Frequency Tuning of Third-Order Distributed Feedback Terahertz Quantum Cascade Lasers by SiO2 and PMMA

We report an extensive study of the effect of an additional dielectric layer on the frequency of terahertz quantum cascade lasers (QCLs). QCLs with third-order distributed feedback structure at frequencies of 3.5 and 4.7 THz are used in our experiment. The applied dielectric layer is either Silicon dioxide (SiO
₂
) or Polymethylmethacrylaat (PMMA). We find that both dielectric layers can shift the lasing frequency by up to -6 GHz on a 3.5-THz QCL, and up to -13 GHz for a 4.7-THz QCL. Full 3-D FEM simulations suggest that the effect is dominated by the effective thickness of the dielectric on the side walls of the laser structure, and also confirm that for a given dielectric layer, the effect is stronger in the 4.7-THz QCL due to its different extension of the electromagnetic field to the free space. This study provides a guideline for shifting the frequency of an existing QCL for frequency critical applications such as spectroscopy or use as a local oscillator.