# Showing papers in "Physical Review Letters in 2007"

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TL;DR: It is found that the energy gap scales inversely with the ribbon width, thus demonstrating the ability to engineer the band gap of graphene nanostructures by lithographic processes.

Abstract: We investigate electronic transport in lithographically patterned graphene ribbon structures where the lateral confinement of charge carriers creates an energy gap near the charge neutrality point. Individual graphene layers are contacted with metal electrodes and patterned into ribbons of varying widths and different crystallographic orientations. The temperature dependent conductance measurements show larger energy gaps opening for narrower ribbons. The sizes of these energy gaps are investigated by measuring the conductance in the nonlinear response regime at low temperatures. We find that the energy gap scales inversely with the ribbon width, thus demonstrating the ability to engineer the band gap of graphene nanostructures by lithographic processes.

4,705 citations

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TL;DR: In this paper, the authors studied three-dimensional generalizations of the quantum spin Hall (QSH) effect and introduced a tight binding model which realized the WTI and STI phases, and discussed its relevance to real materials including bismuth.

Abstract: We study three-dimensional generalizations of the quantum spin Hall (QSH) effect. Unlike two dimensions, where a single ${Z}_{2}$ topological invariant governs the effect, in three dimensions there are 4 invariants distinguishing 16 phases with two general classes: weak (WTI) and strong (STI) topological insulators. The WTI are like layered 2D QSH states, but are destroyed by disorder. The STI are robust and lead to novel ``topological metal'' surface states. We introduce a tight binding model which realizes the WTI and STI phases, and we discuss its relevance to real materials, including bismuth.

2,979 citations

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ETH Zurich

^{1}TL;DR: A new kind of neural-network representation of DFT potential-energy surfaces is introduced, which provides the energy and forces as a function of all atomic positions in systems of arbitrary size and is several orders of magnitude faster than DFT.

Abstract: The accurate description of chemical processes often requires the use of computationally demanding methods like density-functional theory (DFT), making long simulations of large systems unfeasible. In this Letter we introduce a new kind of neural-network representation of DFT potential-energy surfaces, which provides the energy and forces as a function of all atomic positions in systems of arbitrary size and is several orders of magnitude faster than DFT. The high accuracy of the method is demonstrated for bulk silicon and compared with empirical potentials and DFT. The method is general and can be applied to all types of periodic and nonperiodic systems.

1,850 citations

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TL;DR: In this paper, the first observation of Airy optical beams has been reported in both one-and two-dimensional configurations, and they exhibit unusual features such as the ability to remain diffraction-free over long distances while they tend to freely accelerate during propagation.

Abstract: We report the first observation of Airy optical beams. This intriguing class of wave packets, initially predicted by Berry and Balazs in 1979, has been realized in both one- and two-dimensional configurations. As demonstrated in our experiments, these Airy beams can exhibit unusual features such as the ability to remain diffraction-free over long distances while they tend to freely accelerate during propagation.

1,562 citations

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TL;DR: The motion of an artificial microscale swimmer that uses a chemical reaction catalyzed on its own surface to achieve autonomous propulsion is fully characterized experimentally and suggests strategies for designing artificial chemotactic systems.

Abstract: The motion of an artificial microscale swimmer that uses a chemical reaction catalyzed on its own surface to achieve autonomous propulsion is fully characterized experimentally. It is shown that at short times it has a substantial component of directed motion, with a velocity that depends on the concentration of fuel molecules. At longer times, the motion reverts to a random walk with a substantially enhanced diffusion coefficient. Our results suggest strategies for designing artificial chemotactic systems.

1,553 citations

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TL;DR: The theoretical defect model for In(2)O(3) and ZnO finds that intrinsic acceptors have a high Delta H explaining high n-dopability, and the O vacancy V(O) has a metastable shallow state, explaining the paradoxical coexistence of coloration and conductivity.

