Room temperature, broadly tunable, electrically pumped semiconductor sources in the terahertz spectral range, similar in operation simplicity to diode lasers, are highly desired for applications. An emerging technology in this area are sources based on intracavity difference-frequency generation in dual-wavelength mid-infrared quantum cascade lasers. Here we report terahertz quantum cascade laser sources based on an optimized non-collinear Cherenkov difference-frequency generation scheme that demonstrates dramatic improvements in performance. Devices emitting at 4 THz display a mid-infrared-to-terahertz conversion efficiency in excess of 0.6 mW W À 2 and provide nearly 0.12 mW of peak power output. Devices emitting at 2 and 3 THz fabricated on the same chip display 0.09 and 0.4 mW W À 2 conversion efficiencies at room temperature, respectively. High terahertzgeneration efficiency and relaxed phase-matching conditions offered by the Cherenkov scheme allowed us to demonstrate, for the first time, an external-cavity terahertz quantum cascade laser source tunable between 1.70 and 5.25 THz.

We report room-temperature terahertz sources based on Č erenkov difference-frequency generation in dual-wavelength mid-infrared quantum cascade lasers with giant resonant optical nonlinearities originating from intersubband transitions. A Č erenkov difference-frequency generation scheme allows for extraction of THz radiation along the whole length of the laser waveguide and provides directional terahertz emission. Experimentally, our sources demonstrate a conversion efficiency of up to 70 lW/W 2 approximately an order of magnitude improvement over the previous reports.

We discuss the design and operation of widely-tunable terahertz sources based on Cherenkov intra-cavity difference-frequency generation in mid-infrared quantum cascade lasers. Laser chips are integrated into a Littrow-type external cavity system. Devices demonstrate continuous terahertz emission tuning at room temperature with a record tuning range from 1.2 THz to 5.9 THz and peak power output varying between 5 and 90 μW, depending on the operating frequency. Beam steering of terahertz Cherenkov emission with frequency is suppressed and mid-infrared-to-terahertz conversion efficiency is improved by bonding devices onto highresistivity silicon substrates that have virtually no refractive index dispersion and vanishinglysmall optical loss in 1-6 THz range.

Influence of the substrate misorientation on the properties of N-polar GaN films grown by metal organic chemical vapor deposition

Electrically pumped room-temperature semiconductor sources of tunable terahertz radiation in 1-5 THz spectral range are highly desired to enable compact instrumentation for THz sensing and spectroscopy. Quantum cascade lasers with intra-cavity difference-frequency generation are currently the only room-temperature electrically pumped semiconductor sources that can operate in the entire 1-5 THz spectral range. Here we demonstrate that this technology is suitable to implementing monolithic room-temperature terahertz tuners with broadband electrical control of the emission frequency. Experimentally, we demonstrate ridge waveguide devices electrically tunable between 3.44 and 4.02 THz.