A Practical Nanoscopic Raman Imaging Technique Realized by Near-field Enhancement

Near-field scanning optical microscope (NSOM) has the potential to become a very important tool for material characterization due to its ability to investigate the structure and microenvironment of materials in nano-scale by performing spectroscopy as well as topographic mapping. However, near-field Raman results have been rarely reported although Raman spectra are unique in chemical and structural identification. This is due to the fact that Raman signal is intrinsically weak (less than 1 in 107 photons) and the laser power emerging from tip is extremely low (typically 100 nW) because of the low optical throughput of metal coated fiber tips. The long integration time (typically 10 minutes per spectrum) required for collecting good quality Raman spectra makes it impractical to construct a Raman image through this conventional method. In this paper, we report an integration of NSOM and Raman spectrometer using an apertureless configuration, in which the laser is focused onto the sample through a microscope objective and Raman signal is collected by the same objective. This is similar to the conventional microRaman except that a metal tip is brought into the laser spot on sample surface to enhance the Raman signal through surface enhanced Raman scattering (SERS). Raman enhancement of 104 times has been achieved and Raman mapping on real silicon devices has been realized with 1 second exposure time. Furthermore, the reflection scattering geometry employed in our experiments allows the study of any sample without specific sample preparation, unlike the conventional SERS which needs coating samples with metal or growing sample on metal surface.