Ultrafast Plasmonics Beyond the Perturbative Regime: Breaking the Electronic-Optical Dynamics Correspondence.

The transient optical response of plasmonic nanostructures has recently been the focus of extensive research. Accurate prediction of the ultrafast dynamics following excitation of hot electrons by ultrashort laser pulses is of major relevance in a variety of contexts from the study of light harvesting and photocatalytic processes to nonlinear nanophotonics and the all-optical modulation of light. So far, all studies have assumed the correspondence between the temporal evolution of the dynamic optical signal, retrieved by transient absorption spectroscopy, and that of the photoexcited hot electrons, described in terms of their temperature. Here, we show both theoretically and experimentally that this correspondence does not hold under a nonperturbative excitation regime. Our results indicate that the main mechanism responsible for the breaking of the correspondence between electronic and optical dynamics is universal in plasmonics, being dominated by the nonlinear smearing of the Fermi-Dirac occupation probability at high hot-electron temperatures.

All-Optically Reconfigurable Plasmonic Metagrating for Ultrafast Diffraction Management.

Hot-electron dynamics taking place in nanostructured materials upon irradiation with fs-laser pulses has been the subject of intensive research, leading to the emerging field of ultrafast nanophotonics. However, the most common description of nonlinear interaction with ultrashort laser pulses assumes a homogeneous spatial distribution for the photogenerated carriers. Here we theoretically show that the inhomogeneous evolution of the hot carriers at the nanoscale can disclose unprecedented opportunities for ultrafast diffraction management. In particular, we design a highly symmetric plasmonic metagrating capable of a transient symmetry breaking driven by hot electrons. The subsequent power imbalance between symmetrical diffraction orders is calculated to exceed 20% under moderate (∼2 mJ/cm2) laser fluence. Our theoretical investigation also indicates that the recovery time of the symmetric configuration can be controlled by tuning the geometry of the metaatom, and can be as fast as 2 ps for electrically connected configurations.

Tuning temperature gradients in subwavelength plasmonic nanocones with tilted illumination.

Inducing and controlling temperature gradients in illuminated subwavelength plasmonic structures is a challenging task. Here, we present a strategy to remotely induce and tune temperature gradients in a subwavelength metallic nanocone by adjusting the angle of incidence of linearly polarized continuous-wave illumination. We demonstrate, through rigorous three-dimensional numerical simulations, that properly tilting the incident illumination angle can increase or decrease the photoinduced temperature gradients within the nanostructure. We analyze the apex-base photoinduced temperature gradient for different illumination directions, resembling typical illumination schemes utilized in surface or tip-enhanced Raman spectroscopy.

The phototransduction machinery in the rod outer segment has a strong efficacy gradient.

Rod photoreceptors consist of an outer segment (OS) and an inner segment. Inside the OS a biochemical machinery transforms the rhodopsin photoisomerization into electrical signal. This machinery has been treated as and is thought to be homogenous with marginal inhomogeneities. To verify this assumption, we developed a methodology based on special tapered optical fibers (TOFs) to deliver highly localized light stimulations. By using these TOFs, specific regions of the rod OS could be stimulated with spots of light highly confined in space. As the TOF is moved from the OS base toward its tip, the amplitude of saturating and single photon responses decreases, demonstrating that the efficacy of the transduction machinery is not uniform and is 5-10 times higher at the base than at the tip. This gradient of efficacy of the transduction machinery is attributed to a progressive depletion of the phosphodiesterase along the rod OS. Moreover we demonstrate that, using restricted spots of light, the duration of the photoresponse along the OS does not increase linearly with the light intensity as with diffuse light.

Controlling the Heat Dissipation in Temperature-Matched Plasmonic Nanostructures.

