Degenerate four-wave mixing is demonstrated as an effective and sensitive laser analytical spectroscopic method for circular dichroism measurements. A forward-scattering degenerate four-wave mixing optical setup is used to obtain simple optical alignment, highly efficient wave mixing, and very effective use of low laser power. This nonlinear laser-based circular dichroism method offers many advantages, including easy and efficient optical signal collection, use of very short analyte path lengths (e.g., 0.1 mm), and excellent detection sensitivity that is comparable or better than conventional laser-based or non-laser-based circular dichroism methods. Using an analyte path length of only 0.1 mm, and a probe volume of 98 pL, a circular dichroism mass detection limit of 0.68 pg or 2.8 fmol is reported for (+)Co(en)3(3+).
A novel four-wave mixing technique for the measurement
of circular dichroism in optically active liquid
samples is demonstrated. When two cross-polarized continuous-wave
laser beams are crossed at a small
angle in a circular dichroic liquid, a weak thermal grating is produced
with a phase depending on the sign of
the circular dichroism. The polarization of one of the beams can
be modified to allow coherent interference
with an intensity grating-induced thermal grating. A probe beam
scattering from the composite grating results
in a coherent signal beam that reveals the sign and the magnitude of
analyte circular dichroism. The use of
this technique to optimize the signal-to-noise ratio in the presence of
scattered light and laser noise is discussed.
Optical phase conjugation by "forward-scattering" degenerate four-wave mixing in an absorbing liquid analyte solution is reported as a sensitive and simple nonlinear laser spectroscopic method. Since only two input laser beams are used in this nonlinear four-wave mixing setup, it offers important advantages including ease of optical alignment, efficient use of input photon density, low laser power requirements, and high wave-mixing efficiency. In addition, since the phase-conjugate signal is a laser beam, optical signal detection is very efficient and the signal-to-noise is excellent. Important characteristics of this novel nonlinear laser technique, including signal dependence on analyte concentration, individual input beam power, and modulation frequencies, are examined. Excellent detection sensitivity, small detection volume, and convenient sample introduction promise many applications for this nonlinear laser spectroscopic method. Preliminary detection limits of 0.7 amol of eosin B and 45 amol of iodine inside a probe volume of 98 pL are reported using a forward-scattering degenerate four-wave mixing setup.
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