Continuous-wave Highly Efficient Low-Divergence Terahertz Wire Lasers

In recent years, Terahertz (THz) quantum cascade lasers (QCLs) have undergone a fast development, showing high power, ultra-broadband gain and quantum-limited linewidth. For many applications, THz QCLs need to operate in continuous-wave (CW), with a tight control of the emission spectrum and highly collimated beam profiles. These requirements are usually addressed by exploiting distributed feedback (DFB), photonic crystals or micro-cavity architectures, which can allow tailoring either the laser beam divergence or the emission frequency, and eventually both of them simultaneously, as in the case of third-order DFBs. Here we report on an original design in which a wire DFB THz QCL, engineered with a lateral sinusoidal corrugation providing feedback and frequency control, and an array of surface hole provides light outcoupling. This new photonic structure has led to the achievement of low-divergence beams (10°), single-mode emission, high slope efficiencies (250 mW/A), and stable CW operation.