Extraordinary optical transmittance generation on Si3N4 membranes

Metamaterials are attracting increasing attention due to their ability to support novel and engineerable electromagnetic functionalities. In this paper, we investigate one of these functionalities, i.e. the extraordinary optical transmittance (EOT) effect based on silicon nitride (Si3N4) membranes patterned with a periodic lattice of micrometric holes. Here, the coupling between the incoming electromagnetic wave and a Si3N4 optical phonon located around 900 cm-1 triggers an increase of the transmitted infrared intensity in an otherwise opaque spectral region. Different hole sizes are investigated suggesting that the mediating mechanism responsible for this phenomenon is the excitation of a phonon-polariton mode. The electric field distribution around the holes is further investigated by numerical simulations and nano-IR measurements based on a Scattering-Scanning Near Field Microscope (s-SNOM) technique, confirming the phonon-polariton origin of the EOT effect. Being membrane technologies at the core of a broad range of applications, the confinement of IR radiation at the membrane surface provides this technology platform with a novel light-matter interaction functionality

The Viscosity of Carbonate‐Silicate Transitional Melts at Earth's Upper Mantle Pressures and Temperatures, Determined by the In Situ Falling‐Sphere Technique

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Experimental determination of the viscosity of Na2CO3 melt between 1.7 and 4.6 GPa at 1200–1700°C. Implications for the rheology of carbonatite magmas in the Earth's upper mantle

Knowledge of the rheology of molten materials at high pressure and temperature is required to understand magma mobility and ascent rate at conditions of the Earth's interior. We determined the viscosity of nominally anhydrous sodium carbonate (Na2CO3), an analogue and ubiquitous component of natural carbonatitic magmas, by the in situ “falling sphere” technique at 1.7, 2.4 and 4.6 GPa, at 1200 to 1700 °C, using the Paris-Edinburgh press. We find that the viscosity of liquid Na2CO3 is between 0.0028 ± 0.0001 Pa·s and 0.0073 ± 0.0001 Pa·s in the investigated pressure-temperature range. Combination of our results with those from recent experimental studies indicate a negligible dependence on pressure from 1 atm to 4.6 GPa, and a small compositional dependence between molten alkali metal-bearing and alkaline earth metal-bearing carbonates. Based on our results, the viscosity of Na2CO3 is consistent with available viscosity data of both molten calcite (determined at high pressure and temperature) and Na2CO3 at ambient pressure. Molten Na2CO3 is a valid experimental analogue for study of the rheology of natural and/or synthetic near-solidus carbonatitic melts. Estimated values of the mobility and ascent velocity of carbonatitic melts at upper conditions are between 70 and 300 g cm−3·Pa−1·s−1 and 330–1450 m·year−1, respectively, when using recently proposed densities for carbonatitic melts. The relatively slow migration rate allows magma-rock interaction over time causing seismic anomalies and chemical redox exchange

In-situ investigation of the vibrational properties of H2O–CO2-bearing and dry K-rich basaltic glasses at high pressure by mid infrared spectroscopy

The vibrational properties of CO2–H2O-bearing and dry synthetic K-rich basaltic glasses were investigated at room temperature and pressures between 0.0001–5.5 GPa using the diamond anvil cell combined with in-situ reflectance and transmittance Fourier Transform infrared micro-spectroscopy. The absorption coefficient calculated from the Kramers-Kronig relation shows that glasses are dominated by the Q2 aluminosilicate unit followed by Q1, Q3. The variation in Qn concentration upon compression suggests that glasses undergo polymerization from ambient pressure to 2 GPa followed by less marked structural changes up to 4 GPa, above which the structure is further polymerized. Once decompressed, glasses retained a polymerized structure. Our results show that 1.03 wt% CO2-1.42 wt% H2O in a glass with 3.10 wt% K2O and 46.77 wt% SiO2 prevent the formation of fully polymerized connections when cold-compressed. Our results can be used to explain the effect of volatiles and alkali on the rheology of natural basaltic magmas

