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