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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 fromdiffraction (free space), interstellar extinction, and atmospherictransmission. A major limit of current technology is the focusing ofwavelengths $\lambda<300\,$nm (UV). When this technological constraint isdropped, a physical bound is found at $\lambda\approx1\,$nm ($E\approx\,$keV)for distances out to kpc. While shorter wavelengths may produce tighter beamsand thus higher data rates, the physical limit comes from surface roughness offocusing devices at the atomic level. This limit can be surpassed bybeam-forming with electromagnetic fields, e.g. using a free electron laser, butsuch methods are not energetically competitive. Current lasers are not yet costefficient at nm wavelengths, with a gap of two orders of magnitude, but futuretechnological progress may converge on the physical optimum. We recommendexpanding SETI efforts towards targeted (at us) monochromatic (or narrow band)X-ray emission at 0.5-2 keV energies.

Interstellar communication. III. Optimal frequency to maximize data rate