a shade green


A year ago I read that green light had been emitted from silicon. The effect, apparently, was enabled by a substantial reduction of the group velocity of a guided near-infrared pump signal, which led directly to a corresponding spatial compression of the guided field. The light emitted was a third harmonic generation of wavelength 520 nm. In more recent times I have heard of a green comet in Plymouth. I wonder what colour of green the comet was, and under which generation its harmony was achieved; a green light; a shade green.