Widely-tunable room-temperature, electrically-pumped semiconductor sources in 1-5 THz spectral range are highly desired for spectroscopy and imaging application. THz sources based on intra-cavity difference-frequency generation (DFG) in dual wavelength mid-infrared quantum cascade lasers (QCLs) [1-3] with Cherenkov phase-matching  are currently the only monolithic room-temperature semiconductor devices that can operate and provide widely-tunable output in this entire spectral range and beyond [5-7]. We will present our latest results on the development of broadly-tunable THz systems based on this technology. Both monolithic and external cavity configurations are explored. In our external-cavity systems we fix one mid-IR pump frequency by a distributed feedback grating etched into the QCL waveguide and tune the second mid-IR pump frequency with an external grating in a Littrow-type configuration. Due to strong spatial hole burning in QCLs, gain competition between the two mid-infrared pumps is reduced and, with careful balancing of the feedback for the mid-infrared pumps, simultaneous two-color emission can be achieved experimentally. External-cavity THz DFG-QCL systems demonstrate continuous tuning at room temperature from 1.2 THz to 5.9 THz with THz peak power output varying between 5 and 90 W, depending on the operating frequency [5,6]. Beam steering of terahertz Cherenkov emission with frequency is suppressed by bonding devices onto high-resistivity silicon substrates that have virtually no refractive index dispersion and vanishingly-small optical loss in 1-6 THz range. To reduce the size and avoid mechanical complexity of external cavity tuners, we also demonstrate monolithic room-temperature THz DFG-QCL tuners with a tuning range from 3.44 THz to 4.02 THz, which corresponds to 15% of their center frequency. Devices are based on a single ridge-waveguide DFG-QCL source with electrically-separated distributed feedback section and distributed Bragg reflector sections that are used to select and tune the frequencies of the two mid-infrared pumps . We have also fabricated an array of 10 of such monolithic THz DFG-QCL tuners that provide continuous tunability between 2 and 4 THz at room temperature with up to 100 μW of peak THz power output.
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