Blocked Holes Enhance Light Transmission
PRINCETON, N.J., Nov. 28, 2011 — Placing a metal cap over a small hole in a metal film does not stop light from passing through the hole, but rather enhances its transmission — quite contrary to the conventional thought that blocking a hole would allow less light to pass through it.
This finding could have significant implications and uses, particularly in various types of optical instrumentation, said scientists at Princeton University, who carried out the work.
In an example of the “twists” in physics that can occur at very small scales, electrical engineer Stephen Chou and his 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.
These electron microscope images show an experiment demonstrating that blocking a hole in a thin metal film could cause more light to pass through the hole than leaving it 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. (Credit: Stephen Chou/Princeton University)
“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,” Chou said. “We were very surprised.”
Chou said that the result 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 often is 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 a nanometer. 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 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. The research group is continuing to investigate the effect and how it could be applied to enhance the performance of ultrasensitive detectors.
The findings were published Oct. 7 in the journal Optics Express.
For more information, visit: www.princeton.edu
- A lithographic technique using an image produced by photography for printing on a print-nonprint, sectioned surface.
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