The real part Re [ ε T ] as a function of ( δ − ω m ) / ω m for the mechanical resonators with different frequencies. The mechanical driving fields are (a) η = 0 ; (b) η = 1, ϕ = 0 ; and (c) η = 1, ϕ = π . The frequencies of the two mechanical resonators are ω c = 1.005 ω b (red dashed curves); ω c = 1.0 ω b (blue solid curves); ω c = 0.995 ω b (green dotted curves). The other parameters are the same as those in Fig. 2.