Reflectance spectra of Co and Ag/Co/Ag nanograting samples. Measured reflectivity with s-polarized light in the light path configuration shown in inset in panel (b) at different incident angles (right axis) and calculated extinction cross section of a single strip (left axis) for (a) sample Co385, (b) sample Co325, (c) sample ACA385, and (d) sample ACA325. Measured reflectivity with the p-polarized light in the light path configuration shown in inset in panel (e) for (e) sample Co385, (f) sample Co325, (g) sample ACA385, (h) sample ACA325. (i) Calculated incident angle dependence of spectral position to excite SPPs at the air-metal (Ag, Co) interface.