Applied Physical Letters – Reconfigurable gradient index using VO2 memory metamaterials Being able to tune metamaterial devices at this level of precision – repeatedly, as required, and after the metamaterial has been fabricated – opens the door to new techniques, including the ability to manufacture Gradient Index of Refraction (GRIN) devices, that can be used for a variety of imaging and communication technologies.
We demonstrate tuning of a metamaterial device that incorporates a form of spatial gradient control. Electrical tuning of the metamaterial is achieved through a vanadium dioxide layer which interacts with an array of split ring resonators. We achieved a spatial gradient in the magnitude of permittivity, writeable using a single transient electrical pulse. This induced gradient in our device is observed on spatial scales on the order of one wavelength at 1 THz. Thus, we show the viability of elements for use in future devices with potential applications in beamforming and communications.
In this work, we demonstrate a spatially reconfigurable THz metamaterial made from the hybridization of vanadium dioxide (VO2) and split ring resonators (SRRs). The SRR has been the “fruit fly” of metamaterials research, allowing for the convenient implementation of optical characteristics which are unattainable without the use of metamaterials. Our device is composed of an array of 100 nm thick gold SRRs lithographically fabricated on 90 nm thick VO2 grown on a sapphire substrate. VO2 undergoes an insulator to metal transition which can be triggered thermally electronically or optically. The phase transition is hysteretic, and as a result, changes in the conductivity of VO2 generally persist, provided the device temperature is maintained. Hybrid metamaterial-VO2 devices benefit from this memory, as well as the large tuning dynamic range achievable with VO2. Persistent tuning takes advantage of this memory to eliminate the need for continuous stimulation or repeated excitation of the device. Our measurements demonstrate the ability to perform persistent tuning of a hybrid SRR-VO2 device with a spatial conguration, writing a gradient in the permittivity of the hybrid metamaterial.
Our SRR-VO2 device with two electrical contacts is the simplest hybrid metamaterial structure with which to explore advantages of spatial gradient tuning. With it, we have been able to show the ability to induce and spectroscopically probe a persistent spatial gradient within a previously uniform device. Use of finer or even pixel-by-pixel control over switching would allow for creation of precise gradients. Such pixel-level switching nearly necessitates the existence of memory within the device as the difficulty of repeated or sustained excitation would make detailed switching unfeasible. Future flexibility in the optical parameters of metamaterials as a result of tuning occurring on the pixel level will enable the creation of dynamic hybrid metamaterial structures suitable for a wide range of applications.
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