Extraordinary Optical Transmittance Generation on Si3N4 Membranes

Metamaterials have been recently attracting increasing attention thanks to their capability to go beyond the electromagnetic and transport properties of natural materials. Although less developed compared to plasmonic systems, phononic metamaterials offer the advantage of controlling phonon propagation and, consequently, heat transfer at a microscopic level, and strong local field enhancement in the infrared. In this work, extraordinary optical transmittance (EOT) from insulating silicon nitride (Si3N4) membranes patterned with a periodic lattice of micrometric circular holes (3, 5, 7 μm) is achieved. By performing transmittance measurements in the infrared, the coupling between an incoming electromagnetic wave and an optical phonon is obtained, triggering an increase in the transmitted intensity in an otherwise opaque phonon spectral region. This induced transparency effect can be explained in terms of a phonon-polariton generation as also demonstrated by nano-resolved infrared imaging

Schorl breakdown at upper mantle conditions. Insights from an experimental study at 3.5 GPa

Hydrogen and B input throughout the Earth’s mantle is continuously fed through a sequence of dehydration and breakdown reactions of hydrous and B-bearing mineral phases stable at different conditions along the subducting slabs. Therefore, the stability of minerals hosting these elements plays a fundamental role. Tourmaline hosts very large amounts of B (up to 14 wt% of B2O3) along with hydroxyl groups (up to 4 wt% of H2O), thus representing a crucial mineral to investigate the fate of B and H in diverse geological settings. The recent finding of tourmaline minerals in ultra-high pressure metamorphic rocks has raised important questions about the actual tourmaline stability field, paying special attention to the high pressure and temperature stability limits of the various tourmaline species. A single-phase system made of natural schorl with the highest Fe2+ concentration known so far (about 18 wt% of FeO) was studied at a fixed pressure (3.5 GPa) and several temperatures (500, 700, 750, 800, 850 and 950 ◦C) to preliminarily constrain its stability conditions, breakdown mechanisms and breakdown products. Experiments at high pressure-high temperature conditions were performed using a multi anvil apparatus under buffered oxygen fugacity through a Re/ReO2 solid mixture. The experimental products were characterized through a multi-analytical approach consisting in Scanning Electron Microscopy imaging and Energy Dispersive System spectra acquisition, Electron MicroProbe analysis, powder X-Ray Diffraction, 57Fe M ̈ossbauer spectroscopy and reflectance Fourier Transform infrared spectroscopy. At 3.5 GPa and T ranging from 500 up to 700 ◦C, the schorl experienced a partial Fe oxidation coupled with dehydrogenation: Fe2+ + (OH) →Fe3+ + O2 + 0.5H2 (g) The observed Fe oxidation was limited to 30% (significantly lower than the full oxidation observed in ex- periments performed in air at room pressure), suggesting that oxidation-dehydrogenation is indeed a thermally activated process, but both environmental pressure and oxygen fugacity are important governing factors. In the pure schorl system at 3.5 GPa, the structural breakdown started at T = 700 ◦C and ended at 850 ◦C, resulting in the formation of almandine garnet as the first breakdown product together with topaz and a B-rich liquid phase: Na Fe2+ 2 Al) Al5 Fe2+)(Si6O18)(BO3 )3(OH)3 (OH, F) schorl → →Fe2+ 3 Al2 (SiO4 )3 almandine + Fe3+, Al)2SiO4(OH, F)2 topaz + 2SiO2 + Al2O3 + 0.5Na2 O + 1.5B2O3 + H2 O melt At 3.5 GPa and T ≥ 850 ◦ C, tourmaline, garnet and topaz were not observed anymore and kyanite, prismatine- and boromullite-like phases and corundum became stable. Both prismatine-like and boromullite-like phases identified by stoichiometry can incorporate B from the B-rich hydrous melt formed after schorl breakdown and may carry it to lower depths. From our work it follows that the schorl-bearing granitoid rocks (or sediments) have the potential to form hydrous B-bearing metasomatic melts at 3.5 GPa and T ≥ 700 ◦C. In cold subduction environments, between the 700–800 ◦C isotherms, the schorl is expected to be stable up to ~100 km depth along the subducting slab, although an excess SiO2 might be responsible for a reduction in tourmaline stability. The role of tourmaline companion minerals on its breakdown conditions and products is left as future issue when a multi-phase system will be considered