Page 346 - Vibrational Spectroscopic Imaging for Biomedical Applications

320 Cha pte r Ele v e n

of the Raman effect was only the beginning of a broad and growing
branch of molecular spectroscopy, with new applications that continue
to emerge to this day, some of which are presented in this book.
Raman’s excitement was mirrored almost 35 years later when
researchers inserted a cell with transparent liquid, nitrobenzene, in
the cavity of a pulsed ruby laser. To their surprise, they observed that
the laser output consisted of frequency-shifted components in addition
2,3
to the radiation at the lasing frequency. Similar to Raman scattered
light, this new radiation was seen at frequency shifts that correspond
to the frequencies of molecular vibrations. Unlike the incoherently
scattered light seen by Raman, however, the frequency-shifted
emission from the ruby laser exhibited a well-defined propagation
direction and coherence. In analogy with the principle of stimulated
fluorescence on which the ruby laser was based, the name stimulated
Raman scattering was coined to describe this newly discovered
phenomenon. The field of coherent Raman scattering was born.
The frequency-shifted components on the ruby laser output
appeared at lower energies, and are thus Stokes shifted. On closer
inspection, however, coherent components at higher energies were also
found. Minck et al. theorized that these anti-Stokes contributions
4
resulted from a cascaded process in the Raman active medium. In this
process, the ruby laser wavelength at frequency ω was first shifted to
0
lower frequencies ω − ω through the stimulated Raman process,
0 r
where ω are the frequencies corresponding to the Raman active vibra-
r
tions of the molecule. Once a ω − ω , beam has built up in the cavity, it
0 r
is able to combine with the fundamental frequency and generate new
frequency components at ω + ω , i.e., coherent anti-Stokes radiation. 4
0 r
Nonetheless, in Minck et al.’s experiment several stimulated Raman
processes took place in the laser cavity simultaneously, which made it
difficult to isolate the generation of the anti-Stokes components from
the generation of the red-shifted Stokes contributions. Two years later,
Maker and Terhune from the Ford Motor Company were able to gener-
5
ate coherent anti-Stokes radiation outside of a laser cavity. In their
experiment, the required red-shifted ω − ω component was generated
0 r
from the fundamental laser beam through a stimulated Raman process
in a cell of a Raman active medium. Both beams were then collinearly
focused into the sample. Maker and Terhune observed a clear signal at
ω + ω . Moreover, they showed that the strength of the signal depended
0 r
on the presence of a vibrational resonance in the sample at ω , which
r
demonstrated the potential of anti-Stokes generation as a molecular
spectroscopic tool.
The efficiency of the coherent anti-Stokes generation process
depends on the ability of the sample material to respond to three
optical frequencies by producing an oscillatory electronic motion at a
fourth frequency, which is a combination frequency of the three
incoming optical fields. This material property is called the third-order
() 6,7
3
susceptibility and is written as χ . The third-order susceptibility is