High-energy terahertz surface optical rectification

The interest in surface terahertz emitters lies in their extremely thin active region, typically hundreds of atomic layers, and the agile surface scalability. The ultimate limit in the achievable emission is determined by the saturation of the several different mechanisms concurring to the THz frequency conversion. Although there is a very prolific debate about the contribution of each process, surface optical rectification has been highlighted as the dominant process at high excitation, but the effective limits in the conversion are largely unknown. The current state of the art suggests that in field-induced optical rectification a maximum limit of the emission may exist and it is ruled by the photocarrier induced neutralisation of the medium's surface field. This would represent the most important impediment to the application of surface optical rectification in high-energy THz emitters. We experimentally unveil novel physical insights in the THz conversion at high excitation energies mediated by the ultrafast surface optical rectification process. The main finding is that the expected total saturation of the Terahertz emission vs pump energy does not actually occur. At high energy, the surface field region contracts towards the surface. We argue that this mechanism weakens the main saturation process, re-establishing a clearly observable quadratic dependence between the emitted THz energy and the excitation. This is relevant in enabling access to intense generation at high fluences

All-optical two-color terahertz emission from quasi-2D nonlinear surfaces

Two-color terahertz (THz) generation is a field-matter process combining an optical pulse and its second harmonic. Its application in condensed matter is challenged by the lack of phase matching among multiple interacting fields. Here, we demonstrate phase-matching-free two-color THz conversion in condensed matter by introducing a highly resonant absorptive system. The generation is driven by a third-order nonlinear interaction localized at the surface of a narrow-band-gap semiconductor, and depends directly on the relative phase between the two colors. We show how to isolate the third-order effect among other competitive THz-emitting surface mechanisms, exposing the general features of the two-color process

Terahertz emission mediated by ultrafast time-varying metasurfaces

Systems with ultrafast time-varying dielectric properties represent an emerging physical framework. We demonstrate here the observation of subcycle dynamics interacting directly with an electromagnetic source comprised of morphologically constrained photoexcited carriers in a surface nanostructure. A transition to a metallic metasurface state occurs on time scales faster than the terahertz-field period, inducing large nonlinear ultrafast phase shifts in the terahertz emission and exposing an interesting physical setting

Action plan for the conservation of habitats and species associated with seamounts, underwater caves and canyons, aphotic hard beds and chemo-synthetic phenomena in the Mediterranean Sea (Dark Habitats action plan)

Dark habitats are environments where the luminosity is extremely weak, or even absent (aphotic area) leading to an absence of macroscopic autochthonous photosynthesis. The bathymetric extension of this lightless area depends to a great extent on the turbidity of the water and corresponds to benthic and pelagic habitats starting from the deep circa-littoral. Caves which show environmental conditions that favour the installation on of organisms characteristic of dark habitats, are also taken into account. Dark habitats are dependent on very diverse geomorphological structures (e.g. underwater caves, canyons, slopes, isolated rocks, abyssal plains, cold seeps, brine anoxic lakes, hydrothermal springs and seamounts). Dark habitats represent outstanding and potential ecosystems with regard to their: Frailty and vulnerability to any land-based pressure Play an important part in the way the Mediterranean ecosystem functions, insofar as they constitute the main route for transferring matter between the coast and the deep sea Considered as biodiversity hotspots and recruiting areas forming a veritable reservoirs of knowledge and biodiversity Natural habitats that come under Habitat Directive on the conservation of natural habitats and of wild fauna and flora and appear as such as priority habitats requiring protection (Directive 92/43). A certain number of underwater caves enjoy protection status because they fall within the geographical boundaries of Marine Protected Areas (MPAs) Understanding of these functions is necessary for a better understanding and management of the biodiversity of Mediterranean coastal zones and continental shelf.peer-reviewe

Charge Transfer Hybrids of Graphene Oxide and the Intrinsically Microporous Polymer PIM-1

Nanohybrid materials based on nanoparticles of the intrinsically microporous polymer PIM-1 and graphene oxide (GO) are prepared from aqueous dispersions with a re-precipitation method, resulting in the surface of the GO sheets being decorated with nanoparticles of PIM-1. The significant blueshift in fluorescence signals for the GO/PIM-1 nanohybrids indicates modification of the optoelectronic properties of the PIM-1 in the presence of the GO due to their strong interactions. The stiffening in the Raman G peak of GO (by nearly 6 cm^{-1}) further indicates p-doping of the GO in the presence of PIM. Kelvin probe force microscopy (KPFM) and electrochemical reduction measurements of the nanohybrids provide direct evidence for charge transfer between the PIM-1 nanoparticles and the GO nanosheets. These observations will be of importance for future applications of GO-PIM-1 nanohybrids as substrates and promoters in catalysis and sensing

