8,269 research outputs found

### Low Threshold Two-Dimensional Annular Bragg Lasers

Lasing at telecommunication wavelengths from annular resonators employing
radial Bragg reflectors is demonstrated at room temperature under pulsed
optical pumping. Sub milliwatt pump threshold levels are observed for
resonators with 0.5-1.5 wavelengths wide defects of radii 7-8 mm. The quality
factors of the resonator modal fields are estimated to be on the order of a few
thousands. The electromagnetic field is shown to be guided by the defect. Good
agreement is found between the measured and calculated spectrum.Comment: 8 pages, 4 figure

### Isotropic functions revisited

To a smooth and symmetric function $f$ defined on a symmetric open set
$\Gamma\subset\mathbb{R}^{n}$ and a real $n$-dimensional vector space $V$ we
assign an associated operator function $F$ defined on an open subset
$\Omega\subset\mathcal{L}(V)$ of linear transformations of $V$, such that for
each inner product $g$ on $V$, on the subspace
$\Sigma_{g}(V)\subset\mathcal{L}(V)$ of $g$-selfadjoint operators,
$F_{g}=F_{|\Sigma_{g}(V)}$ is the isotropic function associated to $f$, which
means that $F_{g}(A)=f(\mathrm{EV}(A))$, where $\mathrm{EV}(A)$ denotes the
ordered $n$-tuple of real eigenvalues of $A$. We extend some well known
relations between the derivatives of $f$ and each $F_{g}$ to relations between
$f$ and $F$. By means of an example we show that well known regularity
properties of $F_{g}$ do not carry over to $F$.Comment: 13 pages. Added an example to show that loss of regularity is
possible. Extended the bibliography. Comments are welcom

### Quantitative oscillation estimates for almost-umbilical closed hypersurfaces in Euclidean space

We prove $\epsilon$-closeness of hypersurfaces to a sphere in Euclidean space
under the assumption that the traceless second fundamental form is
$\delta$-small compared to the mean curvature. We give the explicit dependence
of $\delta$ on $\epsilon$ within the class of uniformly convex hypersurfaces
with bounded volume.Comment: 11 pages. Comments, discussions or suggestions are welcom

### Making English a New Latin

The paper looks at various aspects of the so-called Latin-English analogy and particularly at the ways in which English may share the fate of Latin in ultimately becoming a victim of its own success. A critical factor in the history of Latin was a conceptual split between its native and non-native varieties, which eventually proved instrumental in establishing its reputation as a dead language. The author wishes to argue that current proposals for a codification of English as a Lingua Franca, aimed at providing vast numbers of L2 learners with a pedagogical alternative that does not emulate L1 standards, may be regarded as major steps towards making English a new Latin: creating a similar split between native versus foreigners' English

### Inverse curvature flows in Riemannian warped products

The long-time existence and umbilicity estimates for compact, graphical
solutions to expanding curvature flows are deduced in Riemannian warped
products of a real interval with a compact fibre. Notably we do not assume the
ambient manifold to be rotationally symmetric, nor the radial curvature to
converge, nor a lower bound on the ambient sectional curvature. The inverse
speeds are given by powers $p\leq 1$ of a curvature function satisfying few
common properties.Comment: 34 pages. Fixed minor errors and typos. Comments welcom

### Gradient estimates for inverse curvature flows in hyperbolic space

We prove gradient estimates for hypersurfaces in the hyperbolic space
$\mathbb{H}^{n+1},$ expanding by negative powers of a certain class of
homogeneous curvature functions. We obtain optimal gradient estimates for
hypersurfaces evolving by certain powers $p>1$ of $F^{-1}$ and smooth
convergence of the properly rescaled hypersurfaces. In particular, the full
convergence result holds for the inverse Gauss curvature flow of surfaces
without any further pinching condition besides convexity of the initial
hypersurface.Comment: 7 pages. Discussions are welcom

### The inverse mean curvature flow in warped cylinders of non-positive radial curvature

We consider the inverse mean curvature flow in smooth Riemannian manifolds of
the form $([R_{0},\infty)\times S^n,\bar{g})$ with metric
$\bar{g}=dr^2+{\vartheta}^2(r){\sigma}$ and non-positive radial sectional
curvature. We prove, that for initial mean-convex graphs over $S^n$ the flow
exists for all times and remains a graph over $S^{n}$. Under weak further
assumptions on the ambient manifold, we prove optimal decay of the gradient and
that the flow leaves become umbilic exponentially fast. We prove optimal $C^2$
estimates in case that the ambient pinching improves.Comment: 29 pages. Thoroughly checked revised version. Fixed some
computational errors and added another small technical assumption to make the
main theorem work. Comments and discussions are very welcom

### Optimal Asset Taxes in Financial Markets with Aggregate Uncertainty

This paper studies Pareto-optimal risk-sharing arrangements in a private information economy with aggregate uncertainty and ex ante heterogeneous agents. I show how to implement Pareto-optima as equilibria when agents can trade claims to consumption contingent on aggregate shocks in financial markets. The first result is that if aggregate and idiosyncratic shocks are independent, the implementation of optimal allocations does not require any interventions in financial markets. This result can be extended to dynamic settings in the sense that, in this case, only savings need to be distorted, but not trades in financial markets. Second, I characterize optimal trading distortions in financial markets when aggregate and idiosyncratic shocks are not independent. In this case, optimal asset taxes must be higher for those securities that pay out in aggregate states in which consumption is more volatile. For instance, this can provide an efficiency justification for the frequently observed differential tax treatment of different asset classes, such as debt and equity claims.

### Two-dimensional optical ring resonators based on radial Bragg resonance

A Bragg-reflection-based ring resonator is proposed and analyzed. Closed-form expressions for the field and dispersion curves for radial Bragg gratings and photonic bandgap crystals are derived. The required gratings exhibit a chirped period and a varying index profile. Small bending radii and strong control over the resonator dispersion are possible by the Bragg confinement. Large free spectral range and low radiation loss are predicted theoretically

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