Frequency mixing, nonlinear optics measurements

Sum-frequency generation (SFG) at second-order and the nonlinear Raman spectroscopy BioCARS at fourth-order can also probe chiral molecules. They have no analog in linear optics. We show that both are only symmetry allowed in a fluid, if the fluid is chiral. However, in contrast to optical activity phenomena, these processes arise entirely from induced electric-dipoles (without magnetic or quadrupolar transitions) and they are not circular differential. All laser beams can be linearly polarized and no polarization modulation is required as the detection of a sum-frequency (yiz. five-wave mixing) photon is in itself a measure of the solution s chirality. Since an achiral solvent can not contribute to the signal, these techniques are sensitive, background-free probes of molecular chirality. The SFG... 

The sample in a DLS experiment is well-dispersed in a saspending medium. In a typical DLS experiment, the sample is illuminated by a single-wavelength laser beam. To measure the Doppler widths, the DLS instrument uses optical mixing or light-beating techniques to translate the optical ftequencies (—6 x 10 Hz with the 488-nm line of an Ar laser) to frequencies near 0 Hz (dc) that can be easily measured. In most DLS instruments, a photomultiplier tube (PMT) is used as a nonlinear mixer because its output is proportional to the square of the electric field falling on its photosensitive surface. 

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