Limitations of rotating-wave approximation in magnetic resonance: Characterization and elimination of the Bloch-Siegert shift in magneto-optics

We present investigations of radio-frequency resonances observed in a rubidium vapor. With measurements of a systematic shift of radio-frequency resonances, induced by an off-resonance component of an oscillating field (the Bloch-Siegert shift), we demonstrate limitations of the rotating-wave approximation. Experimental results are supported with numerical calculations, reproducing all features of the observed signals. Based on the studies, we develop experimental scheme free from the problem. By liberating the measurements from a systematic error (the resonance shift), the new approach will increase accuracy of spectroscopic measurements and magnetic-field detection.