Terahertz emission from an array of tunneling-coupled asymmetric two-level systems

Interacting with a strong electromagnetic field, an asymmetric two-level system radiates at the Rabi frequency ΩR. With a proper choice of the driving amplitude, terahertz emission is achievable (Kibis et al., 2009). A set of two tunneling-coupled asymmetric two-level systems radiates at frequency (ξ1−ξ2)2+ΩR2, where energies ħξ1 and ħξ2 characterize the tunneling interaction (Miri et al., 2016). We show that an array of N tunneling-coupled asymmetric two-level systems radiates at frequency N(ξ1−ξ2)2+ΩR2. N=2,3,2+2,5+52 and 2+3 for N=3,5,7,9 and 11, respectively. The strong impact of ξ1−ξ2 and N on the system allows tuning the terahertz radiation in a broad range. For example, upon increase of the tunneling energy from zero to 2meV, the radiation frequency increases from 3.0 to 6.36THz when N=7. An array of coupled asymmetric quantum wells is promising for the realization of a tunable terahertz emitter. We further show that the system can be used for high harmonic generation.