Using leakage-radiation microscopy, we characterize the efficiency of unidirectional surface-plasmon excitation with periodic (800 nm) arrays of 130-nm-high and 330-nm-wide gold ridges on a thin gold film illuminated with a focused (5-mu m-wide) laser beam. We demonstrate that, at the resonant wavelength of 816 nm, the excitation efficiency of > 0.4 can be obtained with >= 5 ridges by adjusting the beam position. Conducting numerical simulations, we account for the experimental results and calculate the electric-field enhancement achieved near the gold surface. (C) 2009 Optical Society of America