According to a first aspect of the present invention, there is provided a method of making an optically readable element, the method comprising: providing one or more optically readable structures in or on a body, a strain being applied to the one or more optically readable structures as a result of an interaction between the body and the one or more optically readable structures; the one or more optically readable structures each having an intrinsic band structure prior to application of the strain, and wherein the applied strain changes that intrinsic band structure; and wherein the interaction is such that the strain is maintained, after the element has been made, without the need for an external influence.