Ok, I'll take the plunge....it is my understanding vcsel's can be made using indium phosphosp or gallium arsenide.  They provide the light source for our interposer, the silicon substrate the electronic (CMOS)   The interposer becomes key to integrating the two.  I usually don't comment on specific technology, so comments or corrections welcomed.  Also, different light wavelengths would equate to different colors of light.  :)  Bt