Optical fiber meta-tips
Light: Science & Applications
Optical fiber meta-tips
We report on the first demonstration of a proof-of-principle optical fiber 'meta-tip', which integrates a phase-gradient plasmonic metasurface on the fiber tip. For illustration and validation purposes, we present numerical and experimental results pertaining to various prototypes implementing generalized forms of the Snell's transmission/reflection laws at near-infrared wavelengths. In particular, we demonstrate several examples of beam steering and coupling with surface waves, in fairly good agreement with theory. Our results constitute a first step toward the integration of unprecedented (metasurface-enabled) light-manipulation capabilities in optical-fiber technology. By further enriching the emergent 'lab-on-fiber' framework, this may pave the way for the widespread diffusion of optical metasurfaces in real-world applications to communications, signal processing, imaging and sensing. Light: Science & Applications (2017) 6, e16226; doi:10.1038/lsa.2016.226; published online 10 March 2017
Fiber optics; metasurfaces; plasmonics; wavefront manipulation
Metamaterials are artificial composites, which attain their distinctive
properties from a careful structural arrangement of dielectric and/or
metallic subwavelength-sized constituents, rather than their chemical
composition1. Over the past 15 years, they have received an
exponentially growing interest in many scientific and engineering
fields, as a possible route to achieve unconventional light-matter
interaction effects (such as ?negative? refraction2 and ?superlensing?3),
as well as unprecedented field-manipulation capabilities via proper
spatial tailoring of the constitutive parameters4. In spite of such
extremely promising prospects, the practical applications of
metamaterials to optics and photonics remain limited, mainly due to the
significant technological challenges posed by the fabrication process of
three-dimensional (3-D) bulk nanostructures5?7. This has generated a
surge of interest in 2-D implementations (?metasurfaces?), thanks to
the easier fabrication as well as on-chip integrability.
Similar to reflect-arrays and transmit-arrays at radio-frequency8,9,
optical metasurfaces exploit 2-D arrays of resonating elements to
spatially tailor the phase and amplitude distributions of an incident
wave field. However, unlike radio-frequency implementations, optical
implementations can exhibit a deeply-subwavelength profile, by
relying, e.g., on plasmonic10 or dielectric elements11. Stimulated by
the groundbreaking studies by Yu et al.12, which demonstrated the
capabilities of metasurfaces based on V-shaped plasmonic
nanoantennas in terms of anomalous reflection/refraction and wavefront
shaping, several research groups have explored and proposed various
alternative designs13?24. Currently, ?flat? optics and photonics25?27
constitute a very promising research thrust, with a plethora of
potential applications to various fields, ranging from imaging to
As highlighted in recent influential review papers27,34, among the
possible developments, it appears of particular strategic importance
the integration of metasurfaces with fiber-optics technology. Indeed, as
this technology has established itself in communications systems and
has recently expanded to sensing applications, the increasing demand
for better performance and advanced functionalities has led to the
exploration of new fabrication strategies and concepts. Within this
framework, the emergent ?lab-on-fiber? paradigm35?37 envisages
fiberoptics platforms integrated with nanostructured photonic and/or
plasmonic materials capable of controlling the light at the nanoscale.
This represents a very promising pathway to novel ?all-in-fiber?
multifunctional nanoprobes hosting ultracompact labs, which can
disruptively enlarge the conventional fiber-optics functionalities, and
may find a broad variety of applications including optical processing,
environmental and life science, homeland security and so on37. To
deal with unusual substrates such as an optical-fiber tip, several
fabrication processes have been developed36, leading to the realization
of multifunctional and multi-responsive nanoprobes for applications
including optical-fiber tweezers38?40, in vivo single molecule
imaging41,42, scanning near-field optical microscopy43?46, fiber top
cantilevers47 and sensing48?51.
Within the lab-on-fiber framework, the integration of metasurfaces
would constitute a crucial step forward, as it would provide
unprecedented light-manipulation capabilities. Likewise, the
technological maturity and broad diffusion of fiber optics in real-world
applications may substantially boost the practical applicability of
Here, we show that phase-gradient plasmonic metasurfaces can be
successfully fabricated on an optical-fiber tip. As a proof-of-concept of
these ?meta-tips? (MTs), we design and fabricate several prototypes
that implement the steering of an impinging beam in desired
directions. Moreover, with a view toward sensing applications, we
exploit this mechanism to excite surface waves, whose interplay with
the nanoantenna resonances may provide further degrees of freedom
to enhance the sensitivity of plasmonic nano-arrays.
