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Abstract
We present a model for longitudinal mode competition in coupled-cavity (CC) terahertz (THz) quantum cascade lasers (QCLs) by using a scattering matrix method and multi-mode reduced rate equations (RREs). The dependence of the mode selection and tuning characteristics on various device parameters is systematically investigated, including the net waveguide loss, the optical length of the passive cavity, and the heat-sink temperature for different relationship between the active and passive cavity lengths. The changes in eigenmode frequencies due to variations of device parameter are calculated before solving the RREs. The mode selection and tuning results obtained from solving the nonlinear RREs could be well explained by the linear scattering matrix analysis. The mode tuning process simulated by the proposed model is compared with experimentally measured data, yielding good agreement. Comprehensive study of the influence of the key device parameters on the performance of CC THz QCLs provides potential design rules for single-mode operation with either wide frequency tunability or high stability.
Highlights
T HE terahertz (THz) region of the electromagnetic spectrum
Using the scattering matrix method and the multi-mode reduced rate equations (RREs) model, we have investigated the influence of the key parameters on mode selection in CC THz quantum cascade lasers (QCLs), including the net waveguide loss, effective refractive index of the passive cavity, and the heat sink temperature of the CC device
A wider continuous frequency tuning range with a less discrete mode tuning could be obtained for the same variation of THS when La > Lp compared with La ∼ Lp and La < Lp
Summary
T HE terahertz (THz) region of the electromagnetic spectrum THz QCL technology has witnessed rapid development, and THz QCLs can emit output powers >1 W [4], within the 1.2–5.4 THz range [5]–[7], and operate at temperatures of up to ∼200 K in pulsed mode [8]. Single-mode operation THz QCLs with wide frequency tuning range are required for sensing, spectroscopic, and imaging applications [9], [10]. The intrinsic frequency tuning range of single cavity QCLs by adjustment of the injection current and temperature are limited to approximately 0.05 cm−1 and 0.1 cm−1 , respectively [11], [12]. Large tuning range (>20 GHz) THz emission have been achieved by applying external grating, MEMS, or heterogeneous active region in QCLs [13]–[15], but suffer from slow tuning, operation difficulties or device complexity
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