First, the fact that the polarizer doesn’t kill the yellow light is consistent with the idea that it’s fluoresence, since fluoresence in an isotropic medium doesn’t generally preserve the polarization of its exciting light.

Second, there may be a component of Raman scattering (scattering which shifts the energy of the exciting light by creating or annihilating vibrations in the medium) to the observed light. It’s probably several orders of magnitude below the flouresence, but it would likely show up on a spectrometer.

A correctly-adjusted polariser eats all of the green speckle pattern visible along the amber beam, leaving the amber clearer and more visible, but otherwise unchanged.

(And the usual trippy stuff happens if I put the Lichtenberg figure between the polariser and a source of polarised light. Especially if I leave the acrylic block in its protective Ziploc bag!)

So I reckon it’s either fluorescence or maybe even Raman scattering. Not that I’m an expert.

Try it with a polarising filter and see what happens.

Also, blogsome is silently ignoring my posts when I log in as stewpot. Strange.