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 

Posted November 22, 2011; 10:00 A.M. By Steven Schultz

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 

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 

"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 

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 

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.


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