The authors propose the use of optical/near-IR masers as an alternative to radio transmissions for the purposes of searching for extraterrestrial intelligences (ETIs). In that way, they are suggesting new search methods.
As of 1961 (when this paper was published), the “[development of maser oscillators in the optical/near-IR spectral region which would allow transmission across several light-years]” was on the horizon. Interestingly, the authors state that such masers could have been thoroughly developed even 30 years earlier, suggesting that other ETIs may have pursued such avenues (as opposed to radio transmission).
Technology in 1961 suggested that the continuous operation of high power masers was entirely within the realm of possibility, and the outlook has only brightened in the years since then. One issue relates to the directability of such a maser, and the authors suggest that the problem can be overcome by employing masers in tandem with optical systems. They recommend a coordinated system of 25 masers as the optimal configuration.
There are two important factors that must be considered when evaluating the detectability of a maser signal: (1) it must produce a sufficiently large photon flux and (2) it must be distinguishable from the astronomical background. The authors argue that the first condition can be easily satisfied, and, again, the outlook has only become more optimistic over the past half century. The second condition requires more consideration. Due to the small separation between Earth and the Sun, it is likely that a maser signal cannot be spatially separated from the light of the host star around which the signal is originating. The authors suggest transmitting far away from the peak energy output of the Sun (~5000 Angstrom), i.e., either the extreme violet (shortward of ~2000 Angstrom) or in the near-IR. The former choice suffers from limited atmospheric transmission, while the latter suffers more from the diffraction limit at longer wavelengths (if such a limit is applicable for the system being employed). The authors also suggest transmitting in strong absorption features, e.g., the Ca II H or K lines. Since 1961, our knowledge of stellar populations and corresponding exoplanet systems has greatly improved, so perhaps it is more useful to optimize the transmission interval for a typical M dwarf (as opposed to the Sun, which is a G dwarf).
This study proposes a very novel idea. When considering how to find an ETI (and by extension, in trying to envision how they might attempt interstellar communication) it is important to broaden our perspective and consider all possibilities.