MATERIALS AND METHODS
Idea and geometry
The basic idea underlying our MT design is outlined in Figure 1a. We
consider the tip facet of an optical fiber covered (over the entire core
region) by a plasmonic metasurface, which impresses a desired phase
profile in a suitably-polarized field component. As an example, in
the present study we assume a linear-phase distribution along the
x-direction. Accordingly, a light beam normally incident from the core
region will undergo a splitting in transmission (and reflection, not
shown in Figure 1a schematic), with: (i) an ordinary component
experiencing no phase-gradient; and (ii) the emergence of an
anomalous beam (with generally different polarization) steered of an
angle along the x-direction. Such polarization-conversion mechanism
is inherent of single-layer implementations of plasmonic metasurfaces,
as a device to attain a 2? phase span in the transmitted field26. As
schematically illustrated in Figure 1b for the specific case of interest
(normal incidence), the phenomenon can be modeled via generalized
1 l 1 l
sin yt ? next2pgx; sin yr ? nfiber2pgx
which relate the steering angles in transmission (?t) and reflection (?r)
to the metasurface-induced phase-gradient ?x, with nfiber and next
denoting the refractive indexes of the fiber and external regions,
respectively, and ? the vacuum wavelength. Clearly, in the absence of
phase-gradient (?x = 0), Equations (
) trivially reduce to the
conventional Snell?s laws. Our metasurface design, inspired by Babinet?s
principle, features an ?inverted? configuration obtained by patterning a
50 nm gold layer with rectangular nanoholes of variable size. Besides
being particularly suited to focused-ion-beam (FIB) nanofabrication
processes, this inverted design has been shown to provide a higher
polarization efficiency than the more conventional (nanopatch)
configuration17. The unit-cell geometry (Figure 1c) basically comprises
a rectangular nanohole rotated by 45? in the x ? y plane.
The design of Babinet-inverted plasmonic metasurfaces has been
discussed in Ni et al.17. It is well known that the optical properties
of periodically nanopatterned-metal layers generally depend on the
complex interplay between a surface plasmon polariton (which can
couple to the normally impinging light in view of the array-induced
phase-matching mechanism) and the localized waveguide modes
supported by the nanoholes52. As shown in Ni et al.17, by properly
tuning the nanohole sidelengths L1 and L2 so as to work in a
neighborhood of such resonance frequency, an arbitrary phase (within
the full 2? range) can be impressed in the transmitted/reflected
components with suitable polarization. More specifically, as a
consequence of the nanohole symmetry (Figure 1c), the local transmission
coefficient of the cross-polarized component is identical for normal
plane-wave incidence with x- or y-polarized electric field. As
anticipated, this implies that a normally-incident illumination with
linearlypolarized electric field forming an angle ? with the x-axis yields two
transmitted beams (see the pictorial sketch in Figure 1a): an ordinary
one, co-polarized, and (ideally) experiencing zero phase-gradient; and
an anomalous one, with polarization direction rotated by an angle
Incident beam Single-mode fiber core
(90? ? ?), and experiencing the steering effect impressed by the
metasurface26. The two beams are co-polarized for illumination
polarized along the symmetry axis x = y (? = 45?), and orthogonally
polarized for x- or y-polarized illumination (i.e., ? = 0 or ? = 90?,
respectively). In our design procedure, we first compute (numerically)
the co- and cross-polar transmission coefficients of a 2-D periodic
array (with lx = ly = 1 ?m) made of identical nanoholes. More
specifically, assuming x-polarized plane-wave excitation at ? = 1.56 ?m, we
consider moderate variations of L1 and L2 around their resonance
values (which can be roughly estimated via simple analytical
models53), so as to generate some ?look-up? maps, shown in
Supplementary Fig. S1, which directly relate the nanohole dimensions
to the cross-polar transmission-coefficient phase.
As a proof-of-concept, we realized via FIB milling five MT
prototypes, hereafter indicated as MTm (m = 1,?,5). More specifically,
at the telecom wavelength ? = 1.56 ?m, we implemented various
representative values of the phase-gradient ?x, whereas maintaining a
constant phase distribution along y. These phase distributions are
synthesized via a ?supercell? comprising N unit cells as in Figure 1c,
arranged along the x-direction with period lx, and suitably modulated
dimensions L1 and L2. In particular, by exploiting the previously
computed look-up maps, these dimensions are chosen in such a way
an incremental phase difference DF ? 2p=N is attained in the
crosspolar transmission and reflection coefficients between neighbor unit
cells. Replication of such ?supercell? along x (with period Lx ? N lx)
and y (with period Ly ? ly ? 1 mm) yields the desired phase
distribution, with gradient gx ? DF=lx. Further details on the design
procedure are provided in the Supplementary Information.
Table 1 summarizes the main parameters of the five designs. For the
MT1?MT4 designs, which implement the beam steering, the nominal
transmission angles ?t of the anomalous beams are indicated. The MT5
design, instead, implements the excitation of a surface wave. For all
designs, only the sidelengths L1 and L2 of the nanoholes in the first half
of the supercell are explicitly given; the elements in the second half are
obtained by a rotation of 90? in the x?y plane, which provides a ?
phase shift in the cross-polar scattering parameters.26
For the metasurface design, as well as for the computation of the
farfield profiles and the field maps shown hereafter, we rely on the
finiteelement software package COMSOL Multiphysics (www.comsol.com).
For cross-validation, as well as for the computation of reflectivity
spectra, we utilize a 2-D version of the
rigorous-coupled-waveanalysis54 (RCWA) implemented in a public-domain numerical code
(www.sourceforge.net/projects/rcwa-2d/files/). In all simulations, we
consider a standard dispersion model for gold55, and nondispersive,
lossless models for silica (optical fiber) and SiOx (overlay for
surfacesensitivity characterization), with refractive indexes nfiber ? nSiO2 ?
