The conversion of light into free electron-hole pairs constitutes the key process in the fields of photodetection and photovoltaics. The efficiency of this process depends on the competition of ...different relaxation pathways and can be greatly enhanced when photoexcited carriers do not lose energy as heat, but instead transfer their excess energy into the production of additional electron-hole pairs through carrier-carrier scattering processes. Here we use optical pump-terahertz probe measurements to probe different pathways contributing to the ultrafast energy relaxation of photoexcited carriers. Our results indicate that carrier-carrier scattering is highly efficient, prevailing over optical-phonon emission in a wide range of photon wavelengths and leading to the production of secondary hot electrons originating from the conduction band. As hot electrons in graphene can drive currents, multiple hot-carrier generation makes graphene a promising material for highly efficient broadband extraction of light energy into electronic degrees of freedom, enabling high-efficiency optoelectronic applications. PUBLICATION ABSTRACT
Plasmons in graphene nanoresonators have many potential applications in photonics and optoelectronics, including room-temperature infrared and terahertz photodetectors, sensors, reflect arrays or ...modulators. The development of efficient devices will critically depend on precise knowledge and control of the plasmonic modes. Here, we use near-field microscopy between λ0 = 10-12 μm to excite and image plasmons in tailored disk and rectangular graphene nanoresonators, and observe a rich variety of coexisting Fabry-Perot modes. Disentangling them by a theoretical analysis allows the identification of sheet and edge plasmons, the latter exhibiting mode volumes as small as 10-8 λ03 . By measuring the dispersion of the edge plasmons we corroborate their superior confinement compared with sheet plasmons, which among others could be applied for efficient 1D coupling of quantum emitters. Our understanding of graphene plasmon images is a key to unprecedented in-depth analysis and verification of plasmonic functionalities in future flatland technologies.
The precise value of the g factor in graphene is of fundamental interest for all spin-related properties and their application. We investigate monolayer graphene on a Si/SiO_{2} substrate by ...resistively detected electron spin resonance. Surprisingly, the magnetic moment and corresponding g factor of 1.952±0.002 is insensitive to charge carrier type, concentration, and mobility.
Graphene plasmons promise unique possibilities for controlling light in nanoscale devices and for merging optics with electronics. We developed a versatile platform technology based on resonant ...optical antennas and conductivity patterns for launching and control of propagating graphene plasmons, an essential step for the development of graphene plasmonic circuits. We launched and focused infrared graphene plasmons with geometrically tailored antennas and observed how they refracted when passing through a two-dimensional conductivity pattern, here a prism-shaped bilayer. To that end, we directly mapped the graphene plasmon wavefronts by means of an imaging method that will be useful in testing future design concepts for nanoscale graphene plasmonic circuits and devices.
Controlling the energy flow processes and the associated energy relaxation rates of a light emitter is of fundamental interest and has many applications in the fields of quantum optics, ...photovoltaics, photodetection, biosensing and light emission. Advanced dielectric, semiconductor and metallic systems have been developed to tailor the interaction between an emitter and its environment. However, active control of the energy flow from an emitter into optical, electronic or plasmonic excitations has remained challenging. Here, we demonstrate in situ electrical control of the relaxation pathways of excited erbium ions, which emit light at the technologically relevant telecommunication wavelength of 1.5 μm. By placing the erbium at a few nanometres distance from graphene, we modify the relaxation rate by more than a factor of three, and control whether the emitter decays into electron-hole pairs, emitted photons or graphene near-infrared plasmons, confined to <15 nm from the graphene sheet. These capabilities to dictate optical energy transfer processes through electrical control of the local density of optical states constitute a new paradigm for active (quantum) photonics and can be applied using any combination of light emitters and two-dimensional materials.
