Whispering Gallery Modes in a Ferronematic-Infiltrated Fibre Micro-Resonator for Magnetic Field Sensing

Introduction Whispering gallery mode (WGM) micro-resonators are remarkable optical devices whose small mode volumes and high Q-factors make them attractive for a wide range of applications such as filters, lasers, modulators and sensors [1]. WGMs can be excited in dielectric structures with circular symmetry, such as microspheres, micro-disks, and optical fibres, and can be imagined as closed-trajectory rays confined within the cavity by almost total reflections from the curved surface of the resonator. Spectral positions of the WGM resonances are highly sensitive to the geometrical dimensions of the resonator and refractive indices of both the resonator itself and its surrounding medium. Any refractive index perturbation of the interrelated media or a change in diameter results in variations of the WGM spectrum, offering the possibility to design various optical devices [2, 3]. Photonic crystal fibres (PCF) containing micron-sized air holes can be used as cylindrical WGM resonators whose refractive index can be altered by infiltrating them with various materials. In this work we propose a novel magnetic field sensors based on a PCF resonator infiltrated with a ferronematic liquid crystal. Effective refractive index of such a WGM resonator varies in the presence of magnetic field, and thus the value of the field can be related to the spectral shift of the WGMs.