Wave plate and divided prism member

A wave plate comprising:

• a prism member having an entrance surface for receiving a terahertz wave, and an exit surface for emitting the terahertz wave received by the entrance surface;
• wherein
• the prism member is constituted by a plurality of waveguide regions having:
• a partial entrance surface for receiving a part of the terahertz wave, a plurality of total reflection surfaces for totally reflecting the terahertz wave from the partial entrance surface, and a partial exit surface for emitting the terahertz wave totally reflected on the total reflection surfaces;
• each of the partial entrance surfaces combine to constitute the entrance surface of the prism member, and each of the partial exit surfaces combine to constitute the exit surface of the prism member, by stacking the plurality of the waveguide regions;
• a gap portion is arranged between the total reflection surfaces in each of the adjacent waveguide regions, the gap portion being situated along an entire inclined surface;
• and
• contact surfaces of the plurality of waveguide regions on opposites sides from each other in a direction perpendicular to an optical axis have different lengths in a direction along the optical axis.