1:45 and nSiOx ? 1:7, respectively. The external region is assumed as
air (next ? 1). Further details can be found in the Supplementary
We start from a Corning SMF-28 single-mode fiber (with core and
cladding diameters of 8 and 125 ?m, respectively), which is cleaved to
obtain a smooth surface. A segment (of about 10 mm) of the fiber is
first subjected to ethanol rinsing, and subsequently positioned on an
apposite sample holder. The fiber tip is coated with a 50 nm gold layer
via electron beam evaporation (Kenosistec CL400C, Binasco (MI),
Italy), with adhesion enhanced by a 2 nm intermediate chrome layer.
A FIB instrument (Quanta 200 3D FEI, Hillsboro, OR, USA) is used to
pattern the gold layer, by using 50 pA beam current and 30 kV
accelerating voltage. The desired pattern is milled (with 5000 ?
magnification) by rastering the ion beam via parallel writing strategy,
and employing an input text file where all the rectangular holes are
defined in terms of size, spatial coordinates and rotation angles.
Results from the morphological characterization of the samples can be
found in Supplementary Fig. S3 and Supplementary Table S1.
It is worth stressing that the patterned areas (Table 1) are
dimensioned so as to cover a substantial part of the fiber mode
region. It can readily be estimated that at the edges of these areas the
illuminating field profile (approximately a Gaussian beam with waist
size of 5 ?m) has decayed at least 24 dB below its peak value, and is
therefore effectively negligible. This is also consistent with
experimental indications from previous lab-on-fiber studies56.
As previously mentioned, four prototypes (MT1?MT4) implement
the beam steering with various angles. On the other hand, as it will be
clear hereafter, the MT5 prototype (as well as a phase-gradient-free
benchmark) is designed having in mind sensing applications. Within
this framework, to characterize its surface sensitivity, the sample is
placed inside a very high frequency plasma enhanced chemical vapor
deposition (PECVD) chamber using a suitable holder that locates it in
the correct position. A thin SiOx layer is deposited by using an
ultrahigh vacuum cluster tool deposition system (MVSystems Inc., Golden,
CO, USA). The fiber tip is placed at a distance less than 15 mm from
the electrode. The process is carried out at 150? temperature, 2.5 Torr
pressure and 6 W power. Pure silane, hydrogen and carbon dioxide
are used, with a deposition rate of about 1.88 ? s ? 1. Via an
atomicforce-microscope measurement, an overlay thickness value of about
40 nm is verified (see Supplementary Information for details).
The far-field characterization of the fabricated MT samples
implementing the beam steering is carried out by means of an experimental
setup relying on an infrared vidicon camera (see the schematic in
Supplementary Fig. S4). Precise and repeatable positioning of different
samples is ensured by the use of an ad-hoc plastic holder for the fiber
MT. The sample end-face is positioned in the vicinity of the camera by
gx ?rad cm 1?
means of a compact 3-axis micrometer positioning system. Precise
control of the MT distance from the camera receiving window (i.e.,
the input glass face of the vidicon tube) is allowed by a position
reference mounted onto the plastic holder. The fiber MT is
illuminated with a narrowband laser source (centered at ? = 1.56 ?m) by
means of a tunable laser (Yokogawa/Ando AQ4321A, Tokyo, Japan),
and the transmitted far-field is collected by the vidicon camera
(Hamamatsu C2741-03 camera head + C2471 camera controller;
www.hamamatsu.com). The camera head is shielded from visible light
to improve the optical quality of the digital images (786 ? 576 pixels,
256 gray levels), which are acquired via a PCI-1407 IMAQ acquisition
board, sent to a display and transferred to a personal computer for
post-processing. In particular, the field-intensity maps shown hereafter
are measured by positioning the MT at a distance of 4 mm from the
receiving window of the camera; taking into account the distance
between this latter and the photoconducting target inside the vidicon
tube (4 mm), a total distance of 8 mm is estimated between the MT
and the target plane. The transmission angles are estimated via a
differential measurement scheme (Supplementary Fig. S5). For the
polarization measurements, a fiber polarization controller (Thorlabs
FPC560, Newton, NJ, USA) is connected at the output of the tunable
laser, and a linear polarizer (Thorlabs LPIREA100-C, Newton, NJ,
USA) is mounted on a continuous rotation mount (Thorlabs CRM1/
M, Newton, NJ, USA) positioned right before the vidicon camera (see
Supplementary Information for more details). In this case, a distance
of 5.9 mm is estimated between the MT and the target plane.
For the surface-sensitivity characterization of the MT5 prototype
(and its phase-gradient-free benchmark), a standard reflection setup
(schematized in Supplementary Fig. S6) is utilized. The fiber MT is
illuminated by means of a supercontinuum light source (covering the
wavelength range 1.1?2.4 ?m), whereas a 2 ? 1 directional coupler is
used to redirect the reflected signal to an optical spectrum analyzer
(Yokogawa/Ando AQ6317C, having a wavelength range 600?
1750 nm). The acquired spectrum is transferred (via a general purpose
interface bus connection) to a personal computer for post-processing.
Further details are provided in the Supplementary Information.
RESULTS AND DISCUSSION
With specific reference to the MT1 and MT3 designs, Figure 2 shows
the numerically-synthesized magnitude and phase profiles of the
transmission coefficients (for normally-incident x-polarized
illumination) over a supercell. Approximately linear-phase distributions (with
different gradients) are observed for the cross-polarized components,
whereas the co-polar phase as well as the two magnitude distributions
remain more or less uniform. The corresponding distributions for the
MT2 and MT4 design are shown in Supplementary Fig. S2.
