Anisotropic and Chiral Properties

Many metamaterials exhibit anisotropy where their properties depend on the direction of propagation of the electromagnetic wave. Their response also depends on the handedness or chirality of polarized light. This allows manipulating properties like refractive index that depend on polarization or direction. Anisotropic and chiral metamaterials find uses in polarized filters, polarization-sensitive imaging, and optical communication isolation. They also enable novel effects like circular dichroism where left and right circular polarizations experience different indexes of refraction within the same material.

Tunable and Reprogrammable Metamaterials

More recent developments have led to the creation of Metamaterials  whose properties can be externally tuned or reprogrammed after fabrication. This is achieved by integrating active and stimulus-responsive elements into the metamaterial structure. For example, semiconductor elements allow modifying a metamaterial's refractive index using an externally applied voltage. Similarly,by incorporating stimuli-sensitive polymers, liquids crystals or microfluidics, optical, thermal, electronic or chemical inputs can control metamaterial responses in real-time. Tunable metamaterials open new dimensions for applications in sensing, ultrafast switching, and adaptive photonics and transformation optics.

Prospects and Challenges

While metamaterials have demonstrated revolutionary capabilities, scaling up fabrication over large areas to optical frequencies remains challenging. Current bottom-up techniques struggle to assemble perfect periodic nanostructures over macroscopic volumes needed for practical devices. Top-down approaches like nanolithography make progress but remain expensive and low-yield for complex 3D metamaterials. Another limitation stems from losses inherent to metal-based designs at optical frequencies. Further advances in low-loss dielectric and graphene-based designs as well as self-assembly techniques are expected to overcome these issues. Continued exploration of exotic material responses from higher dimensions is also an active area. Overall, metamaterials research holds immense potential to realize capabilities never before possible, advancing fields from healthcare to communications to energy. With progress on scalable fabrication, they may transform technologies in the coming decades.

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