Precise local sensing of magnetic fields is of utmost importance in many disparate areas ranging from power grid monitoring to early detection of earthquakes. Among the different approaches proposed and developed to achieve this task, photonic crystal fibers (PCFs) featuring symmetric arrays of holes running through their length emerged as an optimal platform owing to their ability to tightly confine light in a central glass core while enabling strong interaction with neighboring channels filled with magnetic fluids.

In this work, Tang and coauthors introduce a novel PCF-based sensor capable of measuring variations in both refractive index and external magnetic field accurately. The approach exploits surface plasmon resonances localized in the ultrathin (~30 nm) gold coating of two diametrically opposite holes in the inner array surrounding the core. By injecting a magnetic fluid into these gold-coated holes and launching broadband light into the core, the sensor detects changes in fiber loss due to external magnetic fields. Interestingly, the authors show how imparting slight ellipticity to the inner-array holes enhances the sensitivity over previous designs. The robustness and simplicity of the system makes it a promising solution to the challenge of gauging magnetic fields under different conditions.

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