Figure 3 displays some scanning-electron-microscope (SEM) images
of the MT3 prototype, including the fiber tip and two magnified details
of the metasurface.
With reference to the MT1 and MT3 samples, Figure 4 summarizes
the far-field characterization at the operational wavelength
? = 1.56 ?m, without direct polarization control of the incident and
transmitted fields. More specifically, Figure 4b and 4e show the
measured field-intensity maps at 8 mm from the MT samples. As a
reference, Figure 4a and 4d show the simulated co-polar and
crosspolar (blue and red curves, respectively) intensity profiles at z = 8 mm
and y = 0, averaged over the x- and y-polarized illuminations. In spite
of the lack of polarization control, the comparison between the
numerical profiles and the measured intensity maps still allows a
clearcut interpretation of the main and secondary peaks in terms of the
ordinary (co-polarized) and anomalous beams, respectively. Overall, as
shown in Figure 4c and 4f, a fairly good agreement is observed
between the measured intensity profiles (black-solid curves)
and corresponding simulations (magenta-dashed curves). The
anomalous refraction angles estimated from the experimental data are
yt?exp? ? 11:31 (for MT1) and yt?exp? ? 21:01 (for MT3), in very good
accord with the theoretical estimates from Equation (
) (given in Table 1).
The numerical results also correctly reproduce some secondary side
lobes in the ordinary and anomalous beams, especially visible in the
MT1 case, which can be attributed to slight nonlinearities in the
designed phase profiles (see, e.g., Figure 2a) as well as truncation effects.
As an ultimate validation of the phenomenon, we also carried out
polarization measurements (see Supplementary Information for
details). With reference to the MT3 sample, Figure 5 shows the
farfield maps (at z = 5.9 mm) assuming a y-polarized incident field, and
selecting three representative linear polarization states of the
transmitted field. The assumed incident polarization yields a particularly
clear-cut difference between the ordinary and anomalous beams,
which are co- and cross-polarized, respectively. Accordingly, Figure 5a
shows the y-polarized field map, where a main peak is clearly
observed, representative of the (co-polarized) ordinary beam. As also
observed in the previous measurements (without polarization control),
the peripheral minor peak is attributable to nonidealities. Figure 5c
shows instead the x-polarized field map, where the ordinary beam
(and the peripheral one) disappears, and a different peak appears,
representative of the (cross-polarized) anomalous beam. For an
intermediate case, pertaining to a 45? oblique polarization
(Figure 5b), both peaks are correctly observed. Supplementary
Movie 1 shows the evolution of the measured field map with a finer
sampling (5?) of the selected polarization state in transmission, from
which it can be observed the gradual disappearing of the ordinary
beam and the appearing of the anomalous one.
Discrepancies between experimental and numerical results are
mainly attributable to fabrication tolerances in the gold-layer thickness
as well as in the dimensions of the nanoholes (see the Supplementary
1 2 3 4 5 6
Figure 2 Results from the design procedure (MT1 and MT3). (a, b)
Numerically-synthesized phase and magnitude distributions, respectively, of
the transmission coefficient pertaining to the single nanoholes in the supercell
(shown on top) of the MT1 design (with parameters as given in Table 1), for
the co-polarized (blue square markers) and cross-polarized (red circle markers)
components, assuming an infinite periodic array of period lx ? ly ? 1 mm,
under normally-incident x-polarized plane-wave illumination at l ? 1:56 mm.
Element #1 is chosen as phase reference. Continuous curves are guides to
the eye only. (c, d) Same as above, but for MT3 design.
Information for the morphological characterization). Other potential
sources of uncertainty are associated with the FIB milling process,
which is known to induce a doping of the fiber glass57, and hence
unmodeled shifts in the resonance wavelength of the nanoholes.
Nevertheless, our study indicates that the proposed designs are quite
robust with respect to the above fabrication-related effects.
Qualitatively similar results are observed for the other two
beamsteering design prototypes MT2 and MT4, as shown in Supplementary
We also estimated numerically the efficiency, in terms of the
fraction of the impinging power that gets transferred to the anomalous
beam (see Supplementary Information for details). As summarized in
Table 2 for the four beam-steering prototypes, values ranging within
7?12% are obtained. These results are essentially in line with the 10%
figures observed in previous studies on Babinet-inverted plasmonic
metasurfaces in planar technology17. Nevertheless, there are several
alternatives (e.g., Huygens? dielectric metasurfaces23) that can provide
much higher efficiencies.
Figure 5 Far-field characterization (MT3) with polarization control. (a?c)
Measured field-intensity maps at z ? 5:9 mm for MT3 sample
(with parameters as given in Table 1) pertaining to the y-, oblique (45?) and
x-polarized components, respectively, and assuming a y-polarized incident
field (see also Supplementary Movie 1 for a finer sampling of the selected
Moreover, we highlight that, although the general character of the
underlying resonance phenomenon is not narrowband, our
investigation was inherently restricted within the narrow spectral operational
range (1520?1620 nm) of the tunable laser source utilized. Within this
wavelength range, we observed negligible variations of the efficiency,
and variations up to approximately 7 21 in the anomalous
beamsteering angle. These values are larger than our estimated
measurement uncertainty (see Supplementary Information for details),
and are in line with the theoretical predictions from Equation (
(with the phase-gradient gx assumed as constant).
