4,948 research outputs found
Novel waveguide configuration for convenient and sensitive fluorescence and Raman measurements of liquids over optical fibers
Fluorescence has been measured from a waveguide formed by a PTFE tube with an internal coating of a low-refractive-index amorphous fluoropolymer. The configuration is suited to taking measurements from liquids having a refractive index down to 1.32, including, in particular, aqueous solutions. The parameters which determine the optical collection efficiency have been mathematically modelled. We have produced waveguides up to 1m long, and with 0.955 mm and 0.445 mm internal radii, and measured a (fluorescence) collection enhancement factor of 3 from a 140 mm long, 0.955 mm internal radius waveguide. Work is continuing to increase the enhancement factor
Metal-Insulator Transition Revisited for Cold Atoms in Non-Abelian Gauge Potentials
We discuss the possibility of realizing metal-insulator transitions with
ultracold atoms in two-dimensional optical lattices in the presence of
artificial gauge potentials. Such transitions have been extensively studied for
magnetic fields corresponding to Abelian gauges; they occur when the magnetic
flux penetrating the lattice plaquette is an irrational multiple of the
magnetic flux quantum. Here we present the first study of these transitions for
non-Abelian U(2) gauge fields, which can be realized with atoms with two pairs
of degenerate internal states. In contrast to the Abelian case, the spectrum
and localization transition in the non-Abelian case is strongly influenced by
atomic momenta. In addition to determining the localization boundary, the
momentum fragments the spectrum and the minimum energy viewed as a function of
momentum exhibits a step structure. Other key characteristics of the
non-Abelian case include the absence of localization for certain states and
satellite fringes around the Bragg peaks in the momentum distribution and an
interesting possibility that the transition can be tuned by the atomic momenta.Comment: 4 pages, 4 figures, see http://physics.gmu.edu/~isatija/recentpub.htm
for high resolution figure
The Perceived Impact of a Short-Term Monolingual Karen Language Learning Experience on the Teaching Practices of Low-Literacy Adult English Language Instructors
A limited amount of research on low-educated low-literate adult English language learners exists. An even smaller body of literature addresses innovation and teacher change of the instructors of this demographic. Adult refugees need basic language competency to participate with dignity in a new culture. This study explores the impact of increasing teacher empathy for the monolingual language learning environment that most first-wave refugees encounter upon arrival in the United States and identifies the instructional innovations and modifications that result. Nine participants, all low-level adult ELL instructors from a number of community-based adult language schools, took part in a Karen language class over several weeks. Questionnaires, journaling, audio-recording, and researcher observation were used to study the perceived impact of this professional development language-learning experience on the participants’ teaching practice. The results fall into two categories: Experience and Impact. Within these categories are additional areas where a sharp increase in empathy results as each teacher-participant reports direct and immediate impact on teaching practices. Findings include: a renewed appreciation for the difficulty of the learners’ task, a recognition of the futility of extraneous teacher-talk, a deeper understanding of a variety of classroom behaviors, and a reinforcement of previously learned educational principles such as allowing silent periods for absorption, theme consistency and teaching a limited amount of content per class session. Findings demonstrate that this non-Roman alphabet monolingually-taught class is an experience that deeply challenges the current thought and perspectives of the teacher-participants. Reflective teaching practice is enhanced via this study as a means to create improvements in teaching and in adult learning
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