Heat dissipation in a plasmonic nanostructure is generally assumed to be ruled only by its own optical response even though also the temperature should be considered for determining the actual energy-to-heat conversion. Indeed, temperature influences the optical response of the nanostructure by affecting its absorption efficiency. Here, we show both theoretically and experimentally how, by properly nanopatterning a metallic surface, it is possible to increase or decrease the light-to-heat conversion rate depending on the temperature of the system. In particular, by borrowing the concept of matching condition from the classical antenna theory, we first analytically demonstrate how the temperature sets a maximum value for the absorption efficiency and how this quantity can be tuned, thus leading to a temperature-controlled optical heat dissipation. In fact, we show how the nonlinear dependence of the absorption on the electron-phonon damping can be maximized at a specific temperature, depending on the system geometry. In this regard, experimental results supported by numerical calculations are presented, showing how geometrically different nanostructures can lead to opposite dependence of the heat dissipation on the temperature, hence suggesting the fascinating possibility of employing plasmonic nanostructures to tailor the light-to-heat conversion rate of the system.

Squeezing terahertz light into nanovolumes: nanoantenna enhanced terahertz spectroscopy (NETS) of semiconductor quantum dots.

Terahertz spectroscopy has vast potentialities in sensing a broad range of elementary excitations (e.g., collective vibrations of molecules, phonons, excitons, etc.). However, the large wavelength associated with terahertz radiation (about 300 µm at 1 THz) severely hinders its interaction with nano-objects, such as nanoparticles, nanorods, nanotubes, and large molecules of biological relevance, practically limiting terahertz studies to macroscopic ensembles of these compounds, in the form of thick pellets of crystallized molecules or highly concentrated solutions of nanomaterials. Here we show that chains of terahertz dipole nanoantennas spaced by nanogaps of 20 nm allow retrieving the spectroscopic signature of a monolayer of cadmium selenide quantum dots, a significant portion of the signal arising from the dots located within the antenna nanocavities. A Fano-like interference between the fundamental antenna mode and the phonon resonance of the quantum dots is observed, accompanied by an absorption enhancement factor greater than one million. NETS can find immediate applications in terahertz spectroscopic studies of nanocrystals and molecules at extremely low concentrations. Furthermore, it shows a practicable route toward the characterization of individual nano-objects at these frequencies.

Plasmon resonance tuning in metal nanostars for surface enhanced Raman scattering.

We report the fabrication of Au nanostar arrays by means of electron beam lithography, in which the plasmon resonance energy can be tuned via the nanostar size from the visible into the near-infrared region. The spectral response of the nanostar arrays was investigated by optical extinction (transmittance) experiments, and their surface enhanced Raman scattering performance has been tested at two different excitation wavelengths, 633 nm and 830 nm, using chemisorbed Cresyl violet molecules as analyte. The experimental results are supported by numerical simulations of the spatial and spectral electric field enhancement.

Modified Carbon Nanotubes Favor Fibroblast Growth by Tuning the Cell Membrane Potential.

As is known, carbon nanotubes favor cell growth in vitro, although the underlying mechanisms are not yet fully elucidated. In this study, we explore the hypothesis that electrostatic fields generated at the interface between nonexcitable cells and appropriate scaffold might favor cell growth by tuning their membrane potential. We focused on primary human fibroblasts grown on electrospun polymer fibers (poly(lactic acid)─PLA) with embedded multiwall carbon nanotubes (MWCNTs). The MWCNTs were functionalized with either the p-methoxyphenyl (PhOME) or the p-acetylphenyl (PhCOMe) moiety, both of which allowed uniform dispersion in a solvent, good mixing with PLA and the consequent smooth and homogeneous electrospinning process. The inclusion of the electrically conductive MWCNTs in the insulating PLA matrix resulted in differences in the surface potential of the fibers. Both PLA and PLA/MWCNT fiber samples were found to be biocompatible. The main features of fibroblasts cultured on different substrates were characterized by scanning electron microscopy, immunocytochemistry, Rt-qPCR, and electrophysiology revealing that fibroblasts grown on PLA/MWCNT reached a healthier state as compared to pure PLA. In particular, we observed physiological spreading, attachment, and Vmem of fibroblasts on PLA/MWCNT. Interestingly, the electrical functionalization of the scaffold resulted in a more suitable extracellular environment for the correct biofunctionality of these nonexcitable cells. Finally, numerical simulations were also performed in order to understand the mechanism behind the different cell behavior when grown either on PLA or PLA/MWCNT samples. The results show a clear effect on the cell membrane potential, depending on the underlying substrate.

Direct imaging of DNA fibers: the visage of double helix.