Abstract: Existing defect models for ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ and ZnO are inconclusive about the origin of conductivity, nonstoichiometry, and coloration. We apply systematic corrections to first-principles calculated formation energies $\ensuremath{\Delta}H$, and validate our theoretical defect model against measured defect and carrier densities. We find that (i) intrinsic acceptors (``electron killers'') have a high $\ensuremath{\Delta}H$ explaining high $n$-dopability, (ii) intrinsic donors (``electron producers'') have either a high $\ensuremath{\Delta}H$ or deep levels, and do not cause equilibrium-stable conductivity, (iii) the O vacancy ${V}_{\mathrm{O}}$ has a low $\ensuremath{\Delta}H$ leading to O deficiency, and (iv) ${V}_{\mathrm{O}}$ has a metastable shallow state, explaining the paradoxical coexistence of coloration and conductivity.

1,457 citations

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University of Porto

^{1}, University of Manchester^{2}, University of Minho^{3}, Boston University^{4}, Spanish National Research Council^{5}, Harvard University^{6}TL;DR: It is demonstrated that the electronic gap of a graphene bilayer can be controlled externally by applying a gate bias and can be changed from zero to midinfrared energies by using fields of less, approximately < 1 V/nm, below the electric breakdown of SiO2.

Abstract: We demonstrate that the electronic gap of a graphene bilayer can be controlled externally by applying a gate bias. From the magnetotransport data (Shubnikov-de Haas measurements of the cyclotron mass), and using a tight-binding model, we extract the value of the gap as a function of the electronic density. We show that the gap can be changed from zero to midinfrared energies by using fields of less, approximately < 1 V/nm, below the electric breakdown of SiO2. The opening of a gap is clearly seen in the quantum Hall regime.

1,448 citations

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TL;DR: The pseudospin associated with the valley index of carriers has an intrinsic magnetic moment, in close analogy with the Bohr magneton for the electron spin, forming the basis for the valley-based electronics applications.

Abstract: We investigate physical properties that can be used to distinguish the valley degree of freedom in systems where inversion symmetry is broken, using graphene systems as examples. We show that the pseudospin associated with the valley index of carriers has an intrinsic magnetic moment, in close analogy with the Bohr magneton for the electron spin. There is also a valley dependent Berry phase effect that can result in a valley contrasting Hall transport, with carriers in different valleys turning into opposite directions transverse to an in-plane electric field. These effects can be used to generate and detect valley polarization by magnetic and electric means, forming the basis for the valley-based electronics applications.

1,333 citations

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TL;DR: The main result is a tight bound on the Holevo information between one of the authorized parties and the eavesdropper, as a function of the amount of violation of a Bell-type inequality.

Abstract: We present the optimal collective attack on a quantum key distribution protocol in the "device-independent" security scenario, where no assumptions are made about the way the quantum key distribution devices work or on what quantum system they operate. Our main result is a tight bound on the Holevo information between one of the authorized parties and the eavesdropper, as a function of the amount of violation of a Bell-type inequality.

1,247 citations

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TL;DR: The relaxation hypothesis is confirmed through an ab initio numerical investigation of the dynamics of hard-core bosons on a one-dimensional lattice, and a natural extension of the Gibbs ensemble to integrable systems results in a theory that is able to predict the mean values of physical observables after relaxation.

Abstract: In this Letter we pose the question of whether a many-body quantum system with a full set of conserved quantities can relax to an equilibrium state, and, if it can, what the properties of such a state are. We confirm the relaxation hypothesis through an ab initio numerical investigation of the dynamics of hard-core bosons on a one-dimensional lattice. Further, a natural extension of the Gibbs ensemble to integrable systems results in a theory that is able to predict the mean values of physical observables after relaxation. Finally, we show that our generalized equilibrium carries more memory of the initial conditions than the usual thermodynamic one. This effect may have many experimental consequences, some of which have already been observed in the recent experiment on the nonequilibrium dynamics of one-dimensional hard-core bosons in a harmonic potential [T. Kinoshita et al., Nature (London) 440, 900 (2006)].