It is worth pointing out that, via suitably large values of gx, it is
possible to drive the anomalous transmitted beam in the evanescent
range, so as to couple it with a surface wave that propagates along the
x-direction at the MT interface, and is exponentially bound along
z58,59. In what follows, we explore possible applications of this
mechanism to sensing scenarios.
Perspectives in sensing applications
During the last decade, plasmonic nanosensors have established
themselves as advanced tools for biosensing applications. A broad
variety of configurations have been proposed, and the field is
fastpaced and growing. A common aspect of the ongoing research efforts
remains the continuous quest for improved sensitivity60,61, and
different strategies have been so far adopted, mainly relying on the
proper choice of the nanostructure geometry and dimension, plasmon
coupling effects and amplification mechanisms based on nanoparticle
One might wonder to what extent the aforementioned mechanism
of surface-wave excitation mediated by the phase-gradient could
provide further degrees of freedom to tailor the plasmonic sensitivity
of our MTs. To this aim, we investigated (both theoretically and
experimentally) the possibility to exploit such mechanism to enhance
the light-matter interaction and, hence, to provide an increased
sensitivity to local refractive index changes with respect to standard
(phase-gradient-free) plasmonic benchmarks.
Within this framework, the implied high values of phase-gradient
can be attained in our configuration by reducing the number of
nanoholes per unit cell and/or by increasing the phase difference DF
between neighbor elements. In our implementation (MT5 design in
Table 1), we consider DF ? p (i.e., the maximum value that still
guarantees the correct reconstruction of the linear-phase profile), and
lx ? 0:53 mm (which ensures that the anomalous reflected beam too
is driven in the evanescent range). Figure 6a shows a SEM image of the
fabricated prototype. The resulting supercell (see also the inset in
Figure 7c) is reduced to two identical nanoholes, rotated of 90? in the
x?y plane; the underlying symmetry implies that the excited surface
waves travel along both ? x-directions63. Such configuration is
particularly interesting as it admits a simple phase-gradient-free
counterpart (which will be used as a benchmark to compare the
sensing performance), thereby allowing some insightful considerations
on the effects of the phase-gradient, as discussed hereafter.
Figure 7a and 7b show (red-dashed curves) the measured and
simulated reflectivity spectra, respectively, pertaining to the MT5
sample; they both exhibit a resonant dip centered around
l ? 1:47 mm (l ? 1:46 mm in the simulated spectrum), and are in
quite good agreement both from the qualitative and quantitative
viewpoints. To gain some insight in the nature of the observed
resonance, Figure 7c and 7d show the simulated field maps (in the
x z plane, nearby the MT interface) at the resonant wavelength, for
x-polarized plane-wave illumination. A local field-enhancement,
attributable to the resonance of the single nanoholes, can be observed
in both the co-polarized and cross-polarized components. However,
the transmitted co-polarized field (not affected by the metasurface
phase-gradient) shows a propagating character, whereas the
crosspolarized field clearly exhibits the distinctive features of a surface wave.
This is more evident in the corresponding field-cuts (along the
z-direction) shown in Figure 7e and 7f, respectively.
1.4 1.5 1.6
As a meaningful benchmark configuration (see the SEM image in
Figure 6b and the supercell in the inset of Figure 8e), we selected a
periodic metasurface featuring the same nanoholes as in the MT5
configuration, but without the 90? rotation. In this way, the
phasegradient effects are removed, whereas the resonant effects related to
the rectangular nanoholes are preserved. By comparing the sensing
responses of the MT5 and benchmark design, we can therefore
highlight the possible effects of the phase-gradient.
The measured and simulated reflectivity spectra of the benchmark
sample (shown in Figure 8a and 8b, respectively) exhibit a resonant
dip centered at a higher wavelength (l ? 1:62 mm) than the previous
case. Once again, measurements and simulations agree fairly well.
From the simulated field maps at the resonant wavelength (Figure 8c
and 8d), similarly to the phase-gradient MT5 case (cf. Figure 7c and
7d), we observe a local field-enhancement attributable to the
resonance of the nanohole (chosen as identical in the two designs).
However, an important difference is that now both the co-polarized
and cross-polarized transmitted components exhibit propagating
characteristics; this clearly highlights the impact of the
phasegradient entailed by the MT5 design.
Another important aspect worth exploring is the effect of the
phasegradient in establishing the local field distribution occurring at the MT
surface. To this aim, Figure 9 compares the two resonant field
distributions occurring at the MT interface. From the field maps
(Figure 9a and 9b) and corresponding cuts (Figure 9c and 9d), it is
evident that the MT5 configuration exhibits a sensibly higher
fieldenhancement at the interface, which is crucial in determining the
surface sensitivity to local refractive index variations30.
Having in mind label-free chemical and biological sensing
applications, we evaluate (and compare) the surface sensitivity of both the
phase-gradient MT and its gradient-free benchmark, by considering
the resonance wavelength-shift produced by the deposition of a
nanosized dielectric overlay56,64. Accordingly, we repeat the reflectivity
measurements of both samples after the deposition of a 40 nm SiOx
overlay (refractive index nSiOx ? 1:7, see Supplementary Information
for details). For a more direct comparison with the responses in the
absence of the overlay, these new measured spectra are also shown
(blue-solid curves) in Figures 7a and 8a. A higher redshift (224 nm) is
observed for the phase-gradient MT5 sample when compared with the
gradient-free benchmark (132 nm). This is also consistent with the
numerical predictions (blue-solid curves in Figures 7b and 8b).
