New form of microscopy utilises evanescent waves

Researchers at The University of Tokyo (UTokyo) have developed a new form of microscopy that can probe details in an object’s surface, like the distribution of a material’s lattice and electron temperatures, with nanoscale precision. Described in the journal Scientific Reports, the new technique can observe details that are said to be undetectable by other means.

Conventional microscopes irradiate a sample, usually with light or electrons. Any reflected or scattered radiation can be used to build a detailed image and obtain characteristic information about a material’s surface.

Evanescent waves are short-lived electromagnetic waves that do not transport energy, a little like ripples on a material’s surface. They can be created when light interacts with the surface, but can also be generated thermally — and since all matter contains energy and emits heat, localised heat fluctuations in a material can briefly create strong evanescent waves. The key to the new form of microscopy, developed at UTokyo’s Institute of Industrial Science, is in passively detecting these waves.

“Scanning near-field optical microscopy, using scattered electromagnetic radiation, is one of the most commonly used techniques for examining material properties at the nanoscale level,” said lead author Ryoko Sakuma. “Our new technique uses passive detection of the radiation emitted by the object itself, so the surface doesn’t need any illumination.”

Using their prototype instrument, the researchers examined thermally excited evanescent waves generated in two dielectric materials: aluminium nitride and gallium nitride. The weak scattering, which was unpredicted, can be seen in an absorption band called the Reststrahlen band. This is understood to be the first time such a phenomenon was observed without light exposure.

Most significantly, spectroscopic analysis showed that only the polariton waves (ie, waves caused by surface phonon resonance) exist in the Reststrahlen band, despite theoretical predictions that these polariton waves would be accompanied by a large amount of thermal fluctuation. These results help us to understand thermally excited evanescent waves in this band and lay the groundwork for an improved passive detection model to identify dielectric materials.

“Our instrument is the only one in the world capable of observing nanoscale temperature distributions on surfaces using terahertz wavelengths,” said senior author Yusuke Kajihara. The terahertz wavelength range starts in the mid-infrared, from around 10 µm, extending up to 1 mm.

The team intends to further improve their prototype instrument and refine how the technique works, with Kajihara noting, “This microscope technology is completely new, so we’re still learning specifically how and where it can be applied.” The next step is to develop an improved detection model with greater versatility, resulting in a new and powerful non-destructive characterisation technique allowing highly localised analysis of a material’s surface dynamics.