The Tres Arroyos granite–pegmatite system is located in the SW margin of the Nisa-Alburquerque Variscan batholith. Two granitic facies (monzogranite and marginal leucogranite) and three types of ...aplite–pegmatite dykes (barren, intermediate and highly evolved Li-rich), have been distinguished in the area, with a zoned distribution from the granite southwards. Trace elements in quartz from the five facies have been analysed by LA-ICP-MS in order to obtain information about the petrogenetic links among the different lithologies of this system, as well as to better understand the regional and individual fractionation processes that led to the distinct rocks. Aluminium, Ti, Li and Ge show continuous trends from the monzogranite, through the marginal granitic facies, the barren and intermediate aplite–pegmatites, up to the most evolved Li-rich dykes. Titanium and Ge contents, respectively, decrease and increase gradually with fractionation. In contrast, Al and Li show a more complex trend, with an initial descending trend to the marginal granitic facies, and then showing the highest Al and Li contents in the quartz from the most fractionated Li-rich aplite–pegmatites. This suggests the influence of different competing factors controlling the incorporation of these trace elements in quartz, such as the chemical composition of the melt, the P and T conditions and the rate of crystallization. Based on the good correlation between Al and Li, the substitution Si4+↔Al3++Li+ seems to be the dominant mechanism of Li incorporation into quartz. The negligible amount of other trace elements suggests that the remaining Al was mainly compensated with H+ ions, via the Si4+↔Al3+ + H+ substitution.
A continuous fractionation trend from the monzogranite up to the most fractionated aplite–pegmatites is inferred from geochemical modelling by applying the Rayleigh equation for fractional crystallization. Fractionation rates over 50% are needed to obtain the marginal granite and the barren aplite–pegmatites compositions, and over 99% for the most evolved dykes. No pattern in the chemical variation of the trace elements in quartz from different layers in the layered aplite–pegmatites has been found, suggesting the lack of internal fractionation processes, most probably due to the rapid crystallization of the pegmatitic melt, intruded into a colder country rock.
•Trace elements in quartz reflect magmatic fractionation.•Crystallization conditions influence the content in trace elements in quartz.•Granite–pegmatites from Tres Arroyos belong to the same highly fractionated system.•Chemical composition of the system evolved via fractional crystallization mechanisms.
Tourmaline-rich rocks (up to 60% tourmaline) associated with low–medium grade metamorphic assemblages occur in the Sierra Nevada area (Betic Cordillera, southeastern Spain). Tourmaline appears in a ...variety of forms: (1) stratiform tourmalinites; (2) quartz–tourmaline nodules; (3) porphyroclasts in felsic orthogneisses; and (4) disseminations in psammopelitic metasediments and gneisses. Tourmaline within these lithologic groups exhibits textural and chemical variations that reflect complex premetamorphic growth under open-system conditions, and subsequent changes due to Alpine regional metamorphism.
Microprobe analyses of the tourmalines reveal a wide compositional variation between schorl and dravite end members with variable contents of X-site vacancies (av. 0.084–0.225 apfu), Ca (av. 0.095–0.269 apfu), and excess of Al (up to 6.588 apfu) compared with the theoretical value of 6 in ideal schorl and dravite. The amount of Ca may be significant in porphyroclasts from the gneisses. Fe/(Fe+Mg) ratios for tourmalines in tourmalinites, metasediments, and gneisses range from 0.34 to 0.95, 0.16 to 0.92, and 0.28 to 0.97, respectively. Na/(Na+Ca) ratios are also variable, mostly ranging from 0.5 to 0.9. Many of the tourmalines have complex chemical and colour zoning patterns, including significant fluctuations in Al, Fe, Mg, Na, Ca, Ti, and F.
Based on petrographic and chemical data, three generations of tourmaline have been established. The first generation corresponds to magmatic–postmagmatic tourmaline that is represented by tourmaline porphyroclasts within the orthogneisses. The second generation of tourmaline formed during tourmalinization of psammopelitic rocks giving rise to tourmalinites. The third generation of tourmaline is represented by cellular textures, pale reaction rims and overgrowths developed during the Alpine regional metamorphic overprint.
