FiO Hot Topics: Hot Optics
A key emerging area in biology and medicine, where observations often have to be made in living systems, is developing tools and techniques for molecularly specific imaging, Schaffer explained, with the best being nonlinear optical methods, because they will allow deep tissue imaging. Schaffer discussed nonlinear optical methods such as two-photon excited fluorescence, harmonic generation and coherent anti-Stokes Raman scattering.
Optical scattering results in a loss of image contrast. An ideal way to form a 3-D image is to excite the molecule in a 3-D localized spot, because then scattering doesn't matter. This overcomes a problem with imaging thick samples, where scattering is a problem, he said.
Schaffer talked of "packing the light" into short, femtosecond-length laser pulses for imaging the brain to a depth of 100 µm, with no loss of image quality until a depth of about 1 mm.
Coherent anti-Stokes Raman scattering (CARS) has turned out to be a fantastic way to image lipids, he said. "Lipid is something difficult to fluorescently label, at least in vivo," Schaffer said.
In all the methods mentioned, he said, the nonlinear effects are measured by looking at large changes in color. But some nonlinear effects that don't result in color changes can also be used for imaging, such as modulation transfer. In that method, one beam of a two-beam interaction is modulated and one looks at the effect on the nonmodulated beam.
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