1,194 citations

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TL;DR: It is argued that the experimentally observed saturation of conductivity at low density arises from the charged impurity induced inhomogeneity in the graphene carrier density which becomes severe for n less, similarn(i) approximately 10(12) cm(-2).

Abstract: Carrier transport in gated 2D graphene monolayers is considered in the presence of scattering by random charged impurity centers with density n(i). Excellent quantitative agreement is obtained (for carrier density n>10(12) cm(-2)) with existing experimental data. The conductivity scales linearly with n/n(i) in the theory. We explain the experimentally observed asymmetry between electron and hole conductivities, and the high-density saturation of conductivity for the highest mobility samples. We argue that the experimentally observed saturation of conductivity at low density arises from the charged impurity induced inhomogeneity in the graphene carrier density which becomes severe for n less, similarn(i) approximately 10(12) cm(-2).

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TL;DR: This Letter shows that antenna designs can be transferred to the optical frequency regime by replacing lambda by a linearly scaled effective wavelength lambda(eff)=n(1)+n(2) lambda/lambda/lambda(p), with lambda(p) being the plasma wavelength and n(1), n( 2) being coefficients that depend on geometry and material properties.

Abstract: In antenna theory, antenna parameters are directly related to the wavelength lambda of incident radiation, but this scaling fails at optical frequencies where metals behave as strongly coupled plasmas. In this Letter we show that antenna designs can be transferred to the optical frequency regime by replacing lambda by a linearly scaled effective wavelength lambda(eff)=n(1)+n(2)lambda/lambda(p), with lambda(p) being the plasma wavelength and n(1), n(2) being coefficients that depend on geometry and material properties. It is assumed that the antenna is made of linear segments with radii R << lambda. Optical antennas hold great promise for increasing the efficiency of photovoltaics, light-emitting devices, and optical sensors.

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TL;DR: It is experimentally demonstrate that the magnetization can be reversed in a reproducible manner by a single 40 femtosecond circularly polarized laser pulse, without any applied magnetic field, revealing an ultrafast and efficient pathway for writing magnetic bits at record-breaking speeds.

Abstract: We experimentally demonstrate that the magnetization can be reversed in a reproducible manner by a single 40 femtosecond circularly polarized laser pulse, without any applied magnetic field. This optically induced ultrafast magnetization reversal previously believed impossible is the combined result of femtosecond laser heating of the magnetic system to just below the Curie point and circularly polarized light simultaneously acting as a magnetic field. The direction of this opto-magnetic switching is determined only by the helicity of light. This finding reveals an ultrafast and efficient pathway for writing magnetic bits at record-breaking speeds.

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TL;DR: A graphene bilayer with a relative small angle rotation between the layers is considered and it is found that the low energy dispersion is linear, as in a single layer, but the Fermi velocity can be significantly smaller than the single-layer value.

Abstract: We consider a graphene bilayer with a relative small angle rotation between the layers--a stacking defect often seen in the surface of graphite--and calculate the electronic structure near zero energy in a continuum approximation. Contrary to what happens in an AB stacked bilayer and in accord with observations in epitaxial graphene, we find: (a) the low energy dispersion is linear, as in a single layer, but the Fermi velocity can be significantly smaller than the single-layer value; (b) an external electric field, perpendicular to the layers, does not open an electronic gap.

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TL;DR: The scaling model is developed into a general framework for estimating the reaction energies for hydrogenation and dehydrogenation reactions and it is found that the adsorption energy of any of the molecules considered scales approximately with the adhesion energy of the central, C, N, O, or S atom.

