Noise reduction in a mode-locked semiconductor laser by coherent photon seeding

We have used coherent photon seeding to reduce the noise in an actively mode-locked extended-cavity semiconductor laser. Feedback is provided by an auxiliary cavity terminated either by a wedge plate or by a phase-conjugate mirror. Both conventional and phase-conjugate feedback can reduce the low-frequency noise components (0-5 MHz) by as much as 36 dB, hence drastically reducing the pulse fluctuations. A noise reduction of as much as 18 dB is also observed in the 0.01-3.0-GHz frequency range. We obtained these results while preserving the pulse duration (26 ps) and improving the time-bandwidth product.     

Scaling violation and shadowing corrections at HERA

We study the value of shadowing corrections (SC) in the HERA kinematic region in the Glauber–Mueller approach. Since the Glauber–Mueller approach was proven in perturbative quantum chromodynamics (QCD) in the double-logarithmic approximation (DLA), we develop the DLA approach for the deep inelastic structure function which takes into account the SC. Our estimates show small SC for in the HERA kinematic region while they turn out to be sizable for the gluon structure function. We compare our estimates with those for gluon distribution in leading order (LO) and next to leading order (NLO) in the DGLAP evolution equations. Received: 21 September 1998 /Published online: 14 January 1999     

Allelomimesis as escape strategy of pedestrians in two-exit confinements

We study the efficacy of allelomimesis as an escape strategy of mobile agents (pedestrians) that aim to leave a two-exit room within the shortest possible time. Allelomimesis is the act of copying one’s kindred neighbors. To escape, an agent employs one of the following strategies: (1) It chooses its own route independently (non-copying, α=0), (2) It imitates the actions of its neighbors at all times (blind copying, α=1), or (3) It either copies or acts independently according to a certain probability that is set by the copying parameter . Not more than one agent could occupy a given room location. An agent’s knowledge of the two exit locations is set by its information content . When left alone, an agent with complete knowledge of the exit whereabouts (β=1) always takes the shortest possible path to an exit. We obtain plots of the (group) evacuation time T and exit throughput Q as functions of α and β for cases where the two exits are near (on same room side) and far (on opposite sides of room) from each other. For an isolated agent, T is inversely proportional to β. The chances of escape for an isolated agent with β≤0.2 are higher with adjacent exits. However, for an agent with β>0.4 the chance is better with opposite exits. In a highly occupied room (occupancy rate R=80%) with adjacent exits, agents with β>0.8 escape more quickly if they employ a mixed strategy of copying and non-copying (0.4<α<0.6). On the other hand, blind copying (α≈1) gives agents with β<0.1 a better chance of escaping from the same room. For the same α and R values, opposite exits allow faster evacuation for agents with β<0.1 due to the likelihood of streaming and the lower probability of exit clogging. Streaming indicates an efficient utilization of an exit and it happens when the arcs that are formed are smaller and arch interference is less likely. Allelomimesis provides a simple yet versatile mechanism for studying the egress behavior of confined crowds in a multi-exit room.     

Measurement of the top-Higgs Yukawa coupling at a linear e<Superscript>+</Superscript>e<Superscript>-</Superscript> collider

Understanding the mechanism of electroweak symmetry breaking and the origin of boson and fermion masses is among the most pressing questions raised in contemporary particle physics. If these issues involve one (several) Higgs boson(s), a precise measurement of all its (their) properties will be of prime importance. Among those, the Higgs coupling to matter fermions (the Yukawa coupling). At a linear collider, the process e⁺e^-→tt̄H will allow a direct measurement of the top-Higgs Yukawa coupling. We present a realistic feasibility study of the measurement in the context of the TESLA collider. Four channels are studied and the analysis is repeated for several Higgs mass values within the range 120–200 GeV/c ². PACS 13.66.Jn; 14.65.Ha; 14.80.Bn     

Far infrared optical properties of MoCl5 doped CIS polyacetylene

Far infrared transmission spectra of MoCl₅ homogeneously doped polyacetylene are reported. They show that in this spectral range the optical index is frequency independent but increases strongly with increasing doping level. IR conductivity is compared to d.c. conductivity.     

Nature-inspired dimerization as a strategy to modulate neuropeptide pharmacology exemplified with vasopressin and oxytocin

Vasopressin (VP) and oxytocin (OT) are cyclic neuropeptides that regulate fundamental physiological functions via four G protein-coupled receptors, V_1aR, V_1bR, V₂R, and OTR. Ligand development remains challenging for these receptors due to complex structure–activity relationships. Here, we investigated dimerization as a strategy for developing ligands with novel pharmacology. We regioselectively synthesised and systematically studied parallel, antiparallel and N- to C-terminal cyclized homo- and heterodimer constructs of VP, OT and dVDAVP (1-deamino-4-valine-8-d-arginine-VP). All disulfide-linked dimers, except for the head-to-tail cyclized constructs, retained nanomolar potency despite the structural implications of dimerization. Our results support a single chain interaction for receptor activation. Dimer orientation had little impact on activity, except for the dVDAVP homodimers, where an antagonist to agonist switch was observed at the V_1aR. This study provides novel insights into the structural requirements of VP/OT receptor activation and spotlights dimerization as a strategy to modulate pharmacology, a concept also frequently observed in nature.

Structural and pharmacological study of parallel, antiparallel and N- to C-terminal cyclized homo- and heterodimers of vasopressin and oxytocin. This study spotlights dimerization as a strategy to modulate the pharmacology of neuropeptides.