This first and interesting observation opens up intriguing
perspectives in the exploitation of phase-gradient as a further degree of
freedom in determining and tailoring the surface sensitivity of
plasmonic nanostructures. However, further in-depth investigations
are required to ascertain the role and interplay of the various
phasegradient-induced phenomena (surface wave, field enhancement) in the
observed surface-sensitivity enhancement.
To sum up, we have demonstrated a proof-of-concept MT that
integrates a phase-gradient plasmonic metasurface on an optical-fiber
tip. Our results represent a first, but important step toward endowing
the pervasive fiber-optics technology with unprecedented
metasurfaceenabled light-manipulation capabilities. From one side, this may
dramatically increase the insofar quite limited impact and applicability
of optical metasurfaces in real-world scenarios. From the viewpoint of
the emerging lab-on-fiber paradigm, it represents an enabling factor
with potentially disruptive implications, which significantly broadens
the possible functionalities and application perspectives.
Within this framework, possible direct applications may include
active beam profilers, spatial light modulators and fiber-optic tweezers.
MT-based ?flat-optics? may also open up new venues in biomedical
imaging, including scanning near-field optical microscopy and in vivo
single molecule imaging. Also of great interest is the exploration of
metasurface-based analog computing (along the lines of Silva et al.32),
as well as tunability/reconfigurability mechanisms (e.g., electro-optic,
magneto-optic, fluid-based) for the design of novel active,
reconfigurable optical switches and frequency-agile nanodevices. In this context,
the exploration of more efficient (e.g., dielectric-based) metasurface
implementations is also of great interest, and is currently being
Finally, the results of our prototype study indicate promising
perspectives in sensing applications. Especially, we found that the
metasurface-induced phase distribution may affect the surface
sensitivity in ways unexplored in the past. This brings about additional
degrees of freedom and sophistication in the design and optimization
of nanoplasmonic label-free chemical and biological sensors. We are
currently working on the optimization of the phase distribution for
maximizing the surface sensitivity, as well as on gaining a deeper
insight of the role and interplay of the underpinning phenomena.
CONFLICT OF INTEREST
The authors declare no conflict of interest.
VG and ACus conceived the experiment. MP and GC carried out the design
and numerical simulations. AM, ACre, EE and VLF fabricated the prototypes.
MC performed the experimental characterization and processed the
measurement data. ACut, VG, ACus supervised the study. MP, VG and ACus wrote the
manuscript, with inputs from all other authors.
Supplementary Information for this article can be found on the Light: Science & Applications? website (http://www.nature.com/lsa).
1 Capolino F. Theory and Phenomena of Metamaterials. Boca Raton, FL, USA: CRC Press; 2009 .
2 Smith DR , Padilla WJ , Vier DC , Nemat-Nasser SC , Schultz S. Composite medium with simultaneously negative permeability and permittivity . Phys Rev Lett 2000 ; 84 : 4184 - 4187 .
3 Pendry JB . Negative refraction makes a perfect lens . Phys Rev Lett 2000 ; 85 : 3966 - 3969 .
4 Pendry JB , Schurig D , Smith DR . Controlling electromagnetic fields . Science 2006 ; 312 : 1780 - 1782 .
5 Valentine J , Zhang S , Zentgraf T , Ulin-Avila E , Genov DA et al. Three-dimensional optical metamaterial with a negative refractive index . Nature 2008 ; 455 : 376 - 379 .
6 Liu N , Guo HC , Fu LW , Kaiser S , Schweizer H et al. Three-dimensional photonic metamaterials at optical frequencies . Nat Mater 2008 ; 7 : 31 - 37 .
7 Chanda D , Shigeta K , Gupta S , Cain T , Carlson A et al. Large-area flexible 3D optical negative index metamaterial formed by nanotransfer printing . Nat Nanotechnol 2011 ; 6 : 402 - 407 .
8 Berry DC , Malech RG , Kennedy W. The reflectarray antenna . IEEE Trans Antennas Propagat 1963 ; 11 : 645 - 651 .
9 McGrath D. Planar three -dimensional constrained lenses . IEEE Trans Antennas Propagat 1986 ; 34 : 46 - 50 .
10 Meinzer N , Barnes WL , Hooper IR . Plasmonic meta-atoms and metasurfaces . Nat Photonics 2014 ; 8 : 889 - 898 .
11 Zou LF , Withayachumnankul W , Shah CM , Mitchell A , Bhaskaran M et al. Dielectric resonator nanoantennas at visible frequencies . Opt Express 2013 ; 21 : 1344 - 1352 .
12 Yu N , Genevet P , Kats MA , Aieta F , Tetienne JP et al. Light propagation with phase discontinuities: generalized laws of reflection and refraction . Science 2011 ; 334 : 333 - 337 .
13 Memarzadeh B , Mosallaei H . Array of planar plasmonic scatterers functioning as light concentrator . Opt Lett 2011 ; 36 : 2569 - 2571 .
