  2H-NbSe2 is an archetypal system in which superconductivity and
charge-density-wave (CDW) coexist and compete macroscopically with each other.
In particular, this interplay also manifests in their dynamical fluctuations.
As a result, the superconducting amplitude fluctuations (i.e. Higgs mode) is
pushed below the quasiparticle continuum, allowing it to become a coherent
excitation observable by Raman scattering. In the present study, we coherently
drive the collective oscillations of the two orders and visualize their
interplay in the driven states in the time domain. We find that both collective
modes contribute to terahertz third harmonic generation (THG) and the THG
signals interfere below Tc, leading to an anti-resonance of the integrated THG
signal. The dynamical Ginzburg-Landau model suggests that around the
anti-resonance a periodic energy transfer between the driven Higgs oscillations
and the driven CDW oscillations is possible. In addition to 2H-NbSe2, we also
studied an underdoped La2-xSrxCuO4 (x ~ 0.12) driven beyond the perturbative
regime of THG. A similar interference between two sources of THG is observed
below Tc. While there might be additional sources of THG in these experiments,
our results illustrate the roles of coupled modes in the terahertz THG process
and the tantalizing possibility of coherent control via such couplings.
