"Identification of chemical compounds by vibrational infrared spectroscopy is of great value for physics, chemistry, biology and biochemistry," Rainer Hillenbrand of the Max Planck Institute told nanotechweb.org. "However, the spatial resolution is limited to the wavelength of the infrared light, that is a few microns. To improve the spatial resolution, we combine apertureless near-field optical probing with infrared spectroscopy - this allows infrared spectroscopy and imaging at the nanometre scale."

The researchers illuminated the needle of a scanning probe microscope with infrared light from a laser. The metallic needle acts as an antenna, intensifying the infrared light at its tip.

The scientists brought the tip of the needle to within 30 nm of the surface of a silicon carbide crystal. Then they tuned the laser frequency to the phonon resonance, resulting in a dramatically enhanced infrared signal. The researchers say that this provides conclusive evidence of "near-field-surface-phonon-polariton resonance", a light-matter interaction that is only accessible when the investigation uses nanoscopic probing.

"The infrared response of a polar material changes dramatically when infrared radiation to the sample is applied through our near-field probing tip instead of far-field illumination," explained Hillenbrand. "Compared to the broad spectral response obtained by far-field reflectivity measurements, such a near-field probing process leads to a significant spectral narrowing, resulting in an extremely sharp resonance. The sharpness of the resonance is extremely sensitive to the chemical and structural composition of the sample."

Because the resonance is so sharp it allows scientists to distinguish crystals with resonances slightly shifted because of impurity or non-perfect crystallinity. "Combined with near-field imaging, this effect opens a door to nanoscale infrared analysis of nanocomposite materials," said Hillenbrand. "Because of the sharpness and high signal levels of the resonance, the method is very sensitive."

The technique could also have applications in optical data storage. According to Hillenbrand, the high near-field signals obtained on silicon carbide could allow the reliable and fast near-field optical read-out of 10 nm sized bits.

Now the scientists plan to look at polar materials other than silicon carbide - such as semiconductors and biominerals - and try to apply the effect to investigate nanocomposite materials such as teeth and bones. "We'll also try to apply quantum cascade lasers at different wavelengths to investigate and exploit the phonon-resonance of polar materials not accessible by our CO2 lasers," said Hillenbrand.

The researchers reported their work in Nature.