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5.1 Introduction
Chirality is the property that the spatial geometries of matters, fields, and motions
cannot be identical to their mirror images. There exist various kinds of chiral structures or phenomena for different objects such as nucleons, amino acids, and liquid
crystals. The chiral interaction between light and matter, especially, has been an
important subject in various research domains. For example, the circular dichroism (CD) of materials, which is a difference between the absorptions of left- and
right-circularly polarized lights, has been a standard measure of matter chirality for
a long time. However, the chirality of light is also affected by the interaction with
matter. Optical rotation and circularly polarized luminescence are typical examples.
Recently, chiral interaction has been applied to the generation of light with orbital
angular momentum, sometimes referred to as optical vortex. Accordingly, chiral
interactions have been one of the fundamental principles in germinating, probing,
and designing the nature and the function of matter and light.
Recently, the research on chiral interactions has taken a new step because of the
rapid development of nanofabrication technologies and characterization techniques
of few-molecule systems. For example, the high-accuracy fabrication of metallic
structures enables us to control the localized electric field in nanoscale areas owing
to localized surface plasmon (LSP). By using LSP effect, we can realize circularly
polarized fields with significantly reduced pitch of polarization rotation, i.e., the
so-called superchiral fields [1]. In the past 20 years, various nanostructures have
been reported to generate superchiral fields [2–9]. In addition, the CD signal of
the plasmonic near field was measured via photon scanning tunneling microscopy or
near-field scanning optical microscopy (SNOM) [10–13]. Superchiral fields boost the
molecular CD signal, although the signals of individual molecules, such as proteins,
DNA, and carbon nanotubes, are generally small. The superchiral field is expected
to be a promising tool for performing the sensitive enantioselective detections of
chiral molecules [14–17]. Another interesting application of LSP is the conversion
between the spin angular momentum and orbital angular momentum. A plane wave
light with circularly polarization has spin angular momentum, but not orbital angular
momentum. However, when it is radiated onto metallic nanocomplexes, the induced
plasmonic near field can exhibit optical current with orbital angular momentum [18].
The optical response of a matter manifests not only as optical outputs but also as a
mechanical force exerted on the matter system. Therefore, the plasmonic near field
with the nanoscale radius of gyration is expected to enhance the degree of freedom
to manipulate the center-of-mass motion of nanoobjects.
For the analyses of light–mater chiral interactions, we should note that the conventional scheme of light–mater interaction based on the long wavelength approximation
(LWA) of light or dipole approximation (DA) of matter does not work because the chiral nanostructures have a nanoscale polarization configuration. Moreover, the chiral
light induces non-dipole polarization structures of matter systems. Thus, the nonlocal response is important in nanoscale chiral light–matter interactions, in which
the nanoscale spatial correlation between light and matter plays an essential role.
H. Ishihara et al.
5.1 Introduction
Chirality is the property that the spatial geometries of matters, fields, and motions
cannot be identical to their mirror images. There exist various kinds of chiral structures or phenomena for different objects such as nucleons, amino acids, and liquid
crystals. The chiral interaction between light and matter, especially, has been an
important subject in various research domains. For example, the circular dichroism (CD) of materials, which is a difference between the absorptions of left- and
right-circularly polarized lights, has been a standard measure of matter chirality for
a long time. However, the chirality of light is also affected by the interaction with
matter. Optical rotation and circularly polarized luminescence are typical examples.
Recently, chiral interaction has been applied to the generation of light with orbital
angular momentum, sometimes referred to as optical vortex. Accordingly, chiral
interactions have been one of the fundamental principles in germinating, probing,
and designing the nature and the function of matter and light.
Recently, the research on chiral interactions has taken a new step because of the
rapid development of nanofabrication technologies and characterization techniques
of few-molecule systems. For example, the high-accuracy fabrication of metallic
structures enables us to control the localized electric field in nanoscale areas owing
to localized surface plasmon (LSP). By using LSP effect, we can realize circularly
polarized fields with significantly reduced pitch of polarization rotation, i.e., the
so-called superchiral fields [1]. In the past 20 years, various nanostructures have
been reported to generate superchiral fields [2–9]. In addition, the CD signal of
the plasmonic near field was measured via photon scanning tunneling microscopy or
near-field scanning optical microscopy (SNOM) [10–13]. Superchiral fields boost the
molecular CD signal, although the signals of individual molecules, such as proteins,
DNA, and carbon nanotubes, are generally small. The superchiral field is expected
to be a promising tool for performing the sensitive enantioselective detections of
chiral molecules [14–17]. Another interesting application of LSP is the conversion
between the spin angular momentum and orbital angular momentum. A plane wave
light with circularly polarization has spin angular momentum, but not orbital angular
momentum. However, when it is radiated onto metallic nanocomplexes, the induced
plasmonic near field can exhibit optical current with orbital angular momentum [18].
The optical response of a matter manifests not only as optical outputs but also as a
mechanical force exerted on the matter system. Therefore, the plasmonic near field
with the nanoscale radius of gyration is expected to enhance the degree of freedom
to manipulate the center-of-mass motion of nanoobjects.
For the analyses of light–mater chiral interactions, we should note that the conventional scheme of light–mater interaction based on the long wavelength approximation
(LWA) of light or dipole approximation (DA) of matter does not work because the chiral nanostructures have a nanoscale polarization configuration. Moreover, the chiral
light induces non-dipole polarization structures of matter systems. Thus, the nonlocal response is important in nanoscale chiral light–matter interactions, in which
the nanoscale spatial correlation between light and matter plays an essential role.