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Scientists at the Max-Planck-Institut für Biochemie in Martinsried near München, Germany, have used their infrared near-field microscope to study crystal lattice vibrations (Nature 418, 159, 11. July 2002). They used infrared laser beam illumination of a nano-sized antenna to obtain resonance with the vibrations, the so-called phonon resonance. The new technique makes it possible to find out a crystalŽs chemical identity, and even its structural quality, both with nanometric resolution. This promises interesting applications in materials research in general. It should allow new insight in biominerals such as teeth or bone. Also technical applications e.g. for mass data storage are foreseen.
Infrared antenna for nano-size mapping of crystal vibrations
Scientists at the Max-Planck-Institut für Biochemie in Martinsried near München, Germany, have used their infrared near-field microscope to study crystal lattice vibrations (Nature 418, 159, 11. July 2002). They used infrared laser beam illumination of a nano-sized antenna to obtain resonance with the vibrations, the so-called phonon resonance. The new technique makes it possible to find out a crystalŽs chemical identity, and even its structural quality, both with nanometric resolution. This promises interesting applications in materials research in general. It should allow new insight in biominerals such as teeth or bone. Also technical applications e.g. for mass data storage are foreseen.
Crystals have always been admired for their optical brilliance, but only few specialists have known that crystals can 100% reflect infrared light like a metal. The origin of this reflection is that the lattice atoms vibrate against each other and forbid infrared light waves of the same band of frequencies to enter. 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 had basically been 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.
The metallic needle intensifies the infrared light at its tip (much as a radio antenna enhances faint signals). In the present 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 crystallite 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 oil minerals or in meteorites, to name two. The sharpness of the resonance will allow to distinguish crystals with slightly shifted resonance, thus to detect impurity and non-perfect crystallinity. This could be valuable for the research on the growth and decay of biominerals such as teeth or bones, and help to understand medical processes such as osteoporosis.
On a quite different panel, phonon resonance is thought an element 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 include also the mid-infrared (3 - 30 µm) where the crystal lattice vibrations occur. The active development of quantum cascade semiconductor lasers for the mid-infrared could substantially boost this process. The Martinsried researchers long for obtaining such tailored infrared sources to match the phonon resonance of biological minerals.
For further information please contact:
Dr. Fritz Keilmann
E-Mail: keilmann@biochem.mpg.de
www.biochem.mpg.de/keilmann/
Phone.: +49 (89) 8578 2617 / -2851
Fax: +49 (89) 8578 2641
and:
Dr. Rainer Hillenbrand
E-Mail: hillenbr@biochem.mpg.de
www.biochem.mpg.de/baumeister/personal/Rainer/home.html
Phone: +49 (89) 8578 2455
Fax: +49 (89) 8578 2641
Max-Planck-Institut für Biochemie
Abteilung Molekulare Strukturbiologie
Am Klopferspitz 18a
D-82152 Martinsried
Original publication:
R. Hillenbrand, T. Taubner, F. Keilmann
Phonon-enhanced light-matter interaction at the nanometerscale
Nature (2002) 418, 159-162.
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Figure: 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-Institut für Biochemie)
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