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An alternative pump scheme, named indirect pump one is proposed to clarify its own feasibility. The high device performances of 8 microm quantum cascade lasers with cavity lengths of 4 mm and 1.5 mm are demonstrated: low threshold current densities of 2.7 and 3.3 kA/cm2 and maximum output powers of 362 and 50 mW at room temperature, and high T0-values of 243 and 303 K around room temperature. The higher T0-value, 303 K is the highest record ever reported with quantum cascade lasers. The high stability for temperature changes is interpreted in terms of the indirect pumping model.

Terahertz quantum cascade laser (QCL) sources based on intra-cavity difference frequency generation are currently the only electrically pumped monolithic semiconductor light sources operating at room temperature in the 1–6-THz spectral range. Relying on the active regions with the giant second-order nonlinear susceptibility and the Cherenkov phase-matching scheme, these devices demonstrated drastic improvements in performance in the past several years and can now produce narrow-linewidth single-mode terahertz emission that is tunable from 1 to 6 THz with power output sufficient for imaging and spectroscopic applications. This paper reviews the progress of this technology. Recent efforts in wave function engineering using a new active region design based on a dual-upper-state concept led to a significant enhancement of the optical nonlinearity of the active region for efficient terahertz generation. The transfer of Cherenkov devices from their native semi-insulating InP substrates to high-resistivity silicon substrates resulted in a dramatic improvement in the outcoupling efficiency of terahertz radiation. Cherenkov terahertz QCL sources based on the dual-upper-state design have also been shown to exhibit ultra-broadband comb-like terahertz emission spectra with more than one octave of terahertz frequency span. The broadband terahertz QCL sources operating in continuous-wave mode produces the narrow inter-mode beat-note linewidth of 287 Hz, which indicates frequency comb operation of mid-infrared pumps and thus supports potential terahertz comb operation. Finally, we report the high-quality terahertz imaging obtained by a THz imaging system using terahertz QCL sources based on intra-cavity difference frequency generation.

Terahertz quantum cascade laser sources with intra-cavity non-linear frequency mixing are the first room-temperature electrically pumped monolithic semiconductor sources that operate in the 1.2–5.9 THz spectral range. However, high performance in low-frequency range is difficult because converted terahertz waves suffer from significantly high absorption in waveguides. Here, we report a sub-terahertz electrically pumped monolithic semiconductor laser. This sub-terahertz source is based on a high-performance, long-wavelength (λ ≈ 13.7 μm) quantum cascade laser in which high-efficiency terahertz generation occurs. The device produces peak output power of 11 μW within the 615–788 GHz frequency range at room temperature. Additionally, a source emitting at 1.5 THz provides peak output power of 287 μW at 110 K. The generated terahertz radiation of <2 THz is mostly attributable to the optical rectification process in long-wavelength infrared quantum cascade lasers.

A broad-gain quantum cascade laser design with dual-upper-state is proposed to clarify its own feasibility. Devices employing the proposed active region design exhibit homogeneously wide (&gt;330 cm−1) electroluminescence spectra of which shapes are insensitive to voltage changes. A buried heterostructure laser, emitting at λ∼8.4 μm, demonstrates a high continuous-wave output power of 152 mW together with a high constant slope efficiency of 518 W/A at room temperature. In addition, the device performance is observed to be very insensitive to temperature change; T0-values of ∼306 K and constant slope efficiency over the wide temperature range, 280–400 K.

We report the performance of room temperature terahertz sources based on intracavity differencefrequency generation in mid-infrared quantum cascade lasers with a dual-upper-state (DAU) active region. DAU active region design is theoretically expected to produce larger optical nonlinearity for terahertz difference-frequency generation, compared to the active region designs of the boundto-continuum type used previously. Fabricated buried heterostructure devices with a two-section buried distributed feedback grating and the waveguide designed for Cherenkov differencefrequency phase-matching scheme operate in two single-mode mid-infrared wavelengths at 10.7 lm and 9.7 lm and produce terahertz output at 2.9 THz with mid-infrared to terahertz conversion efficiency of 0.8 mW/W 2 at room temperature. V

Difference frequency generation quantum cascade lasers are well-suited for applications requiring narrow-linewidth emitters.

Device-performances of 3.7 THz indirect-pumping quantum-cascade lasers are demonstrated in an InGaAs/InAlAs material system grown by metal-organic vapor-phase epitaxy. The lasers show a low threshold-current-density of ~420 A/cm2 and a peak output power of ~8 mW at 7 K, no sign of parasitic currents with recourse to well-designed coupled-well injectors in the indirect pump scheme, and a maximum operating temperature of Tmax ~100 K. The observed roll-over of output intensities in current ranges below maximum currents and limitation of Tmax are discussed with a model for electron-gas heating in injectors. Possible ways toward elevation of Tmax are suggested.