Direct imaging becomes important when the knowledge at few/single molecule level is requested and where the diffraction does not allow to get structural and functional information. Here we report on the direct imaging of double stranded (ds) λ-DNA in the A conformation, obtained by combining a novel sample preparation method based on super hydrophobic DNA molecules self-aggregation process with transmission electron microscopy (TEM). The experimental breakthrough is the production of robust and highly ordered paired DNA nanofibers that allowed its direct TEM imaging and the double helix structure revealing.

Swarm of lightsail nanosatellites for Solar System exploration.

This paper presents a study for the realization of a space mission which employs nanosatellites driven by an external laser source impinging on an optimized lightsail, as a valuable technology to launch swarms of spacecrafts into the Solar System. Nanosatellites propelled by laser can be useful for heliosphere exploration and for planetary observation, if suitably equipped with sensors, or be adopted for the establishment of network systems when placed into specific orbits. By varying the area-to-mass ratio (i.e. the ratio between the sail area and the payload weight) and the laser power, it is possible to insert nanosatellites into different hyperbolic orbits with respect to Earth, thus reaching the target by means of controlled trajectories in a relatively short amount of time. A mission involving nanosatellites of the order of 1 kg of mass is envisioned, by describing all the on-board subsystems and satisfying all the requirements in terms of power and mass budget. Particular attention is paid to the telecommunication subsystem, which must offer all the necessary functionalities. To fabricate the lightsail, the thin films technology has been considered, by verifying the sail's thermal stability during the thrust phase. Moreover, the problem of mechanical stability of the lightsail has been tackled, showing that the distance between the ligthsail structure and the payload plays a pivotal role. Some potential applications of the proposed technology are discussed, such as the mapping of the heliospheric environment.

Sustainable Synthesis of Sulfur-Single Walled Carbon Nanohorns Composite for Long Cycle Life Lithium-Sulfur Battery.

Lithium-sulfur batteries are considered one of the most appealing technologies for next-generation energy-storage devices. However, the main issues impeding market breakthrough are the insulating property of sulfur and the lithium-polysulfide shuttle effect, which cause premature cell failure. To face this challenge, we employed an easy and sustainable evaporation method enabling the encapsulation of elemental sulfur within carbon nanohorns as hosting material. This synthesis process resulted in a morphology capable of ameliorating the shuttle effect and improving the electrode conductivity. The electrochemical characterization of the sulfur-carbon nanohorns active material revealed a remarkable cycle life of 800 cycles with a stable capacity of 520 mA h/g for the first 400 cycles at C/4, while reaching a value around 300 mAh/g at the 750th cycle. These results suggest sulfur-carbon nanohorn active material as a potential candidate for next-generation battery technology.

Electrostatic polarization fields trigger glioblastoma stem cell differentiation.

Over the last few years it has been understood that the interface between living cells and the underlying materials can be a powerful tool to manipulate cell functions. In this study, we explore the hypothesis that the electrical cell/material interface can regulate the differentiation of cancer stem-like cells (CSCs). Electrospun polymer fibres, either polyamide 66 or poly(lactic acid), with embedded graphene nanoplatelets (GnPs), have been fabricated as CSC scaffolds, providing both the 3D microenvironment and a suitable electrical environment favorable for CSCs adhesion, growth and differentiation. We have investigated the impact of these scaffolds on the morphological, immunostaining and electrophysiological properties of CSCs extracted from human glioblastoma multiform (GBM) tumor cell line. Our data provide evidence in favor of the ability of GnP-incorporating scaffolds to promote CSC differentiation to the glial phenotype. Numerical simulations support the hypothesis that the electrical interface promotes the hyperpolarization of the cell membrane potential, thus triggering the CSC differentiation. We propose that the electrical cell/material interface can regulate endogenous bioelectrical cues, through the membrane potential manipulation, resulting in the differentiation of CSCs. Material-induced differentiation of stem cells and particularly of CSCs, can open new horizons in tissue engineering and new approaches to cancer treatment, especially GBM.

Photonic Cavity Effects for Enhanced Efficiency in Layered Perovskite-Based Light-Emitting Diodes.

