28
S. Droulias and L. Bougas
interaction, can in principle be arranged into two main categories, as they rely mainly
on either (a) path-length enhancement or (b) chiral-field enhancement.
Path-length enhancement techniques typically rely on the use of optical mirrors
to create either multipass cells or optical cavities, where in both cases spectropolarimetric signals are enhanced by the average number of passes through the
medium. Multipass cells [15, 16] are technically easy to construct and implement,
and optical path-length enhancements as large as ×500 have been demonstrated
[17, 18]; crucially though, multipass techniques cannot be, in principle, employed
for the measurement of optical activity (owing to the effective round-trip cancellation of the polarization effects [19]). The solution is to use stable optical cavities
[20–24], with which one can achieve path-length enhancements of up to 10
5 using
state-of-the-art high quality mirrors, with effective path-lengths of up to several
hundred kilometers (one needs to compare this to the 10 cm-long sample cell in a
single-pass commercial polarimeter), enabling record sensitivities for measurements
of absorption and birefringence (CD and ORD, respectively). Importantly, optical
cavities can easily be made compact and allow for wave-matter interactions in small
volumes. However, cavity-enhanced techniques become inadequate in systems with
losses originating from absorption and/or scattering (e.g. chiral molecules within
complex matrices, thin films, liquid and/or solid systems), because losses hinder the
path-length enhancement. For the case of CD, in particular, path-length enhancement
techniques can be mainly used to probe weak molecular transitions [20, 22, 25].
Chiral-field-enhancement techniques rely primarily on generating probing electromagnetic fields with chiral densities higher than circularly polarized plane waves,
i.e. superchiral fields [26–32]. Chiral and/or achiral nanophotonic systems, such
as plasmonic/dielectric nanostructures and metamaterials, can generate contorted
intense near-fields with high chiral densities around a resonance frequency of the
nanosystem, thus, amplifying the chiral-chiral interactions between them and a
molecule. In general, nanophotonic approaches have proven to be a powerful means
for granting access to weak chiroptical signals not previously attainable with traditional polarimetric techniques, however, the general principle of operation behind
(almost) all contemporary nanophotonic chiral-sensing approaches primarily relies
on the detection of enhanced CD signals in the presence of an optically active chiral substance. To achieve this, right- and left-circularly polarized waves are used
to excite the system and generate these superchiral fields, and enable the ability to
perform CD measurements in transmission. While several works have attributed the
resulting CD signal to be proportional only to the imaginary part of the chirality
parameter, i.e. Im(κ) (see, e.g., [31–33]), in reality, as supported by past and recent
experimental and theoretical results [34–37] the observed CD signals depend on both
the real and the imaginary part of κ [Re(κ) and Im(κ), respectively]. Thus, with most
contemporary nanophotonic approaches, sensing of the magnitude and sign of both
the real and imaginary part of the chirality parameter of a natural optically active substance (complete measurement) has not been possible, while elaborate fabrication is
required and the employed nanosystems typically have intrinsic chiroptical responses
that contribute to the total signal, often precluding direct quantitative measurement
of chirality [26–31].
S. Droulias and L. Bougas
interaction, can in principle be arranged into two main categories, as they rely mainly
on either (a) path-length enhancement or (b) chiral-field enhancement.
Path-length enhancement techniques typically rely on the use of optical mirrors
to create either multipass cells or optical cavities, where in both cases spectropolarimetric signals are enhanced by the average number of passes through the
medium. Multipass cells [15, 16] are technically easy to construct and implement,
and optical path-length enhancements as large as ×500 have been demonstrated
[17, 18]; crucially though, multipass techniques cannot be, in principle, employed
for the measurement of optical activity (owing to the effective round-trip cancellation of the polarization effects [19]). The solution is to use stable optical cavities
[20–24], with which one can achieve path-length enhancements of up to 10
5 using
state-of-the-art high quality mirrors, with effective path-lengths of up to several
hundred kilometers (one needs to compare this to the 10 cm-long sample cell in a
single-pass commercial polarimeter), enabling record sensitivities for measurements
of absorption and birefringence (CD and ORD, respectively). Importantly, optical
cavities can easily be made compact and allow for wave-matter interactions in small
volumes. However, cavity-enhanced techniques become inadequate in systems with
losses originating from absorption and/or scattering (e.g. chiral molecules within
complex matrices, thin films, liquid and/or solid systems), because losses hinder the
path-length enhancement. For the case of CD, in particular, path-length enhancement
techniques can be mainly used to probe weak molecular transitions [20, 22, 25].
Chiral-field-enhancement techniques rely primarily on generating probing electromagnetic fields with chiral densities higher than circularly polarized plane waves,
i.e. superchiral fields [26–32]. Chiral and/or achiral nanophotonic systems, such
as plasmonic/dielectric nanostructures and metamaterials, can generate contorted
intense near-fields with high chiral densities around a resonance frequency of the
nanosystem, thus, amplifying the chiral-chiral interactions between them and a
molecule. In general, nanophotonic approaches have proven to be a powerful means
for granting access to weak chiroptical signals not previously attainable with traditional polarimetric techniques, however, the general principle of operation behind
(almost) all contemporary nanophotonic chiral-sensing approaches primarily relies
on the detection of enhanced CD signals in the presence of an optically active chiral substance. To achieve this, right- and left-circularly polarized waves are used
to excite the system and generate these superchiral fields, and enable the ability to
perform CD measurements in transmission. While several works have attributed the
resulting CD signal to be proportional only to the imaginary part of the chirality
parameter, i.e. Im(κ) (see, e.g., [31–33]), in reality, as supported by past and recent
experimental and theoretical results [34–37] the observed CD signals depend on both
the real and the imaginary part of κ [Re(κ) and Im(κ), respectively]. Thus, with most
contemporary nanophotonic approaches, sensing of the magnitude and sign of both
the real and imaginary part of the chirality parameter of a natural optically active substance (complete measurement) has not been possible, while elaborate fabrication is
required and the employed nanosystems typically have intrinsic chiroptical responses
that contribute to the total signal, often precluding direct quantitative measurement
of chirality [26–31].