Probing the singlet-triplet splitting in double quantum dots: Implications of the ac field amplitude

We consider a double quantum dot whose energy detuning is controlled by an ac electric field. We demonstrate an energy configuration for which the ac-induced current flowing through the double dot directly probes the spin-orbit anticrossing point for small ac field amplitudes. On the contrary, as the ac amplitude increases, a current antiresonance is formed and the direct information about the spin-orbit interaction is lost. This result indicates that a large ac amplitude is not necessarily advantageous for the spectroscopy of spin-orbit-coupled two-spin states. Moreover, we investigate the ac-induced current peaks versus the ac amplitude and show a current suppression when the ac field forms spin-blocked states. This effect gives rise to a characteristic pattern for the current which can be controlled at will by tuning the ac amplitude. Our results can be explored by performing electronic transport measurements in the spin-blockade regime.