Number 604, September 13, 2002
by Phil Schewe, James Riordon, and Ben Stein
"Hyper-Focusing" a Sound Wave
"Hyper-focusing" a sound wave with time-reversed acoustics
has been experimentally demonstrated by researchers in France (Julien
de Rosny, CNRS/ESPCI/University of Paris, julien.derosny@espci.fr),
providing a new way of breaking the so-called "diffraction limit"
when imaging an object.
Even when a sound wave is launched by the tiniest nanomachine, it's
often difficult or impossible to focus the sound wave down to the size
of the machine itself. The same idea holds for any other type of wave,
including light.
That's because conventional lenses don't capture a wave at its source,
but many wavelengths away, in the "far-field." As a result,
the lens cannot focus the wave to a spot smaller than half a wavelength.
This roadblock, called the "diffraction limit," usually dictates
the smallest details one can see with a common optical microscope and
the tiniest circuits that one can carve in a computer chip using light
and lenses.
But researchers can surmount the diffraction limit--and achieve higher-resolution
microscopes, smaller circuits, and better focused sound--by capturing
a wave's "near-field" components, the fields that exist within
a wavelength of the source of the sound or light.
Researchers have now demonstrated a new way of breaking the diffraction
limit by using "time-reversed" (TR) acoustics, a technique
that takes an incident sound wave, produces a backwards-sounding version
of it, and sends the reversed version right back to the source of the
original sound. However, conventional TR acoustics itself is limited
by diffraction, because previous TR devices only captured a sound wave
in the far field rather than at the source.
In the new experiment, researchers connect a loudspeaker to a 1.9-mm-thick
glass plate. From a 100-micron contact point on the plate, they launch
a 5-microsecond-long, 500 KHz sound wave that travels inside the plate
and bounces chaotically from many points on the plate's rounded outer
boundary. A laser interferometer records the initial wave, including
its near-field components, and its trajectory for 1.5 milliseconds.
Using this information, they launch, from the same contact point, a
time-reversed version of the original sound wave. The glass plate's
boundary, which bounced around the initial wave in a chaotic fashion,
acts remarkably as many individual small lenses for the TR wave! It
excellently focuses the wave (albeit only its far-field components),
and sends it back to the tiny 100-micron spot where the sound originated.
However, the focused wave develops an undesirable "diverging"
component that spreads out (see figures
and animations).
To eliminate this component, the researchers generate the missing TR
near-field components at just the right time and this cancels out this
diverging component. What's left is the original wave that focuses on
the 100-micron contact point with a spot size that's 1/14 of the initial
sound's wavelength, 7 times smaller than that allowed by the diffraction
limit. (de Rosny
and Fink, Physical Review Letters, 16 September.)
Self-Assembled Nanotube Networks
In the brownstone neighborhoods of New York City the view out the back
window is often one of myriad telephone wires hanging from a forest
of poles. Now the same thing has been achieved on the nanometer scale.
Scientists at the Nippon Telegraph and Telephone Corporation (NTT)
have created an arbor of nm-wide silicon pillars (with standard lithography
techniques) and then, in a follow-up step, grown a
cobweb of carbon nanotubes, most of which are strung bridgelike between
neighboring silicon pillars (see figure).
The NTT researchers (contact Yoshikazu Homma, 81-46-240-3462,
homma@will.brl.ntt.co.jp) are able to send currents through the suspended
nanotubes, and the goal is to establish interconnection between nanodevices,
and also some kind of nanotube transistor network or even a self-learning
neural network.
Carbon nanotubes have versatile electrical properties. They can, for
example, be made as either n-type or p-type semiconductors through doping.
But the metallic nanotubes are of greater interest right now since electrons
can move ballistically through the tubes (that is, moving in straight
line trajectories, with few disruptive scatterings), even at room temperatures.
Photonic interactions in the suspended nanotube arrays might also be
an attractive possibility. (Homma
et al., Applied Physics Letters, 16 September 2002)
Demagogues and the Prisoner's Dilemma
Charismatic leaders and media personalities can be destabilizing influences
on social groups, according to various "small-world network"
models. This conclusion that seems intuitively consistent with historical
events such as civil uprisings and religious movements.
But, surprisingly, long range connections in a network
(which reduce the degree of separation among members) seem to hinder
the system's return to equilibrium, according to a new model that combines
small-world scenarios with a version of the "prisoner's-dilemma"
proposition, according to which a pair of captured criminals ponder
strategy: if neither criminal confesses, both go free; if one confesses,
the other receives a stiff sentence; if both confess, they each receive
moderate sentences. The study may help us to understand the dynamics
of such social behaviors as smoking among teenagers, which is influenced
by various factors including local social surroundings and the examples
set by media role models.
A collaboration of researchers from Ajou University, Chungbuk National
University, and Seoul University in Korea, and Umea University in Sweden
recently discovered the instability introduced to social systems by
influential persons in a simplified, two-dimensional, small world network.
The researchers (Beom Jun Kim, beomjun@ajou.ac.kr, 82-31-219-2571)
created a 1024-element grid of points that represented an interconnected
group of individuals. Some points in the grid were randomly designated
to be cooperators (e.g., nonsmokers), and others were designated
to be defectors (e.g., smokers). Once the grid was established,
the individuals began playing a version of the prisoner's dilemma game
with their eight nearest neighbors.
The classic prisoner's dilemma is a game involving two players who
each decide whether or not to cooperate with authorities in efforts
to minimize their own prison sentences. In the new small-world/prisoner's-dilemma
model, each individual surveys his nearest neighbors and scores points
depending on their own status as a cooperator or a defector, and the
statuses of their neighbors. The individuals may then change their status
based on their score after each round of the game.
To model the effect of an unusually powerful individual, the researchers
made connections from a single influential member to several distant
network members. In real life, for instance, the influential member
might represent a celebrity or religious demagogue with access to the
media or the Internet. When the influential member was a defector, the
network collapsed into a numerical kind of anarchy, with many cooperators
defecting as well.
Eventually, the benefits of cooperating return the system to equilibrium,
but the more long range connections in the network, the slower the system's
recovery. Although the model is clearly a crude reflection of human
interactions, it suggests that increasing numbers of long range connections
between people may help destabilize communities. The result is in contrast
to the general perception that connections across cultures and nations
is exclusively beneficial to society. (B.J.
Kim et al., Phys. Rev. E, August 2002)
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