Harmonic generation and half flux quantum periodicity in the RF-SQUID

We studied the RF driven SQUID by a numeric simulation and experimentally. The amplitude and the phase of the RF voltage vs RF driving current and DC flux has been obtained both with the SQUID working in the conventional mode and in a new operating mode based on the 2nd harmonic generation in the system. A spectral analysis has been carried out and we report the characteristics of harmonic and subharmonic generation. For the even harmonic components, and in particular for the second one on which the proposed method is based, the VRFvs IRFstaircase pattern is replaced by a series of peaks. We find that the position of the peaks does not depend on the DC flux level and so they result in a stable marker for RF level measurements. The 2nd harmonic output, as a function of DC flux, shows a triangular behaviour with VRF= 0 at . The dependence of VRFon φDCis at least as high as in the conventional operating mode. We have experimentally verified the harmonic generation and the half flux quantum periodicity up to the 4th harmonic. The signal amplitude was enough to allow flux-locked-loop operation.