304
M. J. Huttunen and A. Kiviniemi
discrimination of different types of collagens [74], in wound healing studies and in
cancer research [69, 75–78].
In addition to using linearly polarized excitation beams and polarization-sensitive
detection, also circularly polarized light can be used to provide information of the
objects. A few recent works have studied if nonlinear optical activity (NOA) effects
could provide additional, or more sensitive, morphological information of imaged
objects [79–84]. In particular, NOA effects are related to the small-scale chirality
and anisotropy of the studied objects and could, therefore, provide three-dimensional
structural information. In principle, it is possible to link such information even down
to protein-level structures of the object, however, it is not yet clear how sensitively
the measured NOA effects could truly depend on such small-scale changes. Nevertheless, since changes in the protein-level structure are related to many diseases
and disorders, such modalities could turn out to be highly useful in biomedical or
diagnostic applications.
Another powerful and truly label-free nonlinear imaging modality is based on
THG. Compared to SHG imaging, in THG microscopy the excitation of objects
can be performed at longer wavelengths. Due to this advantage, THG modality has
been used for imaging neurons, white-matter and blood cells deep inside live murine
brain tissues [85]. THG microscopy has also found applications in lipid research,
since lipid molecules act as efficient sources of THG [71, 72, 86]. For example, lipid
bodies in liver tissues and cells have been studied using THG microscopy [70].
Besides THG, also other third-order processes can be utilized in imaging. In particular, CARS microscopy has shown its usefulness as a label-free technique which
can also provide chemical contrast [43, 61]. Applications of CARS microscopy
include lipid composition studies of lipid droplets and investigations of the progression of atherosclerosis in arterial tissues [87, 88]. An illustrative application example
is shown in Fig. 12.10, where combined SHG, 2PEF and CARS microscopy has been
performed to study atherosclerotic lesions of a rabbit model.
As is evident from the overall discussion above, label-free nonlinear microscopy
modalities have already found many interesting and relevant biomedical applications.
These existing applications could undoubtedly benefit from the improved capabilities
that nonlinear super-resolution techniques could offer. In addition, new applications
are bound to emerge once label-free super-resolution techniques establish themselves
as a more mature technology. Finally, we need to mention that in addition to the
aforementioned biomedical applications, nonlinear label-free microscopy has also
been found very useful to investigate artificial metamaterials [90, 91], or one- and
two-dimensional materials, such as carbon nanotubes or graphene [92–96]. Such
applications could also greatly benefit from the improved resolution.
M. J. Huttunen and A. Kiviniemi
discrimination of different types of collagens [74], in wound healing studies and in
cancer research [69, 75–78].
In addition to using linearly polarized excitation beams and polarization-sensitive
detection, also circularly polarized light can be used to provide information of the
objects. A few recent works have studied if nonlinear optical activity (NOA) effects
could provide additional, or more sensitive, morphological information of imaged
objects [79–84]. In particular, NOA effects are related to the small-scale chirality
and anisotropy of the studied objects and could, therefore, provide three-dimensional
structural information. In principle, it is possible to link such information even down
to protein-level structures of the object, however, it is not yet clear how sensitively
the measured NOA effects could truly depend on such small-scale changes. Nevertheless, since changes in the protein-level structure are related to many diseases
and disorders, such modalities could turn out to be highly useful in biomedical or
diagnostic applications.
Another powerful and truly label-free nonlinear imaging modality is based on
THG. Compared to SHG imaging, in THG microscopy the excitation of objects
can be performed at longer wavelengths. Due to this advantage, THG modality has
been used for imaging neurons, white-matter and blood cells deep inside live murine
brain tissues [85]. THG microscopy has also found applications in lipid research,
since lipid molecules act as efficient sources of THG [71, 72, 86]. For example, lipid
bodies in liver tissues and cells have been studied using THG microscopy [70].
Besides THG, also other third-order processes can be utilized in imaging. In particular, CARS microscopy has shown its usefulness as a label-free technique which
can also provide chemical contrast [43, 61]. Applications of CARS microscopy
include lipid composition studies of lipid droplets and investigations of the progression of atherosclerosis in arterial tissues [87, 88]. An illustrative application example
is shown in Fig. 12.10, where combined SHG, 2PEF and CARS microscopy has been
performed to study atherosclerotic lesions of a rabbit model.
As is evident from the overall discussion above, label-free nonlinear microscopy
modalities have already found many interesting and relevant biomedical applications.
These existing applications could undoubtedly benefit from the improved capabilities
that nonlinear super-resolution techniques could offer. In addition, new applications
are bound to emerge once label-free super-resolution techniques establish themselves
as a more mature technology. Finally, we need to mention that in addition to the
aforementioned biomedical applications, nonlinear label-free microscopy has also
been found very useful to investigate artificial metamaterials [90, 91], or one- and
two-dimensional materials, such as carbon nanotubes or graphene [92–96]. Such
applications could also greatly benefit from the improved resolution.