General approach of terahertz achromatic quarter-wave plate composed of stacked quartz plates

Typical waveplates are limited to specific frequencies. We propose a general design of a series of THz achromatic quarter-wave plates that work for different frequency range. The simplified formulas and the basic parameters that are used to calculate the thicknesses and rotating angles with the possibility of choosing the frequency range are provided. The main feature of the design is that the number of the x-cut quartz plates is equal to the frequency expansion factor (f max /f min ) of the resulting waveplate, and by altering the thicknesses of quartz plates following a specific pattern only, the frequency range of the waveplate can be shifted. Two achromatic waveplates working in the frequency range of 0.2-0.6 THz and 0.2-1.2 THz were manufactured for testing. The measurement result demonstrates the reliability of the proposed design.