Near-field Raman probes molecules

17 Jun 2002

Scientists at North Carolina State University in the US have found a new method of measuring the molecular properties of materials that could lead to the development of a wide variety of nanostructure technologies.

The technique, called gradient-field Raman spectroscopy, measures the nanometre scale behaviour of molecules.

It is similar in principle to conventional Raman spectroscopy, in which some of the incident light is scattered at frequencies that are characteristic of the vibrational and rotational energy levels of the probed molecules.

However, when Hans Hallen used a near-field-scanning optical microscope to look at a KTP crystal, he observed vibrational patterns that could not be explained using the rules of conventional Raman spectroscopy.

"We thought that we completely understood Raman, but we didn't," he said. "Near-field Raman was expected to be simply a higher resolution version of Raman, but it isn't."

Hallen then found that the coupling between the light and the molecule in gradient-field Raman spectroscopy is moderated by a strong electric field - caused by the incident light - that shifts the potential energy of the atoms in the KTP crystal.

"This helps you to pick apart the various vibrations at the surface of a sample a little better than you could before," Hallen said. "You can get a good, almost three-dimensional, picture of the vibration modes."

Story courtesy of Opto & Laser Europe magazine.