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

Abstract

Interacting with a strong electromagnetic field, an asymmetric two-level system radiates at the Rabi frequency $\Omega_R$. With a proper choice of the driving amplitude, terahertz emission is achievable [O. V. Kibis {\it et al.}, Phys. Rev. Lett. 102 (2009) 023601]. A set of two tunneling-coupled asymmetric two-level systems radiates at frequency $\sqrt{ (\xi_1-\xi_2)^2+\Omega_R^2}$, where energies $\hbar \xi_1$ and $\hbar\xi_2$ characterize the tunneling interaction [M.F. Miri {\it et al.}, J. Opt. Soc. Am. B 33 (2016) 1873]. We show that an {\it array} of $N$ tunneling-coupled asymmetric two-level systems radiates at frequency $\sqrt{ \mathfrak{N} (\xi_1-\xi_2)^2+\Omega_R^2}$. $\mathfrak{N}= 2, 3, 2+ \sqrt{2}, \frac{5+ \sqrt{5}}{2}$ and $ 2+ \sqrt{3}$ for $N= 3, 5, 7, 9$ and $11$, respectively. The strong impact of $\xi_1-\xi_2$ 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 $2 ~ \mathrm{meV}$, the radiation frequency increases from $3.0$ to $ 6.36 ~\mathrm{THz}$ 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.