Biomedical Engineering Reference
In-Depth Information
Fig. 1.19.
The CARS spectra at
α
and
β
in Fig. 1.18
by the multi-focus CARS microscopy system at 990 cm
−
1
near the peak of
the polystyrene band (a) and 1050 cm
−
1
away from that band (b). The laser
system shown in Fig. 1.16 was also used, and the images were observed with
a2.5cm
−
1
step. The exposure time of the each image was 20 s, which is 100
times shorter than for the stage scanning type.
Figure 1.19 shows the spectra at
α
and
β
in Fig. 1.18. These spectra are
clearly different, and it is found that these are a polystyrene bead and a glass
bead. The polystyrene has an intense band at 1000 cm
−
1
, but the glass has
no high-intensity bands around there. It is possible to draw a clear distinction
between polystyrene and glass.
We have developed a CARS microscopy system for the fingerprint re-
gion. The spectral resolution is about 30 cm
−
1
, and the spatial resolution in
the depth direction is 3.2
μ
m. We have also developed a multi-focus CARS
microscopy system and demonstrated that spectral imaging of a mixture of
polystyrene and glass beads could identify these two substances.
In a CARS spectrum, the nonresonant background without any structure
overlaps with the resonant CARS band, and many bands are closed in the
fingerprint region. As the poor spectral resolution and thelow stability of
laser intensity reduce the contrast of the CARS image nad spectrum, a laser
of narrower spectral bandwidth and higher stability is required for biological
applications.
We used near-infrared lasers to reduce the nonresonant background, be-
cause the intensity of the nonresonant background will be increased when
two-photon absorption exists. However, if the wavelengths of the CARS ex-
citation beams are close to the single-photon absorption of the molecule, the
CARS emission is strongly enhanced by the electronic resonance. It is pos-
sible to obtain information about a specific molecule by electric resonance
enhancement. The wavelength selection of excitation lasers is also important
for CARS microscopy.
