  Any systems with symmetry-breaking eigenstates can effectively radiate
photons with doubled frequency of the incident light, which is known as the
second harmonic generation. Here, we study the second-order nonlinear effects
with the system of surface-state electrons on liquid Helium. Due to the
symmetry-breaking eigenstates, we show that a Rabi oscillation between two
levels of the surface-state electrons can be realized beyond the usual resonant
driving. Consequently, an electromagnetic field with the doubled frequency of
the applied driving could be effectively radiated. This can be regarded as a
frequency-doubled fluorescence, and interestingly, it works in the unusual
Terahertz range.
