Infrared antenna for nano-size mapping of crystal vibrations
Max Planck scientists use a new type of microscope to make crystal vibrations in the nanometre range visible
Crystals have always been admired for their optical brilliance, but only a few specialists know that crystals can reflect 100% of infrared light like a metal. The origin of this reflection is that the lattice atoms vibrate against each other and hinder infrared light waves of the same band of frequencies from entering. The team of physicists consisting of Rainer Hillenbrand (postdoc), Thomas Taubner (doctoral student) and Fritz Keilmann (senior researcher) now shows that the infrared behaviour changes dramatically when the infrared is applied through their antenna, which is the probing needle of their microscope: the known metal-like reflectivity transforms into a monochromatic or single-colour resonant response. This effect was predicted 19 years ago by Aravind and Metiu (University of California, Santa Barbara, USA) but not yet been experimentally observed.
The infrared near-field microscope of the same group had already been in the news three years ago, for its ability to resolve details as small as 1/100th of the wavelength, and further its unique ability to distinguish chemical composition ("chemical microscope", see Nature 399, 134, 1999). The basic technique is to illuminate the needle of a scanning probe microscope with infrared light. While the needle scans over the sample the surface relief builds up on the computer screen. Simultaneously, the recorded infrared light generates an infrared image of the same area, valuable for interpreting local material composition.
Fig.: A partly gold (Au) covered silicon carbide crystal (SiC) studied by the near-field infrared microscope. a: Schematic view of the probing needle and the laser beam illumination. The arrows indicate the infrared laser beam focused to the needle and the strongly enhanced reflection. b: Topography image. c: Infrared near-field images on and off resonance (the color scale encodes the signal amplitude): the phonon resonance of SiC happens at 10.8 micrometer wavelength, generating strongly enhanced brightness compared to gold. At 10.2 micrometers the image contrast reverses and gold reflects stronger than the SiC crystal.Graphic/images: R. Hillenbrand / Max Planck Institute for Biochemistry
The metallic needle intensifies the infrared light at its tip (much as a radio antenna enhances faint signals). In the current experiment the researchers studied a silicon carbide (SiC) crystal. When the tip of the needle came within 30 nm of the crystal surface they observed a dramatically enhanced infrared signal once they had tuned the laser frequency to the phonon resonance. This was indicative of extreme local intensity. Compared with a gold surface the SiC appears 200-fold brighter. The experiment is conclusive evidence of "near-field-surface-phonon-polariton resonance", as it is correctly called, a light-matter interaction that is only accessible when the investigation uses nanoscopic probing.
Practical applications of the phonon resonance rest on either the high signal level or the narrowness of the resonance, or both. In a mixed-crystal sample any individual component is expected to show up very brightly when the infrared illumination happens to hit its phonon resonance. Such multicomponent nanocomposites abound in, to name two examples, oil minerals or in meteorites. The sharpness of the resonance allows crystals with slightly shifted resonance to be distinguished and thus the detection of impurities and non-perfect crystallinity. This could prove valuable for research on the growth and decay of biominerals such as teeth or bones, and help to understand medical processes such as osteoporosis.
In quite a different field, phonon resonance is proposed as a possibility for future optical integrated circuits and ultra-high density data storage. By exploiting phonon resonance, photonics and microscopy are seen to expand from the traditional visible spectrum (0.4 - 0.7 µm), or the near infrared of telecommunication (1.5 µm), to also include the mid-infrared (3 - 30 µm) where crystal lattice vibrations occur. The active development of quantum cascade semiconductor lasers for the mid-infrared spectrum could substantially boost this process. The Martinsried researchers long for obtaining such tailored infrared sources to match the phonon resonance of biological minerals.
Original publication: R. Hillenbrand, T. Taubner, F. Keilmann, Phonon-enhanced light-matter interaction at the nanometre scale, Nature 418, 159-162, (2002)