2 Surface Plasmons for Chiral Sensing
47
the near-field optical chirality density [65–70]. Moreover, using F ± /F inc = R ± it
follows that:
δ F =
F + − F −
F + + F −
=
R + F inc − R − F inc
R + F inc + R − F inc
=
R + − R −
R + + R −
= ρ DR ,
(2.22)
i.e. the differential chirality flux δ F is equal to ρ DR , as we also demonstrate in
Fig. 2.14.
2.6 Discussion and Conclusions
In this chapter, we start from the observation that plasmonic near fields at metalchiral interfaces exhibit non-vanishing optical chirality density, which generates a
far-field chirality flow. By measuring this chirality flow, which we show to be simply
the reflectance in our measurement protocol, we are able to detect changes in the
near-field chirality and we demonstrate how this can be exploited for chiral-sensitive
measurements using SPR instrumentations. From this, we proceed to demonstrate
the following: (a) how chiral-sensitive SPR measurements allow for the complete
determination of chirality of a natural optically active substance (handedness and
magnitude) and for the detection of the near-field wave at the metal-chiral interface;
(b) how CHISPR is able to detect both the real and imaginary part of the chiral index
of refraction, i.e. detect both circular birefringent and circular dichroism effects; and
(c) how CHISPR is particularly sensitive for the case of sub-wavelength chiral layers,
for which traditional (commercial) polarimetry has typically insufficient sensitivities
to detect. A particularly significant advantage of the angle-resolved CHISPR protocol
we present here, is that it can be employed for spectroscopy of a molecule simply by
tuning the frequency of the incident laser radiation over a molecular absorption line
[71, 72]. In this case, one can record either the output of an angular split (Figs. 2.6
and 2.8) or a differential measurement (Fig. 2.10) as a function of frequency, and
the outcome of such an experiment would be the molecular spectrum of a chiral
molecule through angle-resolved CHISPR measurements. Furthermore, in a SPRbased chiral-sensing scheme the whole evanescent-wave volume is sensitive to the
probed chiral substance (due to the mobility of the propagating SPPs), contrary to
contemporary chiral-sensing nanophotonic schemes that typically rely on localised
surface plasmons, where the sign and magnitude of the chiroptical response can possess a complex dependence on sample geometry [26–31, 73]. Crucially, the CHISPR
signals we demonstrate are within the sensitivity of current SPR instrumentation for
the realistic values of the chirality parameter we consider and, therefore, CHISPR
measurements can be directly realized on existing SPR measurement instrumentations with slight modifications on the analysis stage. As a final remark, we wish to
emphasize here that since the observed relationships between κ and the measured
quantities (θ , ρ DR and φ DR ) will effectively impose a lower limit of chiral detection, we expect that this can be further improved by enhancing the local fields, for
47
the near-field optical chirality density [65–70]. Moreover, using F ± /F inc = R ± it
follows that:
δ F =
F + − F −
F + + F −
=
R + F inc − R − F inc
R + F inc + R − F inc
=
R + − R −
R + + R −
= ρ DR ,
(2.22)
i.e. the differential chirality flux δ F is equal to ρ DR , as we also demonstrate in
Fig. 2.14.
2.6 Discussion and Conclusions
In this chapter, we start from the observation that plasmonic near fields at metalchiral interfaces exhibit non-vanishing optical chirality density, which generates a
far-field chirality flow. By measuring this chirality flow, which we show to be simply
the reflectance in our measurement protocol, we are able to detect changes in the
near-field chirality and we demonstrate how this can be exploited for chiral-sensitive
measurements using SPR instrumentations. From this, we proceed to demonstrate
the following: (a) how chiral-sensitive SPR measurements allow for the complete
determination of chirality of a natural optically active substance (handedness and
magnitude) and for the detection of the near-field wave at the metal-chiral interface;
(b) how CHISPR is able to detect both the real and imaginary part of the chiral index
of refraction, i.e. detect both circular birefringent and circular dichroism effects; and
(c) how CHISPR is particularly sensitive for the case of sub-wavelength chiral layers,
for which traditional (commercial) polarimetry has typically insufficient sensitivities
to detect. A particularly significant advantage of the angle-resolved CHISPR protocol
we present here, is that it can be employed for spectroscopy of a molecule simply by
tuning the frequency of the incident laser radiation over a molecular absorption line
[71, 72]. In this case, one can record either the output of an angular split (Figs. 2.6
and 2.8) or a differential measurement (Fig. 2.10) as a function of frequency, and
the outcome of such an experiment would be the molecular spectrum of a chiral
molecule through angle-resolved CHISPR measurements. Furthermore, in a SPRbased chiral-sensing scheme the whole evanescent-wave volume is sensitive to the
probed chiral substance (due to the mobility of the propagating SPPs), contrary to
contemporary chiral-sensing nanophotonic schemes that typically rely on localised
surface plasmons, where the sign and magnitude of the chiroptical response can possess a complex dependence on sample geometry [26–31, 73]. Crucially, the CHISPR
signals we demonstrate are within the sensitivity of current SPR instrumentation for
the realistic values of the chirality parameter we consider and, therefore, CHISPR
measurements can be directly realized on existing SPR measurement instrumentations with slight modifications on the analysis stage. As a final remark, we wish to
emphasize here that since the observed relationships between κ and the measured
quantities (θ , ρ DR and φ DR ) will effectively impose a lower limit of chiral detection, we expect that this can be further improved by enhancing the local fields, for