In this work we review the concept of optical fiber meta-tips, recently introduced by the authors. Optical fiber meta-tips come from the integration of plasmonic optical metasurfaces and the fiber optics technology. In particular, this integration is a major breakthrough in the recently emerged “Lab-On-Fiber” technology, where nanostructured platforms with special capabilities of light manipulation are integrated onto the tip of an optical fiber. Optical fiber meta-tips can, on one side, boost the real world applicability of optical metasurfaces, and, on the other side, bring about more advanced capabilities of on-fiber control and manipulation of light wavefront and polarization. With a view towards possible practical applications to label-free chemical and biological sensing, we inspect the potentialities of optical fiber meta-tips to work as sensors of local refractive index variations, and aim at studying the effect that the phase gradient impressed by the metasurface has on the surface sensitivity of proposed devices.
We realize the first optical-fiber “meta-tip” that integrates a metasurface on the tip of an optical fiber. In our proposed configuration a Babinet-inverted plasmonic metasurface is fabricated by patterning (via focused-ion-beam) an array of rectangular aperture nanoantennas in a thin gold film. Via spatial modulation of the nanoantennas size, we properly tune their resonances so as to impress abrupt arbitrary phase variations in the transmitted field wavefront. As a proof-of-principle, we fabricate and characterize several prototypes implementing in the near-infrared the beam-steering with various angles. We also explore the limit case where surface waves are excited, and its capability to work as refractive index sensors. Notably, its sensitivity overwhelms that of the corresponding gradient-free plasmonic array, thus paving the way to the use of metasurfaces for label-free chemical and biological sensing. Our experimental results, in fairly good agreement with numerical predictions, demonstrate the practical feasibility of the meta-tip concept, and set the stage for the integration of metasurfaces, and their exceptional capabilities to manipulate light, in fiber-optics technological platforms, within the emerging “lab-on-fiber” paradigm.
In this study, the possibility to excite Bloch surface waves (BSWs) on the tip of a single-mode optical fiber is explored
for the first time. In particular, we first show the possibility to achieve an on-tip excitation of BSWs, with optimized
characteristic of the arising resonances, via an “all-fiber” grating-coupled configuration. Furthermore, envisioning novel
high-performance fiber tip nanoprobes for label-free biosensing, we introduce an ad hoc design aimed at maximizing the
refractive-index sensitivity. Numerical results indicate that the estimated sensitivities are comparable with those
exhibited by current plasmonic lab-on-tip bio-probes, but are accompanied by a higher spectral selectivity. Therefore,
this preliminary work paves the way to the development of new classes of miniaturized surface-wave optical fiber
devices for low-detection-limit label-free chemical and biological sensing.
Maria Principe, Alberto Micco, Alessio Crescitelli, Giuseppe Castaldi, Marco Consales, Emanuela Esposito, Vera La Ferrara, Vincenzo Galdi, Andrea Cusano
We report on the first example of a “meta-tip” configuration that integrates a metasurface on the tip of an optical fiber. Our proposed design is based on an inverted-Babinet plasmonic metasurface obtained by patterning (via focused ion beam) a thin gold film deposited on the tip of an optical fiber, so as to realize an array of rectangular aperture nanoantennas with spatially modulated sizes. By properly tuning the resonances of the aperture nanoantennas, abrupt variations can be impressed in the field wavefront and polarization. We fabricated and characterized several proof-of-principle prototypes operating an near-infrared wavelengths, and implementing the beam-steering (with various angles) of the cross-polarized component, as well as the excitation of surface waves. Our results pave the way to the integration of the exceptional field-manipulation capabilities enabled by metasurfaces with the versatility and ubiquity of fiber-optics technological platforms.
This work provides numerical and experimental evidence that a significant difference exists between bulk and surface sensitivities in nanodevices based on Rayleigh anomalies (RAs) when employed for sensing the surrounding refractive index. In particular, recently proposed sensing schemes based on RAs in optical nanogratings are shown to be applicable only in the presence of bulk analytes. In the presence of thin overlays of analytes, instead, the (surface) sensitivity deteriorates up to two orders of magnitude by comparison with its bulk value, as well as with typical surface sensitivities exhibited by sensors based on surface plasmon resonances. This aspect is of fundamental importance as it severely limits the practical applicability of these devices to chemical and label-free biological sensing.
We report the evidence of plasmonic-photonic resonances in hybrid metallo-dielectric quasi-crystal nanostructures
composed of aperiodically-patterned low-contrast dielectric slabs backed on a metal layer. Via both experimental and
numerical studies, we characterize these resonant phenomena with specific reference to the Ammann-Beenker (quasiperiodic,
octagonal) tiling lattice geometry, and investigate the underlying physics. In particular, we show that, by
comparison with standard periodic structures, a richer spectrum of resonant modes may be excited. Such modes are
characterized by a distinctive plasmonic or photonic behavior, discriminated by their field distribution. Concerning the
possible applications, we also explore the structure functionalization via nanosized high refractive index overlays (for
resonance tuning and quality-factor enhancement), as well as its surface sensitivity to deposition of nanolayers of
materials mimicking bio-molecular binding. Overall, our results indicate the possibility of exciting a wealth of resonant
modes with state-of-the-art quality factors and sensing/tuning efficiencies, of potential interest for the development of
high-performance optical devices for communications, energy and sensing applications.
