National Institute of Standards and Technology

Gaithersburg, MD 20899

Near-field Raman spectroscopy can be used to obtain chemical specificity with the subwavelength spatial resolution of near-field scanning optical microscopy (NSOM). We report detailed measurements of near-field Raman spectra from a single crystal diamond sample. These measurements allow us to access the limits of using conventional aluminum coated apertured probes for near-field Raman spectroscopy. In order to discriminate between the near-field contributions to the Raman signal and bulk scattering, the Raman intensity has been measured as a function of the sample-probe separation. The Raman intensity shows about a factor of seven increase for sample-probe separations of approximately 10 nm compared to signals measured at separations greater than 100 nm, indicating that near-field contributions are present in these Raman spectra. The functional form of the increase in the Raman signal with decreasing sample-probe separation is expected to depend on the aperture size of the near-field probe. This has the potential to provide a simple in situ means of measuring aperture sizes of NSOM probes. Due to both the relatively low Raman cross sections and the poor throughput of aluminum coated probes, relatively long integration times (~ 5 minutes) are required to obtain high quality spectra. For this reason we are investigating methods of modifying the probes to enhance the near-field Raman signal. Preliminary experiments indicate that an aluminum coated apertured probe modified by over-coating it with a rough layer of silver shows a greater enhancement in the near-field Raman intensity than is observed for typical NSOM probes coated only with aluminum.