ENSEMBLE MONTE CARLO SIMULATION OF THE 4.8MICROMETER DEEP WELL QUANTUM CASCADE LASERS
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As early as 1971, the possibility of emission between confined subbands of semiconductor quantum wells has been proposed . Adopting such idea, the first electrically pumped Quantum Cascade Laser (QCL) was experimentally demonstrated in 1994 . The QCL is made by a periodic series of thin layers with different bandgaps to form a multiple-quantum-well structure. Each period containing several tens of layers is called a stage. Each stage contains an electron-injecting region (injector) and an active region where the optical emissions occur. Schematics of the bandstructure and the lasing process for two successive stages of a QCL under the applied bias are shown in Fig. 1.1. An electron injected into the left end, from the previous stage, first experience several transitions through non-radiative scatterings, and reaches the bottom of the miniband. Then it tunnels through a relatively thick barrier at the end of the injector to fill the upper lasing level 3 (the upper red line) localized in the active region. Due to the large optical coupling strength between the upper lasing level 3 and the lower lasing level 2, the electron has large chance to make a radiative transition (the yellow wavy lines) between the two levels, and consequently emits a photon. The electron in the lower lasing level 2 is quickly relaxed to level 1, which might be assisted by phonon emission. Finally, the electron tunnels to the injector in the next stage and triggers another photon emission. This process repeats in each stage.