New Optical Probes of Chiral Molecules
Conventional optical activity phenomena, such as optical rotation and circular
dichroism, are based on the interference of induced oscillating electric- and magnetic (and
electric-quadrupole) moments, and arise from a differential response to left and right
circularly polarized light. Circularly polarized light is clearly chiral, and so it is remarkable
that the action of three linearly polarized electric fields can also be a chiral probe in a homogeneous
and isotropic liquid. This is indeed the case, when two optical fields of different frequency interact
coherently to generate light at their sum- or difference frequency [1-3]. SFG is in
general a purely electric-dipolar nonlinear-optical process and the signal photons from an
optically active liquid are only generated, if the molecules are chiral.
In the study of SFG from liquids, we have observed a new electro-optic effect,
i.e. the influence of a static electric field on the intensity of SFG. The effect arises when a static
electric field is applied to coherent sum-frequency generation in an optically active liquid.
The static field does not change the phase matching conditions of the sum-frequency process, but
it gives rise to an electric-field induced contribution to the signal. The beat between
chirality-sensitive SFG (a second-order process) and achiral electric-field
induced sum-frequency generation (a third order process) yields a contribution to the
intensity that is linear in the static electric field and that changes sign with the enantiomer.
The effect can therefore be used to determine the absolute sign of the isotropic part of the
sum-frequency hyperpolarizability and hence the handedness of chiral molecules in solution [4].
[1] P. Fischer, D.S. Wiersma, R. Righini, B. Champagne, and A.D. Buckingham,
Three-wave mixing in chiral liquids, Phys. Rev. Letters, 85 (2000),
4253-4256.
[2] P. Fischer, K. Beckwitt, F.W. Wise, and A.C.Albrecht, The chiral specificity of
sum-frequency generation in solutions, Chem. Phys. Letters, 352
(2002), 463-468.
[3] P. Fischer, F.W. Wise, and A.C. Albrecht, Chiral and achiral contributions to sum-frequency
generation from optically active solutions of binaphthol, J. Phys. Chem. A, 107
(2003), 8232-8238.
[4] P. Fischer, A.D. Buckingham, K. Beckwitt, D.S. Wiersma, and F.W. Wise, A new electro-optic
effect: Sum-frequency generation from optically active liquids in the presence of a dc electric
field, Phys. Rev. Letters, 91 (2003), 173901.
For a recent review article, see: P. Fischer and F. Hache, “Nonlinear optical spectroscopy of
chiral molecules”, Chirality, 17 (2005), 421.
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