85 research outputs found
Observation of 2nd band vortex solitons in 2D photonic lattices
We demonstrate second-band bright vortex-array solitons in photonic lattices.
This constitutes the first experimental observation of higher-band solitons in
any 2D periodic system. These solitons possess complex intensity and phase
structures, yet they can be excited by a simple highly-localized vortex-ring
beam. Finally, we show that the linear diffraction of such beams exhibits
preferential transport along the lattice axes
Robust optical delay lines via topological protection
Phenomena associated with topological properties of physical systems are
naturally robust against perturbations. This robustness is exemplified by
quantized conductance and edge state transport in the quantum Hall and quantum
spin Hall effects. Here we show how exploiting topological properties of
optical systems can be used to implement robust photonic devices. We
demonstrate how quantum spin Hall Hamiltonians can be created with linear
optical elements using a network of coupled resonator optical waveguides (CROW)
in two dimensions. We find that key features of quantum Hall systems, including
the characteristic Hofstadter butterfly and robust edge state transport, can be
obtained in such systems. As a specific application, we show that the
topological protection can be used to dramatically improve the performance of
optical delay lines and to overcome limitations related to disorder in photonic
technologies.Comment: 9 pages, 5 figures + 12 pages of supplementary informatio
Afferent signalling from the acid-challenged rat stomach is inhibited and gastric acid elimination is enhanced by lafutidine
<p>Abstract</p> <p>Background</p> <p>Lafutidine is a histamine H<sub>2 </sub>receptor antagonist, the gastroprotective effect of which is related to its antisecretory activity and its ability to activate a sensory neuron-dependent mechanism of defence. The present study investigated whether intragastric administration of lafutidine (10 and 30 mg/kg) modifies vagal afferent signalling, mucosal injury, intragastric acidity and gastric emptying after gastric acid challenge.</p> <p>Methods</p> <p>Adult rats were treated with vehicle, lafutidine (10 â 30 mg/kg) or cimetidine (10 mg/kg), and 30 min later their stomachs were exposed to exogenous HCl (0.25 M). During the period of 2 h post-HCl, intragastric pH, gastric volume, gastric acidity and extent of macroscopic gastric mucosal injury were determined and the activation of neurons in the brainstem was visualized by c-Fos immunocytochemistry.</p> <p>Results</p> <p>Gastric acid challenge enhanced the expression of c-Fos in the nucleus tractus solitarii but caused only minimal damage to the gastric mucosa. Lafutidine reduced the HCl-evoked expression of c-Fos in the NTS and elevated the intragastric pH following intragastric administration of excess HCl. Further analysis showed that the gastroprotective effect of lafutidine against excess acid was delayed and went in parallel with facilitation of gastric emptying, measured indirectly via gastric volume changes, and a reduction of gastric acidity. The H<sub>2 </sub>receptor antagonist cimetidine had similar but weaker effects.</p> <p>Conclusion</p> <p>These observations indicate that lafutidine inhibits the vagal afferent signalling of a gastric acid insult, which may reflect an inhibitory action on acid-induced gastric pain. The ability of lafutidine to decrease intragastric acidity following exposure to excess HCl cannot be explained by its antisecretory activity but appears to reflect dilution and/or emptying of the acid load into the duodenum. This profile of actions emphasizes the notion that H<sub>2 </sub>receptor antagonists can protect the gastric mucosa from acid injury independently of their ability to suppress gastric acid secretion.</p
Mechanisms of Intragastric pH Sensing
Luminal amino acids and lack of luminal acidity as a result of acid neutralization by intragastric foodstuffs are powerful signals for acid secretion. Although the hormonal and neural pathways underlying this regulatory mechanism are well understood, the nature of the gastric luminal pH sensor has been enigmatic. In clinical studies, high pH, tryptic peptides, and luminal divalent metals (Ca2+ and Mg2+) increase gastrin release and acid production. The calcium-sensing receptor (CaSR), first described in the parathyroid gland but expressed on gastric G cells, is a logical candidate for the gastric acid sensor. Because CaSR ligands include amino acids and divalent metals, and because extracellular pH affects ligand binding in the pH range of the gastric content, its pH, metal, and nutrient-sensing functions are consistent with physiologic observations. The CaSR is thus an attractive candidate for the gastric luminal sensor that is part of the neuroendocrine negative regulatory loop for acid secretion
Toward a geography of black internationalism: Bayard Rustin, nonviolence and the promise of Africa
This article charts the trip made by civil rights leader Bayard Rustin to West Africa in 1952, and examines the unpublished âAfrica Programâ which he subsequently presented to leading American pacifists. I situate Rustinâs writings within the burgeoning literature on black internationalism which, despite its clear geographical registers, geographers themselves have as yet made only a modest contribution towards. The article argues that within this literature there remains a tendency to romanticize cross-cultural connections in lieu of critically interrogating their basic, and often competing, claims. I argue that closer attention to the geographies of black internationalism, however, allows us to shape a more diverse and practiced sense of internationalist encounter and exchange. The article reconstructs the multiplicity of Rustinâs black internationalist geographies which drew eclectically from a range of Pan-African, American and pacifist traditions. Though each of these was profoundly racialized, they conceptualized race in distinctive ways and thereby had differing understandings of what constituted the international as a geographical arena. By blending these forms of internationalism Rustin was able to promote a particular model of civil rights which was characteristically internationalist in outlook, nonviolent in principle and institutional in composition; a model which in selective and uneven ways continues to shape our understanding of the period
Ultra-low-power hybrid light-matter solitons.
New functionalities in nonlinear optics will require systems with giant optical nonlinearity as well as compatibility with photonic circuit fabrication techniques. Here we introduce a platform based on strong light-matter coupling between waveguide photons and quantum-well excitons. On a sub-millimetre length scale we generate picosecond bright temporal solitons at a pulse energy of only 0.5âpJ. From this we deduce a nonlinear refractive index three orders of magnitude larger than in any other ultrafast system. We study both temporal and spatio-temporal nonlinear effects and observe dark-bright spatio-temporal polariton solitons. Theoretical modelling of soliton formation in the strongly coupled system confirms the experimental observations. These results show the promise of our system as a high speed, low power, integrated platform for physics and devices based on strong interactions between photons
Contextualising Apartheid at the End of Empire: Repression, âDevelopmentâ and the Bantustans
This article examines the global dynamics of late colonialism and how these informed
South African apartheid. More specifically, it locates the programmes of mass
relocation and bantustan âself-governmentâ that characterised apartheid after 1959 in
relation to three key dimensions. Firstly, the article explores the global circulation of
idioms of âdevelopmentâ and trusteeship in the first half of the twentieth century and its
significance in shaping segregationist policy; secondly, it situates bantustan âselfgovernmentâ
in relation to the history of decolonisation and the partitions and
federations that emerged as late colonial solutions; and, thirdly, it locates the
tightening of rural village planning in the bantustans after 1960 in relation to the
elaboration of anti-colonial liberation struggles, repressive southern African settler
politics and the Cold War. It argues that, far from developing policies that were at odds
with the global âwind of changeâ, South African apartheid during the 1960s and 1970s
reflected much that was characteristic about late colonial strategy
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