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In terahertz (THz) technologies, generation and manipulation of THz waves are two key processes usually implemented by different device modules. Integrating THz generation and manipulation into a single compact device will advance the applications of THz technologies in various fields.Here, we demonstrate a hybrid nonlinear plasmonic metasurface incorporating an epsilon-nearzero (ENZ) indium tin oxide (ITO) layer, for seamlessly combining efficient generation and manipulation of THz waves across a wide frequency band. The coupling between the plasmonic

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Abstract-Applications of metallic metamaterials have generated significant interest in recent years. Electromagnetic behavior of metamaterials in the optical range is usually characterized by a locallinear response. In this article, we develop a finite-difference time-domain (FDTD) solution of the hydrodynamic model that describes a free electron gas in metals. Extending beyond the locallinear response, the hydrodynamic model enables numerical investigation of nonlocal and nonlinear interactions between electromagnetic waves and metallic metamaterials. By explicitly imposing the current continuity constraint, the proposed model is solved in a self-consistent manner. Charge, energy and angular momentum conservation laws of high-order harmonic generation have been demonstrated for the first time by the Maxwell-hydrodynamic FDTD model. The model yields nonlinear optical responses for complex metallic metamaterials irradiated by a variety of waveforms. Consequently, the multiphysics model opens up unique opportunities for characterizing and designing nonlinear nanodevices.

The nonlinear metamaterials have been shown to provide nonlinear properties with high nonlinear conversion efficiency and in a myriad of light manipulation. Here we study terahertz generation from nonlinear metasurface consisting of single layer nanoscale split-ring resonator array. The terahertz generation due to optical rectification by the second-order nonlinearity of the split-ring resonator is investigated by a time-domain implementation of the hydrodynamic model for electron dynamics in metal. The results show that the nonlinear metasurface enables us to generate broadband terahertz radiation and free from quasi-phase-matching conditions. The proposed scheme provides a new concept of broadband THz source and designing nonlinear plasmonic metamaterials.

Abstract-The interaction between electromagnetic field and plasmonic nanostructures leads to both strong linear and nonlinear behaviors. In this paper, a time-domain hydrodynamic model for describing the motion of electrons in plasmonic nanostructures is presented, in which both surface and bulk nonlinearity are considered. A coupled Maxwell-hydrodynamic system capturing full-wave physics and free electron dynamics is numerically solved with the parallel finite-difference time-domain (FDTD) method. The validation of the proposed method is presented by simulating a plasmonic metasurface. The linear response is compared with the Drude dispersion model and the nonlinear terahertz emission from a difference-frequency generation process is theoretically analyzed. The work is fundamentally important to design nonlinear plasmonic nanodevices, especially for efficient and broadband THz emitters.Index Terms-Hydrodynamic model, nonlinear plasmonic nanodevice, finite-difference time-domain (FDTD), terahertz emission, metasurface.

Plasmonic metamaterials and metasurfaces offer new opportunities in developing high performance terahertz emitters/detectors beyond the limitations of conventional nonlinear materials. However, simple meta-atoms for second-order nonlinear applications encounter fundamental trade-offs in the necessary symmetry-breaking and local-field enhancement due to radiation damping that is inherent to the operating resonant mode and cannot be controlled separately. Here we present a novel concept that eliminates this restriction obstructing the improvement of terahertz generation efficiency in nonlinear metasurfaces based on metallic nano-resonators. This is achieved by combing a resonant dark-state metasurface, which locally drives nonlinear SRRs in the near field, with a specific spatial symmetry that enables destructive interference of the radiating linear moments of the SRRs, and perfect absorption via simultaneous electric and magnetic critical coupling of the pump radiation to the dark mode. Our proposal allows eliminating linear radiation damping, while maintaining constructive interference and effective radiation of the nonlinear components. We numerically demonstrate a giant second-order nonlinear susceptibility ∼ 10 −11 m/V, one order improvement compared to previously reported split-ring-resonator metasurface, and correspondingly, two orders of magnitude enhanced terahertz energy extraction should be expected with our configuration under the same conditions. Our study offers a paradigm of high efficiency tunable nonlinear meta-devices and paves the way to revolutionary terahertz technologies and optoelectronic nanocircuitry.Following the emergence of quantum-cascade lasers [1-3] and ultrafast photoconductive switches [4], terahertz (THz), a historically mysterious electromagnetic spectrum bridging microwaves and optics, in the past two decades has gained significant progresses in technologies, leading to tremendous transformative applications in sensing, imaging and communication etc [4][5][6][7][8][9]. The blooming of THz technologies and related applications at current stage demand urgently efficient and compact THz emitters/detectors, yet the development of which remains challenging. Many state-of-the-art THz sources are based on nonlinear optical rectification or difference-frequency generation (DFG) in inorganic crystals, which suffer from the drawbacks of either narrow bandwidth, subtle phasematching limitations, inevitable spectrum gaps or low emission intensity [1-3, 10-13]. Surpassing natural materials, the freedom of designing our own subwavelengthscale atoms as building blocks of metamaterials or metasurfaces has shown great flexibilities in achieving versatile functions in different disciplines [14][15][16][17][18][19][20][21][22][23][24][25][26]. The coexistence of resonant nonlinearity and local field enhancement in properly designed meta-atoms [23,[27][28][29][30][31][32] open up an alternative route for high-efficiency nonlinear devices that eliminates those common restrictions. Screening effects o...