Hydrogen (H₂) has attracted lots of attention as a future energy source, especially due to its high energy-density, carbon-free, and pollution-free characteristics^[1,2]. However, H₂ is flammable and has low ignition energy. At room temperature and pressure, H₂ exhibits a wide explosion range for concentrations (4%–75% by volume) coupled with a large flame propagation speed. Furthermore, as a colorless, odorless, extremely volatile gas, it diffuses very fast and easily, so it may eventually leak out of its container due to its small molecular size^[3]. Thus, fast leaks detection equipment for all H₂-related systems is required because of such safety reasons.

Electrical H₂ gas sensors with an electrical readout employ the change in electrical conductivity upon H₂ absorption in metal. However, such electrical sensors can only show enhanced sensitivity at high working temperatures, thus raising safety issues. Alternatively, in optical sensing schemes, the change in reflectance and/or transmittance of H₂-absorbing materials is detected^[4]. A fiber-based readout has arisen as well, providing numerous advantages such as safety, corrosion resistance, and suitability for remote sensing. To date, optical fiber H₂ sensors have demonstrated high performance, which propelled the technique to the hazardous areas of certain industrial environments, such as H₂ fuel filling stations or nuclear waste repository environments^[5–7]. However, such optical fiber sensors have to use special architectures to interact with the outer medium. Typical optical fiber structures, including fiber gratings^[8,9], tapered fibers^[10], D-shape^[11,12], and U-bent^[13], limit large-scale production and promotion.

In contrast with other materials solving hydrogen sensitivity such as tungsten trioxide (WO3)^[14] and zinc oxide (ZnO)^[15], palladium (Pd) is a particularly suitable and widely used functional material for specific H₂ detection because the reversible phase transition depends on the ambient H₂ concentrations at room temperature^[16]. Hydrogenation happens due to a change in the dielectric function with the change from Pd to PdHx. Since the H atoms occupy interstitial lattice sites (α-phase) and even saturate the Pd lattice (β-phase) at high H₂ concentrations, an increase of the lattice constant of over 10% would be achieved^[17].

In recent years, two main types of Pd-based H₂ sensors are thin films and plasmonic nanoparticles^[18–21]. In these two cases, the transmittance/reflectance and wavelength shift monitor can be adapted to analyze the H₂ concentration, respectively. The relative simplicity of reflectance measurements makes such Pd-material-based H₂ sensing systems more suitable for large-scale, integral sensor applications. However, Pd thin films suffer strongly from undesirable effects such as a hysteresis in the loading and unloading of H₂ and deactivation through poisoning by other gases. On the other hand, sensors based on Pd nanoparticles, which are synthesized by chemical synthesis, are limited by poor reproducibility due to the uncontrollable surface morphology of nanoparticles in chemical synthesis. Using nanoantennas is an effective alternative because of the controllable surface morphology owing to the fabrications with nanometer level accuracy, such as lithography, nanoimprinting, and others. Additionally, different nanostructure geometries and advanced optimization methods based on mathematics can be investigated to enhance the H₂ sensitivity and to calculate the suitable parameters for the complex nature of the array manufacturing to ensure the reproducibility of the sensor. Recently, H₂ detection using nanoantennas and hydrogenation of thin films have been demonstrated as well^[17,22–26].

In the present work, we demonstrate an optical fiber H₂ gas sensor consisting of a commercial multi-mode optical fiber and a metasurface attached to the fiber tip. The optimized metasurface is composed of a layer of Pd nanoantennas suspended above a gold (Au) mirror layer. The reflectance of the Pd-based metasurface is ∼0.6, and it shows a clear reflectance or a power change of ∼0.28 or ∼3.26dB after it is exposed to H₂, indicating an excellent hydrogenation response performance. Moreover, a fiber-based reflectance readout method can be adopted to analyze H₂ concentration in our schemes. Thus, an integral remote monitoring H₂ sensing device composed of a compact optical fiber readout system and a Pd-based metasurface for harsh access H₂ detection is achieved.

