Giant Enhancement of Nonlinear Optical Processes With Split-Ring Resonators for THz Applications

Plasmonic resonances have been widely applied in nonlinear optics due to their unprecedented capability of concentrating and manipulating light at the deep subwavelength scale. Doubly resonant cavities can confine light of different wavelengths in the same cavity with large modal overlap and strong field enhancement. However, designing of such cavities is usually difficult when the resonant wavelengths differ by orders of magnitude. Here, by using plasmonics to match the modal sizes, we propose a split-ring resonator that resonates in both the near-infrared and terahertz (THz) bands simultaneously, with their fields colocalized within a deep subwavelength gap to achieve strong nonlinear interactions. THz wave generation through difference-frequency generation can be significantly enhanced in this resonator, yielding a 4-order-of-magnitude enhancement of the effective nonlinear susceptibility from the material's intrinsic value. Therefore, such structures can serve as an ideal platform for nonlinear optical processes between the near-infrared and THz waves, which can be employed for THz applications such as THz wave generation and detection.