[LINK] EMR
stephen at melbpc.org.au
stephen at melbpc.org.au
Thu Nov 24 23:52:55 AEDT 2011
Discussing electromagnetic radiation, here's a new EMR oddity ..
Blocked holes can enhance rather than stop light going through, engineers
find
Posted November 22, 2011; 10:00 A.M. By Steven Schultz
<http://www.princeton.edu/main/news/archive/S32/19/71M69/index.xml?
section=science>
Conventional wisdom would say that blocking a hole would prevent light
from going through it, but Princeton University engineers have discovered
the opposite to be true.
A research team has found that placing a metal cap over a small hole in a
metal film does not stop the light at all, but rather enhances its
transmission.
In an example of the extraordinary twists of physics that can occur at
very small scales, electrical engineer Stephen Chou and colleagues made
an array of tiny holes in a thin metal film, then blocked each hole with
an opaque metal cap.
When they shined light into the holes, they found that as much as 70
percent more light came through when the holes were blocked than when
they were open.
"The common wisdom in optics is that if you have a metal film with very
small holes and you plug the holes with metal, the light transmission is
blocked completely," said Chou, the Joseph Elgin Professor of
Engineering.
"We were very surprised."
Chou said the result could have significant implications and uses. For
one, he said, it might require scientists and engineers to rethink
techniques they have been using when they want to block all light
transmission.
In very sensitive optical instruments, such as microscopes, telescopes,
spectrometers and other optical detectors, for example, it is common to
coat a metal film onto glass with the intention of blocking light. Dust
particles, which are unavoidable in metal film deposition, inevitably
create tiny holes in the metal film, but these holes have been assumed to
be harmless because the dust particles become capped and surrounded by
metal, which is thought to block the light completely.
"This assumption is wrong the plug may not stop the leakage but rather
greatly enhance it," Chou said.
He explained that in his own field of nanotechnology, light is often used
in a technique called photolithography to carve ultrasmall patterns in
silicon or other materials. Thin metal film patterns on a glass plate
serve as a mask, directing light through certain locations of the plate
and blocking other locations. Given the new finding, engineers ought to
examine whether the mask blocks the light as expected, Chou said.
Conversely, Chou said, the newly discovered "blocking" technique might be
used in situations when a boost in light transmission is desired.
In near-field microscopy, for example, scientists view extremely fine
details by passing light through a hole as tiny as billionths of a meter
in diameter. With the new technique, the amount of light passing through
the hole and thus the amount of information about the object being
viewed can be increased by blocking the hole.
Chou and colleagues stumbled on the phenomenon of enhanced light
transmission through a blocked hole in their research on developing
ultrasensitive detectors that sense minute amounts of chemicals, with
uses ranging from medical diagnostics to the detection of explosives.
In one of their experimental detectors, the researchers studied
transmission of light through an array of tiny holes that were 60
nanometers (billionths of a meter) in diameter and 200 nanometers apart
in a gold film that was 40 nanometers thick. Each tiny hole was capped
with a gold disk that was 40 percent larger than the hole. The disks sat
on top of the holes with a slight gap between the metal surface and the
disks.
The researchers pointed a laser at the underside of the film and tested
to see if any of the laser light went through the holes, past the caps,
and could be detected on the other side.
To their surprise, they found that the total light transmission was 70
percent higher with the holes blocked by the metal disks than without
blockers. The researchers repeated the same experiment shining the light
in the opposite direction pointing at the side with the caps and
looking for transmitted light under the film and found the same results.
"We did not expect more light to get through," Chou said. "We expected
the metal to block the light completely."
Chou said the metal disk acts as a sort of "antenna" that picks up and
radiates electromagnetic waves. In this case, the metal disks pick up
light from one side of the hole and radiate it to the opposite side. The
waves travel along the surface of the metal and leap from the hole to the
cap, or vice versa depending on which way the light is traveling.
Chou's research group is continuing to investigate the effect and how it
could be applied to enhance the performance of ultrasensitive detectors.
The researchers published their findings Oct. 7 in the journal Optics
Express, and it quickly became one of the most downloaded papers.
The work is sponsored in part by the Defense Advanced Research Agency and
the National Science Foundation.
--
Cheers
Stephen
More information about the Link
mailing list