A method and system for writing a label (defined within a predetermined region of the sample 110), the label displaying a visible layout of light-modified regions in a predetermined spatial arrangement. The method comprises: modifying regions of a material within the label using light, wherein the modifying comprises using light of a first polarisation state to provide photo-induced optically active regions of a first type having a first optical activity state which is characteristic of having been formed by light of the first polarisation state, in order to encode covert information in the label using the locations of the first type of light-modified regions within the spatial arrangement of the label.