Name	Info	Provider	Link
	free resource	Optica Publishing Group	https://opg.optica.org/abstract.cfm?URI=oe-29-18-29095
Full text from	free resource	Enlighten: Publications, University of Glasgow	https://eprints.gla.ac.uk/276142
	free resource	Strathprints - the University of Strathclyde	https://strathprints.strath.ac.uk/77688/