Abstract: Density functional theory calculations are presented for CHx, x=0,1,2,3, NHx, x=0,1,2, OHx, x=0,1, and SHx, x=0,1 adsorption on a range of close-packed and stepped transition-metal surfaces. We find that the adsorption energy of any of the molecules considered scales approximately with the adsorption energy of the central, C, N, O, or S atom, the scaling constant depending only on x. A model is proposed to understand this behavior. The scaling model is developed into a general framework for estimating the reaction energies for hydrogenation and dehydrogenation reactions.

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TL;DR: An operational definition is provided, from which it is proved that steerable states are a strict subset of the entangled states, and a strict superset of the states that can exhibit Bell nonlocality.

Abstract: The concept of steering was introduced by Schrodinger in 1935 as a generalization of the Einstein-Podolsky-Rosen paradox for arbitrary pure bipartite entangled states and arbitrary measurements by one party. Until now, it has never been rigorously defined, so it has not been known (for example) what mixed states are steerable (that is, can be used to exhibit steering). We provide an operational definition, from which we prove (by considering Werner states and isotropic states) that steerable states are a strict subset of the entangled states, and a strict superset of the states that can exhibit Bell nonlocality. For arbitrary bipartite Gaussian states we derive a linear matrix inequality that decides the question of steerability via Gaussian measurements, and we relate this to the original Einstein-Podolsky-Rosen paradox.

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TL;DR: Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long-wavelength optical phonons of graphene and demonstrate optically the particle-hole symmetry about the charge-neutral Dirac point.

Abstract: Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long-wavelength optical phonons of graphene. The EFE in this two-dimensional honeycomb lattice of carbon atoms creates large density modulations of carriers with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display the interactions of lattice vibrations with these unusual carriers. The changes of phonon frequency and linewidth demonstrate optically the particle-hole symmetry about the charge-neutral Dirac point. The linear dependence of the phonon frequency on the EFE-modulated Fermi energy is explained as the electron-phonon coupling of massless Dirac fermions.

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TL;DR: In this article, the quasiparticle energies and band gaps of graphene nanoribbons were calculated using a first-principles many-electron Green's function approach within the GW approximation.

Abstract: We present calculations of the quasiparticle energies and band gaps of graphene nanoribbons (GNRs) carried out using a first-principles many-electron Green's function approach within the GW approximation. Because of the quasi-one-dimensional nature of a GNR, electron-electron interaction effects due to the enhanced screened Coulomb interaction and confinement geometry greatly influence the quasiparticle band gap. Compared with previous tight-binding and density functional theory studies, our calculated quasiparticle band gaps show significant self-energy corrections for both armchair and zigzag GNRs, in the range of 0.5-3.0 eV for ribbons of width 2.4-0.4 nm. The quasiparticle band gaps found here suggest that use of GNRs for electronic device components in ambient conditions may be viable.

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TL;DR: It is reported that a resonance response with a very high quality factor can be achieved in a planar metamaterial by introducing symmetry breaking in the shape of its structural elements, which enables excitation of trapped modes, i.e., modes that are weakly coupled to free space.

Abstract: We report that a resonance response with a very high quality factor can be achieved in a planar metamaterial by introducing symmetry breaking in the shape of its structural elements, which enables excitation of trapped modes, i.e., modes that are weakly coupled to free space.

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TL;DR: The shape of the conductivity curves indicates that high mobility samples contain some short-range disorder whereas low mobility samples are dominated by long-range scatterers.

Abstract: The conductivity of graphene samples with various levels of disorder is investigated for a set of specimens with mobility in the range of 1-20x10(3) cm2/V sec. Comparing the experimental data with the theoretical transport calculations based on charged impurity scattering, we estimate that the impurity concentration in the samples varies from 2-15x10(11) cm(-2). In the low carrier density limit, the conductivity exhibits values in the range of 2-12e2/h, which can be related to the residual density induced by the inhomogeneous charge distribution in the samples. The shape of the conductivity curves indicates that high mobility samples contain some short-range disorder whereas low mobility samples are dominated by long-range scatterers.

