Near-field manipulation of terahertz surface waves by metasurfaces [Invited]

Surface waves（SWs） are a special form of electromagnetic waves that travel along the boundary between a metal and a dielectric. The special optical properties of SWs render them very attractive in applications, such as subdiffractional lithography, novel biochemical sensors, and ultrafast integrated circuitries. Herein, we present a review of our recent progress in excitation and manipulation of terahertz SWs due to interference or coupling between a pair of slit resonators in metasurfaces, showing the ability to devise ultrathin and compact plasmonic components.

Tunable and multichannel terahertz perfect absorber due to Tamm surface plasmons with graphene

In this paper, we have shown that perfect absorption at terahertz frequencies can be achieved by using a composite structure where graphene is coated on one-dimensional photonic crystal(1 DPC) separated by a dielectric. Due to the excitation of optical Tamm states(OTSs) at the interface between the graphene and 1 DPC, a strong absorption phenomenon occurs induced by the coupling of the incident light and OTSs. Although the perfect absorption produced by a metal–distributed Bragg reflector structure has been researched extensively, it is generally at a fixed frequency and not tunable. Here, we show that the perfect absorption at terahertz frequency not only can be tuned to different frequencies but also exhibits a high absorption over a wide angle range. In addition,the absorption of the proposed structure is insensitive to the polarization, and multichannel absorption can berealized by controlling the thickness of the top layer.

Linear polarization conversion of transmitted terahertz wave with double-layer meta-grating surfaces

In this Letter, we demonstrate a linear polarization conversion of transmitted terahertz wave with double-layer meta-grating surfaces, which integrated the frequency selectivity of a split ring resonator metasurface and the polarization selectivity of a metallic grating surface. Since the double-layer can reduce the loss, and the Fabry- Perot like resonant effect between the two layers can improve the conversion efficiency, this converter can rotate the incident y-polarized terahertz wave into an x-polarized transmitted wave with relatively low loss and high efficiency. Experimental results show that an average conversion efficiency exceeding 75% from 0.25 to 0.65 THz with the highest efficiency of 90% at 0.43 THz with onlv -2 dB loss has been achieved.

Tunable terahertz wave difference frequency generation in a graphene/AlGaAs surface plasmon waveguide

Graphene-based surface plasmon waveguides(SPWs) show high confinement well beyond the diffraction limit at terahertz frequencies. By combining a graphene SPW and nonlinear material, we propose a novel graphene/AlGaAs SPW structure for terahertz wave difference frequency generation(DFG) under near-infrared pumps.The composite waveguide, which supports single-mode operation at terahertz frequencies and guides two pumps by a high-index-contrast AlGaAs∕Al Oxstructure, can confine terahertz waves tightly and realize good mode field overlap of three waves. The phase-matching condition is satisfied via artificial birefringence in an AlGaAs∕Al Ox waveguide together with the tunability of graphene, and the phase-matching terahertz wave frequency varies from 4 to 7 THz when the Fermi energy level of graphene changes from 0.848 to 2.456 eV. Based on the coupled-mode theory, we investigate the power-normalized conversion efficiency for the tunable terahertz wave DFG process by using the finite difference method under continuous wave pumps, where the tunable bandwidth can reach 2 THz with considerable conversion efficiency. To exploit the high peak powers of pulses, we also discuss optical pulse evolutions for pulse-pumped terahertz wave DFG processes.