Simulation of optical and electronic characteristics of terahertz quantum cascade laser

Abstract

Nowadays terahertz spectroscopy is fast becoming a method of choice both in industrial and security applications. This spectroscopic method requires a device that produces broadband THz radiation. Currently one of the techniques used to generate this radiation is by using a heterogeneous AlGaAs/GaAs terahertz quantum cascade laser (THz QCL). Typically the device combines a few independent designs of active region that needs to be fabricated using very special, costly instruments and tested several times to improve its performance. A simulation model is critically needed to study the electronic transport properties of the individual designs and the optical performance of the waveguide used to support the broadband operation. Therefore the aim of this work is to establish a new model to study the electronic transport properties of four (4) experimentally established designs using the density matrix (DM) method and the optical characteristics of a narrow double metal waveguide using the finite element (FE) method. From these calculations several critical characteristics of THz QCL such as the threshold current densities, the average gain, the gain spectra, the optical mode confinement factor and the figure of merit (F.O.M) can be determined and their dependence on relevant design parameters can be studied. The results from the DM calculations show that the threshold current densities for these individual designs are in the range of 350 to 600 A cm-2 with the average gain of about 20 cm-1 and 100 cm-1, without and with the stimulated emission respectively. The calculated gain spectra exhibit Lorentzian pattern over a frequency range of 1 to 5 THz. The gain spectrum of the whole system based on a combination of each individual gain spectra leads to a bandwidth of around 3 THz which is bigger than the reported bandwidth from previous experimental broadband QCL designs. Meanwhile, the result from FE method for the calculated confinement factor value obtained for the waveguide increases from 90 % to 100 % in frequency range of 2 to 4 THz. The combined effects of all the design parameters lead to a figure of merit (F.O.M) that decreases with the increase of the operating frequency. As a conclusion, a reliable prediction tool based on the DM method and FE method has been demonstrated whereby it can be used not only as a guide for broadband QCL design but also to simulate its performance before the actual design is to be fabricated.