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. These detectors use a thin metal film 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 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. In addition to Chou, the team included graduate student Wen-Di Li and postdoctoral researcher Jonathan Hu in the Department of Electrical Engineering. The work is sponsored in part by the Defense Advanced Research Agency and the National Science Foundation.
Steven Schultz | EurekAlert!
Fraunhofer starts development of refrigerant-free, energy-efficient electrocaloric heat pumps
09.12.2019 | Fraunhofer IPM
A solution for cleaning up PFAS, one of the world's most intractable pollutants
06.12.2019 | Colorado State University
Vaccinia viruses serve as a vaccine against human smallpox and as the basis of new cancer therapies. Two studies now provide fascinating insights into their unusual propagation strategy at the atomic level.
For viruses to multiply, they usually need the support of the cells they infect. In many cases, only in their host’s nucleus can they find the machines,...
More than one hundred and fifty years have passed since the publication of James Clerk Maxwell's "A Dynamical Theory of the Electromagnetic Field" (1865). What would our lives be without this publication?
It is difficult to imagine, as this treatise revolutionized our fundamental understanding of electric fields, magnetic fields, and light. The twenty original...
In a joint experimental and theoretical work performed at the Heidelberg Max Planck Institute for Nuclear Physics, an international team of physicists detected for the first time an orbital crossing in the highly charged ion Pr⁹⁺. Optical spectra were recorded employing an electron beam ion trap and analysed with the aid of atomic structure calculations. A proposed nHz-wide transition has been identified and its energy was determined with high precision. Theory predicts a very high sensitivity to new physics and extremely low susceptibility to external perturbations for this “clock line” making it a unique candidate for proposed precision studies.
Laser spectroscopy of neutral atoms and singly charged ions has reached astonishing precision by merit of a chain of technological advances during the past...
The ability to investigate the dynamics of single particle at the nano-scale and femtosecond level remained an unfathomed dream for years. It was not until the dawn of the 21st century that nanotechnology and femtoscience gradually merged together and the first ultrafast microscopy of individual quantum dots (QDs) and molecules was accomplished.
Ultrafast microscopy studies entirely rely on detecting nanoparticles or single molecules with luminescence techniques, which require efficient emitters to...
Graphene, a two-dimensional structure made of carbon, is a material with excellent mechanical, electronic and optical properties. However, it did not seem suitable for magnetic applications. Together with international partners, Empa researchers have now succeeded in synthesizing a unique nanographene predicted in the 1970s, which conclusively demonstrates that carbon in very specific forms has magnetic properties that could permit future spintronic applications. The results have just been published in the renowned journal Nature Nanotechnology.
Depending on the shape and orientation of their edges, graphene nanostructures (also known as nanographenes) can have very different properties – for example,...
03.12.2019 | Event News
15.11.2019 | Event News
15.11.2019 | Event News
13.12.2019 | Physics and Astronomy
13.12.2019 | Physics and Astronomy
13.12.2019 | Materials Sciences