Tapered terahertz quantum cascade lasers

Terahertz quantum cascade lasers with tapered waveguide structure are reported. It is found there is an optimized tapered angle with which the peak output power of the devices with same total length reaches the maximum. The far-field divergence of the laser beam depends on the tapered angle as well as the driven current. The existence of the optimized tapered angle and the current-dependent far-field pattern may be attributed to the self-focusing effect in the devices.