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Register a new userTunable and Dual-broadband Giant Enhancement of SHG and THG in a Highly-engineered Graphene-Insulator-Graphene Metasurface
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We demonstrate a novel scheme to dramatically enhance both the second- and third-harmonic generation in a graphene-insulator-graphene metasurface. The key underlying feature of our approach is the existence of a double-resonance phenomenon, namely the metasurface is designed to possess fundamental plasmon resonances at both the fundamental frequency and the higher harmonic. In particular, this dual resonant field enhancement at the two optical frequencies, combined with a favorable spatial overlap of the optical near-fields, lead to the increase of the generated higher harmonic by several orders of magnitude. Remarkably, we demonstrate that by tuning the Fermi energy of the graphene gratings the dual-resonance property can be locked-in over a broad spectral range of ~20 THz, and equally important, the enhanced nonlinear frequency generation process can be readily switched in the same device between the second and third harmonic. This new type of graphene metasurface could open up new avenues towards the development of novel ultra-compact and multi-frequency active photonic nanodevices.
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We present a graphene-based metasurface that can be actively tuned between different regimes of operation, such as anomalous beam steering and focusing, cloaking and illusion optics, by applying electrostatic gating without modifying the geometry of the metasurface. The metasurface is designed by placing graphene nano-ribbons (GNRs) on a dielectric cavity resonator, where interplay between geometric plasmon resonances in the ribbons and Fabry-Perot resonances in the cavity is used to achieve 2$pi$ phase shift. As a proof of the concept, we demonstrate that wavefront of the field reflected from a triangular bump covered by the metasurface can be tuned by applying electric bias so as to resemble that of bare plane and of a spherical object. Moreover, reflective focusing and change of the reflection direction for the above-mentioned cases are also shown.
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Metasurfaces incorporating graphene hold great promise for dynamic manipulation of terahertz waves. However, it remains challenging to design a broadband graphene-based terahertz metasurface with switchable functionality of half-wave plate (HWP) and quarter-wave plate (QWP). Here, we propose a graphene-metal hybrid metasurface for achieving broadband switchable HWP/QWP in the terahertz regime. Simulation results show that, by varying the Fermi energy of graphene from 0 eV to 1 eV, the function of the reflective metasurface can be switched from an HWP with polarization conversion ratio exceeding 97% over a wide band ranging from 0.7 THz to 1.3 THz, to a QWP with ellipticity above 0.92 over 0.78-1.33 THz. The sharing bandwidth reaches up to 0.52 THz and the relative bandwidth is as high as 50%. We expect this broadband and dynamically switchable terahertz HWP/QWP will find applications in terahertz sensing, imaging, and telecommunications.
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