THz quantum cascade lasers operating on radiative states of a 2D photonic crystals resonator

Summary form only given. 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. Unfortunately, to date this device architecture can be implemented only in one dimension (i.e. in surface-emitting DFB lasers). The extension to 2D would lead to improved far fields and much higher output powers. As a matter of fact, it can be intuitively shown that the output power scale more than linearly with the lateral size dimension.In this talk we demonstrate a novel PhC design which allows one to operate the lasers on the radiative modes of a 2D photonic structure. Note: traditional 2D PhC band-edge lasers - regardless of the operating wavelength - operate on non-radiative modes [6] (i.e. high Q-factor modes), consequently achieving poor power extraction efficiencies. By introducing a judiciously designed anisotropy in the lattice spacing parameters (period and radius) of a 2D PhC, we are able to independently control the in-plane and out-of-plane components of the resonator Q factor. This novel ability permits to separately address the non-radiative and radiative modes total Q factors, and to reverse the mode competition in favor of the high-power extraction mode. We experimentally demonstrate the concept using a THz QC laser based on a modified phonon-resonant active region [7]. We successfully obtain ope- ation on the radiative mode of the 2D PhC structure. The surface emission has a very low divergence (14x10 degrees), and it is naturally single lobed, without recurring to S phase-shift insertions. This is a natural advantage of radiative modes. Peak output powers of 42mW (average power ~10 mW), corresponding to a plug-in efficiency of ~0.2%, are obtained in this first generation of devices. According to a simple model, peak output powers of ~300 mW should be achievable using this architecture.