The single-electron transport in a three-ion magnetic molecule modulated by a transverse field

We study single-electron transport in a three-ion molecule with strong uniaxial anisotropy and in the presence of a transverse magnetic field. Two magnetic ions are connected to each other through a third, nonmagnetic ion. The magnetic ions are coupled to ideal metallic leads and a back gate voltage is applied to the molecule, forming a field-effect transistor. The microscopic Hamiltonian describing this system includes inter-ion hopping, on-site repulsions, and magnetic anisotropies. For a range of values of the parameters of the Hamiltonian, we obtain an energy spectrum similar to that of single-molecule magnets in the giant-spin approximation where the two states with maximum spin projection along the uniaxial anisotropy axis are well separated from other states. In addition, upon applying an external in-plane magnetic field, the energy gap between the ground and first excited states of the molecule oscillates, going to zero at certain special values of the field, in analogy to the diabolical points resulting from Berry phase interference in the giant spin model. Thus, our microscopic model provides the same phenomenological behavior expected from the giant spin model of a single-molecule magnet but with direct access to the internal structure of the molecule, thus making it more appropriate for realistic electronic transport studies. To illustrate this point, the nonlinear electronic transport in the sequential tunneling regime is evaluated for values of the field near these degeneracy points. We show that the existence of these points has a clear signature in the I-V characteristics of the molecule, most notably the modulation of excitation lines in the differential conductance.Comment: 10 pages, 13 figure

EXPROPRIATIONS. Literary Confidences between Life and Death

This paper proposes the delimitation of a literary territory, or of certain speech acts as a form of expression specifically dissociated from religious and philosophical discourses, and the corresponding adaptation of such acts to the small and big screens. Expropriations, or confidences by characters on the verge of death, are used as a trope to convey this specific idea. They refer to a kind of speech that no longer bears the weight of worldly events, but that does not attempt to ignore the consequences of having been in the world. With this reconceptualization of the term, this article seeks to identify an ethics of the human intensity in three specific sequences: two stories for cinema and television –Visconti’s The Leopard, and the final episode of Brideshead Revisited– and André Gide’s “literary testament,” Et nunc manet in te.Este ensayo propone acotar un terreno literario, o ciertos actos de habla como forma de expresión específicamente deslindada de lo religioso y lo filosófico, y su correspondiente adaptación en términos visuales a la pequeña y gran pantalla. La expropiación, o confidencia de personajes a las puertas de la muerte, sería el tropo que puede transmitir esa especificidad. Se trata de un tipo de discurso que ya no soporta el peso de los hechos, pero que no trata de eludir las consecuencias de haber estado en el mundo. Con esta reconceptualización del término, el texto busca identificar una ética de la intensidad humana en tres secuencias específicas: dos relatos para el cine y la televisión –El Gatopardo, de Visconti, y el capítulo final de Retorno a Brideshead–, y el “testamento literario” de André Gide, Et nunc manet in te

Fabrication of Tapered Circular Depressed-Cladding Waveguides in Nd:YAG Crystal by Femtosecond-Laser Direct Inscription

Crystalline materials are excellent substrates for the integration of compact photonic devices benefiting from the unique optical properties of these materials. The technique of direct inscription with femtosecond lasers, as an advantage over other techniques, has opened the door to the fabrication of true three-dimensional (3D) photonic devices in almost any transparent substrate. Depressed-cladding waveguides have been demonstrated to be an excellent and versatile platform for the integration of 3D photonic circuits in crystals. Here, we present the technique that we have developed to inscribe tapered depressed-cladding waveguides with a circular section for the control of the modal behavior. As a proof of concept, we have applied the technique to fabricate structures in Nd:YAG crystal that efficiently change the modal behavior from highly multimodal to monomodal, in the visible and near infrared, with reduction factors in the waveguide radius of up to 4:1. Our results are interesting for different devices such as waveguide lasers, frequency converters or connectors between external devices with different core sizes.This research was funded by Consejería de Educación, Junta de Castilla y León, grant number SA287P18, and Ministerio de Economía y Competitividad, grant number FIS2017-87970R. F.C. thanks support from National Natural Science Foundation, grant number 6177512

Superficial waveguide splitters fabricated by femtosecond laser writing of LiTaO 3 crystal

