{HEAD ~ Gain} in the concentrated solids occurs at energies equal to or lower than {TAIL ~ the band gap photoluminescence}, i.e., emission 
{ HEAD ~ Gain } in the concentrated solids occurs at energies equal to or lower than { TAIL ~ the band gap photoluminescence } , i.e. , emission
HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH