Polarization dynamics in vertical-cavity surface emitting lasers

Experiments and their interpretation on polarization dynamics and polarization switching in vertical-cavity surface-emitting lasers operated in the fundamental transverse mode regime are reviewed. Important observations are switching events to a mode with the lower unsaturated gain and the existence of elliptically polarized dynamical transition states after the destabilization of the low-frequency polarization mode. The observations demonstrate the need to consider explicitly the phase properties of the optical field as well as nonlinear effects affecting polarization selection above threshold. Good qualitative agreement is found with a model which takes into account the spin degrees of freedom of the light field as well as of the carriers (`spin-flip model'), if the spin-flip rate is taken to be some tens of $10^9$~s$^{-1}$. This constitutes a strong -- though indirect -- indication that spin dependent processes are important in polarization selection in the devices investigated

Time reversal symmetry in optics

The utilization of time reversal symmetry in designing and implementing (quantum) optical experiments has become more and more frequent over the past years. We review the basic idea underlying time reversal methods, illustrate it with several examples and discuss a number of implications.Comment: 5 pages, 3 figures, typeset with revte

Lognormality of Rates and Term Structure Models

A term structure model with lognormal type volatility structure is proposed. The Heath, Jarrow and Morton (HJM) framework, coupled with the theory of stochastic evolution equations in infinite dimensions, is used to show that the resulting rates are well defined (they do not explode) and remain positive. They are bounded from below and above by lognormal processes. The model can be used to price and hedge caps, swaptions and other interest rate and currency derivatives including the Eurodollar futures contract, which requires integrability of one over zero coupon bond. This extends results obtained by Sandmann and Sondermann (1993), (1994) for Markovian lognormal short rates to (non-Markovian) lognormal forward rates.Term structure of interest rates, lognormal volatility structure, Heath, Jarrow and Morton models.

Placemaking in action: Factors that support or obstruct the development of urban community gardens

The paper examines factors that support or obstruct the development of urban community garden projects. It combines a systematic scholarly literature review with empirical research from case studies located in New Zealand and Germany. The findings are discussed against the backdrop of placemaking processes: urban community gardens are valuable platforms to observe space-to-place transformations. Following a social-constructionist approach, literature-informed enablers and barriers for the development of urban community gardens are analysed against perceived notions informed by local interviewees with regard to their biophysical and technical, socio-cultural and economic, and political and administrative dimensions. These dimensions are incorporated into a systematic and comprehensive category system. This approach helps observe how the essential biophysical-material base of the projects is overlaid with socio-cultural factors and shaped by governmental or administrative regulations. Perceptual differences become evident and are discussed through the lens of different actors

Two-frequency emission and polarization dynamics at lasing threshold in vertical-cavity surface-emitting lasers

11 pages, 16 figures.-- PACS nrs.: 42.55.Px, 42.60.Mi, 42.65.Sf.We investigate experimentally and theoretically the polarization dynamics in vertical-cavity surface-emitting lasers in the vicinity of the lasing threshold. If the net gain anisotropy between the two orthogonally polarized modes is close to zero, the laser operates in a partially polarized state. This state occurs at and slightly above the lasing threshold. It is characterized by anticorrelated dynamics of the two modes which result from completely anticorrelated dynamics at low frequencies and correlated dynamics on the time scale of the relaxation oscillations. If the net anisotropy is changed by a change of the detuning between the cavity resonance and the gain maximum, one of the two linear polarization states is selected depending on the sign of the net gain. For one sign the well-known phenomenon of polarization switching is recovered at higher currents. The experimental results show very good agreement with simulations based on a model that takes into account nonlinear spin dynamics and linear temperature-dependent detuning effects.M.S., M.W., and T.A. gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft and the Deutsche Akademische Austauschdienst and thank W. Lange for his support and stimulating discussions. S.B. and J.M. gratefully acknowledge financial support by VISTA HCTMR BFM-2000-1108 and TIC 99-0645-C05-02.Peer reviewe

The phase shift induced by a single atom in free space

In this article we theoretically study the phase shift a single atom imprints onto a coherent state light beam in free space. The calculations are performed in a semiclassical framework. The key parameters governing the interaction and thus the measurable phase shift are the solid angle from which the light is focused onto the atom and the overlap of the incident radiation with the atomic dipole radiation pattern. The analysis includes saturation effects and discusses the associated Kerr-type non-linearity of a single atom

The Inhibitory Site of a Diguanylate Cyclase Is a Necessary Element for Interaction and Signaling with an Effector Protein

Many bacteria contain large cyclic diguanylate (c-di-GMP) signaling networks made of diguanylate cyclases (DGCs) and phosphodiesterases that can direct cellular activities sensitive to c-di-GMP levels. While DGCs synthesize c-di-GMP, many DGCs also contain an autoinhibitory site (I-site) that binds c-di-GMP to halt excess production of this small molecule, thus controlling the amount of c-di-GMP available to bind to target proteins in the cell. Many DGCs studied to date have also been found to signal for a specific c-di-GMP-related process, and although recent studies have suggested that physical interaction between DGCs and target proteins may provide this signaling fidelity, the importance of the I-site has not yet been incorporated into this model. Our results from Pseudomonas fluorescens indicate that mutation of residues at the I-site of a DGC disrupts the interaction with its target receptor. By creating various substitutions to a DGC\u27s I-site, we show that signaling between a DGC (GcbC) and its target protein (LapD) is a combined function of the I-site-dependent protein-protein interaction and the level of c-di-GMP production. The dual role of the I-site in modulating DGC activity as well as participating in protein-protein interactions suggests caution in interpreting the function of the I-site as only a means to negatively regulate a cyclase. These results implicate the I-site as an important positive and negative regulatory element of DGCs that may contribute to signaling specificity

Region-Resolved Quantitative Proteome Profiling Reveals Molecular Dynamics Associated With Chronic Pain in the PNS and Spinal Cord

To obtain a thorough understanding of chronic pain, large-scale molecular mapping of the pain axis at the protein level is necessary, but has not yet been achieved. We applied quantitative proteome profiling to build a comprehensive protein compendium of three regions of the pain neuraxis in mice: the sciatic nerve (SN), the dorsal root ganglia (DRG), and the spinal cord (SC). Furthermore, extensive bioinformatics analysis enabled us to reveal unique protein subsets which are specifically enriched in the peripheral nervous system (PNS) and SC. The immense value of these datasets for the scientific community is highlighted by validation experiments, where we monitored protein network dynamics during neuropathic pain. Here, we resolved profound region-specific differences and distinct changes of PNS-enriched proteins under pathological conditions. Overall, we provide a unique and validated systems biology proteome resource (summarized in our online database painproteome.em.mpg.de), which facilitates mechanistic insights into somatosensory biology and chronic pain—a prerequisite for the identification of novel therapeutic targets

Generation of entangled matter qubits in two opposing parabolic mirrors

We propose a scheme for the remote preparation of entangled matter qubits in free space. For this purpose, a setup of two opposing parabolic mirrors is considered, each one with a single ion trapped at its focus. To get the required entanglement in this extreme multimode scenario, we take advantage of the spontaneous decay, which is usually considered as an apparent nuisance. Using semiclassical methods, we derive an efficient photon-path representation to deal with this problem. We also present a thorough examination of the experimental feasibility of the scheme. The vulnerabilities arising in realistic implementations reduce the success probability, but leave the fidelity of the generated state unaltered. Our proposal thus allows for the generation of high-fidelity entangled matter qubits with high rate