Terahertz (THz) quantum-cascade lasers (QCLs) based on GaAs/AlAs heterostructures have been developed for application-defined emission frequencies between 3.4 and 5.0 THz. These THz QCLs can be used as local oscillators in airborne or satellite-based astronomical instruments or as radiation sources for high-resolution absorption spectroscopy. The GaAs/AlAs THz QCLs can be operated in mechanical cryocoolers and even in miniature mechanical cryocoolers due to their comparatively high wall-plug efficiency of around 0.2% and typical current densities below 500 A/cm2. These lasers exhibit output powers of more than 1 mW at operating temperatures up to about 70 K, which is sufficient for most of the abovementioned applications.
Due to their narrow line width and rather large intrinsic tuning range, QCLs for the THz spectral region are promising radiation sources for high-resolution spectroscopy of molecules, atoms, and ions utilizing rotational or ﬁne-structure transitions. In atmospheric science, the rotational transition of the hydroxyl radical (OH) at 3.55 THz and the ﬁne-structure line of atomic oxygen (OI) at 4.75 THz are of particular interest. Both can be measured with QCL-based heterodyne receivers. For fundamental research and industrial applications, high-resolution absorption spectroscopy based on ﬁne-structure transitions in Al, N+, and O at 3.36, 3.92, and 4.75 THz, respectively, is expected to allow for the quantitative determination of the atom and ion densities in plasma processes. Furthermore, QCLs emitting in the atmospheric windows around 3.43, 4.32, and 4.92 THz are of interest for applications if the THz radiation has to be transmitted through air over a longer distance up to about 10 m.
Starting from recently developed GaAs/AlAs QCLs for 4.75 THz with a wall plug efficiency which is by a factor of 3 larger than the one for corresponding GaAs/Al0.25Ga0.75As QCLs, the layer structure of this design was gradually scaled toward lower or higher frequencies. The target frequencies for the gain maxima are achieved by an appropriate adjustment of the quantum well thicknesses and a corresponding ﬁne-tuning of the thicknesses of some particular barriers. Although the growth of the QCLs with very thin AlAs barriers remains challenging, the operating parameters of 21 lasers fabricated from 11 different wafers have been analyzed. The output powers of Fabry-Pérot lasers based on single-plasmon waveguides reach values of up to 6 mW at an operating temperature of 30 K. The threshold current densities vary between 100 and 300 A/cm2. Optimized lasers exhibit competitive wall plug efﬁciencies and emission powers of more than 1 mW at rather high operating temperatures in continuous-wave operation. For 4.75 THz, we demonstrated an output power of more than 1 mW for an operating temperature larger than 70 K using laser ridge dimensions of 0.08 × 0.87 mm2 when operated in a mechanical cryocooler.
Author: L. Schrottke , X. Lü , B. Röben , K. Biermann , T. Hagelschuer , M. Wienold , H.-W. Hübers , M. Hannemann , J.-P. H. van Helden , J. Röpcke , H. T. Grahn
Title: High-performance GaAs/AlAs terahertz quantum-cascade lasers for spectroscopic applications
Source: IEEE Trans. Terahertz Sci. Technol. , 10 , 133 ( 2019 )