A terahertz pulse emitter monolithically integrated with a quantum cascade laser
A terahertz pulse emitter monolithically integrated with a quantum cascade laser (QCL) is demonstrated. The emitter facet is excited by near-infrared pulses from a mode-locked Ti:sapphire laser, and the resulting current transients generate terahertz pulses that are coupled into an electrically isolated QCL in proximity. These pulses are used to measure the gain of the laser transition at ∼ 2.2 THz, which clamps above threshold at ∼ 18 cm−1 and has a full width at half-maximum linewidth of ∼ 0.67 THz. The measurement also shows the existence of absorption features at different biases that correspond to misalignment of the band structure and to absorption within the two injector states. The simplicity of this scheme allows it to be implemented alongside standard QCL ridge processing and to be used as a versatile tool for characterizing QCL gain media.