Blocked holes can enhance rather than stop light going through

Nov 22, 2011
These electron microscope images show an experiment in which Princeton electrical engineer Stephen Chou showed that blocking a hole in a thin metal film could cause more light to pass through the hole than leaving the hole unblocked. The top image shows an array of holes with gold caps, each of which is 40 percent bigger than the hole on which it sits. The bottom image shows a cross-section view of one hole with the cap sitting on top. The hole covered with the cap surprisingly allows more light to be transmitted through the film than a hole without the cap, Chou's research team found. Credit: Stephen Chou/Princeton University

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 , 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 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. These detectors use a thin with an array of holes and metal disks to boost faint signals produced when laser light encounters a molecule, allowing much greater sensitivity in identifying substances.

In one of their experimental detectors, the researchers studied transmission of light through an array of 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 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 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.

Explore further: Three's a charm: NIST detectors reveal entangled photon triplets

More information: The researchers published their findings Oct. 7 in the journal Optics Express.

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User comments : 25

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Royale
5 / 5 (12) Nov 22, 2011
This is a great example of why Physics is so fascinating.
enginarc
1 / 5 (14) Nov 22, 2011
More light passing through?

I think, the observer is detecting more light due to position, orientation to surface, as the amount of light passing though the hole - with or without capped - shouldn't change.

imo light waves, when struct to the cap is playing tricks, and this is detected as more light by the sensor.
Vendicar_Decarian
Nov 22, 2011
This comment has been removed by a moderator.
axemaster
2 / 5 (3) Nov 22, 2011
Let us not forget that light is electromagnetic waves. It's actually not so surprising to see that it would interact with tiny blobs of metal in this way - the metal is probably transmitting the waves past the insulating barrier. Kind of like tiny waveguides or antennas.
Callippo
1.2 / 5 (9) Nov 22, 2011
I do agree - as a distributed field analogy of this system can serve the transmission of signal in coaxial cable, which gets actually worse, when the rest of coax cable remains open - this is the true reason of these termination plugs, used for older Ethernet cabling.

http://media.digi...29-6.jpg
bluehigh
2.2 / 5 (10) Nov 22, 2011
I'm not so sure the co-ax analogy is relevant. With co-ax its about impedance matching. When the cable is not correctly terminated the signals are reflected back up the cable. This is not what happens with light in the article.

I find the results extraordinary enough to require independent verification. How big is the 'slight gap' between the caps and surface? At what scale does this effect start and stop? Is it wavelength dependent? Since when does gold re-emit photons at the same wavelength?

Experimental error.

RayWilson
1.8 / 5 (4) Nov 22, 2011
The caps are 40% larger(?). Could this be a simple "bending" effect by the caps hence directing light that ordinarily misses the holes, bending the light through the holes. The cross-sectional area of the caps is almost 2X the hole areas.
Subtract a bit for absorption by the gold and the maybe 70% more light gets through.
Callippo
1.9 / 5 (9) Nov 22, 2011
I'm not so sure the co-ax analogy is relevant. With co-ax its about impedance matching. When the cable is not correctly terminated the signals are reflected back up the cable. This is not what happens with light in the article.
The caps actually work in narrow range of wavelength. It's example of band gap filter, common in resonators.
Subtract a bit for absorption by the gold and the maybe 70% more light gets through.
The EM waves do travel around surface of gaps in form of surface plasmons. At the nanoscale metals are behaving like elastic materials, covered with liquid mercury surface of electrons.
bluehigh
3.4 / 5 (13) Nov 22, 2011
covered with liquid mercury surface of electrons.


A what? Did you just pick out some words in the hope that they made any sense?
powerup1
3 / 5 (4) Nov 23, 2011
More light passing through?

I think, the observer is detecting more light due to position, orientation to surface, as the amount of light passing though the hole - with or without capped - shouldn't change.

imo light waves, when struct to the cap is playing tricks, and this is detected as more light by the sensor.


In what journal did you publish your research proving your assertions?
rawa1
1.8 / 5 (5) Nov 23, 2011
covered with liquid mercury surface of electrons.
Sorry, sometimes I'm mixing the analogies with real systems in my posts too much. The free electrons in metals are covering the atoms in layer, which is similar to layer of liquid mercury. It's elastic and it can spread the electromagnetic waves in form of surface plasmonic waves like ripples at pond. These waves enable to spread the light around/across the gold caps covering the holes.

The absorption of various molecules to the cap surface affects the spreading of surface plasmon waves significantly, which is why this system is so sensitive for organic compounds. This is just the consequence of the fact, the light is forced to use these surface waves for its spreading instead of bulk ones.

