Polarization conversion element

A polarization conversion element comprising:

• a first light guide including a first main surface, a second main surface on a side opposite to the first main surface, and a side surface;
• a transmissive first polarization grating opposed to the side surface;
• and
• a transmissive second polarization grating opposed to the second main surface, wherein
• the first polarization grating contains nematic liquid crystal aligned along a normal direction of the side surface, and is configured to diffract first circularly polarized light of unpolarized incident light and to diffract second circularly polarized light which rotates in a direction opposite to the first circularly polarized light in a direction different from the first circularly polarized light, and
• the second polarization grating contains nematic liquid crystal aligned along a normal of the second main surface, and is configured to diffract the first circularly polarized light and the second circularly polarized light.