  We develop the microscopic theory of a terahertz (THz) laser based on the
effects of resonant tunneling in a double quantum well heterostructure embedded
in both optical and THz cavities. In the strong coupling regime the system
hosts dipolaritons, hybrid quasiparticles formed by the direct exciton,
indirect exciton and optical photon, which possess large dipole moments in the
growth direction. Their radiative coupling to the mode of a THz cavity combined
with strong non-linearities provided by exciton-exciton interactions allows for
stable emission of THz radiation in the regime of the continuous optical
excitation. The optimal parameters for maximizing the THz signal output power
are analyzed.