Tourmaline in the Martinamor antiform occurs in tourmalinites (rocks with >15–20% tourmaline by volume), clastic metasedimentary rocks of the Upper Proterozoic Monterrubio formation, quartz veins, ...pre-Variscan orthogneisses and Variscan granitic rocks. Petrographic observations, back-scattered electron (BSE) images, and microprobe data document a multistaged development of tourmaline. Overall, variations in the Mg/(Mg + Fe) ratios decrease from tourmalinites (0·36–0·75), through veins (0·38–0·66) to granitic rocks (0·23–0·46), whereas Al increases in the same order from 5·84–6·65 to 6·22–6·88 apfu. The incorporation of Al into tourmaline is consistent with combinations of xχAl(NaR)−1 and AlO(R(OH))−1 exchange vectors, where xχ represents X-site vacancy and R is (Mg + Fe2+ + Mn). Variations in xχ/(xχ + Na) ratios are similar in all the types of tourmaline occurrences, from 0·10 to 0·53, with low Ca-contents (mostly <0·10 apfu). Based on field and textural criteria, two groups of tourmaline-rich rocks are distinguished: (1) pre-Variscan tourmalinites (probably Cadomian), affected by both deformation and regional metamorphism during the Variscan orogeny; (2) tourmalinites related to the synkinematic granitic complex of Martinamor. Textural and geochemical data are consistent with a psammopelitic parentage for the protolith of the tourmalinites. Boron isotope analyses of tourmaline have a total range of δ11B values from −15·6 to 6·8‰; the lowest corresponding to granitic tourmalines (−15·6 to −11·7‰) and the highest to veins (1·9 to 6·8‰). Tourmalines from tourmalinites have intermediate δ11B values of −8·0 to +2·0‰. The observed variations in δ11B support an important crustal recycling of boron in the Martinamor area, in which pre-Variscan tourmalinites were remobilized by a combination of mechanical and chemical processes during Variscan deformation, metamorphism and anatexis, leading to the formation of multiple tourmaline-bearing veins and a new stage of boron metasomatism.
The Nevado–Filabride complex, the lowest complex in the Betic hinterland, forms a stack of Alpine nappes. The tectonic units consist of metasedimentary sequences whose ages are not well constrained. ...Gneiss bodies included in the sequences have been one of the few sources of geochronological data in this metamorphic complex. New radiometric data from U-Pb zircon dating on gneisses from the western Sierra Nevada confirm the presence of late Carboniferous intrusive rocks in the Betic hinterland. These results, combined with available data from the literature and a detailed structural analysis, suggest that the gneisses represent a single late Variscan magmatic event. Evidence for a close genetic relation between gneisses and tourmalinites is provided by field and petrographic observations, in conjunction with geochemical data, U-Pb zircon (314
±
7; 304
±
23
Ma) and
40Ar/
39Ar tourmaline (319.85
±
5.81; 317.85
±
3.67
Ma) geochronology. A pre-late Carboniferous age for the basal formation of the Nevado–Filabride sequence can be inferred. The gneiss protolith and the graphite schist are considered to be the boron source and the precursor of the tourmalinites, respectively. Superposed tectonic units rather than a continuous Palaeozoic sequence is supported by the occurrence of Palaeozoic rocks at the top of the Nevado–Filabride complex. The nappe tectonics, as evidenced from the contractional character of the unit boundaries and the superposition of higher-grade on lower-grade metamorphic rocks, is additionally supported by the superposition of older on younger rocks as revealed from radiometric data.
► Tourmalinites in the host rocks confirm the intrusive nature of the Nevado–Filabride gneisses. ► U-Pb zircon dating reveals a late Carboniferous age for the gneiss protoliths. ►
40Ar/
39Ar tourmaline dating also supports the late Carboniferous intrusion age. ► Structural and radiometric data accord with the proposed nappe stack structure.
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