86,979 research outputs found
On a rank-unimodality conjecture of Morier-Genoud and Ovsienko
No full textLet α=(a,b,…) be a composition. Consider the associated poset F(α), called a fence, whose covering relations are x1◁x2◁…◁xa+1▷xa+2▷…▷xa+b+1◁xa+b+2◁…. We study the associated distributive lattice L(α) consisting of all lower order ideals of F(α). These lattices are important in the theory of cluster algebras and their rank generating functions can be used to define q-analogues of rational numbers. In particular, we make progress on a recent conjecture of Morier-Genoud and Ovsienko that L(α) is rank unimodal. We show that if one of the parts of α is greater than the sum of the others, then the conjecture is true. We conjecture that L(α) enjoys the stronger properties of having a nested chain decomposition and having a rank sequence which is either top or bottom interlacing, the latter being a recently defined property of sequences. We verify that these properties hold for compositions with at most three parts and for what we call d-divided posets, generalizing work of Claussen and simplifying a construction of Gansner
Passively mode-locked diode-pumped surface-emitting diode laser
No full textWe demonstrate to our knowledge the first passively mode-locked surface-emitting semiconductor laser. We used a 3-W high-brightness diode laser as pump source and a semiconductor saturable absorber mirror (SESAM [1,2]) as modelocker. The laser generates two stably mode-locked output beams with 11 mW average power each, 26ps pulse duration, and a repetition rate of 4.4 GHz. In contrast to edge-emitting semiconductor optically pumped semiconductor vertical external cavity surface emitting lasers [OPS-VECSELs) allow one to scale up the mode area in order to generate a high average power and high pulse energies, while the external cavity enforces a diffraction-limited output. A diffraction-limited output with >0.5 W in cw operation has been demonstrated for a similar kind of device [3]. Thus our concept promises to be scalable to much higher average and peak powers than can be obtained from electrically pumped surface-emitting mode-locked diode lasers or from edge emitting diode lasers. Only synchronously pumped surface emitting semiconductor lasers have generated pulses with high average power and pulse energy, but these require a powerful pulsed pump source [4]. Compared to mode-locked laser based on ion-doped crystals or glasses. mode-locked semiconductor lasers can generate high repetition rate (multi-GHz) pulse trains without Q-switching instabilities [5]. Their broad amplification bandwidth is sufficient for pulse durations in the femtosecond regime. <br/
EXTREMAL SET THEORY, CUBIC FORMS ON F n 2 AND HURWITZ SQUARE IDENTITIES
No full textInternational audienceWe study cubic forms on F n 2 using the Hurwitz-Radon theory of square identities. As an application, we obtain the following elementary statement. Given a family F of subsets of an n-set such that the cardinality of the symmetric di↵erence of any two elements F, F 0 2 F is not a multiple of 4, the maximal size of F is bounded by 2n, unless n ⌘ 3 mod 4 when it is bounded by 2n + 2. We also apply this theory to obtain some information about Boolean cubic forms and so-called additive quadruples
Review of Mark F. Chingono. The State, Violence and Development. The Political Economy of War in Mozambique, 1975–1992.
No full textBook review of: Mark F. Chingono. The State, Violence and Development. The Political Economy of War in Mozambique, 1975–1992. Avebury (Aldershot, Brookfield USA, Hong Kong, Singapore, Sydney), 1996. 291 pp. Foreword by Keith Hart. Tables. Appendix. Selected Bibliography. Index. £42.00. $71.95. Cloth
Passively mode-locked diode-pumped surface-emitting semiconductor laser
No full textA surface-emitting semiconductor laser has been passively mode locked in an external cavity incorporating a semiconductor saturable absorber mirror. The gain medium consists of a stack of 12 InGaAs-GaAs strained quantum wells, grown above a Bragg mirror structure, and pumped optically by a high-brightness diode laser. The mode-locked laser emits pulses of 22 ps full-width at half maximum duration at 1030 nm, with a repetition rate variable around 4.4 GHz
High power femtosecond source based on passively mode-locked 1055nm VECSEL and Yb-fibre power amplifier
No full textWe report 5 ns pulses at 160 W average power and 910 repetition rate from a passively mode-locked VECSEL source seeding an Yb-doped fibre power amplifier. The amplified pulses were compressed to 291 fs duration
18 December 2012 -Portuguese President of FCT M. Seabra visiting the Computing Centre with IT Department Head F. Hemmer, ATLAS experimental area with Collaboration Spokesperson F. Gianotti and A. Henriques Correia, in the LHC tunnel at Point 2 and CMS experimental area with Deputy Spokesperson J. Varela, signing an administrative agreement with Director-General R. Heuer; LIP President J. M. Gago and Delegate to CERN Council G. Barreia present.
No full textVariations on the Author
No full text“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Lasing in Bose-Fermi mixtures
No full textA.K. acknowledges the support from the EPSRC Established Career Fellowship. V.K., M.D., V.F.S. and A.K. acknowledge support from the Russian Ministry of Science and Education, contract (contract No. 11.G34.31.0067). P.G.S. acknowledges support from Greek GSRT program Aristeia (grant No. 1978). C.S., M. A. J.F., M.K and S.H. acknowledge support from the state of Bavaria.Light amplification by stimulated emission of radiation, well-known for revolutionising photonic science, has been realised primarily in fermionic systems including widely applied diode lasers. The prerequisite for fermionic lasing is the inversion of electronic population, which governs the lasing threshold. More recently, bosonic lasers have also been developed based on Bose-Einstein condensates of exciton-polaritons in semiconductor microcavities. These electrically neutral bosons coexist with charged electrons and holes. In the presence of magnetic fields, the charged particles are bound to their cyclotron orbits, while the neutral exciton-polaritons move freely. We demonstrate how magnetic fields affect dramatically the phase diagram of mixed Bose-Fermi systems, switching between fermionic lasing, incoherent emission and bosonic lasing regimes in planar and pillar microcavities with optical and electrical pumping. We collected and analyzed the data taken on pillar and planar microcavity structures at continuous wave and pulsed optical excitation as well as injecting electrons and holes electronically. Our results evidence the transition from a Bose gas to a Fermi liquid mediated by magnetic fields and light-matter coupling.Peer reviewe
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