Polarization converters are key elements for optical and photonic technologies. Transmissive or reflective elements that enable rotating the polarization plane of linearly-polarized light (linear-to-linear conversion) or turning linearly-polarized light into circularly-polarized (linear-to-circular conversion) are particularly relevant and already ubiquitous. However, to meet the demand for more compact and performant devices, novel elements combining multiple polarization functionalities on a small-footprint platform are needed. To create them, scientists work on designing artificial materials, called metamaterials, which could enable the targeted functionalities while remaining very thin.

In this context, Zhen Qiao and coauthors have found theoretically an appealing metamaterial design for applications in the THz spectral region. It combines gold microstructures, polyimide layers, and a phase-change vanadium oxide structure, to enable multiple, temperature and frequency switchable functionalities within a less-than-50 μm thickness. At room temperature, where vanadium oxide is transparent, the metamaterial behaves as a transmissive linear-to-circular converter. Above 68ºC, where vanadium oxide becomes metallic, the metamaterial is switched into a reflective polarization converter. In this configuration, polarization conversion can be switched from linear-to-circular to linear-to-linear by changing the frequency of the incident light, as a result of the excitation of distinct electromagnetic modes. Remarkably, the conversion performance remains excellent from normal incidence to oblique incidence at very large angles.

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