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Since the first demonstration of quantum-cascade lasers (QCLs) in 1994, remarkable progress has been made from the mid-infrared (mid-IR) to terahertz (THz) spectral range. The 1–6 THz spectral range is very attractive for many applications, such as imaging, chem-/bio-sensing, heterodyne detection, and spectroscopy. However, this spectral range still lacks high-performance compact continuous-wave (CW) light sources operable at room temperature. Recently, THz sources based on intracavity difference-frequency generation (DFG) in dual-wavelength mid-IR QCLs have been demonstrated. These devices, known as THz DFG-QCLs, have their active region engineered to exhibit giant intersubband nonlinear susceptibility χ(2) for THz DFG. Recently, we developed THz DFG-QCLs containing an homogeneous active region with dual-upper states (DAU), which exhibit a THz output power of 301 μW with a high mid-IR-to-THz conversion efficiency of 1.2 mW/W2. The DAU active region approach provides a broadband gain bandwidth, and as a result, two wavelength emissions can be obtained without a heterogeneous cascade that has been used previously; this leads to a low threshold current density compared with that obtained from the use of a heterogeneous active region. Here, we present a low threshold THz DFG-QCL based on a λ~6.8 μm DAU active region. The λ~6.8 μm DAU-QCLs have exhibited very low threshold current density as well as broad gain bandwidth. By applying the λ~6.8 μm DAU design approach, the device demonstrates room temperature CW operation without an epidown mounting scheme, where a threshold current density for THz emission has been shown to be low, at 1.3 kA/cm2. Besides, ultra-broadband emission covering 1.6–3.5 THz has been obtained in CW mode below 200 K.
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