Near-field Raman spectroscopy can be used to obtain images with both chemical specificity and the subwavelength spatial resolution of near-field scanning optical microscopy (NSOM). In the absence of signal intensification factors, such as surface enhancement or electronic resonance in the specimen, Raman scattering suffers from a small cross section (ς = 10-28 cm2 to 10-31 cm2). Since most reports of Raman-NSOM to date involve exploitation of a specimen-specific intensification, an assessment of the general applicability of Raman-NSOM to a wider variety of un-enhanced samples is of great interest. We report here on several approaches to increasing the sensitivity of near-field Raman spectroscopy that do not rely on specimen properties. The use of chemically etched aperture probes as an illumination source has been investigated and compared to probes fabricated by the traditional heat and pull method. Raman spectra of a single crystal diamond specimen were recorded as a function of tip-to-sample separation (Z). The exponential decay of the Raman signal with Z yields information about the lateral extent of the aperture, allowing for meaningful comparisons between probes with equivalent aperture dimensions. Novel probe structures that utilize plasmon excitation to enhance the near-field Raman signal are also being explored. A Raman spectral image of a dye cast on a thin silver film is used to illustrate the present chemical imaging capabilities of Raman-NSOM.
Proceedings Title: Optical Devices and Diagnostics in Materials Science, Conference | | Optical Devices and Diagnostics in Materials Science | SPIE
Conference Dates: August 1, 2000
Conference Location: San Diego, CA
Conference Title: Proceedings of SPIE--the International Society for Optical Engineering
Pub Type: Conferences
chemical imaging, near-field microscopy, near-field spectroscopy, NSOM, Raman microscopy, raman spectroscopy, vibrational spectroscopy