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Novel Technique Stamps Out Nanoprobes

The Molecular Foundry at Lawrence Berkeley Lab is scaling up production of nanophotonic devices

4 min read
Photo of Stefano Cabrini, head of nanofabrication at The Molecular Foundry
Stefano Cabrini, director of the nanofabrication facility at the Lawrence Berkeley National Laboratory's Molecular Foundry
Image: Lawrence Berkeley National Laboratory

One of the cornerstones of the U.S. National Nanotechnology Initiative was the establishment of nanotechnology research facilities that would be available to outside users.  One of the five user facilities operated by the U.S. Department of Energy (DOE) is located at Berkeley National Lab and is known as The Molecular Foundry.

In a visit to The Molecular Foundry, we had the opportunity to discuss the state of nanofabrication with Stefano Cabrini, who is the director of its nanofabrication facility. “The mission of The Molecular Foundry is dedicated to nanoscience in general, but it is a user facility,” Cabrini told IEEE Spectrum. “So we are hosting here people from all over the world, from companies and from academia, from Europe, from Asia, from everywhere. They come here to work with us, to use or our instruments and facilities, but also to have access to our expertise.”

The goal of the research facility is to split the research 50-50 between outside users and the resident experts at the lab. However, Cabrini acknowledges that the boundaries quickly become blurry, especially when pursuing the best science.

While nanofabrication is Cabrini’s main bailiwick, much of his work of late has been focused on nanophotonics, especially in the emerging fields of metamaterials and plasmonics

Plasmonics exploits the waves of electrons (plasmons) that are created on the surface of a metal when it is struck by photons. Metamaterials are artificially structured materials fabricated by assembling different objects in place of the atoms and molecules that make up a conventional material. The resulting material has very different electromagnetic properties than those found in naturally occurring or chemically synthesized materials. Both plasmonics and metamaterials are often used in antenna technologies for essentially squeezing down the size of light waves.

Back in 2012, Cabrini and his colleagues described, in the journal Science, a device that operates like a transducer for far-field light to near-field light. The near-field light overcomes the diffraction limit of light and provides a much higher resolution than far-field light in microscopy.

Prior to this work, electron microscopy was the way to collect information about nanomaterials, but the data it collects about the nanomaterials is only at the sub-atomic level. But information about the nanomaterial’s chemical makeup requires analysis at the larger molecular level. While optical or vibrational spectroscopy typically provides chemical information on materials on the macroscale, the problem with these light-based techniques is that the light comes up against its diffraction limit when it tries to focus in on the nanoscale. You simply cannot focus a light down to a spot smaller than half its wavelength.

By using plasmonics, the researchers were able to exploit the surface plasmons that are created on a metal surface when light hits it. When the plasmons on two surfaces are separated by a small gap, it’s possible to collect and amplify the optical field in the gap, making a stronger signal for scientists to measure.

“We saw that with plasmonics it was possible to focus light onto a very small spot—a few nanometers—and normally you can’t squeeze the light in that small a dimension,” said Cabrini.  

When the light is squeezed down to these very small spots, it begins to interact with the matter it is focused on. Once you have collected the photons that are either scattered or emitted because of this interaction with the near-field light and the material, it is possible to turn it back into far-field light.

The key to making all this work was the design and fabrication of the near-field probe. Cabrini and his colleagues were able to fabricate a tapered, four-sided tip on the end of an optical fiber. The researchers dubbed the resulting tip “campanile” after the tower that resembles a church campinile on the UC Berkeley campus. Two of the campanile's gold-coated sides are separated by just a few nanometers at the tip. The three-dimensional taper enables the device to channel light of all wavelengths down into an enhanced field at the tip. The size of the gap determines the resolution.

Research has continued at the Foundry with this device and has been led by Keiko Munechika, the manager for nanofabrication at aBeam Technologies, a small company that is working on next-generation nanofabrication.

In the latest research, described in the journal Scientific Reports, Munechika and her colleagues have developed a simplified and robust method for fabricating the campanile near-field probe. The nanoimprinting technique operates much like a stamp used on sealing wax. There is a mold with the desired shape that is pressed down into a polymer that sits on the optical fiber. Once the polymer cools, the tip of the optical fiber is in the shape of the near field probe .

“The original version of the campanile was made one-by-one, by curving out an optical fiber into a campanile structure consisting of micro and nanometer features using a focused ion beam,” explained Munechika, in an email interview with IEEE Spectrum. “It requires both time and expertise (and a lot of patience). The throughput was only a few campanile probes per month. Many researchers have been interested in using the probes but there was no way to scale up the fabrication. So, we developed a new way to fabricate campanile probes using nanoimprint lithography. Now it’s possible to make several probes per day.”

The probes will be used for nano-spectroscopic imaging of various materials including new-generation photovoltaic materials to learn optoelectronic processes at the nanometer scale resolution. “We are hoping that many new discoveries will be made, now that these probes will be more accessible to researchers,” said Munechika.

This latest research highlights what Cabrini believes is one of the foundations of The Molecular Foundry’s mission: that technologies be developed for widespread use.

Cabrini adds: “Our aim here is not just to make the smallest things while maintaining the same quality, but to transfer these devices to the widest spectrum of applications so that everyone can use them.”

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