Got it?
Royale
5 / 5 (4) Nov 23, 2011
Why do you people have two names? In the response in the post above me it clearly shows that Callippo = rawa1. Why do you do it? To agree with yourself and upvote? Seems pointless to me. Just see how others respond to your posts. It really helps you judge the accuracy or relevance of posts. Don't slur those votes by agreeing with yourself.
Bond Number
not rated yet Nov 23, 2011
I think it would be more interesting if they used another material than a noble metal which are known to have negative permittivities around ir/vis
definitude
1 / 5 (1) Nov 23, 2011
The nature of lights tendency to skirt around materials, does not surprise me, in fact if validates my understanding of how the atomic structure interfaces with photons from the Young experiment.

In fact, not long ago the detection of light at the edge of the Young experiment was furthered by detection of eddy like patters in the photon and atomic interaction. This is something I predicted, albeit with no great accuracy, but from an understanding that matter is but a collection of interacting fields. A fact that still has legs even after CERN has thrown as much energy as it has, at "particles."

Cris Smith
rawa1
1 / 5 (6) Nov 23, 2011
It really helps you judge the accuracy or relevance of posts.
This is just the point. You should always use the post content, not the name of person in judging of relevancy of the posts. The later way is not scientific and not objective. If you're not able to do so, then you're incompetent to discuss scientific problems here. BTW I'm using multiple accounts from historical reasons.
bewertow
3 / 5 (6) Nov 24, 2011
^HAHA rawa1 has a sock puppet account! What a huge loser making multiple accounts to agree with himself.
Callippo
1 / 5 (4) Nov 24, 2011
Well, at least it's funny.
Egleton
2.3 / 5 (3) Nov 25, 2011
Someone repeat this experiment.
Take three very thin sheets of polarised material.I used shards of liquid crystal from a calculator display.

A and B are placed in contact and turned to extinguish the light coming through.
Place C on top of the now extinguished A and B.
Maintaining the relative positions of A and B, rotate C.

The light which used to be extinguished is now allowed through.

My explanation. The light takes a very short time to traverse the three thin sheets. Because the future causes the present to come into existence, and the very near future has the strongest "Pull" The light "senses" that C is in open position and so is free to continue through in spite of being blocked by A and B.
Phaze
not rated yet Nov 25, 2011
what holds the gold up from the film surface, is it transparant
MorituriMax
2.6 / 5 (5) Nov 26, 2011
Callippo
The EM waves do travel around surface of gaps in form of surface plasmons. At the nanoscale metals are behaving like elastic materials, covered with liquid mercury surface of electrons.
rawa1
Sorry, sometimes I'm mixing the analogies with real systems in my posts too much. The free electrons in metals are covering the atoms in layer, which is similar to layer of liquid mercury.
Okkkk.... so WTF are you posting under different names? Is this how you get ratings greater than 1? You just stroke yourself under different aliases?
Well, at least it's funny.
Only in a shutin pathetic kind of way.
bluehigh
2.1 / 5 (9) Nov 27, 2011
Such a lack of imagination sock puppets show!

I find the results extraordinary enough to require independent verification.
- bluehigh

Yes bluehigh, seems less controversial results are often required to be verified by independent tests.

seems less controversial results are often required to be verified
- bluehigh

I agree bluehigh, like the FTL neutrinos that everyone wants to have re-checked again and again because of cognitive dissonance.

everyone wants to have re-checked again and again
- bluehigh

yep - well done bluehigh, I completely agree with you. I give you a 5.

I completely agree with you.
- bluehigh

yep, I'm with you. 5 to you too.

I suppose not quite so much fun!
rwinners
2.8 / 5 (4) Nov 28, 2011
This doesn't sound right. Perhaps the 'caps' are acting as lenses.
rawa1
1 / 5 (1) Nov 28, 2011
The don't act as a lenses, but the surface of metal is conductive for EM waves at short distances, as explained above.
jakack
not rated yet Nov 28, 2011
Does this allow all wavelengths of light through or only certain portions of the spectrum? For this experiment they used a laser. I'm assuming it's all visible light, but am just curious if there were limitations.
rawa1
1 / 5 (2) Nov 29, 2011
Does this allow all wavelengths of light through or only certain portions of the spectrum?
Of course, this effect works in just narrow range of wavelengths, outside of this band it can be completely reversed - and you can write another publication about it.

http://physicswor...ws/40955
jakack
1 / 5 (1) Nov 29, 2011
How much light gets through the 40 nm thick metal surface itself without holes or plugs?