Figure 1(a) presents the schematic of the H₂ sensing metasurface architecture. In this design, promising geometry consists of an array of Pd nanostructures, which is separated by a dielectric spacer layer from the Au film. The Au film, here, acts as a mirror, and the magnesium fluoride (MgF2) is employed as a dielectric spacer layer. The dielectric spacer layer is used to generate a metal–insulator–metal (MIM) meta-atom, which can be treated as a Fabry–Pérot cavity to increase reflectivity^[27]. Figure 1(b) shows the scheme of the H₂ sensor, which consists of a metasurface and a fiber flange plate that serves as a connector to the optical fiber. Thus, a simple optical fiber-based readout mechanism can be used, which is based on the broadband light source with a working range of 1250–1600 nm and the power meter to monitor the reflection intensity. Moreover, a polarization controller (PC) was placed upstream of the circulation to adjust and orient the state of the polarization of light so as to provide linear polarization.

Using this nanostructure layout and optical fiber-based readout mechanism, the optical reflectance R can be read out and optimized through careful tuning of the design parameters. In fact, the highest H₂ sensitivity would be achieved by maximizing the absolute reflectance difference. In this work, we thus focus on optimizing the relevant design parameters, e.g., the diameter of Pd nanodisks, the thickness of a MgF2 spacer, to achieve a maximum ΔR. Specifically, we perform numerical calculations on a model system^[28], where the Pd nanoantenna is on a MgF2 spacer and an Au mirror, to obtain a better understanding of the optical response and the underlying physical principles and to predict an optimum sensor design.

Numerical calculations with the finite difference time domain (FDTD) method have been carried out to gain better insight into the reflectance of the Pd nanoantennas. To investigate the behavior of the reflectance difference ΔR due to the hydrogeneration of the Pd-based metasurface, we perform simulations in Lumerical FDTD on the same system as in Fig. 1(a) with the dielectric function of the Pd disks (α-phase) and PdH disks (β-phase), where the refractive index of Pd and PdH is from the open access refractive index database^[29]. Additionally, the size of the PdH disk, which we considered here in the calculation, is approximated as isotropic volume expansion of 10% compared to Pd. Because the phase transition typically occurs in Pd particles in H₂ atmosphere with a concentration of ∼2% in volume^[30], the Pd-based metasurface thus represents a Pd absorber structure in the presence of a H₂ concentration that is below the explosive limit (4% in volume).

Figure 2 presents a pseudocolor plot of the reflectance difference ΔR as a function of wavelength λ, the diameter of Pd disk ∅disk, and thickness of the spacer layer lspacer. The Pd disk thickness hdisk considered in the calculations is 20 nm, and the structures are periodically arranged with a period of 360 nm. As it can be seen, the reflectance difference ΔR of the Pd disk could reach ∼0.3 when the diameter ranges from 150 to 250 nm, the spacer thickness ranges from 50 to 250 nm, and the wavelength range is 1200–1500 nm. Additionally, such high reflectance difference of ∼0.3 can also be achieved when the Pd disk diameter is beyond 250 nm, and the wavelength range is 1500–1600 nm. Considering the period of 360 nm in this calculation, it is in fact approximated as a Pd film. Thus, such a metasurface would suffer more strongly the influence of hysteresis in the loading and unloading of H₂ due to the thermodynamics of the hybrid interaction^[31,32]. Moreover, these wavelength ranges have a high loss in a multi-mode optical fiber, which we use in this case (as will be discussed below).

To explore the difference between Pd film and Pd metasurface during the progress of hydrogenation, Fig. 3 shows the reflectance of Pd (red line) as a function of varying wavelengths of incident light. Additionally, the wavelength spectrum of the reflectance of Pd hydride (black line), which considered the parameters of the β-phase, was displayed. Both of these two curves indicate that the reflectance increases with the red-shifted wavelength of incident light. The reflectance difference ΔR is also displayed in Fig. 3, and a blue curve is ΔR as a function of wavelength λ with the phase transition of Pd. The variance of the film thicknesses is ignored because there is little light passing through the metal film, and the reflectance does not change when the thickness varies. As shown in the blue curve, the reflectance difference is higher when the wavelength of the incident light is shorter. These results show that a higher reflectance difference ΔR can be obtained at a short wavelength of the incident light. As Fig. 3 shows, the highest reflectance difference ΔR corresponding to 0.168 can be achieved when the wavelength of the incident light is 586 nm. Compared with the results of nanoantennas, this maximum ΔR is much lower. Furthermore, unfortunately, the reflective spectrum is fixed for Pd film, and commercial communication optical fibers have a high loss at such lower wavelength. On the other hand, the working wavelength for a Pd-based metasurface can be shifted artificially to match the working wavelength of optical fibers by changing the size parameters of the nanoantenna.

