Mid-infrared-pumped, electrically driven terahertz quantum laser: Enhanced pumping efficiency and prevention of electric-field domains

We investigated GaAs/(Al,Ga)As structures for mid-infrared-pumped, electrically driven terahertz quantum lasers (TQLs) based on the concept of Waldmueller et al. [Phys. Rev. Lett. 99, 117401 (2007)]. We developed a design with the gain maximum appearing at a relatively low field strength of 6.5 kV/cm. The present TQL design avoids pumping-induced current instabilities, which prevented a previous design from operating at the required field strength as discussed by Giehler et al. [J. Appl. Phys. 110, 103104 (2011)]. We applied photocurrent (PC) spectroscopy to examine the pumping process. The measured PC spectra are found to be in agreement with simulated spectra in the vicinity of the desired operating field strength. We demonstrate that electric-field domain formation can be suppressed using a continuous-wave CO2 laser as the pump.