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K. Fujita
is affected by the refractive index distribution of the sample. Owing to this coherent
imaging property in CARS and SRS microscopy, it is difficult to determine a point
spread function (PSF) that can be convoluted with a sample distribution to calculate
the resultant images, as typically done for the incoherent image formation in fluorescence imaging. However, as an image of a point object, the CARS/SRS excitation
distribution shown in Fig. 8.1 is typically used as the PSF of CARS/SRS microscopy.
Although the spatial resolution of CARS/SRS microscopy suppresses the classical
diffraction limit, the practical spatial resolution is still low since these techniques use
near-infrared (NIR) light. The spatial resolution realized by the nonlinear interaction
is equivalent to or even lower than that in conventional microscopy using visible
light.
In laser scanning CARS microscopy, Raghunathan described a method to reduce
the Raman excitation volume by using the multiplicative interaction of phase shaped
pump and Stokes beams [16]. In typical CARS microscopy, both pump and Stokes
beams have a Gaussian profile. In the proposed technique, the profile of Stokes beam
is modified to have a more complicated phase distribution in order to control the
spatial distribution of CARS effect. The technique was demonstrated by Kim et al.
with the observation of polystyrene beads, where a spatial resolution of 130 nm was
confirmed (Fig. 8.2) [17]. In their technique, Toraldo-style pupil filter was introduced
to modulate the intensity distribution of the Stokes laser spot so that the center
Fig. 8.2 CARS images a without and b with the multiplicative focus field engineering. The images
were reconstructed by using the Raman signal at 1000 cm −1 of polystyrene beads with a diameter
of 300 nm. 728 nm pump and 1785 nm Stokes beams are used for generation of CARS signal. The
3D-rendered images within the dotted rectangles in (a) and (b) are shown in (c) and (d), respectively.
Reproduced from [17] with permission from The Optical Society
K. Fujita
is affected by the refractive index distribution of the sample. Owing to this coherent
imaging property in CARS and SRS microscopy, it is difficult to determine a point
spread function (PSF) that can be convoluted with a sample distribution to calculate
the resultant images, as typically done for the incoherent image formation in fluorescence imaging. However, as an image of a point object, the CARS/SRS excitation
distribution shown in Fig. 8.1 is typically used as the PSF of CARS/SRS microscopy.
Although the spatial resolution of CARS/SRS microscopy suppresses the classical
diffraction limit, the practical spatial resolution is still low since these techniques use
near-infrared (NIR) light. The spatial resolution realized by the nonlinear interaction
is equivalent to or even lower than that in conventional microscopy using visible
light.
In laser scanning CARS microscopy, Raghunathan described a method to reduce
the Raman excitation volume by using the multiplicative interaction of phase shaped
pump and Stokes beams [16]. In typical CARS microscopy, both pump and Stokes
beams have a Gaussian profile. In the proposed technique, the profile of Stokes beam
is modified to have a more complicated phase distribution in order to control the
spatial distribution of CARS effect. The technique was demonstrated by Kim et al.
with the observation of polystyrene beads, where a spatial resolution of 130 nm was
confirmed (Fig. 8.2) [17]. In their technique, Toraldo-style pupil filter was introduced
to modulate the intensity distribution of the Stokes laser spot so that the center
Fig. 8.2 CARS images a without and b with the multiplicative focus field engineering. The images
were reconstructed by using the Raman signal at 1000 cm −1 of polystyrene beads with a diameter
of 300 nm. 728 nm pump and 1785 nm Stokes beams are used for generation of CARS signal. The
3D-rendered images within the dotted rectangles in (a) and (b) are shown in (c) and (d), respectively.
Reproduced from [17] with permission from The Optical Society