We modeled, designed, and built a 500-MHz acousto-optic mode locker with a diffraction efficiency of 28% per 1 W drive power. The transducer is zinc oxide sputtered onto a sapphire substrate. A new figure of merit is defined for the mode-locker design, which indicates that sapphire is a good substrate material. Pulse widths of less than 10 psec with an average power of 150 mW were achieved from a 500-MHz pulse-rate, diode-pumped, cw mode-locked Nd:YLF laser using a pump power of 700 mW.