Abstract
Hydrogen (H2) sensing is crucial for modern energy storage technology, which looks to hydrogen as the most promising alternative to fossil fuels. In this respect, magnesium (Mg) offers unique possibilities, since magnesium and hydrogen easily undergo a reversible hydrogenation reaction where Mg reversibly converts into MgH2. From an optical point of view, this process produces an abrupt refractive index change, which can be exploited for sensing applications. To maximize this opportunity, we envision an architecture composed of two Ag/ITO/Mg metal/dielectric resonators facing each other and displaced by 200 nm of vacuum. This structure forms a so-called Epsilon-Near-Zero (ENZ) multi-cavity resonator, in which the two internal Mg layers, used as tunneling coupling metals, are accessible to environmental agents. We demonstrate that the hydrogenation of the two Mg layers leads to substantial changes in the strong coupling between the cavities composing the entire resonator, with a consequent abrupt modification of the spectral response, thus enabling the sensing mechanism. One of the main advantages of the proposed system with respect to previous research is that the proposed multilayered architecture avoids the need for lithographic processes. This feature makes the proposed architecture inexpensive and wafer-to-chip scalable, considering that each kind of substrate from common glass to silicon can be used. Therefore, our sensing architecture offers great promise for applications in embedded H2 sensors.
Highlights
The ever-increasing demand for innovative energy sources that can replace fossil fuels is encouraging the scientific community to make increasingly significant efforts in hydrogen-related technology
hyperbolic metamaterials (HMMs) made of Pd nanowires immersed in nanoporous anodized aluminum oxide (AAO) were demonstrated to be useful for lowconcentration H2 sensing [39]
The optical response of these multilayers has been described in the framework of the analogy between quantum mechanics and classic electromagnetism that revealed that resonances arising in these structures can be considered as resonant tunneling modes, in correspondence to which the multilayer manifests a so-called “Epsilon-Near-Zero” (ENZ) effective permittivity [40]
Summary
The ever-increasing demand for innovative energy sources that can replace fossil fuels is encouraging the scientific community to make increasingly significant efforts in hydrogen-related technology. Sensors based on an optical readout can, overcome some of the most common drawbacks of electrically driven sensors, such as compactness and operativity in harsh conditions In this respect, photonic nanotechnology can offer plenty of innovative optical-based solutions, especially those developed in the fields of liquid crystals [13,14,15,16], photonic crystals [17,18,19,20,21], and plasmonics [22,23,24,25,26,27,28,29]. They remain exposed to environmental conditions; they are potentially accessible to H2 molecules, whose presence can determine their hydrogenation to form MgH2 This process is reversible and it is responsible for an abrupt refractive index change (Mg is a metal while MgH2 is a dielectric) that switches the system from a three-cavity to a single-cavity resonator. Considering its ease of fabrication, inexpensiveness, and wafer-to-chip-size scalability, the proposed structure holds great promise for applications in embedded H2 sensors
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