Increasing the output power or the wall-plug efficiency (WPE) of THz quantum cascade (QC) lasers is a topic of great interest, since it could have a major impact on applications. Once a high-performance active region is employed, the key ingredient is the laser resonator. Several effective solutions have been developed in by the scientific community [1, 2, 3]. More generally, periodic metal/semiconductor based structures permit to strongly confine the light and simultaneously tailor the emitted far-field [4]. Recently our team introduced the concept of graded photonic heterostructures [5]. In these devices, the laser is forced to operate on symmetric, radiative modes of a 1D grating, hence obtaining elevated radiation efficiency.
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