Instrumentation & Measurement Magazine 24-5 - 63

Multi-Plasmonic Resonance Based
Sensor for the Characterization
of Optical Dispersion Using a
D-Shaped Photonic Crystal Fiber
Markos P. Cardoso, Anderson O. Silva, Amanda F. Romeiro, M. Thereza R. Giraldi,
João C.W.A. Costa, José L. Santos, José M. Baptista, and Ariel Guerreiro
S
urface plasmon-polaritons are electromagnetic
modes that can be excited at a conducting-dielectric
interface [1]. The engineering of surface plasmon
resonance (SPR) based devices is a milestone in the development
of optical sensors. The ability to construct an all-optical
system to confine lightwave power at subwavelength dimensions
with higher levels of sensitivity and resolution in a broad
spectral range are the central features that have attracted a
rapid-growing interest in SPR sensors [2]. Particularly, minute
variations in the refractive index of the surrounding medium
(also known as analyte) change significantly the characteristics
of the electromagnetic fields of a surface plasmon mode.
As a consequence, the spectral shifts in the mode phase and
also losses variations in the associated confined power can be
used to detect analyte properties that are described in terms of
the refractive index [3].
Development of SPR Sensors
The phase-matching to achieve plasmonic resonance is a major
challenge in the development of SPR sensors. Prism-coupling
techniques comprise the focusing of a beam of light into a
metal layer deposited on the face of the prism [4]-[5]. Even
though of relevant efficiency to excite surface plasmon waves,
they comprise an apparatus that is difficult to miniaturize, has
a high fabrication cost and are not designed to be carried from
one place to another, which are drawbacks for practical remote
sensing applications. A more suitable alternative employs optical
fibers to achieve surface plasmon resonance. A common
configuration consists of removing a section of the fiber cladding
for the deposition of a thin conducting layer. A plasmonic
resonance is reached whenever the phases of the core guided
mode and the surface plasmon at the conducting interface are
equal. Tapered fibers and D-shaped fibers are proper examples
of such configurations [6]-[8]. For operating as an optical
sensor, it is expected that the thickness of the conducting slab
be fine controlled to allow a large fraction of optical power to
reach the surrounding medium.
Unique characteristics such as low loss, high mode confinement
with a large mode area and fine control of the evanescent
field penetration into the metallic medium make photonic
crystal fibers (PCF) able to be quite adequate to excite and enhance
surface plasmon resonances [9]-[11]. A PCF is composed
of a set of air holes extending along the fiber length and periodically
distributed over its cross-sectional area. The light
waveguiding is favored by inserting a defect in the array of air
holes. The modal properties are remarkably influenced by the
structural parameters such as the diameter of the air holes and
the distance between them.
SPR sensors constructed on a PCF find have a large variety
of applications. For refractive index sensing, the
D-shape configuration is an efficient alternative to take advantage
of the mode fiber properties for optimally exciting
plasmonic resonances. Indeed, several designs of SPR sensors
based on a D-shaped PFC with increasingly levels of
sensitivity and resolution have been reported over the last
decade [12]-[14]. However, to the best of our knowledge,
all of these sensors are scrutinized to interrogate the average
refractive index or the refractive index at a specific
wavelength of the surrounding medium, with no further
information regarding the dispersive character of the analyte
such as phase velocity, group velocity, or slow-light and
fast-light regimes.
A dispersion sensor finds potential applications for the investigation
of the constituents of fluids, for instance. Where
small variations in chemical composition produce subtle
changes in the dispersion related to the medium, that cannot
be roughly captured by a sensor monitoring an average refractive
index. Indeed, the optical properties of a medium depend
on the atomic structure of the constituents in terms of a set of
natural frequencies ωj
, damping coefficients γj
and strength
This research was supported in part by the: CAPES Foundation, Ministry of Education, Brazil, Finance code 001; Brazilian
National Council for Scientific and Technological Development; European Regional Development Fund through the
COMPETE 2020 Program; and Portuguese funding agency within project POCI-01-0145-FEDER-032257.
August 2021
IEEE Instrumentation & Measurement Magazine
1094-6969/21/$25.00©2021IEEE
63

Instrumentation & Measurement Magazine 24-5

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 24-5