Layered architectures for light-emitting diodes (LEDs) are the standard approach for solution-processable materials such as metal-halide perovskites. Upon designing the composition and thicknesses of the layers forming the LED, the primary focus is typically on the optimization of charge injection and balance. However, this approach only considers the process until electrons and holes recombine to generate photons, while for achieving optimized LED performance, the generated light must also be efficiently outcoupled. Our work focuses on the latter aspect. We assume efficient photon generation and analyze the effects of the geometrical configuration together with the dipole orientation, mimicking the light emission, on the main characteristics defining the LED, such as the Purcell effect and the outcoupling efficiency. We find that in-plane dipoles result in significantly increased outcoupling efficiency. Furthermore, the mismatch in refractive index among the layers and their different thicknesses can be tuned to maximize the Purcell effect and minimize internal losses. The combined optimization of dipole orientation and layer thicknesses can improve the efficiency of the LED up to a factor 10, hence highlighting the importance of considering also the photonic properties of the LED structures if the objective is to maximize the LED performance.

Nanoporous Metals: From Plasmonic Properties to Applications in Enhanced Spectroscopy and Photocatalysis.

The field of plasmonics is capable of enabling interesting applications in different wavelength ranges, spanning from the ultraviolet up to the infrared. The choice of plasmonic material and how the material is nanostructured has significant implications for ultimate performance of any plasmonic device. Artificially designed nanoporous metals (NPMs) have interesting material properties including large specific surface area, distinctive optical properties, high electrical conductivity, and reduced stiffness, implying their potentials for many applications. This paper reviews the wide range of available nanoporous metals (such as Au, Ag, Cu, Al, Mg, and Pt), mainly focusing on their properties as plasmonic materials. While extensive reports on the use and characterization of NPMs exist, a detailed discussion on their connection with surface plasmons and enhanced spectroscopies as well as photocatalysis is missing. Here, we report on different metals investigated, from the most used nanoporous gold to mixed metal compounds, and discuss each of these plasmonic materials' suitability for a range of structural design and applications. Finally, we discuss the potentials and limitations of the traditional and alternative plasmonic materials for applications in enhanced spectroscopy and photocatalysis.

Validity of the V parameter for photonic quasi-crystal fibers.

We have investigated the validity of the V parameter to identify the single-mode operation regime for photonic quasi-crystal fibers. Our results show that the V parameter can be considered only for smoothly changing fibers. On the other hand, for fibers showing isolated high refractive regions around the core, only a mode area analysis can allow the estimation of the threshold for the single-mode operation regime. Finally, these findings can be generalized to any kind of optical fiber.

Bottom-Gate Approach for All Basic Logic Gates Implementation by a Single-Type IGZO-Based MOS Transistor with Reduced Footprint.

Logic functions are the key backbone in electronic circuits for computing applications. Complementary metal-oxide-semiconductor (CMOS) logic gates, with both n-type and p-type channel transistors, have been to date the dominant building blocks of logic circuitry as they carry obvious advantages over other technologies. Important physical limits are however starting to arise, as the transistor-processing technology has begun to meet scaling-down difficulties. To address this issue, there is the crucial need for a next-generation electronics era based on new concepts and designs. In this respect, a single-type channel multigate MOS transistor (SMG-MOS) is introduced holding the two important aspects of processing adaptability and low static dissipation of CMOS. Furthermore, the SMG-MOS approach strongly reduces the footprint down to 40% or even less area needed for current CMOS logic function in the same processing technology node. Logic NAND, NOT, AND, NOR, and OR gates, which typically require a large number of CMOS transistors, can be realized by a single SMG-MOS transistor. Two functional examples of SMG-MOS are reported here with their analysis based both on simulations and experiments. The results strongly suggest that SMG-MOS can represent a facile approach to scale down complex integrated circuits, enabling design flexibility and production rates ramp-up.

Textured nanofibrils drive microglial phenotype.

