Microwave frequency doubling in ruby

Investigations of quantum mechanical frequency doubling in ruby are reported. Experimental results are given for the 4.04-GHz output from a ruby sample driven at 2.02 GHz when 4.04 GHz corresponds to paramagnetic resonance. The variation of output power with orientation differs appreciably from the well-known variation of the linear (maser) effect. Good agreement with theoretical results has been achieved.