For example, {HEAD ~ the maximum gain} can occur at {TAIL ~ an energy} at or below the maximum band gap emission 
occur at
For example , { HEAD ~ the maximum gain } can occur at { TAIL ~ an energy } at or below the maximum band gap emission
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH