The grating can have {HEAD ~ a periodicity} from 200 nm to {TAIL ~ 500 nm} 
to
The grating can have { HEAD ~ a periodicity } from 200 nm to { TAIL ~ 500 nm }
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL