![]() ![]() Full control over the polarization of electromagnetic waves requires dynamic control over these polarization effects. In contrast, directionally asymmetric transmission of linearly polarized waves occurs in anisotropic structures with 3D chirality (the 3D twist of helices) and arises from different conversion efficiencies between orthogonal linear polarizations that interchange for opposite propagation directions (linear conversion dichroism) 17, 18, 19, 20, 21, 22. Directionally asymmetric transmission of circularly polarized waves occurs in lossy, anisotropic structures with 2D chirality (the 2D twist of spirals) and arises from different conversion efficiencies between left- and right-handed circularly polarized waves that interchange for opposite propagation directions (circular conversion dichroism) 13, 14, 15, 16. Metamaterials, which derive enhanced or novel electromagnetic properties from artificial structuring on the sub-wavelength scale, have enabled orders-of-magnitude enhancements of linear anisotropy 3, 4, 5, 6 and optical activity 7, 8, 9, 10, 11, 12 from microwave to optical frequencies, and led to the discovery of directionally asymmetric effects that are reciprocal. The Faraday effect is characterized by interchanged phase delays and interchanged transmission levels for opposite propagation directions of its circularly polarized eigenstates (used in optical isolators and circulators) 2. Directional asymmetries arise from the broken reciprocity of the wave-matter interaction under static magnetic field, which is known as the Faraday effect. Linear anisotropy and optical activity are the same for opposite wave propagation directions. ![]() media that are different from their mirror image, and manifests itself as different phase delays (circular birefringence, used in polarization rotators) and different transmission levels (circular dichroism, used in circular polarizers) for left-handed and right-handed circular eigenpolarizations. Optical activity occurs in 3D-chiral media, i.e. Linear anisotropy results from preferred directions in the material’s structure and manifests itself as different phase delays (linear birefringence, used in wave plates) and different transmission levels (linear dichroism, used in linear polarizers) for orthogonal, linear eigenpolarizations. Established methods for polarization manipulation employ linear anisotropy, optical activity and the Faraday effect in crystals, which require long interaction lengths (compared to the wavelength) to accumulate significant polarization changes 1. Polarization is a fundamental property of electromagnetic waves and the ability to manipulate polarization states underpins numerous applications throughout the electromagnetic spectrum. Potential applications include directionally asymmetric active devices as well as intensity and polarization modulators for electromagnetic waves. We demonstrate the effect numerically and experimentally, describe it analytically and explain the underlying physical mechanism based on simulated surface current distributions. As the metamaterial is heated, the insulator-to-metal phase transition of vanadium dioxide effectively renders the structure achiral and the transmission asymmetry vanishes. The chiral structure exhibits pronounced asymmetric transmission at room temperature when vanadium dioxide is in its insulator phase. The effect is observed in a terahertz metamaterial containing 3D-chiral metallic inclusions and achiral vanadium dioxide inclusions. Here we demonstrate thermal switching of asymmetric transmission of linearly polarized terahertz waves. Chiral materials can exhibit different levels of transmission for opposite propagation directions of the same electromagnetic wave. ![]()
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