Metasurface-based broadband terahertz polarization converters*

A phase modulating device, such as a phase retarder, allows conversion of the polarization state of electromagnetic waves and is at the core of various disciplines of optics including optical chemistry, bio optics, and optical engineering. In the terahertz spectral range, however, due to the lack of natural substances possessing strong birefringence, metasurfaces have been adapted to obtain desired polarization states in this region. The idea of employing metasurfaces mainly relies on the artificial designing of birefringence in the metasurfaces and the utilization of resonant light interaction with unit resonators. In particular, the latter allows the emergence of asufficient phase retardation in the transmitted light for the polarization conversion, but simultaneously it limits the operating spectral range to the proximity of the resonance frequencies of metasurfaces.

In this work, we introduce a broadband terahertz polarization converter based on ultrathin metasurfaces. Our polarization converter, consisting of anisotropic achiral units, can alter a linearly polarized incident into either left- or right-circularly polarized one depending on the rotation angle of the converter, and operates in a broad spectral range as our proposed design does not require resonant light-unit interaction in the metasurface. The feasibility of our design is demonstrated by performing the terahertz spectroscopy on circular dichroism of amino acids in which our polarization converter enabled distinguish of target molecules from their enantiomers.