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S. Droulias and L. Bougas
Fig. 2.1 SPR principle of operation. a Experimental implementation for SPR measurements
(Kretschmann configuration). b Explanation of SPP excitation. The glass substrate is used to provide the necessary tangential wavenumber (k inc ) to match that of the SPP (k SPP ). This is shown as
a crossing between the SPP dispersion (solid) and the light line in glass (dashed) (the light line in
the dielectric is also shown). c Excitation of surface plasmons in the Kretschmann configuration
for a Au-H 2 O interface at 633 nm. The shaded area marked with ‘TIR’ (Total Internal Reflection)
denotes the region below the critical angle (41.8 ◦ )
the x-direction at the interface between the metal and the dielectric medium located
above; the evanescent field is confined in the z-direction, as also shown schematically
in Fig. 2.1a. To excite the SPP wave, first an incident wave must match the polarization
properties of the SPP and, therefore, a TM(p)-polarized wave is required (components
E x , H y , E z ). Second, the incident wave must also match the tangential wavenumber
of the SPP, k SPP ; this is provided by the substrate, as shown schematically in Fig. 2.1a
and explained in Fig. 2.1b. There it is shown that the dispersion of the SPP lies below
the lightline in the dielectric (k SPP > k d ; k d : wavenumber in the dielectric) and, hence,
the incident wave must have a high tangential wavenumber k inc to match k SPP and
achieve efficient power transfer to the SPP wave. This can be achieved, for example,
via the incident angle θ in an angle-resolved experiment, as shown in Fig. 2.1c. For
this example the wavelength of the incident wave is 633 nm, a typical wavelength
employed in SPR spectroscopy, and the calculations have been performed for H 2 O
on a 50 nm Au layer and a prism of refractive index n sub = 1.5 used as substrate. By
expressing the tangential wavenumber of the incident wave as k inc = k 0 · n sub · sin θ ,
where k 0 is the free-space wavenumber, it becomes clear that k inc can be controlled
by both by θ and n sub . Therefore, as θ is scanned, maximum power transfer from the
incident wave to the SPP wave can be achieved at a certain angle where the condition
k inc = k SPP is met. At this angle, the excitation of the SPP wave becomes the most
efficient and the reflected optical power is therefore minimized. This is manifested
as a dip in the angle-resolved measured reflection.
The CHISPR sensing scheme is an extension of the typical SPR configuration, in
which the dielectric layer is replaced by a chiral medium. Therefore, to understand
the principles of CHISPR it is instructive first to examine the properties of SPPs
at a metal-dielectric interface and how these are modified when the dielectric layer
becomes chiral.
S. Droulias and L. Bougas
Fig. 2.1 SPR principle of operation. a Experimental implementation for SPR measurements
(Kretschmann configuration). b Explanation of SPP excitation. The glass substrate is used to provide the necessary tangential wavenumber (k inc ) to match that of the SPP (k SPP ). This is shown as
a crossing between the SPP dispersion (solid) and the light line in glass (dashed) (the light line in
the dielectric is also shown). c Excitation of surface plasmons in the Kretschmann configuration
for a Au-H 2 O interface at 633 nm. The shaded area marked with ‘TIR’ (Total Internal Reflection)
denotes the region below the critical angle (41.8 ◦ )
the x-direction at the interface between the metal and the dielectric medium located
above; the evanescent field is confined in the z-direction, as also shown schematically
in Fig. 2.1a. To excite the SPP wave, first an incident wave must match the polarization
properties of the SPP and, therefore, a TM(p)-polarized wave is required (components
E x , H y , E z ). Second, the incident wave must also match the tangential wavenumber
of the SPP, k SPP ; this is provided by the substrate, as shown schematically in Fig. 2.1a
and explained in Fig. 2.1b. There it is shown that the dispersion of the SPP lies below
the lightline in the dielectric (k SPP > k d ; k d : wavenumber in the dielectric) and, hence,
the incident wave must have a high tangential wavenumber k inc to match k SPP and
achieve efficient power transfer to the SPP wave. This can be achieved, for example,
via the incident angle θ in an angle-resolved experiment, as shown in Fig. 2.1c. For
this example the wavelength of the incident wave is 633 nm, a typical wavelength
employed in SPR spectroscopy, and the calculations have been performed for H 2 O
on a 50 nm Au layer and a prism of refractive index n sub = 1.5 used as substrate. By
expressing the tangential wavenumber of the incident wave as k inc = k 0 · n sub · sin θ ,
where k 0 is the free-space wavenumber, it becomes clear that k inc can be controlled
by both by θ and n sub . Therefore, as θ is scanned, maximum power transfer from the
incident wave to the SPP wave can be achieved at a certain angle where the condition
k inc = k SPP is met. At this angle, the excitation of the SPP wave becomes the most
efficient and the reflected optical power is therefore minimized. This is manifested
as a dip in the angle-resolved measured reflection.
The CHISPR sensing scheme is an extension of the typical SPR configuration, in
which the dielectric layer is replaced by a chiral medium. Therefore, to understand
the principles of CHISPR it is instructive first to examine the properties of SPPs
at a metal-dielectric interface and how these are modified when the dielectric layer
becomes chiral.