Which type of electromagnetic radiation has the greatest frequency and lowest wavelength?

What we currently observe as gamma radiation from nuclear events in stars is something like 10 to 100 exahertz. That's about the upper frequency limit of the electromagnetic radiation we know of, and it will have the shortest wavelength (something at or under 30 nano millimeters). It isn't a stretch to imagine frequencies higher than that, but what is it that can generate electromagnetic oscillations in that range?