Localized surface plasmon resonances (LSPRs), arising from the interaction of light with noble metal nanostructures, can be described as the collective oscillations of free conduction electrons. In the past decade, LSPRs in noble metal nanostructures have been combined with nonlinear optics that brings a new concept named “nonlinear plasmonics”. Many nonlinear processes can be boosted based on the strong local field enhancement, including Raman scattering, second- and third-harmonic generation (SHG and THG) and four-wave mixing. As a fundamental and important nonlinear optical effect, SHG in metal nanostructures has been studied widely. Generally, the most popular way to promote SHG conversion efficiency in well-designed plasmonic nanostructures is making use of the resonance enhancement effect. However, SHG is limited by the non-centrosymmetry requirement of the material structure as the nature of even-order optical nonlinearity, leading to a rather weak SHG signal in centrosymmetric nanostructures even in the strong resonance condition. Since the resonance enhancement effect is usually prominent, this relatively small contribution from the asymmetry of the nanostructure is easily neglected or covered by the strong resonance enhancement factors.

The ultrafast optics group led by Prof. Peixiang Lu presents a novel approach to quantitatively extract the contribution of asymmetric local-field from the strong resonance enhancement effect by a white-light supercontinuum signal. The experimental and calculated results all reveal that the pure effective χ(2) increases as the asymmetric degree of local-field distribution increase. This illustrates the importance of asymmetric local-field to χ(2), which provides a new evidence for the nonlinear plasmonics design in metal nanostructures.

This work is published on Opt. Express Vol. 25, No. 2, 1296 (2017). This work was supported by the 973 Programs (2014CB921301), National Natural Science Foundation of China (NSFC) (11204097), Doctoral fund of Ministry of Education of China (20130142110078) and Fundamental Research Funds for the Central Universities (HUST: 2016YXMS015).

Figure The local-field distribution and the SH intensity as a function of the asymmetric degree of the cross-shape nanohole.