Achromatic THz wave plates are a prerequisite for THz polarization operation, especially in THz time domain spectroscopy techniques. We demonstrated in this paper the design of quartz achromatic THz quarter wave plate by simulated annealing algorithm. The phase retardation, retardation detuning, thickness and absorption loss for a quartz quarter wave plate that are combined by up to 8 quartz plates are discussed. It is shown that as the number of the combined quartz plates increasing, a remarkable retardation stability over an ultrabroadband frequency range (0.2–3 THz) could be achieved. It is found that there should be a compromise between the retardation detuning and absorption loss of THz wave plate. For a practical THz achromatic quarter wave plate operated from 0.2 THz to 3 THz, the optimized quartz plate number is about five.
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