An optical device includes a small and low-cost variable polarization plane rotator that can control a rotation angle of the polarization plane easily. The object of the invention is to provide a device as referred to in the introduction that enables a more precise orientation of the ultrasound transducer relative to the target region. The carbon nanotube film is placed on a surface of a softened polymer substrate. Such devices can be formed from nanoscale wires such as nanowires or nanotubes. A second period M is determined for the partial scan section according to the time interval p, the first period T, and the number of channels N.