Precise control of evanescent scattering by self-assembled ferromagnetic particles for optical sensing with tunable sensitivity.

We propose an optical sensing platform that uses evanescent scattering through precise manipulation of self-assembled ferromagnetic particle columns. The movement of the column tips can be controlled dynamically down to a submicron range by an external actuation, namely, a magnetic field, for interacting with evanescent wave propagation along an optical waveguide that causes a change in its output intensity for optical sensing. To demonstrate the idea, an AC current sensor with only a 5 mm interaction length is proposed and realized. Furthermore, its sensitivity is tunable within 9-20 dB/A by varying a DC-biased signal. The platform shows favorable signal reversibility, stability, broadband operation, and real-time response.