Optical pulse shaping for selective excitation of coherent molecular vibrations by stimulated Raman scattering
2009
Coherent anti-Stokes Raman scattering (CARS) can be used to identify biological molecules from their vibrational
spectra in tissue. A single double-chirped broadband optical pulse can excite a broad spectrum of resonant
molecular vibrations in the fingerprint spectral region. Such a pulse also excites nonresonant CARS, particularly
from water. We describe a theoretical technique to design an optical pulse to selectively excite coherent vibrations
in a target molecular species so that the CARS signal generated is increased. The signal from other molecules
is reduced, since the incident pulse does not excite them to have coherent vibrations. As an example, we apply
the technique to design pulses to elicit increased CARS signal from a mixture of one or more of the alcohols
methanol, ethanol, and isopropanol. We also show how such pulse designs can be used to selectively excite one
member of closely related complex biological species. As measured interferometrically, the CARS signal from
three phosphodiester stretch modes of DNA can be increased to more than ten times that of the analogous signal
from RNA when the pulse design technique is used.
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