  We study the origin of the symmetry of the fluorescence spectrum from the
two-level system subjected to a low-frequency periodic modulation and a
near-resonant high-frequency monochromatic excitation by using the analytical
and numerical methods based on the Floquet theory. We find that the fundamental
origin of symmetry of the spectrum can be attributed to the presence of the
generalized parity of the Floquet states, which depends on the driving
parameters. The absence of the generalized parity can lead to the asymmetry of
the spectrum. Based on the generalized parity, the conditions for the symmetry
and asymmetry of the spectrum can be derived, which succeeds in predicting
symmetry and asymmetry of the spectrum for the harmonic, biharmonic, and
multiharmonic modulations. Moreover, we find that the secular approximation
widely used in the analytical calculation may lead to artifact symmetry of the
spectrum that vanishes when such approximation is avoided. The present study
provides a significant perspective on the origin of the symmetry of the
spectrum.
