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