Holes blocked with cap transmit more light, researchers find

It seems intuitive: Blocking a hole with a solid object should prevent light from moving through it. But it turns out that obstructing a tiny metal hole with a metal cap actually makes more light penetrate it instead, according to new research by electrical engineering professor Stephen Chou and his team of researchers.

“If you have a tiny hole in a metal film, we knew that when you put a little piece of metal either at the back or in front of the hole, you can block the light,” Chou said. “But what we discovered is that, in fact, you enhance the light going through the hole.”

Chou’s team made its discovery when they created tiny holes in a thin sheet of metal, then used an opaque metal cap to block each hole. Upon shining light into the holes, they found that the light transmission through a blocked hole was 70 percent higher than through an open one.

Chou said that this finding could change our understanding of how light propagates in nanostructures: On normal scales, a piece of metal is thought to block light transmission, but on a microscopic level, the metal does not block light at all.

“This is opposite from classical theories,” Chou said.

This discovery may aid the development of technologies that require total light blockage as well as those that need enhanced light transmission, according to Chou.

For example, it is common for highly sensitive optical instruments—including everyday ones such as cameras—to use a metal film to block light. When the metal is deposited on the glass, dust particles inevitably create holes in the metal—but these holes were thought to be safe because the metal cap would block light from going through them.

However, Chou’s discovery tells us that the light will actually go through these holes, which means that dust particles could actually create leaky films. Dust particles cause the same problem in the nanofabrication area called photolithography, a techniquefor the printing of nano-patternsthat uses a glass mask as a negative to block part of the light while allowing certain amounts of light to pass through. As such, these new findings on blocked holes are “making scientists rethink how they develop designs of these instruments, such as perhaps making the coating many layers rather than a single layer,” Chou explained.

Chou’s discovery also offers a solution for instruments that require enhanced light transmission. For example, in microscopy, scientists use high-sensitivity microscopes with very small holes to pick up optical signals from single molecules in small locations of their samples. The signals depend on how much light goes through the small hole, Chou explained, “so they want to get as much light through the small hole as possible. With our discovery, you can put a small block on the hole to allow more light transmission.”

The team is currently testing various biological applications of their findings, such as enhancing the bio-detection limit. He cited the example of cancer detection: Doctors may be able to detect cancer much earlier than today’s technology allows because the sensitivity of their equipment could potentially increase by three orders of magnitude, according to Chou.

The team’s findings were published in an October issue of the journalOptics Express and its work became one of the month’s most downloaded papers.

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They have previously worked on developing new nanotechnology and inventing innovative electronic devices—optical, biological and magnetic—that are extremely small so that conventional theory may no longer apply.

“This means we have lots of opportunity to discover and innovate,” Chou explained.