Bloch - Siegert shift in the Rabi oscillations on the “dressed” electron spin states

Using electron paramagnetic resonance (EPR) at Rabi frequencies, we study the Bloch-Siegert effect in the coherent dynamics of electron spin states driven by a bichromatic radiation consisting of a transverse microwave (MW) and longitudinal radio frequency (RF) fields. The Rabi oscillations on the electron spin states “dressed” by the MW field are observed at the double resonance, when the MW frequency equals the Larmor frequency of the spin system and the frequency ωrf of the RF field is close to the Rabi frequency ω1 in the MW field. The dispersive (|ωrf – ω1| ~ ωrf, ω1) and near-resonant (ωrf ≈ ω1) regimes are investigated. We demonstrate that the Bloch-Siegert shift increases with increasing RF amplitude and at the fixed RF amplitude it has the largest values in the “low-frequency” (ωrf < ω1) range of the dispersive regime. Experimental results were obtained in the time-resolved EPR for E’1 centres in quartz.