New Hybrid Properties of TiO2 Nanoparticles Surface Modified With Catecholate Type Ligands

Surface modification of nanocrystalline TiO2 particles (45 Å) with bidentate benzene derivatives (catechol, pyrogallol, and gallic acid) was found to alter optical properties of nanoparticles. The formation of the inner-sphere charge–transfer complexes results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites. The binding structures were investigated by using FTIR spectroscopy. The investigated ligands have the optimal geometry for chelating surface Ti atoms, resulting in ring coordination complexes (catecholate type of binuclear bidentate binding–bridging) thus restoring in six-coordinated octahedral geometry of surface Ti atoms. From the Benesi–Hildebrand plot, the stability constants at pH 2 of the order 103 M−1 have been determined

Measuring the stability of fundamental constants with a network of clocks

The detection of variations of fundamental constants of the Standard Model would provide us with compelling evidence of new physics, and could lift the veil on the nature of dark matter and dark energy. In this work, we discuss how a network of atomic and molecular clocks can be used to look for such variations with unprecedented sensitivity over a wide range of time scales. This is precisely the goal of the recently launched QSNET project: A network of clocks for measuring the stability of fundamental constants. QSNET will include state-of-the-art atomic clocks, but will also develop next-generation molecular and highly charged ion clocks with enhanced sensitivity to variations of fundamental constants. We describe the technological and scientific aims of QSNET and evaluate its expected performance. We show that in the range of parameters probed by QSNET, either we will discover new physics, or we will impose new constraints on violations of fundamental symmetries and a range of theories beyond the Standard Model, including dark matter and dark energy models

Combined effects of human pressures on Europe’s marine ecosystems

Marine ecosystems are under high demand for human use, giving concerns about how pressures from human activities may affect their structure, function, and status. In Europe, recent developments in mapping of marine habitats and human activities now enable a coherent spatial evaluation of potential combined effects of human activities. Results indicate that combined effects from multiple human pressures are spread to 96% of the European marine area, and more specifically that combined effects from physical disturbance are spread to 86% of the coastal area and 46% of the shelf area. We compare our approach with corresponding assessments at other spatial scales and validate our results with European-scale status assessments for coastal waters. Uncertainties and development points are identified. Still, the results suggest that Europe’s seas are widely disturbed, indicating potential discrepancy between ambitions for Blue Growth and the objective of achieving good environmental status within the Marine Strategy Framework Directive

Role of deep sponge grounds in the Mediterranean Sea: a case study in southern Italy

The Mediterranean spongofauna is relatively well-known for habitats shallower than 100 m, but, differently from oceanic basins, information upon diversity and functional role of sponge grounds inhabiting deep environments is much more fragmentary. Aims of this article are to characterize through ROV image analysis the population structure of the sponge assemblages found in two deep habitats of the Mediterranean Sea and to test their structuring role, mainly focusing on the demosponges Pachastrella monilifera Schmidt, 1868 and Poecillastra compressa (Bowerbank, 1866). In both study sites, the two target sponge species constitute a mixed assemblage. In the Amendolara Bank (Ionian Sea), where P. compressa is the most abundant species, sponges extend on a peculiar tabular bedrock between 120 and 180 m depth with an average total abundance of 7.3 +/- 1.1 specimens m(-2) (approximately 230 gWW m(-2) of biomass). In contrast, the deeper assemblage of Bari Canyon (average total abundance 10.0 +/- 0.7 specimens m(-2), approximately 315 gWW m(-2) of biomass), located in the southwestern Adriatic Sea between 380 and 500 m depth, is dominated by P. monilifera mixed with living colonies of the scleractinian Madrepora oculata Linnaeus, 1758, the latter showing a total biomass comparable to that of sponges (386 gWW m(-2)). Due to their erect growth habit, these sponges contribute to create complex three-dimensional habitats in otherwise homogenous environments exposed to high sedimentation rates and attract numerous species of mobile invertebrates (mainly echinoderms) and fish. Sponges themselves may represent a secondary substrate for a specialized associated fauna, such zoanthids. As demonstrated in oceanic environments sponge beds support also in the Mediterranean Sea locally rich biodiversity levels. Sponges emerge also as important elements of benthic-pelagic coupling in these deep habitats. In fact, while exploiting the suspended organic matter, about 20% of the Bari sponge assemblage is also severely affected by cidarid sea urchin grazing, responsible to cause visible damages to the sponge tissues (an average of 12.1 +/- 1.8 gWW of individual biomass removed by grazing). Hence, in deep-sea ecosystems, not only the coral habitats, but also the grounds of massive sponges represent important biodiversity reservoirs and contribute to the trophic recycling of organic matter