A method and system for authenticating an item includes providing the item including a polymer substrate comprising a polymer material and a doping material, the polymer material and the doping material configured to transmit radiation laterally through the polymer substrate, and the doping material capable of scattering radiation and absorbing radiation of at least one specific wavelength to generate a spectral signature in a spectral band of wavelengths of the transmitted radiation, irradiating the item with incident radiation characterized by a spectral band of wavelengths spanning a band of wavelengths including the at least one specific wavelength absorbed and scattered by the doping material, detecting the spectral signature after the radiation is transmitted laterally through the polymer substrate, and determining a code associated with the spectral signature.