Increasing the nonlinear optical response at nanometer length scale is a very important issue due to the wide applications in various disciplines such as information science, bio-medicine and quantum computation technology. Second harmonic generation (SHG) arising from the metal nanostructures has provide a very powerful tool in studying the surface and interface properties of these materials. The SHG from various kinds of asymmetric geometric configurations such as V and L shape structures, imperfect nano-spheres, metal/insulator/metal multilayer structures, and planar split ring resonators have been proposed. However, all the previous studies in plasmonic nonlinear optical behavior rely on the enhancement of the electric field and seldom considered the magnetic field effect.
In this work, we present a vertical split ring resonator (SRR) based metamaterial to generate SHG. By adopting such a novel structure, both the electric and magnetic field will be significantly enhanced due to the localized surface plasmon resonance, hence the generation of the second-harmonic and its re-emission into the far field are dramatically increased several orders comparing with that of the planar SRR. We simulated and fabricated the reflective type vertical SRR, and optimized the aspect ratio to maximize the SHG signal. We further systematically studied the nonlinear optical response in the vertical SRR dimers and trimers and found that the gap distance between two SRRs plays a very important role in the SHG intensity. This work paves a new way in increasing the nonlinear transition quantum efficiency and provides a new insight in designing new nonlinear sources.