Figure 4(a) presents the wavelength spectra of reflectance as a function of varying thicknesses of the spacer layer lspacer. The diameter and thickness of the Pd disk are 250 nm and 20 nm, respectively. After considering the parameters of the β-phase, the counterpart’s pseudocolor plot of Pd hydride can be seen in Fig. 4(b). Furthermore, reflectance difference ΔR as a function of wavelength λ and varying thicknesses of spacer lspacer can be seen in Fig. 4(c).

The positions of two exemplary ΔR maxima of ∼0.25 are indicated in Fig. 4(c). Both of these positions are located at the wavelength of 1200 nm and 1320 nm with the same spacer thicknesses of 150 nm. Nevertheless, these positions are not the highest or lowest reflectance of the Pd or Pd hydride, as shown in Figs. 4(a) and 4(b). A key feature of the system can find that the highest reflectance differences can be obtained via tuning the thicknesses of the spacer layer. Even though, the thickness of the spacer layer should be chosen carefully, because it could decrease the interaction as well. In particular, a response maximum very close to a region of low response cannot be chosen, since a deposition technique with high accuracy is required. The envisioned readout wavelength of the sensor, the working range of the optical fiber, should be taken into account at the same time. In this case, the reflectance difference ΔR of ∼0.23 can be achieved when the spacer thickness is 150 nm and the wavelength is 1300 nm, which is located at the communication window of the common multi-mode optical fiber, as shown in Fig. 4(c). Instead of the single-mode optical fiber, here, a multi-mode fiber optical fiber should be used, because the mode field distribution, which covers the area of the metasurface, is required. A multi-mode optical fiber for the working wavelength of 1300 nm (OM1, Changfei Optical Fiber and Cable Co., Ltd.) would be used.

A further simulation was performed to investigate the influence of the mode field distribution of the optical fiber. The mode field distribution was firstly calculated when the wavelength of incident light is 1300 nm using the COMSOL Multiphysics for influence evaluation. As the results show, the diameter of the mode field distribution is ∼50μm, taking into account that the numerical aperture of the optical fiber is 0.275, and this mode field distribution could cover the whole area of the metasurface. Although the ΔR of the optimized position is not the maximum, it is the best parameter for a Pd-based metasurface for H₂ detection via a simple and economic optical fiber readout system. After that, incident light with 1300 nm and its mode field distribution is the same as the above-calculated result and was used as the incidence of a Pd-based metasurface with 61×61 array, 360 nm period, 20 nm thickness of Pd, 100 nm diameter of Pd, and 150 nm thickness of MgF2. The calculated results show that a reflectance of the Pd-based metasurface is 0.529355, and the one after the Pd-based metasurface reacting with H₂ at 0.249729 can be achieved. In other words, a reflectance difference ΔR of ∼0.28 can be reached when the light from a multi-mode optical fiber is incident to a Pd-based metasurface. Thus, an optical fiber-based readout system would be suitable for this optimized Pd-based metasurface H₂ sensor. In addition, the reflectance difference is higher than that when the incident light was a plane wave because the wavefront from the fiber output is an inhomogeneous plane wave, and it directly impacts the result^[17].

The simple optical fiber readout mechanism and an optimization of the Pd-based metasurface, which is next to the fiber tip, have been demonstrated. Suitable parameters of the metasurface, including diameter ∅disk and thickness of the Pd disk, thickness of the spacer layer lspacer, and wavelength of the incident light, were calculated using the Fourier modal method. The optimized reflectance difference ΔR of ∼0.28 can be obtained when exposed to the sensor in the presence of H₂. Together with the intrinsic features of optical fibers^[33], this Pd-based metasurface with an optical fiber readout system provides a promising platform for remote and harsh access H₂ detection. Moreover, the mechanism of sensor performance is a possibility to tune its spectral range of operation by the diameter and thickness of the specificity recognition materials to make this design applicable to other molecular detection applications or to positively affect the interaction between biomaterials and cells.