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human tissues. (3) Raman spectroscopy is provided with high time reso-
lution (several picoseconds) and high spatial resolution (several hundred
nanometers). It is possible to investigate changing molecular systems in
detail by measuring the time- and space-resolved Raman spectra.
10.2 DEEP NEAR-INFRARED EXCITED RAMAN
SPECTROSCOPY
In spite of the many advantageous features discussed above, Raman
spectroscopy is by no means used more extensively than infrared
absorption and nuclear magnetic resonance spectroscopy. The reason
is the diffi culty caused by the interference from fl uorescence. In order
to measure Raman scattering, the excitation laser has to illuminate the
sample. If the sample itself or the impurities contained in the sample
absorb the excitation laser light and emit fl uorescence, the much stron-
ger fl uorescence totally masks the weak Raman scattering signals. In
physical and chemical applications, it is possible to purify samples and
therefore, by using highly purifi ed samples, the fl uorescence problem
can be bypassed. However, in biological, medical, and industrial appli-
cations, it is not possible to purify samples because they have to be
measured as they are. Therefore, the fl uorescence interference becomes
more problematic and hinders extensive use of Raman spectroscopy in
these applications.
In order to overcome this problem of fl uorescence in Raman spec-
troscopy, many attempts have been made. They can be classifi ed into
three categories: (1) the near-infrared excitation method [27 - 29] , (2)
the nonlinear Raman method, and (3) the time-resolved method. The
near-infrared excitation method will be discussed in detail below. The
nonlinear Raman method includes coherent anti- Stokes Raman scat-
tering (CARS) spectroscopy, coherent Raman gain spectroscopy, and
inverse Raman spectroscopy [30] . The time - resolved method utilizes
the different temporal behaviors of Raman scattering and fl uorescence.
Time - gated photon counting [31,32] , streak - camera [33] and optical
Kerr gating [34] have been used to eliminate fl uorescence that is emitted
with a time delay from the excitation laser pulse. It is fair to say that
the near-infrared excitation method is the most versatile among the
three.
The near-infrared excitation reduces the possibility of photoexcita-
tion of fl uorescent species in the sample and thereby suppresses the
fl uorescence background to a great extent. It also makes Raman mea-
surements more biofriendly because long- wavelength excitation causes
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