  We propose a practical approach to manipulate the counter-rotating (CR)
interactions in the quantum Rabi model by introducing a sinusoidal modulation
to the transition frequency of the quantum two-level system in this model. By
choosing appropriate modulation frequency and amplitude, enhancement and
suppression of the CR interactions can be achieved in the Jaynes-Cummings
regime (including both weak- and strong-coupling cases) as well as the
ultrastrong-coupling regime. In particular, we calculate the output photon
emission of the cavity vacuum state under enhanced CR terms. Our results show
that continuous and steady photon emission from the cavity vacuum can be
observed in the Jaynes-Cummings regime as a consequence of this enhancement.
Our approach can be realized in superconducting quantum circuits.
