A device includes a substrate and nanoscale fin formed from a first material, a RF emitter that emits energy in a range of radio frequencies, and a waveguide formed from a second material. The device further includes a bichromatic directional coupler configured to couple pump and probe laser light into the waveguide. The waveguide is positioned proximate to the nanoscale fin along a coupling length such that the pump laser light propagating within the waveguide is coupled into the nanoscale fin from evanescent wave overlap along the coupling length. The pump laser light causes the first material to absorb the probe laser light when energy emitted by the RF emitter is at one or more frequencies dependent on a magnetic field. The device further includes a processor configured to determine a magnetic field strength of the magnetic field based on an identification of the one or more frequencies.