A method of forming a non-optically detectable identifiable mark at an outer surface of an article formed from a solid state material, said method including the steps of forming a plurality of recesses within a predetermined region of a photoresist 5 applied to an outer surface of an article formed from a solid state material, wherein said plurality of recesses is formed by two-photon absorption lithography and wherein said one or more recesses extend at least partially through the photoresist and from an outer surface of the photoresist and towards said outer surface of the article 10 formed from a solid state material; and applying an etching process such that at least a portion of the outer surface of said article is exposed and etched so as to form a plurality of etched portions extending into said article from the outer surface of the article and corresponding to said plurality of recesses; wherein said predetermined region of said photoresist defines an identifiable mark to be applied to the outer 15 su
