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J. E. Vázquez-Lozano and A. Martínez
ticles, in very distinct arrangements, have also been suggested as suitable systems to
boost near-field chirality [10, 11]. Besides displaying lower absorption losses than
its plasmonic counterparts, dielectric resonators are proving to give promising results
with a number of important practical advantages for chiral applications such as sensing, spectroscopy, and enantioselectivity [12]. Specifically, they can provide large
areas of high and uniform-sign chirality [13, 14], spectral accessibility and tunability
of chiral interactions [15], and switchability upon reversing the input polarization
[16]. Ultimately, all these approaches, either based on metallic or dielectric nanostructures, have been strongly fostered on account of the early proposal of superchiral
fields [17], as they can be used to get high levels of resolution and sensitivity in order
to check and characterize the chirality of matter or nanostructures.
1 This necessity of
enhancing chiroptical responses arises from the fact that interaction between chiral
light and chiral analytes (typically biomolecules such as sugars or proteins [20], or
artificial metamolecules at the micro or even nanoscale [21]) is in general extremely
weak. Indeed, leaving aside the complexity that the experimental arrangements might
entail by themselves, this smallness is mainly due to the large scale difference (and
the subsequent mismatch) between the operational wavelength of the input light and
the typical size of the chiral objects [22]. It is here where plasmonic nanostructures and metamaterials come into play, as they ease the full spatiotemporal control
of light-matter interactions [23], thereby leading to sculpted and highly contorted
three-dimensional electromagnetic (EM) fields [24]. In fact, it should be noted that
the main requirement for the occurrence of strong optical chirality, and thus for
enhancing the chiroptical interactions, actually lies on the complexity in the shapes
traced out by the EM field distributions [24–26].
It is very well known that plasmonic nanostructures and metamaterials are inherently dispersive systems [27]. In fact, all the media and materials, except the vacuum,
are dispersive, and, consequently, feature absorption losses [28–30]. These characteristics are to be accounted for when addressing dynamical properties such as, for
example, EM energy. Hence, as regards the matter we are concerned, the same should
be done for the optical chirality [31, 32]. However, it is certainly surprising that, in
most of the previous studies on optical chirality and its interaction with matter, contributions of material dispersion as well as dissipation have mostly been ignored.
Rather, it is a common practice to apply the earliest definition originally derived for
monochromatic optical fields in free space [17, 31–33]:
C vacuum (r, t) ≡
1
2
ε 0 E · (∇ × E) + μ 0 H · (∇ × H)
,
(13.1)
1 It is worth pointing out that, superchirality (or superchiral light), is well defined only in the case
of plane-wave propagation in free space, because it is actually defined with respect to the chirality
of circularly polarized light. Notice that, for example, in the case of waveguiding systems, the term
of superchirality may be misunderstood, as it would depend on the specific structure [18]. So, in
lieu of superchirality, henceforth we shall refer to it simply as the enhanced chirality. In any case, it
is noteworthy to mention that there exists a subtle controversy regarding superchiral fields and its
apparent unlimited enhancement factor (for further details on this issue see, e.g., [19]).
J. E. Vázquez-Lozano and A. Martínez
ticles, in very distinct arrangements, have also been suggested as suitable systems to
boost near-field chirality [10, 11]. Besides displaying lower absorption losses than
its plasmonic counterparts, dielectric resonators are proving to give promising results
with a number of important practical advantages for chiral applications such as sensing, spectroscopy, and enantioselectivity [12]. Specifically, they can provide large
areas of high and uniform-sign chirality [13, 14], spectral accessibility and tunability
of chiral interactions [15], and switchability upon reversing the input polarization
[16]. Ultimately, all these approaches, either based on metallic or dielectric nanostructures, have been strongly fostered on account of the early proposal of superchiral
fields [17], as they can be used to get high levels of resolution and sensitivity in order
to check and characterize the chirality of matter or nanostructures.
1 This necessity of
enhancing chiroptical responses arises from the fact that interaction between chiral
light and chiral analytes (typically biomolecules such as sugars or proteins [20], or
artificial metamolecules at the micro or even nanoscale [21]) is in general extremely
weak. Indeed, leaving aside the complexity that the experimental arrangements might
entail by themselves, this smallness is mainly due to the large scale difference (and
the subsequent mismatch) between the operational wavelength of the input light and
the typical size of the chiral objects [22]. It is here where plasmonic nanostructures and metamaterials come into play, as they ease the full spatiotemporal control
of light-matter interactions [23], thereby leading to sculpted and highly contorted
three-dimensional electromagnetic (EM) fields [24]. In fact, it should be noted that
the main requirement for the occurrence of strong optical chirality, and thus for
enhancing the chiroptical interactions, actually lies on the complexity in the shapes
traced out by the EM field distributions [24–26].
It is very well known that plasmonic nanostructures and metamaterials are inherently dispersive systems [27]. In fact, all the media and materials, except the vacuum,
are dispersive, and, consequently, feature absorption losses [28–30]. These characteristics are to be accounted for when addressing dynamical properties such as, for
example, EM energy. Hence, as regards the matter we are concerned, the same should
be done for the optical chirality [31, 32]. However, it is certainly surprising that, in
most of the previous studies on optical chirality and its interaction with matter, contributions of material dispersion as well as dissipation have mostly been ignored.
Rather, it is a common practice to apply the earliest definition originally derived for
monochromatic optical fields in free space [17, 31–33]:
C vacuum (r, t) ≡
1
2
ε 0 E · (∇ × E) + μ 0 H · (∇ × H)
,
(13.1)
1 It is worth pointing out that, superchirality (or superchiral light), is well defined only in the case
of plane-wave propagation in free space, because it is actually defined with respect to the chirality
of circularly polarized light. Notice that, for example, in the case of waveguiding systems, the term
of superchirality may be misunderstood, as it would depend on the specific structure [18]. So, in
lieu of superchirality, henceforth we shall refer to it simply as the enhanced chirality. In any case, it
is noteworthy to mention that there exists a subtle controversy regarding superchiral fields and its
apparent unlimited enhancement factor (for further details on this issue see, e.g., [19]).