Biomedical Engineering Reference
In-Depth Information
Fig. 1.8.
The phase-matching conditions of CARS
is given by
k
3
=
k
1
+
k
1
−
k
2
,
(1.1)
k
1
where
k
1
and
are the wave vectors of the
ω
1
beam,
k
2
is that of the
ω
2
beam, and
k
3
is that of the CARS radiation in the sample. In CARS
spectroscopy, the configurations shown in Fig. 1.8 are used to satisfy the
phase-matching condition to observe the isotropic sample.
1.3.2 3DResolutionbyCARSMicroscopy
The combination of CARS spectroscopy and microscopy was proposed by
Duncan et al. [20]. Their system does not have axial resolution because the
incident beams were not so tightly focused. We have proposed that three-
dimensional imaging properties should be obtained to focus the excitation
beams tightly [21]. Independent of us, A. Zumbusch et al. succeeded in ob-
taining the 3D resolved image and applied it to the observation of biological
samples [22].
Figure 1.9 shows the optical layout of tightly focused CARS microscopy.
The
ω
1
and
ω
2
beams are tightly focused into the sample by an objective
with a high numerical aperture (NA), and then the yielded CARS radiation
is collected by another objective though a filter. The tightly focused beam
has a variety of wave vectors; the phase-matching condition is not so severe.
When the collimated beam is tightly focused by an objective, the electric
field is given by gathering of plane waves that have various directions of
progress. Since the wave vector indicates the direction of progress, the wave
vector of the focused beam is distributed spherically, with radius 1/
λ
, limited
by the NA of the objective. Figure 1.10 shows this wave vector distribution
of the focused beams, where
n
is the refractive index of the sample,
μ
is the
Fig. 1.9.
The optical layout of a tightly focused CARS microscopy system
