Multiple-photon transitions in electrically detected magnetic resonance measurements of 4H-SiC transistors

We report an ultralow-field frequency-swept electrically detected magnetic resonance (fsEDMR) measurement scheme sensitive to so-called ultrastrong coupling in paramagnetic systems, which arises from comparatively strong driving fields and weak Zeeman interaction with small static fields. We observe multiple-photon transitions in the EDMR spectrum of a 4H-SiC transistor. The multiphoton transitions are a strong function of the linearly polarized driving field and of the static field. The observation of both field-swept EDMR at a constant frequency and fsEDMR demonstrate that the transitions we observe are caused by multiphoton transitions. In the small static field and large driving-field regime, Bloch-Siegert effects cause small changes to the resonant frequency. We observe these Bloch-Siegert shifts in the resonance frequency in the ultralow-field fsEDMR scheme and verify the observation by also measuring the driving field directly using Faraday's law of induction and a sensing coil. Multiphoton transitions are important for quantum engineering applications.