  The frequency-dependent nonlinear dielectric increment of dipolar fluids in
nonpolar fluids is often measured by using a stationary relaxation method in
which two electric fields are used: The static direct current (DC) field of
high strength causing the dielectric nonlinearity, and the probing alternating
current (AC) field of low strength and high frequency. When a nonlinear
composite is subjected to a sinusoidal electric field, the electric response in
the composite will, in general, consist of AC fields at frequencies of
higher-order harmonics. Based on the Froehlich model, we present a theory to
investigate nonlinear AC responses of dipolar fluids containing both
polarizable monomers and dimers. In the case of monomers only, our theory
reproduces the known results. We obtain the fundamental, second-, and
third-order harmonics of the Froehlich field by performing a perturbation
expansion. The even-order harmonics are induced by the coupling between the AC
and DC fields although the system under consideration has a cubic nonlinearity
only. The harmonics of the Froehlich field can be affected by the field
frequency, temperature, dispersion strength and the characteristic frequency of
the dipolar fluid, as well as the dielectric constant of the nonpolar fluid.
The results are found to be in agreement with recent experimental observations.
