Fang Applies Optical Technique to Ultrasound

Nicholas Fang focuses on the smallest of materials but his imagination is expansive.

When his optical microscope couldn't produce the kind of high resolution, micron-scale images he wanted, Fang improved its capabilities through a relativity new technique called superlensing that enhances resolution by incorporating layered nanomaterials in the imaging process.

Satisfied with the results of that optical technique, Fang decided to apply those same principles to acoustics. This latest venture resulted in a paper in Nature Materials and is ultimately geared toward greatly improving the quality of ultrasound images for medical or industrial uses.

Fang is an assistant Professor of Mechanical Science and Engineering at the University of Illinois and part-time faculty member at Beckman. One of his main research foci is on the design and manufacturing of photonic metamaterials (nanoscale materials that resonate with light effectively) and devices in which artificial atoms serve as building blocks to form a composite with light-refracting qualities. These structures can then be used for "superlensing" or imaging structures around the scale of 50 nanometers, or about 1/8th the size of a normal optical wavelength.

Fang said superlensing uses non-magnetic materials like copper to form composites that greatly enhance the imaging capabilities of a traditional lens.

"We want to build atoms and building blocks that are much smaller than the wavelength of the electromagnetic wave, yet when they are connected together they effectively give a refraction of light," he said.

Fang is now expanding those concepts into acoustics in what the Nature Materials article calls the first investigation into an "acoustic analogue to electromagnetic metamaterials."

"Photonic metamaterials is a relatively new idea that came out about 10 years ago," Fang said. "The concept is to take the analogy of how atoms refract light and build artificial atoms or building blocks that are much smaller than the wavelength of light, or in this case, the wavelength of acoustics. Here we are talking about a totally different wave."

When he talks about this latest research into acoustics, Fang is able to picture a sound wave coming from a musical instrument: the wave flowing out, he says, fills a space greater than the parts of the instrument, just as a sub-wavelength of sound could produce more robust images than those from a conventional ultrasound.

"Think about a flute," said Fang, a member of the Molecular and Electronic Nanostructures research initiative at Beckman. "When we play music we are using some instrument that carries sound waves, and yet each of those buttons on the instrument are much smaller than the wavelengths they produce."

Fang said a sub-wavelength of sound - along the lines of 100 kilohertz to a few megahertz - is a good wavelength for finding micron scale stress fractures in building materials, or for more detailed ultrasound images in medical screenings.

"We're using very tiny elements, tiny resonator cavities, and in an analog to the flute, we try to create those structures in order to manipulate or control the resonating frequencies of ultrasound," he said. "Now imagine we use a flute to do imaging. The unique thing is we need something that is coming out of the resonance but also carries a very unique surface wave. This surface wave is essential to create the image we want with very high resolution."

Possible applications for high-resolution ultrasonic imaging are many. Fang said if the method is used for medical screening it could improve the resolution of ultrasound images without increasing risks to patients. Tumors, for instance, could be caught at their earliest stages. Other applications could include non-destructive testing in industry for quality control, such as non-destructive acoustical searches for stress fractures in building materials, or for underwater acoustics. Fang said their targeted frequency of 30 KHz is perfect for underwater imaging and detection that could be useful in military and civilian applications, especially for deep-sea object detection.

Both superlensing and ultrasonic metamaterials, Fang said, are about "improving the application's capabilities."

Fang said he first got interested in superlensing after becoming dissatisfied with the images from traditional optical microscopes.

"The traditional lens only gives us the resolution of optical wavelengths," he said. "However, if we can one day convert the picture with superlensing, we will be able to see and make nanostructures in a three-dimensional fashion.

"It always is about bridging the light and materials actions," Fang added. "How are we going to improve the light, further confine the light and how are we going to push the fabrication and basically form the loop to create materials? How do we use the confinement to further improve the material application's capabilities?"

Fang said more work is required in order to integrate the capabilities of superlensing with available optics. He said a few terahertz frequency are a good frequency range for molecular detections.

"A lot of molecules show up or leave their fingerprints at the terahertz wavelength yet there is not a very good design of the optics for them," Fang said. "Most of the development so far has concentrated on the transceivers and detectors. The optics to transfer this spectrum is lacking.

"By creating the terahertz metamaterials we are filling the gap. In terms of optical wavelengths we are shooting for building a microscope that allows us to image sub-hundred nanometer features. Our goal is to create a way to quickly and accurately observe nanoscopic pictures of living organisms."

The article by Fang and his collaborators in Nature Materials is titled "Ultrasonic metamaterials with negative modulus" and is available online.