15th International Symposium on Space Terahert: Technology A Phase-Locked Terahertz Quantum Cascade Laser

Progress in coherent receiver development at terahertz frequencies has been hindered by the lack of a suitable local oscillator source that can be qualified for space missions. Space systems now in development rely on harmonic multipliers of microwave sources. Multipliers have achieved usable performance up to 1.6 THz, and probably, with a lot of work can be extended to 2.0 THz, but above that hope dims. The science driving the development of THz spectral line receivers requires capability up to at least 4.75 THz (the neutral oxygen line). Ideally, the THz LO should be some type of tunable semiconductor laser that can be locked to a harmonic of a millimeter-wave source. In the early 1990's a new type of semiconductor laser was developed called the quantum-cascade-laser. This device is unipolar in that only transitions of electrons between sub-levels in the conduction band are needed to produce laser emission. Although the QCL was originally developed as an infrared laser operating well above the 8-9 THz reststrahl band of GaAs/GaAlAs, it was eventually realized that operation below the reststrahl band was also possible, but much more difficult. The first terahertz QCL was demonstrate