We report on Y-branch superficial depressed-cladding waveguides fabricated by femtosecond laser writing of MgO∶LiTaO3 crystal. The cladding waveguides with a rectangular cross-section are single mode for both transverse electric and transverse magnetic polarization, and show good transmission properties at a telecommunication wavelength of 1.55 μm. Divergence angles as large as 2.6 deg are successfully achieved in the splitters with nearly equalized splitting ratios (1:1). The fabricated shallow structures are excellent photonic elements for optoelectronic applications.This work was supported by the National Natural Science Foundation of China (Grant No. U1332121) and Ministerio de Economía y Competitividad under Project FIS2013-44174-P, Spain

Y-junctions based on circular depressed-cladding waveguides fabricated with femtosecond pulses in Nd:YAG crystal: A route to integrate complex photonic circuits in crystals

e have designed and fabricated photonic structures such as, Y-junctions (one of the basic building blocks for construction any integrated photonic devices) and Mach-Zehnder interferometers, based on circular depressed-cladding waveguides by direct femtosecond laser irradiation in Nd:YAG crystal. The waveguides were optically characterized at 633 nm, showing nearly mono-modal behaviour for the selected waveguide radius (9 μm). The effect of the splitting angle in the Y structures was investigated finding a good preservation of the modal profiles up to more than 2°, with 1 dB of additional losses in comparison with straight waveguides. The dependence with polarization of these splitters keeps in a reasonable low level. Our designs pave the way for the fabrication of arbitrarily complex 3D photonic circuits in crystals with cladding waveguides.We acknowledge support from Junta de Castilla y León (Projects UIC016, SA046U16) and MINECO (FIS2013-44174-P, FIS2015-71933-REDT

Femtosecond laser irradiation on Nd:YAG crystal: Surface ablation and high-spatial-frequency nanograting

In this work, we systematically study the surface modifications of femtosecond (fs) laser irradiated Nd:YAG crystal in stationary focusing case (i.e., the beam focused on the target in the steady focusing geometry) or dynamic scanning case (i.e., focused fs-laser beam scanning over the target material). Micro-sized structures (e.g. micro-craters or lines) are experimentally produced in a large scale of parameters in terms of pulse energy as well as (effective) pulse number. Surface ablation of Nd:YAG surface under both processing cases are investigated, involving the morphological evolution, parameter dependence, the ablation threshold fluences and the incubation factors. Meanwhile, under specific irradiation conditions, periodic surface structures with high-spatial-frequency (<λ/2) can be generated. The obtained period is as short as 157 nm in this work. Investigations on the evolution of nanograting formation and fluence dependence of period are performed. The experimental results obtained under different cases and the comparison between them reveal that incubation effect plays an important role not only in the ablation of Nd:YAG surface but also in the processes of nanograting formation.This work is supported by National Natural Science Foundation of China (No. 11404194), Junta de Castilla y León (Project UIC016), and MINECO (FIS2013-44174-P)

Graphene-based Y-branch laser in femtosecond laser written Nd:YAG waveguides

We report on Q-switched waveguide lasers on the graphene-based crystalline Y-branch platform. By applying direct femtosecond laser writing of Nd:YAG laser crystal, a surface waveguide splitter with Y-branch geometry has been fabricated with depressed cladding configuration. The Q-switched lasing operation at 1064 nm is achieved in transmission mode, by attaching a two-layer graphene on the resonator output mirror, as well as by using interaction between the evanescent field and a few-layer graphene that was positioned right above the Y-type waveguide. Q-switched laser with a maximum average power of 173 mW, pulse energy and duration of 63 nJ and 90 ns is obtained. This work opens a way for laser-written crystalline devices as compact, direct-pump laser sources for diverse applications.The work is supported by the National Natural Science Foundation of China (No. 11274203), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20130131130001), and Ministerio de Economía y Competitividad under Project FIS2013-44174-P, Spain

Spontaneous micro-modification of single-layer graphene induced by femtosecond laser irradiation

Permanent micro-modifications in single-layer graphene with a spontaneous periodic structural change have been induced by femtosecond (fs) laser irradiation. These modifications present a regular variation along the radial direction from a central ablated region. Based on the obtained micro-Raman spectrum and the reflective micro-spectrum of laser-irradiated graphene, structural modification with periodic variations containing several spectral regimes has been observed, which was in good agreement with periodic topography of the structure observed using an atomic force microscope. It has also been found that several regions of the laser induced structures were with different optical properties, which were identified to be correlated with different modification mechanisms. In addition, after fs laser processing, graphene still maintains crystallinity. This work may be helpful for the development of graphene-based microstructures or devices by fs laser pulsesThis work was supported by the National Natural Science Foundation of China (No. 11535008), Junta de Castilla y León (Project SA046U16), and MINECO (FIS2013-44174-P, FIS2015-71933-REDT)