A photonics device for threshold magnetometry includes an absorbent material with nonlinear optical susceptibility, such as a diamond material with nitrogen vacancy defects, that is disposed in an optical resonator. The optical resonator receives light from an input source and includes nonlinear optical properties that enable the resonator to undergo a nonlinear photon generation process at a certain threshold power level to generate photons at distinct frequencies from the input light. The absorbent material absorbs photons entering the resonator when excited, which causes the threshold power level to shift as a function of the absorption. This may cause the optical resonator to stop generating photons via the nonlinear photon generation process and output a change in power. The change in power can be used to determine the characteristics of a present magnetic field.