  In two recent Letters [Phys. Rev. Lett. 90, 046806; Phys. Rev. Lett. 94,
057408], Hirakawa et al. investigate the emission of coherent terahertz (THz)
radiation from optically excited superlattices. The authors interpret the
spectra of the THz transients in terms of a semiclassical transport model and
conclude that the data provide evidence for the existence of the so-called
Bloch gain. We show that the interpretation of the data in terms of a
conductivity is, however, not performed correctly, thus making the conclusions
of cited authors void.

Mesoscale and Nanoscale Physics