EXAMINATION OF THE -1 SURFACE-PLASMON MINI-GAP ON A GOLD GRATING(1, )

An energy gap in the excitation of surface plasmons is found for light at normal incidence to a gold grating. This gap occurs at the crossin g of the plus and minus first order surface plasmons. It arises direct ly as a consequence of distortion of the grating from sinusoidality, t he first harmonic of the grating providing coupling between the plus a nd minus one orders. Experiments have been performed using both wavele ngth scans, where at a fixed angle of incidence the wavelength of exci tation is varied, and angle of incidence scans, where for a fixed wave length the angle of incidence is varied a few degrees either side of n ormal to the grating. By fitting the angular dependent reflectivity sc ans using grating modelling theory the gold grating is characterized a t all wavelengths. This then allows a detailed comparison of the theor etical dispersion curve with that obtained experimentally. The agreeme nt for both p-polarized light (for angle dependence with the plane of incidence normal to the grating grooves) and for s-polarized light (an gle dependence with the plane of incidence perpendicular to the gratin g grooves) is excellent. An apparent momentum gap in the lower energy branch of the dispersion curve, attributed to the loss of coupling str ength, is found to move to the upper branch if the grating profile is inverted.