The Optics Laboratory

Nano-Raman Spectroscopy

Raman spectroscopy enables the study of vibrations in molecules and solids through the interaction of light with the vibrations. Some of the energy of a photon sets off a quantum of vibration, a phonon, in the material. Due to energy conservation, the photon leaves with less energy than it came with -- its color is shifted towards the red, or longer, wavelengths. We measure this (slight) change in color to find the energy of the vibration. Some vibration modes of a material do not interact with light in this manner, or do so only under special conditions. This information about the vibration also brings deeper understanding. A complementary technique in which the photon is completely absorbed (looses all its energy) interacts with the vibrations in a different manner. It is called infrared (IR) spectroscopy.

Raman spectroscopy was first invented about 75 years ago, in 1928, by Chandrasekhara Venkata Raman. The advent of the laser popularized the method, and use grew. About 30 years ago, Edgar Etz and his colleagues invented micro-Raman spectroscopy, which allowed study of small sample regions. One of the concerns at the time (private communication with Dr. Etz) was that micro-Raman spectroscopy might not be the same as traditional Raman spectroscopy, since it was performed through a microscope and on such a small (micron-sized) volume of material. They concluded and it is now universally agreed that micro-Raman and traditional Raman spectroscopies are the same. About 5 years ago, we were the first to push Raman spectroscopy one step further in resolution, to nano-Raman spectroscopy and nano-Raman imaging (also called near-field Raman). We faced a similar question: is nano-Raman the same as the other Raman spectroscopies? The answer this time is "No!" There are important differences in the way the light interacts with the vibrations, such as which vibration modes can be excited with the Raman technique. Some of the differences are easily understood, and are described here. Others required looking at Raman in a fundamentally different manner, which we call Gradient Field Raman (GFR) and describe on