Artificially engineered materials\char22{}metamaterials\char22{}typically alter only one heat or electromagnetic parameter at once. Researchers test a metamaterial that behaves simultaneously like a thermal concentrator and an electrical invisibility cloak.

In this paper, we report on recent experimental results obtained with fiber-Bragg-grating (FBG) hydrophones for underwater acoustic detection. The optical hydrophones under investigation consist of FBGs coated with ring-shaped polymers of different size and mechanical properties. The coating materials were selected and designed in order to provide mechanical amplification, via judicious choice of their acousto-mechanical properties and by exploiting selected resonances occurring in different frequency ranges. Our underwater acoustic measurements, carried out within the range 4–35 kHz, reveal the resonant behavior of these optical hydrophones, as well as its dependence on the coating size and type of material. These experimental data are also in good agreement with our previously published numerical results. By comparison with bare (i.e., uncoated) FBGs, responsivity enhancements of up to three orders of magnitude were found, demonstrating the effectiveness of polymeric coatings in tailoring the acoustic response of FBG-based hydrophones.

Multisection quarter‐wave impedance transformers are widely applied in microwave engineering and optics to attain impedance‐matching networks and antireflection coatings. These structures are mostly designed in the spatial domain (time harmonic) by using geometries of different materials. Here, we exploit such concepts in the time domain by using time‐varying metamaterials. We derive a formal analogy between the spectral responses of these structures and their temporal analogs, i.e., time‐varying stepped refractive‐index profiles. We show that such space‐time duality grants access to the vast arsenal of synthesis approaches available in microwave engineering and optics. This allows, for instance, the synthesis of temporal impedance transformers for broadband impedance matching with maximally flat or equi‐ripple responses, which extend and generalize the recently proposed quarter‐wave design as an antireflection temporal coating. Our results, validated via full‐wave numerical simulations, provide new insights and deeper understanding of the wave dynamics in time‐varying media, and may find important applications in space‐time metastructures for broadband frequency conversion and analog signal processing.

In this paper, we present the study of the acousto-optic behavior of underwater-acoustic sensors constituted by fiber Bragg gratings (FBGs) coated by ring-shaped overlays. Via full-wave numerical simulations, we study the complex opto-acousto-mechanical interaction among an incident acoustic wave traveling in water, the optical fiber surrounded by the ring shaped coating, and the FBG inscribed the fiber, focusing on the frequency range 0.5-30 kHz of interest for SONAR applications. Our results fully characterize the mechanical behavior of an acoustically driven coated FBG, and highlight the key role played by the coating in enhancing significantly its sensitivity by comparison with a standard uncoated configuration. Furthermore, the hydrophone sensitivity spectrum exhibits characteristic resonances, which strongly improve the sensitivity with respect to its background (i.e., away from resonances) level. Via a three-dimensional modal analysis, we verify that the composite cylindrical structure of the sensor acts as an acoustic resonator tuned at the frequencies of its longitudinal vibration modes. In order to evaluate the sensor performance, we also carry out a comprehensive parametric analysis by varying the geometrical and mechanical properties of the coating, whose results also provide a useful design tool for performance optimization and/or tailoring for specific SONAR applications. Finally, a preliminary validation of the proposed numerical analysis has been carried out through experimental data obtained using polymeric coated FBGs sensors revealing a good agreement and prediction capability.

Significance: The recently introduced concept of photonic doping in epsilon-near-zero media enables the tailoring and control of the effective magnetic response over broad regions of the parameter space, by resorting to a limited number of arbitrarily placed, nonmagnetic doping particles. Here, we theoretically extend this idea to non-Hermitian scenarios with either doping particles or host media characterized by tailored distributions of gain and loss. This considerably enlarges the attainable effective permeability parameter space, granting access to the entire complex plane. In this extended vision, losses are not considered as second-order, detrimental effects, and their interplay with gain is instrumental to enabling several intriguing effects of potential interest in reconfigurable nanophotonics and optical sensing, whose practical implementation is deferred to future studies.