Black & white continuous tone printing using multiple negative working plates, so that each plate prints an equal segment of a determined density range

Continuous tone printing, either collotype or screenless lithography, are processes that are in limited use. The halftone method of reproduction is still the most economical way to reproduce a continuous tone original. This study dealt with the mechanism of screenless lithography, more specifically on how to make the use of negative working plates a feasible alternative to positive working plates. It is known that the positive working plate can print a longer range of tones than a negative working plate. So the main point of the experiment dealt with how to extend the range of a negative working plate. A negative plate is of high contrast, and so multiple negative working plates were used to print one image. Each plate in itself can print three to four steps of a continuous tone step wedge. In this experiment, three plates were used in the hopes of printing twelve steps in a continuous gradation. A continuous tone step wedge along with a black and white photograph were divided among the three plates. Twelve steps on the step wedge were identified, and so each film had four of the possible thirty steps which existed on the step wedge. These films were exposed to plates, and mounted on the press. Each image was run in registration with one another, and the solid ink density was varied to produce the continuous tone effect. From the press sheets examined, in which there are examples in this report, a high contrast image was produced. One of the main problems in the experiment was differences in emulsion coating on each individual plate. Which resulted in inconsistent exposure times from one plate to another. From repeated plate testing using a .15 density increment continuous tone step wedge as a control guide, it was found that for the same exposure a hardening of the plate coating (and consequently of ink receptivity) corresponding to two steps equaling .30 density difference was to be expected. This made it virtually impossible to get a handle on the correct exposure time needed for the plate. Results from this experiment show, that it is not possible to effectively print a continuous tone image with a negative working plate and or multiple negative plates. This is due to the high contrast of the negative working plate, thus making it suitable for mainly halftone lithographic purposes

Direct evidence for efficient ultrafast charge separation in epitaxial WS2_2/graphene heterostructure

We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS$_2$ and graphene. This heterostructure combines the benefits of a direct gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS$_2$, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS$_2$ layer. The resulting charge transfer state is found to have a lifetime of $\sim1$\,ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS$_2$ and graphene bands as revealed by high resolution ARPES. In combination with spin-selective excitation using circularly polarized light the investigated WS$_2$/graphene heterostructure might provide a new platform for efficient optical spin injection into graphene.Comment: 28 pages, 14 figure

Direct evidence for efficient ultrafast charge separation in epitaxial WS₂/graphene heterostructures

We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS2 and graphene. This heterostructure combines the benefits of a direct-gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS2, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS2 layer. The resulting charge-separated transient state is found to have a lifetime of ∼1 ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS2 and graphene bands as revealed by high-resolution ARPES. In combination with spin-selective optical excitation, the investigated WS2/graphene heterostructure might provide a platform for efficient optical spin injection into graphene

Foraging behavior of leaf cutting ants: How do workers search for their food?

Forager ants search for adequate food sources in nature and, after their discovery, decide whether the source is suitable or not for the colony. However, we asked “How do workers seek out the substrate for cultivation of the symbiontic fungus on which they feed? To answer this question, we evaluated the distance traveled by individual workers in the search of food and the distance traveled to return to the nest, as well as the time and velocity necessary for these activities. The results showed that the distance traveled by the leaf cutting ant, Atta sexdens rubropilosa (Linneus, 1758), in the search of food was greater than the distance traveled to return with the substrate to the colony. On the other hand, the mean time and velocity were similar for food search and return to the colony. These results support the hypothesis of information transfer, according to which the worker needs to return to the nest at the beginning of foraging to transfer information to other workers and thus to establish the process of worker ant foraging. It can be concluded that workers travel large distances in a random manner until finding their substrate, but the return to the nest is efficient considering the shorter distance traveled.

Grid services for the MAGIC experiment

Exploring signals from the outer space has become an observational science under fast expansion. On the basis of its advanced technology the MAGIC telescope is the natural building block for the first large scale ground based high energy gamma-ray observatory. The low energy threshold for gamma-rays together with different background sources leads to a considerable amount of data. The analysis will be done in different institutes spread over Europe. Therefore MAGIC offers the opportunity to use the Grid technology to setup a distributed computational and data intensive analysis system with the nowadays available technology. Benefits of Grid computing for the MAGIC telescope are presented.Comment: 5 pages, 1 figures, to be published in the Proceedings of the 6th International Symposium ''Frontiers of Fundamental and Computational Physics'' (FFP6), Udine (Italy), Sep. 26-29, 200