Crystal structure of human serum albumin at 2.5 A resolution.

A tunable terahertz achromatic wave plate including at least three phase retarders and a tuning device is provided. The at least three phase retarders are sequentially arranged along a first direction, and configured to provide an achromatic range in a terahertz band, so as to reduce a chromatic aberration of a terahertz wave through the tunable terahertz achromatic wave plate in the achromatic range. The at least three phase retarders respectively include a liquid crystal cell. The tuning device is configured to tune a liquid crystal orientation angle of the liquid crystal cell, so as to correspondingly change a birefringence of the at least three phase retarders in the terahertz band. The terahertz wave through the tunable terahertz achromatic wave plate has same phase retardation in the achromatic range. The achromatic range in the terahertz band is determined according to the birefringence of the at least three phase retarders.