Infrared-lit needle senses crystal vibrations

By scanning a light source of subwavelength dimension across a sample and observing near-field backscatter, an optical image of a surface can be constructed that has a resolution considerably smaller than the diffraction limit of about a half-wavelength for ordinary optical microscopes. This technique is used in the well-known near-field scanning optical microscope (NSOM), in which light is guided to the sample via a fiber tip. A few years ago, scientists at the Max Planck Institute for Biochemistry (Martinsried, Germany) developed an infrared (IR) NSOM based upon an antenna-type tip that resolved details as small as 1/100 of a wavelength and allowed it to sense chemical composition (a "chemical microscope").¹

Now, this group has used the instrument to obtain resonance with crystal vibrations—the so-called phonon resonance.² The technique makes it possible to find out a crystal's chemical identity and structural quality, both to nanometer resolution. The results promise interesting applications in materials research and should allow insight into biological minerals such as teeth or bone. Technical applications such as optical integrated circuits and mass data storage are also foreseen.

In the technique, the needle of a scanning-probe microscope is illuminated with IR light. The metallic needle concentrates the optical field at its tip, much as a radio antenna enhances faint signals. At the same time that the mechanical interaction of the scanning probe with the sample provides surface-relief information, the light scattered from the near-field region of the antenna provides data for an IR image of the same area.

The researchers studied a silicon carbide (SiC) crystal, which reflects 100% of IR light as would a metal (see figure). This reflection occurs because lattice atoms vibrate against each other and hinder IR lightwaves of the same band of frequencies from entering. When IR light is applied through the microscope's antenna probe, the IR behavior of the crystal changes dramatically—the usual metal-like reflectivity transforms into a single-color resonant response. This effect was predicted 19 years ago by Aravind and Metiu (University of California, Santa Barbara) but had not yet been experimentally observed.

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An infrared near-field scanning optical microscope includes a laser-illuminated probe needle (far left). A silicon carbide (SiC) crystal partly coved with gold (Au) is imaged by the microscope; mechanical tip contact provides a topographic image (left center). Optical images are also obtained, both on and off resonance (near and far right, false-color amplitude scale). The 10.8-µm resonance of SiC generates strongly enhanced brightness compared to Au; at 10.2-µm the image contrast reverses and Au reflects more strongly.

When the tip of the needle came within 30 nm of the crystal surface, the Martinsried scientists observed a dramatically enhanced IR signal once they had tuned the frequency of the laser—either a line-tunable carbon dioxide or a quantum-cascade laser—to the phonon resonance at 920 cm^-1. To suppress background light scattered from the tip and the surface, the researchers took advantage of the nonlinear dependence of the near-field induced signal on the distance of the needle to the surface by using a lock-in detection frequency that was a harmonic of the needle's mechanical resonance frequency.

Brighter than gold

The signal showed an extremely high local intensity—compared to a gold surface, the SiC appeared 200 times more intense. The experiment is conclusive evidence of near-field surface-phonon-polariton resonance—a light-matter interaction that is only accessible when an intense IR field interacts with optical phonons. The near-field probing singles out a sharp resonance within the phonon band, resulting in a spectral response very different from the far-field response. This type of resonance is much stronger and sharper than the well-known resonance by conduction electrons (the surface-plasmon polariton), which can be excited by visible light.

The authors foresee practical applications based 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 brightly and unambiguously when the IR illumination happens to hit its phonon resonance. Such multicomponent nanocomposites abound in, to name two examples, oil minerals and meteorites. The sharpness of the resonance allows crystals with slightly shifted resonance to be distinguished, enabling the detection of impurities and imperfect crystallinity. This effect could prove valuable for research on the growth and decay of teeth or bones and help to understand medical processes such as osteoporosis. The introduction of quantum-cascade lasers operating in the mid-IR where more crystal-lattice vibrations occur will help to expand the range of this instrument.