Terahertz Quantum Cascade Lasers: Determining the Optimal Waveguide for 1.5 THz
Abstract: A quantum cascade laser (QCL) involves the cascading of electrons through quantum wells. Terahertz QCL’s emit waves that can detect manufacturing defects, cancerous tissue, etc. However, due to waveguide effects, there are no high-power terahertz QCL’s that emit at a frequency between 1 and 2 THz. Optimizing the waveguide for these frequencies can address this issue. In this frequency range, 1.5 THz has the best atmospheric transmission, and was therefore chosen in this study. We built and validated a numerical code that calculates the laser beam pattern in the QCL for any waveguide structure and outputs the threshold gain as the figure of merit. The ideal waveguide type at low THz frequencies is the semi-insulating surface plasmon (SISP) waveguide, which traps the laser beam between a metal layer and a thin plasma layer. Our code was used to score all possible SISP waveguide structures (within standard parameters for GaAs) at 1.5 THz by varying the plasma layer width and doping. The optimal SISP waveguide at 1.5 THz was found to have a plasma layer width of 1430 nm and doping of 5.9 × 1017 cm-3. For this waveguide, the gain threshold is 27.1 cm-1, indicating that a high-power, low-frequency terahertz QCL can be constructed using these parameters.
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