LACK OF MAGIC FREQUENCIES AT OPTICAL WAVELENGTHS
In the microwave regime it has been relatively easy to identify a range of frequencies where ETIs are likely to place their beacon signals. It is not obvious where the corresponding magic frequencies or wavelengths would be located in the optical spectrum.
When Optical SETI was first suggested by Charles Townes in 1961, the Carbon Dioxide wavelength of 10.6 microns was selected as being an optical magic wavelength. This laser transition is very efficient, there is plenty of carbon dioxide in space, and our atmosphere is reasonably transparent at that infrared wavelength.
Other efficient laser transitions would also be suitable as so-called magic wavelengths. However, if we assume that ETIs would use pulsed laser beacons, as suggested by Monte Ross in the 70's, then we don't need to know or guess the magic frequencies - just the broad magic frequency regime. This means that we can substantially reduce the frequency space we need to search. Indeed, although visible frequencies are five orders of magnitude higher than microwave frequencies, the amount of frequency search space needed in the optical regime need not be much more than in the microwave regime. Our search bins at optical wavelengths can be five or more magnitudes wider than those employed at microwave wavelengths.