William Angerer - Department of Physics and Astronomy - University ...

176
7.1 Historical Context of Second-Harmonic Generation Microscopy
In 1974 Robert Hellwarth and Paul Christensen imaged second-harmonic generation
from a polycrystalline ZnSe film [26. 27]. Fig. 7.1 displays their experimental setup
for SHG microscopy. The coupling of the fundamental light to the various orientations
of the polycrystalline crystal axes produced a quasi-phase matching condition
for certain propagation directions. Fundamental light incident on a particular grain
produced SHG along the quasi-phased matched direction. Since the fundamental
laser light covered many grains, second-harmonic light was scattered in various directions.
These small beams of scattered SHG were called "beam lets" by Hellwarth and
Christensen. By examining the distribution and intensity of these SHG beamlets.
they determined the average size of the polycrystalline grains, the thickness of the
grains, the preferred orientation of the normal to the grains, and the coverage of the
grains. This experiment introduced techniques to probe local structure with secondharmonic
generation. SHG microscopy has been used more recently to examine the
local structure of Langmuir monolayers [103]; to probe the domain structure of magnetic
garnet films [104]' BaTi03 [105], and LiTa03 [105]; to measure concentration
gradients of Sb on Ge (111) [106]; and to probe tissue [107].
The recent activity in nonlinear optical microscopy is probably a result. at least
in part, of the success of conventional nonlinear optics as a probe of surfaces and
interfaces. In addition, the introduction of Ti:Ah03 lasers into the nonlinear optical
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.