Frequency-doubled scattering of symmetry-breaking surface-state electrons on liquid helium

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 an approximate 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.