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Ludwig Maximilian University of Munich

^{1}, Yale University^{2}, Cornell University^{3}, McGill University^{4}TL;DR: It is found that reaching the quantum limit of arbitrarily small phonon numbers requires going into the good-cavity (resolved phonon sideband) regime where the cavity linewidth is much smaller than the mechanical frequency and the corresponding cavity detuning.

Abstract: We present a quantum-mechanical theory of the cooling of a cantilever coupled via radiation pressure to an illuminated optical cavity. Applying the quantum noise approach to the fluctuations of the radiation pressure force, we derive the optomechanical cooling rate and the minimum achievable phonon number. We find that reaching the quantum limit of arbitrarily small phonon numbers requires going into the good-cavity (resolved phonon sideband) regime where the cavity linewidth is much smaller than the mechanical frequency and the corresponding cavity detuning. This is in contrast to the common assumption that the mechanical frequency and the cavity detuning should be comparable to the cavity damping.

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TL;DR: Spin Hall conductivities obtained from both the direct and inverse spin Hall effects are experimentally confirmed to be the same, demonstrating the Onsager reciprocal relations between spin and charge currents.

Abstract: Reversible spin Hall effect comprising the direct and inverse spin Hall effects was electrically detected at room temperature. A platinum wire with a strong spin-orbit interaction is used not only as a spin current absorber but also as a spin-current source in the specially designed lateral structure. The obtained spin Hall conductivities are $2.4\ifmmode\times\else\texttimes\fi{}{10}^{4}\text{ }\text{ }(\ensuremath{\Omega}\mathrm{m}{)}^{\ensuremath{-}1}$ at room temperature, ${10}^{4}$ times larger than the previously reported values of semiconductor systems. Spin Hall conductivities obtained from both the direct and inverse spin Hall effects are experimentally confirmed to be the same, demonstrating the Onsager reciprocal relations between spin and charge currents.

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TL;DR: Relativistic viscous hydrodynamic fits to Brookhaven Rel ativistic Heavy Ion Collider data on the centrality dependence of multiplicity, transverse, and elliptic flow for square root s = 200 GeV Au+Au collisions seem to favor a much smaller viscosity over entropy ratio, below the bound from the anti-de Sitter conformal field theory conjecture.

Abstract: Relativistic viscous hydrodynamic fits to Brookhaven Relativistic Heavy Ion Collider data on the centrality dependence of multiplicity, transverse, and elliptic flow for square root s = 200 GeV Au+Au collisions are presented. For standard (Glauber-type) initial conditions, while data on the integrated elliptic flow coefficient v(2) are consistent with a ratio of viscosity over entropy density up to eta/s approximately 0.16, data on minimum bias v(2) seem to favor a much smaller viscosity over entropy ratio, below the bound from the anti-de Sitter conformal field theory conjecture. Some caveats on this result are discussed.

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TL;DR: It is found with 95% confidence that the inverse-square law holds (|alpha|

Abstract: We conducted three torsion-balance experiments to test the gravitational inverse-square law at separations between 9.53 mm and $55\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$, probing distances less than the dark-energy length scale ${\ensuremath{\lambda}}_{d}=\sqrt[4]{\ensuremath{\hbar}c/{\ensuremath{\rho}}_{d}}\ensuremath{\approx}85\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. We find with 95% confidence that the inverse-square law holds ($|\ensuremath{\alpha}|\ensuremath{\le}1$) down to a length scale $\ensuremath{\lambda}=56\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ and that an extra dimension must have a size $R\ensuremath{\le}44\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$.

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TL;DR: The role of excitonic coupling on the nature of photoexcitations in the conjugated polymer regioregular poly(3-hexylthiophene) is addressed by means of temperature-dependent absorption and photoluminescence spectroscopy.

