We have developed terahertz (THz) quantum-cascade lasers (QCLs) based on GaAs/AlAs heterostructures, which exhibit single-mode emission at 3.360, 3.921, and 4.745 THz. These frequencies are in close correspondence to fine-structure transitions of Al atoms, N+ ions, and O atoms, respectively. Due to the low electrical pump power of these THz QCLs, they can be operated in a mechanical cryocooler in continuous-wave mode, while a sufficient intrinsic tuning range of more than 5 GHz is maintained. The single-mode operation and the intrinsic tuning range of these THz QCLs allow for the application of these lasers as radiation sources for high-resolution absorption spectroscopy to determine the absolute densities of Al atoms, N+ ions, and O atoms in plasmas.
For plasma science, the density of the atoms and ions of interest is determined by evaluating the spectral profiles of the respective high-resolution absorption features. In this configuration, the QCLs have to fulfill similar operating parameters as for the local oscillators in heterodyne spectrometers: (a) the lasers have to exhibit single-mode emission at a frequency matching the transition frequency with a precision of several GHz and a sufficient tuning range in order to cover the complete absorption profile. (b) The output power has to reach values of about 1 mW. (c) The pump power has to be located in the range between 1 and 2 W. Therefore, the strategy for the development of the lasers is very similar to the approach for the local oscillators. Furthermore, the lasers have to allow for a continuous frequency tuning, which is achieved by a fast ramping of the injection current without abrupt changes. Therefore, instabilities such as mode jumping and discontinuous changes of the output power have to be avoided for the entire tuning range. Such a stable operation in the fast-ramping mode is an additional challenge for the development of THz QCLs for high-resolution spectroscopy.
Figure 1 shows the conduction band profile and subband structure of an exemplary QCL based on a hybrid design for GaAs/AlAs heterostructures for 3.360 THz. Owing to the small electrical pump power of only a few watts, the THz QCLs exhibit stable continuous-wave operation in mechanical cryocoolers. Figure 2 exhibits light output-current density-voltage characteristics for several operating temperatures and lasing spectra of this QCL. The dynamic range is sufficiently large for the evaluation of the spectral profiles of the respective absorption features. Short Fabry-Pérot lasers based on single-plasmon waveguides exhibit single-mode operation with tuning ranges of more than 5 GHz and output powers of several mW. Based on the QCL for 4.745 THz, the Leibniz Institute for Plasma Science and Technology (INP) has developed an absorption spectrometer for the determination of the absolute density of oxygen atoms in a plasma.
Author: X. Lü, B. Röben, K. Biermann, J. R. Wubs, U. Macherius, K.-D. Weltmann, J. H. van Helden, L. Schrottke, and H. T. Grahn
Title: Terahertz quantum-cascade lasers for high-resolution absorption spectroscopy of atoms and ions in plasmas
Source: Semicond. Sci. Technol. 38, 035003 (2023)