Combined forces: AFM boosts IR

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  • Published: May 1, 2013
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Combined forces: AFM boosts IR

Instrumental resonance

A technique that combines atomic force microscopy (AFM) with infrared spectroscopy has allowed researchers, for the first time, to study the infrared absorption of semiconductor plasmonic microparticles on the nanometre scale.

In work supported by the National Science Foundation (NSF), Jonathan Felts, Stephanie Law, Daniel Wasserman, and William King of the University of Illinois at Urbana-Champaign, working alongside Christopher Roberts and Viktor Podolskiy of the University of Massachusetts, have demonstrated how nanometre-scale heating effects can reveal surface plasmon resonance.

Experimental optoelectronic devices based on plasmonic structures have come to the fore in recent years, offering up to technologists a new way to carry out high-resolution optical sensing and other applications. Unfortunately, until now, there were was a dearth of instruments capable of measuring the phenomena on the nanoscale to allow them to assess and understand device performance more precisely to confirm or refute the theoretical models built around such phenomena.

King and colleagues have now found a way around that problem. "For the first time, we have measured nanometre-scale infrared absorption in semiconductor plasmonic microparticles using a technique that combines atomic force microscopy with infrared spectroscopy," he explains King is an Abel Bliss Professor in the Department of Mechanical Science and Engineering (MechSE) at Illinois. "Atomic force microscope infrared spectroscopy allows us to directly observe the plasmonic behaviour within microparticle infrared antennae," he adds.

Sustenance

Plasmonic metamaterials are usually composites that generate self-sustaining, propagating, electromagnetic waves, surface plasmons, when light interacts with the metal-dielectric. The phenomena exhibited by such materials are not known to occur naturally, and scientists hope to exploit them in controlling light as they can overcome the fundamental limits of conventional materials restricted by the shortest wavelengths in microscopy and nanotechnology for imaging and sensors, for instance. Negative refractive index materials and three-dimensional optical materials have already made their way into the popular psyche as "invisibility cloaks", for instance, although the applications are much wider than that.

Colleague Wasserman, assistant professor of electrical and computer engineering at Illinois explains further how, "Highly doped semiconductors can serve as wavelength flexible plasmonic metals in the infrared. However, without the ability to visualize the optical response in the vicinity of the plasmonic particles, we can only infer the near-field behaviour of the structures from their far-field response." What the present study gives researchers is a "clear window into the optical behaviour of this new class of materials on a length scale much smaller than the wavelength of light."

In the field

The researchers compared near-field and far-field measurements on indium arsenide microparticles heavily doped with silicon with electromagnetic simulations to confirm the presence of localized plasmonic resonance. They were also able to obtain high-resolution maps of the spatial distribution of absorption within single plasmonic structures and variation across plasmonic arrays. An infrared absorption peak at 5.75 micrometres corresponds to a localized surface plasmon resonance within the microparticles, the team reports in the journal Applied Physics Letters. Having access to measurements on near field behaviour in plasmonic structures will help researchers expand the design parameters for plasmonic meta materials, which will open up the design of more complex optical materials.

Related Links

Appl Phys Lett 2013, 102, 152110: "Near-field infrared absorption of plasmonic semiconductor microparticles studied using atomic force microscope infrared spectroscopy"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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