Label-free sensing of ultralow-weight molecules
with all-dielectric metasurfaces supporting
bound states in the continuum
SILVIA ROMANO,
†,1,
*GIANLUIGI ZITO,
†,2
STEFANIA TORINO,
1
GIUSEPPE CALAFIORE,
3
ERIKA PENZO,
3
GIUSEPPE COPPOLA,
1
STEFANO CABRINI,
3
IVO RENDINA,
1
AND VITO MOCELLA
1
1
Institute for Microelectronics and Microsystems, Unit of Naples, National Council of Research, Via Pietro Castellino, 80131 Naples, Italy
2
Institute of Protein Biochemistry, National Council of Research, Via Pietro Castellino, 80131 Naples, Italy
3
Lawrence Berkeley National Laboratory, Molecular Foundry Division, 67 Cyclotron Road, Berkeley, California 94720, USA
*Corresponding author: silvia.romano@na.imm.cnr.it
Received 16 April 2018; revised 21 May 2018; accepted 21 May 2018; posted 24 May 2018 (Doc. ID 328323); published 21 June 2018
The realization of an efficient optical sensor based on a photonic crystal metasurface supporting bound states in
the continuum is reported. Liquids with different refractive indices, ranging from 1.4000 to 1.4480, are infiltrated
in a microfluidic chamber bonded to the sensing dielectric metasurface. A bulk liquid sensitivity of 178 nm/RIU
is achieved, while a Q-factor of about 2000 gives a sensor figure of merit up to 445 in air at both visible and
infrared excitations. Furthermore, the detection of ultralow-molecular-weight (186 Da) molecules is demon-
strated with a record resonance shift of 6 nm per less than a 1 nm thick single molecular layer. The system exploits
a normal-to-the-surface optical launching scheme, with excellent interrogation stability and demonstrates
alignment-free performances, overcoming the limits of standard photonic crystals and plasmonic resonant
configurations.
© 2018 Chinese Laser Press
OCIS codes: (050.5298) Photonic crystals; (280.4788) Optical sensing and sensors; (260.5740) Resonance.
https://doi.org/10.1364/PRJ.6.000726
1. INTRODUCTION
Optical sensors can reveal layer thickness changes, bulk fluid
properties, molecular bindings, or cellular interactions by meas-
uring changes of the refractive index (RI), light absorption, or
scattering at the sensing surface. Surface plasmon resonance
(SPR) and localized surface plasmon resonance (LSPR) sensors
are considered a gold standard in label-free and real-time sur-
face sensing [1–4]. The large spectral shift Δλ
res
of the reso-
nance peak in response to a relatively small change Δn of
the refractive index n of the surrounding medium corresponds
to high values of the bulk sensitivity S Δλ
res
∕Δn [5–7].
However, the figure of merit FOM S∕FWHM (where
FWHM is the full width at half maximum) of plasmonic sen-
sors reaches around 8 [8,9] for LSPR-based sensors and around
23 [9] for SPR devices, being strongly limited by the large plas-
monic resonance linewidth due to the high optical absorption
losses in the metal. Low values of FOM negatively affect the
performance of a sensing device and the limit of detection.
In complex plasmonic metamaterial-based sensors, large sensi-
tivity can be reached. FOM values of 330 have been reported
[10]. A high FOM value is typically associated with large
Q-factors, for which engineering of complex designs is manda-
tory. In this regard, several efforts have been put forward relying
on Fano resonance engineering, yet mainly in the case of plas-
monic structures [11]. These last are affected by optical losses in
the visible range and are more efficient in the terahertz range
[12], where resistive losses are minimal. However, in this case,
detector efficiency is poor, and a large footprint may be a
technological limitation.
A promising class of optical sensors characterized by a high
Q-factor and large values of FOM is based on photonic crystal
(PhC) nanocavities [13]. They typically consist of punctual de-
fects in a PhC lattice [14] and provide strong interaction be-
tween the confined optical field and the analyte, finding direct
application in the realization of gas, liquid, temperature, stress,
humidity, refractive index, biochemical sensors, etc. [15–17].
However, a PhC cavity requires a high-precision nanofabrica-
tion process for contro lling position and size of the air holes and
precise near-field coupling [18,19], for instance, as with optical
fiber coupling [20].
Herein, we explore a different sensing scheme approach
overcoming the aforementioned limits. The device is based
on plasmon-like surface waves, delocalized on a wide area to
provide a large interaction volume with matter but excited
in an all-dielectric photonic crystal metasurface (PhCM). It ex-
ploits a normal-to-the-surface optical launching scheme to
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Vol. 6, No. 7 / July 2018 / Photonics Research
Research Article
2327-9125/18/070726-08 Journal © 2018 Chinese Laser Press