Biomedical Engineering Reference
In-Depth Information
(a)
(b)
y
E
ω
x
θ
THG
y
z
x
(c)
Experiment
eory
10
5
0
50
100
150
200
250
Angle (deg.)
-5
-10
-15
FIgurE 3.8
THG as a function of the polarization direction. (a) Geometry. (b) THG image of a coverslip show-
ing
xz
and
yz
glass/water interfaces (
NA
= 1.2). (c) Ratio between the signal obtained from the
xz
-oriented interface
and the signal from the
yz
-oriented interface as a function of the direction of the excitation polarization.
If we now consider the case of an interface along the
x
or
y
axis (Figures 3.7b and 3.7c), the emission
diagrams evolve from a single-peaked emission in the case of an interface perpendicular to the optical
axis, to the aforementioned symmetric double-peaked emission in the case of an interface parallel to the
optical axis, with intermediate positions showing an asymmetric double-peaked emission.
This orientation dependence can be confirmed experimentally by imaging the top corners of a glass
coverslip (Figure 3.8a). In this geometry, signals from glass/water interfaces with three different orienta-
tions can be detected. It would be complex to compare the signal obtained from the vertical interfaces
and the horizontal one because of the presence of aberrations caused by the focusing on an interface.
However, signals from the two vertical interfaces can be compared because the aberrations are similar
in both cases. Figure 3.8b shows the ratio of THG intensities from the vertical interfaces as a function of
the polarization of the excitation beam. This experiment is fully consistent with the theoretical predic-
tion plotted on the same graph.
3.2.9.1 influence of the nA on orientation effects
The relative visibility of structures with different orientations is also dependent on the excitation NA.
Let us now consider a centered slab with a nonlinear susceptibility χ
(3)
oriented along the axial or trans-
verse directions. Figure 3.9 presents the corresponding calculated THG signal as a function of both the
orientation and the size of the slabs.
The THG signal dependence as a function of the width of an
xy
-oriented slab is similar to the case of
a centered sphere (see Section 3.2.5), because phase matching is also dominated by the Gouy phase shift.
For the same reason, it is also strongly dependent on the excitation NA. In the case of an
xz
-oriented
slab, THG efficiency is not dominated by axial phase matching and THG signal exhibits a lesser depen-
dence on the NA. This implies that the relative signals from structures having different orientations
change with the focusing conditions: THG is more orientation-dependent for lower NAs, as illustrated
by Figure 3.9 which compares the orientation visibility ratio for NAs of 0.8 and 1.2.
