A structure for measuring energy absorption by a surface plasmon receptor or NFT on a waveguide comprises a first waveguide, a first input grating for coupling light comprising a first wavelength into the first waveguide, a first output grating for coupling light out of the first waveguide, a first plurality of surface plasmon receptors in cooperation with the first waveguide to receive light energy and located between the first input grating and the first output grating. The structure may further comprise a second waveguide, a second input grating for coupling light into the second waveguide, a second output grating for coupling light out of the second waveguide, a second plurality of surface plasmon receptors between the second input grating and the second output grating and in cooperation with the second waveguide to receive light energy, wherein the second plurality may be less than or greater than the first plurality.