We report terahertz frequency (3.5 THz, λ≃87 μm) emission from quantum-cascade lasers employing a bound-to-continuum transition in the active region. The maximum operating temperature is in excess of 90 K. Peak powers of 20 mW at 20 K and 10 mW at 77 K are achieved. The same devices show continuous-wave operation up to 55 K with measured optical powers of 15 mW at 10 K.

A quantum-cascade laser operating at λ=66 μm is demonstrated. It consists of a three-quantum-well chirped-superlattice active region embedded in a waveguide based on a single interface plasmon and a buried contact. A threshold current density of 210 A/cm2 at T=12 K, a maximum peak optical power of 4 mW, and operation up to T=44 K are achieved in a 2.7 mm long device with a high reflectivity backfacet coating.

We have measured the spectral linewidths of three continuous-wave quantum cascade lasers operating at terahertz frequencies by heterodyning the free-running quantum cascade laser with two far-infrared gas lasers. Beat notes are detected with a GaAs diode mixer and a microwave spectrum analyzer, permitting very precise frequency measurements and giving instantaneous linewidths of less than -30 kHz. Characteristics are also reported for frequency tuning as the injection current is varied.

We report high power quantum cascade lasers operating above liquid nitrogen temperature at λ≃87 and 130μm based on a bound-to-continuum transition. For λ≃87μm, 56mW peak power in pulsed operation and 50mW continuous wave operation at 10K are demonstrated. At λ≃130μm, a peak power of 50mW was achieved and devices operated in continuous wave reached a maximum temperature of 53K with an optical power of 11.5mW at T=10K. Lifetimes are extracted from the scaling of the transport and laser parameters as a function of size using a simple rate equation model.

We demonstrate heterodyne mixing of two free-running, multimode, 3.3-THz quantum cascade lasers by use of a point-contact Schottky diode. By temperature tuning the emission wavelength of one laser, a difference frequency signal spanning the 2-4-GHz range is obtained, with a signal-to-noise ratio of 30 dB. The frequency of the heterodyne signal is subject to random fluctuations of a few megahertz, principally from instabilities in the temperatures of the devices. From single-shot measurements we derive an instantaneous linewidth for a single Fabry-Perot mode of 20 kHz, corresponding to an integration time of 3.6 ms.