14 Ni XJ , Emani NK , Kildishev AV , Boltasseva A , Shalaev VM . Broadband light bending with plasmonic nanoantennas . Science 2012 ; 335 : 427 .
15 Pfeiffer C , Grbic A. Metamaterial Huygens' surfaces: tailoring wave fronts with reflectionless sheets . Phys Rev Lett 2013 ; 110 : 197401 .
16 Monticone F , Estakhri NM , Al? A . Full control of nanoscale optical transmission with a composite metascreen . Phys Rev Lett 2013 ; 110 : 203903 .
17 Ni XJ , Ishii S , Kildishev AV , Shalaev VM . Ultra-thin, planar, Babinet-inverted plasmonic metalenses . Light Sci Appl 2013 ; 2 : e72; doi:10.1038/lsa2013. 28 .
18 Farmahini-Farahani M , Mosallaei H . Birefringent reflectarray metasurface for beam engineering in infrared . Opt Lett 2013 ; 38 : 462 - 464 .
19 Lin DM , Fan PY , Hasman E , Brongersma ML . Dielectric gradient metasurface optical elements . Science 2014 ; 345 : 298 - 302 .
20 Kim M , Wong AMH , Eleftheriades GV . Optical Huygens' metasurfaces with independent control of the magnitude and phase of the local reflection coefficients . Phys Rev X 2014 ; 4 : 041042 .
21 Cheng JR , Ansari-Oghol-Beig D , Mosallaei H . Wave manipulation with designer dielectric metasurfaces . Opt Lett 2014 ; 39 : 6285 - 6288 .
22 Ding XM , Monticone F , Zhang K , Zhang L , Gao DL et al. Ultrathin Pancharatnam-Berry metasurface with maximal cross-polarization efficiency . Adv Mater 2015 ; 27 : 1195 - 1200 .
23 Decker M , Staude I , Falkner M , Dominguez J , Neshev DN et al. High-efficiency dielectric Huygens' surfaces . Adv Opt Mater 2015 ; 3 : 813 - 820 .
24 Arbabi A , Horie Y , Bagheri M , Faraon A . Dielectric metasurfaces for complete control of phase and polarization with subwavelength spatial resolution and high transmission . Nat Nanotechnol 2015 ; 10 : 937 - 943 .
25 Kildishev AV , Boltasseva A , Shalaev VM . Planar photonics with metasurfaces . Science 2013 ; 339 : 1232009 .
26 Yu NF , Genevet P , Aieta F , Kats MA , Blanchard R et al. Flat optics: Controlling wavefronts with optical antenna metasurfaces . IEEE J Select Topics Quantum Electron 2013 ; 19 : 4700423 .
27 Yu NF , Capasso F . Flat optics with designer metasurfaces . Nat Mater 2014 ; 13 : 139 - 150 .
28 Aieta F , Genevet P , Kats MA , Yu NF , Blanchard R et al. Aberration-free ultrathin flat lenses and axicons at telecom wavelengths based on plasmonic metasurfaces . Nano Lett 2012 ; 12 : 4932 - 4936 .
29 Aieta F , Kats MA , Genevet P , Capasso F . Multiwavelength achromatic metasurfaces by dispersive phase compensation . Science 2015 ; 347 : 1342 - 1345 .
30 Yang YM , Kravchenko II , Briggs DP , Valentine J . All-dielectric metasurface analogue of electromagnetically induced transparency . Nat Commun 2014 ; 5 : 5753 .
31 Zhang XJ , Wu Y . Effective medium theory for anisotropic metamaterials . Sci Rep 2015 ; 5 : 7892 .
32 Silva A , Monticone F , Castaldi G , Galdi V , Alu A et al. Performing mathematical operations with metamaterials . Science 2014 ; 343 : 160 - 163 .
33 Pors A , Nielsen MG , Bozhevolnyi SI . Analog computing using reflective plasmonic metasurfaces . Nano Lett 2015 ; 15 : 791 - 797 .
34 Yu NF , Capasso F . Optical metasurfaces and prospect of their applications including fiber optics . J Lightwave Technol 2015 ; 33 : 2344 - 2358 .
35 Albert J. A lab on fiber . IEEE Spectr 2014 ; 51 : 48 - 53 .
36 Kostovski G , Stoddart PR , Mitchell A. The optical fiber tip: An inherently light-coupled microscopic platform for micro- and nanotechnologies . Adv Mater 2014 ; 26 : 3798 - 3820 .
37 Cusano A , Consales M , Crescitelli A , Ricciardi A . Lab-on-Fiber Technology . New York, NY, USA: Springer; 2015 .
38 Berthelot J , A?imovi? SS , Juan ML , Kreuzer MP , Renger J et al. Three-dimensional manipulation with scanning near-field optical nanotweezers . Nat Nanotechnol 2014 ; 9 : 295 - 299 .
39 El Eter A , Hameed NM , Baida FI , Salut R , Filiatre C et al. Fiber-integrated optical nano-tweezer based on a bowtie-aperture nano-antenna at the apex of a SNOM tip . Opt Express 2014 ; 22 : 10072 - 10080 .
40 Gelfand R M , Wheaton S , Gordon R . Cleaved fiber optic double nanohole optical tweezers for trapping nanoparticles . Opt Lett 2014 ; 39 : 6415 - 6417 .
