In this work, we propose broadband and switchable terahertz (THz) polarization converters based on either graphene patch metasurface (GPMS) or its complementary structure (graphene hole metasurface, GHMS). The patch and hole are simply cross-shaped, composed of two orthogonal arms, along which plasmonic resonances mediated by Fabry-Perot cavity play a key role in polarization conversion (PC). An incidence of linear polarization will be converted to its cross-polarization (LTL) or circular polarization (LTC), as the reflected wave in the direction of two arms owning the same amplitude and π phase difference (LTL), or ±π/2 phase difference (LTC). Such requirements can be met by optimizing the width and length of two arms, thickness of dielectric layer, and Fermi level EF of graphene. By using GPMS, LTL PC of polarization conversion ratio (PCR) over 90% is achieved in the frequency range of 2.92 THz to 6.26 THz, and by using GHMS, LTC PC of ellipticity χ ≤ −0.9 at the frequencies from 4.45 THz to 6.47 THz. By varying the Fermi level, the operating frequency can be actively tuned, and the functionality can be switched without structural modulation; for instance, GPMS supports LTL PC as EF = 0.6 eV and LTC PC of χ ≥ 0.9 as EF = 1.0 eV, in the frequency range of 2.69 THz to 4.19 THz. Moreover, GHMS can be optimized to sustain LTL PC and LTC PC of |χ| ≥ 0.9, in the frequency range of 4.96 THz to 6.52 THz, which indicates that the handedness of circular polarization can be further specified. The proposed polarization converters of broad bandwidth, active tunability, and switchable functionality will essentially make a significant progress in THz technology and device applications, and can be widely utilized in THz communications, sensing and spectroscopy.

Graphene and its relatives, such as bilayer and trilayer graphene, are promising plasmonic materials. Very recently, graphene has been demonstrated to be precisely folded (Chen et al 2019 Science 365 1036–40), thus folded graphene provides another appealing platform for plasmonics. In folded graphene nanodisks, we find fundamental dipole modes (DMs) will exhibit mode splitting, with one parallel and another perpendicular to the folding axis. The two DMs show differences in field patterns and folding angle dependence, but they both can be tuned by the size of structures and the Fermi level of graphene. Some interesting high order modes are introduced as well, which can be further engineered by folding. Our studies enrich the current research of graphene plasmonics, and pave the way for particular plasmonic device applications.

Plasmonically induced transparency (PIT), a classical analogue of electromagnetically induced transparency in quantum systems, could offer an efficient way to engineer light-matter interaction and thus achieve particular manipulation of light. To realize PIT, the crucial point is the design of two strongly coupled modes, with one bright and the other dark. The common practice often takes dipole resonance as the bright mode, while higher order resonance as the dark one. We present here an alternative scheme to realize PIT in coupled graphene nanodisks through mechanical folding, where only fundamental dipole resonances are involved, and the dark mode is derived from two nondegenerate dipole resonances in folded nanodisk. Two parameters (folding angle and rotating angle ) arising from folding can be utilized to tune PIT. They determine two different PIT windows separately. By varying either one of them, single-window PIT can be switched to double-window PIT, and vice versa. Besides the mechanical operations, the proposed PIT can also be actively tuned by electrostatic gating. To understand the mechanism properly, a theoretical model of two coupled harmonic oscillators is presented, from which the predictions show good agreement with simulations.