Locally enhanced laser spot makes molecules visible
Until recently researchers such as Michler and his colleague Silke Christiansen of the Max Planck Institute in Halle were, however, confronted with a problem. In order to use the spectroscopic technique to detect mechanical stresses in minute components the laser spot needed to be locally enhanced. This required a metallic tip, ideally manufactured to exactly the right size and shape with nanometer accuracy, something which to date was not possible technically. The technique was therefore not sensitive enough to allow quality control of materials on the nanometer scale and to uncover minute structural faults.
For decades, scientists have faced the problem of how to make molecules visible with the help of laser beams. The wavelengths of the light emitted by lasers are still much longer than the nanometer scale dimensions of molecules, which therefore cannot be «seen» – resolved, in scientific jargon – by the laser light. One way to «trick» the visible light waves was discovered in 1974 by the American chemist Martin Fleischmann, the same scientist who, fifteen years later, would claim to have discovered cold fusion in the laboratory. While his cold fusion experiments could not be reproduced by other researchers (and are today considered to be erroneous), his optical experiments have revolutionized several scientific fields including Raman spectroscopy.
Fleischmann discovered that a rounded tip coated with gold or silver can be used to efficiently couple laser light into molecules or crystals. Doing so makes the Raman technique significantly more sensitive, theoretically enabling individual molecules to be identified. In practice, to date the factor limiting the resolution of the method has been the fineness of the tip which can be achieved. The sputtered gold surface of the tips tip takes on randomly different shapes, so that each one is slightly different in shape and size. «These tips look like the Matterhorn under the microscope», says Michler. To be able to follow surface features with a spatial resolution in the nanometer range – such as for example how microelectronic components change over time – requires that the tips must be identical to the nanometer. Only then the measurements are comparable.