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The non-linear responses of optical materials offer useful mechanisms for optical switching, novel optical sources, and harmonic frequency conversion. However, the non-linear response of traditional materials is usually extremely weak and requires high input power for excitation. In this study, we theoretically propose a scheme for enhancing the third harmonic generation (THG) efficiency and output power of layered graphene disks array by introducing a plasmonic antibonding state with enhanced oscillation strength due to plasmonic coupling. We verify that, the THG efficiency of a double-layer stacked graphene/SiO2 disk structure under relatively low input intensity can be significantly enhanced more than one order of magnitude with appropriate design, as compared with monolayer patterned graphene nanostructure. We also demonstrate that the THG efficiency can be further improved by optimizing the geometry parameters such as spacer distance and Fermi energy. Our results offer an effective mechanism for significantly improving THG efficiency in the mid-infrared and terahertz ranges, thereby paving the way for new frequency converters and modulators in optical communication and signal processing.

Tungsten oxide is regarded as the most promising electrochromic material owing to its continuously tunable optical properties, low cost, and high coloration efficiency.

We present a unique scheme to efficiently enhance terahertz third harmonic generation (THG). By exploiting the high-order guided-mode resonance of graphene plasmonic grating, the off-plane transverse localized field can be suppressed, which enables the electromagnetic energy to be tightly concentrated and significantly enhanced in the monolayer graphene. Meanwhile, high density of in-plane hot spots can be excited to form a quasi-continuous enhanced-field profile called as hot surface. This can bring about the giant nonlinearity enhancement of terahertz field-graphene interaction, greatly pushing forward the enhanced THG in monolayer graphene to breakout the limitation of various patterned metamaterials. It is anticipated that the graphene plasmonic grating has promising application for nonlinear terahertz spectroscopy, imaging and communication.

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