1.Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, Guangzhou Key Laboratory for Special Fiber Photonic Devices, School of Information Optoelectronic Science and Engineering, South China Normal University, Guangzhou 510006, China 2.Jiangmen Zhuxi Laser and Smart Co. Ltd., Jiangmen 529000, China
Fund Project:Project supported by the Natural Science Foundation of Guangdong Province, China (Grant No. 2015A030313383).
Received Date:22 October 2018
Accepted Date:21 November 2018
Available Online:01 January 2019
Published Online:20 January 2019
Abstract:Terahertz quantum cascade laser is a semiconductor laser that effectively obtains terahertz waves. It uses the semiconductor heterojunction to have a quantum cascade effect under an applied voltage, and then the phonon assists the electron resonance from the upper stage to the next stage, so that a single electron injected externally can emit multiple photons. However, some electrons will deviate from the transport path during transportation and these electrons are called leakage electrons. Electron leakage comes from three ways. The first way is the scattering of electrons from the upper laser level through the long longitudinal phonon to the low energy level; the second way is the scattering of electrons from the lower laser level to the high energy bound level and the continuous level; and the third way is the scattering of electrons from the upper laser level to high energy bound levels and continuous levels. These leakage electrons directly reduce the number of population inversions in the laser system, making the laser output power limited. At present, most of researchers explain the electron leakage through indirect measurements, and there are few studies in which the electron leakage is analyzed by establishing theoretical models. In this paper, the electron leakage model in THz QCL is established by using thermodynamic statistical theory and laser output characteristic theory. The degree of electron leakage is measured by output power. The influence of lattice temperature and quantum well barrier height on electron leakage are studied. It is found that when the lattice temperature rises and the electrons in the upper laser state leak to higher energy levels, the number of electrons leaking to the adjacent bound state and the continuous state increases, and the number of electrons leaking to the next near-bound level is relatively small. In the case of electron leakage, the utilization of electrons becomes lowered, and the laser output power is also lowered. The study also shows that an appropriate increase in the height of the quantum barrier can suppress the leakage of electrons. Using the established theoretical model to optimize the quantum well barrier height of the previously reported laser system, an 8 mW terahertz quantum cascade laser (THz QCL) laser output at 210 K is obtained. Compared with the reported experimental results, the temperature and output power are improved. These results provide a theoretical basis for studying the electron leakage temperature characteristics of THz QCL and also optimally designing the THz QCL active region structure. Keywords:terahertz/ quantum cascade laser/ electron leakage/ active region
图 1 THz QCL级联的能级结构
图 2 泄漏电流密度与晶格温度的关系
图 3 归一化输出功率与晶格温度的关系
图 4 归一化输出功率与势垒高度参数的关系
