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Infrared imager reveals near-field structure

02 Jan 2007

Researchers in France have created a nanoscopic version of an infrared night-vision camera that could be used to optimize a new generation of photonic components.

The new instrument, dubbed a thermal radiation scanning tunnelling microscope (TRSTM), detects near-field thermal radiation from surfaces and has a resolution of around 100 nm.

"I was strongly motivated not only by the challenge of creating a scanning near field optical microscope which operates without any external illumination, but also by the fact that there is significant new physics related to the study of thermal radiation at very short distances from the radiating surfaces," Yannick De Wilde of the Ecole Supérieure de Physique et de Chimie Industrielles de la Ville de Paris told nanotechweb.org.

"In this context, an important motivation was to investigate the coherence of near-field thermal radiation, which had been predicted and indirectly observed a few years before by some of my colleagues."

De Wilde and colleagues at Laboratoire de Spectroscopie en Lumière Polarisée, Laboratoire EM2C, Institut Fresnel, Laboratoire d'Etudes Thermiques, and Laboratoire de Photonique et de Nanostructures used an apertureless infrared near-field scanning optical microscope to image silicon carbide samples patterned with gold. The microscope consisted of an atomic force microscope (AFM) with a hot sample holder, an infrared optical microscope and a mercury-cadmium-telluride infrared detector.

The AFM's tungsten tip acts as a scattering centre that picks up the infrared evanescent fields emitted by the surface and radiates a related signal in the far field. As a result, the kit acts as an optical scanning tunnelling microscope, leading the team to dub it the thermal radiation scanning tunnelling microscope.

"TRSTM images of gold patterns on a silicon carbide substrate prepared by Yong Chen of LPN-ENS exhibit structures which are the signature of interferences of thermally excited surface waves called surface plasmons on gold," said De Wilde. "This finding is the first direct evidence that thermal radiation changes dramatically its properties in the near-field, and can exhibit the same coherence properties as a laser, due to evanescent surface waves."

According to De Wilde, the observation of fringes in the TRSTM image is the result of confinement effects on the gold patterns which play the role of a planar cavity for surface plasmons. "They are the electromagnetic analogues of the stationary wave patterns which have been observed in STM images on an atomic corral, with the important difference that in TRSTM one deals with photonic waves instead of electronic waves," he explained.

The team says that as well as for investigating new physical phenomena, the device could be used to investigate operating micro or nano-electronic devices, in order to map their temperature and understand more about their heat transfer mechanisms.

"In the field of nano-optics, studying the electromagnetic local density of states around nanostructures such as nano-antennas should be very useful in the process of optimizing these new optical components," said De Wilde. "Such components are meant to manipulate electromagnetic fields or to control the radiative emission of dipoles at the scale of a few nanometres, which requires a tool for their investigation on a subwavelength scale."

Now the researchers hope to improve the device's optical detection so that they can detect the energy spectrum of photons which are scattered by the tip of the TRSTM at the surface of the sample. This should reveal even more information about the physical properties of the material located directly under the tip.

The researchers reported their work in Nature.

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