We propose a metal-vanadium dioxide (VO₂) metamaterial with broadband and functionality-switchable polarization conversion in the terahertz regime. Simulation results show that the function of the proposed metamaterial can be switched from a half-wave plate (HWP) to a quarter-wave plate (QWP) over a broad bandwidth of 0.66–1.40 THz, corresponding to a relative bandwidth of 71.8%. The HWP obtained when VO₂ is in the insulating state has reflection of 90% and linear polarization conversion ratio exceeding 98% over the bandwidth of 0.58–1.40 THz. By transiting the phase of VO₂ into the conducting state, the obtained QWP can convert the incident linearly-polarized wave to circularly-polarized wave with an ellipticity of 0.99 over 0.66–1.60 THz. Additionally, results show that the proposed broadband switchable HWP/QWP has a large angular tolerance. We expect that this broadband and switchable multi-functional wave plate will find applications in polarization-dependent terahertz systems including sensing, imaging, and telecommunications.

We propose a switchable broadband and wide-angular terahertz asymmetric transmission based on a spiral metasurface composed of metal and VO₂ hybrid structures. Results show that asymmetric transmission reaching up to 15% can be switched on or off for circularly polarized terahertz waves when the phase of VO₂ transits from the insulting state to the conducting state or reversely. Strikingly, we find that relatively high asymmetric transmission above 10% can be maintained over a broad bandwidth of 2.6–4.0 THz and also over a large incident angular range of 0^°–45^°. We further discover that as the incident angle increases, the dominant chirality of the proposed metasurface with VO₂ in the conducting state can shift from intrinsic to extrinsic chirality. We expect this work will advance the engineering of switchable chiral metasurfaces and promote terahertz applications.

Plasmonic surface lattice resonances (SLRs) supported by metal nanoparticle arrays exhibit narrow linewidths and enhanced localized fields and thus are attractive in diverse applications including nanolasers, biochemical sensors and nonlinear optics. However, it has been shown that these SLRs have much worse performance in a less symmetric environment, hindering their practical applications. Here, we propose a novel type of narrow SLRs that is supported by metal-insulator-metal nanopillar arrays and that has better performance in a less symmetric dielectric environment. When the dielectric environment is as asymmetric as the air/polymer environment with a refractive index contrast of 1.0/1.52, the proposed SLRs have high quality factors of 62 under normalincidence and of 147 under oblique incidence in the visible regime. We attribute these new SLRs to Fano resonance between an in-plane dipole and an out-of-plane quadrupole (or dipole) that are respectively supported by the two metal ridges under normal (or oblique) incidence. We also show that the resonance wavelength can be tuned by varying the geometric sizes or by changing the angle of incidence. By doing this, we clarify the conditions for the formation of the proposed SLRs and illustrate their attractive merits in sensing applications. We expect that this new SLR can open up applications in asymmetric dielectric environments.

We demonstrate Fourier single-pixel imaging in the terahertz regime. The experimental system is implemented using a photoinduced coded aperture setup, where monolayer graphene on a high-resistance silicon substrate illuminated by a coded laser beam works as a terahertz modulator. The results show that high-quality terahertz images can be reconstructed using a greatly reduced number of measurements. We further find that deep photoinduced terahertz modulation, by adding a monolayer graphene on the silicon substrate and by using high laser power, can significantly improve the image quality. Compared to Hadamard single-pixel imaging with a reordered Hadamard matrix, the Fourier approach has higher image quality. We expect that this work will speed up the efficiency of single-pixel terahertz imaging and advance terahertz imaging applications.