Abstract
Electrolytically tunable graphene “building blocks” for reconfigurable and optically transparent microwave surfaces and absorbers have been designed and fabricated by exploiting Deep Eutectic Solvents (DESs). DESs have been first explored as electrolytic and environmentally friendly media for tuning sheet resistance and Fermi level of graphene together with its microwave response (reflection, transmission and absorption). We consider the tunability of the reconfigurable surfaces in terms of transmittance, absorption and reflectance, respectively, over the X and Ku bands when the gate voltage is varied in the −1.4/+1.4 V range. The numerical simulations and experimental measurements also show the ability of the absorber, in the Salisbury screen configuration, to achieve near perfect absorption with a modulation of about 20%. These results could find applications in several technological fields, ranging from electromagnetic pollution to integrated multi-physical regulation systems, thereby helping the advance of the performance of microwave cloaking systems, stealth windows, frequency selective surfaces, modulators and polarizers.
Highlights
The quest for optically transparent microwave devices has increased over the last years
We demonstrate that Deep Eutectic Solvents (DESs) represent a valid, low-cost alternative to ionic liquids for realizing a graphene capacitor with a suitable electrochemical window
We demonstrate the possibility to realize reconfigurable and optically transparent microwave absorbers
Summary
The quest for optically transparent microwave devices has increased over the last years. The possibility to integrate an optically transparent absorber with an acoustic absorber has been reported[14], leading to a multi-physical regulation system These devices offer new functionalities, they share a common limitation: they are “static” and their properties mainly relate to their geometry. Electrolytic gating allows the controlled switching from pristine (high Rs) to doped graphene (low Rs) This opens the possibility (for an optimized configuration) to switch from a microwave transparent window to an absorbing or reflecting one. Common mixtures derive from a quaternary ammonium salt (e.g. choline chloride, ChCl) and a neutral hydrogen-bond donor such as glycerol (Gly), urea and natural carboxylic acids Because of their minimal ecological footprint, in the last few years DESs have been progressively replacing common and toxic volatile organic compounds in several fundamental and applied processes[21,22]. The realization of “quasi-metal” graphene microwave mirror takes advantage of the chemical protocol based on SOCl2 that allows the realization of graphene sheets with very low sheet resistance (down to less than 20 ohm/sq)[27] that can be implemented as efficient microwave mirror (supplying the need of metallic components)[13]
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