linewidth of these peaks is lower than that of the passive modes of the scattering medium. A method called fraction of absorbed pumping allowed us to infer that this peak's mode (localized modes) is confined to a shallow region near the input-pumping border.

Shape Memory Cellulose-Based Photonic Reflectors.

Biopolymer-based composites enable to combine different functionalities using renewable materials and cost-effective routes. Here we fabricate novel thermoresponsive photonic films combining cellulose nanocrystals (CNCs) with a polydiolcitrate elastomer exhibiting shape memory properties, known as hydroxyl-dominant poly(dodecanediol-co-citrate) (PDDC-HD). Iridescent films of CNCs are first made by evaporation-induced self-assembly, then embedded in the PDDC-HD prepolymer, and finally cured to obtain a cross-linked composite with shape memory properties. The fabricated samples are characterized by polarized optical microscopy, scanning electron microscopy, and thermomechanical cycling. The obtained hybrid material combines both intense structural coloration and shape memory effect. The association of stiff cellulose nanocrystals and soft polydiolcitrate elastomer enhances the overall mechanical properties (increased modulus and reduced brittleness). This hybrid nanocomposite takes advantage of two promising materials and expands their possibilities to cover a wide range of potential applications as multiresponsive devices and sensors. As they perform from room to body temperatures, they could be also good candidates for biomedical applications.

Large fluctuations at the lasing threshold of solid- and liquid-state dye lasers.

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.

Emission regimes of random lasers with spatially localized feedback.

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.

Seeded Synthesis of Monodisperse Core-Shell and Hollow Carbon Spheres.

Monodisperse carbon spheres between 500 and 900 nm are hydrothermally synthesized from glucose on polystyrene seeds. Control over temperature, time, glucose concentration, and seed size yields hybrid spheres without aggregation and no additional spheres population. Pyrolysis transforms the hybrid into hollow carbon spheres preserving monodispersity. This approach provides a basis for functional carbon spheres applicable in photonics and energy storage.

Colloidal crystals and water: Perspectives on liquid-solid nanoscale phenomena in wet particulate media.

Solid colloidal ensembles inherently contain water adsorbed from the ambient moisture. This water, confined in the porous network formed by the building submicron spheres, greatly affects the ensemble properties. Inversely, 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, we summarize these advances, along with new ones, that are linked to general interfacial water 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 on water in colloidal crystals provides new opportunities for multidisciplinary study of interfacial and nanoconfined liquids and their essential role in the physics of utmost important systems such as particulate media.

Photophysical Analysis of the Formation of Organic-Inorganic Trihalide Perovskite Films: Identification and Characterization of Crystal Nucleation and Growth.

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.

Large area resonant feedback random lasers based on dye-doped biopolymer films.

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.