  We report on the operation of a double metal waveguide far-infrared quantum
cascade laser emitting at 28 $\mu$m, corresponding to the AlAs-like phonon
Reststrahlen band. To avoid absorption by AlAs-like optical phonons, the
Al-free group-V alloy GaAs$_{0.51}$Sb$_{0.49}$ is used as a barrier layer in
the bound-to-continuum based active region. Lasing occurs at a wavelength of
28.3 $\mu$m, which is the longest wavelength among the quantum cascade lasers
operating from mid-infrared to far-infrared. The threshold current density at
50 K is 5.5 kA/cm$^{2}$ and maximum operation temperature is 175 K. We also
discuss the feasibility that operation wavelength cover the whole spectral
range bridging between mid-infrared and terahertz by choosing suited group
III-V materials.
