  We report the development of the achromatic half-wave plate (AHWP) at
millimeter wave for cosmic microwave background polarization experiments. We
fabricate an AHWP consisting of nine a-cut sapphire plates based on the
Pancharatnam recipe to cover a wide frequency range. The modulation efficiency
and the phase are measured in a frequency range of 33 to 260 GHz with incident
angles up to 10 degrees. We find the measurements at room temperature are in
good agreement with the predictions. This is the broadest demonstration of the
AHWP at the millimeter wave.
