Waveplate

Waveplates are used to change the polarization type of the incident radiation by unequally phase-shifting its orthogonally polarized components.
Sinocera Photonics manufactures Multiple Order  and Cemented Zero Order Quarter and Half Waveplates from high quality artificial crystal quartz. 
Multiple Order Waveplates are fabricated from a single 1 mm thick plate,  The retardence at the specified wavelength is equal to an integral namber of waves plus 1/4 or 1/2 λ.
Zero Order Waveplates are made from two individual plates of crystalline quartz, cemented or air spaced with the direction of retardation opposed,. The final retardation is the net difference in retardation of the two plates, 1/4 or 1/2 λ.

Waveplates can modify the Polarization State of light which is very useful in many applications. It was also called retardation plates (or just wave plates or retarders for short).