In this work, we report the first evidence of the resonant behavior of underwater acoustic sensors constituted by a Fiber
Bragg Grating (FBG) coated by a ring shaped overlay. The complex opto-acousto-mechanical interaction among an
incident acoustic wave travelling in water, the optical fiber surrounded by the ring shaped coating and the FBG inscribed
in an optical fiber is numerically analyzed by means of a commercial multiphysics software (COMSOLtm) implementing
the finite element method. The numerical analysis has been performed in the frequency range 0.5-30 kHz, typically used
in sonar applications. The retrieved numerical results, describing the opto-acoustic response of the optical hydrophone,
highlighted that the coating is able to notably improve the sensitivity of the hydrophone, in the whole investigated
frequency range, when compared to a FBG without coating. Furthermore, the hydrophone sensitivity versus the
frequency presents characteristic resonances, which strongly improve the hydrophone opto-acoustic sensitivity with
respect to the sensitivity background far from the resonances. As ascertained by means of a three-dimensional modal
analysis of the hydrophone, the composite cylindrical structure of the sensor acts as an acoustic resonator accorded to the
frequencies of the longitudinal vibration modes of the composite structure. In order to provide a full description of the
sensor performances, we carried out also a parametric analysis by varying the geometrical and mechanical properties of
the coating. The numerical results, besides outlining the resonant behavior of the coated FBG, also provided a tool for
the design and optimization of the sensor performances which in turn can be tailored for specific Sonar applications.
In this paper, we present a comparative study of the emission properties of line sources embedded in two-dimensional
finite-size aperiodically-ordered "photonic-quasicrystal" slabs made of dielectric cylinders arranged
according to representative categories of aperiodic tilings. Our study, based on a rigorous full-wave numerical
method, indicates the possibility of achieving directive low-sidelobe emission at several frequencies. In this connection,
parametric studies are presented, and similarities and differences with the periodic case are highlighted.
For the baseline design of future gravitational wave detection interferometers, use of optical cavities with nonsphericalmirrors supporting flat-top ("mesa") beams, potentially capable of mitigating the thermal noise of
the mirrors, has recently drawn a considerable attention. To reduce the severe tilt-instability problems affecting
the originally conceived nearly-flat, "Mexican-hat-shaped" mirror configuration, K. S. Thorne proposed a
nearly-concentric mirror configuration capable of producing the same mesa beam profile on the mirror surfaces.
Subsequently, Bondarescu and Thorne introduced a generalized construction that leads to a one-parameter family
of "hyperboloidal" beams which allows continuous spanning from the nearly-flat to the nearly-concentric
mesa beam configurations. This paper is concerned with a study of the analytic structure of the above family
of hyperboloidal beams. Capitalizing on certain results from the applied optics literature on flat-top beams,
a physically-insightful and computationally-effective representation is derived in terms of rapidly-converging
Gauss-Laguerre expansions. Moreover, the functional relation between two generic hyperboloidal beams is investigated.
This leads to a generalization (involving fractional Fourier transform operators of complex order)
of some recently discovered duality relations between the nearly-flat and nearly-concentric mesa configurations.
Possible implications and perspectives for the advanced Laser Interferometer Gravitational-wave Observatory
(LIGO) optical cavity design are discussed.
The limit sensitivity of interferometric gravitational wave antennas is set by the thermal noise in the dielectric mirror
coatings. These are currently made of alternating quarter-wavelength high/low index material layers with low
mechanical losses. The quarter-wavelength design yields the maximum reflectivity for a fixed number of layers, but not
the lowest noise for a prescribed reflectivity. This motivated our recent investigation of optimal thickness
configurations, which guarantee the lowest thermal noise for a targeted reflectivity. This communication provides a
compact overview of our results, involving nonperiodic genetically-engineered and truncated periodically-layered
configurations. Possible implications for the advanced Laser Interferometer Gravitational wave Observatory (LIGO) are
discussed.
In this paper, we work towards a robust approach for imaging weak-contrast objects buried under a rough soil/air interface using data from an electromagnetic GPR array. A major source of variability in observed GPR signals is due to reflection from a rough and random ground. Our approach to imaging is based on us of physical and statistical modeling techniques to estimate and compensate for this rough soil/air interface. An approximate physical model based on Gaussian beams is used to model the interaction of the illumination with the ground and estimate the surface profile. This estimated surface profile is then used to correct the raw data for the effects of the rough surface. The corrected data may subsequently be used to reconstruct the subsurface region and localize anomalies. In this stage, statistical models can be used to account for both noise and residual unmodeled effects.
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