Abstract: We address the role of excitonic coupling on the nature of photoexcitations in the conjugated polymer regioregular poly(3-hexylthiophene). By means of temperature-dependent absorption and photoluminescence spectroscopy, we show that optical emission is overwhelmingly dominated by weakly coupled H aggregates. The relative absorbance of the 0-0 and 0-1 vibronic peaks provides a powerfully simple means to extract the magnitude of the intermolecular coupling energy, of approximately 5 and 30 meV for films spun from isodurene and chloroform solutions, respectively.

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TL;DR: A quantum theory of cooling of a mechanical oscillator by radiation pressure-induced dynamical backaction is developed, which is analogous to sideband cooling of trapped ions, and it is shown that the final average occupancy can be retrieved directly from the optical output spectrum.

Abstract: A quantum theory of cooling of a mechanical oscillator by radiation pressure-induced dynamical backaction is developed, which is analogous to sideband cooling of trapped ions. We find that final occupancies well below unity can be attained when the mechanical oscillation frequency is larger than the optical cavity linewidth. It is shown that the final average occupancy can be retrieved directly from the optical output spectrum.

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TL;DR: It is shown numerically that vector antenna arrays can generate radio beams that exhibit spin and orbital angular momentum characteristics similar to those of helical Laguerre-Gauss laser beams in paraxial optics.

Abstract: We show numerically that vector antenna arrays can generate radio beams that exhibit spin and orbital angular momentum characteristics similar to those of helical Laguerre-Gauss laser beams in paraxial optics. For low frequencies (1 GHz), digital techniques can be used to coherently measure the instantaneous, local field vectors and to manipulate them in software. This enables new types of experiments that go beyond what is possible in optics. It allows information-rich radio astronomy and paves the way for novel wireless communication concepts.

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TL;DR: A new formulation of the subtraction method is proposed to numerically compute arbitrary infrared-safe observables for this class of processes and exploits the universal behavior of the associated transverse-momentum distributions in the small-qT region to cancel the infrared divergences.

Abstract: We consider higher-order QCD corrections to the production of colorless high-mass systems (lepton pairs, vector bosons, Higgs bosons, etc.) in hadron collisions. We propose a new formulation of the subtraction method to numerically compute arbitrary infrared-safe observables for this class of processes. To cancel the infrared divergences, we exploit the universal behavior of the associated transverse-momentum (qT) distributions in the small-qT region. The method is illustrated in general terms up to the next-to-next-to-leading order in QCD perturbation theory. As a first explicit application, we study Higgs-boson production through gluon fusion. Our calculation is implemented in a parton level Monte Carlo program that includes the decay of the Higgs boson into two photons. We present selected numerical results at the CERN Large Hadron Collider.

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TL;DR: Angular and spectrally resolved luminescence show that the polariton emission is beamed in the normal direction with an angular width of +/-5 degrees and spatial size around 5 microm.

Abstract: We observe a room-temperature low-threshold transition to a coherent polariton state in bulk GaN microcavities in the strong-coupling regime. Nonresonant pulsed optical pumping produces rapid thermalization and yields a clear emission threshold of 1 mW, corresponding to an absorbed energy density of 29 mu J cm(-2), 1 order of magnitude smaller than the best optically pumped (In,Ga)N quantum-well surface-emitting lasers (VCSELs). Angular and spectrally resolved luminescence show that the polariton emission is beamed in the normal direction with an angular width of +/- 5 degrees and spatial size around 5 mu m.

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TL;DR: It is shown that in a coflowing stream this transition from dripping to jetting is characterized by a state diagram that depends on the capillary number of the outer fluid and the WeberNumber of the inner fluid.

Abstract: A liquid forced through an orifice into an immiscible fluid ultimately breaks into drops due to surface tension. Drop formation can occur right at the orifice in a dripping process. Alternatively, the inner fluid can form a jet, which breaks into drops further downstream. The transition from dripping to jetting is not understood for coflowing fluid streams, unlike the case of drop formation in air. We show that in a coflowing stream this transition can be characterized by a state diagram that depends on the capillary number of the outer fluid and the Weber number of the inner fluid.