  The optimal frequency for interstellar communication, using "Earth 2017"
technology, was derived in papers I and II of this series (arXiv:1706.03795,
arXiv:1706.05570). The framework included models for the loss of photons from
diffraction (free space), interstellar extinction, and atmospheric
transmission. A major limit of current technology is the focusing of
wavelengths $\lambda<300\,$nm (UV). When this technological constraint is
dropped, a physical bound is found at $\lambda\approx1\,$nm ($E\approx\,$keV)
for distances out to kpc. While shorter wavelengths may produce tighter beams
and thus higher data rates, the physical limit comes from surface roughness of
focusing devices at the atomic level. This limit can be surpassed by
beam-forming with electromagnetic fields, e.g. using a free electron laser, but
such methods are not energetically competitive. Current lasers are not yet cost
efficient at nm wavelengths, with a gap of two orders of magnitude, but future
technological progress may converge on the physical optimum. We recommend
expanding SETI efforts towards targeted (at us) monochromatic (or narrow band)
X-ray emission at 0.5-2 keV energies.