Microglia are highly plastic cells that change their properties in response to their microenvironment. By using immunofluorescence, live-cell imaging, electrophysiological recordings and RNA sequencing, we investigated the regulation of modified bacterial cellulose (mBC) nanofibril substrates on microglial properties. We demonstrate that mBC substrates induce ramified microglia with constantly extending and retracting processes, reminiscent of what is observed in vivo. Patch-clamp recordings show that microglia acquire a more negative resting membrane potential and have increased inward rectifier K+ currents, caused by an upregulation of Kir2.1 channels. Transcriptome analysis shows upregulation of genes involved in the immune response and downregulation of genes linked to cell adhesion and cell motion. Furthermore, Arp2/3 complex activation and integrin-mediated signaling modulate microglial morphology and motility. Our studies demonstrate that mBC nanofibril substrates modulate microglial phenotype, paving the way for a microglia-material interface that may be very valuable for anti-neuroinflammatory drug screening.

Towards enhanced sodium storage of anatase TiO2via a dual-modification approach of Mo doping combined with AlF3 coating.

Recent studies on anatase TiO2 have demonstrated its capability of performing as an anode material for sodium-ion batteries (SIBs) even though, due to poor conductivity, realistic applications have not yet been foreseen. In order to try to address this issue, herein, we shall introduce a cost effective and facile route based on the co-precipitation method for the synthesis of Mo-doped anatase TiO2 nanoparticles with AlF3 surface coating. The electrochemical measurements demonstrate that the Mo-doped anatase TiO2 nanoparticles deliver an ∼40% enhanced reversible capacity compared to pristine TiO2 (139.8 vs. 100.7 mA h g-1 at 0.1 C after 50 cycles) due to an improved electronic/ionic conductivity. Furthermore, upon AlF3 coating, the overall system can deliver a much higher reversible capacity of 178.9 mA h g-1 (∼80% increase with respect to pristine TiO2) with good cycling stability and excellent rate capabilities of up to 10 C. The experimental results indicate that the AlF3 surface coating could indeed effectively reduce the solid electrolyte interfacial resistance, enhance the electrochemical reactivity at the surface/interface region, and lower the polarization during cycling. The improved performance achieved using a cost-effective fabrication approach makes the dually modified anatase TiO2 a promising anode material for high-performance SIBs.

Galvanic Replacement Reaction as a Route to Prepare Nanoporous Aluminum for UV Plasmonics.

There is a growing interest in extending plasmonics applications into the ultraviolet region of the electromagnetic spectrum. Noble metals are commonly used in plasmonic, but their intrinsic optical properties limit their use above 350 nm. Aluminum is probably the most suitable material for UV plasmonics, and in this work we fabricated substrates of nanoporous aluminum starting from an alloy of Al2Mg3. The porous metal is obtained by means of a galvanic replacement reaction. Such nanoporous metal can be exploited to achieve a plasmonic material suitable for enhanced UV Raman spectroscopy and fluorescence. Thanks to the large surface to volume ratio, this material represents a powerful platform for promoting interaction between plasmonic substrates and molecules in the UV.

Facile Synthesis of Highly Graphitized Carbon via Reaction of CaC2 with Sulfur and Its Application for Lithium/Sodium-Ion Batteries.

In the present work, we report, for the first time, a novel one-step approach to prepare highly graphitized carbon (HGC) material by selectively etching calcium from calcium carbide (CaC2) using a sulfur-based thermo-chemical etching technique. Comprehensive analysis using X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and N2 adsorption-desorption isotherms reveals a highly graphitized mesoporous structure for the CaC2-derived carbon with a specific surface area of 159.5 m2 g-1. Microscopic analysis displays micron-scale mesoporous frameworks (4-20 µm) with a distinct layered structure along with agglomerates of highly graphitized nanosheets (about 10 nm in thickness and 1-10 µm lateral size). The as-prepared HGC is investigated for the role of an anode material for lithium- and sodium-ion batteries. We found that HGC exhibits good lithium storage performance in the 0.01-1.5 V range (reversible capacity of 272.4 mA h g-1 at 50 mA g-1 after 100 cycles and 214.2 mA h g-1 at 500 mA g-1 after 500 cycles), whereas, when sodium is considered, we observed a drop in the overall electrochemical performance owing to the high graphitization degree. More importantly, the present study provides a perspective approach to fabricate HGC via a simple, cost-effective, and efficient synthetic route using CaC2 and sulfur as reactants.