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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 micro-environment 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 100nW) because of the low
optical throughput of metal coated fiber tips. The long integration
time (typically 10 minutes per spectrum) required for collection 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 micro-Raman 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.
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