(refractive index measured close to 1.356 at 589 nm). The red
curve corresponds to azimuthally-polarized modes that are not
able to penetrate in the gold coating. The black curve is obtained
with radially-polarized modes and exhibits the typical SPR signature
around 1541 nm, which is due to the maximum phase
matching of the cladding mode to the surface plasmon mode of
the gold water interface, according to [8]. The Bragg wavelength
(not displayed in this part of the graph) appears at the right end
side, centered at 1602 nm. In the following, the Bragg wavelength
is used to remove any effect from surrounding temperature
changes by monitoring its shift. This intrinsic feature is very interesting
in practice as a change of 0.1 °C induces an SRI change of
105
, thus susceptible to generate erroneous spectral
modifications.
It is well know that the SPR mode is the most responsive in
response to refractometric changes. A sensitivity close to 550 nm/
RIU in the range between 1.32 and 1.42 has been reported previously.
However, for high-resolution refractometric sensing over
an SRI range limited to 103 typically, accurate measurements of
the SPR mode are not possible from the radially polarized spectrum
taken alone. Indeed, the SPP is only revealed by its absence
from the spectrum and it cannot be reliably measured for wavelength
shifts limited to a few picometers. Hence, different methods
have been developed to track the SPR shift, mainly based on a
comparison between both orthogonally-polarized amplitude
spectra as reported in our previous works [9–10]. In practice,
modes slightly off the SPR are used, because they combine relatively
high sensitivity with a narrow spectral width and they can
be “followed” by a combination of their changes in amplitude and
wavelength. Indeed, as they stand on the shoulder of the SPR
envelope, a slight change of the SPP location yields a modification
of the peak-to-peak amplitude of these modes, as shown below.
The mode labeled ‘0’ in Fig. 3 corresponds to the cut-off mode
(mode whose effective refractive index is the closest to the one of
the surrounding medium). It was also demonstrated that it is
polarization-insensitive, i.e. the wavelengths of the radial and
azimuthal components are perfectly matched for this mode.
Therefore, it can be used as a reference to locate the mode labeled
‘2’, which is actually the most sensitive one in response to refractometric
changes due to its position over the shoulder of the
SPR envelope, as demonstrated in [11]. High resolution sensing
based on the mode labeled ‘2’ is demonstrated in Fig. 4 with
spectra measured during a dilution process (controlled change of
the SRI in a limited range).