Refractive Index Enhancement with Vanishing Absorption in an Atomic Vapor
Since the birth of quantum and nonlinear optics, one of the key challenges has
been if one can achieve a very large refractive index for a laser beam. A key
application of a large refractive index is to optical imaging science. A large
refractive index corresponds to a reduced wavelength inside the medium and
enhanced imaging and lithographic resolution. A simple and an efficient way to
achieve a large refractive index in a gaseous medium is likely to have
significant practical implications since lithographic resolution currently
determines the size and the processing power of every semiconductor integrated
circuit.
In this work, we demonstrated a new scheme for increasing the refractive index
of an atomic vapor without increasing absorption to the beam. By exciting two
resonances in atomic Rubidium and using the interference between these two
resonances, we observed an enhanced refractive index while maintaining
vanishing absorption. In the future, our scheme may allow a refractive index
approaching a value of 10 in a gas. Such a large refractive index will allow
construction of optical microscopes with spatial resolution at the 10
nanometer scale using visible light.
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LT11046
Fluid Instabilities and Untangling in Biological Cell Sorting
Pattern formation in embryogenesis, cancer metastasis and tissue engineering all depend strongly on the mechanical interactions between biological cells. These interactions can be and often are modeled using analogs from fluid mechanics like viscosity and surface tension. In our paper, we show a fascinating new role for the surface tension analog in the process of cell sorting - i.e. the ability of random mixtures of two cell types to spontaneously sort themselves out into two distinct domains, one enclosing the other. When the fraction of each cell type in a 3D aggregate is greater than about 30%, there is a very high probability that even a random mixture will already have each cell connected to all other cells of the same type. However, the two cell-type domains are topologically tangled (like a block of Swiss cheese where one cell type is the cheese and the other the holes). Under such circumstances, cell sorting involves untangling the domains by breaking or sealing off loops and tunnels of each cell type. The cell-cell analog of surface tension can drive such events through a process akin to the Plateau-Rayleigh instability - the one that breaks a thin stream of water into individual drops. This fluid-like instability does not change the lowest (mechanical) energy configuration of a cell aggregate, but does determine whether that configuration can be reached on accessible time scales.
Summary Figure 1. Model of cell sorting in a pseudo-random aggregate. One cell type is shown with filled cells and the other with thin grey outlines. As time progresses, the loop at P seals off (breaking the thread of cells that ran through the loop) and the loop at Q breaks. Panels B and C are close-ups of the overview shown in A.
Summary Figure 2. Model of a Plateau-Rayleigh instability in a thin chain of cells.
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LQ11196
Theory tells how to distinguish mechanisms of actin bundling
Living cells explore their environment using finger-like protrusions.
These protrusions are formed by the bundles of filaments made of a protein
called actin. Filamentous actin is an important component of cell
cytoskeleton and the mechanisms by which actin filaments form a bundle are
an area of active research. Recently, formation of actin bundles has been
reconstituted in vitro in the presence of a linker protein. In these
experiments, the bundles form star-like structures (asters). We present a
theoretical analysis showing that relatively simple measurements could
distinguish between various scenarios of bundle initiation. Based on the
results, we propose to measure how the number of bundles depends on the
size of the aster. Our theory predicts that this dependence is described
by a power law with an exponent having different values depending on the
initiation mechanism. The exponent value around 3 would indicate that the
bundles arise from binding of filament tips into a tip complex, while the
value between 1 and 2 would point to a mechanism in which the bundles are
initiated from binding of the tip of one filament to the side of the
other.
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LL11340B
From ultra-smoothness to nano-patterning by ion beam erosion
Ultra-smoothness and well-ordered pattern formation can be obtained on a single surface by ion beam erosion and
simply with different angles of incidence. Our systematic investigations reveal that a mechanism where collisions of
energetic ions with near-surface atoms produce an effective downhill atomic displacement plays a leading role in the
transition of the two distinct scenarios. Energetic ion bombardment on surface is known to produce well-ordered
low-dimensional (ripples or dots) structures at the submicron or nanoscale. However, in certain cases, a pronounced
ultra-smoothening effect is observed, which overwhelms all roughening effects to form any patterns. In this work, we
implement real time monitoring of surface evolution upon ion beam erosion, enabling by x-ray scattering. The quantitative
determination of surface kinetics supports the idea that a collision-induced mechanism is the dominant smoothening process
for near normal incidence. Quantitative agreement is obtained using ion-collision simulations to compute the magnitude
of the surface atomic flux. The results lead to predictions for the surface morphology phase diagram as a function of
ion energy and incidence angle that substantially agree with experimental. The resulting theory is applicable to many
surfaces that become non-crystalline during ion beam erosion.
