Resonant-phonon terahertz quantum-cascade lasers using metal-metal waveguides

We report the development and progress of terahertz quantum cascade lasers (QCLs) that use the resonant-phonon depopulation scheme to obtain population inversion and where a low-loss, high-modal-confinement metal-metal waveguide is used. These waveguides were fabricated using an indium-gold reactive wafer bonding technique followed by substrate removal, and have been used to obtain lasing up to 137 K in pulsed mode and 93 K in continuous wave (cw) mode. Also, lasing has been obtained at frequencies as low as 2.1 THz (/spl lambda/=141 /spl mu/m), which is the longest wavelength achieved in a QCL without the use of a magnetic field. Recently, an alternative copper-copper thermocompression wafer bonding technique was used to extend cw operation up to a temperature of 97 K.