196
K. Fujita
Recent developments in optical microscopy have paved various ways toward
breaking the limit of the spatial resolution and visualizing structures or distributions
of materials smaller than the half of the wavelength. The techniques, such as superresolution microscopy, use light–material interactions to overcome the diffraction
limit. Since the diffraction limit originated from the property of light propagation in
the optical system, the deviation from the conventional framework of optical imaging can be achieved by taking the light–material interaction into account for image
formation.
In particular, super-resolution microscopy has been developed intensively for
fluorescence imaging [1]. The control of fluorescence emission property allows
several different approaches to break the diffraction barrier. For example, stimulated emission depletion (STED) microscopy uses stimulated emission of fluorescence to reduce the volume of fluorescence emission after excitation, which
obtains super-resolution images by scanning the detection volume over the sample [2]. Another super-resolution technique, known as single-molecule localization
microscopy, manipulates the fluorescence emission or light absorption property to
avoid simultaneous observation of two or more fluorescent molecules located within
the diffraction limit [3–5]. Other super-resolution imaging techniques also use the
relationship between fluorescence excitation and emission to realize the spatial resolution beyond the resolution limit determined by the wavelength [6, 7].
The approaches mentioned above can also be adopted to realize super-resolution
imaging in Raman microscopy. Control of Raman scattering properties of the sample
is not as easy as that with fluorescence molecules. However, understanding of the
optical processes in Raman scattering allows us to manipulate the imaging property
of optical systems. Actually, there are various approaches to induce Raman effect,
such as coherent anti-Stokes Raman scattering, stimulated Raman scattering, and
surface-enhanced Raman scattering, offering different strategies to break the diffraction limit. It has been well known that the use of the enhancement of Raman scattering
at the apex of metallic probe for super-resolution imaging can realize the spatial resolution at nanometer scale [8–10]. The technique is known as tip-enhanced Raman
scattering (TERS) microscopy and has found its applications in the material industry.
Although TERS is a powerful technique for super-resolution Raman imaging, this
chapter focuses on the super-resolution technique that detects signal light at far-fields
where the property of imaging optics and the spatial and temporal control of Raman
scattering take important roles in image formation. In this chapter, we categorize
super-resolution techniques based on the strategies for breaking the diffraction limit
and describe the implementation of each technique that achieves super-resolution
imaging in Raman microscopy.
8.2 Point Spread Function (PSF) Engineering
Among the many different types of Raman microscopy, laser scanning is a typical
approach to produce a Raman scattering image of a sample. In laser scanning tech-
K. Fujita
Recent developments in optical microscopy have paved various ways toward
breaking the limit of the spatial resolution and visualizing structures or distributions
of materials smaller than the half of the wavelength. The techniques, such as superresolution microscopy, use light–material interactions to overcome the diffraction
limit. Since the diffraction limit originated from the property of light propagation in
the optical system, the deviation from the conventional framework of optical imaging can be achieved by taking the light–material interaction into account for image
formation.
In particular, super-resolution microscopy has been developed intensively for
fluorescence imaging [1]. The control of fluorescence emission property allows
several different approaches to break the diffraction barrier. For example, stimulated emission depletion (STED) microscopy uses stimulated emission of fluorescence to reduce the volume of fluorescence emission after excitation, which
obtains super-resolution images by scanning the detection volume over the sample [2]. Another super-resolution technique, known as single-molecule localization
microscopy, manipulates the fluorescence emission or light absorption property to
avoid simultaneous observation of two or more fluorescent molecules located within
the diffraction limit [3–5]. Other super-resolution imaging techniques also use the
relationship between fluorescence excitation and emission to realize the spatial resolution beyond the resolution limit determined by the wavelength [6, 7].
The approaches mentioned above can also be adopted to realize super-resolution
imaging in Raman microscopy. Control of Raman scattering properties of the sample
is not as easy as that with fluorescence molecules. However, understanding of the
optical processes in Raman scattering allows us to manipulate the imaging property
of optical systems. Actually, there are various approaches to induce Raman effect,
such as coherent anti-Stokes Raman scattering, stimulated Raman scattering, and
surface-enhanced Raman scattering, offering different strategies to break the diffraction limit. It has been well known that the use of the enhancement of Raman scattering
at the apex of metallic probe for super-resolution imaging can realize the spatial resolution at nanometer scale [8–10]. The technique is known as tip-enhanced Raman
scattering (TERS) microscopy and has found its applications in the material industry.
Although TERS is a powerful technique for super-resolution Raman imaging, this
chapter focuses on the super-resolution technique that detects signal light at far-fields
where the property of imaging optics and the spatial and temporal control of Raman
scattering take important roles in image formation. In this chapter, we categorize
super-resolution techniques based on the strategies for breaking the diffraction limit
and describe the implementation of each technique that achieves super-resolution
imaging in Raman microscopy.
8.2 Point Spread Function (PSF) Engineering
Among the many different types of Raman microscopy, laser scanning is a typical
approach to produce a Raman scattering image of a sample. In laser scanning tech-