Modeling of Surface-Emitting Terahertz Quantum Cascade Laser Based on Difference Frequency Generation

In this paper, we propose a simple method to calculate the THz field and THz power generated via intracavity difference frequency generation in a dual-wavelength mid-infrared quantum cascade laser. The approach is based on using a surface emission scheme in the nonlinear waveguide. The results show that the THz output power varies linearly with the product of the tow mid-infrared pump powers, and strongly depends on the cover refractive index for the THz waves. Specifically, a higher refractive index leads to a significant increase in THz power, while a low product of the pump powers levels result in weak output power. Additionally, the effect of nonlinear film thickness on optical efficiency is considered. The efficiency decreases as the film thickness increases. This is due to the dependence of the THz field amplitude in the cover region on the film-substrate and film-cover interfaces, which is significantly affected by the film thickness. Our model generates output power through surface emission, in agreement with existing experimental results, highlighting the importance of optimizing the cover material and film thickness to improve output power and THz efficiency.