41 H?ppener C , Novotny AL . Antenna-based optical imaging of single Ca2+ transmembrane proteins in liquids . Nano Lett 2008 ; 8 : 642 - 646 .
42 van Zanten TS , Lopez-Bosque MJ , Garcia-Parajo MF . Imaging individual proteins and nanodomains on intact cell membranes with a probe-based optical antenna . Small 2010 ; 6 : 270 - 275 .
43 Taminiau TH , Moerland RJ , Segerink FB , Kuipers L , van Hulst NF. ?/4 resonance of an optical monopole antenna probed by single molecule fluorescence . Nano Lett 2007 ; 7 : 28 - 33 .
44 Mivelle M , Ibrahim IA , Baida F , Burr GW , Nedeljkovic D et al. Bowtie nano-aperture as interface between near-fields and a single-mode fiber . Opt Express 2010 ; 18 : 15964 - 15974 .
45 Vo T-P , M ivelle M , Callard S , Rahmani A , Baida F et al. Near-field probing of slow Bloch modes on photonic crystals with a nanoantenna . Opt Express 2012 ; 20 : 4124 - 4135 .
46 Mivelle M, van Zanten TS , Garcia-Parajo MF . Hybrid photonic antennas for subnanometer multicolor localization and nanoimaging of single molecules . Nano Lett 2014 ; 14 : 4895 - 4900 .
47 Iannuzzi D , Heeck K , Slaman M , de Man S , Rector JH et al. Fibre-top cantilevers: design, fabrication and applications . Meas Sci Technol 2007 ; 18 : 3247 - 3252 .
48 Smythe EJ , Dickey MD , Bao JM , Whitesides GM , Capasso F . Optical antenna arrays on a fiber facet for in situ surface-enhanced Raman scattering detection . Nano Lett 2009 ; 9 : 1132 - 1138 .
49 Consales M , Ricciardi A , Crescitelli A , Esposito E , Cutolo A et al. Lab-on-fiber technology: Toward multifunctional optical nanoprobes . ACS Nano 2012 ; 6 : 3163 - 3170 .
50 Ricciardi A , Crescitelli A , Vaiano P , Quero G , Consales M et al. Lab-on-fiber technology: a new vision for chemical and biological sensing . Analyst 2015 ; 140 : 8068 - 8079 .
51 Atie EM , Xie ZH , El Eter A , Salut R , Nedeljkovic D et al. Remote optical sensing on the nanometer scale with a bowtie aperture nano-antenna on a fiber tip of scanning nearfield optical microscopy . Appl Phys Lett 2015 ; 106 : 151104 .
52 Catrysse PB , Fan SH . Propagating plasmonic mode in nanoscale apertures and its implications for extraordinary transmission . J Nanophoton 2008 ; 2 : 021790 .
53 Puscasu I , Spencer D , Boreman GD . Refractive-index and element-spacing effects on the spectral behavior of infrared frequency-selective surfaces . Appl Opt 2000 ; 39 : 1570 - 1574 .
54 Moharam MG , Grann EB , Pommet DA , Gaylord TK . Formulation for stable and efficient implementation of the rigorous coupled-wave analysis of binary gratings . J Opt Soc Am A 1995 ; 12 : 1068 - 1076 .
55 Johnson PB , Christy RW . Optical constants of the noble metals . Phys Rev B 1972 ; 6 : 4370 - 4379 .
56 Ricciardi A , Consales M , Quero G , Crescitelli A , Esposito E et al. Versatile optical fiber nanoprobes: from plasmonic biosensors to polarization-sensitive devices . ACS Photonics 2014 ; 1 : 69 - 78 .
57 Micco A , Ricciardi A , Pisco M , La Ferrara V , Cusano A . Optical fiber tip templating using direct focused ion beam milling . Sci Rep 2015 ; 5 : 15935 .
58 Sun SL , He Q , Xiao SY , Xu Q , Li X et al. Gradient-index meta-surfaces as a bridge linking propagating waves and surface waves . Nat Mater 2012 ; 11 : 426 - 431 .
59 Sun SL , Yang KY , Wang CM , Juan TK , Chen WT et al. High-efficiency broadband anomalous reflection by gradient meta-surfaces . Nano Lett 2012 ; 12 : 6223 - 6229 .
60 Brolo AG . Plasmonics for future biosensors . Nat Photonics 2012 ; 6 : 709 - 713 .
61 Li M , Cushing SK , Wu NQ . Plasmon-enhanced optical sensors: a review . Analyst 2015 ; 140 : 386 - 406 .
62 Guo LH , Jackman JA , Yang HH , Chen P , Cho NJ et al. Strategies for enhancing the sensitivity of plasmonic nanosensors . Nano Today 2015 ; 10 : 213 - 239 .
63 Zou C , Withayachumnankul W , Shadrivov IV , Kivshar YS , Fumeaux C . Directional excitation of surface plasmons by dielectric resonators . Phys Rev B 2015 ; 91 : 085433 .
64 Yu XD , Shi L , Han DZ , Zi J , Braun PV . High quality factor metallodielectric hybrid plasmonic-photonic crystals . Adv Funct Mater 2010 ; 20 : 1910 - 1916 . This work is licensed under a Creative Commons Attribution 4.0 International License . The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material . To view a copy of this license , visit http://creativecommons.org/licenses/by/4.0/ r The Author(s) 2017